Technical Questions and Answers Archive2020-02-26T17:30:07+00:00

The ASA Technical Questions and Answers is a free service offered to all users, but primarily intended for engineers, architects, owners and anyone else who may be specifying the shotcrete process and/or has need for a possible answer to a technical question.

User agreement: The answers provided to submitted questions are intended for guidance in planning and executing shotcrete applications. This information is intended only for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations, and who will accept responsibility for the application of the material it contains. The American Shotcrete Association provides this information based on the best knowledge available to them and disclaims any and all responsibility for the information provided. The American Shotcrete Association will not be liable for any loss or damage arising therefrom.

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I have used ASA’s Position Statements from the Pool and Recreational Committee and find them very useful. Are there any design guides or books on shotcrete pool design that are available? I am a structural engineer and tend to design pools as retaining walls, but I believe some of my designs could be “value engineered” to reduce rebar in the case of walls with a vertical curve (base of the wall is curved and not straight) and possibly the use of a bond beam at the top.2023-06-07T16:35:16+00:00

The current International Swimming Pool and Spa Code (ISPSC) has no provisions for design of concrete pool shells. Many structural engineers use ACI 350 Code Requirements for Environmental Engineering Concrete Structures for pool structures, especially commercial pools. ACI 350 is based on ACI 318 Building Code Requirements for Structural Concrete but has modifications to provide a design for concrete structures that are normally exposed to water, and thus need more crack control for watertightness. ACI 350 also addresses requirements for durability for concrete exposed to liquid so that structures will be expected to be serviceable for at least 50 to 100 yrs. Some engineers feel ACI 350 is overkill for pools and may use ACI 318, or just use their past experience. Generally, use of ACI 350 will require a higher percentage of reinforcing steel, have closer steel spacing and somewhat reduced tension in bars to control cracking. ACI 350’s concrete cover provisions may also be somewhat higher than ACI 318, to provide more corrosion protection of reinforcing. ACI Committee 506 is developing a guide document for construction of shotcrete pools, but does not directly address design. ACI has recently authorized a new technical committee to develop a Code for Design of Pools and Watershapes. However, staffing the committee and then developing a consensus standardized document will take several years.

Regarding the cove of the floor-wall joint, if you have a cove or the bond beam you can use the additional “d” distance for your vertical steel from external loads on the walls though the moment has to be carried in the thinner sections of the floor and wall adjacent to the cove. If you consider the bond beam is a stiffening element for the top of a straight wall acting as a panel between the ends in a rectangular plan pool you may be able to reduce some of the vertical bending stresses. However, that may be hard to quantify for a freeform pool without a more advanced analysis.

Regarding reference books you may find David Billington’s Thin Shell Concrete Shells useful for analysis and design of concrete shells. It is an old book (1982 for 2nd Edition) and may be hard to find but may be helpful. Hopefully ACI’s new Code Committee for Pools will set the standard of practice in the pool industry and be specifically referenced by ISPSC.

What would be the right concrete mixture for a swimming pool? I found out that a few concrete plants have different mixes, so I wanted to know what would be the correct one.2023-06-07T16:34:18+00:00

There is no special requirement for shotcrete placement in pools. Basically, you are building a watertight structural concrete shell. Concrete should be a minimum 28-day compressive strength of 4000 psi (28 MPa). Most shotcrete uses a 3/8 in. (10 mm) maximum coarse aggregate size due to our small diameter delivery lines. The coarse aggregate (rock) being about half the weight of the sand content. We typically need a cement-rich paste so minimum cementitious content (cement, fly ash, slag) of 700 lb/yd3. We also need a low w/cm ratio for the ability to stack in the wall so maximum of 0.45, with most wet-mix concrete 0.42 or less. You may find our ASA Position Statements for our Pool and Recreational Committee helpful in providing more detail. Our current statements include:

  • Compressive Strength Values of Pool Shotcrete
  • Shotcrete Terminology
  • Sustainability of Shotcrete in the Pool Industry
  • Watertight Shotcrete for Swimming Pools
  • Monolithic Shotcrete for Swimming Pools (No Cold Joints)
  • Forming and Substrates in Pool Shotcrete
  • Curing of Shotcrete for Swimming Pools

You can find the position statements freely available at www.shotcrete.org/products-services-information/resources/.

A new gunite pool of ours was sprayed in an irregular fashion by a non-certified worker in the Bahamas. For the most part, the pool looks good, but one wall was measured at 3 in. The rest of the pool is 6 in. The rebar in the thin area was encapsulated which was good, but a couple of linear cracks in the wall formed even after ample wetting during the initial cure period. Six months have gone by. Our plan now is to pressure wash the cracked areas and add an additional 6 to 8 in. of gunite thickness which may not look too bad since the pool is a natural lagoon style pool. Staples with gunite over the top would be the other option. What is your opinion?2023-06-07T16:33:47+00:00

Properly prepared surfaces along with proper shotcrete materials, equipment and placement techniques will produce a construction joint that acts monolithically and not be a “cold” joint. Shotcrete placed onto an existing concrete surface will provide an excellent bond IF the following conditions are met:

  1. Make sure the surface is roughened and clean.
    1. The amplitude the of roughness should be +/- 1/16th in. (1.6 mm) or more.
    2. If the surface was not roughened when it was shot be sure to have the contractor roughen it.
    3. A high-pressure water blaster (5000 psi [34 MPa] or more) or abrasive blasting can help to roughen and clean the surface.
  2. Bring the concrete surface to saturated surface dry (SSD) condition. This means the surface feels damp, but water is not picked up on a hand.
  3. Make sure the shotcrete placement is properly executed with high velocity placement and quality materials.
    1. The dry-mix shotcrete (Gunite) should have a minimum 28-day compressive strength of 4000 psi (28 MPa).
    2. Be sure the shotcrete contractor is using an air compressor able to produce at least 385 CFM (11 m3/min) of air flow at 120 psi (0.83 MPa).
    3. Use of an ACI-certified shotcrete nozzleman is recommended.
  4. No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.

This article on the excellent bond between shotcrete provides more detail: https://shotcrete.org/wp-content/uploads/2020/05/2014Spr_TechnicalTip.pdf

The pool wall thickness is a part of the pool engineer’s design. Adding 6 to 8 in. to the existing 3 in. (75 mm) would be making the overall thickness 9 to 11 in. (225 to 275 mm). This would require more reinforcing steel, so you should verify with the pool designer the additional reinforcing steel requirements.

You may also find our ASA Position Statements on Pool Construction helpful as a reference for proper pool shotcrete construction. They are freely available at: https://shotcrete.org/products-services-information/resources/

ACI 318-14 (Building Code Requirements for Structural Concrete) requires post-installed expansion anchors to meet the testing criteria of ACI 355.2-07 (Qualification of Post-installed Mechanical Anchors in Concrete). ACI 355.2 specifies certain anchor testing and evaluation requirements to verify suitable anchor performance and to determine other aspects (such as failure mode) to use in conjunction with Chapter 17 of ACI 318 when designing the post-installed expansion anchors. Anchor testing is required largely to be performed by an independent agency and normally is conducted in normal weight and/or light weight concrete that meet pertinent ACI, ASTM and other requirements. Some post-installed expansion anchor manufacturers (like Hilti) have not had their anchors tested per ACI 355.2 in shotcrete type concrete, only tested in normal weight and light weight concrete. As such, these anchor manufacturers typically do not publish/offer any permitted load ratings, installation torques or other design and installation requirements for their expansion bolts when used in shotcrete. Instead, they recommend site testing to determine anchor performance or that the responsible design engineer can make an engineering judgment on anchor acceptability, as appropriate, if site testing is not performed. Do you have knowledge of any expansion bolt manufacturers that have tested their products is typical shotcrete? If yes to #1 above, do you know if the testing was done per ACI 355.2 requirements?2023-06-07T16:32:55+00:00

Shotcrete is a placement method for concrete. With proper equipment and placement techniques, concrete shotcreted in place will have strength, unit weight, permeability, and other hardened properties equivalent or superior to cast concrete consolidated by vibration. Due to delivering concrete material through relatively small diameter lines (1.5 to 2 in.) [38 to 51 mm] concrete mixtures for shotcrete placement typically limit the maximum coarse aggregate size to a nominal 3/8 to ½ in. (9.5 – 13 mm) size.

Thus, answering your specific questions:

  1. We are not aware of any expansion bolt manufacturers tests that used shotcrete placement of concrete for their test samples. However, tests on cast concrete should be equivalent with a given compressive strength and aggregate size/type in the concrete mixture.
  2. We expect that as answered in #1, that the tests run with ACI 355.2 requirements in cast concrete would have similar results when used with shotcrete placement of the concrete mixtures with similar hardened properties.

Also, note that ACI 318-19 directly includes shotcrete as a placement method for structural concrete.

We have a wet-mix shotcrete steel fiber overhead application progressing in our state. The question is about the use of a steel trowel finish, as opposed to say a magnesium or wood float finish. In the ASA Shotcrete Inspector seminar, it was stated that a steel trowel is less durable, reduces freeze-thaw resistance and shows cracking more proximately. As this particular application is overhead and, in a tunnel, there is not as much of a concern with water infiltration and the associated freeze-thaw exposure. We usually don’t allow steel trowels for flat work, due to deicing salts, but that concern wouldn’t apply here. My superintendent has asked me to reach out to you to see if you might have any further detailed advice on this type of application. Construction is wanting a smooth finish and looks do matter here as it is a high-profile project. If the DOT were to allow the steel trowel for finishing, what would be your concerns or suggestions to this approach?2023-06-07T16:31:59+00:00

Freeze-thaw deterioration is dependent on the concrete being saturated in multiple freezing/thawing cycles. In an overhead application, where water can’t stand on the surface, the concrete can’t be saturated unless water permeates through from the upper surface. And with good quality concrete in the tunnel, water shouldn’t permeate through, so it should be functionally watertight. As a result, freeze-thaw likely isn’t a critical durability issue.

A steel trowel finish does require extra working of the surface and would require the contractor to be very attentive to the proper time to obtain the finish yet not overly disturb the fresh concrete. Gravity is working against the overhead concrete staying in place.

Having a smooth steel trowel finish would make minor shrinkage cracks more noticeable. However, in the tunnel without exposure to sunlight or much wind exposure, and with proper attention to curing, perhaps surface cracking will be minimal.

I have a question regarding shotcrete pools. Does the ASA have a position on how to detail reinforcement at bulky elements that are shot interior to the main pool shell? This would typically involve stairs or large stoops. I notice a lot of contractors shoot these as unreinforced bulk elements, but this practice appears to promote cracking at the face of the pool shell. I’m only asking because I saw a few of these this past summer.2023-06-07T16:31:15+00:00

Shotcrete is a placement method for concrete. Thus, any concrete structure using shotcrete placement should be designed using appropriate concrete design codes and standards. ASA does not have a published position on reinforcement of these types of pool elements though we are in the midst of developing one. As most concrete experiences drying shrinkage and associated cracking, most designers would include some level of reinforcement in these types of sections to control tensile stresses from shrinkage and temperature changes creating volume change in the concrete. ACI 350 (Code Requirements for Environmental Engineering Concrete Structures) is the ACI Code that deals with concrete liquid-containing structures with provisions specifically for providing liquid tightness and durability in continuously wet environments. The ACI 350 Code requires up to 0.5% reinforcement for shrinkage and temperature stresses. ACI 318 is the Concrete Code for Structures and requires 0.18% minimum reinforcement for shrinkage and temperature. Designers may choose to use the lower ACI 318 value since they consider the benches and steps not part of the water retaining pool shell. Other designers would consider the higher ACI 350 values as they are interested in better crack control. Overall, having a substantially unreinforced thickness of concrete would lead to more cracking that would be problematic in the pool.

I was taught in engineering courses that conventional concrete should not be counted on to carry tensile stress. For steel reinforced concrete, the reinforcing bar is designed to carry all tensile loads. Although concrete obviously has some tensile strength, it is too low and prone to cracking failure to consider it in design. In fact, I believe you can assume it is cracked from the shrinkage during curing. Is gunite treated the same way? I have a pool that is developing a crack through an elevated wall/beam and down into the plaster to the bottom floor at the sun shelf. I witnessed the plumbers cutting some rebar in the beam to allow for PVC plumbing to water sheer (up at top of beam, just under the tile topping) and I worry this is the root cause along with settlement that put the top of the beam in tension. The rebar down low should be intact and I hope the crack width may stay minor down in the plaster. On top of the tiles beam where the maximum tensile stress would have been, the crack is fairly wide. The crack movement opened up a gap in the grout line between tiles of about 0.08 to 0.10 in. (2-2.5 mm). I think it was a real sin for them to have cut the rebar. If it is necessary to reinforce the tensile side to halt future movement, I would think cutting a slot or two in the gunite across the crack (say 12 in. [300 mm] each side. Up high just under the water sheer) and epoxy a rebar in the slots.2023-06-07T16:30:31+00:00

Shotcrete, both dry-mix (gunite) and wet-mix are a placement method for concrete. Wet-mix uses premixed concrete while dry-mix simply adds water to the concrete materials at the nozzle. Both dry-mix and wet-mix with proper materials, equipment, and placement with produce quality concrete sections. The embedded reinforcement in the pool shell is designed to carry tensile loads. This may be bending stresses from structural loadings (settlement or water/backfill), or volume changes from drying shrinkage and temperature changes. Cutting a reinforcing bar would certainly negate its ability to carry loads in the vicinity of the cut and reduce the load carrying capacity until the development length allows the reinforcing bar to start carrying it full load.

The layout of your cracked section isn’t clear from your description. An 8 to 10 mil (2 to 2.5 mm) crack is sizable in a water-containing structure. Fixing the existing crack with a reinforcing bar epoxied in place across the crack may be effective. However, that solution would only carry any additional load on the section (structural or volume change), as the existing loads have already created cracks. Thus, you should also address filling the crack as part of the solution. This may be with epoxy injection or swellable polyurethane grouts. You should consult with the pool design engineer for their recommendation on the best method for repair.

We are reinforcing an exterior pool wall with shotcrete, and wanted to know what preparations need to take place for the shotcrete to adhere correctly, and what the minimum thickness needs to be? We also need to level out the floor up to 5 in. (125 mm) that will gradually go to zero to meet other side. Can shotcrete be used in this application?2023-06-06T23:45:29+00:00

When shooting onto existing concrete sections the surface must be properly prepared and then shotcreted with proper shotcrete materials, equipment, and placement techniques. This will produce a construction joint that acts monolithically and not be a “cold” joint. Shotcrete placed onto an existing concrete surface will provide an excellent bond IF the following conditions are met:

  1. Make sure the surface is roughened and clean.
    1. The amplitude of roughness should be +/- 1/16th in. (1.6 mm) or more.
    2. If the surface was not roughened when it was chipped out, be sure to have the contractor roughen it.
    3. A high-pressure water blaster (5000 psi [35 MPa] or more) or abrasive blasting can help to roughen and clean the surface.
  2. Bring the concrete surface to saturated surface dry (SSD) condition. This means the surface feels damp, but water is not picked up on a hand.
  3. Make sure the shotcrete placement is properly executed with high velocity placement and quality materials.
    1. The shotcrete should have a minimum 28-day compressive strength of 4000 psi (28 MPa).
  4. Be sure the shotcrete contractor is using an air compressor able to produce at least 185 ft3/min (5.2 m3/min) for wet-mix and 385 ft3/min (11 m3/min) for dry-mix (gunite) of air flow at 120 psi (0.8 MPa).
  5. Use of an ACI-certified shotcrete nozzleman is recommended.
  6. No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.
  7. A minimum thickness of no less than ½ in. (13 mm) is recommended.

This article on the excellent bond between shotcrete provides more detail: www.shotcrete.org/wp-content/uploads/2020/05/2014Spr_TechnicalTip.pdf:

Regarding the additional floor thickness, though it may be shotcreted by an experienced nozzleman, it is difficult to properly shoot horizontal surfaces and control rebound and overspray from the shotcrete placement. We suggest that casting and vibrating for consolidation the horizontal sections is preferred to shotcreting. You should consider a bonding agent since the concrete is cast against the existing concrete floor without any impact velocity. We would also not recommend tapering down to 0 in. thickness. The feather edge will tend to be an area that may easily spall over time. Thus, we recommend cutting an ½ to ¾ in. (13 to 19 mm) deep shoulder so the concrete can have some thickness at its thinnest locations.

I’m in the market for a saltwater pool and was wondering what the ASA recommended concrete compressive strength should be. I’ve read California mandates a 2500 psi (17 MPa) minimum, but some construction companies use 5000 psi (35 MPa). What is the ASA standard for a saltwater pool?2023-06-06T23:44:48+00:00

Our ASA Pool Position Statement on Compressive (Strength) Values of Pool Shotcrete states pool and shotcrete contractors have a responsibility to provide a pool structure that not only meets certain design specifications, but also meets basic durability values expected with shotcrete applications. The American Shotcrete Association’s (ASA) Pool and Recreational Shotcrete Committee and ASA Board of Direction have reaffirmed a 4000-psi minimum for in-place compressive strength pool concrete.

As a saltwater pool has a higher chloride content than fresh water you may want to consider using the requirements of ACI 350 (Code Requirements for Environmental Engineering Concrete Structures) where Table 4.2.2 – Requirements for Special Exposure Conditions has this requirement:

“For corrosion protection of reinforcement in concrete exposed to chlorides in tanks containing brackish water and concrete exposed to deicing chemicals, seawater, or spray from seawater – maximum water-cementitious. Materials ratio, by weight is 0.40 with a minimum 28-day compressive strength of 5000 psi.”

In summary the minimum 28-day compressive strength should be no less than 4000 psi, but for enhanced durability you may consider 5000 psi as required by ACI 350. You may find more guidance in our ASA Pool Position Statement “Watertight Shotcrete for Swimming Pools” at www.Shotcrete.org/Resources.

 

I am a TBM Tunnel Engineer from India, and I was looking for information on the applicable compressed air pressure range required for a wet-mix shotcrete application (small shotcrete pump-capacity 7 CU.M/Hr) hand spraying with a 30m hose for a better-compacted mix. I would kindly request you to please send me information on the pressure range to be expected for good quality shotcrete placement of the concrete mix on the rock substrate in NATM Tunneling.2023-06-06T23:44:11+00:00

Wet-mix shotcrete depends on air flow at the nozzle to accelerate the concrete to 60 to 80 mph (100 kph to 130 kph). Most air compressors produce their air flow capacity at 100 to 120 psi (7 to 8.4 kg/cm2) at the compressor. However, depending on the size length and couplings in the air hose, there may significant pressure drops when the air reaches the nozzle. Here’s what ACI 506R-16 Guide to Shotcrete Section 4.4.2 states for wet-mix:

“The recommended ft3/min (m3/min) needed for the wet-mix process is between 200 to 400 ft3/min (5.7 to 11.3 m3/min) air volume at 100 psi (7 bar). Higher air volume capacities are needed for higher volume and higher-velocity shotcrete applications. If a blowpipe is to be used during the shooting process, more air will be required to run both operations simultaneously. Conducting a test during the preconstruction testing phase using a blowpipe while gunning the wet-mix material will indicate if the air compressor has enough air volume capacity to perform both tasks at the same time. Long, small-diameter lines may not provide sufficient air volume capacity, even with a large air compressor. Test and consider increasing the size of the air line.”

Though there is no direct guidance for air pressure at the wet-mix nozzle you may consider the guidance for dry-mix air pressure in ACI 506R Section 4.4.1:

“The operating air volume (ft3/min [m3/min]) drives the material from the gun into the hose, and the air pressure is measured at the material outlet or air inlet on the gun. The operating pressure varies directly with the hose length, the density of the material mixture, the height of the nozzle above the gun, and the number of hose bends. Experience has shown that operating pressures should not be less than 60 psi (4 bar) when 100 ft (30 m) or less of material hose is used, and the pressure should be increased 5 psi (0.34 bar) for each additional 50 ft (15 m) of hose and 5 psi (0.34 bar) for each additional 25 ft (7.5 m) the nozzle is above the gun.”

