Core testing for strength
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. In this case the strength results can be more variable and a greater number of cores should be extracted. For strength testing, the length to diameter ratio should be;
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.
When cores are received in the laboratory they may be examined for degree of compaction, cracks, voids, honeycombing and the presence of reinforcement.
Before testing cores for 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 or, more usually, capping with high alumina cement (calcium aluminate cement) mortar or a sulphur/sand mixture. Cores should be tested in a dry state. This is a 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