The compressive strength of concrete test is one of the post-test widely doing at the site to find out the compression strength of particular concrete batch.
To construct a building, we have to take care of the quality and strength. So to ensure the quality and strength of a material some kind of test has to do in the field like slump test of concrete, bulking of sand test, silt content test for sand, etc.,
Purpose the Test
A slab was designed to cast by M25 grade of concrete but we can’t find out the strength of the concrete in the semi-solid state. Even though we do some preliminary test to ensure the quality of the concrete but that cannot define its strength.
A certain quantity of concrete shall cast as a cube while pouring time of the concrete to find the compressive strength of concrete. Then the cube is to be tested by the Compression Testing Machine after completion of curing period 7 days and 28 days.
Now, we are going to discuss the Compressive Strength of concrete with the detailed procedure.
Before getting into the procedure we shall clarify some basic doubts happening while doing the test.
Compressive strength of concrete
The compressive strength of concrete is defined as the maximum crushing stress can withstand by the concrete.
You may aware of concrete cube test, but you may have some question like why this test conducting or any other question. We listed out the common doubt happening in concrete cube test.
- Why the cube test is doing at 7 days, 14 days and 28 days?
- Why was the size of mould chosen 150mm x 150mm x 150mm?
- Purpose of cube weighing?
- How many samples should be taken for testing
- The minimum value of Compressive Strength for a cube
- How to avoid the loss of concrete strength.
Why the cube test is doing at 7 days, 14 days and 28 days?
Everybody has asked the first question why the cube test is doing at 7 days, 14 days and 28 days?. Since the concrete will not attain its strength suddenly. The concrete takes 24 hours to attain the 16% of strength and the strength will increase gradually. The concrete attains its full strength in 28 days.
In the time period, proper curing should be done for the concrete to avoid the shrinkage cracks.
You may refer to the below table
|Days||Percentage of Strength|
So we can predict the crushing strength of concrete at the stage of 3 days, 14 days until wait for 28 days.
Gradual Difference of Compressive Strength of Concrete
|Grade||Crushing Strength @ 3 Days||Crushing Strength @ 7 Days||Crushing Strength @ 28 Days|
Why was the size of mould chosen 150mm x 150mm x 150mm?
The standard mould size 150mm x 150mm x 150mm has used for casting the cube because when we increase the size of mould then it will be difficult to handle the cube.
Purpose of Cube weighing?
You may know that the unit weight of concrete is 2400 kg/cum. How could it be measured?
Assume, approx weight of one concrete cube = 8 kg
Then the volume of concrete = 0.15 x 0.15 x 0.15 = 0.0033 cum
So for 1 cum = 8/0.0033 = 2400 kg/ cum
The cube has weighted to find the unit weight of the concrete.
How many samples should be taken for the Test?
As per IS Code, the sample selection for concrete cube test has listed below.
|Quantity of concrete (Cum)||Number Of Sample|
|51 & above||4+1 additional sample for each additional 50 Cum concrete.|
The minimum value of Compressive Strength for a cube
As per IS 456, the individual compressive value of a concrete cube should not be less than 75% from its grade.
If the concrete grade is M20, then the individual crushing value is should not be less than 15 N/Sqmm.
The individual crushing value difference between the cube should not be greater than 15%.
How to avoid the loss of concrete strength
The Compressive Strength of Concrete test should be do based on the following things and also ensure that before casting the concrete.
- Proper reinforcement design
- Do a proper material test
- The proper water cement ratio
- Ensure the proper formwork
- Mixing of material
You may interest to read Honeycomb rectification.
Compressive Strength of Concrete Test
We hope you may feel free from your all doubts and if anything else doesn’t forget to comment. Now we are going to see how to do the compressive strength of concrete test.
- Steel Mould (150mm x 150mm x 150mm)
- Tamping rod
- CTM Machine
- Weighing machine
- Estimate the adequate ingredients to prepare the concrete with a proper water-cement ratio.
- Ensure that the cube mould should be free from dust and rust.
- Now, pour the concrete into the mould by proper compaction with the help of tamping rod.
- Finish the top surface as smooth by the trowel.
- The mould should be covered with gunny bag and place undisturbed for 24 hours at a temperature of 27 ° Celsius ± 2.
- After 24 hours the cube specimen shall remove from the mould and it should be immersed into the water for 7 or 28 days based on the test.
Method of Testing
- The cube shall be removed from the water 30 minutes prior to the test and it should be in dry condition.
- The specimen should be weighed prior to the test.
- The specimen should place between the plate with proper alignment.
- Now, apply the load (kilonewton) gradually to the specimen.
- The specimen will break at maximum load (1 Division = 5 KN)which is noted down as the crushing value of the cube.
The average (Minimum Three Sample) crushing value should be note down as the compression value of the selected concrete batch.
How to calculate the compressive value of a Cube?
Compressive Strength of concrete cube = Maximum load/Area of the cube
Assume that the compression load is 375 KN (1 Kg = 9.81 N)
Cross Sectional Area – 15 x 15 = 225 Sqcm.
Compressive Strength = (375 x 1000/225) = 1666/9.81 = 169.82 Kg/Sqcm.