The minimum 60 psi (4 bar) necessary for dry-mix could be applied to the wet-mix air supply as the velocity created by the air flow is similar.

I would like to know the fire rating information on a shotcrete wall, 8 in. thick.2023-06-06T23:43:22+00:00

Shotcrete is simply a placement method for concrete. Thus, fire resistance of any shotcreted concrete section can be evaluated by consulting ACI CODE-216.1- 14(19) Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies. There are many factors that affect the fire resistance so you will need to review the code to establish what’s appropriate for your structure.

Is there a guide to determining pressure applied to a form during shotcrete placement? I have seen references to 50 lb/ft2 (240 kg/m2) but no backup to this. Is the ACI formwork design guide applicable to shotcrete in some way?2023-06-06T23:42:54+00:00

Since shotcrete is pneumatically placed against a one-sided form there is no liquid concrete pressure against the form. When bench shooting vertical walls the majority of pressure from the high velocity impact is carried by the previously placed concrete as the concrete is stacked. Plus, this pressure is very localized, only affecting a small zone immediately adjacent to the impact area of the material stream. Here is an article on a research project that quantified the force https://shotcrete.org/wp-content/uploads/2020/05/2007Sum_TechnicalTip.pdf

From the paper conclusion “In normal spraying conditions, wet- and dry-mix shotcretes produced a force on the panel of about 45 and 20 lb (200 and 90 N), respectively. The maximum load recorded is 87 lb (389 N) and it was observed in simulating a water plug.”

If your shotcrete form is exposed to wind forces before placing shotcrete, you may want to consider the wind pressure that may be expected across the entire form during construction to keep the form intact. This should be much less than the 50 lb/ft2 pressure you mention. ACI formwork design is intended for cast concrete where liquid concrete is contained within a two-sided form and is NOT applicable to shotcrete placement. Some sources show a 60 mi/hr (100 km/ hr) wind exerting about 10 lb/ft2 (50 kg/m2) on a vertical wall.

What is the efficiency of dry-mix shotcrete? How much is over spray vs. how much sticks to the surface?2023-06-06T23:42:21+00:00

Dry-mix shotcrete may have more rebound (coarse aggregate that bounces off the surface) than wet-mix so it may be considered a little less efficient. However, predampening and the use of special nozzles can increase wetting and reduce rebound which makes the dry-mix efficiency approach wet-mix. Rebound may be estimated as 5 to 15%, with an average of 10% of the weight of the concrete materials. The experience and placing technique of the nozzleman can substantially affect the amount of rebound in either dry-mix or wet-mix. Overspray is much less and may depend on wind conditions and placing techniques. When looking at the overall efficiency of shotcrete placement in a given section, dry-mix materials can be tailored much closer to the actual need while wet-mix may have minimum concrete delivery volume and time constraints that would end up not using all the material delivered. Also, dry-mix has approximately ¼ the production rate of wet-mix, so in high-volume placements, wet-mix may have a natural advantage in productivity.

 

Is it true that shotcrete compression tests are more accurate and are likely to be higher (better) results? Basically, is there a difference when testing wet-mix shotcrete applied pneumatically as compared to simply being taken directly in cylinders from the batch plant or ready-mix delivery?2023-06-06T23:40:42+00:00

Shotcrete placement provides full consolidation of the concrete by high-velocity impact. Concrete placed into cylinders for testing is consolidated by multiple rodding in three layers. Shotcreting also has some percentage of rebound so the concrete mixture that remains in the panel is more paste-rich than the mixture entering the pump. Thus, shotcrete placement may provide better consolidation, and a more paste-rich in-place mixture resulting in higher compressive strengths. However, shotcrete compressive strength is evaluated by cores extracted from panels. The coring process can create microcracking in the exterior surface of the core and produce slightly lower compressive strength than cylinders that have no damage to the outer surface when removed from the cylinder form and tested. Overall, there doesn’t seem to be a significant difference when evaluating the concrete material’s strength by shotcrete placement or concrete cylinders taken before pumping.

You could establish a correlation on a specific project by taking cylinders before pumping and then shooting material test panels. Then testing the cylinders and cores from the panel at the same age.

I was wondering if ASA has any safety meeting/ tailgate presentations on high-pressure shotcrete work. I need to ensure my team knows how to properly disconnect hoses and align proper fitting procedures. Furthermore, do you have any guidance on a repair procedure for the rubber hose?2023-06-06T23:41:42+00:00

We have an ASA document, “Safety Guidelines for Shotcrete,” that addresses shotcrete safety. It is not a job specific safety plan but gives you guidance on the information you may include in your plans. You can find it on our bookstore at https://shotcrete.org/bookstore/?productpage=2. A free copy of the Safety Guidelines is provided to all of our corporate and sustaining members. Membership also provides many other benefits, including discounts on shotcrete nozzleman certification and participation in our committees. We have a committee specifically devoted to Education and Safety for shotcrete that is very active.

Regarding the repair of the rubber hose, there is no procedure to repair any breaches in the hose itself. A damaged or excessively worn hose should never be used as the pressure that builds when a delivery line plugs during pumping is extremely dangerous. Modern pumps can reach 2000 psi (14 MPa) internal concrete pressure when experiencing a plug and the hose must be capable of carrying that high pressure. The wet-mix shotcrete hose is heavily reinforced, and the couplings are designed for high pressure. Your crew must be sure that all clamps are fully engaged on the heavy-duty couplings, and that safety pins are in place.

We have an existing historic 4 in. (100 mm) hollow clay tile wall that is finished with a 3-coat gypsum plaster. We would like to spray shotcrete on the non-finished side of the wall to strengthen it. Will the application of shotcrete on the back side of the wall possibly damage the historic plaster and paint? Will there be too much of a vapor drive from the application for the plaster to hold? Have you experienced plaster deterioration or loose keys from the application of shotcrete on the backside of a wall?2023-06-06T23:39:44+00:00

As long as the hollow clay tile wall is rigid and stable, the shotcrete application should not impact the finished side plaster. Shotcrete impacts in a very localized area directly where the material stream is hitting the substrate. Research shows that the localized force is about 90 to 100 lbs (40 to 45 kg) when shooting directly on the substrate. If shooting a thicker wall using a benching method, most of the force is carried by the previously shot material, so it would have less impact. If the clay tile needs to be stiffened, an initial thin layer could be shot to provide additional thickness before the final thickness is placed. Though we don’t have any specific information about vapor transmittal, concrete used in shotcrete placement inherently has a low w/cm and less water in the mixture to bleed or create vapor. We haven’t had any reports of plaster or grout falling off the inside of masonry walls that have been structurally enhanced with shotcrete.

We have a client that has requested a shotcrete application for a dirt crawlspace. The facility is located in the Northeast part of New York. I’m not the designer, but I believe we are looking at a wet-mix, applied at a 4 in. (100 mm) depth throughout the crawlspace. I had a few questions I was hoping could get answered as we move through the technical specifications process. 1) Is there an off-season for the shotcrete product? Does it have to be applied in warm weather? Again, this is upstate NY. 2) What kind of equipment is used to dispense/place the product? (same as concrete?) 3) What is the noise level of the dispensing equipment? Typical of any concrete pour or much louder? The facility is a childcare center, so they are concerned about noise levels.2023-06-06T23:38:53+00:00

Here are the answers in the same order as your list.

  1. Shotcrete is just a placement method for concrete, so there is no “off-season.” However, as with cast concrete, cold weather placement needs more planning and material delivery controls. As wet-mix shotcrete material is predominately delivered by ready-mix trucks, the concrete should be delivered at 50°F (10°C) or higher. Then once shot, if temperatures are less than 50°F, the concrete should be protected by insulated blankets or the area enclosed by vented heaters to keep temperatures above 50°F. We also don’t want to shoot onto frozen surfaces.
  2. We use small line concrete pumps standard in the concrete industry.
  3. Concrete pumps and air compressors are generally diesel-powered engines. Pump engines range in horsepower from 75 to over 200 hp, depending on the pump size. Your application is of relatively low volume, so one of the smaller pumps should do fine. We also use relatively small air compressors, and many contractors who work in residential or metropolitan areas use equipment equipped with sound reduction. The sound at the point of placement where the air-accelerated material stream exits the nozzle is generally just the sound of the airflow.
I would like to ask if there are any articles, references, etc, which reference procedures to determine the maturity of the concrete applied via shotcrete? Basically, how to generate the validation curves?2023-06-06T23:28:35+00:00

Shotcrete is a placement method for concrete. Thus, generating the maturity curves would be based on the concrete mixtures. There are several online resources about the maturity method. One that discusses production of the curves is from the Minnesota DOT and available in PDF format at www.dot.state.mn.us/materials/concretedocs/MaturityMethodProcedure.pdf.

I am working on a project that has existing tunnels made with shotcrete. I am needing to hang 12 in. (300 mm) duct and trying to figure out the best type or suggested anchors to use.2021-07-26T17:54:27+00:00

Shotcrete is just a placement method for concrete. Shotcrete placement with proper materials and application techniques should provide monolithic concrete with a 28-day compressive strength of at least 4000 lb/in2 (28 MPa). Thus, any systems that work in concrete should be fine. Either mechanically-fixed or epoxy-set anchors are commonly used in concrete. You should consult with the anchor suppliers for the size and type of anchor appropriate for your specific application.

We have shotcrete test panels, 12 by 12 by 6 in. (300 by 300 by 150 mm). My question is it OK to move the panels right after shooting to different location for initial curing or should the panels be left untouched for certain period of time at the spot of shooting?2021-07-26T17:53:30+00:00

In normal weather conditions test panels should be undisturbed for at least 24 hours. In cold weather you must protect the panels from freezing and preferably keep the concrete surface temperature above 50 to 55 °F. That allowsthe young concrete to gain enough strength to tolerate movement. Also, your test panels are very small in comparison to current ACI 506.2 Specification for Shotcrete requirements that have a minimum of 16 by 16 by 51⁄2 in. (400 by 400 by 140 mm) dimensions. When coring your smaller panels you should be sure to have the nearest edge of the core 3 in. (75 mm) from the sides to preclude the effect that rebound collection in the corners may cause.

I am in the process of hiring a contractor to install a pool at my home. I have a long driveway, about 350 ft (100 m) and the pool will be constructed behind my house. The contractor wants to remove a substantial amount of plantings to allow the shotcrete truck to get closer to the pool area. How far can you reasonably pump shotcrete horizontally and what type/brand pump would be needed?2021-07-26T17:51:51+00:00

Shotcrete placement conveys concrete materials through a delivery line that ranges from 11⁄2 in. (38 mm) to 21⁄2 in. (64 mm) in diameter. A 2 in. (50 mm) diameter at the nozzle is most common. Wet-mix shotcrete pumps wet concrete through the line. Dry-mix conveys dry concrete materials through the line and adds water at the nozzle to make concrete. Using proper concrete materials, equipment and placing techniques both wet-mix and dry-mix should provide high strength, durable concrete for your pool. Wet-mix is more difficult to pump as it has more internal friction pushing the low-slump concrete through the line. Shotcrete contractors will try to minimize the delivery line length in wet-mix to make pumping easier and reduce the chance for plugs in the line. With proper planning, concrete mixture selection, and delivery line choices, wet-mix shotcrete can be pumped 500 ft (150 m) or more. The shotcrete contractor can increase the pumping distance by using steel pipe for most of the delivery line with rubber hose the last 50 to 75 ft (15 to 23 m), or using larger diameter hose or pipe and then reducing to the final size closer to the nozzle. If the shotcrete contractor is using dry-mix (often called Gunite) they should easily be able to reach over 500 ft as the delivery hose is mostly full of air conveying the dry concrete materials. This means you don’t have nearly the internal friction that would tend to cause plugging of the line, and thus allows for longer delivery lines.
Thus, if using wet-mix, check with the shotcrete contractor and see if they can make adjustments to their delivery lines to allow a greater pumping distance to accommodate your lot. If using dry-mix, they should have no problems running the hose to your back yard assuming they have enough hose.

We have a cast-in-place wall with extensive rock pockets and voids from inadequate vibration during casting. One option is to tear down the wall and replace, however we are wondering if shotcrete can be used to repair the deficiencies. If so, what are the recommended procedures to prepare and shotcrete the repairs?2021-05-26T18:14:16+00:00

Shotcrete is a great solution to your wall casting issue. In all shotcrete repair to get the best bond you need to:
1. Chip back to sound substrate – all the rock pockets and voids should be chipped out (or you can use hydrodemolition) to sound concrete.
2. If the chipped-out area is deep into the wall, make sure to have the opening at about a 45° angle from the back of the chipped out area to the surface so that the air flow providing shotcrete’s high velocity can escape and not be trapped.
3. Do not feather edge the perimeter of the repaired opening. Provide a ¾ to 1 in. (19 to 25 mm) roughly square shoulder at the perimeter edge. If this is sawcut make sure the sawed surface is roughened before shotcreting.
4. Thoroughly clean the chipped-out area to remove all dust.
5. Bring the entire chipped out area to a saturated surface dry condition.
6. Do NOT use a bonding agent. It will detract from the inherent excellent bond of shotcrete.
7. Use an experienced shotcrete nozzleman (ACI-certified in the vertical orientation for the process being used) with a quality concrete mixture, and proper shotcrete equipment.
8. Make sure the shotcrete finishers are experienced and do not tear or delaminate the shot sections.
9. Protect the freshly shot and finished sections from freezing or extremely hot weather.
10. Cure the shot sections for a minimum of 7 days. A water cure is preferred to a curing membrane. Either wet-mix or dry-mix would be suitable for your project.

The shotcrete contractor you select for the project should recommend the process they are best suited for based on their crew experience and equipment. Appropriate testing for this type of repair may include compression testing of the materials from shotcreted panels (ASTM C1140 Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels, ASTM C1604 Standard Test Method for Obtaining and Testing Drilled Cores of Shotcrete, and ACI 506.2 Specification for Shotcrete), and bond pull-off tests to verify the bond of the shotcreted material to the original substrate. For more guidance on shotcrete and its use in concrete repairs, you may want to review ACI 506R-16 Guide to Shotcrete, as it can give you more detailed information about shotcrete materials, surface preparation, shotcrete crews and placement, testing, protection and curing.

Can carbon nanotubes be used in shotcrete?2021-05-26T18:11:39+00:00

Shotcrete is a placement method for concrete so most admixture or supplemental cementitious materials that can be used in cast concrete will work with shotcrete placement. In fact, shotcrete contractors have been some of the most innovative adopters of new concrete technologies. Silica fume (microsilica) is a ultrafine particle 100 times smaller than cement that enjoys early and wide use in shotcrete due to its ability to make concrete stickier and more cohesive. This facilitates overhead placements and can provide thicker layers. Shotcrete has also used other ultrafine and nanoparticles, like colloidal silica, clay-based particles and the carbon nanotubes you mentioned.

The ultrafine and nanoparticles can provide many benefits to fresh concrete, as well as hardened properties. This may include:
• Improve the pumpability of wet-mix concrete;
• Reduce rebound and dust due to increased “stickiness”;
• Ease the finishing process;
• Reduced permeability by filling pores between cement in the paste;
• Enhanced corrosion resistance for embedded reinforcement;
• Enhanced resistance to chemical attack.

Can a gunite bridge surface be painted? And if so, what preparation/materials are recommended.2021-05-26T18:10:39+00:00

Yes, shotcrete is a placement method for concrete, so any coating appropriate for concrete would be applicable specify new concrete should be a certain age before applying their coating. Generally, the concrete surface should be clean and dry before coating. The surface texture provided on the shotcrete can affect the coating application. A hard, smooth steel trowel finish will tend to be quite slick, and the coating may not bond as well as a floated or sponge finish. A light abrasive blast may be considered to roughen the surface and give more bond. If using a gun or rodded finish for the shotcrete, the coating will generally require quite a bit more material to be able to fill the depressions in the surface.

Please discuss the differences in curing/shrinking cracking challenges for wet-mix shotcrete versus dry-mix shotcrete (gunite). In my limited experience, wet-mix shotcrete tends to crack easier than dry-mix shotcrete. In general terms, or on average is gunite more porous, but less likely to show cracks?2021-01-04T21:20:05+00:00

Both wet-mix and dry-mix shotcrete when using proper materials, equipment and placement techniques will produce high strength, low permeability concrete in-place. Dry-mix shotcrete will tend to have a lower w/cm since water is added to the dry concrete materials at the nozzle. Wet-mix needs a higher w/cm and a fairly high cement paste content to facilitate pumpability. This can make wet-mix more susceptible to plastic or drying shrinkage cracking than dry-mix. However, cracking in either dry-mix or wet-mix shotcrete can be controlled by using fogging of freshly finished surfaces and then early curing of exposed shotcrete surfaces.

Wet-mix still has very low w/cm (0.40 to 0.45) to allow vertical and overhead placement without sloughing or falling out and is lower than much of the form-and-poured concrete. The low w/cm and high velocity impact produces excellent compressive strength and low permeability. Properly placed dry-mix will have similar compressive strength and permeability as wet-mix. strength gain.

I would like to know how could I obtain an expert opinion from ASA stating that a particular person specializes in gunite (dry-mix shotcrete) operations and that the person is amply qualified to perform such work?2021-01-04T21:18:26+00:00

ASA is the international trade association dedicated to advancement of shotcrete placement. The American Concrete Institute (ACI) has a long history of certification of individual craftsmen in concrete construction. Nearly 20 years ago ASA worked closely with ACI to establish the ACI Shotcrete Nozzleman certification program to evaluate a nozzleman’s ability to place quality shotcrete. ASA is the primary group that conducts the ACI Nozzleman Certification sessions around the world. We provide a full day of education before the formal ACI certification and have a team of examiners with proven shotcrete expertise to rigorously conduct the sessions to ACI’s high standards. Many engineers specify that all the nozzlemen on their projects must have ACI Nozzleman Certification. Many concrete-related codes, specifications and standards require ACI-certified shotcrete nozzleman for shotcrete placement of structural concrete.

Thus, ASA doesn’t offer an expert opinion on individuals as an association but provide the ASA education and ACI certification upon request. You will find full details of the session services we can provide on our website at shotcrete.org/ education then clicking on the “Shotcrete Nozzleman Certification Program” link. An article that covers in greater detail the overall shotcrete nozzleman education and certification process ACI Nozzleman Certification—Why, Who, When, and How can be found on our website: shotcrete.org/ wp-content/uploads/2020/05/2018Win_Hanskat.pdf

Also, many of our corporate members offer consulting services. You can find them in our free, online Buyer’s Guide at Shotcrete.org/BuyersGuide. On the web page you can select the category, subcategory and geographic location to narrow your search.

 

We are working on a design-build project located in southern California of which the perimeter walls of the underground parking structure are planned to be shotcrete. Do you recommend any particular ACI publication that would be applicable to the shotcrete operation or does ASA have their own publications, similar to ACI, that are more appropriate to shotcreting?2021-01-04T21:17:22+00:00

ACI Committee 506 is the technical committee responsible for creating and maintaining the ACI documents related to shotcrete. ACI 506R-16 Guide to Shotcrete is an excellent resource for shotcrete information including materials, equipment, crew composition and placing techniques. It is a non-mandatory document that is very descriptive and readable. ACI 506.2-13 (18) Specification for Shotcrete is another excellent resource that has mandatory requirements for the contractor’s shotcrete placements. Additionally, the ACI 318-19 Building Code Requirements for Structural Concrete directly addresses shotcrete for use in buildings.

