Compressive strength of concrete cubes

Compressive  strength  of  concrete cubes

Concrete cube testing to determine the strength of concrete can be done in both destructive and non-destructive methods of testing in civil engg. Jobs, civil engg. Construction practice cube test is one of the vital test which is performed to determine the strength of concrete which has been already used in the construction. Cube testing is a destructive testing method of concrete testing, as cubes are crushed in compression testing machine. The mostly tested cubes in practice are of 150x150x150 size in mm.

 * Principal :-

The tests are required to determine the strength of concrete and therefore its suitability for the job.

 *  Required apparatus :

-

       I.            6 no. of cube moulds of 150x150x150 mm in size

     II.            One slide wrench

  III.            One trowel

  IV.            One temping rod of length 600 mm & 16 mm dia

    V.            One nail

  VI.            One water tank

VII.            One steel rule

VIII.            Compression testing machine

  IX.            One soft cloth

     X.            One mallet

  XI.            Lubricating agent

 * Theory :-

Compressive strength of concrete depends on many factors such as water-cement ratio, cement strength, quality of concrete material, quality control during production of concrete etc.

The concrete is poured in the mould and tempered properly so as not to have any voids. After 24 hours these moulds are removed and test specimen are put in water for curing. The top surface of these specimen should be made even and smooth. This is done by putting cement paste and spreading smooth on whole are of specimen.

 *  Procedure :-

the proportion and material for making these test specimen are from the same concrete used in the field. Taking 6 cubes of 15 cm mix M15 or above. Mixing the concrete either by hand or in a laboratory batch mixer. Now clean the moulds and apply oil. Fill the concrete in the moulds in layers approximately 5 cm thick. Compact each layer with not less than 35 strokes per layer using a tamping rod. Now, level the top surface and smoothen it with a trowel. The specimens are stored in moist air for 24 hrs and after this period the specimens are marked and removed from the moulds and kept submerged in clear fresh water until taken out prior to test.

Now following procedure are done to determine the compressive strength of concrete –

       i.            Removed the specimen from water after specified curing time and wipe out excess water from the surface.

     ii.            Take the dimension of the specimen to the nearest 0.2 m.

  iii.            Clean the bearing surface of the testing machine.

  iv.            Place the specimen in the machine in such a manner that the load shall be applied to the opposite sides of cube cast.

     v.            Align the specimen centrally on the base plate of the machine.

  vi.            Rotate the movable portion gently by hand so that it touches the top surface of the specimen.

vii.            Apply the load gradually without shock and continuously at the rate of 140 kg/cm²/min till the specimen fails.

viii.            Record the maximum load and note any unusual features in the type of failure.

  *  Calculations :-

Size of the cube = 150 x 150 x 150 in mm

Area of the specimen = 22500 mm²

The strength of concrete increases with increase in age.

Crushing strength of cube tested at 7 days = 310 KN/22500mm² = 13.78 N/mm²

 *Compressive strength of concrete at various ages :-

The strength of concrete increases with age. Table shows the strength of concrete at different ages in comparison with the strength at 28 days after casting.

  *Compressive strength of different grades of concrete at 7 and 28 days :-

  *Safety & precautions :-

       i.            Use handle gloves, safety shoes and apron at the time of test.

     ii.            After test switch off the machine.

  iii.            Keep all the exposed metal parts greased.

  iv.            Keep the guide rods firmly fixed to the base and top plate.

Equipment should be cleaned throughtly before testing and after testing.






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