Concrete pool tanks or spas are rarely smooth, clean and level enough for bonding of waterproofing membranes and/or ceramic tile. Improper preparation and/or preparation products are a major cause of failure in pools. Cast-in-place concrete walls often have defects such as form release, curing compounds, and surface laitance from finishing techniques plus all the normal construction surface contamination. Mechanical means such as shot blasting, high pressure washing or grinding are the preferred methods to prepare the surface. Acids or chemicals are not recommended, and if used must be completely neutralized so as not to affect the bond of the waterproofing or thinset. Patching or leveling compounds must be carefully selected when used in a pool application and should be allowed to thoroughly dry. As with any surface to receive tile, it is always easier to make it flat ahead of time. When using a surface applied membrane it is imperative the substrate be made flat to avoid excessive thinset buildup.
You may also find submerged applications of waterproofing carry a different set of recommendations for drying times than you are accustomed to. Often this is not noted in the product data sheet as it is very difficult to address all possible scenarios in just a few pages of instructions and cautions. When not absolutely sure, call the manufacturer. That seems so simple and if it were done more often it might make for a few less inspectors in the world. It is very prudent to always flood test any waterproof installation prior to application of tile. If you are not the one doing the waterproofing make sure the method that was employed does not interfere with your products or methods and was flood tested. Do not settle for the verbal assurance of “I never had a problem.” Recently I was on such a job where the concrete tank was chemically treated by some means. The contractor then floated a bonded mortar screed on the walls, applied waterproofing and tile, much of which later ended up in the bottom of the pool. It was never a problem before; need I say more?
As you may expect, there are special considerations for the tile used in pools and spas as well. Low absorbtion tile is preferred and mandatory in freeze/thaw climates. Using paper face-mounted ceramic mosaic tile is ideal. While there is limited selection in face-mounted tile, this system offers the ultimate in bonding capabilities. Use caution when considering back-mounted sheets using PVC dot mounting or adhesive mounted mesh mosaic tile. The backings and quality of mounting methods vary. This can result in bond failure or bond strengths being compromised. When pools go out of balance, some mounting systems can be subjected to chemical attack by the pool water. Check with the manufacturer of the selected tile to verify their recommendation for not only use in submerged installations, but the chemical resistance of their mounting system to the pool or spa water.
Our recommendations for grout should follow those of thinset. Always use a polymer modified grout for increased chemical resistance at a minimum. If not fully and firmly packed in the grout joint, it won’t take long before the surrounding area loosens with the constant water pressure. Some epoxies are also suitable for use in pool environments, however, if exterior, not all are UV stable and may fade over time. Movement joints are mandatory in pools as with all tile installations. In the long term, all tile experiences a degree of moisture expansion. It may well be many years before the benefits of installing movement accommodation joints become of any value. Those pools that have them will experience many additional years of serviceable life over those that don’t. A word of caution about use of sealants for caulking these joints: very few sealants are capable of providing long trouble free service in a chemically harsh environment like a pool or spa. Every urethane or silicone is not up to the task. Sometimes Poly Sulfides are recommended. Whatever your choice, verify it is specifically recommended for use in chemical environments, not just submerged applications.
The balance of the pool water is responsible for many problems in the maintenance of tiled swimming pools. Acidity, alkalinity, and water hardness (mineral content) in swimming pool water must be kept in balance to prevent contamination and deterioration of Portland cement mortars and grouts. Chlorine treatments are not typically the cause of mortar attack and deterioration in tile swimming pools. Swimming pool water needs to be maintained between a pH of 7.2 & 8.0. If the pH is too high (alkaline) then mineral deposits will form on tile and grout, especially at the waterline. If pH is too low (acidic) then etching and deterioration of Portland cement-based materials will occur. If this condition remains constant then the cement bonding mortar and grout can deteriorate to the point of tile failure.
Pools and spa installations require careful planning, execution, and solid viable ongoing maintenance to provide long term service. It can be difficult to convince a homeowner realizing their dream or a school board with swim meets scheduled that they need to wait just a little bit longer to fill and enjoy. I have been there and understand the challenge. However, you need to stand firmly on the needs of tile installations in submerged applications to avoid costly and catastrophic failure.