Q. I've heard about an equation that allows you to get a fairly decent prediction of concrete flexural strength based on compressive strength. Is there such an equation, and, if so, how can I get a copy?

A. The equation, which you can find in ACI 330R-92, 'Guide for Design and Construction of Concrete Parking Lots,' is: Modulus of Rupture = 2.3 times the two-thirds power of compressive strength.