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Core testing for compressive strength

When required, cores are cut using a drill with a hollow barrel tipped with industrial diamonds. The whole rig has to be firmly fixed in position by weights, anchor bolts, vacuum pads or bracing against other parts of the structure. The preferred core diameter is at least 75mm (in UK 100mm is the normal diameter) except where not practical and then not less than 50mm. The diameter should be at least 3.5 x the maximum aggregate size. Sometimes even smaller diameter cores have to be used for strength testing e.g. in situations where there is significant reinforcement present. In this case the strength results can be more variable and a greater number of cores should be extracted. For compressive strength testing, the length to diameter ratio of the extracted core should be of sufficient size such that after end preparation they are within the following limits;

For comparing against a 1:1 specimen (cube) the length to diameter ratio is to be within 0.90:1 and 1.10:1

For comparing against a 2:1 specimen (cylinder) the length to diameter ratio is to be within 1.95:1 and 2.05:1.

On extraction, the core should be retained in a sealed container between site and the laboratory e.g. plastic bag. When cores are received in the laboratory they should be examined for degree of compaction (excess voidage), cracks, discontinuities, honeycombing and the presence of reinforcement. This should be noted on the test certificate.

Before testing cores for compressive strength, they have to be trimmed to length and the ends prepared so that they are flat and perpendicular to the longitudinal axis. This is achieved by grinding (recommended) or capping with high alumina cement (calcium aluminate cement) mortar or a sulphur/sand mixture when anticipated strength is <60MPa. However, when the strength is anticipated to be >60MPa then grinding only should be used. Cores should be tested in a laboratory dry state within 10 hours after end preparation. This is air dry, not oven dry.

The important Standards are;

BS EN 13791: 2019. Assessment of compressive strength in structures and precast concrete component

BS EN 12504: 2019. Testing concrete in structures. Part 1 Cored specimens - taking, examining and testing in compression

Typical core sample
Acknowledgement: The Concrete Society

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