The diameter of core specimens for the determination of concrete compressive strength should preferably be at least three times the nominal maximum size of the coarse aggregate used in the concrete, and must be at least twice the maximum size of the coarse aggregate in the core sample.
The length of the specimen, when capped, should be nearly as practicable twice its diameter. A core having a maximum height of less than 95% of its diameter before capping or a height less than its diameter after capping must be rejected. It is preferable to test the cores in moist condition.
The ASTM standard prescribes the following procedure: “Submerge the test specimens in lime-saturated water at 23.0 +/- 1.7° C for at least 40 h immediately prior to making the compression test. Test the specimens promptly after removal from water storage.
During the period between removal from water storage and testing, keep the specimens moist by covering with a wet blanket of burlap or other suitable absorbent fabric.” If the ratio of the length to diameter of the specimen is less than 1.94 apply correction factors shown in the Table 1.
Table-1: Correction Factor for Ratio of Length of Cone to its Diameter
Ratio of length of cylinder to diameter (L/D)
Strength correction factor
The procedures for properly removing concrete samples by core drilling are given in ASTM C 42. Cores must be taken using diamond-studded core bits when the cores are to be tested for strength.
A shot drill may be acceptable for other applications when the core is drilled vertically. However, diamond-studded core bits are recommended for other drill orientations.
Fig: Core sampling and testing of concrete
The following guidelines are of particular importance in core sampling:
The number, size, and location of core samples should be carefully selected to permit all necessary laboratory tests. If possible, use virgin samples for all tests so that there will be no influence from prior tests.
For determination of strength the core must have a minimum diameter of greater of three times the maximum nominal size of the coarse aggregate, or 50 mm.
For strength tests, the cores must have a length of at least twice their diameter.
Reinforcing steel should not be included in a core to be tested for strength.
During drilling of cores, the electrical conduits or prestressing steel must not be disturbed.
It is better to drill core through full depth of member to avoid the need of its breaking for extraction. An extra 50 mm is usually drilled to allow for possible damage at the base of the core.
At least three cores must be removed at each location in the structure for strength determination.
The hole drilled to take the core is filled by packaged repair material. A thixotropic material is required for repair of slabs as it must not fall down under gravity. In some cases, a precast cylinder of concrete may be fitted in the core hole by using cement grout or epoxy resin.
Minimum core diameter is usually 100 mm but 75 mm and 50mm diameters may be used in special cases. The number of 50 mm diameter cores must be three times the number of 100 mm diameter cores to get the same accuracy.
The 20 percent top portion of member with a minimum of 50 mm and maximum of 300 mm and a side cover of 50 mm within the member is preferably not included in the portion of the core to be tested.
Before testing the cores, dimensions, density, shape, information whether aggregates appear to be well-graded or gap-graded, position of any cracks, damage due to drilling and presence of steel must be recorded. Ends are prepared in high speed wet grinding machine using diamond-faced grinding wheels.
Otherwise, capping using strong materials may be carried out. Water soaking of 40 hours is recommended prior to the testing. According to the British standard CSTR No. 11, the effect of the length to diameter ratio of the core (R) on the strength is given as under: