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, water-cement 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 Cubes by Compression Testing Machine


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


Compressive Strength of Concrete Cubes by Compression Testing Machine

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 air-dried 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
  1. The specimens will be removed from the water after specified curing time and wiped out excess water from the surface.
  2. The dimension of the specimens will be noted to the nearest 0.2m.
  3. The bearing surfaces of the testing machine will be wiped clean.
  4. 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.
  5. 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.
  6. 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.
  7. The movable portion will be rotated gently by hand so that it will touch the top surface of the specimen.
  8. 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.
  9. 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 mm2



Crushing load KN



Compressive strength N/mm2





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 (fck) at 7 days =
  • Expected maximum load = fck × 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
Compressive Strength of Concrete Cubes by Compression Testing Machine

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 cross-sectional area of specimen gives the compressive strength of the concrete cube.


Read also:- 
Compressive Strength Test on Concrete Surface using Rebound Hammer