Q.: Our ready mixed concrete company has received a request for concrete with a flexural strength of 670 psi. We have never had this kind of order before. Can you tell us what equivalent compressive strength such a concrete represents?

A.: This sounds like an order for paving concrete, which is commonly specified in terms of flexural strength. Flexural strength can be related to compressive strength of concrete by the following formula:

This says that the flexural strength is roughly equal to 9 times the square root of the compressive strength. On this basis a flexural strength of 670 psi would be equivalent to 5540 psi compressive strength. The formula is not exact because it is influenced by the type of aggregate. On paving jobs the modulus of rupture is determined by ASTM C 78 "Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading)," and cylinders for compressive strength are not likely to be made.