Concrete is the most used material in the construction industry for various structures like building, bridges, PQC roads, etc. We use concrete of different grades as per the structural design.
Concrete is best for taking the compressive load in any structure. So during construction, we take concrete cubes of 15cm X 15cm X 15cm for testing the compressive strength of the concrete.
Required apparatus: -
A) At Site:
1) Cube mould of size 150mm X 150mm X 150mm (internal dimension)
2) A tamping rod of 16 mm diameter and 600mm long steel bar.
B) At Laboratory:
1) A Curing Pond
2) Weight balance
3) Compression Testing Machine
Procedure:
The procedure used
for the compressive strength test of concrete is conducted as per IS 516 (1959), IS 456-2000.
A) Sampling Stage of cubes for compressive strength test
1) Find out a levelled surface at the site & place the cube moulds.
2) Oil should be applied inside the cube mould after cleaning it properly.
3) Fill the fresh concrete inside the cube mould after checking the Slump test.
4) The concrete should be filled in three layers and each layer should be properly compacted by using the tamping rod.
5) Each layer should not exceed 50mm & should be tamped for
6) After filling the moulds remove excess concrete and finish the top surface of the concrete.
7) For a single test we need a minimum six (6) numbers of cube mould, out of which three (3) numbers cube test in seven (7) days and another three (3) numbers cube test for twenty-eight (28) days for compressive strength test for concrete.
8) Total numbers of cube mould required on the concrete quantity and the frequency of the test.
B) Post-Sampling Stage of cubes for compressive strength test
1) Remove the concrete cubes from the cube moulds after Twenty-four (24) hours.
2) permissible time for removing the cubes from cube mould is Sixteen (16) hours to Seventy-two (72) hours.
3) After removing the cubes, place them inside a curing pond & let them gain strength with time.
C) Compressive strength test of concrete cubes of size 15CM X 15CM X 15CM: -
1) When the curing time completes remove the cubes from the curing pond and let the cube become dry in a few hours.
2) Take the weight of the dry cubes to know whether the cube has been filled properly or not.
3) Place the cube on the compression plate in the compressive testing machine & align it properly so that during testing the load will be distributed properly.
4) After placing the cubes properly start the compressive test machine to apply load on the cube at an increasing rate of 140 kg/cm2 without any sudden shock to find out the proper value.
5) The reading on the meter of the compression testing machine is in ‘kilo newton’ (kn) and we have the surface area of the cube in ‘square milli meter’ (mm2).
6) Note down the highest load applied to the cube before crushing.
The Complete Procedure of Cube Test for The Compressive Strength of Concrete Is Explained in The Video Below.
Calculation: -
1) Now we have the load value in ‘KN’ so 1st we have to convert it in to newton.
For Example, if we have the noted value = 520 KN then after multiplying 1000 with it we will find out the value in ‘Newton’ (N)
Value in N = 520 X 1000 = 520000 N
2) The surface area of the cube = 150 X 150 = 22500 MM2
3) The final result: –
(520000 N / 22500 MM2 ) = 23.11 KN/MM2
Test Frequency of cube test for compressive strength of concrete: -
The number of samples required with respect to the quantity of concrete:
1) For 1 to 5 cum : 1 set
2) For 6 to 15 cum : 2 set
3) For 16 to 30 cum : 3 set
4) For 31 to 50 cum : 4 set
5) Above 51 Cum : 4 set + 1 set extra for each 50 cum of cum of concrete
CUM :- Cubic Meter (m3)
The cube test for compressive strength of concrete must be conducted as per the above frequency during the construction of any structure to ensure the grade of concrete. The cube test for compressive strength of concrete should be carried out only if we found a satisfying result in the Slump test for workability.
Click here to read about the Slump test for the workability of concrete.
