We are proposing a vertical support of excavation structure using reinforced shotcrete to retain a 10 ft (3 m) high sandy soil. What are the design criteria to choose the reinforcement and the thickness of the shotcrete? Also, what is the minimum reinforcement and shotcrete thickness you would recommend?

FThe shoring design should be done by a competent licensed professional engineer who specializes in earth retention systems. This is not a question that can properly be answered by ASA. You can search for such a professional in our Buyers Guide section of the ASA website shotcrete.org. Another resource is the FHWA Manual for Design and Construction of Soil Nail Walls.

I recently hired a pool contractor to build a residential pool. The contractor has been in business for more than 30 years and has a great reputation. The progress so far is that the pool has been installed using shotcrete. The shotcrete has been curing for the last 9 days. Within the last 9 days, it has rained heavily twice. On the second rainy day, immediately after the rain finished, I walked outside to see the amount of water that had collected inside the pool. I noticed that the water was muddy looking. Upon closer inspection, as the rest of the pool was dry, except for the deep end, there were two trails of water coming from the shallow end and running into the pool of water in the deep end. After getting down into the pool, I noticed that these trails of water were from water bubbling up through the shotcrete floor on the slope closer to the shallow end. The bubbling was like a small stream of water coming up out of the shotcrete in two places. I suppose it is from the hydrostatic pressure from the groundwater under the concrete? My question is should this be concerning? The plaster has not been installed. How should these holes be filled? The holes certainly do not look like they were intentional, as you can’t really even see them, except for the water coming out of them. Is there a problem with the shotcrete installation? Does this mean that my pool will leak when it is filled with water? I would image that if water can come up through the shotcrete, the water can also go down through the shotcrete, resulting in erosion of the soil under the pool? Before the shotcrete was installed, there was no groundwater present and the dirt was dry. (Referenced images can be found here)

It is difficult to make an assessment of a situation like this with a few photos and the description given. Based on your statement that the contractor has an excellent 30-year reputation, we’d suggest you discuss these concerns with the contractor. If his response does not give you a full explanation, we suggest you engage an independent professional familiar with shotcrete installations and swimming pools to give you an opinion. You can use ASA’s Buyers Guide at shotcrete.org to find a consultant.
There certainly is reason to question the quality of the installation based on the description. However, it is not unusual for the pool shell contractor to leave a temporary opening in the shell to relive potential groundwater pressure and prevent floating of the empty pool shell. These holes are, or should be done, in a professional manner to allow complete watertight sealing when filled. In some cases, the openings may include a pressure relief valve.

I am an architect working on a project in which a masonry building was “coated” with a shotcrete or gunite material. It is reported that this was troweled on approximately 20 years ago. 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?

From what you have described, the original work may have been done by the shotcrete process or hand-applied. Typically in the shotcrete process, over a large area at the thickness of 5 in. (127 mm), you would expect to see reinforcing mesh or reinforcing bars.
The material can be patched. If the areas are small, it may be prudent to hand patch. Shotcrete is most effective in larger applications. It is important to select a good repair mortar or mixture design.
ASA does not have a recommended finish. Generally in this type of application, the goal would be to match the existing surfaces as close as reasonably possible.
Many patches are done on bridges and other structures without regard to water penetration.
We do not have certified contractors; however, many of our corporate contractor members have ACI Certified Nozzlemen on their teams and the Buyers Guide on the ASA website can be searched by location and specialty. Also on our website, we have a position paper on “Shotcrete Contractor and Crew Qualifications” with a handy checklist summary that would be useful in qualifying potential contractors for your project.

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.

We have a project in the Los Angeles, CA, area that we have designed as poured-in-place concrete. The contractor would like to shotcrete all basement walls. However, we have columns that are integrated with the wall. A City of Los Angeles technical bulletin does not address shotcrete columns in a wall where the bar spacing varies from the wall. Do you have any experience with this issue and is there a way to shotcrete both the wall and the column?

There have been many prior projects in the city of Los Angeles where columns and pilasters are incorporated into perimeter basement walls. This would likely require a preconstruction mockup panel and might require a waiver from the City Engineering Department. It is extremely important to ensure that the work is done by a qualified and experienced shotcrete specialty contractor who has documented experience doing similar work.

I am currently working on a project that involves repairing the concrete walls of a sanitary sewer interceptor structure and line pipe and was wondering if you had any reference information regarding shotcrete being used to repair similar items and how well it has held up. Any information you send will be greatly appreciated.

Shotcrete, both wet- and dry-mix, have been used to repair and reline sewer structures for many decades and has held up well as a lining or repair material. It is important to have the work done by a qualified shotcrete specialty contractor and to specify a durable concrete mixture design. The following are links to articles which may also be of interest to you:

We’re proposing to install a new shotcrete shear wall against existing 8 in. (203 mm) CMU walls for a project of ours. STRUCTURAL has provided a steel reinforcement cage across each wall plane, which is tied to the existing CMU wall with a series of anchors running horizontally and vertically. Between this new shotcrete wall and existing CMU wall, we are calling for a vapor barrier layer to retard moisture migration from the exterior to the interior. At this point, we are considering going with a PROSOCO Cat-5 fluid-applied vapor barrier system over the CMU but were curious how this might interface with the shotcrete. Have you dealt with this vapor barrier issue before, or seen similar installations? Would the steel reinforcing cage be adequate to support the shotcrete by itself during application, or should we be concerned about the surface of the backup wall?

Shotcrete has been placed against fluid-applied waterproofing and other smooth surfaces, which would likely be similar to the surface you are concerned with. An experienced shotcrete contractor should be able to install this shear wall, taking care to apply the shotcrete in a sequence which inhibits the material from sagging or sloughing. The key is hiring a good, well-experienced shotcrete contractor.
The reinforcing bar size and spacing can help provide support for building up the shotcrete. The spacing should be no greater than 12 in. (305 mm) on center each way and must be rigid.

We are the Architects of Record for a multi-family rehabilitation here in Portland, OR. The existing building is three stories plus a half basement. It is a historic building and is also an unreinforced masonry building. As a result, it requires significant seismic upgrades. Our structural engineer is proceeding with a design that uses several 4 in. (102 mm) thick shotcrete walls as shear elements throughout the building. In most instances, these walls are being constructed adjacent to an existing wall to minimize their impact to the existing floor plans. This seems fine against the exterior masonry walls but we have concerns where the shotcrete is to be installed directly adjacent to a standard architectural partition (gypsum wall board and wood studs). We are planning on plywood sheathing to act as a one-sided form to prevent the shotcrete application from harming the existing stud wall. However, we were curious if we should also include a water barrier so that the application did not allow moisture to migrate into the existing walls during the installation. Is this assumption correct? And if so, is there a performance or product recommendation that you can offer for this purpose?

Shotcrete has been used in similar structural upgrades for decades quite successfully. There are many ways to ensure that the moisture from the fresh shotcrete would not impact the existing partitions. Methods for protection include use of green board (as used in a shower or bath), plywood, painted coatings, moisture-resistant sheeting, and so on.
Shotcrete is placed with a low water-cement ratio (w/c) and the water is needed by the shotcrete to hydrate the cement and harden. As long as the existing surface or treated surface does not actively absorb the moisture, the shotcrete will use the available internal moisture to hydrate the cementitious materials.

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.