Q.: We placed a concrete retaining wall that was 1 foot thick, 12 feet high, and over 300 feet long. The testing lab made one set of cylinders during the placement. Breaking of the 28-day cylinders showed low concrete strengths. The engineer would like to take cores from the wall to investigate the strength. Where in the wall should cores be taken?

A.: Coring is generally necessary when cylinder strengths are unacceptably low. Therefore, core strengths should be obtained from the in-place concrete that corresponds to the low cylinder strengths. Unfortunately, this often is not the case in practice.

ACI 318, "Building Code Requirements For Reinforced Concrete," requires the cores to be removed from the area in question. This means finding the suspect concrete from the truckload from which the cylinders were taken. If accurate placement records are available, the area of presumed low strength can be located and cores taken. If accurate placement records are not available, use a nondestructive testing technique to locate the area of suspected low strength.

These procedures satisfy ACI 318 requirements, but other locations may need testing. The truckload of concrete from which the cylinders were taken may represent the concrete quality in other trucks as well. Using a nondestructive technique to locate the suspect truckload of concrete provides a comparison for locating other potential low-strength areas.