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. 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 sulpher 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 Core
The strength of a 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 dia 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.
Following are the factors which affect the compressive strength of extracted concrete cores:
- Size of stone aggregate: If the ratio of diameter of core to maximum size of stone aggregate is less than 3, a reduction in strength is reported. For concrete with 20mm size aggregate, 50mm dia core has been tested to give 10% lower results than with 10mm dia cores.
- Presence of transverse reinforcement steel: It is reported that the presence of transverse steel causes a 5 to 15% reduction in compressive strength of core. The effect of embedded steel is higher on stronger concrete and as its location moves away from ends, i.e. towards the middle. However presence of steel parallel to the axis of the core is not desirable.
- H/D ratio: This has been already discussed above. However its value should be minimum 0.95 and maximum 2. Higher ratio would cause a reduction in strength.
- Age of concrete: No age allowance is recommended by the Concrete Society as some evidence is reported to suggest that in-situ concrete gains little strength after 28 days. Whereas others suggest that under average conditions, the increase over 28 days’ strength is 10% after 3 months, 15% after 6 months. Hence it is not easy to deal the effect of age on core strength.
- Strength of concrete: The effect in reducing the core strength appears to be higher in stronger concretes and reduction has been reported as 15% for 40 MPa concrete. However a reduction of 5 50 7% is considered reasonable.
- Drilling operations: The strength of cores is generally less than that of standard cylinders, partly as a consequence of disturbance due to vibrations during drilling operations. Whatever best precautions are taken during drilling, there is always a risk of slight damage.
- Site conditions vis-a-vis standard specimens: Because site curing is invariably inferior to curing prescribed for standard specimens, the in-situ core strength is invariably lower than the standard specimens taken and tested during concreting operations.