Source: National Concrete Masonry Association, Herndon, Va.
Preliminary results from NCMA Research and Development Laboratory tests on concrete masonry units and prism specimens indicate a one-to-one correlation in compressive strength at the 2,000-psi range. The measurement of 2,000-psi unit strength yielding 2,000-psi prism strength is one-third higher than widely referenced tables in current codes acknowledge. While NCMA lab analysis continues, the unit strength-prism strength observation may prompt a change to the minimum compressive strength requirements for concrete masonry units in ASTM C90. A 100-psi increase in the standard’s minimum compressive strength of 1,900 psi—a decades-long threshold—has the potential for significant returns in design economy when coupled with proposed code changes to remove the inherent conservatism built into the unit strength method of verifying specified compressive strength.