Concrete cores are used for testing of actual properties of concrete in existing structures such as strength, permeability, chemical analysis, carbonation etc. Sampling of concrete cores and testing its strength is described.
While Rebound Hammer, CAPO/Pullout, Windsor probe and ultrasonic pulse velocity tests give indirect evidence of concrete quality, a more direct assessment on strength can be made by core sampling and testing.
Core Sampling and Testing of ConcreteConcrete cores are usually cut by means of a rotary cutting tool with diamond bits. In this manner, a cylindrical specimen is obtained usually with its ends being uneven, parallel and square and sometimes with embedded pieces of reinforcement. The cores are visually described and photographed, giving specific attention to compaction, distribution of aggregates, presence of steel etc. The core should then be soaked in water, capped with molten sulphur to make its ends plane, parallel, at right angle and then tested in compression in a moist condition as per BS 1881: Part 4: 1970 or ASTM C 42-77. The core samples can also be used for the following:
- Strength and density determination
- Depth of carbonation of concrete
- Chemical analysis
- Water/gas permeability
- Petrographic analysis
- ASHTO Chloride permeability test
Fig: Instrument showing core cutting
Fig: Concrete CoreThe strength of a concrete core test specimen depends on its shape, proportions and size. The influence of height/diameter (H/D) ratio on the recorded strength of cylinder is an established fact. Strength of core have to be related to the standard cylinder strengths, i.e. for H/D ratio of 2. Thus core should be preferably have this ration near to 2. For values of H/D less than 1, between 1 and 2, a correction factor has to be applied. Cores with H/D ratio less than 1 yield unreliable results and BS 1881: Part-4:1970 prescribes a minimum value as 0.95. The same standard specifies the use of 150mm or 100mm cores. However, cores as small as 50mm are also permitted in the standards. Very small diameter cores exhibit more variability in results than larger diameter cores, hence their use is generally not recommended. The general rule adopted for fixing the core size, besides the H/D ratio, is the nominal size of stone aggregate and the dia should be not less than 3 times the maximum size of stone aggregate. For diameter of core less than 3 times the size of the stone aggregate, an increased number of cores have to be tested.