We all talk about the compressive strength of concrete all the time. And also know concrete is strong at compressive strength and weak at tensile strength.
But what is meant by characteristic strength?
The strength of concrete, where not more than 5% of the test results are expected to fall, is known as the characteristic strength of concrete.
Little Confusing? (Please pay attention to the below paragraph to understand)
In Simple words, Only 5% chances are there that the concrete will fail at its characteristic strength. If a block of M25 concrete has a characteristic strength of 23 MPa means that the chances of failure are only 5% at that load(23 Mpa). 95% chance is there; it will survive.
The compressive strength test will be determined by the strength of concrete at 28 days as per IS standard.
Based on the cube test results, the below graph was plotted. The number of cube samples represents the vertical axis & the compressive strength of the concrete cube represents the horizontal axis.
The target mean strength Fcm = fck + (K x S)
Fck = Characteristic strength of concrete.
K = 1.65 (Constant)
S = Standard deviation of concrete cube
Every material has its own resistance strength. For example, concrete is strong against compression and weak against tension. The resistance capacity where the chances of failure occur is below 5% is called the characteristic strength.
The characteristic load is the maximum load act on a structure that has a 95% chance of not being exceeded during the structure’s lifespan.
The compressive strength of concrete has been determined by both Non-Destructive Tests such as the
Let us discuss the short procedure of the compressive strength test of the concrete cube in a destructive method.
As per IS Code, the number of cube samples that have to be cast based on the concrete volume is given below.
QUANTITY OF CONCRETE (M3) | NUMBER OF SAMPLES |
1-5 | 1 |
6-15 | 2 |
16-30 | 3 |
31-50 | 4 |
51 & above | 4+1 additional sample for each additional 50 m3 concrete. |
Compressive Strength of Concrete Cube = (Maximum load/Area of the cube)f
The concrete takes 24 hours to attain 16% of its strength, and the strength will increase gradually.
DAYS | PERCENTAGE OF STRENGTH |
Day 1 | 16% |
Day 3 | 40% |
Day 7 | 65% |
Day 14 | 90% |
Day 28 | 100% |
The concrete attains its full strength at 28 days. The gradual strength increase has been listed below.
GRADE | CRUSHING STRENGTH @ 3 DAYS | CRUSHING STRENGTH @ 7 DAYS | CRUSHING STRENGTH @ 28 DAYS |
M10 | 4 | 6.5 | 10 |
M15 | 6 | 9.7 | 15 |
M20 | 8 | 13 | 20 |
M25 | 10 | 16.25 | 25 |
M30 | 12 | 19.5 | 30 |
The grade of concrete has been listed below based on the compressive strength 28 days cube test.
Type of Concrete | Grade of concrete | Mix Ratio | Compressive strength @ 28 days in N/Sqmm |
Ordinary concrete | M5 | 1:5:10 | 5 |
M7.5 | 1 : 4 : 8 | 7.5 | |
M10 | 1 : 3 : 6 | 10 | |
M15 | 1 : 2 : 4 | 15 | |
M20 | 1 : 1.5 : 3 | 20 | |
Standard Concrete | M25 | 1:1:2 | 25 |
M30 | Design Mix | 30 | |
M35 | Design Mix | 35 | |
M40 | Design Mix | 40 | |
M45 | Design Mix | 45 | |
M50 | Design Mix | 50 | |
High Strength Concrete | M55 | Design Mix | 55 |
M60 | Design Mix | 60 | |
M65 | Design Mix | 65 | |
M70 | Design Mix | 70 | |
M75 | Design Mix | 75 | |
M80 | Design Mix | 80 |
The characteristic strength of concrete is the result of the compressive strength of the concrete cube test. The design strength is the required strength of concrete to be designed as per the IS code.
For example, assume that the strength of concrete required M25, and the target design strength is 28.5 N/Sqmm. Now the chances of failure of concrete cubes fail at 25N/Sqmm is only 5%.
The design strength is 28.5 N/Sqmm, and the characteristic strength of concrete is 25 N/Sqmm.
Hope you’ve enjoyed the topic. Happy Learning 🙂
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