Theory:
One of the important properties of concrete is its strength in compression. The strength in compression has a definite relationship with all other properties of concrete i.e. these properties improved with the improvement in compressive strength. Thus, with this single test one judge that whether Concreting has been done properly or not. In India cubical moulds of size 15 cm × 15cm × 15 cm are commonly used. The concrete is prepared with definite proportion is poured in the mould and tempered properly so as not to have any voids. After 24 hours these moulds are removed and test specimens are put in water for curing. The top surface of this specimen should be made even and smooth. This is done by putting cement paste and spreading smoothly on whole area of specimen. These specimens are tested by compression testing machine after 3, 7 or 28 days curing. Load should be applied gradually at the rate of 14 N/mm2 per minute till the Specimens fails. Load at the failure divided by area of specimen gives the compressive strength of concrete. At least three specimens are tested at each selected age. The failure of the specimen is called as ‘hour glass’ type failure. This happens due to lateral restraint provided by the plates to the cubes.
Practically, the compression testing system develops a complex system of stresses due to end restraints provided by steel platens of compression testing machine (CTM). Under compression loading, due to “poisons effect”, the cube specimen also undergo lateral expansion. However, the steel platens don’t undergo lateral expansion to the same extent that of concrete. Thus, there exist a differential tendency of lateral expansion between steel platens and concrete cube faces. As a result of this, tangential forces are induced between the end surfaces of the concrete specimen and the adjacent steel platens of CTM. Therefore, in addition to the applied compressive stress; lateral shearing stresses are also effective in these specimens. Effect of this shear decreases to words the centre of the cube. Thus, the cube has near vertical crack at cubes centre and sometimes, the cube may completely disintegrate leaving a relatively undamaged central core. Figure 8 shows, typical failure patterns of the concrete cube
To determine the cube strength of the concrete of given properties.
Reference:
IS: 516 - 1959,
IS: 1199-1959,
SP: 23-1982,
IS: 10086-1982.
Apparatus:
Moulds for the test cubes, tamping rod, metallic sheet, Compressive testing
machine.
Material:
Cement, sand, aggregate and water, grease
Procedure:
1. Calculate the material required for preparing the concrete of given proportions
2. Mix them thoroughly in mechanical mixer until uniform colour of concrete is
obtained
3. Pour concrete in the lightly greased cube moulds.
4. Fill concrete in two layers each of approximately 75 mm and ramming each
layer with 35 blows evenly distributed over the surface of layer.
5. Struck off concrete flush with the top of the moulds.
6. Level the concrete at the top of the mould by means of trowel and give proper
identification mark of the specimen.
7. Immediately after being made, they should be covered with wet mats.
8. Specimens are removed from the moulds after 24hrs and cured in water. Keep it for curing up to 28 days.
Testing of concrete cubes:
9. Take the cube out of water at the end of three days with dry cloth. Measure the
dimensions of the surface in which the load is to be applied. Let be ‘L’ and ‘B’
respectively.
10. Place the cube in compressive testing machine and apply the load uniformly at
the rate of 35N/mm2.
11. Note the load at which the cube fails. Let it be ‘P’. Also note the type of
failure and appearance cracks
12. Calculate the compressive strength of the cube by using formula P/A. Where
A is the area of loaded surface (i.e. L×B).
13. Repeat the same procedure (steps 9 to 12) for other two cubes.
14. Repeat the whole procedure (Step 9 to 13) to find the compressive strength of
the cube at the end of 7 days and 28 days.
Observation:
Results:
The type and grade of concrete:
The compressive strength of cement ate the end of
i) 3 days: ___________. N/mm2.
ii) 7 days: ___________. N/mm2.
iii) 28 days: ___________. N/mm2.
Precautions:
(Discuss about the precautions to be taken while conducting this experiment)
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Questions:
1. What is the effect of W/C ratio on compressive strength of concrete?
2. Mention the factors those affect the compressive strength of concrete?
3. What is butting of concrete mixture? Why is it done?
4. How does strength correlate with other properties of hardened concrete?
