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. The assessment of concrete strength in existing structures adopts an approach which determines an ´equivalent concrete in-situ 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.
Until 2019, the assessment has been based on BS EN 13791:2007 complimented by the UK guidance document BS 6089:2010 together with BS EN 12504-1:2009. All three documents have been extensively reviewed to ensure the procedures and test methods used provide a sound and reliable method for assessing in-situ compressive strength. This resulted in the publication of a totally revised BS EN 13791:2019 with a National Forward and a National Annex which coexists with the revised BS EN 12504-1:2019 Test method for taking and testing of core samples including its own National Annex. Previous versions together with BS 6089: 2010, have been withdrawn. A guidance document PDCEN/TR 17086 was also published in 2020.