A past article in Shotcrete magazine describes the additions to cover shotcrete in ACI 319-19: shotcrete.org/wp-content/ uploads/2020/06/2019Fal_HanskatHollandSuprenant.pdf

Another past article dealing with Shotcrete Testing—Who, Why, When, and How can be found on our website here: shotcrete.org/wp-content/uploads/2020/01/2011Sum_Hanskat.pdf

 

 

Do you have any case studies or applications where Ground Granulated Blast Furnace Slag (GGBFS) cement was successfully used in the concrete mix?2021-01-04T21:16:22+00:00

Shotcrete is a placement method for concrete. Concrete mixtures that have been shotcreted have used all different supplemental cementitious materials (SCM) including slag. Silica fume, and fly ash are also widely used. Generally geographic availability and cost of the slag are a deciding factor on if slag is included in a concrete mixture design. Concrete using alternative binders or cements have been used for specialized applications like refractory, or extremely high early strength gain. Here are links to past articles from our Shotcrete magazine that have considered slag in shotcreted concrete mixtures:

The Use of Slag Cement, Alkali-Free Accelerator, and Macro-Synthetic Fibers https://shotcrete.org/wp-content/ uploads/2020/05/2016Spr_Yurdakul-etal.pdf

Sustainable Shotcrete Using Blast-Furnace Slag https://shotcrete.org/wp content/uploads/2020/05/2013Fal_Sustainability.pdf

Our shotcrete contractor started shooting our residential swimming pool on a Friday and stopped 1/3 of way through the process. They plan to restart and complete shooting the pool the following Monday. This site is in Houston and has been averaging 90+ °F (32 °C) average daily temperatures. Aside from soaking the existing pool structure during the hiatus and additional continuous watering through the 10 days after completion to let concrete cure, what steps do I and the contractor need to make to ensure a cold joint will not fail and leak in the future? I will be able to supervise the second shot. What can I do, or make sure the contractor does, to ensure the structural integrity and quality of my investment?2021-01-04T21:15:29+00:00

Wet curing of newly placed concrete is certainly important. Your 10-day cure sounds great. Properly placed shotcrete against a properly prepared construction joint will provide concrete that acts monolithically and will not be a “cold joint” as is common in cast concrete. There are three key factors for joint preparation:

  1. The joint must be roughened. This should be done when the crew finished for the day using a stiff broom or raking with a trowel.
  2. On the subsequent shoot the surface should be cleaned (usually using a high pressure, pressure washer).
  3. Before shooting bring the surface to a saturated surface dry (SSD) condition. SSD means the pores of the concrete have been filled with water but there is no running water on the surface.

Following these 3 steps are essential for creating a monolithic, watertight joint. Here’s a link to an article on why shotcrete doesn’t have cold joints: https://shotcrete.org/wp-content/uploads/2020/05/2014Spr_TechnicalTip.pdf

Quality shotcrete placement requires a well-designed concrete mixture, proper equipment, attention to the surface prep, proper placement techniques, and curing. One aspect that can help you ascertain the nozzleman’s expertise is to require they have a current American Concrete Institute Shotcrete Nozzleman Certification. You can verify an individual’s certification status at: https://www.concrete.org/certification/verifyacertification.aspx?d=Ask

 

 

I am looking for information on where, when and why welded wire mesh (WWF) would be recommended in lieu of structural steel reinforcement, or with steel reinforcement, or guidance on how to determine which option or combination is the preferred one.2023-06-06T23:33:18+00:00

WWF should only be used when the area of reinforcing provides the required reinforcement area. It may serve as supplemental reinforcement to bars, or entirely replace reinforcing bars as long as the required area of reinforcing steel is provided in the concrete section. Where multiple sheets (more than 2 at say a corner) overlap you may need to cutout one or more of the layers to allow good encasement. Don’t use steel fibers with WWF as they tend to bunch up where the wires cross.

WWF can be used to provide reinforcing in double curved sections where it would be hard to bend and place reinforcing bars. Also, it may be used where you have a thick concrete cover or unreinforced thickness of a repair area that you wanted to get some reinforcing in place to control depth of any surface or shrinkage cracking.

If the mesh is just being provided to help support shotcrete being shot overhead you may not worry about the area of steel, just adding it in addition to the original design reinforcing. Then a lighter mesh might be used since you aren’t really worried about its ability to be reinforcing.

 

 

I work for a Commercial pool builder. We are doing a Jr. Olympic pool (competition pool) 40 ft by 70 ft (12 m by 21 m) with a 20 ft by 10 ft (6 m by 3 m) swim out area with steps. I would rather have the entire pool shot with shotcrete. The office wants the floor poured first then shoot the walls the next day. Can you talk about any advantages or disadvantages to either? Normally we are in a remote location with no power or water. We’ve had issues with shotcrete cracking on the floors and I understand that can happen because of many factors but poured concrete can as well.2021-01-04T21:13:11+00:00

Casting the floor first is recommended. Note that rebound and trimmings from vertical wall placement are not fully consolidated concrete and must not be used as part of a structural concrete section. Thus, casting first allows:

  • No collection of rebound and trimmings from wall construction to get trapped below floor reinforcing steel.
  • Provides a stable surface to collect and remove rebound and trimmings.
  • Is generally faster placement and allows full consolidation of concrete throughout the floor thickness by immersion vibrators.
  • If using wet-mix shotcrete the shotcrete pump can serve as a line pump for pumping concrete into the floor thus requiring no extra equipment.
  • Allows proper joint preparation at the floor/wall joint to provide a watertight joint.

 

 

Our pool company just completed the shotcrete today and temperatures are expected to reach near freezing tonight. Should the shotcrete still cure properly? Should we be concerned?2021-01-04T21:12:11+00:00

You say near freezing, so we assume it did not reach freezing. Though the newly place concrete will not gain strength quickly it should not be damaged. As temperatures rise during the day the warmer temperatures should allow more strength gain. Concrete has a chemical reaction to build strength, that generates internal heat. If freezing weather was expected, we would recommend use of a protective blanket on the concrete surface to help hold the heat in and promote quicker strength gain. Generally, in concrete we like to see the concrete surface temperature at 50 °F (10 °C) or higher in cold weather to encourage strength gain.

 

 

I am working on a fire re-build project near Napa, CA. The existing building has 12 in. (300 mm) thick shotcrete basement walls. There are many areas where the concrete cover spalled off from the rebar. The rebar appears to be ingood condition with no visible buckling. Is it possible to repair these walls with a 1 1/2 to 2 in. (38 to 50 mm) layer of shotcrete to restore the concrete cover? If so, what sort of surface prep is required? Is a concrete bonding agent required?2023-06-06T23:35:43+00:00

Yes, this is definitely a good application for shotcrete. To restore the cover:

  • Chip or hydrodemo back to sound concrete. If a reinforcing bar is more than half exposed, chip at least 1 in. (25 mm) back behind the bar.
  • Make sure the surface is roughened and clean.
  • Bring the concrete surface to saturated surface dry (SSD) condition. This means the surface feels damp but water is not picked up on a hand.
  • Make sure the shotcrete placement is properly executed. Use of an ACI-certified shotcrete nozzleman is recommended.
  • No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.
  • If chipping out a section do not feather edge. Provide a ¾ to 1 in. (19 to 25 mm)) depth of cut at the edge to provide adequate thickness for the integrity of the shotcreted material at the edge.

 

This article on the excellent bond between shotcrete provides more detail: https://shotcrete.org/wp-content/uploads/2020/05/2014Spr_TechnicalTip.pdf

 

I’m interested in any information you can provide about recommended expansion/contraction joint spacing for lazy river concrete flumes.2020-08-12T17:28:32+00:00

Shotcrete is a placement method for concrete. As the lazy river is a long concrete trough that is intended to be functionally watertight, the best guidance is ACI, 350-06 Code Requirements for Environmental Engineering Concrete Structures. As the predominate stresses in the horizontal direction result from shrinkage and temperature, Section 7.12.2.1 provides guidance on joint spacing based on the provided reinforcement ratio in the concrete section. The closer the movement joints, the less reinforcement required. Chapter 5 of ACI 350.4R-04, Design Considerations for Environmental Engineering Concrete Structures, has guidance on design of joints for water containing structures to help maintain water tightness.

We are working on a large infrastructure project in New York state. We have access and approval issues for a new concrete mixture design. Our understanding is we can specify a Pre-Bagged or Pre-Packaged Shotcrete mix for a wet-mix application. Is this the correct terminology? Are there companies that produce this type of product?2020-08-12T15:25:43+00:00

We see both Pre-Bagged and Pre-Packaged terminology used. ASTM C1480 / C1480M – 07(2012) uses the rather unwieldy Standard Specification for Packaged, Pre-Blended, Dry, Combined Materials for Use in Wet or Dry Shotcrete Application.”  Most suppliers of packaged dry concrete materials for shotcrete have formulations designed for wet-mix applications. You can find our corporate members who supply pack-aged materials on our website in the Buyers Guide (shotcrete.org/BuyersGuide), select the Category, Shotcrete Materials-Mixture Sales” and the Subcategory, Wet Mix. 

We are working on a repair/renovation project in Boston. A long-concealed wall next to an adjacent property is now visible as the adjacent property is being renovated. We have been told that our wall must have a 2-hour fire rating. Our wall is composed of CMU masonry and exposed structural steel members. Applying shotcrete to the CMU and steel is a good solution for several reasons. Would you provide shotcrete specifications that will have a 2-hour fire rating on CMU and structural steel?2020-08-12T15:24:43+00:00

Shotcrete is a placement method for concrete. Thus, the fire resistance for shotcrete placement is the same as concrete. The primary reference for fire resistance of concrete is ACI 216.1-14(19) Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies. 

would like to ask if there are any articles, references, etc, which reference procedures to determine the maturity of the concrete applied via shotcrete? Basically, how to generate the validation curves?2020-08-12T15:24:00+00:00

Shotcrete is a placement method for concrete. Thus, generating the maturity curves would be based on the concrete mixtures. There are several online resources about the maturity method. One that discusses production of the curves is from the Minnesota DOT and available in PDF format at  www.dot.state.mn.us/materials/concretedocs/MaturityMethodProcedure.pdf. 

I have a 24 in (600 mm) thick shotcrete wall that will have to be scanned for voids. The project has been struggling to locate a local expert who has the capability to scan this thick of a wall. In addition, this new 24 in thick shotcrete wall was dowelled and made “as one unit” together with an existing 24 inches thick cast in place wall. What technique should I use to get the right proportion?2020-08-12T15:23:14+00:00

Shotcrete is a placement method for concrete. Thus, all non-destructive testing (NDT) applicable to concrete walls would be usable on your wall. However, it is difficult to get good results with a scanning system for heavily reinforced concrete walls of your thickness. Impact Echo and Impulse Response are two one-sided techniques that can provide good results for a portion of the 24 in thickness though would likely not be able to scan the entire depth. Ultrasonic Pulse Velocity is a potential if you can access both sides of the wall. For one-sided investigation at greater depth you may be able to use a MIRA system. It is sophisticated tomographic system that says it can test from 50mm to 800mm (32 in) thickness. There are national consulting firms that provide these investigation systems. Each requires a highly trained, experienced operator so be sure to verify the firm can document successful experience with the method.

Can shotcrete be considered as structural concrete with wire mesh and rockbolts in tunnel linings?2020-04-09T18:52:53+00:00

Shotcrete is a placement method for concrete. It is routinely used for a wide variety of structural applications. It has been used for both initial and final linings in tunnels where it is commonly reinforced with wire mesh, fibers, or reinforcing steel. You may want to review our past Shotcrete magazine articles on tunnel shotcrete at https://shotcrete.org/archive-search/
using keywords such as “tunnel,” “underground,” and “linings.” Also, we have two position papers from our underground committee: “Spraying Shotcrete Overhead in Underground Applications,” and “Spraying Shotcrete on Synthetic Sheet Waterproofing Membranes,” that you may find informative. Also, ACI 506.5R-09, “Guide for Specifying Underground Shotcrete,” can provide insight into topics important for using and specifying underground shotcrete.

Do you have any reference documentation for the application of using concrete stamps/textures or hand carving on vertical shotcrete applications for architectural finishes?2020-04-09T18:52:07+00:00

Unfortunately, I’m not aware of any specific references to guide you in stamping or carving of fresh shotcrete for vertical surfaces. In many ways carving and stamping is a technique that requires more of an artistic flare that a finisher develops by hands-on experience. One key aspect is to minimize the working of the surface as much as possible to prevent tearing or delaminating the shotcrete surface layers. Carving or stamping when the concrete has reached an appropriate level of set (not too hard or soft) must also be judged by the experienced shotcrete finisher.

I live in a 10-story condo building built in the 1960s. The entire building is made of gunite. We use masonry drill bits for holes, but nothing seems to grip well. We have tried concrete screws, metal drywall anchors (which seem to work the best), and a plethora of other anchors and screws, but nothing seems to work. They all cause mushrooming and they either don’t grip or turn the wall to powder. What are the best tools to affix things to the walls?2020-04-09T18:51:32+00:00

Shotcrete is a placement method for concrete. Dry-mix shotcrete (the old tradename is gunite) using proper materials, equipment, and application techniques should have easily been able to reach a strength of 4000 psi (28 MPa) in the first month. After 10 years in-place the concrete should be even stronger. The level of concrete strength developed by quality shotcrete should easily accommodate drilling in anchors or concrete screws. In my experience it would have been highly unusual to build an entire 10-story building with shotcrete even in the 1960s. I’d suggest based on the extremely weak material properties and the wall turning to dust what you think is shotcrete may be sprayed plaster or stucco. Those materials don’t have near the same strength as shotcrete and would exhibit many of the problems you have mentioned.

I am not sure if this is the right place to go… 15 months ago I contracted with a company to remove my pavers, pour a concrete pad, and install spraycrete. It was beautiful! The contractor even has photos on his website. Now, I have hairline cracks all over the place and there are more weekly. My contractor is definitely giving me the runaround regarding this. I explained that I would not have paid more than $16,000 if my pool deck would be cracked a year later. Hurricane Irma took out my pool cage and the insurance money was not enough to replace it, so I used the money for my pool deck. Can someone please tell me what to do? Should these hairline cracks be everywhere? What is the lifespan of the material? Help!2020-01-30T19:50:24+00:00

Spray-crete is NOT shotcrete. Shotcrete is high-velocity placement of concrete in thicker structural sections. It appears Spray-crete is a low-velocity sprayed mortar product generally applied in a very thin layer to provide texture to an existing concrete substrate. Since you mentioned the underlaying concrete pad was cast and then the Spray-crete added the cracking could well be originating in the underlaying concrete. Concrete cracks for a variety of reasons, such as drying shrinkage, thermal volume change (summer/winter cycles), inadequate curing, insufficient reinforcing steel, or settlement of the subgrade. You should locate a local professional engineer experienced in concrete slab evaluation who can evaluate your site, materials used, application techniques, and potential causes of the cracking. You may find the Florida Engineering Society and ACEC-FL has a list of firms who offer evaluation services.

I’m hoping you’d be willing to answer a couple of questions I have about gunite. I am having a swimming pool installed at my house in Florida. I was away when the gunite was shot a few weeks ago and didn’t know at the time that the gunite should be sprayed with water for a few times a day for about a week according to what I have read online. The pool company owner knew I would be away and never mentioned the need for the gunite to be periodically moistened. The owner also made no provision for any of his employees to hose it down or install a sprinkler. When I learned after the fact of the watering requirement and asked him about it, he said it was unnecessary because the gunite was shot at 4000 psi (28 MPa) and not the “industry standard” of 3000 psi (21 MPa). Then he added that with the almost daily rain in Florida at this time of year, all was okay. I’m concerned about the gunite’s integrity—its permeability and the possibility of shrinkage and cracking. Could you tell me if I have a reason to be concerned, and if so, what do you suggest I should do about it?2020-01-30T19:50:07+00:00

Gunite is the original tradename for what we now call dry-mix shotcrete. Shotcrete is a placement method for concrete, so recommendations for curing and protection follow general ACI guidelines for exposed concrete. ASA recommends a minimum of 7 days of continuous (not just a few times a day) wet curing to help control shrinkage issues, increase strength, and reduce permeability in young concrete sections. Lack of curing and exposure to windy, hot, or dry conditions will certainly increase the potential for shrinkage and cracking of the concrete. Lack of curing will prevent the concrete from achieving its maximum potential strength.

Shotcrete placement with quality materials and proper application techniques generally exceeds the minimum 4000 psi 28-day compressive strength ASA recommends. The statement that 3000 psi is the “industry standard” is not true, as the ACI 350 Code for concrete liquid-containing requires a minimum 28-day compressive strength of 4000 psi for concrete intended to have low permeability when exposed to water. The required strength depends on the pool design. If you want to confirm the compressive strength of your in-place concrete, cores taken from the pool should be tested for compressive strength by a qualified testing lab. ASTM C1604/C1604M provides guidance on taking cores from existing structures. A minimum 3 in. (76 mm) diameter core is recommended. Before coring, it is recommended to use ground-penetrating radar (GPR) or similar equipment to identify the location of reinforcement in the pool section, and then take cores to avoid cutting through the reinforcement wherever possible. The core holes would then need to be filled with a high-strength, non-shrink cementitious grout. Once you learn the actual strength, you would need to check with the pool design engineer to verify the strength is adequate for the design. If the strengths are not adequate, you should consult with the pool designer or a licensed professional engineer experienced in pool design for potential solutions.

Regarding cracking, the lack of curing will increase the concrete’s shrinkage and correspondingly the potential for cracking. You should verify that there are no significant cracks in the pool shell before the plaster or other interior coating is applied. If there are cracks, the pool contractor should repair those before proceeding with the plaster or coating. Although proper curing would certainly decrease the concrete’s permeability, generally good-quality shotcrete with proper placement and a strength of 4000 psi will be functionally watertight and not allow any significant amount of water to flow through the uncracked concrete thickness. You will find more detailed information on pool compressive strengths and watertightness of pool shells in our ASA Position Statements.

I have a project where we are designing a shotcrete wall. The contractor plans to mix the shotcrete on site and we need to work with them to come up with a mixture design. Do you have any suggested wet-mix shotcrete designs for on-site mixing applications or could you point me where some may be published?2020-01-30T19:49:50+00:00

Shotcrete is simply a placement method for concrete. Most wet-mix shotcrete contractors use a 2 in. (50 mm) diameter delivery hose, so maximum coarse aggregate size should be limited to 3/8 in. (10 mm) nominal. Pumpability usually requires a good paste content. Here’s a link to an article, “Understanding Wet-Mix Shotcrete: Mix Design, Specifications, and Placement,” that should answer many of your questions. It is rare to have wet-mix concrete mixed on site from bulk aggregate and cementitious materials. Most site-batched wet-mix uses dry prepackaged materials that have metered water addition to provide a specific water-cementitious materials ratio (w/cm) for the provided bagged mixture. We also see volumetric mixers used on site that can precisely meter the dry concrete materials and water often with needed water-reducing air-entraining admixtures.

We are applying shotcrete in a slope (8 in. [200 m]) to make it stable. The shotcrete wall has a length of 330 ft (100 m). Do we need to specify construction and an isolation joint? In case it is required as well as an isolation joint, does the separation between joints need to be 30 ft (9 m)? Where can I find information about joints for shotcrete?2020-01-30T19:49:30+00:00

Shotcrete is a placement method for concrete. Thus, you should place movement joints (contraction, expansion, isolation) as would be required by your local design codes for concrete. Joint spacing will depend on the amount of reinforcement used in the section to resist temperature and shrinkage volume changes. Construction joint spacing can be determined by the contractor depending on their production rates. Properly prepared construction joints will act as monolithic concrete as long as the joints are properly prepared and proper concrete materials, equipment, and placement techniques are used by the shotcrete contractor. Construction joints should be roughened, cleaned, and then dampened to saturated surface-dry conditions before subsequent shotcrete placement.

The U.S. Bureau of Reclamation has a nice summary document that you may find helpful in your design.

I place shotcrete and I use the wet-mix method. I have been asked by a contractor to repair a pool that was shot with the dry-mix method. He is having trouble convincing his client that with the proper preparation we can shoot the repair with the wet method. Am I missing anything?2020-01-30T19:49:02+00:00

Shotcrete is a placement method for concrete. Both wet-mix and dry-mix produce quality in-place concrete when using quality materials and proper equipment and placement techniques. There are no compatibility problems with shooting wet-mix over dry-mix. Basically, it is just shooting shotcrete on top of already placed concrete. For proper bond, the surface of the existing dry-mix must be roughened, cleaned, and brought to a saturated surface-dry moisture condition before shooting the wet-mix lining.

