I own a home on a very busy street and the house placement borders the street, approximately 75 ft (23 m) from the curb. Traffic flow has increased over the past year, and I have tried all suggested and approved soundproofing wall systems with varied results. It would seem that a shotcrete product sprayed within a wall cavity would work great. My assumption is that I would need to insulate the outward face of the cavity enough to prevent excessive condensation and moisture buildup. This would certainly solve the sound problem (depending on the mass sprayed). Are there any suggestions or references you might be able direct me to?

A properly designed shotcrete composite wall system would certainly create a quiet atmosphere in the interior of the house. There are 3-D shotcrete wall systems on the market which use shotcrete on both the interior and exterior surfaces with a foam material in the center for insulation and vapor barrier. Attempting to do something like this to an existing structure would require a lot of analysis. Shotcrete is a method of placing concrete and adding shotcrete to an existing wall would significantly increase the weight of the wall and could overload the footings or impact other parts of the system. A structural engineer well-versed in residential construction should be consulted before attempting such a modification.

We have a client who is looking to make a relatively deep cut in a mixture of soils and rock (approximately. 50 ft [15.2 m] high, maximum; the structures will be placed on a pad at the bottom of the cut). We are looking to provide a shotcrete facing for the entire cut area. The upper portion of the cut will be in soil; therefore, the design of a soil nail wall with temporary and permanent facing in the soil region seems to be relatively straightforward using design guidance in FHWA publications, Geotechnical Engineering Circular #7, and some software programs. Significant portions of the exposed cut face, however, consist of nondurable bedrock (claystone). We want to stabilize this area with shotcrete to prevent weathering and the generation of overhang conditions where the claystone is overlain by a more durable sandstone. I have been unable to find design procedures or guidance on specifying shotcrete (thickness, reinforcement type, etc.) and whether or not rock bolts should be used. If so, how do you select the size, spacing, resin type, etc.?

Soil and rock stabilization is an excellent application for shotcrete. However, ASA as an association does not provide engineering design. We recommend consulting with a geotechnical engineer familiar with the local geology and soil conditions to evaluate potential lateral earth forces from the claystone. Once potential loads are established, a consulting engineer experienced with shotcrete in soil nailing applications will be able to design the soil nail facing. You can check our online Buyers Guide to find a consulting engineer experienced with shotcrete.

An inspection report on our home indicated there was evidence of past rodent infiltration. An engineering consulting firm recommended that, to prevent rodents from burrowing underneath the foundation, we have a contractor apply shotcrete across the entire crawlspace bottom, then have a 2 oz. (60 mL) vapor barrier installed on top of it. The barrier would be glued or taped up the sides of the crawlspace. As there is some shrinkage of the concrete during the curing process, I would expect creatures could later emerge between the shotcrete and crawlspace sides. Have you heard this type of shotcrete application in a crawlspace as a structural pest barrier? What thickness should the shotcrete be? Is this use of shotcrete effective? Are there any potential drawbacks to using shotcrete in this way, such as possible problems with the house later on?

Shotcrete is a method of placing concrete and the properties of shotcrete are equivalent to those of cast concrete. The type of work you are describing is done in many cases with the shotcrete process and is commonly called “ratproofing.” As you have engaged an engineer, we would suggest you follow his advice and he should determine the thickness required. Providing a row of dowels around the stem wall to tie the shotcrete to the wall should eliminate any significant separation between the shotcrete and the stem wall. Shrinkage of concrete between the walls may cause some minor hairline cracking, but nothing to allow ingress of rodents or insects.

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.”

