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?

## What is the characteristic strength of concrete?

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 chances of failure is 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

## What is characteristic strength and characteristic load?

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.

## How to calculate the compressive strength of concrete?

The compressive strength of concrete has been determined by both Non-Destructive Tests such as the Rebound Hammer Test and by destructive methods as per IS code 456.

Let us discuss the short procedure of the compressive strength test of the concrete cube in a destructive method.

• The concrete should be cast in steel mould size 150mm x 150mm x 150mm & immerse in water for curing up to 28 days.
•  Then it should be tested through the compression testing machine to find the load which breaks the cube.

As per IS Code, the number of cube samples has to be cast based on the concrete volume is given below.

 QUANTITY OF CONCRETE (CUM) 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 Cum concrete.

Compressive Strength of Concrete Cube = (Maximum load/Area of the cube)

### Example Calculation

• 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.

### Compressive strength of concrete at days

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

### Characteristic Compressive strength of different grades concrete after 28 days

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

## What is the difference between characteristic strength and design strength?

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 🙂 Satheesh is a Civil Engineer who has more than 9+ Years of experience in residential construction. He is the author, editor of Civil Planets

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