One of the most absurd conclusions by the NPC/NPIRC study has to do with the causes of craze cracking (shrinkage cracks) in pool plaster.
It has been known for decades in the cement/concrete industry that high water to cement ratios, calcium chloride additions, excessive wet troweling, and hot and windy weather leads to increased shrinkage cracks and other problems (ACI 302.1R-04).
Yet, the NPC and two Cal Poly professors disregarded that established science and tried to connect certain sanitizers as causing increased craze cracking! The Phase 2 study report states that Salt systems resulted in the greatest amount of craze cracking (and also etching and discoloration); with Bleach, Trichlor, and Cal Hypo somewhere in the middle, and Dichlor with having the least amount of craze cracking (pg. 43 & 85).
How in the world could the chemistry professor think that different sanitizers affect plaster differently? He would know that all chlorine sanitizers add the same thing (chlorine) to the water. And since the pH was maintained within the same range in all of the 12 test pools, how could there be any different affect?
And how could the concrete engineer professor go along with that conclusion knowing that shrinkage cracks involve materials, workmanship, and weather? Where in the ACI literature does it say that aggressive rain water causes shrinkage and craze cracking? It doesn’t. In fact submerging plaster in water helps to reduce cracking.
The Phase 3 study report states that the Dichlor sanitized pools had the most crazing cracks (pg. 57). That is opposite of what occurred in Phase 2! Wouldn’t that inconsistency raise a red flag? What is their excuse for suggesting that sanitizers increase crazing, and not mentioning a word about workmanship issues as likely causes? And the NPC claims that their study is credible.
Check out the link to what an NPC Technical Advisor wrote on a pool with cracking and calcium nodules.
The dark colored plaster pool above is an older pool with delamination cracks and calcium nodules growing on them.
It is not the newer "Salt" pool mentioned in the link above.