In today’s concrete preparation world large planetary grinding has largely taken the place of shot blasting even when specifications and standards require a shot blasted profile.
Recently, I was on a job site where a nationally recognized manufacturer of moisture vapor emission primers was in use. The specified product primer data sheet required a concrete surface profile (CSP) meeting ICRI (International Concrete Repair Institute) CSP 3. I reviewed the concrete prepared with diamond grinders and could see that it matched the visual standard of a CSP 2 but the manufacturer’s representative passed it as a CSP 3. I was shocked and questioned the representative who replied, “it was close enough”! I wondered if he would say that if the system failed down the road.
This experience prompted me to perform side by side adhesion testing to compare adhesion of a 20 mil, 2 coat epoxy system over concrete prepared per ICRI CSP 2, and CSP 3. My testing showed that a CSP 2 yields adhesion pull numbers 10-15% less than a CSP 3. Additionally, the amount of concrete on the back of the dollies pulled from the CSP 2 test areas was thinner and of less mass than the amount of concrete removed from the test areas prepared to a CSP 3.
Proponents of grinding point to the fact that the test dollies have concrete on the back side claiming that the failure mode was at the concrete level and based on standards like ASTM D 7234 they can make that argument, but claims like these are misleading.
ASTM D 7234 was derived from its sister standard, ASTM D 4541, which was designed to test and measure the pull off adhesion strength of coatings on steel. Steel has far higher tensile strength than concrete and a coating tested on steel results in much higher adhesive pull off numbers and does not result in substrate failure like concrete. Understanding this should cause further discussion by the ASTM D 7234 committee and refinement of the standard.
Summary
The failure mode of testing per ASTM D 7234 does not prove that diamond grinding meets the requirements of a shot blasted concrete surface. The profile created by shot blasting contains increased surface area for a floor coating to adhere, which appears to increase the overall bond strength to concrete. Responsible flooring contractors, manufacturers, and testing agencies should not accept, “close enough” and demand that the concrete preparation visually meets the ICRI CSP standards.
Chris O’Brien is a NACE Coating Inspector and founder and CEO of Prime Coat Coating Systems
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