The ASA Technical Questions and Answers is a free service offered to all users, but primarily intended for engineers, architects, owners and anyone else who may be specifying the shotcrete process and/or has need for a possible answer to a technical question.
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PoolQ The Park District Department of our city is in the process of designing a new swimming pool. One of the prospective bidders made a presentation in which they said they would use shotcrete instead of conventional cast in place concrete. Their design is to use 6 in.-thick walls instead of the 12 in.-thick walls as proposed for the cast in place design. They claim that 6 in. of shotcrete is as strong as 12 in. of formed concrete. Is this a true statement?
If this statement was true, there would be a lot more shotcrete projects! The truth is that shotcrete is a method of concrete placement, not a special material. The materials, mix designs, and mix proportions may vary between the shotcrete method and the conventional concrete form and pour method, but the thickness and reinforcing of the structure will be very similar.
There is a subtle difference between the two methods that might affect thickness requirements. Shotcrete is generally placed directly onto the undisturbed soil, joining with the soil to provide the shell for the pool. To use the form and pour method, over-excavation would be required to accommodate two-sided forming. The walls would then have to withstand the forces of backfilling. This may result in a thicker wall requirement. The final decision regarding wall thickness, however, should be made by a structural engineer.
Shotcrete is widely used for swimming pool construction. In some areas it is virtually the only method used. Successful shotcrete swimming pool construction is a result of having an appropriate design, selecting a qualified contractor with certified nozzlemen, selecting appropriate materials and shotcrete mixture design, and following industry recommendations for placing, finishing, and curing.
PoolQ The plaster color installed in my pool was the wrong color. The plaster has been chipped out. My concern is damage to the shotcrete shell in the process. There are deep holes and gauges and there was water seepage in a few areas behind the shotcrete. There is also evidence of honeycombed areas in the shotcrete, as well as some other shotcrete concerns since reading up on the shotcrete process. Please help, as I’m being told that they will just plaster over these concerns. However, the plastering information reads that plaster thickness should not exceed 7/8 in. (22 mm) thickness (but can be a little thicker around plumbing fixtures).
Shotcrete is a placement method for concrete. When the pool shotcrete contractor uses quality materials, properly sized and maintained equipment, and experienced crew members, the shotcreted pool shell should be watertight. Proper shotcrete application would also not exhibit voids, honeycomb, or major seepage through the pool shell. Before replastering, the pool shell should be evaluated and all defects (voids, cracks, porous sections, deep holes, and so on) repaired using industry-standard methods and materials. You may consider retaining a professional engineer or experienced pool consultant to evaluate the pool shell and make specific recommendations on the appropriate repair for your specific issues. You can use our Buyers Guide (shotcrete.org/products-services-information/buyers-guide/) to locate consultants in your area. You also may want to review our pool-specific Position Statements on our website at shotcrete.org/products-services-information/resources/. The “Watertight Shotcrete for Swimming Pools” would be particularly informative for you to assist in discussions with your pool contractor.
PoolQ 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.
PoolQ We are building a pool using shotcrete, and our pool design team has been asked to use the Aquron pool shell protector. The info from Aquron says for best results, spray the CPSP the morning after the shotcrete has been applied. However, our shotcrete company told us to keep the shell damp for a minimum of 7 days. Could you weigh in on this?
The ICC (International Code Council) Building Code requires a 7-day wet cure, which is good practice for concrete or shotcrete. The Aquron Technical Data Sheet mentions prewetting the surface prior to application, but we do not see the direction on the timing. You may want to question the product representative regarding how to accomplish the ICC curing requirements and also get the best results from their product.
PoolQ We are designing a 41 x 60 ft (12 x 18 m), 2.5 to 4.5 ft (0.8 to 1.4 m) deep swimming pool. The walls and floor are 8 in. (203 mm) thick with No. 4 (No. 13) bars. We are specifying shotcrete to build the pool. What should be the minimum spacing for the expansion joint?
