Posted by Kim Skinner on August 12, 2009 at 11:00am
Our field and lab research, coupled with consulting and analysis services from two of the best cement failure-analysis labs in the country, has led to newer and better explanations for the causes and mechanisms of many plaster failures, including etching, scaling, nodules, delaminations, spotting, streaking, staining, discolorations, and general surface deteriorations. Together with the professional labs, we have been able to demonstrate that known concrete/cement science applies to pool plaster.Here is what we would offer as a sort of “Ten-point checklist” of issues leading to either good or bad pool plaster.First, the best cement/aggregate ratio is about 1 part cement to 1.5 – 1.75 parts aggregate (marble sand or limestone). For quartz finishes, the mix is a little richer. This produces a higher quality pool plaster product than either richer (more cement) or leaner (more sand) mixes. Plaster that is too rich tends to shrink and crack, and plaster that is too lean is unworkable and isn’t durable. The cement and aggregate should also be selected with care – it needs to be of good quality, consistent, and of an appropriate grade for use in pool plaster. (This also applies to any admixtures).Second, a water/cement ratio of .50 or less is best when mixing plaster. The American Concrete Institute (ACI) and the Portland Cement Association (PCA) have both established that lower water/cement ratios (less than .50) provide for better quality cement which can resist occasional exposure to mild acids. Lower water/cement ratios increase density and reduce permeability, porosity, shrinkage (craze cracking), and movement of water within the cement product. Higher water/cement ratios can result in a finished product that does not have adequate protection or long-term durability against the natural effects of water and the environment.Third, plaster should contain as little calcium chloride set-accelerant as possible, and never more than 2% to the amount of white cement. (Colored plaster, of course, should not contain any calcium chloride.) It has been documented by the PCA and other cement laboratories that using more than this amount increases discoloration, mottling, and shrinkage of cement. Alternatives to calcium chloride, which do not have these downside effects, are available.Fourth, a plaster mix should be mixed thoroughly, but also not too long. It is recommended that if the plaster has been mixed for more than 90 minutes, the plaster mix should be discarded.Fifth, water should not be added to plaster surfaces while troweling because of the strong risk potential. It has been documented by the ACI and PCA that doing so (“skewing the surface water-to-cement ratio”) increases porosity, shrinkage (cracking), streaking, spotting, and variable discoloration. Small amounts of water for trowel lubrication doesn’t necessarily seem to have a significant adverse effect on the plaster surface – most visible problems require a combination of water finishing, chloride abuse, and other improper practices, and take time to become exposed. But it is very important not to “work” or force additional water into the plaster surface while troweling. This particular practice can weaken the surface and show deterioration and discoloration (or color variation) much sooner than usual. Dark colored plaster is even more susceptible to discolorations from water additions than white plaster.Sixth, well-timed hard troweling can help produce a good dense plaster finish. However, if the plaster surface has overly hardened before a smooth surface is obtained, then late hard troweling can cause discoloration and color variation, especially when calcium chloride and extra water is also used. This also has been documented by the PCA.Seventh, plastering in extreme weather conditions can lead to durability problems. The ACI and PCA both mention that placing cement-based products on days or in conditions that are either too cold or too hot adversely affects the quality and durability. However, these problems can be avoided by “tenting” the pool, thus protecting the plaster surface (and the plastering finishers!) from the extreme temperatures. For example, tenting a pool in extreme high temperatures, and perhaps even directing air from an evaporative cooler under the tent, can help the plaster to retain its moisture and thereby properly cure and harden.
As long as the shell (substrate) of the pool is 40 degrees and above, and the outside temperature stays above that until the pool is filled with water, you should be in good shape. Fortunately, when water is added to cement, it will generate a little heat and will help for a durable product.
Hi Kim, Thanks for the great recap of Plaster technics. I have recently relocated from Cal (Sac. area) to the southwest. Could you quantify what you thinik is a temperature extreme on the cold end of the scale? We absolutely love the cooler working conditions here at 7000 feet,we are experiencing great results and with a new crew we are assembling. Our guys are well seasoned non-pool applicators but new to pool finishes and they are absolute masters. I don't think I've ever done a job below about 60 degrees and we will be seeing much lower temps than that. Any suggestions? Thanks, Bill
Thank you Dana,
I blogged this entry in an effort to help all pool industry members (including the NPC) understand what is required for a durable pool plaster product. Most of this information has been published by onBalance in years past, but I note that many of these plastering issues are not properly addressed in the current NPC Technical Manual. As a current NPC member, I welcome your support and effort to have these sound principles and standards adopted and incorporated into the manual. That would be a benefit for the entire pool industry and make NPC membership more worthwhile.
Every person in the pool industry that has any connection to cementitious pool surfaces of any kind absolutely needs to be a member of the National Plasterer's Council.
Among many other Member benefits, NPC members recieve recent, relevant, quality research data from on-going studies performed in actual swimming pools at Cal Poly San Luis Obispo.
Thank you Kim for your hard work and support of the Pool Industry's professional pool plasterer's organization!
Pleased to say that marginal applicators are pretty rare around here these days. The contractors are leaving us with a beautiful product and I'm really looking forward to Lesson 2.
Comments
I blogged this entry in an effort to help all pool industry members (including the NPC) understand what is required for a durable pool plaster product. Most of this information has been published by onBalance in years past, but I note that many of these plastering issues are not properly addressed in the current NPC Technical Manual. As a current NPC member, I welcome your support and effort to have these sound principles and standards adopted and incorporated into the manual. That would be a benefit for the entire pool industry and make NPC membership more worthwhile.
Among many other Member benefits, NPC members recieve recent, relevant, quality research data from on-going studies performed in actual swimming pools at Cal Poly San Luis Obispo.
Thank you Kim for your hard work and support of the Pool Industry's professional pool plasterer's organization!