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.
Historically, Concrete Society Technical Report 11 (CSTR 11) was used in the UK to interpret core strength results, but it was superseded in part by BS EN 13791 and BS 6089. Following extensive research, BS 13791 was again revised in 2019 to incorporate features of BS 6089 and current thinking. At the same time, the concrete core testing standards were revised. The BS EN 12390 standards changes were primarily editorial and minor technical and did not significantly affect the test methodology. However, the changes to BS EN 12504-1 were significant. In particular, handling/storage/moisture conditions at test are clearly defined. There are no longer corrections for length/diameter ratio and reinforcement but defined rules relating to core length/diameter and reinforcement content.
BS EN 12504-1:2019 Testing concrete in structures, Part 1: Cored specimens – Taking, examining and testing in compression This includes a National Annex NA, which gives a method for visual assessment of excess voidage (formerly in BS 1881-120 and before that CSTR 11).
BS EN 13791:2019 Assessment of in-situ compressive strength in structures and precast concrete components.
Core samples tested in the laboratory are used to obtain a ‘core compressive strength’ which provides a measure of the actual strength of the concrete as it presently exists in the structure and is the value that is then used for any assessment of structural integrity.
The BS EN 12504 series also covers indirect non-destructive methods – Rebound Hammer to measure surface hardness and Ultrasonic Pulse Velocity (UPV). Guidance for establishing the relationships between test results from indirect test methods and the in-situ strength is included in BS EN 13791.
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
Two assessments can be carried out – one for the in situ strength and the other for conformity:
- Estimation of compressive strength for structural assessment of an existing structure – For use where there is none or very little information about the concrete used in the structure or where the supplier has declared a non-conformity and cannot provide data to support an acceptable estimated characteristic in-situ compressive strength.
- Assessment of compressive strength class of concrete in case of doubt – For use where there is doubt over the compressive strength of recently supplied concrete resulting from identity testing or problems suspected in the execution of the works.
The two assessments use different design principles and analyses and should be treated separately as there will be significantly different outcomes depending on the method used.
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.
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