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?

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.

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.

Is it possible to apply shotcrete on glass? Can shotcrete or concrete bond with glass? If it is an issue due to the smooth surface properties of glass, will sandblasting help? We have made test holes and it is adhered very well, except where moisture penetration through the parapet has compromised adhesion. This is installed with five layers approximately 1 in. (25 mm) thick. We are considering leaving the material in place, patching as required, and removing the existing nonpermeable paints (which are peeling). Our thought is to coat the surface with a variegated colored stain, allowing for permeability of vapor. Can the material be patched as necessary? What is the recommended finish? Do we need to be concerned that the material could permit water penetration? Are there certified contractors in the Cleveland, OH, area?

We are not aware of any specific applications of shotcrete to glass. The glass would have to be tough or strong enough to withstand the sprayed application. We have seen shotcrete stick to smooth surfaces like glass and glossy paint, but have not seen any data on the bond. In general terms, better bond is achieved with roughened surfaces so sandblasting the glass would likely improve the bond.

We are looking at constructing climbing boulders made with shotcrete and using a polystyrene core. Does the polystyrene need to be covered with a protective membrane to protect it during the spraying process? Also, could the polystyrene contribute to the structural integrity of the boulders, or should it just be used as a void-forming material?

Polystyrene is available in various densities and the denser the product, the less likely that the shotcrete process will cause any damage to it. The question of the structural value of the polystyrene needs to be addressed by a competent structural engineer. It is not something that we, as a shotcrete association, feel qualified to comment on.

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.