Compressive
Strength of Cement Concrete
Compressive strength is the strength of concrete against crushing due to
direct compressive load. It is considered to be the most important property of
concrete and is often taken as the index of the overall quality of the
concrete.
Factors Affecting
Compressive Strength of Concrete
1.Type
and fineness of cement
2.Strength
size, shape, surface, texture and grading of aggregate
3.Water
to cement ratio
4.Aggregate
to cement ratio
5.Curing
time and temperature
6.Age
of concrete
Compressive Strength Test
Compressive the strength test is performed on 15 x 15 x 15 cm cube, Metalic mould is used
for casting concrete cube. Fresh concrete is filled in three layers in a mould
and each layer is compacted 25 times by tamping road, sometimes vibration table
is used for compaction of the cube. After that cube is kept in a wet room whose
temperature is about 27degree Celsius. After that crushing is done in the
usual way.
The cube can be tested after 3,7,14
and 28 days. Cube is placed in compression testing machine and axial load is
applied at the rate of 140 kg/ cm2. In actual concrete is required sufficient
time to gain strength. Usually 28 days strength is considered as full strength
of concrete. Sometimes the strength of concrete may be required to be tested
earlier, generally 7 days.
Apparatus
for Compressive Strength Test
1.Cube
Mould (150mm *150mm *150mm or 100mm*100mm*100mm)
2.Tamping
Rod (Dia. 16mm, Height 600mm)
3.Comprssion
Testing Machine
Example:
Grade of concrete = M25
Cube size = 150mm*15mm*150mm
Age of cube = 28 days
S.No.

Date of
Casting

Date of
Testing

Load (
KN )

Area of
Cube
(150*150)
mm2

Compressive
Strength
(
N/mm2)= Load/Area

Remark

1

11/10/18

08/11/18

610

22500

(610*1000)/22500
= 27.11


2

11/10/18

08/11/18

580

22500

(580*1000)/22500
= 25.77

Avg=26.96N/mm2

3

11/10/18

08/11/18

630

22500

(630
*1000)/22500
= 28

Notes:
1.The
strength of any individual cube should not vary by more than 15% of
average % the set of three cubes. If it is more than 15% then it should be
considered fail.
2.Three
specimens shall be made for each sample.
Frequency of Sampling
S.No.

Quantity of concrete in the work (m3)

Number of samples

1

15

1

2

615

2

3

1630

3

4

3150

4

5

51 and above

4
+ one additional sample for each additional 50 m3

Compressive Strength of Concrete at various Ages
Concrete Age

Strength %

1 Day

16%

3 Days

40%

7 Days

65%

14 Days

90%

28 Days

99%

Compressive
Strength of Different Grade of Concrete at 7 days and 28 days
Concrete Grade

Minimum compressive strength at 7 days (N/mm2)

Characteristic compressive strength at 28 days
( N/mm2)

M15

10

15

M20

13.5

20

M25

17

25

M30

20

30

M35

23.5

35

M40

27

40

M45

30

45

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