What is Compressive Strength Test of Concrete Cube?
Compressive strength of concrete cube test is the most important
strength test for concrete. This single test gives an idea about all the
characteristics of concrete. Concrete are very strong in compression. It is
assumed that whole of the compression will be taken up by the concrete at the
time designing any RCC structure.
Compressive strength of concrete depends on many factors such as cement
strength, watercement ratio, quality of concrete material, quality control during
the production of concrete etc.
What is Compressive Strength?
Compressive strength is the ability of
material or structure to carry the maximum loads on its surface without
any crack or deflection. Under compression test a material tends to reduce the
size, while in tension, size elongates.
Compressive Strength of Concrete by Compression Testing Machine as per IS: 516– 1959
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IS Code For Compressive Strength Test Of Concrete Cube:
 IS: 1199 – 1959, Method of sampling and analysis of concrete
 IS: 516 – 1959, Method of test for compressive strength of concrete
Apparatus for Compressive Strength Test of Concrete Cube:
 Compression Testing Machine
 Specimen mould
 Tamping rod
 Weighing device
 Tools and containers for mixing
Procedure for Compressive Strength Test of Concrete Cube:
Preparation of Concrete Cube
Specimens
Sampling of Materials – Samples of aggregates and cement for each batch of concrete will
be of the desired grading and will be in an airdried condition.
Proportioning – The proportions of the materials (aggregates, cement, water, etc.)
will be similar in all respects to those to be employed in the work.
Weighing – The quantities of cement, each size of aggregate and the
amount of water for each batch will be determined by weight to an accuracy of
0.1 % of the total weight of the batch.
Hand
Mixing –
 The cement and fine aggregate are mixed by hand or preferably in a laboratory batch mixer in such a manner as to avoid loss of water or other materials. It is done until the mixture ensures the greatest possible blending and of uniform color.
 The coarse aggregate is added and mixed with the mixture of cement and fine aggregate until the coarse aggregate is uniformly distributed throughout the batch.
 Water is added and mixed with the mixture until the concrete appears to be homogeneous and of the desired consistency.
 Each batch of concrete will be of such a size as to leave about 10 % excess after moulding the desired numbers (6) of test specimens.
Mould – 6 concrete cubes test
specimens in shape 15 cm × 15 cm × 15 cm are taken.
Compacting –
 The moulds will be wiped clean and applied oil here.
 The obtained concrete will be filed in the moulds in layers approximately 5 cm thick.
 Each layer will be compacted with not less than 35 strokes per layer using a tamping rod (a steel bar of 16 mm diameter and 60 cm long, bullet pointed at lower end).
 The top surface will be leveled and smoothed with a trowel.
Curing of Cubes – The test specimens will be stored in a place free from vibration
in moist air of at least 90 percent relative humidity and at a temperature
of 25 °C to 29 °C for 24 hours ± ½ hour from the time
of addition of water to the dry ingredients. After this period the
specimens will be marked and removed from the moulds and kept submerged in
clear freshwater until taken out prior to the test.
Precautions – The water for curing should be
tested every 7 days till 28 days at the temperature of 25 °C to
29 °C.
Procedure for Concrete Cube Test
 The specimens will be removed from the water after specified curing time and wiped out excess water from the surface.
 The dimension of the specimens will be noted to the nearest 0.2m.
 The bearing surfaces of the testing machine will be wiped clean.
 Any loose sand or other materials will be removed from the surfaces of the specimens which are to be in contact with the compression machine platens.
 In the case of cubes the specimen will be placed in the machine in such a manner that the load will be applied to opposite sides of the cubes as cast that is not to the top and bottom.
 The axis of the specimen will be carefully aligned with the centre of thrust of the spherically seated platen. No packing will be used between the faces of the test specimen and the steel platen of the testing machine.
 The movable portion will be rotated gently by hand so that it will touch the top surface of the specimen.
 The load will be applied gradually without shock and increased continuously at a rate of
approximately 140 kg/cm²/minute until the resistance of the specimen to the
increasing load breaks down and no greater load can be sustained.
 The maximum load applied to the specimen will be recorded and the appearance of the fractured faces of concrete cube and fracture if they are unusual in the type of failure will be noted.
NOTE:
At
least three specimens should be tested at each selected age. If compressive strength
of any specimen varies by more than 15 percent of average strength, results of
such specimen should be rejected. Average of the specimens gives the compressive
strength of concrete. The strength requirements of concrete.
Observation:
 Mix proportion = ………………….
 Date of casting = …………………
 Date of Testing = …………………
 Age of concrete = ………………...
 Curing history = ............................
 Appearance of fractured faces of concrete cube and the type of fracture if they are unusual in the type of failure
Table
1: Compressive
strength test of concrete
Trail
No. 1

