Shotcrete is a method of placing concrete. The basic guidelines for placing concrete or shotcrete can be found in documents ACI 305R-10, “Guide to Hot Weather Concreting,” and ACI 306R-10, “Guide to Cold Weather Concreting.” There are some basic “rules of thumb,” such as 40°F (4°C) and rising for starting concrete/shotcrete operations and 40°F (4°C) and falling for stopping concrete/shotcrete operations. With proper planning and procedures, concrete or shotcrete can be placed at below-freezing temperatures and at very hot temperatures, but only with proper planning, procedures, and likely at some cost.
We are in the process of building a shotcrete pool and are required to wet-test the pool before set, waterproofing, and tile. What is the expected water-loss percentage? We are required to achieve 1%.
We are not aware of a specific standard for pools. However, ACI 350.1-10, “Specification for Tightness Testing of Environmental Engineering Concrete Containment Structures,” specifies a volume loss of 0.05% of volume per day conducted over a 72-hour test period for “hydrostatic tightness testing of open liquid containment structures. Specifics of conducting the test can be found in the ACI 350.1-10 document.
It should be noted that the pool should be filled and allowed to saturate for 3 days before beginning the measurements.
We just shot a wet-mix swimming pool for a customer. The shallow end depth starts at 39 in. (991 mm) to the top of the beam and over 10 ft (3 m) linear slopes down to 54 in. (1372 mm). From there we maintain our 1 to 3 ft (0.3 to 0.9 m) slope down to 8 ft (2 m) for the diving end of the pool. The customer would like to raise the entire shallow end pool floor up to the 39 in. (991 mm) depth. We prefer to use wet-mix shotcrete. The overlay would be tapered from the 39 in. (991 mm) start to 15 in. (381 mm) thick at the 54 in. (1372 mm) depth. What would you recommend for this overlay to bond and not “pop loose” or cause crack transfer to pool plaster?
The proposed overlay will be similar to any repair where shotcrete is placed over existing concrete. Proper surface preparation is essential for allowing good bond. Guidance on surface preparation can be found in ACI 506R-05, “Guide to Shotcrete”. It also appears you are suggesting tapering the thickness from 15 to 0 in. (381 to 0 mm). Feathering thickness down to 0 in. (0 mm) is not encouraged, and a minimum thickness should be established. Because the overlay section will be quite thick and experience differential shrinkage from the previously shot material, the overlay will require additional reinforcement to accommodate temperature and shrinkage stresses. You should consult with an engineer experienced in shotcrete design to establish the proper amount of reinforcement. The required reinforcement and cover over the reinforcement will control your minimum overlay thickness.
We have a two-story shotcrete wall enclosing an indoor community pool. We are specifying a board-form finish for the interior and the exterior will have a parge finish coat. Are there any issues with the consistent moisture from the pool that should be addressed in the concrete mixture or topical sealant? How should we deal with the exterior versus interior finishes in regards to water intrusion protection and allowing the green concrete to “dry out” over time?
Shotcrete is a method of placing concrete and the characteristics of shotcrete are those of cast concrete. Although the enclosed swimming pool will increase the interior humidity, the high humidity should have no detrimental effects on the exposed shotcrete, and may even be beneficial in reducing long-term drying shrinkage of the wall.
Both cast-in-place and shotcreted concrete are commonly used for construction of water tanks with constant exposure to water under significant hydraulic pressure. Using good construction techniques with good-quality concrete to build the tank’s walls produces walls with no moisture evident on the exterior face of the tanks. Simply having a high-humidity atmosphere is a much less severe exposure and should not result in interior air moisture being transmitted into and through the shotcrete wall. Any coatings considered for aesthetics should follow the manufacturer’s recommendations for drying time of concrete before application. If there is a concern about the permeability of the shotcrete wall, a premium shotcrete mixture including silica fume might prevent some issues on this application.
Can you send me a document with ASA specifications for gunite coverage of reinforcing bar for swimming pools, please?
ASA does not have such a document. The concrete cover for embedded reinforcing steel is subject to the local Building Codes and may be increased by the structural plans and specifications produced by an Engineer or Architect for a specific project. ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures and Commentary,” covers concrete structures intended for water containment and would be applicable to pools. The following is a link to ACI’s bookstore: www.concrete.org/bookstore/ProductDetail.aspx?itemid=35006.
