Portal: Pool

This inquiry involves a lot more than just mix design. First, additives to the mix by themselves will not keep the shotcrete from cracking. To minimize leakage for the proposed application, he will have to use either a waterproofing membrane on top of the shotcrete, or plaster like would be used on a swimming pool. Putting a membrane behind the shotcrete would only serve to keep ground water from entering the pond through the back side. The other aspect to be addressed is the fact that all concrete shrinks, and that is what causes the cracks. So anything that can be done to minimize shrinkage should help. To name just a few items: avoid shooting on a windy and or low humidity day; use aggregates in the mix that have a good record regarding shrinkage; avoid excessive cement content in the mix; use reinforcing steel (mesh or rebar); synthetic fibers help reduce early plastic shrinkage; proper curing is absolutely essential!

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?

There are two ways to insulate the outside of concrete spas. The first way is to shotcrete the spa and then glue Styrofoam to the outside of the concrete shell or to spray the insulated foam to the outside surface. The second way is to use the ICF (insulated concrete form). You would only have to use one side of this form system. This system would act as the outside form so that the shotcrete could bond to the foam. This type of system has foam insulation thicknesses from 1 to 4 inches thick. Yes, it can be fitted to form circles. Each ICF system is different, so some research would be needed to see which system would work the best. Since most spas are formed up before they are shot, the ICF system would serve two purposes: forming and insulation in one step.

On large swimming pools, it is not unusual to have joints that are left over a weekend or longer. The key is the means by which the joint is dealt with. As with any concrete joint, the surface needs to be clean and free of laitance or other contamination. This can be accomplished by cleaning the joint while it is green on the first day or by cleaning with waterblasting, sandblasting, or wire brushing after the surface has gotten hard. As long as the joint is clean, all gloss has been removed, and the joint is dampened the structure should not be impacted by the joint. Also, 27 cubic yards in 4 hours is not necessarily slow production. Depending upon the circumstances, I would think that 27 cy in 4 hours was quite productive.