Compressive strength is the ability of material or structure to carry the loads on its surface without any crack or deflection. A material under compression tends to reduce the size, while in tension, size elongates.
To determine the compressive strength of concrete cores, casted, stored and cured under specified conditions.Concrete cores to be tested after 28days of concreting. Cores to be tested for compression strength shall have ends that are reasonably even, perpendicular to the axis and of the same diameter as the body of the specimen.
A core which before capping has a maximum height of less than 95 percent of the diameter or after capping a height less than its diameter shall not be used. Diameter of the core extracted shall not be less than three times the nominal maximum size of the aggregate in the concrete and in no case shall be less than two times the nominal maximum size of the aggregates in the respective concrete.
Compression testing machine capable of loading at 140Kg/Sqcm/min with a permissible error of ± 2% of the maximum load.
Preparation of Core sample:
The ends of the specimen shall be capped before testing. The material used for the capping shall be such that its compressive strength is greater than that of the concrete in the core. Caps shall be made as thin as practicable and shall not flow or fracture before the concrete fails when the specimen is tested. The capped surfaces shall be at right angles to the axis of the specimen and shall not depart from a plane by more than 0·05mm. The capping of specimens shall be carried out with a sulphur mixture consisting of 2 or 3 parts sulphur to 1 part of inert filler, such as fire-clay.
The specimens shall be securely held in a special jig so that the caps formed have true plane surfaces. Care shall be taken to ensure that the sulphur compound is not over-heated as it will not then develop the required compressive strength. Sulphur caps shall be allowed to harden for at least 2 hours before applying the load. After checking for irregularities, the core shall be placed in water at a temperature of 24° to 30°C for 48 hours before testing. The overall height of the specimens with capping & diameter of the concrete core shall be measured to the nearest millimetre
Age of test:
Tests shall be made at recognized ages of the tests specimens, the most usual being 7 and 28 days. Ages of 13 weeks and one year are recomended if tests at greater ages are required. Where it may be necessary to obtain the early strengths. tests may be made at the ages of 24 hours ± 1 hour and 72 hours ± 2 hours. The ages shall be calculated from the time of the addition of water to the dry ingredients.
Concrete cores to be tested after 28days of concreting. Cores to be tested for compression strength shall have ends that are reasonably even, perpendicular to the axis and of the same diameter as the body of the specimen. A core which before capping has a maximum height of less than 95 percent of the diameter or after capping a height less than its diameter shall not be used. Diameter of the core extracted shall not be less than three times the nominal maximum size of the aggregate in the concrete and in no case shall be less than two times the nominal maximum size of the aggregates in the respective concrete.Cores are to be tested immediately after removing from the water and while they are in SSD condition. Wipe off the surface water and remove the fins if any. Check the dimension and noted down the weight. Wipe the bearing platens of the testing machine clean. The faces of the test specimen and the steel plattens of the testing machine shall not be packed. Rotate the movable portion gently by hand, so that uniform seating is obtained. Apply the load without any shock continuously at the rate of 140kg/sqcm/minute until the resistance of specimen to the increase of load breaks down and no greater load can be sustained. Record the maximum load applied to the specimen.
Compressive strength of the core shall be calculated as ratio of maximum load applied divided by the cross-sectional area of the specimen to be reported to the nearest 0.1 N/mm2
Average of three specimens shall be taken as sample of representative batch provided individual variation is not more than 15% of the average, otherwise repeat test is required.
A correction factor according to the height/diameter ratio of specimen after capping shall be obtained from the graph below
If the diameter of the core extracted is less than 100mm, then fixed correction of 1.08 is also applied to the core strength.The corrected core strength is calculated by multiplying core strength with correction factor for height/diameter ratio and diameter correction, this being the equivalent strength of a cylinder having a height/diameter ratio of two.
The equivalent Cube strength of the concrete shall be determined by multiplying the corrected core strength by 5/4