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regarding the “grading” of cores based on 506.2: I imagine the 1995 standard-based 1 through 5 grading system is no longer valid?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?

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.

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?

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?

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?

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.

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 shotcrete.org/Resources and 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?

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?

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?

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.

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.