Core testing is the most direct method to determine the compressive strength of concrete in a structure. Generally, cores are obtained either to assess whether suspect concrete in a new structure complies with strength-based acceptance criteria or to evaluate the structural capacity of an existing structure based on the actual in-place concrete strength. In either case, the process of obtaining core specimens and interpreting the strength test results is often confounded by various factors that affect either the in-place strength of the concrete or the measured strength of the test specimen. The scatter in strength test data, which is unavoidable given the inherent randomness of in-place concrete strengths and the additional uncertainty attributable to the preparation and testing of the specimen, may further complicate compliance and evaluation decisions.

This guide summarizes current practices for obtaining cores and interpreting core compressive strength test results. Factors that affect the in-place concrete strength are reviewed so locations for sampling can be selected that are consistent with the objectives of the investigation. Strength correction factors are presented for converting the measured strength of non-standard core-test specimens to the strength of equivalent specimens with standard diameters, length-to-diameter ratios, and moisture conditioning. This guide also provides guidance for checking strength compliance of concrete in a structure under construction and methods for determining an equivalent specified strength to assess the capacity of an existing structure.