The compressive strength test of concrete is one of the destructive tests widely done at the site to find out the compression strength of a particular concrete batch.
The ability of hardened concrete cube to resist the compression loads applied on the surface is known as
Otherwise, the compressive strength of concrete is defined as the maximum crushing stress endured by the concrete.
Assume a slab at our site is designed to cast M25 grade of concrete, but we could not define its strength in the semi-solid state.
For that matter, A certain quantity of concrete shall be cast as a cube while concrete pouring time to find the compressive strength of that concrete batch. The cube will be preserved and curated well for almost 28 days to be tested by the Compression Testing Machine.
Video Courtesy – NCTEL
Compressive Strength of concrete cube = Maximum load/Area of the cube
Example Calculation
| Grade | Age of Test | Specimen | Load (KN) | Tensile Strength (Kg/Sq.cm) | Average Strength |
| M10 | 28 Days | Sample 1 | 300 | 136 | 138 |
| 28 Days | Sample 2 | 290 | 131 | ||
| 28 Days | Sample 3 | 325 | 147 |
Simple. The concrete takes 24 hours to attain 16% of its strength, and the strength will increase gradually.
The concrete attains its full strength in 28 days. Experts already recorded the concrete strength change at specific time intervals like below.
Gradual Difference of Compressive Strength of Concrete
So it is easy for us to compare the strength with that reference value at a specific time interval.
There is no such thing that we only have to use 150mm moulds.
We can also mould any square cubes. But it requires a higher capacity CTM machine to calculate the values such as 300 tonnes, which does not cost-friendly. So we stick to the 150mm mould.
Also, it is easy for us to carry or transfer the cubes inside the site/field.
To identify the unit weight of concrete. You may know that the unit weight of concrete is 2400 kg/m3.
How could it be measured?
Simple. Assume, the approx weight of one concrete cube = 8 kg
Then the volume of concrete = 0.15 x 0.15 x 0.15 = 0.0033 m3; So for 1 m3 = 8/0.0033 = 2400 kg/ m3. Since the concrete weight itself has also been assumed for the structural calculations, if it exceeds the assumed value, then the load on the structure will likely increase.
As per IS Code, the sample selection for the concrete cube test has listed below.
As per IS 456, the individual compressive value of a concrete cube should not be less than 75% of its grade.
Example – If the concrete grade is M20, then the individual crushing value should not be less than 15 N/Sqmm. The individual crushing value difference between the cube should not be greater than 15%.
Hope you enjoyed the topic. Happy Learning 🙂
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Why should the concrete specimens be immersed in water before testing?
For curing purposes
from what time the days are calculated for testing like 7 days 28 days...is it from the time of casting or after 24hrs of casting like removal of cubes from moulds and putting in curing tank
Is very educative