Q.: For a long time we have considered taking the plunge and going in for accelerated strength testing. We know that cylinders tested in accordance with ASTM C 684 are broken at age 24 hours by the warm water method, at 28 ½ hours by the boiling water method, and at 49 hours by the autogenous curing method. No matter which method we adopt, our problem is how to be sure that we interpret the data properly so that we don't either accept strengths that are too low or require strengths that increase the cost unnecessarily. Can you suggest some guidelines?

A.: Perhaps you should study the way strength data were obtained and interpreted in the construction of the CN Tower in Toronto. Our April 1975 issue, pages 128 to 132 outlined the testing program. Further details on setting up a program are given in Peter Smith's 39-page article "Opening the Oyster of Concrete Quality," Stanton Walker Lecture, National Sand and Gravel Association and National Ready Mixed Concrete Association, 900 Spring Street, Silver Spring, Maryland 20910.

Since 1978 the U.S. Army Corps of Engineers has been using data from the warm water method to aid in control of concrete construction on civil works projects. Anyone planning to use any accelerated method should study the American Concrete Institute committee report, Accelerated Strength Testing, ACI 214.1R-81.