In this article, the compressive strength of the concrete cube test will be discussed.
To determine the compressive strength of the concrete cube.
Apparatus & Equipment
1. Compression test machine.
2. Cube mold of 15 cm size.
3. Curing Tank.
4. Tamping bar.
5. Weighing balance.
Specimen For Test
The cube samples shall be 15 x 15 x 15 cm. If the largest nominal size of the aggregate does not exceed 2 cm, then cube samples of 10 cm may be used as an alternative.
Casting Of Concrete Cubes For Compressive Strength Test
The ingredients should be proportioned as per design standards.
Mixing Of Concrete
The concrete should be mixed either by hand or in a laboratory machine mixer in such a manner as to reduce wastage of water and other materials.
The mixing should be done as follows:
1. Add one-half of the coarse aggregate into the mixing drum.
2. Then add cement and fine aggregates and finally add the remaining coarse aggregates.
3. Add the required amount of water.
4. Start the mixing machine until the resulting concrete gives a uniform texture.
Hand mixing is also adopted for preparing concrete cubes for the compressive strength test. The mixing should be done on a watertight, non-absorbent platform with a trowel or shovel using the following procedure:
1. Mix the cement and fine aggregates thoroughly in dry conditions.
2. Add Coarse aggregates and thoroughly mix with cement and fine aggregates.
3. Add the required amount of water and mix it properly until the concrete becomes homogeneous of the desired consistency.
Sampling Of Concrete Cubes
1. Fill the mold with freshly mixed concrete in layers approximately 5 cm deep.
2. Compact the concrete either by vibrator or by using the tamping bar. ( Minimum 35 strokes per layer for 15 cm cubes and 25 strokes for 10 cm cubes).
3. Finish the top level of the mold using a trowel.
4. Cover the mold with a glass plate or gunny bag to protect against evaporation.
Curing Of Concrete Cubes For Test
1. Keep the specimens in a place, free from vibration, in moist air, and at a temperature of 27°C ±2° C for 24 hours.
2. After 24 hours, mark and remove the samples from the mold.
3. Submerge the cubes immediately in fresh and clean water until taken out prior to the test. The water should be renewed every 7 days.
Procedure Of Compressive Strength Test
1. Remove the specimens from the water before 30 minutes of testing.
2. Remove any loose sand or other material from the surface of the specimens and let them dry.
3. Clean the bearing surface of the compression testing machine.
4. Now place the cube in the testing machine in such a manner that the load is applied o the opposite sides of the cubes.
5. Align the axis of the specimen with the center of thrust of the spherically seated platen.
6. Apply the load increasingly at a rate of 140 kg/cm² per minute until the cube collapse.
7. Note down the maximum load applied to the specimen and any other unusual activities at the time of failure.
Points To Be Remembered
Few points must be remembered for the compressive strength of concrete testing.
1. Age Of Test
The test should be done at 7 days and 28 days.
2. Number Of Specimen
At least three specimens, preferably from different batches for testing at each selected age.
Calculation For Compressive Strength Concrete
Let assume the maximum applied load is 400 KN = 400000 N
Cross-sectional area of cube =15 x 15 = 225 cm²
Compressive strength of concrete = 400000/225 = 1778 N/cm² = 1778/9.81 = 181 Kg/cm²
[1kg =9.81 N]
|Specimen 1||Specimen 2||Specimen 3|
|Compressive Load(KN)||380 KN||400 KN||425 KN|
|Compressive Strength(Kg/Cm²)||(380000/225) / 9.81= 172 kg/cm²||(400000/225) / 9.81= 181 kg/cm²||(425000/225) / 9.81= 192 kg/cm²|
|Average Compressive Strength||= (172+181+192)/3 = 181.66 Kg/cm²|
This compressive strength test of concrete should be done at a temperature of 27°C ± 2°C.
Compressive Strength Of Concrete Over Time
Compressive Strength Of Concrete At 7 & 28 Days