Core Extraction & Testing
Extracting the concrete core samples of minimum 65 mm diameter and 100-150 mm length (approx.) from a selected location forms RCC members of the structures covered under the study and evaluation of the properties in the laboratory from the core sample selected. IS 456:2000; IS 516:1959; IS 1199:2002; ASTM C-42.
Core drilling method is the most direct way of measuring the actual strength of concrete in the structure. The examination and compression testing of cores cut from hardened concrete is a well – established method, enabling visual inspection of the interior regions of a member to be coupled with strength estimation. Other properties which can be measured is also given in this paper.
IS: 456-2000 specified that the points from which cores are to be taken and the number of cores required shall be at the discretion of the engineer-in-charge and shall be representative of the whole of concrete concerned in no case, however, shall fewer than three cores be tested. Core shall be prepared and tested as described in IS: 516.
Concrete in the member represented by a core test shall be considered acceptable, if the average equivalent cube strength of the cores is equal to at least 85 percent of the cube strength of the grade of concrete specified for the corresponding age and no individual core has a strength less than 75 percent. In case the core test results do not satisfy these requirements, or where such tests have not been done, load test may be resorted to.
CORE CUTTING PROCEDURE
CORE DRILLING
A core to be tested for strength shall not be removed from structure until the concrete has become hard enough to permit its removal without disturbing the bond between the mortar and the coarse aggregate. Normally the concrete shall be 14 days old before the specimens are removed. It is preferred the concrete should be 28 days old for drilling cores.
A core is usually cut by means of a rotary cutting tool with diamond bits. The concrete core drilling machine is portable, but it is heavy and must be firmly supported and braced against the concrete to prevent relative movement which will result in a distorted or broken core, and a water supply is also necessary to lubricate the cutter. Hand-held equipment is available for cores up to 75 mm diameter.
A core specimen taken perpendicular to a horizontal surface shall be located, when possible, with its axis perpendicular to the bed of the concrete as originally placed. A specimen taken perpendicular to a vertical surface, or perpendicular to a surface with a batter, shall be taken from near the middle of a unit of deposit.
If the ends of the cores do not conform to the perpendicularity and planeness requirements, they shall be sawed or ground to meet those requirements or capped as per standard procedure.