When applying 6 in. (152 mm) of shotcrete on an exterior concrete surface, how long should that cure before coating with an acrylic elastomeric roof coating? What is the moisture content of that thickness after 7 days, or after 14 days?2020-01-30T19:48:42+00:00

Shotcrete is just a placement method for concrete. Thus, the in-place material will have the same characteristics as concrete. You should consult the coating supplier to establish how long they want the concrete surface cured. They may also specify a certain limit for surface moisture conditions. There are several methods for evaluating the moisture content of the in-place concrete. Here are the ASTM standards that deal with surface moisture:

  • ASTM F710, Preparing Concrete Floors to Receive Resilient Flooring; Section 5.2, pH Testing;
  • ASTM F1869, Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride;
  • ASTM F2170, Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes;
  • ASTM F2659, Preliminary Evaluation of Comparative Moisture Condition of Concrete, Gypsum Cement and Other Floor Slabs and Screeds Using a Non-Destructive Electronic Moisture Meter; and
  • ASTM F3191, Field Determination of Substrate Water Absorption (Porosity) for Substrates to Receive Resilient Flooring.

In addition, The International Concrete Repair Institute (ICRI) has a certification program, “ICRI Concrete Slab Moisture Testing Technician—Grade 1.”

How thick would you recommend a shotcrete application be to make bedrock reservoir waterproof?2020-01-30T19:48:25+00:00

There are many variables that would need to be considered to answer your question. Water flow, depth of the application, and overall geometry can influence the required thickness. This is a question that should be addressed by an experienced engineer who can ascertain the required structural properties to resist the hydraulic loads for the depth and geometry of your application. You may find consultants who are ASA Corporate Members in our Buyers Guide.

I am helping to design radiation shielding for a cyclotron and we do not have much space. The machine produces both neutrons that must be shielded for as well as gamma rays. I have been told that we could get a density of 3.2 g/cc, which helps for gamma rays, but I need to find out more about the water content of the finished product for the neutron shielding part so I can scale between regular 2.4 g/cc concrete and the high-density shotcrete product. Do you have knowledge of this?2020-01-30T19:39:40+00:00

Shotcrete is a placement method for concrete. We have shot a wide variety of concrete mixtures from lightweight to heavyweight to the use of alternative cements. Reviewing ACI 304.3R-96, “Heavyweight Concrete: Measuring, Mixing, Transporting, and Placing,” the key to producing heavyweight concrete is the aggregates used. The document also mentions that more cement paste is needed in heavyweight concrete and that helps to increase pumpability, which is important to use with shotcrete placement. Shotcrete will allow you to minimize or eliminate formwork, so it may have benefits of reducing cost and time for construction. Wet-mix shotcrete generally has a low water-cementitious materials ratio (w/cm) of 0.38 to 0.45. Dry-mix shotcrete tends to be even lower with a 0.35 to 0.40 w/cm. Shotcrete materials achieve this using high-range water-reducing admixtures. As concrete hydrates and gains strength, the available free water in the concrete is consumed, and so may be less of a problem for shielding. Reviewing the 304.3R document, it appears reaching the 3.2 g/cc density is fairly easy, and even higher densities may be achievable. You may want to review the ACI 304.3R document, as it provides a good background of heavyweight concrete mixture design and use.

I am working with a private club in Connecticut and we are are trying to determine what the life expectancy is of a concrete commercial pool shell. There is no evidence of failure or cracking and the pool surface is painted, not plaster. We think it is reinforced shotcrete. Are there any tests or rules we should take into consideration?2020-01-30T19:39:13+00:00

Shotcrete is a placement method for concrete. Thus, evaluating your pool shell life expectancy will be the same as any concrete structure exposed to water. You may find the technical document ACI 201.1R-08, “Guide for Conducting a Visual Inspection of Concrete in Service,” helpful in evaluating your pool shell. Generally, shotcreted concrete that uses quality materials, proper equipment, and placement techniques will serve for at least 50 to 60 years. ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures,” states: “When all relevant loading conditions are considered, the design should provide adequate safety and serviceability, with a life expectancy of 50 to 60 years for the structural concrete.” This ACI 350 Code is for liquid-containing concrete structures such as tanks for water and wastewater treatment but the original pool design may or may not meet the Code requirements.

Is there any article or reference referring to the procedure of maturity of the concrete applied to the shotcrete? Basically, explaining how to generate the validation curves?2020-01-30T19:38:15+00:00

Shotcrete is a placement method for concrete. Thus, generating the maturity curves are based on the concrete mixtures. There are several online resources about the maturity method. The Minnesota DOT provides a PDF resource that discusses production of the curves.

We are working on a repair/renovation project in Boston, MA. A long, concealed wall next to an adjacent property is now visible, as the adjacent property is being renovated. We have been told that our wall must now have a 2-hour fire rating. Our wall is comprised of concrete masonry units (CMUs) and exposed structural steel members. Applying shotcrete to the CMUs and steel is a good solution for several reasons. Can you provide or point me to a shotcrete specification that will have a 2-hour fire rating on CMUs and structural steel?2020-01-30T19:37:57+00:00

Shotcrete is a placement method for concrete. Thus, the fire resistance for shotcrete placement is the same as concrete. The primary reference for fire resistance of concrete is ACI 216.1-14(19), “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”

I have a 24 in. (610 mm) thick shotcrete wall that needs to be scanned for voids. The project has been struggling to locate a local expert who has the capability to scan this thick of a wall. In addition, this new 24 in. thick shotcrete wall was dowelled and made an “as-one-unit” together with an existing 24 in. form-and-pour wall. Any thoughts and ideas would be greatly appreciated.2020-01-30T19:37:42+00:00

Shotcrete is a placement method for concrete. Thus, all nondestructive testing (NDT) applicable to concrete walls would be usable on your wall. However, it is difficult to get good results with a scanning system for heavily reinforced concrete walls of your thickness. Impact echo and impulse response are two one-sided techniques that can provide good results for a portion of the 24 in. thickness, although they would likely not be able to scan the entire depth. Ultrasonic pulse velocity is a potential if you can access both sides of the wall. For one-sided investigation at greater depth, you may be able to use a MIRA system. It is a sophisticated tomographic system that says it can test from 50 to 800 mm (32 in.) thickness. There are national consulting firms that provide these investigation systems. Each requires a highly trained, experienced operator—so be sure to verify the firm can document successful experience using the method.

A contractor has proposed using shotcrete to repair the concrete in the elbows of a draft tube. I have not heard of shotcrete being used in a draft tube. Velocities would range from 10.5 to 3.8 ft/s (3.2 to 1.2 m/s). I am concerned about whether the shotcrete would delaminate after time or be abraded away, as there is considerable abrasion present where the concrete cover has been abraded away on the floor of the elbow. Any guidance would be helpful.2020-01-30T19:31:22+00:00

Shotcrete has been used in many dam repairs, including large-diameter draft tubes. Shotcrete is high-velocity (60 to 80 mph [100 to 130 km/h]) placement of concrete. When shotcreting with proper concrete materials, equipment, placement, and curing techniques, along with complete surface preparation, you can expect a tensile bond strength of at least 150 psi (1 MPa) between the existing concrete and the newly shotcreted material. Original Portland Cement Association research by Felt from 1956 showed that 200 psi (1.4 MPa) bond shear strength is required for bonded overlays to act monolithically in flexure. Research by Silfwerbrand in 2003 showed that the ratio of bond shear strength to direct tensile bond strength ranges from 1.9 to 3.1. Thus, using the low value of the range with a 150 psi tensile bond strength yields a shear strength of at least 285 psi (2 MPa), well above the 200 psi needed. You may find more information on the bond between concrete and shotcrete layers in the article “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints” that can be found in our article archive.

Regarding the abrasion, shotcrete displays good toughness in a wide variety of demanding applications. Quality shotcrete should have at least a 4000 psi 28-day compressive strength and, with attention to mixture design using silica fume and a low water-cementitious materials ratio (w/cm), can comfortably reach 6000 to 8000 psi (40 to 55 MPa) or more. Shotcrete also can easily use steel or synthetic fiber to significantly increase the toughness of the in-place concrete.

Finally, because shotcrete requires no formwork or bonding agent for a high-quality repair, you will find the shotcrete process provides an economical solution.

An article about a draft tube modification project can be found in our article archive.

We are constructing a new custom roundabout with water running through the bridges on the Coast of Zintan, Tripoli, Libya, using a three-dimensional (3-D) panel system. It’s basically a system with an expanded polystyrene (EPS) panel with a wire mesh and shotcrete on both sides. Because of the heat, sea salt, and high humidity of the region, we are looking for a mixture formula for a waterproof shotcrete for the exterior coating with the right aggregate size to help prevent moisture migration to the interior and prevent cracks. Do you have any recommendations for the shotcrete?2020-01-30T19:30:59+00:00

If looking for a low-permeability concrete mixture for shotcrete placement, you should consider using supplementary cementitious materials (SCMs) such as silica fume, fly ash, or slag to reduce the permeability. You should also be sure to require a minimum 4000 psi 28-day compressive strength to assure good paste content and the ability to fully encase your reinforcement. The addition of microfibers and early wetting of the finished shotcrete surface will help to reduce the potential for early-age plastic shrinkage cracking. Proper curing for at least 7 days is also important to help increase strength gain and reduce the potential for drying shrinkage cracking. You can consult our Buyer’s Guide to locate our corporate members, who may consult with you on the mixture design. However, please be aware that many of the panel systems with an EPS core don’t use high-velocity (60 to 80 mph) shotcrete for consolidation and compaction of the sprayed concrete, but use a low-velocity sprayed mortar (LVSM). Because LVSM doesn’t have the compaction of shotcrete impact, it depends on a more sophisticated and expensive cementitious mixture, often with a latex or other admixture to improve adhesion and reduce permeability. You’ll need to consult with the material supplier of the LVSM product to verify the permeability of their in-place product.

We’re building a pool 25 x 45 ft (7.6 x 14 m) and had a massive cave-in on our deep end. The builder wants to build temporary walls to shoot the shotcrete against, then remove the plywood walls and backfill with gravel. I’m wondering if this will work and if they will be able to remove the plywood without damaging the shotcrete walls. I know with typical forms you would prep the form with oil so the concrete doesn’t stick. Would that be necessary for shotcrete, too? Also, is there a recommended wait time for curing before we backfill?2020-01-30T19:30:43+00:00

Shooting shotcrete against a one-sided form (what you called a temporary wall) is a common way to build a shotcrete wall. Once the shotcrete sets and builds strength, the plywood form can easily be stripped off the back of the wall. Form release agents (not oil) can be applied to the plywood to make the stripping easier. Once the forms are removed and the concrete has gained adequate strength, the walls can be backfilled with compacted soil or gravel, depending on the drainage needs.

We recommend 7 days of curing. Continuous water curing is best, but if impractical, applying a curing compound on the exposed surface at twice the manufacturer’s recommended rate for a good seal is acceptable. If they remove the forms before 7 days, they should also water cure or apply curing compound to that newly exposed surface. The shotcrete needs to build up enough strength to resist the external force of the backfill, so check with the pool designer to see what they need for the required strength of the concrete before backfilling. With most good-quality shotcrete materials and placement techniques, you can expect about 4000 psi (28 MPa) compressive strength in 7 days.

I am working on a restoration of a small 1870s train station constructed of serpentine stone in the Philadelphia, PA, area. In many areas the stone has deteriorated, leaving deep “divets” in the exterior wall faces and, in some cases, there is no stone at all. Our intent is to build (infill) the walls back to a flush face for stucco treatment for the lower portions of the wall and to repair or replace stone above that point. Is there a minimum amount of treatment recommended for a shotcrete application? If it can be used for such an application, is reinforcement required? The stone is rather friable and I don’t want to attach too much to it for fear of further damaging the stone.2020-01-30T19:30:23+00:00

This is a great application for shotcrete placement of high-quality concrete without formwork. If you are merely adding shotcrete to fill out to a uniform surface profile without any structural requirements you may not need reinforcement. However, it may still be advisable to include fibers in the shotcrete mixture to help control plastic shrinkage cracking. Generally, you would want to keep a minimum thickness of 1 in. (25 mm) to provide enough thickness for finishing. If you need the shotcreted sections to be self-supporting and carry loads as structural concrete, you should consult with a structural engineer to determine the appropriate thickness and reinforcement for the expected loads. Shotcrete is a placement method for concrete, so standard reinforced concrete design is appropriate for shotcreted sections.

How much shotcrete coverage over No. 4 reinforcing bar is required?2020-01-30T19:30:04+00:00

Shotcrete is simply a placement method for concrete. The specified concrete cover over reinforcing bar is usually included in contract documents for construction and values vary depending on exposure conditions. ACI 318 provides cover requirements for structural concrete in buildings, and ACI 350 provides cover requirements for concrete liquid-containing structures. Local building codes and fire codes may also require specific cover in concrete construction. If your project doesn’t specify the cover requirements, we recommend you consult with a professional engineer experienced in the type of project you are working on to learn what the code requirements may be.

What is the best way to check the sand-to-cement ratio in gunite batch trucks? Is it normal (common) to get up to 10% air straight from the mixing auger? I had a gunite truck fill a 5 gal. (19 L) bucket with mixed material (sand and cement) then put it in my lab mixer and wetted it up to a 3 in. (75 mm) slump and ran a test for air, unit weight, and cast a set of cylinders. I am trying to figure out the sand-to-cement ratio, but I don’t have a cement diverter to run a real-time sand-to-cement ratio test, so I am testing dispensed material to see if it makes 70% strength in 7 days, which indicates a good mixture. Are there any other ways of checking sand-to-cement proportions? I tested the mixed material in a lab about 5 minutes after it was dispensed.2020-01-30T19:29:50+00:00

By “gunite batch truck” we assume you are referring to a volumetric mixer supplying material for a dry-mix shotcrete operation. Gunite is the original tradename for what we now call dry-mix shotcrete. Here’s the description from an ACI Materials Journal (January-February 1991 issue) article about the calibration of volumetric mixers:

“To insure production of quality concrete, each volumetric-measuring unit must be calibrated for each respective concrete ingredient, following the manufacturer’s recommendations and ASTM C 685. These ingredients must be the same as those to be used in actual concrete production. The measuring devices for aggregates, cement, and dry admixtures are calibrated by weighing the discharged ingredient. Devices for water, latex modifier (if required), and liquid admixtures such as air-entraining and water-reducing admixtures generally are calibrated by weighing or measuring the volume of the discharged ingredient. The objective of calibration is to coordinate the discharge of all concrete ingredients to produce the proper mixture.”

ASTM C685/C685M states, “The proportioning and indicating devices shall be individually checked by following the equipment manufacturer’s recommendations as related to each individual concrete batching and mixing unit. Adequate standard volume measures, scales, and weights shall be made available for the checking accuracy of the proportioning mechanism.” Thus, you need to check with your equipment supplier for their recommended procedures to verify batching. Because concrete mixtures always are based on weight of ingredients there you will need to weigh a given volume to confirm the batching is accurate.

The air content test is a measure of total air so includes both entrapped and entrained air. Ten percent is definitely high. The 10% air is likely not representative of the in-place shotcrete. It may have been an issue with the lab mixer introducing more entrapped air for some reason. Estimating the air content from the unit weight test requires a good value for the theoretical unit weight. I’m not sure if you have that with the volumetric batching. You should run the air meter test (ASTM C231/C231M) to measure the air content to get a more accurate assessment.

Regarding the verification of mixture proportions, this is from ASTM C685/C685M:

“7.5 Proportioning Check—Whenever the sources or characteristics of the ingredients are changed, or the characteristics of the mixture are noted to have changed, the purchaser is permitted to require a check of the fine aggregate content and the coarse aggregate content by use of the washout test. Essentially, in the washout test, 1 ft3 [0.03 m3] of concrete is washed through a No. 4 [4.75-mm] sieve and through a No. 100 [150-µm] sieve; that retained on the No. 4 sieve is normally considered coarse aggregate whereas that passing the No. 4 and retained on the No. 100 sieve is considered fine aggregate. Corrections to the quantity of aggregates (per cubic foot or cubic meter of concrete) shall be made if the original sieve analysis of each aggregate is available.”

Because you are only interested in the sand and cement, you can simply weigh the sample of concrete, then wash out all the cement, and then weigh the remaining sand. You would need to bring the sand to roughly the same moisture content as the sand in the truck, so you aren’t including in the weight of excess water in the sand. You should note that with shotcrete impact during placement we will generally lose 50% of the air content, so your final in-place air should be around 5%. That is a reasonable value for good freezing-and-thawing durability.

I would like to better understand the limitations related to the height of install when it comes to the gunite application. Because gunite is a dry concrete mixed with water at the nozzle of the applying apparatus, I have been told by others in the industry that the application is only intended for use of walls less than 4 or 5 ft tall. If that is the case, is it safe to assume that the gunite application strategy should not be used for below-grade vaults exceeding a height of 5 ft? I am looking for design literature specific to gunite.2020-01-30T19:29:32+00:00

Dry-mix shotcrete adds mixing water to the dry concrete materials as the concrete materials flow through and out the nozzle. Gunite is the original tradename for dry-mix shotcrete. Though you may not find design information using the old gunite name, you will find numerous current design references to dry-mix shotcrete. This includes ACI 506R-16, “Guide to Shotcrete”; ACI 506.2-13, “Specification for Shotcrete”; ACI 506.6T-17, “Visual Shotcrete Core Quality Evaluation Technote”; ACI 372, “Design and Construction of Circular Wrapped Prestressed Concrete Tanks”; ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures”; ACI 350.5, “Specifications for Environmental Concrete Structures”; and seven ASTMs that directly cover shotcrete. ACI 318-19, “Building Code Requirements for Structural Concrete,” has also added specific shotcrete provisions. Dry-mix shotcrete has been used for decades to build structural concrete walls over 50 ft high in circular prestressed concrete tanks that withstand a full head of water pressure. This is a substantially greater water pressure than your 5 ft vault wall would experience. There are no limitations in the dry-mix placement process that would preclude use in high walls. Both dry-mix and wet-mix shotcrete using quality materials, proper equipment, and experienced placement crews will produce in-place concrete of equal strength, durability, and low permeability. However, generally wet-mix shotcrete can offer placement rates up to four times higher than dry-mix. Thus, in thicker, longer walls, wet-mix shotcrete may be more cost-effective because it can be placed faster.

One quick question regarding the “grading” of cores based on 506.2: I imagine the 1995 standard-based 1 through 5 grading system is no longer valid? In one case the special inspector has failed a core (score of 5) simply because a piece of it broke off during the coring and removal from the coring cylinder. I’m leery of that type of failed rating. What do you think about that, and core grading in general?2020-01-30T19:29:14+00:00

The current version of the ACI “Specification for Shotcrete” is ACI 506.2-13 and purposely eliminated the core grading due to many problems that arose in the field in trying to equitably apply the procedure. ACI 506 has a new document ACI 506.6T-17, “Visual Shotcrete Core Quality Evaluation Technote,” that is intended to assist specifiers with evaluating shotcrete core quality for an intended use. So yes, the 506.2-95 version of the specification and the grading system it included is no longer supported as an industry standard.

Regarding grading, a shotcrete score of 5 under the deprecated grading system just because the core broke during coring is unreasonable. The coring operation exerts a massive force on the core, as it basically rips it out of the concrete. We have seen many instances where the core snapped where a large reinforcing bar horizontally crossed the core and effectively created a slip plane that reduced the shear resistance of the core section to the torque created by the coring. In those cases, examining the core hole for any evidence of shotcrete problems would be appropriate and then discounting the core itself.

In summary, my position and the ACI 506 Committee’s position is that core grading should not be used for evaluating shotcrete quality. The ACI 506.6T-17 Technote document should be the current reference for making a reasonable evaluation of shotcrete quality for the intended use.

