I am involved with an above-grade canal replacement project that used shotcrete. The forms were recently pulled from one of the vertical walls, and extensive defects were visible throughout the wall length. Reinforcing bar shadowing occurred at nearly every vertical bar as well as numerous void pockets of various depths (some even up to 3 in. [76 mm]). Demolishing and replacing the wall is not a viable option. I am responsible for developing repair recommendations to the client and would like to know if there are any standards for shotcrete tolerances, specifically in regards to defects. I am going to specify that a repair mortar be used for repairing the defects and would appreciate any insight into this, as well as any wisdom that could be offered in dealing with reinforcing bar shadowing.

This seems to be two questions:
1. The repairs should be accomplished as recommended in ICRI Technical Guidelines: the area to be repaired should be chipped out to sound concrete, sand­blasted or waterblasted to remove any bruised material, dampened to a saturated surface-dry condition, and patched with a reputable and known repair mortar.
2. ACI 506 defines “shadow” as any porous areas behind an obstacle, such as reinforcement. Proper shotcrete application by an experienced nozzleman with adequate equipment and crew will create shotcrete sections that have minimal shadowing and voids. However, sometimes upon stripping of the backside forms there is a noticeably darker coloration of the concrete surface immediately behind reinforcing bars. This darker coloration often does not have porous areas, but is an indication that the reinforcing bar is close to the form and, although good, dense material has been placed, it has a slightly higher cement paste content and thus appears darker on the surface. Sounding the area of discoloration should identify any porous areas that should be repaired, as mentioned previously.
It should be noted that this is not unique to shotcrete. Cast concrete that is not properly placed and fully consolidated often exhibits voids and rock pockets that need to be repaired upon stripping of the forms.

Would epoxy-coated reinforcing bar be required for a shotcrete site retaining wall that does not have waterproofing behind it?

Shotcrete is a method of placing concrete and the properties of properly placed shotcrete are equal to those of cast concrete. We have seldom seen the use of epoxy-coated reinforcing bar in retaining walls with or without waterproofing. (The California Department of Transportation builds many retaining walls with cast concrete and with shotcrete and seldom uses epoxy-coated reinforcing bar or waterproofing.)
It is important that you use a contractor qualified and experienced in this type of work and who uses ACI Certified Nozzlemen and a qualified crew.

I am considering using shotcrete for lining stormwater conveyance ditches at a project site, and am trying to find any possible information on the potential for shotcrete (or other cement products for that matter) to leach selenium. Please advise if you have any information regarding this topic.

Shotcrete is a method of placing concrete. Thus, testing for selenium appropriate for concrete is suitable for shotcrete. The Portland Cement Association (PCA) has published a paper on testing of cement for various constituent components, including selenium, to meet NSF 61 requirements. Their testing showed “Values for arsenic, cadmium, selenium, and silver were all below detection limits.” The paper can be found at PCA’s website.

I have a project in Fort Worth, TX, where we will be placing shotcrete on some interior walls that will have steel embeds for other structural supports. Do you have an article or literature regarding good practices of shotcrete placement around steel embeds?

Shooting around embeds can be very challenging. The most important factor is to have the work done by a shotcrete subcontractor who has done this successfully in the past and has ACI Certified Nozzlemen who are also experienced in this type of work. It has been done successfully on many projects in the past, but we do not have a published procedure to do this work.

I am searching for criteria/guidelines or ratings on what different profiles are achieved by shotcrete. I am hoping there are installed shotcrete profile requirements with respect to final surface roughness. We manufacture a waterproofing system and are often asked to be installed over shotcrete, to which we have no objections. However, I am hoping there are criteria/guidelines/ratings on achieved profile of the finished surface. For example: The concrete industry often talks about roughness achieved by shotblasting and the surfaces getting to various degrees between CSP-1 to CSP-9. Is there a criteria/guideline/rating system, or something similar with shotcrete? Here is the link to some such guidelines.

Shotcrete is a method of placing concrete. The surface texture of roughness varies considerably depending on the application and the abilities of the installer. The surface can vary from a rough nozzle finish to a smooth trowel finish and many variations between these two extremes. In buildings, the typical finishes are wood float, rubber float, or trowel finish. The owner and the architect determine what finish will be required and generally specify the finish in the construction documents. The documents which might be of help to you are ACI 506R, “Guide to Shotcrete,” ACI 301, “Specifications for Structural Concrete,” and ACI 117, “Specification for Tolerances for Concrete Construction and Materials.”

Is shotcrete applied to hardened cast-in-place concrete considered monolithic by the American Concrete Institute (ACI)? Is shotcrete-to-shotcrete considered monolithic by ACI? Can either of these connections be made watertight? Or at least as watertight as the concrete? Is shotcrete without admixtures truly watertight or waterproof? (My definition of “watertight” would be a measurable amount or more than leaching of moisture and calcium.)

Shotcrete properly applied to a well-prepared existing concrete surface will create an excellent bond and structurally act as a monolithic system without joints or layers. Pulloff testing of shotcrete applied to concrete will often fail in the underlying concrete substrate, and not at the bond interface or within the shotcrete section.
Shotcrete is a method of placing concrete and should have similar watertightness characteristics. These characteristics can be enhanced with admixtures and supplementary cementitious materials such as silica fume. Please refer to the images of cores from shotcrete applied to existing concrete.

Do we need to coat reinforcing steel after sandblasting and prior to placement of product?

The answer is no; shotcrete will bond well to sandblasted reinforcing bar on overhead or vertical applications. Shotcrete, like conventionally placed concrete, can be placed over uncoated black bar or bar that is coated with rust inhibitors. In repair areas where there is heavy scale on the reinforcing bar and spalling of the concrete, the repair can sometimes include some sort of reinforcing bar treatment or inclusion of a rust inhibitor in the shotcrete mixture. It depends on the situation and the assessment of the design engineer as to what is necessary.

Why is there not more extensive use of fiberglass reinforcing bars? It seems like it would be a natural choice for most projects involving shotcrete in wet applications, as well as conventionally placed concrete, especially in the types of jobs we do, such as the rehabilitation of existing concrete channels that usually contain acidic waters. I understand that anything other than steel is more expensive, but isn’t prevention now cheaper than remediation later?

Although similar in dimensions, fiberglass reinforcing has distinctly different structural properties when compared to conventional steel reinforcement. This is a question better answered by the fiberglass reinforcing industry or the structural engineering community. As the American Shotcrete Association, we do not get involved in the engineering design of structural sections. However, it should be pointed out that properly designed and applied shotcrete provides a very corrosion-resistant environment around embedded steel reinforcement, providing excellent long-term durability in normal exposure conditions.