COMPRESSIVE STRENGTH OF CONCRETE

 COMPRESSIVE STRENGTH OF CONCRETE

AIM:

To determine the compressive strength of the concrete.


ctm


APPARATUS REQUIRED:

1. Compressive testing machine
2. Curing Tank 
3. Cube mould

SAMPLE:

The composite sample shall be truly representative of the batch and shall not be less than 0.02 m3 in volume. It shall be composed of the mixture of proportions taken from the different points in the batch. When continuous mixtures are used, a batch shall be regarded as the discharge from the mixture during one minute.

PROCEDURE:

CUBE CASTING

  1. Assembling the mould for use, clean and apply the oil thin coating entire contact surfaces of the bottom of the mould and the base plate in order to ensure that no water escapes during the filling.
  2. Fill the concrete in the moulds in layers approximately 5cm thick
  3. When compacting by hand, the standard tamping bar shall be used and the strokes of the bar shall be distributed in a uniform manner over the cross-section of the mould.
  4. Compact each layer with not less than 35 strokes per layer using a tamping rod. 
  5. The strokes shall penetrate into the underlying layer and the bottom layer shall be rodded throughout its depth. Where voids are left by the tamping bar, the sides of the mould shall be tapped to close the voids.
  6. Level the top surface and smoothen it with a trowel
  7. The casted cubes shall be stored on the site at a place free from vibration, under damp matting, sacks or other similar material for 24 hours ± ½ hour from the time of adding the water to the other ingredients. 
  8. The temperature of the place of storage shall be within the range of 22°to 32°C. 
  9. After the period of 24 hours casted cubes shall be marked for later identification, removed from the moulds and unless required for testing within 24 hours, stored in clean water at a temperature of 24° to 30°C until they are transported to the testing laboratory. 

CURING - (at LABORATORY)

  1. On arrival at the testing laboratory, the specimens shall be stored in water at a temperature of 27° ± 2°C until the time of test. 
  2. Cubes stored in water shall be tested immediately on removal from the water and while they are still in the wet condition. Surface water and grit shall be wiped off the specimens and any projecting fins removed. 
  3. The specimen should be removed from the water 30 minutes prior to the testing.
  4. The specimen should be in dry condition before conducting the testing.

TESTING

  1. The dimensions of the specimens to the nearest 0.2 mm and their weight shall be noted before testing.
  2. Clean the bearing surface of the testing machine
  3. The Cube weight should not be less than 8.1 Kg’s
  4. Place the specimen in the machine in such a manner that the load shall be applied to the opposite sides of the cube cast.
  5. Align the specimen centrally on the base plate of the machine.
  6. Rotate the movable portion gently by hand so that it touches the top surface of the specimen.
  7. Apply the load gradually without shock and continuously at the rate of 140 kg/cm2/minute till the specimen fails
  8. Record the maximum load and note any unusual features in the type of failure.

NOTE:

In curing tank the temperature of water should be maintain at 27ºc ± 2ºc. Water should be replaced for every seven days. 
Minimum three specimens should be tested at each selected age. If result of any specimen varies by ± 15 per cent of average strength, results of such specimen should be rejected. Average of three specimens gives the crushing strength of concrete.

CALCULATIONS:

Weight of the cubes =............ Kg
Maximum load applied =……….tones = ………….N
Compressive strength =Load/Area N/mm2

Note: Acceptable value

GRADE OF CONCRETE

COMPRESSIVE STRENGTH AT 28 DAYS

M15

15 N/mm²

M20

20 N/mm²

REFERENCE: IS: 456 – 2000 & IS: 516 – 1959.



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