What is the minimum spacing between reinforcing bar recommended in a shotcrete swimming pool deep-end wall. The engineer is calling out for 12 in. (300 mm) walls with a double mat of No. 5 bars at 4 in. (100 mm) on center each way. In my opinion, this does not give enough space to properly encase the reinforcing bar with shotcrete without creating voids and trapping uncontrollable rebound.2020-01-30T19:28:56+00:00

The engineer designs the wall based on the loads anticipated on it throughout its service life. The minimum spacing for noncontact lap splices as indicated in ACI 506.2, “Specification for Shotcrete,” is:

“Clearance of at least three times the diameter of the largest reinforcing bar; three times the maximum size aggregate; or 2 in., whichever is least”

In your case with No. 5 bars, the 2 in. (50 mm) minimum probably controls and would then provide a 2.6 in. (64 mm) center-to-center spacing. No. 5 bars at 4 in. spacing can be shot properly with proper materials and technique. An ACI-certified shotcrete nozzleman will have had education that explains how this configuration or even closer spacing can be shot properly. Experienced shotcrete contractors doing structural concrete walls do this type of work routinely with excellent results. If you are concerned about encasement of the reinforcing steel in the back curtain of steel you may consider erecting only the back curtain of steel, shooting the wall out to the location of the outer curtain of reinforcement, erecting the outer curtain, and then shooting out to the final surface. Experienced shotcrete contractors have shot in thick sections (36 in. [900 mm] thick or more) with heavy reinforcement (No. 11 at 4 in.) using this technique.

What is the cure time for shotcrete before paint can be sprayed? And what type of paint (latex or oil-based) would work best?2020-01-30T19:28:37+00:00

Shotcrete is a placement method for concrete. Thus, cure time is the same as what the coating manufacturer recommends for new concrete surfaces. Because moisture in the concrete surface is part of the concern of bond from manufacturers, a water-based latex may be preferable to oil-based, but you should discuss the appropriate time and materials with your coating supplier.

With structural shotcrete walls, what is the anticipated shear strength of the sprayed shotcrete to a vertical wall with geotextile fabric applied to a retention system? What is the anticipated dead load of the sprayed shotcrete at the base of the sprayed wall when the shotcrete wall is sprayed to a nominal thickness of 6 in. (150 mm)? Also, what is the maximum wall thickness that can be sprayed in a single pass application with a normal mixture design to achieve 4800 to 5000 psi strength?2020-01-30T19:28:19+00:00

Shotcrete is a placement method for concrete. Thus, structural properties are equivalent to those of concrete with specified strength. Shotcrete compressive strengths will normally range from 4000 to 7000 psi at 28 days, so your 4800 to 5000 psi (33 to 35 MPa) is very normal. Density of shotcrete placed concrete is the same as cast concrete ranging from 145 to 150 lb/ft3. Walls can generally be built at any required thickness because we will create the wall by bench shooting building the wall from the bottom to the top in vertical lifts.

We are applying a new shotcrete shell to an existing building. We are calling for a 5 in. (125 mm) base coat with a 1/4 in. (6 mm) brush-finished flash coat. We are specifying a color admixture for all of the shotcrete. It is a large building and we anticipate several days of shotcrete operations. Would it be wise to allow (or even specify): 1) the color admixture to be used in the flash coat only? 2) all of the flash coat to be applied at once, possibly days after all of the base coat has been installed, to achieve a more uniform final appearance?2020-01-30T19:27:39+00:00

If you only need color in the exposed shotcrete surface, just putting color in the flash coat should be adequate as long as the flash coat is continuous across the entire area. You may want to specify the final “flash” layer to be thicker, perhaps 1/2 in. (12 mm) to 1 in. (25 mm) thick. This would help assure consistent color and still allow the finishers to produce a consistent final texture. As long as the concrete materials are the same in the production, you wouldn’t need to shoot the final layer all in one day. Also, be sure curing methods are the same for the entire area as varying moisture can sometimes affect the early appearance of the coloring, though it usually balances out over time.

I am currently researching shotcrete machines for a sewerage channel. Would dry-process or wet-process be more functional and efficient? I am new to the industry, so could you please give me some background information regarding the equipment used as well as information about the techniques and processes you would use? Could you also inform me about the factors like humidity and what effect they would have on what process you would use and how you would apply it? Please also let me know about any other information that you would consider as having a significant effect on the overall process.2020-01-30T19:27:24+00:00

The thickness of the shotcrete placement, site logistics, and the availability of ready-mixed concrete, as well as expertise of the shotcrete contractor would likely determine the most cost-effective method for shotcrete placement. Both dry-mix and wet-mix will produce structural concrete in place with similar physical properties and durability. Wet-mix can generally produce more volume of material placed per hour than dry-mix. You had several basic questions about shotcrete that can be answered by visiting sections of Shotcrete.org. On our website, you will find the informational pages www.shotcrete.org/Resources and www.shotcrete.org/ArchiveSearch beneficial. The Resources page lists many shotcrete-specific articles and web resources. The Archive Search allows you to search through our Shotcrete magazine archive for past articles using keywords. You may also find ACI 506R-16, “Guide to Shotcrete,” an informative primer on all aspects of shotcrete. You can purchase a PDF copy of the Guide from ACI at www.concrete.org/store/productdetail.aspx?ItemID=50616. ACI Committee 506, Shotcreting, also has several other technical documents available for specification for shotcrete, evaluation of shotcrete cores, specifying underground shotcrete, and fiber-reinforced shotcrete you can find on the ACI website.

ACI 506R-16 discusses surface preparation requirements for various substrate surfaces and notes that for earth surfaces shotcrete shall not be placed on frozen ground. There does not appear to be any specific temperature requirement for other substrate materials, however. For concrete or masonry sub-straight surfaces, are there temperature requirements for shotcrete application in situ­ations where bond is not required?2020-01-30T19:27:04+00:00

All surfaces receiving shotcrete should be above freezing. The mandatory requirements of ACI 506.2-13, “Specification for Shotcrete,” specifies:

“3.4.5 Cold weather shotcreting—Unless otherwise specified, shooting may proceed when ambient temperature is 40°F and rising. Stop shooting when ambient temper­ature is 40°F and falling, unless measures are taken to protect the shotcrete. Shotcrete material temperature, when shot, shall not be less than 50ºF. Do not place shotcrete against frozen surfaces.”

Your question then asks about substrate temperatures for sections not requiring bond. The concern of frozen concrete is not only bond, but an issue with freezing of some thickness of the concrete that would prevent strength gain. For requirements on this, ACI 301-16, “Specifications for Structural Concrete,” would likely apply and 5.3.2.1(b) requires:

“5.3.2.1(b) Cold weather—Concrete temperatures at delivery shall meet the requirements of 4.2.2.5. Do not place concrete in contact with surfaces less than 35°F. Unless otherwise specified, this requirement shall not apply to reinforcing steel.”

There is a discrepancy between ACI 301 and ACI 506.2. The ACI 301 value (35°F) is somewhat more conservative, though ACI 506.2 provisions (32°F) have proven to produce quality shotcrete. You may consider asking the Engineer of Record for your project what minimum substrate temperature is acceptable on your specific job.

I am working on a restoration of a small 1870s train station constructed of serpentine stone in the Philadelphia, PA, area. In many areas, the stone has deteriorated, leaving deep “divets” in the exterior wall faces and in some cases, there is no stone at all. Our intent is to build (infill) the walls back to a flush face for stucco treatment for the lower portions of the wall and to repair or replace stone above that point. Is there a minimum amount of treatment recommended for a shotcrete application? If it can be used for such an application, is reinforcing required? The stone is rather friable, and I don’t want to attach too much to it for fear of further damaging the stone. If shotcrete is not an appropriate approach for this repair, can you advise of other repair methods?2020-01-30T19:26:00+00:00

This is a great application for shotcrete placement of high-quality concrete without formwork. If you are merely adding shotcrete to fill out to a uniform surface profile without any structural requirements, you may not need reinforcement. However, it may still be advisable to include fibers in the shotcrete mixture to help control plastic shrinkage cracking. Generally, you would want to keep a minimum thickness of 1 in. (25 mm) to provide enough thickness for finishing. If you need the shotcreted sections to be self-supporting and carry loads as structural concrete, you should consult with a structural engineer to determine the appropriate thickness and reinforcement for the expected loads. Shotcrete is a placement method for concrete so standard reinforced concrete design is appropriate for shotcreted sections.

How much shotcrete coverage is required over No. 4 reinforcing bar?2020-01-30T19:25:42+00:00

Shotcrete is simply a placement method for concrete. The specified concrete cover over reinforcing bar is usually included in contract documents for construction and values vary depending on exposure conditions. ACI 318 provides cover requirements for structural concrete in buildings, and ACI 350 provides cover requirements for concrete liquid-containing structures. Local building codes and fire codes may also require specific cover in concrete construction. If your project doesn’t specify the cover requirements, we recommend you consult with a professional engineer experienced in the type of project you are working on to learn what the code requirements may be.

I would like to better understand the limitations related to the height of install when it comes to gunite application. Since gunite is a dry concrete mixed with water at the nozzle of the applying apparatus, I have been told by others in the industry that the application is only intended for use on walls less than 4 or 5 ft tall. If that is the case, it is safe to assume that the gunite application strategy should not be used for below-grade vaults exceeding a height of 5 ft? I am looking for design literature specific to gunite.2020-01-30T19:25:27+00:00

Dry-mix shotcrete adds mixing water to the dry concrete materials as the concrete materials flow through and out the nozzle. Gunite is the original tradename for dry-mix shotcrete. Though you may not find design information using the old gunite name, you will find numerous current design references to dry-mix shotcrete. This includes ACI 506R-16, “Guide to Shotcrete”; ACI 506.2, “Specification for Shotcrete”; ACI 506.6T-17, “Visual Shotcrete Core Quality Evaluation Technote”; ACI 372, “Guide to Design and Construction of Circular Wire-and-Strand-Wrapped Prestressed Concrete Structures”; ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures”; ACI 350.5, “Specifications for Environmental Concrete Structures”; as well as seven ASTM standards that directly cover shotcrete. ACI 318-19, “Building Code Requirements for Structural Concrete,” has also added specific shotcrete provisions. Dry-mix shotcrete has been used for decades to build structural concrete walls over 50 ft (15 m) high in circular prestressed concrete tanks that withstand a full head of water pressure. This is substantially greater water pressure than your 5 ft vault wall would experience. There are no limitations in the dry-mix placement process that would preclude use in high walls. Both dry-mix and wet-mix shotcrete using quality materials, proper equipment, and experienced placement crews will produce in-place concrete of equal strength, durability, and low permeability. However, generally wet-mix shotcrete can offer placement rates up to four times higher than dry-mix. Thus, in thicker, longer walls, wet-mix shotcrete may be more cost effective because it can be placed faster.

I’m not satisfied with my subtrades blowout procedures for the shotcrete lines. I’m not an expert in this; however, the way they are doing it does not look safe. I’ve tried to Google and reach out to other shotcrete companies but have not had any luck. They use a hopper that concrete is pumped into and is disbursed through lines that are moved manually; this step is normal. But when they have a blockage or cleaning the line when done, they have two workers sit on the end. They use compressed air. There must be a better way to clean and clear a blockage than having human bodies as weights. Please give me some guidance.2020-01-30T19:25:10+00:00

ASA’s “Safety Guidelines for Shotcrete” specifically addresses hose blockages. Use of compressed air to clear blockages or for cleaning the lines is not recommended. The Guidelines state:

“With the variety of shotcrete material delivery systems available, and their placement on individual job sites, the Contractor should establish site-specific safety procedures applicable to the specific delivery systems and site conditions for blockage removal. Any field procedures for clearing blockages should not use compressed air as means to remove or dislodge blockages.”

Using water to clear blockages or the delivery lines when finishing shooting is the recommended procedure. However, if compressed air is used, the hose end must be securely fastened with a substantial fastening system that can routinely and safely handle the forces created if the concrete is discharged explosively. Shotcrete contractors have developed cleanout bins that clamp the hose end into a heavy steel tank and collect the waste concrete from the line for disposal. Others have created clamps that firmly hold the end of the hose to a loader bucket or other heavy piece of equipment, thus depending on the weight of the equipment to hold the hose end. Simply having two people sitting on the hose end is not safe and can result in injury to crew members. Even when holding the hose end with a clamping system on heavy equipment, clearing the line can cause an explosive discharge of concrete with material flying in a wide path from the hose. Unless planned for and contained, the material stream can hit adjacent workers, facilities, equipment, and vehicles.

Surely you are aware of the recent OSHA regulations regarding Respirable Crystalline Silica (RCS). Does ASA have any information about typical levels of RCS generated during indoor shotcreting? Do you have any recommendations for an apparatus to test levels to ensure safety of our workers and OSHA compliance?2020-01-30T19:24:51+00:00

ASA has closely followed the development of the new OSHA rules for a couple of years before they were put into effect. You can find several articles in Shotcrete magazine that specifically address the rules in consideration of shotcrete application. In the Summer 2016 issue, an article, “OSHA’s New Crystalline Silica Rule–Potential Impact on Shotcrete Operations,” addressed many of the concerns. Unfortunately, with the wide variety of shooting conditions, there are no generic values. The reason for this is that the levels can vary significantly due to a variety of factors, including:

  1. The materials used—this includes comparing wet-mix to dry-mix and the variations in variability of concrete mixture design ingredients (for example, silica fume, fly ash, and accelerator).
  2. Equipment
    1. Dry-mix gun type (rotary or chamber), using a predampener or not (type of wet-mix pump likely doesn’t make much difference)
    2. Size of air compressor (more air might result in more dust)
    3. Delivery line and hose (1.5 in. [40 mm] hose versus 2 in. [50 mm]) can change volume of flow, and then level of acceleration and nozzle stream dispersion as a function of air volume)
    4. Nozzle type can significantly affect the material stream
  3. Shooting location (inside or enclosed, or open air)

With so many variables it is difficult, if not impossible, to get any reliable “generic” number for shotcrete as a whole. Many of our shotcrete contractors are using air quality consulting firms or testing labs who have the monitoring equipment. You may want to note that silica fume is amorphous silica, not crystalline, so it is not hazardous. Most exposure to crystalline silica is through sawing, cutting, or grinding of hardened concrete. We expect that most shotcrete contractors will need to establish a reliable, accurate level by on-site testing because shotcrete is not directly covered in Table 1 of the OSHA rule.

I am looking through Chapter 20 of ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures and Commentary,” and I don’t see any suggested methods for repair—only that the structure (or any repair) must meet specified criteria (strength, load testing, etc.) for serviceability. I am trying to find some reference allowing shotcrete to be used to reestablish the thickness of an existing structure that has inadequate cover over reinforcement. Can you provide any guidance?2020-01-30T19:24:30+00:00

The ACI 350 Code doesn’t really cover repair in detail because it is more about new environmental structures. Shotcrete is concrete; it provides excellent bond to properly prepared concrete substrates and it inherently creates a composite section with the existing concrete that acts monolithically. The supplemental shotcrete could be considered an additional layer of concrete that acts monolithically and thus provides the needed concrete cover. Reference-wise, you could refer to the article “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints” in the Shotcrete magazine archive, as it discusses the bond issue between layers. There are research papers that discuss a 200 psi (1.4 MPa) bond shear stress is needed for a bonded concrete overlay to act monolithically, and that bond pulloff (tensile) test results should be multiplied by 2 or 3 to represent the bond shear capacity. Because we typically well exceed a 145 psi (1.0 MPa) tensile bond pulloff strength, properly applied shotcrete will easily exceed the 200 psi requirement for the bond to act monolithically.

An additional factor in providing additional cover with shotcrete is that shotcrete has very cement-rich paste, along with low permeability, and thus provides a better alkaline environment to combat corrosion of embedded reinforcement, so is effectively giving even better cover than normal form-and-pour concrete.

Shotcrete is very rarely used in Belize. I am building a new home and purchased a shotcrete machine to apply exterior finishes to houses, and most importantly to my new pool. A very good contractor here (who has built many hand-plastered pools) has told me that we cannot use shotcrete in Belize because of the type of sand that we have. Is this true? Can the consistency of the sand make it impossible to use shotcrete?2020-01-30T19:24:08+00:00

Shotcrete is a placement method for concrete. If the sand can be used for concrete, it should be acceptable for shotcrete placement. To some extent, the type of shotcrete equipment may make a difference. If shooting wet-mix shotcrete, the concrete mixture must be able to be pumped. For pumpability, we do recommend a smooth gradation of the fine aggregate. If shooting dry-mix shotcrete, the sand gradation has much less effect, and you should be able to shoot most any sand in your concrete mixture.

I live in NW Florida, Fort Walton Beach. In 1987 a previous owner gutted and substantially rebuilt my house. The exterior brick walls were covered with wire mesh and over 1 in. (25 mm) of coquina shell shotcrete. I bought the house in 1993. I am forever trying to reduce my bills. I would like to find any information I can use to reduce my home insurance. Do you have any links to anyone that has data about my siding being more fire resistant or wind resistant than ordinary masonry/brick veneers? Any guidance you can provide will be greatly appreciated.2020-01-30T19:23:45+00:00

“Coquina shell shotcrete” is not a well-defined concrete material. Though it may have been promoted to you as shotcrete, it may well have been a stucco-like application accomplished with low-velocity plastering equipment, and thus not high-velocity shotcrete placement. Without physical characteristics of the in-place material (strength, density, type, and amount of reinforcement), it is hard to delineate the structural enhancement the coating may provide. You could have an evaluation by an engineer or testing lab to ascertain the characteristics of the in-place material. Then refer to ACI 216.1-14, “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies,” to see whether your composite system has enhanced fire resistance as compared to brick alone. Regarding wind resistance, you would need to have a structural engineer evaluate the composite system for any potential increase in strength against wind loadings.

I have some hairline cracks visible in the outer 3 in. (75 mm) thick shotcrete outer shell of a monolithic dome home. How concerned should I be about them? The outer coat was applied 10 months ago and they seem to be spreading. The shotcrete was applied over chain-link fencing. These cracks are several feet long and most are from doorway or window openings. What is the best/easiest way to repair these?2020-01-30T19:23:28+00:00

The design of any concrete structure must consider loadings and environmental conditions. This includes drying shrinkage of the concrete, and daily and seasonal thermal changes that introduce stresses within the concrete sections. Domes are thin-shell concrete structures and stress concentrations are expected around any openings through the shell. Typically, the design engineer will provide additional reinforcing bars around openings and especially at corners of openings to accommodate the buildup of stresses in these locations. Also, chain-link fence is not considered as acceptable concrete reinforcement, as it cannot accept tension in a straight orientation within the concrete. Proper concrete reinforcement is either deformed reinforcing bars or steel mesh with smooth wires laid out in an orthogonal pattern.

You should have a professional engineer with experience in concrete shell design and construction evaluate your dome home for structural integrity. If the cracking is determined to not affect the structural integrity, epoxy or polyurethane grout injection is routinely used to seal cracks in concrete.

What is the maximum lift for an 8 in. (200 mm) wall against wood lagging temporary shoring?2020-01-30T19:22:54+00:00

There is not a fixed value that one can use for maximum lift height. The maximum lift height when bench shooting is controlled by the concrete mixture (admixtures, aggregate, slump), concrete temperature, size and layout of reinforcement, substrate being shot against, and ambient temperatures. These factors must be evaluated by the nozzleman during the placement. The maximum height is constrained by the ability of the fresh concrete in the lower portion of the lift to carry the weight of concrete in higher portions without creating sagging or sloughing. Hot weather conditions will allow higher lift heights than cold weather in non-accelerated concrete mixtures.