Trail
No. 2

Trail
No. 3


Wt. of
specimen kg


Dimensions of the specimen

L in mm


B in mm


H in mm


Cross
sectional area mm^{2}


Crushing
load KN


Compressive
strength N/mm^{2}

Calculations:
 Size of the cube = 15 cm × 15 cm × 15 cm
 Area of the specimen (calculated from the mean size of the specimen) = 225 cm²
 Characteristic compressive strength (f_{ck}) at 7 days =
 Expected maximum load = f_{ck} × area × f.s
 Range to be selected is …………………..
Similar calculation for compressive strength should be done after 28 day
 Maximum load applied to specimen = ……… tones = ………… N
 Compressive strength of specimen = ………… N/mm²
Formula for compressive strength
Result:
Average compressive strength of the concrete cube = ………….N/ mm² (at 7 days).
Average compressive strength of the concrete cube =……….
N/mm² (at 14 days).
Average compressive strength of the concrete cube =……….
N/mm² (at 28 days).
Reports of Cube Test:
 Identification mark
 Date of test
 Age of specimen
 Curing conditions, including date of manufacture of specimen
 Appearance of fractured faces of concrete cube and the type of fracture if they are unusual in the type of failure
NOTE: The compressive strength of concrete increases with its age.
The below two tables show the strength of concrete at different ages.
Table 2: Comparison of Compressive Strength of Concrete at Various
Ages
Age

Strength percentage

1 day

16%

3 days

40%

7 days

65%

14 days

90%

28 days

99%

Table 3: Compressive
Strength of Different Grades of Concrete Cube at 7 and 28 Days
Grade
of
Concrete

Minimum
compressive strength N/mm² at 7 days

Specified
characteristic compressive strength (N/mm²) at 28 days

M15

10

15

M20

13.5

20

M25

17

25

M30

20

30

M35

23.5

35

M40

27

40

M45

30

45

Why Compressive
Strength Test of Concrete is Important?
The most important strength test for concrete is the compressive
strength test of concrete cube. It gives an idea about all the
characteristics of concrete. By the help of this single test one can judge that
whether concreting has been done properly or not.
What is the Size of
Concrete Cubes Used for Compressive Strength Test?
For the compressive strength test of concrete cube two types of
specimens either cube of 15 cm ×15 cm × 15 cm or 10cm × 10 cm × 10 cm,
depending upon the size of aggregate, are used. For most of the construction works
cubical moulds of size 15 cm × 15 cm × 15 cm are commonly used.
Summary of the
compressive strength test of concrete:
Concrete cube of size 15 cm × 15 cm × 15 cm are poured in the
mould and tempered properly so as not to have any voids. After twenty four
hours these moulds are removed and test specimens are put in water for curing.
By putting cement paste and spreading smoothly on whole area of
specimen the top surface of these specimen is made even and smooth.
These specimens are tested by compression testing machine after 7 days
curing, 14 days curing and 28 days curing. Maximum load is applied gradually at
the rate of 140 kg/cm² per minute till the specimens fails. Load at the failure
divided by crosssectional area of specimen gives the compressive
strength of the concrete cube.
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