An estimate of the in-situ strength of concrete may be required for several reasons:

• when an assessment of the in-situ concrete strength is needed during construction
• when an existing structure is to be modified or redesigned
• to assess structural adequacy when doubt arises about the compressive strength in the structure due to defective workmanship or deterioration of concrete due to fire or other causes
• to assess structural adequacy in the case of non-conformity of the compressive strength obtained from standard cube test specimens (or when cube results are not available)

The most contentious issues normally arise when cube results are not available or when they are below specified strength.

Two standards are key to this process:

BS EN 13791:2007, 'Assessment of in-situ compressive strength in structures and pre-cast concrete components'

BS 6089:2010, ‘Assessment of in-situ compressive strength in structures and precast concrete components. Complementary guidance to that given in BS EN 13791'

Between them they cover both direct (cores) and indirect (Rebound Hammer to measure surface hardness, Ultrasonic Pulse Velocity (UPV) of the concrete, Pull out strength) methods for assessing the in-situ compressive strength of concrete. They also provide guidance for establishing the relationships between test results from indirect test methods and the in-situ core strength, and hence the in-situ strength of the concrete in the structure.

Historically, Concrete Society Technical Report 11 (CSTR 11), Concrete core testing for strength, was used in the UK to interpret core strength results but it is no longer regarded as best practice and is superseded in part by BS EN 13791. In places CSTR 11 also conflicts with BS EN 13791. Consequently, aspects of the approach have been adopted within the overall system described in BS 6089.

Core samples tested in the laboratory are used to obtain a ‘Corrected insitu cube strength’

The ‘Corrected insitu cube strength’ is the measured core compressive strength expressed as the strength of an equivalent cube. It is intended to provide a measure of the actual strength of the concrete as it presently exists in the structure with specific allowance in calculation for the geometrical differences between cores and standard cubes. Correction is made for shape, the ratio of length to diameter of the specimen under test and the influence of any embedded reinforcing steel.

Sandberg can offer a full testing and consultancy service comprising

• Initial review
• Planning a suitable test programme
• UKAS accredited core sampling and laboratory testing
• Full interpretation of results

An important point to remember is that the compressive strength of cores and the in-situ strength will generally be less than that measured on standard test specimens taken from the same batch of concrete. This is due to a range of factors including the degree of compaction and curing in practical site conditions and dependent on the location in the member where in-situ strength is determined