I recently had a project that requires a velocity of ±400 psi (2.8 MPa) to be applied to prepared areas of deteriorated concrete. My questions is: What is the velocity of the material being applied if I am using 50 ft x 1-1/2 in. (15 m x 38 mm) shooting hose with 60 psi (0.4 MPa) supplied pressure? I would also like to know how that is calculated.2020-01-30T19:22:20+00:00

Shotcrete placement requires high velocity for full consolidation and compaction of the concrete. Standard shotcrete equipment with a properly sized air compressor will produce a velocity of 60 to 80 mph (95 to 130 km/h). The 400 psi you stated is not a velocity, but a pressure. Sixty psi is a very low air pressure and may not create the velocity needed for proper shotcrete placement. Shotcrete generally uses compressors that create 100 to 125 psi of air pressure. Also, depending on the shotcrete process used (wet-mix or dry-mix), ACI 506R-16, “Guide to Shotcrete,” states you need an air compressor that can produce a flow rate of at least 200 ft3/min (5.7 m3/min) for wet-mix to 600 ft3/min (17 m3/min) for dry-mix for your 1-1/2 in. diameter hose.

Per contract I have to reinforce first-floor walls with 5 in. (125 mm), 4000 psi (28 MPa) concrete. Due to poor condition of the backup wall, conventional form-and-pour is not an option. I have proposed the use of shotcrete to the architect. What type of shotcrete will meet design criteria for this type of work?2020-01-30T19:21:58+00:00

Shotcrete is a placement method for concrete. It is routinely used to strengthen existing masonry or concrete walls. The 5 in. thickness can be easily achieved with either dry-mix or wet-mix shotcrete. If using dry-mix, you may want to review available prepackaged bagged dry concrete materials. A comprehensive listing of our ASA member companies supplying bagged dry concrete materials can be found in our online Buyers Guide. If using wet-mix, the local concrete supplier should be able to provide concrete mixture designs that will achieve the 4000 psi compressive strength requirement. Dry-mix and wet-mix processes use different equipment and skills of nozzlemen are somewhat different. We encourage requiring use of an ACI-certified shotcrete nozzlemen in the process to be used on the project. If looking to select a qualified shotcrete contractor, you may want to review our ASA position paper on Shotcrete Contractor and Crew Qualifications.

Are there cases of shotcrete being sprayed on the underside of metal floor decking for the purpose of sound transfer reduction and dampening between levels? If so, are there special application methods and formulations?2020-01-30T19:21:32+00:00

We could not identify any specific cases of shotcreting onto metal decking for sound dampening. However, shotcrete is used in a wide variety of overhead applications, and as long as we can get good bond to the underside of the metal decking, should perform well. Depending on the surface profile of the metal decking, you may want to consider attaching studs to the decking to enhance the overall bond of the concrete. You may also want to consider using lightweight shotcrete since the concrete sounds like it is more for acoustic purposes, than for structural.

The company installing our pool ran short of wet mix by about 4 yd3 (3 m3). The pressure truck left, leaving one finisher behind. It was about an hour before the last of the material arrived and what had already been placed had pretty much set. He added about 20 gal. (76 L) of water so he could get it in place. The fully exposed areas, I feel, would be OK. However, the long tapered overlays concern me for strength; it seems like more of a patch. Since it was not shot in place to compress and integrate it, will it bond and strengthen properly?2020-01-30T19:18:59+00:00

Proper shotcrete placement of concrete depends on high-velocity impact of the concrete materials for full consolidation. Though not clear from your inquiry, it sounds like the additional concrete was just dumped in place without high-velocity impact. Thus, you simply have normal cast concrete that would require some type of external vibration to densify and properly consolidate the concrete. The bond of cast concrete would be inferior to the bond from shotcrete since you do not have the high-velocity impact driving the cement paste into the previously shot material. Also, adding 20 gal. of water at the site would substantially weaken the concrete from the original design strength of the concrete mixture. Further, feathering edging in a joint is not recommended since you end up with a very thin overlay at some point that may tend to spall or delaminate much more easily at the thinner section. A better approach when running short of concrete is to stop and prepare a joint for later shotcreting. For best bond, joints should be cut at a 45-degree angle, roughened, and then cleaned and wetted immediately before shooting.

Thus, answering your final question, in summary the bond will be reduced, and with the higher water content, the concrete will be weaker than properly shotcreted concrete.

We are currently working on a job that requires integral color for a sculpted rock facing, but the plant close to the job does not supply color. The DOT we are working for has informed us they will not approve the use of retarder in the mixture so we can order the colored mixture from a plant that is 50 minutes away from the site because they are worried about long-term strength of the material decreasing. Do you know of any literature that we can provide to the DOT regarding effects of retarder on 28-day strength of shotcrete?2020-01-30T19:18:40+00:00

Shotcrete is concrete. PCA’s Design and Control of Concrete Mixtures, 16th Edition states: “In general, some reduction in strength at early ages (one to three days) accompanies the use of retarders. However, increased long-term strength may result from retarding the initial rate of hydration. Excessive addition rates of a retarding admixture may permanently inhibit the hydration of cement.” Thus, if you closely follow the admixture manufacturer’s recommendations for dosage, you may get higher long-term strength than non-retarded mixtures. You may also consider use of the newer hydration control admixtures that essentially stop hydration until activated, and can theoretically put the concrete “to sleep” for up to 3 days.

I have three apartment buildings (with three, four, and five units). Their basements were insulated just over 10 years ago with closed cell spray foam with a class one fire rating. Now for some reason, the building inspector says I have to cover the foamed walls of all three large basements with 0.5 in. (12 mm) drywall and build stud walls to hang the drywall for fire protection. This would reduce the width of the basement stairs significantly, and they would become so narrow that they would then be out of compliance. These basement walls are uneven field­­stone foundations, with bumps, protrusions, and even some curves. I was thinking that shotcrete-applied concrete might work much better and be more appropriate than wood and drywall for a sometimes-moist basement. According to the local Building Code, a 2 in. (50 mm) layer of concrete would suffice. How can a shotcrete application be made to adhere to closed cell foam? The wall heights are approximately 7 ft (2 m) plus bond.2020-01-30T19:18:23+00:00

Shotcrete is just a placement method for concrete. So shotcreting will provide the fire resistance of concrete. For securing the concrete, you can place anchors through the foam into the original basement wall. You can consult with an engineer experienced with shotcrete on anchor size and spacing required for supporting the shotcrete layer.

There is a dam rehabilitation project where stepped reinforced cement concrete (RCC) was used for overflow protection. The surface is spalling rather badly and testing indicated that the RCC was marginal for long-term durability. Is it feasible to place shotcrete over RCC to improve aesthetics and provide additional strength and durability?2020-01-30T19:18:01+00:00

Shotcrete has been used for over 100 years for slope stabilization on natural soils. Your self-consolidating concrete (SCC) sounds like it could be considered a high-grade soil, so stabilization and protection of the SCC with shotcrete is certainly a good application for shotcrete. The fresh surface of the shotcrete can receive a wide variety of finishes, from an as-shot finish (rough) to a floated or even stamped or carved look. You should consider appropriate amounts of reinforcement (reinforcing bars and/or fibers) to control potential shrinkage cracking, as appropriate for your exposure and serviceability requirements.

Are there industry-standard design guides for the design of formwork for shotcrete? I have reviewed the technical questions and found the article “Dynamic Forces during Shotcreting Operations” by Frederic Gagnon and Marc Jolin, but the impact loads in this article are quite small (approximately 90 lb [40 kg] per nozzle application), and I feel uncomfortable using that as the sole design load. However, since there is little to no hydrostatic load from shotcrete, I am having a difficult time coming up with a reasonable design load in lieu of this.2020-01-30T19:17:42+00:00

Shotcrete placement produces very localized pressure on the one-sided forms we typically use. The 90 lb (40 kg) is a reasonable figure when shooting directly against the form. If shooting thick walls, we typically use a benching approach for placement that puts most of the force directly on the supporting floor (or earth) and putting little force on the form. Thus, the form is more of a way to define the back surface. You will often see shotcrete forms in lower-height applications using thin forming materials such as Masonite, pegboard, or even a stay-form for the formed surface. Often, the larger load controlling the design on a form may be the wind loads expected during the construction period.

I have a project where the foundation sub is planning to shotcrete foundations walls instead of pouring them. They’ve submitted all the procedural things necessary to prove their competence and know what they’re doing. For a portion of our foundation, we’re immediately adjacent to an existing building. The sub mentioned today on site that they were not planning to put Styrofoam or anything between our new wall and the existing wall that would resist lateral pressure from the fluid concrete and the question was raised whether this is ok or not (FYI there is still rigid insulation, waterproofing, etc.). The argument is that the concrete is obviously stiff enough to stay in place, thanks to the nozzle-applied admixture, without an interior form that it wouldn’t be exerting any lateral pressure on the adjacent wall. I can follow that logic and almost buy it but I’m wondering if we still need something to resist the force applied from actually shooting the concrete in place?2020-01-30T19:17:25+00:00

Shotcrete is a high-velocity placement of concrete. In most thick walls, as I imagine your foundation walls are, the shotcrete contractor will be bench shooting the walls. This means they will be shooting the full wall thickness in 3 to 4 ft (0.9 to 1.2 m) high lifts where most of the impact forces and weight of the shotcrete is carried by the previously shot material. This results in very low impact forces on the back side of the section. When creating a section with a one-sided form, shotcrete contractors have used thin material, like Masonite, pegboard, or even an expanded mesh material, as we just need to have a surface to define the back of the section.

What is the cure time for shotcrete?2020-01-30T19:17:10+00:00

Shotcrete is a placement method for concrete construction. Shotcrete-placed concrete should be properly cured to provide desired strength and reduce potential shrinkage. ACI 308.1-11, “Specification for Curing Concrete,” and ACI 308R-16, “Guide to External Curing of Concrete” are excellent reference documents. ASA recommends curing a minimum of 7 days, and prefers curing with water, maintaining a continuously wet surface condition for the 7-day period. If using a curing membrane instead of water curing, ASA recommends applying the curing membrane at twice the curing membrane manufacturer’s recommended application rate, and applying in two layers with the second perpendicular to the first. If applying a coating over the final concrete surface, you should check with the coating supplier to verify the duration and properties required before application of the coating.

We are looking for the application of shotcrete on tidal waters. We are located on Lower Puget sound in Washington state and need examples where this has been used and is holding up under the moving tides. The examples don’t have to be applied to our area; the same conditions may apply to other saltwater areas.2020-01-30T19:16:54+00:00

Shotcrete is a placement method for high-quality concrete. Here’s a link to an article of a rehabilitation of a concrete-supported lighthouse in the Saint Lawrence Seaway (Pointe de la Prairie Lighthouse) that provides a lot of detail on an installation similar to yours, including salt-water exposure in a tidal zone: (www.shotcrete.org/wp-content/uploads/2020/01/2014Sum_Sustainability.pdf). Plus, this project also has extreme freezing-and-thawing exposure.

We had a spa added to an existing pool. The shotcrete was too liquid and sloughed off to the bottom. The shotcrete contractor scooped the sloughed material from the bottom with his hands and put it back on the wall. There are fissures and holes in the wall. There was also reinforcing steel close to the surface of the wall. The cold joint at the existing pool wall wasn’t prepared. They added a shotcrete seat to the existing pool over the old Marcite with no removal or roughening up of the surface. They then refused to water cure it. There wasn’t enough reinforcing steel and formwork from the pool company, so the shotcrete contractor had to stop and add more steel from steel I had lying around. So, the shotcrete sat in the truck for quite a while before shooting. We are concerned about the quality of the pool.2020-01-30T19:16:39+00:00

You are correct in suspecting quality issues with your pool. These are the specific issues that lead to poor quality, that can affect the serviceability and durability of your pool.

  1. Shotcrete placement requires high velocity and impact for compaction of the concrete. Hand-applying “sloughed-off” concrete would not provide proper compaction needed for producing monolithic concrete sections. The resultant fissures and voids in your pool reflect the lack of proper velocity and compaction.
  2. Proper preparation of the substrate is essential for good bond and creating a concrete section that acts monolithically. The surface needs to have any materials that would interfere with the bond removed, be roughened, cleaned, and brought to a saturated surface-dry condition before shotcrete placement. This article from Shotcrete magazine gives more details on how and why surface preparation is important (www.shotcrete.org/wp-content/uploads/2020/01/2014Spr_TechnicalTip.pdf).
  3. Concrete cover over the reinforcing bar is critical for maintaining corrosion protection of the embedded steel, and thus providing long-term durability. Low cover will often result in premature corrosion and subsequent spalling of the concrete cover, reducing the serviceability and life of the pool concrete.
  4. Curing is important for all concrete, and especially for the relatively cement-rich concrete we use for wet-mix shotcrete. Curing essentially provides additional water to hydrate the cement in the concrete, and produces stronger, less permeable concrete. Not curing concrete yields concrete that is weaker, more permeable, and ultimately less suitable for creating a watertight pool shell.
  5. You haven’t indicated the actual time concrete sat for “a while.” Industry standards are that concrete should be placed within 90 minutes of the introduction of water to the mixture unless special precautions are taken. Water is usually added at the ready mix plant. If concrete sits too long it can start to lose workability. At the point of losing workability, some contractors will add additional water on site over and above the concrete mixture design requirements, but this “retempering” produces concrete that is weaker and more permeable than the original design mixture.

Based on your input, you have many good reasons to ask the contractor to provide full remediation of these quality issues.

We are a small community in south central Illinois with a deteriorating masonry building issue that drastically needs to be addressed. I have been looking at your shotcrete product and I am thinking that this may be the most efficient and economical way to protect these exposed surfaces from the elements and stabilize these structures. My question to you is: Have any other communities used this product for this purpose, can this product be used in a situation such as ours, or am I just barking up the wrong tree? The wall in question is three stories tall, about 40 ft (12 m); the exposed wall was interior multi-course thick masonry from the late 1800s-era.2020-01-30T19:16:21+00:00

Your proposed project is a great application for shotcrete. We’ve seen shotcrete used for enhancing structural integrity of historic masonry structures across the country. Often shotcrete is used on the back side of the wall to add structural strength while preserving the exterior appearance. In effect, we build a structural concrete wall in-place behind the old wall. Shotcrete has the natural advantage of not requiring any formwork, and can create a good bond to the existing wall, letting the structure elements work together. Here’s a link to a past article from Shotcrete magazine documenting the restoration of a historic brick building (www.shotcrete.org/wp-content/uploads/2020/01/2009Win_SCM01pg08-12.pdf). If you don’t need to preserve the exterior appearance, you can certainly shotcrete the exterior of the wall using the same approach. You can finish the interior (or exterior) surfaces in a variety of ways to provide the architectural appearance you desire. Shotcrete is a placement method for concrete, so using shotcrete will provide a final structure with the strength and durability of cast concrete. By the way, the term “gunite” is the old tradename for what we currently call “dry-mix shotcrete.” Let us know if you have any further questions.

We are in the swimming pool design and construction industry as a general contractor. We subcontracted a large percentage of the work to complete a project — namely, the shotcrete of the pool shell, and were very unhappy with the results. The walls are not plumb and areas are not shot to the full thickness. We didn’t check out the contractor’s current work and he is incapable of making any repairs. We have done corrective chipping and bush hammering to get the walls plumb and areas at the proper grade. However, many areas need to be filled to the proper thickness up to 2 in. (50 mm). Can this be done with either a dry or wet mix? Do you need to bush hammer a recessed area to accept a minimum amount of new material in lieu of a feather edge? Is a wet mix acceptable to fill these areas given that the aggregate in it is generally up to 0.375 in. (9.5 mm) or so? With the dry mix being primarily concrete sand and cement it would seem more practical.2020-01-30T19:16:02+00:00

As you discovered, experience of the shotcrete contractor is key to a successful project. It takes an experienced and knowledgeable shotcrete team (the project manager, supervisor, pump or gun operator, nozzlemen, and finishers) to get a quality job. Answering your specific questions:

  1. Can this be done with either a dry or wet mix? Yes, either wet or dry mix will produce good results. You must be sure to properly prepare the substrate including chipping/bush hammering back to sound concrete, fully cleaning the surface and then bringing the surface to a saturated surface-dry condition (SSD).
  2. Do you need to bush a recessed area to accept a minimum amount of new material in lieu of a feather edge? Feather edging will create a very thin layer that would have more potential to spall when exposed to shrinkage or seasonal thermal movements. We recommend creating a square shoulder at least 0.75 in. (19.0 mm) deep to create an acceptable thickness of the patching layer.
  3. Is a wet mix acceptable to fill these areas given that the aggregate in it is generally up to 0.375 in. or so? With the dry-mix being primarily concrete sand and cement it would seem more practical. Wet-mix with a coarse aggregate can be shot in thin layers, but with a 0.375 in. coarse aggregate may require more finishing due to impact depressions of the aggregate in the shot surface. A dry-mix material without coarse aggregate may be easier to fill in the thin layers. Dry mix is typically less productive in volume placed per hour than wet mix, but in this relatively small-volume repair application, either process should be fine.
Are there published tolerances for shotcrete, specifically wall thickness, plumbness, and irregularities in surface, or should these tolerances be provided on the construction drawings? ACI 117 provides these tolerances for cast-in-place concrete, but specifically states it does not apply to shotcrete.2020-01-30T19:15:22+00:00

ACI 117 provides an excellent guide for tolerances for concrete structures. Although shotcrete is concrete, ACI 117 specifically excludes shotcrete because shotcrete’s unique method of placement permits a wider variety of applications and uses than that of form-and-pour concrete. Shotcrete can generally be finished to the tolerances required for the application. For example, lining a channel might not need close tolerance control, while an Olympic luge/sledding track or skateboard park may require very exacting tolerances. ACI 506.2-16, “Specification for Shotcrete,” in the Tolerances section (and the Mandatory checklist item) requires the specifier producing the contract documents to provide the tolerances required for the project. ACI 506R-16, “Guide to Shotcrete,” Section 3.8, Tolerances, provides a more descriptive commentary. Pertinent portions of that section include:

  • Tolerances provide an indication of the finished product expected by the owner, but meeting tolerances may require additional effort and cost. Tolerances given by ACI 117.1R, for placement of reinforcing steel, cover over reinforcing steel, and overall alignment of cast-in-place structural members should be generally the same for shotcrete. Tolerances that require distinct values for shotcrete construction are cross-sectional dimensions, cover, and surface finish (or flatness). Therefore, specifying tolerances that can be consistently achieved are needed so that project expectations can be met at a reasonable cost.
  • Specified tolerances should be based on use and function and can be the same as concrete, but are typically broader. Some finished surface tolerances may be waived to achieve proper coverage over existing reinforcement.

Although some shotcrete structures have been allowed greater tolerances than allowed for concrete, shotcrete structures can be built to the same degree of accuracy and tolerance as cast-in-place concrete.

You should review the entirety of Section 3.8, Tolerances, in ACI 506R-16 to get a complete description of tolerances for shotcrete placements.

We have a question about the shotcrete setup strength for the exclusion zone in underground shotcrete work. New York and others are requiring a shotcrete exclusion zone (an area excluded from personnel) based upon either time or strength. The UK asks for an engineered approach to this minimum strength. Do you have any information on this?2020-01-30T19:04:36+00:00

The minimum strength for safety must be established by the designer based on the specific structural and geotechnical aspects of the project. The minimum strength value may also be influenced on whether using fiber-reinforced shotcrete or plain shotcrete. The early-age strength was tested with a Meyco Needle Penetrometer after creating a time (early) strength curve with minimum tests at 10 minutes and 30 minutes with Needle Penetrometer; then at 3 hours and 6 hours with Hilti Studs; then at 1, 7, and 28 days with cores. Thus, the curve was calibrated for the specific mixture and environmental conditions. It was also useful to identify when early strength (and potentially long-term strength) was lacking.

Will the new ACI Tech Note for core evaluation (ACI 506.6T-17, “Visual Shotcrete Core Quality Evaluation”) replace ACI 506.2-95, “Specification for Shotcrete”? That document has some sections about core evaluation.2020-01-30T19:04:17+00:00

ACI 506.2-95, “Specification for Shotcrete,” is the deprecated version, and is no longer published (it isn’t readily available on the ACI website). Thus, the Tech Note along with the current ACI 506.2-13, “Specification for Shotcrete,” is the current industry standard for evaluating cores. An engineer may specify use of the outdated ACI 506.2-95; however, they are opening up their exposure because it isn’t the current standard. This is similar to the exposure if an engineer uses a much older version of ACI 318, and not the current one.

