Procedures to be followed when low strength measurements occur can be programmed to minimize delays in construction without endangering either the safety of the structure or the interests of those involved in the construction. Three aspects of concrete strength evaluation are considered in this article: What degree of apparent strength deficiency justifies further investigation? When investigation is indicated, how should it be performed? How should results of the investigation be translated into action with respect to the structure and to the allocation of costs?
It must be assumed that a measured strength of 500 psi below that specified represents a deficiency of some sort either in the concrete or in its testing (although it should not be overlooked that such measurements will occur on an average of once or twice in every 100 tests simply due to normal variability). The significance of the low strength measurement should be investigated in the following sequence, taking as many of the six steps as necessary: Verify testing accuracy; compare structural requirements with measured strength; non-destructive tests; core tests; load tests; and corrective measures.
In those rare cases where a structural element fails the load test or where structural analysis of untestable members indicates an inadequacy, appropriate corrective measures must be taken. The alternatives, depending on individual circumstances, are: Reduce the load rating to a level consistent with the concrete strength actually obtained; augment the construction to bring its load-carrying capacity up to original expectations (this might involve adding new structural members or increasing the size of existing member); or replace the unacceptable elements.