Assessment of in situ concrete strength
There may be a need to assess the strength of the concrete in an existing structure, perhaps to determine its ability to carry additional loads or following modification. This is generally done by taking cores at representative locations in the structure, to give a good overall indication of the strength.
If details of the Standards to which the structure was designed and constructed are available, these can obviously be used to obtain an indication of the expected concrete strength. Further information on the development of specified concrete strengths over the years is given in Concrete Society Technical Report 70, Historical approaches to the design of concrete buildings and structures.
Clearly cores should NOT be taken at highly stressed locations, which could lead to structural damage. The cores are tested in compression. Guidance may be found in BS 6089, Guide to assessment of concrete strength in existing structures on the approach to be adopted to determine the equivalent concrete cube strength. This cube strength is akin to a cube taken from the structure and is not the same as a standard 28 day cured cube strength. This can then be used to recalculate the strength of the structure (see Assessment of existing structural strength).