We have a client who wants to reline the surface of a brick drying room made from bricks and used to dry bricks. The temperature doesn’t exceed 400°F (200°C). Apart from the usual shotcrete best practice, do you recommend the addition of any additives to make the shotcrete/reinforcement more suitable for the heating and cooling cycles?2020-01-30T19:03:53+00:00

Generally, sand/cement in standard concrete mixtures starts breaking down around 400 to 500°F (200 to 260°C) because the portland cement starts to dehydrate. It loses strength with every phase and will have no strength after 1000°F (540°C). Putting it on brick depends on the shape of the brick surface. Uneven surfaces with holes will hold much better than flat brick. If the brick is flat, placing more than a couple of inches (±50 mm) may delaminate and fall off the wall without anchors of some sort. You could probably use a 2 x 2 in. (50 x 50 mm) or 3 x 3 in. (75 x 75 mm) mesh. In refractory, we consider low temperatures at 1700 to 1800°F (930 to 980°C). In the higher temperature refractory, we use calcium aluminate cement and “traprock,” which is a fine crushed limestone aggregate. The mixture ratios would be the same as a typical sand and cement gunning mixture. This is what traditionally has been for coal bunkers and coal dryers or any other sections in the 1700°F (930°C)-plus temperature range.

I am an architect and we have a client that is planning to do some major landscaping to his yard. He would like to consider shotcrete to create stone-type walls and outcroppings. Can you advise us on this? Would this be an appropriate application? Should he also just consider having a landscape company install boulders in lieu of a shotcrete-type landscape?2020-01-30T19:03:37+00:00

Shotcrete placement for a concrete wall gives the owner the advantages of concrete durability and the appearance of rock. Shotcrete has been used extensively for cre­­ating false rock faces for zoos, water parks, highways, and retaining walls. However, because shotcrete placement and carving to look like natural rock requires quite a bit of labor, the costs to do so may be more extensive than simply placing boulders.

Can we find an appropriate and easy way to evaluate the shrinkage performance of shotcrete?2020-01-30T18:59:26+00:00

Shotcrete is a placement method for concrete. So, standard concrete tests for shrinkage are applicable. You will find an article from Shotcrete magazine, “Shotcrete Testing—Who, Why, When, and How,” helpful. The specific section on drying shrinkage tests says:

“Drying shrinkage of the shotcrete can be tested using general provisions of ASTM C157. Because the shotcrete is shot into a large panel and not into the relatively small mold specified by ASTM for the shrinkage test beam, it is recommended that a beam approximately 11.25 in. (285 mm) in length be sawed from a test panel. As most shotcrete uses coarse aggregate less than 1 in. (25 mm), a 3 in. (75 mm) thick panel with a 3 in. (75 mm) wide cut should approximate the ASTM requirements. The A/E should specify in the contract documents drying shrinkage limits that are appropriate for the design of the structure.”

We are looking for the application of shotcrete on tidal waters. We are located on Lower Puget Sound in Washington state and need examples where this has been used and is holding up under the moving tides.2020-01-30T18:58:40+00:00

Shotcrete is a placement method for high-quality concrete. Here’s a link to an article of a rehabilitation of a concrete-supported lighthouse in the St. Lawrence Seaway (Pointe de la Prairie Lighthouse) that provides a lot of detail on an installation like yours, including saltwater exposure in a tidal zone. Additionally, this project also has regular freezing-and-thawing exposure (www.shotcrete.org/wp-content/uploads/2020/01/2014Sum_Sustainability.pdf).

Another project with tidal zone exposure involved repair of bridge pier pile caps on the East Coast of Florida. The article details the project parameters and testing conducted to verify the quality of the shotcrete placement (www.shotcrete.org/wp-content/uploads/2020/01/2012Win_White.pdf).

Can we find an appropriate and easy way to evaluate the2020-01-30T18:56:59+00:00

Shotcrete compressive strength should be tested using cores from test panels. Shotcrete cannot be shot into closed cylinder forms and be representative of the in-place shotcrete. ASTM C1140-11 and C1604-05(2012) provides the panel configuration and compression testing requirements. However, if using the wet-mix process for shotcreting and one wants to verify the compressive strength, air content, or temperature of ready mixed concrete materials as delivered, rather than as shotcreted in place, samples can be taken from the truck before pumping. These cylinders would follow ASTM C31-15 for making and curing concrete test specimens.

We want to know if it’s possible to apply a 1 in. (25 mm) lift of shotcrete to a berm (2:1 slope). The berm will be treated with emulsion (oil and water) prior to the shotcrete. We are only looking for long-term erosion control. Will the emulsion be required or will it cause a bonding problem with the soil?2020-01-30T18:56:30+00:00

Shotcrete is a placement method for concrete. If the berm is composed of granular materials, one wouldn’t expect the shotcrete to actually bond to the soil. Rather, placing a thin shotcrete layer would create a uniform, relatively impermeable layer of concrete to prevent water from penetrating through and washing out the soil underneath. The emulsion may help to stabilize the soil to help withstand the pressure of shotcreting directly against the berm, but many similar soil stabilization projects will shotcrete directly onto the natural soils. Also, when considering using the emulsion, be aware that some oils may contain ingredients (like sulfur) that can attack the concrete over time. You should consult with an engineer or concrete materials specialist to ascertain whether the specific oil you want to use will have a long-term effect on the shotcreted layer.

I am a structural engineer and we have recently begun work with a shoring contractor. We have been designing soil nails, micropiles, soldier piles, and so on with temporary and permanent shotcrete facings. The contractor has requested that some of our future designs use chain-link mesh in lieu of welded wire mesh, particularly in temporary situations with walls under 10 ft (3 m). I understand that chain link is a cost-effective alternative and, according to the contractor, handles the shotcrete well. Do you have any experience with this type of design/installation process? Can you point me to any literature on the use of chain-link reinforcement in shotcrete walls?2020-01-30T18:56:15+00:00

Some mines have used chain link mesh in shotcrete in severely deforming ground and claim that it is better in holding the ground than mesh after large deformations, in which the shotcrete sustains major cracking with deformations. Other than for such unusual applications, we do not recommend the use of chain-link reinforcement in shotcrete. It cannot be fixed “tight” and as such is susceptible to vibration and movement during shooting, resulting in shotcrete sloughing and formation of voids in the shotcrete. Also, the mesh interconnections are conducive to the formation of voids during shooting. Additionally, there don’t appear to be any consistent material standards on the strength, flexibility, or brittleness of the steel (or other materials) used in the fencing material, so a designer has no way to establish the tensile or flexural strength of the concrete sections. In brief, don’t use chain-link mesh if you want to produce quality, durable shotcrete.

The plaster color installed in my pool was the wrong color. The plaster has been chipped out. My concern is damage to the shotcrete shell in the process. There are deep holes and gauges and there was water seepage in a few areas behind the shotcrete. There is also evidence of honeycombed areas in the shotcrete, as well as some other shotcrete concerns since reading up on the shotcrete process. Please help, as I’m being told that they will just plaster over these concerns. However, the plastering information reads that plaster thickness should not exceed 7/8 in. (22 mm) thickness (but can be a little thicker around plumbing fixtures).2020-01-30T18:55:58+00:00

Shotcrete is a placement method for concrete. When the pool shotcrete contractor uses quality materials, properly sized and maintained equipment, and experienced crew members, the shotcreted pool shell should be watertight. Proper shotcrete application would also not exhibit voids, honeycomb, or major seepage through the pool shell. Before replastering, the pool shell should be evaluated and all defects (voids, cracks, porous sections, deep holes, and so on) repaired using industry-standard methods and materials. You may consider retaining a professional engineer or experienced pool consultant to evaluate the pool shell and make specific recommendations on the appropriate repair for your specific issues. You can use our Buyers Guide (www.shotcrete.org/products-services-information/buyers-guide/) to locate consultants in your area. You also may want to review our pool-specific Position Statements on our website at www.shotcrete.org/products-services-information/resources/. The “Watertight Shotcrete for Swimming Pools” would be particularly informative for you to assist in discussions with your pool contractor.

Is there a way for me to find out what the standard or appropriate repair for exposed reinforcing bar in a shotcrete swimming pool shell?2020-01-30T18:55:39+00:00

Because shotcrete is a placement method for concrete, standard concrete repair techniques are applicable. If the repair is being done by shotcreting new concrete on the existing concrete, the repair should include these steps:

  1. Chip the poor or weak concrete back to sound concrete.
  2. If reinforcing bars are exposed, they should be cleaned of any rust.
  3. If more than half of the perimeter of a reinforcing bar is exposed, the concrete should be chipped back to provide at least a 0.75 in. (19 mm) space behind the bar to allow shotcrete to flow around the back of the bar.
  4. Before shooting any additional shotcrete, the surface should be cleaned and then wetted to a saturated surface-dry condition (SSD).
  5. SSD means the substrate is damp, but there is no flowing water. It should feel damp to the touch, but no water is picked up on your hand when placing it against the surface.
  6. Do not use bonding agents, as they can interfere and reduce the bond of the new shotcrete to the old shotcrete surface.
  7. When the new shotcrete is in place, the contractor should initiate water curing as soon as possible. In warm, dry, or windy weather, fogging of the fresh surface immediately after finishing would be helpful.
  8. Water curing (where the concrete is kept continuously moist) should be supplied for at least 7 days.
Can we find an appropriate and easy way to evaluate the shrinkage performance of shotcrete?2020-01-30T18:54:18+00:00

Shotcrete is a placement method for concrete. So, standard concrete tests for shrinkage are applicable. You will find an article from Shotcrete magazine, “Shotcrete Testing—Who, Why, When, and How,” helpful. Here’s a link to the archived PDF of the article (www.shotcrete.org/wp-content/uploads/2020/01/2011Sum_Hanskat.pdf). The specific section on drying shrinkage tests says:

“Drying shrinkage of the shotcrete can be tested using general provisions of ASTM C157. Because the shotcrete is shot into a large panel and not into the relatively small mold specified by ASTM for the shrinkage test beam, it is recommended that a beam approximately 11.25 in. (285 mm) in length be sawed from a test panel. As most shotcrete uses coarse aggregate less than 1 in. (25 mm), a 3 in. (75 mm) thick panel with a 3 in. (75 mm) wide cut should approximate the ASTM requirements. The A/E should specify in the contract documents drying shrinkage limits that are appropriate for the design of the structure.”

We are building an area of a park that is on an existing pier in Brooklyn, NY. We are researching using shotcrete to form contours on certain areas. Weighting of the pier is an issue. Our question is, can lightweight concrete, or cellular concrete, be used in shotcrete? If so, what are the weights?2020-01-30T18:53:26+00:00

Shotcrete is a placement method for concrete. Lightweight concrete usually ranges from 90 to 115 lb/ft3 (1400 to 1800 kg/m3). In wet-mix shotcreting, lightweight concrete should use presoaked aggregate to make the mixture pumpable. When it’s pumpable, as with conventional concrete mixtures, it is then accelerated to a high velocity by air at the nozzle and projected onto the surface. Lightweight aggregates can also be used directly in dry-mix, and there you don’t need to worry about pumpability because the dry materials are conveyed through the delivery hose. Water is added at the nozzle.

Here’s the specific reference on lightweight from ACI 506R-16, “Guide to Shotcrete”:
2.1.3.2 Lightweight aggregates—Lightweight aggregates should conform to ASTM C330/C330M if used in shotcrete. The aggregate should meet one of the gradations shown in Table 1.1.1. Wet-mix shotcrete with lightweight aggregate is seldom used and is difficult to pump because the aggregate absorbs water, which reduces the consistency of the mixture. Presaturating the lightweight aggregate before batching improves pumpability. Lightweight aggregate mixtures have been shot for wall and floor construction. Shotcrete is frequently employed for fireproofing structural steel members using lightweight aggregates in the mixture.

We suspect that cellular concrete cannot be shot because it uses injection of a pre-formed foam into a cement slurry and is highly fluid. That would preclude any stacking of material to make a vertical surface, and would instead just be pumped in place like a high slump concrete.

Can you please provide me a technical recommendation on whether or not expansion joints should be used in a large shotcrete pool that is approximately 230 ft x 135 ft (70 m x 41 m)? In my design I am calling for two expansion joints which would break the pool into three approximately 76 ft-8 in. (23 m) sections. The contractor is telling me that he typically does not use expansion joints in the pool and that they are unnecessary. I do not typically work with shotcrete and have limited pool design but given the size of the structure I would think it would be best to include expansion joints. Can you please recommend whether or not the expansion joints should be used? Any help would be greatly appreciated.2020-01-30T18:53:04+00:00

Shotcrete is a placement method for concrete. All normal concrete experiences drying shrinkage that creates a volume change in the hardened concrete. Pools will also experience volume change in the concrete due to thermal changes, especially summer to winter seasonal swings. Contraction and expansion joints are common in all kinds of concrete liquid-containing structures especially with walls of this length. Though we can’t provide a firm design recommendation you should consider these factors:

  • What are the weather conditions when the pool is anticipated to be built? If during hot summer months, could there be enough seasonal temperature swing to require expansion joints?
  • If expansion joints wouldn’t be needed would contraction joints be needed to handle anticipated temperature swings, and drying shrinkage?
  • Will the pool be empty for extended times (this could lead to more shrinkage or direct exposure to solar gain or cold conditions?
  • Is the pool to be kept full or empty during the winter months? (if the pool is in a geographic region where extending freezing conditions are prevalent)

Overall, the design for a shotcrete pool should be the same as one for a cast concrete pool.

I am a homeowner who is having a pool built in my backyard. The company used shotcrete last Thursday, but didn’t tell us we needed to keep it wet for the next few days. We found out on the following Monday that we should have been keeping the shotcrete wet. The 4 days that passed before we began wetting the shotcrete were very windy and hot (low 80s). The pool company is now telling us that it’s probably not a big deal that the shotcrete wasn’t kept wet for 4 days. My question is this: How has the shotcrete been compromised by not keeping it wet for 4 days? What can I expect to happen to the shotcrete (cracks?) What would you recommend as far as a fix??2020-01-30T18:52:33+00:00

ASA recommends a minimum of 7 days curing to help control shrinkage issues in young concrete sections. Lack of curing, and exposure to windy, hot or dry conditions will certainly increase the potential for shrinkage and cracking of the concrete. Lack of curing will prevent the concrete from achieving its maximum potential strength. However, shotcrete generally exceeds the minimum 4000 psi (28 MPa) 28-day compressive strength ASA recommends, and required strength depends on the pool design. If you want to confirm the compressive strength of your in-place concrete, cores taken from the pool should be tested for compressive strength by a qualified testing lab. ASTM C1604 Standard Test Method for Obtaining and Testing Drilled Cores of Shotcrete gives guidance on taking cores from existing structures. A minimum 3” diameter core is recommended. Before coring it is recommended to use ground penetrating radar (GPR) or similar equipment to identify the location of reinforcing in the pool section, and then take cores to avoid cutting through the reinforcing wherever possible. The core holes would then need to be filled with a high strength, non-shrink cementitious grout. Once you learn the actual strength, you would need to check with the pool design engineer to verify the strength is adequate for the design. If the strengths are not adequate, you should consult with the pool designer or a licensed professional engineer experienced in pool design for potential solutions.

I modified an existing pool and had a new 20 ft (6 m) wall built that was subsequently back filled. The rebar was epoxied and tied into existing pool wall/floor. The wall is 4.5 ft to 6 ft (1.3 to 1.8 m) tall. 60 days later we have two vertical hairline cracks that run top to bottom. I watered the wall properly and there are no cracks in the other sections we shot (spa, etc). We backfilled 12 days after the wall was shot with hand equipment only. The sample test taken when shooting came back at 6500 psi (4.1 MPa). The original pool bottom is below the wall and has no issues. The wall appears to be 12 to 14 in. (300 to 350 mm) thick from top to bottom. My question is if the wall was shot too thick would the lack of additional rebar cause the wall to fail? And is the necessary course of action to demo the entire wall and reinforce the rebar, then shotcrete again?2020-01-30T18:52:17+00:00

There are many variables that can cause cracking. Vertical cracking is often the result of drying shrinkage of the concrete. You said you cured (watered) the wall properly, but don’t give any specifics. ASA recommends a minimum of 7 days of curing, with a wet cure preferred over a spray-applied membrane. You should have a licensed engineer evaluate the structural sections, and determine if there were any problems with the amount or placement of reinforcing in your wall section.

What is the R-Value per inch of shotcrete?2020-01-30T18:51:58+00:00

Since shotcrete is simply a placement method for concrete, the R-value is the same as cast concrete. ACI 122R-14, “Guide to Thermal Properties of Concrete and Masonry Systems,” would be a good reference.

We have a backwash tank on a waste water treatment plant which is made by a secant wall. The lower area for this structure will receive a shotcrete liner approximately 12 ft (3.7 m) tall on average, the interior perimeter of the structure includes 104 lineal ft (32 m) of unreinforced and 440 ft (134 m) reinforced sections which are a 12 in (300 mm) minimum thickness. The drawings call for vertical control joints with waterstop approximately every 30 ft (9 m). We don’t believe the control joints are necessary and could achieve the same desired performance with one monolithic installation of the shotcrete. Are the control joints really necessary when you are installing the shotcrete against a solid secant wall which does not contain any control joints?2020-01-30T18:51:43+00:00

By control joints, we assume you mean contraction joints. Shotcrete is a placement method for concrete. All normal concrete experiences drying shrinkage that creates a volume change in the hardened concrete. Though shotcrete has a lower w/cm than most form and pour concrete, it will still undergo shrinkage. In being shot on an existing concrete wall the shotcrete liner will be restrained by the bond to the substrate and the restraint of the horizontal volume change from shrinkage can create internal tensile stresses in the concrete. This is likely the reason the designer has specified contraction joints in the section. 30 ft (9 m) spacing between joints is common in new construction of concrete tanks. The question becomes whether the bond of the shotcrete to the existing substrate is high enough to restrain the volume change and prevent cracking along the hundreds of lineal feet of wall you will be lining. The thickness of the lining, the type and duration of curing, the concrete mix design, the strength of the concrete, the strength of the substrate, the quality of shotcrete application, proper surface preparation and exposure to seasonal temperature changes will impact the effect of the volume change of the lining. With the many variables we’ve pointed out you can see there isn’t a clear answer that covers all situations. We recommend you discuss your opinion with the designer or consult with a professional engineer experienced in shotcrete repairs to fully evaluate the specific structural sections you’re shotcreting.

Our company has been working on the design of a concrete pond for winery wastewater and the contractor proposed to replace the concrete liner with a geomembrane (canal 3) covered by shotcrete. Have you seen cases of this application being successful for wastewater holding? As an alternative we are considering applying the shotcrete over a clay liner. Are there any concerns or recommendations for this approach?2020-01-30T18:51:27+00:00

Shotcrete is a placement method for concrete. Shotcrete has been successfully used for over 70 years in thousands of industrial wastewater treatment/storage tanks, as well as replacement lining of sewers and manholes. Thus, exposure of the shotcreted pond to wastewater should be as good or likely even better than the original cast concrete liner. Long term durability of the shotcreted section will be dependent on the concrete mixture design. Many contractors use supplemental cementitious materials (SCMs) like silica fume or fly ash to improve the pumping or shooting characteristics of the mix. These SCMs also help to reduce permeability, increase strength, and thus make the concrete more durable. Fly ash also has the benefit of adding some sulfate resistance that would be beneficial in wastewater exposure conditions. Shotcrete is often shot on geomembranes or directly on the subgrade soils if they are stable enough to hold the impact, and weight of the shotcrete.