To keep shrinkage cracking sufficiently tight so as not to cause reflective cracking in the plaster layer, what is the proper depth of reinforcing steel from the shotcrete surface in swimming pool applications?
This question should be addressed by a qualified engineer with experience in designing swimming pools and well-versed in shotcrete technology. Shotcrete is a method of placing concrete and the parameters that work for concrete cover work for shotcrete.
Good practices for placing shotcrete or concrete include:
- Predampening the soil that the concrete/shotcrete is placed against;
- Ensuring that the reinforcing bar temperature is not too high; and
- Early curing of the shotcrete surface and maintenance of curing for the specified duration. If no duration is specified, a 7-day wet cure is recommended.
Using fibers in the shotcrete/concrete can also help control surface early-age plastic shrinkage cracking.
I have a swimming pool that appears to have shrinkage cracks in the floor. I have tried to inject an epoxy, but the cracks are too small. Do you have any suggestions?
There are a wide variety of epoxies and polyurethanes used for crack injection. Smaller crack widths would require a lower-viscosity material to penetrate the crack. You should contact an engineer or injection specialist experienced in shotcrete and cracking issues to evaluate the cracking and make a specific recommendation for repair. Proper concrete mixture design, placement techniques, and early water fogging and curing can help to reduce plastic shrinkage and drying shrinkage cracking in the future.
Are there tolerance standards for the use of shotcrete in pool construction? For example, in regard to the pool depth, what is the accepted variation from the depth specified?
We are not aware of specific tolerances for shotcrete in swimming pools. Shotcrete is a method of placing concrete and the cover over reinforcing steel should be the same as that for cast concrete. With respect to the depth of the pool, this would be a building code issue, not a shotcrete issue.
We recently contracted with a shotcrete company to install a shotcrete structure for a swimming pool. After the pool was completed and filled with water, rust stains began emerging through the plaster surface. When we broke out a section of the pool structure, we found that there was little to no coverage of shotcrete over the steel reinforcement. The shotcrete company’s excuse is that they shot the pool to maintain the desired finished depths and widths and there was little to no coverage because the steel was set too high (even if that were the case, they never alerted anyone during the installation). This sounds like an excuse to me. Shouldn’t the shotcrete company we hired make sure that the concrete coverage met or exceeded what the structural engineer called for? Is there any credibility to their explanation of why they didn’t cover the reinforcing bar enough? What is the standard practice for shotcrete installation?
In short, the shotcrete contractor is responsible for maintaining proper cover over the reinforcing steel. The reinforcing bar installer should set the steel in the proper location for achieving the required cover corresponding to the final desired shape. If the shotcrete contractor finds that he cannot maintain proper cover with the reinforcing as placed, however, he needs to communicate to the designer/owner/general contractor that the reinforcing needs to be fixed before he shoots the section in place. There is no excuse for placing shotcrete with less than the specified cover, as shooting it with reduced cover will obviously create a section that has much less durability than intended by the designer.
I have been in the swimming pool industry for 30 years and I deal with a lot of different engineers on my commercial projects who want a wet test to verify water tightness before the finish is applied to the pool. In my experience, air-entrained shotcrete tends to be porous and leak. Are there any engineering specifications that state that air-entrained shotcrete is porous and will leak if the surface is not trowel-finished?
Properly added and mixed air-entraining admixture in concrete will actually reduce the permeability of concrete. This is because the small, well-formed air bubbles from air-entraining admixtures are not interconnected as larger, entrapped air bubbles may be in non-air-entrained concrete. Thus, the reported higher permeability of the air-entrained shotcrete is not a material flaw but must be from poor shotcrete application. Air entraining from 4 to 7% air is advantageous for enhanced resistance to the freezing-and-thawing cycles of saturated concrete and should be specified by the designer in areas subject to significant numbers of freezing-and-thawing cycles annually. The reported high permeability and resultant failure to pass a water-tightness test could be investigated by taking cores of the “porous” material and conducting a petrographic analysis of the core. Based on the reported results, I strongly suspect that the in-place shotcrete has major issues with sand pockets, overspray, and rebound.