You need to address this question to a professional engineer who is knowledgeable in the characteristics of shotcrete and concrete. Shotcrete is a method of placing concrete and the same parameters used in concrete design apply to shotcrete placements. Many pools of this size are designed and built without expansion joints, but it is beyond the scope of our association to provide further guidance.
PoolQ We are having a pool built with shotcrete. The pool company has asked us to change the contract to allow them to use the wet method instead of the dry method of shotcrete. I have read through your website and found it helpful in understanding the difference between the two, but I would like to know if one is better or more sound than the other.
Pools are built with both processes. Some find it easier to shoot pools with the wet method. But, when properly done, there should be no difference in performance between wet and dry process shotcrete. Depending on the complexity of the pool, the wet method placement can be faster than the dry method. It comes down to the experience of the contractor and their crews, for a good quality placed pool shell. The nozzleman plays a key role in the placement of well placed shotcrete in both methods. The geographical area may determine the economics of which method is used. Curing of the in-place concrete shell is the same for both processes (water curing for 7 days). Wet concrete has a 90 minute window from the time it is batched at the plant until it placed. Temperature of the material and the air temperature can increase or decrease the set times of the concrete. Typically Ready-Mix companies hold back 10-15 gallons of water in the mix so that the contractor can adjust the slump of the concrete on site. Adding 1 gallon of water over the design mix (amount of gallons of water per yard of concrete) can decrease the strength of the concrete by 200psi.
If you have additional concerns, the following questions should be asked:
- Does the contractor have a good track record of shooting pools with the wet method?
- How many pools have they completed with the wet method?
- Can you provide a list of past completed jobs?
- How do they plan to incorporate the trimmed concrete into the shell? (The rebound and the trimmed concrete play a key role in the final quality of the pool shell.)
- What concrete mix design do they plan to use?
PoolGeneralQ We are having a pool installed and would like to know if shotcrete should be placed when severe weather is forecasted for late in the day or the next day? Is it possible to cover the area after application to protect it?
Shotcrete is a method for placing concrete. Concrete has a set time that provides hardness of the in-place concrete. Final set may take anywhere from an hour to several hours depending on the concrete mixture and the weather.
Hot weather has a faster set time than cold weather with the same concrete mixture. If the pool is shot in the morning, it may be hard enough to tolerate a rainstorm in the late afternoon. The concrete should certainly be set by the next day unless there is a very cold overnight temperature or a problem with retarder in the concrete mixture. Tarps could be placed over the vertical walls to protect them from a sudden rainstorm. The floor may be harder to protect as the rainwater wouldn’t necessarily drain off the floor.
If concrete is exposed to rain and the surface isn’t washed off in any way, the concrete should be good. Similarly, if you are expecting freezing temperatures overnight, the fresh concrete must be protected from freezing. This may require using vented heaters and insulated blankets. The goal of the cold weather protection is to keep the surface temperature of the fresh concrete above 50°F (10°C). More details on cold weather protection can be found in ACI PRC-306-16 Guide to Cold Weather Concreting available directly from ACI’s bookstore (concrete.org/store.aspx).
PoolQ We are having a swimming pool built with shotcrete. Our question is, what is the required curing time for shotcrete prior to exposure to heavy rain? We are trying to plan the shotcrete installation when the weather looks most favorable.
Shotcrete needs to be protected from rain until it obtains its final set, usually 4 or 5 hours. Following final set, it should be wet cured for at least 4 days, preferably 7 days if possible. The exposure to rain would prove beneficial as the rain would assure the presence of moisture for continued curing.
PoolQ We are having a swimming pool constructed. The pool consultant is concerned about cold joints during construction if walls and the floor are shotcreted on different days. The shotcrete subcontractor states that there is no problem as the next layer of shotcrete will knit itself to the previous placement and form a solid bond. Is the shotcrete subcontractor correct?
Yes, if care is taken to prepare the receiving surface properly. The receiving shotcrete edge must be sound (no loose or unconsolidated material), clean (no traces of laitance or gloss), rough, and dampened to a saturated surface-dry condition. If these steps are followed, there should be no concern about the soundness of the joint.
PoolQ 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.