In-situ core strength compared to standard cube strength
There is no unique relationship between the strength of in-situ concrete, obtained from a core taken from a structure, and that of a standard cured cube cast from the same concrete. Any relationship is a function of many variables.
A standard test cube is cast from a concrete sampled from throughout a delivered load. It is carefully prepared and stored under standardised conditions until the age of test, normally 28 days. In this way results from sets of cubes can be compared and a statistical approach can be undertaken to develop a characteristic strength for a particular concrete mix design.
The concrete’s strength development in-situ will be influenced by the curing and temperature profile of the concrete as dictated by the chemical interaction of the cement, aggregate and admixture. This profile depends on the season, the element dimensions and exposure. Once a core is cut, the measured strength will be affected by; the age at the time of coring and testing, length diameter ratio, presence of reinforcement, orientation relative to casting, compaction and voidage.
It is difficult, if not impossible, to provide a precise factor for these variables. It is accepted that differences in specimen dimensions can be factored and a core strength converted to an in-situ cube strength. It is not surprising that in most structures the in-situ cube strength is expected to be lower than the standard cube strength of the same concrete. These inherent differences between standard cube strengths and in-situ strengths are allowed for by way of partial material safety factors for concrete in the structural design codes.