The concrete cube test is performed to determine the compressive strength of concrete and it also provides an idea about all the characteristics of the experimented concrete. One can easily judge based on the results of this test whether the concreting process has been done properly or not. Proper concreting refers to standard practices that need to be followed while preparation, pouring and curing of the concrete mix.
The test for computing the compressive strength of concrete can be performed by using either a cubical or a cylindrical concrete sample as the standard specimen. The cubical or cylindrical shape of the standard specimen used for the test is recommended by standard construction codes(IS:516-1959).
In India we prefer computing the compressive strength of concrete using concrete cubes and consider 80% of the computed result as the original compressive strength of concrete. This is because the compressive strength computed in the case is approximately 25% more than the original strength of the concrete. The compressed strength computed in the case of cylindrical samples used as standard specimens is approximately equal to the actual strength of the concrete. American Society for Testing Materials(ASTM) C39/C39M provides the Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens for computing the compressive strength of concrete.
Figure 2: Some of the steps involved in Compressive testing of Concrete
The compressive strength of concrete used for construction generally varies from 15MPa to 30MPa which can be higher for industrial and commercial structures. The compressive strength of concrete depends on a number of factors such as water cement(w/c) ratio, grade of cement, quality of concrete material components, quality control, curing process and more.
What is Compressive Strength?
Figure 3: Cube being Tested in a Compressive Testing Machine(CTM)
Compressive strength can be defined as the potential of a material to carry loads on its surface until it cracks or gets deflected. A material under compression tends to get reduced in the direction in which the compression is being applied.
Expression for Compressive Strength
Compressive strength of any material can be expressed as the ratio of the applied load compressing the material at the point of failure to the cross-sectional area of the surface on which the load was acting. Therefore,
Compressing Load(P)
Compressive Strength = 
Area of X-section(A) on which load is acting
Cube Test Procedure for Computing Compressive Strength of Concrete
The Cube Test is performed in the Compression Testing Machine(CTM). The proportion of materials taken to form the test specimen is the same as that used in the field conditions and must be sufficient in quantity to form 6 specimen cubes. 3 specimen cubes make a sample and if the strength of any cube of a sample varies by more than 15% of the average strength of the 3 cubes, the result obtained from that specific sample must be rejected.
The concrete materials are mixed together either by hand or in a laboratory batch mixer. Hand mixing must be performed by taking proper precautions to avoid inappropriate concrete mix formation. The cement and fine aggregate must be mixed on a non-absorbent and watertight platform until the mixture is thoroughly blended and forms a uniform color. Thereafter the coarse aggregates are added and mixed until it gets uniformly distributed within the batch. Lastly water is added and the batch is mixed until the concrete mix develops a homogeneous density and the desired consistency.
For the cube test, two specimen sizes of cubes, either 15cm x 15cm x 15cm or 10cm x 10cm x 10cm are used depending on the size of the aggregate being used.
After selecting the appropriate mold, the concrete is poured into it and tempered in such a way that there are no air voids present in it. The molds have to be removed 24 hours after the concrete has been poured and the test specimens obtained from these molds have to be put in water for the curing process.
The top surface of these specimens should be made even and smooth by applying cement paste, spread evenly on the whole top surface area.
After 7 days or 28 days of curing, the concrete cubes are taken out from under water and are tested under the compression testing machine. The load exerted by the testing machine is applied gradually at a rate of 140kg/cm2 per minute until the specimen fails. The value of the load(P) exerted at the point of failure is recorded. This load when divided by the surface area of the specimen cube gives the compressive strength of concrete. The average of the 3 specimens in a sample give the crushing strength of concrete.
Gain in Strength of Concrete over Time
Concrete develops its strength with time after being poured in the mold and during the curing stage. Even after the curing is complete, the strength of concrete still increases very slowly for its entire life. The concrete achieves more than 60% of its strength in the first 7 days after casting and more than 99% in 28 days. For different grades of concrete the development in strength is almost same for all in terms of percentage of strength gained with time as shown in the following Compressive Strength vs Age graph.
