Q.: We placed some concrete that was supposed to reach 2000 psi compressive strength at 28 days and 1000 psi at 7 days. It didn't meet either requirement but sometime between 40 and 50 days the strength surpassed 2000 psi. Since that time we have pinpointed the problem and corrected it. However, now the question arises, is concrete that reaches 2000 psi at about 50 days as good as concrete that reaches 2000 psi at 28 days?

A.: There are really two possible questions here:

  1. If two concretes reach a strength of 2000 psi at 28 days and 50 days respectively, are they equally good at the times they reach that strength? The answer is a qualified yes. They both have the same strength, though there may be minor differences in absorption and permeability.
  2. If two concretes reach a strength of 2000 psi at 28 days and 50 days respectively, will they be equally good at some later age? The answer depends on the conditions.

(a) If the reason for slow strength gain in one concrete is that it was made of Type II or Type IV cement, or that it contained too much retarder, or was cured at a low temperature, it may actually end up being a better concrete than the other. It may develop more ultimate strength and better properties in general. The key factor is that if the curing conditions are such that the cement has plenty of time to hydrate, the newly forming cement gel will deposit itself uniformly throughout the matrix. When cement must hydrate rapidly, the cement gel tends to deposit itself on and around the original cement grains. This eventually tends to choke off these grains from reacting with the surrounding water. It also causes the gel to distribute itself less evenly so that the whole matrix is weaker. Slow, steady curing, whether achieved by low (but above-freezing) temperature, by a retarder, or by a slow-hydrating type of cement, is likely to produce a higher ultimate strength, other things being equal. One place where the effects of temperature and rates of curing are discussed is in A. M. Neville's book Properties of Concrete, John Wiley and Sons, New York, 1973, pages 276 to 280.

(b) If, however, the reason for the slow gain in strength is that the mix was improperly proportioned, or contains deleterious impurities such as clay, or was not cured properly but was allowed to dry out, or was abused in some other way, the concrete will not gain its full strength potential. If so, the concrete is not as good as the concrete that gained strength at the rate intended.