PoolQ I’m hoping you’d be willing to answer a couple of questions I have about gunite. I am having a swimming pool installed at my house in Florida. I was away when the gunite was shot a few weeks ago and didn’t know at the time that the gunite should be sprayed with water for a few times a day for about a week according to what I have read online. The pool company owner knew I would be away and never mentioned the need for the gunite to be periodically moistened. The owner also made no provision for any of his employees to hose it down or install a sprinkler. When I learned after the fact of the watering requirement and asked him about it, he said it was unnecessary because the gunite was shot at 4000 psi (28 MPa) and not the “industry standard” of 3000 psi (21 MPa). Then he added that with the almost daily rain in Florida at this time of year, all was okay. I’m concerned about the gunite’s integrity—its permeability and the possibility of shrinkage and cracking. Could you tell me if I have a reason to be concerned, and if so, what do you suggest I should do about it?
Gunite is the original tradename for what we now call dry-mix shotcrete. Shotcrete is a placement method for concrete, so recommendations for curing and protection follow general ACI guidelines for exposed concrete. ASA recommends a minimum of 7 days of continuous (not just a few times a day) wet curing to help control shrinkage issues, increase strength, and reduce permeability in young concrete sections. Lack of curing and exposure to windy, hot, or dry conditions will certainly increase the potential for shrinkage and cracking of the concrete. Lack of curing will prevent the concrete from achieving its maximum potential strength.
Shotcrete placement with quality materials and proper application techniques generally exceeds the minimum 4000 psi 28-day compressive strength ASA recommends. The statement that 3000 psi is the “industry standard” is not true, as the ACI 350 Code for concrete liquid-containing requires a minimum 28-day compressive strength of 4000 psi for concrete intended to have low permeability when exposed to water. The required strength depends on the pool design. If you want to confirm the compressive strength of your in-place concrete, cores taken from the pool should be tested for compressive strength by a qualified testing lab. ASTM C1604/C1604M provides guidance on taking cores from existing structures. A minimum 3 in. (76 mm) diameter core is recommended. Before coring, it is recommended to use ground-penetrating radar (GPR) or similar equipment to identify the location of reinforcement in the pool section, and then take cores to avoid cutting through the reinforcement wherever possible. The core holes would then need to be filled with a high-strength, non-shrink cementitious grout. Once you learn the actual strength, you would need to check with the pool design engineer to verify the strength is adequate for the design. If the strengths are not adequate, you should consult with the pool designer or a licensed professional engineer experienced in pool design for potential solutions.
Regarding cracking, the lack of curing will increase the concrete’s shrinkage and correspondingly the potential for cracking. You should verify that there are no significant cracks in the pool shell before the plaster or other interior coating is applied. If there are cracks, the pool contractor should repair those before proceeding with the plaster or coating. Although proper curing would certainly decrease the concrete’s permeability, generally good-quality shotcrete with proper placement and a strength of 4000 psi will be functionally watertight and not allow any significant amount of water to flow through the uncracked concrete thickness. You will find more detailed information on pool compressive strengths and watertightness of pool shells in our ASA Position Statements.
