We are a shotcrete company in Southern California, and I have a technical question about an issue that has recently come up. Is there any official ASA or ACI guide/spec that recommends or lays out the use of a certified nozzleman on a job that he was not “job site” approved for? For instance, a nozzleman who did not shoot a test panel was overseen by the approved nozzleman for that job site. I understand that the approved nozzleman can oversee another ACI-certified nozzleman on the job so that he can rest and oversee production. Seeing as the approved man is overseeing the nozzleman, is it the same as him shooting the wall himself?

It appears you are talking about a nozzleman who has not been pre-qualified by shooting a mockup panel on a specific project. There is no official ASA or ACI document that addresses this issue. However, this situation is similar to what ACI guidance provides regarding ACI-certified Shotcrete Nozzlemen and Nozzlemen-in-training. ACI’s Certification Program Policy for Shotcrete Nozzleman and Nozzleman-In-Training Section 1.04 states: “ACI Shotcrete Nozzleman-In-Training (NIT) certification is available to applicants who do not possess 500 hours of work experience but do possess a minimum of 25 hours of hand nozzling work experience in the process being sought (vertical orientation only). The examiner of record may require an ACI-certified nozzleman to be present during the performance exam; however, the ACI-certified nozzleman’s interaction with the examinee is strictly limited to the safety of the shooting operation.”

We are having a pool installed and would like to know if shotcrete should be placed when severe weather is forecasted for late in the day or the next day? Is it possible to cover the area after application to protect it?

Shotcrete is a method for placing concrete. Concrete has a set time that provides hardness of the in-place concrete. Final set may take anywhere from an hour to several hours depending on the concrete mixture and the weather.

Hot weather has a faster set time than cold weather with the same concrete mixture. If the pool is shot in the morning, it may be hard enough to tolerate a rainstorm in the late afternoon. The concrete should certainly be set by the next day unless there is a very cold overnight temperature or a problem with retarder in the concrete mixture. Tarps could be placed over the vertical walls to protect them from a sudden rainstorm. The floor may be harder to protect as the rainwater wouldn’t necessarily drain off the floor.

If concrete is exposed to rain and the surface isn’t washed off in any way, the concrete should be good. Similarly, if you are expecting freezing temperatures overnight, the fresh concrete must be protected from freezing. This may require using vented heaters and insulated blankets. The goal of the cold weather protection is to keep the surface temperature of the fresh concrete above 50°F (10°C). More details on cold weather protection can be found in ACI PRC-306-16 Guide to Cold Weather Concreting available directly from ACI’s bookstore (concrete.org/store.aspx).

Please direct me to the proper guidelines for the hydration of shotcrete.

By hydration, I’m assuming you are referring to curing of freshly placed concrete. A minimum of 7 days of continuous wet curing is recommended. Continuous means the exposed concrete surface is wet the entire duration. Not just “watered” twice a day as has sometimes been suggested by contractors. Use of a spray-on curing membrane is an option if for some reason water curing is not practical.

If you are looking specifically at swimming pools, this ASA Pool and Recreational Shotcrete Committee Position Statement “Curing of Shotcrete for Swimming Pools” provides more details: shotcrete.org/wp-content/uploads/2022/01/218216_ SCMSummer21_PS7v2.pdf Also, for a more detailed look at curing, the American Concrete Institute (ACI) document, ACI PRC-308-16 Guide to External Curing of Concrete is an excellent reference.

You can find this at ACI’s bookstore (concrete.org/store.aspx). Finally, a good reference on the basics of concrete, from materials to placement and curing, is the Portland Cement Association’s “Design and Control of Concrete Mixtures, 17th Edition.” You can find this on the PCA website bookstore.

I have a project with a segmental retaining wall. The project is located in Maryland. The CMUs of the retaining wall have significantly deteriorated in several areas. The geo-grid fabric appears to be in good condition in the areas where it could be observed. I suspect the deterioration is caused by saltwater runoff during snow events combined with the freeze/thaw cycles. There are some localized areas where the masonry units have completely disintegrated. The largest area being approximately 4 ft2 (0.37 m2). Is it feasible to remove the deteriorated masonry material, down to a sound surface, and shotcrete the face to restore (or exceed) the structural integrity of masonry units? If so, will this restore the structural integrity of the segmental retaining wall?

Shotcrete is routinely used for the repair of deteriorated concrete masonry and brick. Shotcrete should easily have a minimum compressive strength of 4000 psi (28 MPa) when properly shot and cured. According to the National Concrete Masonry Association, current CMU units have a minimum 2000 psi (14 MPa) compressive strength. Older CMU had a lower 1500 psi (10 MPa) minimum. Thus, the shotcrete placement will be significantly stronger and less permeable than the in-place CMU. This should give the wall better resistance to saltwater and freeze-thaw exposures thus extending useful life.

Depending on the depth of the new shotcrete placement, you may consider mechanically tying the shotcreted layer back to the sound CMU with epoxy or mechanically embedded anchors or j-bolts. Thicker sections may also benefit from the use of a steel wire mesh or fibers.

When shooting onto existing CMU sections, the surface must be properly prepared and then shotcreted with proper shotcrete materials, equipment, and placement techniques. Shotcrete placed onto an existing CMU surface will provide an excellent bond IF the following conditions are met:

  • Make sure the surface is roughened and clean.
  • The amplitude of roughness should be +/- 1/8th in. (3 mm) or more.
  • A high-pressure water blaster (5000 psi [34 MPa] or more) or abrasive blasting can help to roughen and clean the surface.
  • Bring the CMU surface to a 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 with high-velocity placement and quality materials.
  • The shotcrete should have a minimum 28-day compressive strength of 4000 psi (28 MPa).
  • Be sure the shotcrete contractor is using an air compressor able to produce at least 185 CFM (5.2m3/min) for wet-mix and 375 CFM (10.6 m3/min) for dry-mix (gunite) of air flow at 100 to 120 psi (0.7 to 0.8 MPa).
  • 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.

Finally, without details on the loading conditions, and structural details of the original construction, we cannot comment on the structural integrity of the repaired wall. You should contact a professional engineer experienced in concrete repair to evaluate the structural integrity of the wall when repaired.

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.

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.

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 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?

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?

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?

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.

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.