I have a 24 in. (610 mm) thick concrete dome that serves as an enclosure to protect extremely sensitive and important equipment that needs to withstand high impact demands such as tornados or missiles. The contractor is proposing to use the shotcrete method with the following sequence: shoot approximately 1 in. (25 mm) (to achieve reinforcing bar cover); let stand for 8 hours; then place a reinforcing mat; then shoot the majority of the dome thickness; let stand for 8 hours; then place the other mat of reinforcing; then shoot the remaining concrete cover. I am concerned that, with an 8-hour duration between concrete placements, the three layers of concrete will not be adequately bonded such that they behave monolithically. In particular, I would be concerned that the aggregate of the concrete that is shot onto a mat of reinforcement will not be able to make its way “behind” the bar’s deformation, thus causing voids. Please let me know your thoughts on the aforementioned concerns, whether it would be reasonable to shoot a 24 in. (610 mm) dome with a minimum of two layers of reinforcement all at once, and whether any of the ACI codes or standards speak to shotcrete joints parallel to reinforcement.

Multi-layer buildout of shotcrete sections is very common and has decades of successful performance in existing structures. Shotcrete applied to a properly prepared, existing hardened concrete substrate (such as a previously shot shotcrete layer) develops an excellent bond. The high-velocity impact of shotcrete on the surface is in effect like sandblasting, and opens up the receiving surface immediately before exposing it to the fresh cementitious paste. Cores taken through multiple layered shotcrete sections exhibit no signs of reduced bond. Often it is nearly impossible to identify where one layer stops and the next starts.
Incremental placement of reinforcing bars in layered application is also common. Proper shotcrete consistency, nozzleman technique, and air velocity will force fresh cement paste around the back of the bar and fully encase the reinforcing bar, even when in contact with the previous hardened concrete surface.
Shooting a 24 in. (610 mm) thickness at one time with two layers of reinforcement in the mostly overhead orientation of a dome would require use of special concrete mixture designs with chemical accelerators, and would be very difficult to execute with consistent quality. Also, depending on the formwork design, unbalanced loading on the dome by shooting very thick sections adjacent to sections not yet shot would be a potential concern.
For more information on the performance of shotcrete in layers, you can review this article from Shotcrete magazine, “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints.”

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.

We are replacing an undersized box culvert carrying a creek under a road with a vehicular bridge. To reduce excavation limits, we are using top-down caisson wall construction with shotcrete facing between caissons for abutments and wingwalls. The shotcrete will be placed in lifts as soil is excavated between abutment/wingwall caissons. The architectural pattern for the face of the abutments and wingwalls is a rectangular pattern of an indented, V-shaped notch. The notches have a maximum depth of 2 in. (51 mm). The structural portion of the shotcrete wall will be 12 in. (305 mm) thick with steel reinforcement. Can this horizontal and vertical V-notch pattern be formed or stamped into the face of the structural wall (with additional thickness as required for pattern) in one wall placement? Or does the pattern have to be a separate placement after the structural wall is cured? If this is done in two placements, I assume that we would need reinforcing bars from the structural portion of the wall into the architectural placement and reinforcement within the architectural placement to lock it in place. What is the minimum required thickness of the architectural layer to account for reinforcing bar embedded from the structural layer and the required reinforcing bar in the architectural layer?

There are many ways to approach this situation. It would be difficult, but not impossible, to install all of the work in a top-down sequence and end up with an architecturally uniform surface.
Approach 1: Install a minimal initial layer top-down with either fibers or welded wire reinforcement. Install dowels from the caissons into the structural facing layer. Install the facing from the bottom up with preplaced V-strips to make the pattern. Finish to the outermost face of the detail strips. Alternately tool the details, but likely more like 1 in. (25 mm) instead of 2 in. (51 mm).
Approach 2: Install the structural wall top-down, encapsulating the outer reinforcing steel to a plane at the depth of the detail strips. Prepare the surface by sandblasting or water blasting to create a favorable bonding surface. Install detail strips to the face of the roughened wall. Place and finish the finish layer to the depth of the detail strips.
If the base layer is properly prepared, the bond should be very good and adding dowels would be redundant. There is nothing wrong with redundancy and if so, the minimum layer thickness would be 2 to 3 in. (51 to 76 mm).
The nature of this work will mandate the use of a highly qualified shotcrete subcontractor who has experience in installing similar-quality architecturally significant walls.

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.