The conventional way of determining strength potential of concrete is to cast cylinders which are moist-cured at 73 degrees F and tested 28 days after casting. By relying on the 28-day results to assure a designer that adequate strength has been developed, there is a chance that large quantities of inferior concrete can be placed in a structure before detection. Upon discovery, it may be impossible to replace the defective concrete without also removing material of acceptable quality. Consequently, the extra cost incurred is even greater.
In ASTM C 684, three procedures for accelerated testing are given. These are: (A) warm water method; (B) boiling water method; and (C) autogenous curing method. Procedure A consists of curing the cylinders without moisture loss, immediately after the concrete is cast in metal or plastic molds, for 23 « hours while immersed in water at 95 degrees F. In Procedure B, cylinders cast in metal or plastic molds are initially cured for 23 hours at 70 degrees F, followed by curing in boiling water for 3 « hours. By Procedure C, cylinders are stored immediately after casting in an insulated and sealed curing container which preserves the evolved heat of hydration and prevents moisture loss for 48 hours. Following the accelerated curing, cylinders are capped and tested with a total elapsed time from casting of 24 hours for Method A; 28 « hours for Method B; and 49 hours for Method C.
When accelerated strengths are to be used to make predictions of strength at 28 days, sufficient data are necessary to cover a strength range which ACI 214.1R-81 recommends be equal to at least 75 percent of the specified 28-day strength. For the correlation equations to be considered adequate, this same report suggests that at least 30 sets of test data representing at least three water-cement ratios be used.