5/21/2023 0 Comments Plaster pool![]() In disagreeing with the above statement, the National Pool Plaster Council (NPC) has actually claimed in their 2002 & 2005 Technical Manuals that blisters and spalls can be caused by aggressive pool water chemistry. In fact, as we have mentioned, the inherent weakness leading to spalling is cre- ated before the pool is ever filled with water. One thing that does not play a role in plaster spalling (or peeling) is water chemistry aggressive water is not required to form spalls. The step radiuses are occasionally over worked to insure smoothness and straightness, and the floors sometimes get too much troweling due to being walked on by the pool finishers while troweling. Spalling is relatively rare, and when it does occur it is most commonly found on a step radius, on shallow end floors, and around the main drain these being places where finishing can be an additional challenge for a number of reasons. Since the vast majority of pool plaster does not spall, blister, or flake off, we can see that most plasterers properly apply, cure, trowel, and finish the pools they plaster. (We in the pool industry use the terms delamination or bond failure forÄeeper bond separation between new and old plaster or between plaster and the substrate, and we use the term scaling to describe calcium carbonate depositing from pool water to plaster surface) But in the pool plastering industry, the term spalling has become a commonly used term to describe this shallow surface loss. It is not referred to as spalling, since spalling is generally reserved by the cement/concrete industry to describe deeper delamination. It is referred to by many names, including surface flaking, buckles, blistering, scaling, or delamination. The general cement/concrete industry has documented this loss of a thin surface layer as a surface defect caused by improper application and troweling. Note that the newly exposed surface (where the flaking occurred) is extremely rough and coarse, while the older existing surface surrounding the missing plaster area is much smoother. This could be described as a moon like crater look, or a shallow and sunken pit look. You can see the round indented area with the loss of plaster. Often, these weak areas do not flake off until the pool is drained of its water and the plaster dries out. However, after time this weakness is manifest by a 16th to an 8th inch layer of the surface flaking off, generally in small areas or spots. If this happens, the top of the finished plaster will look fine, even for awhile after the pool is filled. It can also include too much drying between trowel passes, hard troweling, and overworking the finish. This can happen when plastering on dry, hot days, during low humidity, in wind, etc. If the surface becomes too dry, with a dry crust on the surface but a wet paste underneath, a weakened zone can be formed just subsurface. The opposite problem of forcing too much water into the hydrating surface is allowing the surface to dry too fast. ![]() Finishers learn to time the troweling passes so that bleed water is evaporated, and the surface workable. If troweling is performed when bleed water is present, forcing bleed back into the plaster paste causes excessively high water/cement ratios in the surface finish, weakening it. One of the arts or skills of the plastering trade is properly timing the troweling passes. Swimming pool plaster is commonly troweled in three to five separate passes, the early passes placing the material where it needs to go and consolidating it, and final passes creating the smooth, final surface. Bleed water then evaporates from the surface. This process is called bleeding, and the rising water is referred to as bleed water. What happens to the excess water? As the solids (sand and hydrated, crystallized cement grains) settle, the excess water rises to the surface. This is done because if the mixture is too dry and thick, it is almost impossible to place and trowel to a smooth finish. When cement, sand, and water are mixed to form pool plaster, about twice as much water is added as is really needed to hydrate all of the cement.
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