We have a pool designed with the cast-in-place concrete construction method in mind. The project has been awarded to a dry-mix gunite contractor. To accommodate the contractor’s placement method, we have been working with him on the details. They are planning on casting the floor and shooting the walls. There are two main areas of concern/questions that we have. First is in regard to the air entrainment and the admixtures that are appropriate for gunite. They have not used air-entrainment admixtures prior to this project. What is the effect to durability without using air? What are the workability effects of adding air entrainment? Which product is recommended? Second, we have specified a hydrophilic waterstop between the cast-in-place floor and the wall. With the walls using a gunite application, what is the best method for preventing water infiltration in the construction joint? Does it hurt the integrity of the joint by installing a hydrophilic waterstop? If the water stop is omitted, what does the surface roughness need to be to provide a monolithic-type connection?2020-01-30T18:51:06+00:00

Air entrainment will generally slightly reduce the compressive strength of concrete, but significantly increase the resistance to freezing-and-thawing exposure. Dry-mix shotcrete (gunite) is generally a very paste-rich mixture. With modern cements, the normal 28-day compressive strengths easily exceed commonly specified compressive strengths. ASA recommends a minimum of 4000 psi (28 MPa) for shotcrete, and 4000 psi to 5000 psi (28 to 34 MPa) strengths are routinely specified.

  1. Air entrainment increases the workability. The small air bubbles act as a form of lubricant to ease internal friction between the concrete mixture components.
  2. You should contact one of our material supplier members to see what they offer. You can use our Buyers Guide at www.shotcrete.org/products-services-information/buyers-guide/, and limit your search to “Admixture Sales” with the “Air-Entraining” subcategory.
  3. Quality shotcrete shot against a properly prepared concrete substrate should produce a watertight interface. The hydrophilic waterstop at the joint could be considered a secondary method of making the joint watertight. Though not necessary, it is kind of a “belt and suspenders” approach with a relatively low cost to place.
  4. In shotcrete construction, surface preparation between layers to provide full bond is important. ACI 506.2-13, “Specification for Shotcrete,” specifically addresses this in the requirements of Sections 3.4.2.1 and 3.4.2.2 that state:

“3.4.2.1 When applying more than one layer of shotcrete, use a cutting rod, brush with a stiff bristle, or other suitable equipment to remove all loose material, overspray, laitance, or other material that may compromise the bond of the subsequent layer of shotcrete. Conduct removal immediately after shotcrete reaches initial set.
“3.4.2.2 Allow shotcrete to stiffen sufficiently before applying subsequent layers. If shotcrete has hardened, clean the surface of all loose material, laitance, overspray, or other material that may compromise the bond of subsequent layers. Bring the surface to a saturated surface-dry condition at the time of application of the next layer of shotcrete.”

For more details on bond between shotcrete layers, you may want to refer to an article in the Spring 2014 issue of Shotcrete magazine, “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints.” A PDF of the article can be found at www.shotcrete.org/wp-content/uploads/2020/01/2014Spr_TechnicalTip.pdf.

I had a concrete pool shell installed using gunite (dry shotcrete method) in July 2013. It was never finished due to unfortunate circumstances and has been exposed to the elements of weather over the last 2 years, mostly filled up with water from rain and, in the colder months, frozen like a pond. We would like to finish the pool but were told by the pool company that the concrete looked odd and we should have it strength tested. We had core samples taken from the walls and floor from a certified testing lab. The results from the six samples ranged from 1700 to 2200 psi (12 to15 MPa). When the pool was blown on July 3, 2013, it was to achieve 4000 psi (28 MPa) compressive strength in 28 days. Is it normal for the shotcrete strength to have weakened so much?2020-01-30T18:50:37+00:00

Properly produced concrete material shotcreted in place should gain strength over time, not lose strength. ASA recommends that concrete placed by the shotcrete method have a minimum compressive strength at 28 days of 4000 psi (28 MPa). Coring does damage the sample somewhat, so it is common to require core strength to meet 85% of the specified compressive strength. Cores should be no less than a nominal 3 in. (76 mm) in diameter for representative results because smaller cores (less than 3 in. [76 mm] diameter) are more subject to damage from the core extraction, affecting the reported strength. Thus, at 85% of 4000 psi (28 MPa) the minimum should be 3400 psi (23 MPa). Based on the reported values, and assuming a 3 in. (76 mm) diameter or greater core, the concrete strength is well below ASA’s recommended strength, and the strength you originally specified in 2013.

I am currently involved with the design of an unreinforced masonry building retrofit. Could you point me toward resources concerning the seismic behavior of a reinforced shotcrete masonry wall? I am interested in learning more about the force (shear) transfer between the masonry/shotcrete interfaces.2020-01-30T18:50:11+00:00

Shotcrete is a placement method for concrete. Thus, seismic design for concrete is applicable to shotcrete placement. Here’s a link to an article in the Winter 2009 issue of Shotcrete magazine, titled “Seismic Retrofit of Historic Wing Sang Building,” that details the seismic retrofit of a brick building in Vancouver, BC, Canada: https://www.shotcrete.org/wp-content/uploads/2020/01/2009Win_SCM01pg08-12.pdf.

A second article from 1999, “Seismic Reinforcing of Masonry Walls with Shotcrete,” also gives some input on the design: https://www.shotcrete.org/wp-content/uploads/2020/01/1999Fal_Snow.pdf. In general, the structural engineer must evaluate the condition of the existing masonry structure and determine whether the added shotcrete sections will be supplementing the existing capacity or providing the full resistance to seismic loads.

Our shotcrete mixture needs to be NSF 61 certified. We have been able to obtain certification of all components with the exception of reinforced fiber. Is there or does fiber reinforcing need NSF 61 approval?2020-01-30T18:25:24+00:00

Each manufacturer of concrete constituents needs to have their products tested by NSF if they want NSF 61 certification. Whether the fibers need NSF 61 certification is an issue with the local authority having jurisdiction for exposure of components to potable water supply systems in your state. Generally, this is the state EPA-type agency, but maybe a federal agency if on a federal project.

We don’t maintain a database of manufacturer products that meet NSF 61. However, you can readily identify contacts for the fiber manufacturers who are ASA corporate members with our Buyers Guide at https://www.shotcrete.org/products-services-information/buyers-guide/. When entering the Buyers Guide, you can select “Fiber + Reinforcement Sales” and the fiber type subcategory to get a list of our member fiber suppliers.

I just had a swimming pool built and everything I have read online says that the gunite shell has to be sprinkled with water for several days after the pour. My pool builder says they never do it, and, when I asked why, I’m just told that’s the way they do it. I told them I am worried because every other pool builder says to do that except the one I hired and I can get no answer as to why. Is this an acceptable practice? I am worried that years down the line I may have a problem. I live in Oviedo, FL, and the weather has been in the low 70s and the humidity not particularly high. They did hit the water table and have a pump running—would any of this have an impact?2020-01-30T18:24:59+00:00

ASA recommends a minimum of 7 days curing for all exposed shotcrete surfaces. Wet curing is preferred to supply additional water to the concrete surface. If a spray-on curing membrane is used instead of water curing, the material should be applied at twice the manufacturer’s recommended rate for formed surfaces. Curing is important to allow the concrete to develop as much strength as possible and to help resist cracking from internal shrinkage of the concrete. Low humidity, wind, and exposure to sun will increase the need for proper curing. If the site is dewatered, the groundwater is below the concrete work, and not effective in curing the exposed shotcrete surface. The American Concrete Institute (ACI) has an excellent reference, ACI 308R-01, “Guide to Curing Concrete.” It appears your contractor is not following the industry standards as documented by ACI.

Why is shotcrete not applied to general housing? It could be applied against outer insulation reinforced panels, forming both pillars and walls at the same time. Higher cost of shotcrete should be more than compensated by much shorter time and lower need of cranes.2020-01-30T18:24:32+00:00

Shotcrete has been used for residential concrete construction, often for domed or other curved shapes. It has also been used to provide a structural skin over internal insulation panels in more rectangular layouts. Since shotcrete is concrete projected at high speed (between 60 and 80 mph [97 to 129 km/h]) many insulation products do not withstand the impact and abrasion associated with shotcrete impact. In many cases when shooting over an insulating foam panel that cannot withstand the high velocity impact, shotcrete isn’t used, but a low-velocity plaster/grout mix is spray applied over the insulation. Here’s a link to the PCA website with a page on residential housing using a foam inner panel: https://www.cement.org/think-harder-concrete-/homes/building-systems/shotcrete. You can also find similar system information doing a web search for “shotcrete foam panel.”

We have an existing cut slope approximately 328 ft (100 m) high (3:1 vertical:horizontal) with cut benches and need to apply shotcrete onto the slope surface at a height of approximately 230 ft (70 m) from the road level. Is it possible to reasonably transfer and apply shotcrete mixture at such a height from the road level? What type of transfer hoses and equipment is preferable? Is dry-mix or wet-mix shotcrete preferable?2020-01-29T16:28:38+00:00

Yes, either dry-mix or wet-mix process can and have been used at this height. Because either process can be used in these conditions, you should use an experienced shotcrete contractor who will pick the best method based on their firm’s shotcrete capabilities. Factors such as the shotcrete contractor’s specific shotcrete equipment, material availability, site constraints, remoteness of the location, and crew experience will influence their choice.

I would like to know if there is any parameter for test panel dimensions and inclination of proposed shotcrete surfaces represented by the test panels. I also would like to know if there is any specification for frequency of making test panels during the shotcrete project duration. In general, what is the specified type of panels and number of them to be specified and to what time frequency should a contractor assemble them for quality control purposes?2020-01-29T16:28:20+00:00

ACI 506.2-13, “Specification for Shotcrete,” is an excellent resource for answering your questions. ACI 506.2 addresses both preconstruction panels and test panels used for material quality control during construction. ACI 506.2, Section 1.5.1.4, requires that when preconstruction panels are required, the Contractor shall “Construct test panels for each proposed shotcrete mixture, each anticipated orientation, and each proposed nozzleman.” Preconstruction test panels vary in size to adequately represent the embedded reinforcement and section thicknesses in the work to be done.

Test panels shot during construction for evaluation of material properties are covered in ACI 506.2, Section 1.6.3.1, which states: “Construct a test panel for each mixture, each nozzleman, and each work day or for every 50 yd3 (38 m3) placed—whichever results in the most panels. The face dimensions of a test panel shall be a minimum of 16 x 16 in. (406 x 406 mm) with a minimum depth of 5 in. (127 mm). For toughness testing in accordance with ASTM C1550, the face dimension shall be 30.5 in. (775 mm) in diameter and 3 in. (76 mm) thick. Shoot test panels in a vertical orientation only unless otherwise specified.”

ACI 506.2-13 has extensive provisions for the submittals, testing, materials, and execution of shotcrete work by a shotcrete contractor. It cites many of the ASTM testing standards appropriate for shotcrete construction. We recommend you review the document in its entirety to become familiar with current industry standards.

We have 18 ft high, 12 in. thick (5.5 m high, 205 mm thick) walls to shotcrete and need horizontal cold joints to place the shotcrete in three pours (three height sections). How do we create the joint?2020-01-29T16:28:00+00:00

Shotcrete is routinely used in creating retaining walls or soil-nailed walls in this fashion. Designers and inspectors often confuse placement of multiple layers of shotcrete in building out a section with cold joints experienced in cast-in-place concrete construction. Unlike cast-in-place concrete, shotcrete provides thorough consolidation and densification by high-velocity impact of fresh concrete material on the receiving surface. The high-velocity impact of shotcrete on a hardened, previous shot layer (or existing concrete surface) provides a strong abrasive blast to open up the surface, and then provides an immediate exposure of that hardened surface to fresh cement paste. As a result, shotcrete exhibits excellent bond to concrete and previously shot surfaces. Thus, the structural action between the sections acts as a monolithic section without any weakened planes.

In shotcrete construction, surface preparation between layers to provide full bond is important. ACI 506.2-13, “Specification for Shotcrete,” specifically addresses this in the requirements of Section 3.4.2.1 and 3.4.2.2 that require:

3.4.2.1 When applying more than one layer of shotcrete, use a cutting rod, brush with a stiff bristle, or other suitable equipment to remove all loose material, overspray, laitance, or other material that may compromise the bond of the subsequent layer of shotcrete. Conduct removal immediately after shotcrete reaches initial set.

3.4.2.2 Allow shotcrete to stiffen sufficiently before applying subsequent layers. If shotcrete has hardened, clean the surface of all loose material, laitance, overspray, or other material that may compromise the bond of subsequent layers. Bring the surface to a saturated surface-dry condition at the time of application of the next layer of shotcrete.

An experienced shotcrete contractor should routinely provide proper surface preparation between shotcreted sections, and use skilled crews with ACI certified nozzlemen to place and cure the shotcrete placements.

We have a concrete tunnel repair project, where expansion joints in a 10 x 10 ft (3 x 3 m) tunnel need to be repaired. The joints are on the top, bottom, and both sides of the tunnel; therefore, we have vertical and overhead applications. The detail calls for deteriorated concrete to be removed to a depth of about 6 in. (152 mm), existing reinforcing steel to be cleaned and preserved, and old waterstop removed and new PVC waterstop installed. How far can shotcrete be pumped for an application in a tunnel? We are looking at about 200 ft (61 m) for the shotcrete to be pumped from the supply to the repairs. Is this constructible?2020-01-29T16:27:37+00:00

Yes, this is definitely a great application for shotcrete. Either wet- or dry-mix shotcrete can easily be used in tunnels with hose lengths of 200 ft (61 m). An experienced shotcrete contractor will be able to select the appropriate process based on the site, availability of material, and their particular equipment and trained shotcrete crews.

Is there a reference where I can obtain some guide regarding the tensile strength of fiber-reinforced shotcrete (steel and poly)?2020-01-29T16:27:18+00:00

ACI 506.1R-08, “Guide to Fiber-Reinforced Shotcrete,” is a good reference for general use of fibers in shotcrete. If looking at underground applications for shotcrete, ACI 506.5R-09, “Guide to Specifying Underground Shotcrete,” will also offer guidance. The ACI 506 documents are available in hard copy format on our bookstore website with ASA member discounts (www.shotcrete.org).
Because shotcrete is a method for placing concrete, many, if not most, of the properties of fiber-reinforced concrete, as found in ACI 544 committee documents, are applicable to shotcrete.

I have a very small job to do that would normally be handled by a gunite or sprayed-on application. I need to form the inside of a concrete box into a cylindrical shape. I would like to get a few hints or suggestions on how I might accomplish the “gunite” solution using my hands/tools only. Does this require a special mix of the mortar mixture?2020-01-29T16:26:50+00:00

Shotcrete is concrete placed at high velocity to achieve compaction. It is dependent on the projection of material with air velocity of 60 to 80 mph (97 to 129 km/h) from the nozzle to consolidate the concrete material in place. It cannot be hand-applied. You may consider hand-applied pre-packaged mortar mixtures to achieve your results, although strength and durability may be less than a similar section with shotcrete because the hand-applied material is not fully compacted. Another alternative is to create an inner cylinder and cast concrete in the space between the box and the form. The concrete could then be vibrated for consolidation.

We are planning on placing shotcrete over rock face and are concerned about water seeping out of the rock and forming ice behind the shotcrete. Could you please offer some insight?2020-01-29T16:26:09+00:00

Shotcrete is a functionally impermeable material when properly designed and placed. Thus, if water accumulates between the rock substrate and shotcrete, depending on temperatures and thickness of the shotcrete, it may freeze. As with all concrete, shotcrete material can be air-entrained to enhance resistance to freezing-and-thawing exposure. You should consult with an experienced engineer to determine whether the type of rock, geometry of the sections, and anchoring of the shotcrete (such as with soil nails or rock bolts), along with the bond of shotcrete to the rock, will provide the desired performance.

What is the standard materials delivery rate velocity for shotcrete applications?2020-03-03T15:50:20+00:00

Recent research into velocity of the material stream shot from the nozzle is approximately 60 to 80 mph (97 to 129 km/h) in the middle of the stream. Outer portions of the stream are slowed and show speeds of about 45 mph (72 km/h). Here’s a link to the Technical Tip published in the Fall 2013 Shotcrete magazine that provides more complete documentation of the research: www.shotcrete.org/wp-content/uploads/2020/01/2013Fal_TechnicalTip.pd.

I’ve been testing shotcrete cores for compression strength according to ACI 506, ASTM C1604, and ASTM C1385. The only thing that we have been doing out of specifications is the panel. Our panels are 18 x 18 in. (457 x 457 mm). We have been coring at the center of the panel 2 days after it has been cast. We test these cores at 7 and 28 days, and the strength of cores reflects passing at 7 days but failing at 28 days. Can you please tell me what could be the cause of this?2020-01-29T16:23:27+00:00

ASTM C1140/C1140M-11, “Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels,” is the appropriate ASTM standard for producing and coring test panels. ASTM C1140 specifies panel size as a minimum of 24 x 24 in. (610 x 610 mm) with a minimum 3.5 in. (89 mm) depth. Without more information on the materials used in the shotcrete and the type of shotcrete, it is impossible to identify what may be causing the lower compressive strength tests.
The compressive strength should increase between 7 and 28 days on a curve equivalent to cast concrete. Strength degradation between 7 and 28 days may be a result of poor shotcrete application or problems with coring or curing of the samples.

We have a school project in California. It is for a structure with shotcrete walls and a shotcrete dome roof shot with an inflatable form. There is disagreement on the nozzlemen qualification panels. One group says that a panel should be shot for each nozzleman for each position (three panels: one vertical, and two for different slopes of the dome) in a single layer with the most congested reinforcing bar in any single layer to simulate job conditions. A second group maintains that the same three panels should be shot, but they should be built up over a period of 6 days in gradual layers to represent the layering of the actual shooting. I think that the first group is correct and complies with the intent of ACI 506. Shooting one-layer panels with the most congested reinforcing bar to be placed in any one layer would best simulate the job placement conditions. I don’t see any added advantage in shooting qualification panels over a period of days in layers and seems to be reading too much into “simulating” jobsite conditions.2020-01-29T16:22:55+00:00

In construction of shotcrete dome roofs with inflatable forms, the structural thickness of the dome is built out in layers to prevent overloading the support offered by the inflatable form and foam. Thus, your nozzleman qualification panels should be representative of the dome construction methods. This would include shooting orientation (vertical and varying slopes), shooting procedures (layers), and with the most congested reinforcing. When shotcrete is applied in layers, all you need to do is wait for the first layer to stiffen sufficiently (usually called initial set), before applying the next layer. It is not necessary to wait for days before applying the next lift.

We are considering sealing off the openings of some existing water tunnels by constructing reinforced concrete walls within the openings. One wall, for instance, is 9.25 ft wide by 12 ft high (3 x 4 m) and is to be 22 in. (559 mm) thick. Other walls will be in the range of 18 to 26 in. (457 to 660 mm) thick. What are the limits as to the maximum thickness/size wall that shotcrete can be used to construct? Also, I noticed that in the latest revision of ACI 506R-05 that the previous recommended limits on the reinforcing bar size have been removed. I believe in the past, the reinforcing bars were limited to the smaller-size bars to reduce the development of sand pockets behind the bars. Is there a recommended limit on the size bars that can be used? Perhaps it was in the ACI standard but I just overlooked it.2020-01-29T16:22:36+00:00

Experienced shotcrete contractors with qualified crews have often shot structural concrete sections 24 to 30 in. (610 to 762 mm) thick with reinforcing bars up to No. 11 in size. There is no real limit to how thick you can build a shotcrete “wall.” Though earlier versions of ACI 506R, “Guide to Shotcrete,” did recommend limiting reinforcing bars sizes, modern equipment, concrete mixtures, and shooting techniques have proven that large-diameter steel reinforcing bars can be properly encased on a routine basis by experienced shotcrete crews. Thus, ACI 506R was updated to reflect the successful industry practice.

There is a pre-stabilized steep slope (nailing is used along with shotcrete). The client wishes to benefit from the maximum achievable area in plan and is asking for excavation of the stabilized slope to shift back the wall face around a few meters. The new stabilization plan shall include a safe gradual excavation of the existing wall from top to bottom along with the destruction of the existing shotcrete face, reinforcement, and the installed nails. What is the most common destruction method for the existing shotcrete? At the same time, I think pulling out the nails using hydraulic jacks would be applicable.2020-01-29T16:21:48+00:00