The following letter was received from Bryce Simons, State Concrete Engineer with the New Mexico State Highway &
Transportation Department concerning the article on “Capping Cylinders for Testing High Strength Concrete” in Issue No. 14.
I find it extremely encouraging that all this work and the associated questions are being asked and researched. However, I do not understand why much of the information, and the associated “lessons learned” in the private construction sector (especially high-rise construction) during the mid to late 1980’s are not being utilized. When we developed the very high strength concrete mixes with compressive strengths exceeding 20,000 psi (138 MPa) for use in Seattle, we had to deal with the issue of how to test the cylinders. At that time, the best sulfur capping compound that we could buy seemed to result in an apparent maximum compressive strength of approximately 16,000 psi (110 MPa), regardless of the quality of the concrete.
When we switched over to the use of pad-caps, we immediately removed the ceiling, and the identical mixes all of a sudden exhibited strengths of 18,000 and 19,000 psi (124 and 131 MPa). After we were able to develop an acceptable grinding system (none existed when e started this work), we also saw the ceiling disappear. When we finally issued the specifications, we allowed the testing laboratories to use either grinding only, pad caps only, or a combination of grinding and pad caps.
On the Two-Union Square project, the cylinders were ground only. On the Pacific First Center project, the cylinders were first ground, nd then tested using the pad caps. Although there were some other basic differences between the two mixes, it was interesting to note that the test results between the two systems were remarkably similar. The results were also very consistent, and statistically sound. I believe that the coefficient of variation for each of the mixes was approximately 4 percent.
Consequently, when I see statements that indicate that these systems do not work with strengths above 12,000 psi (83 MPa), I can’t help but discount these statements, since I tend to believe what I have seen with my own eyes.