Procedures:
a. The broken portions of the beams selected for the compressive strength test shall be have a length not less than 50mm greater than the width of the beam, be free from cracks, chipped surfaces or other obvious defects within the length to be tested.
b. Unless this test is carried out immediately after the flexural strength test on the beam, the broken portions should be returned to the curing tank until required.
c. Place the broken portion of the beam in the test jig as per the sketch.
d. Place the jig centrally in the compression test machine (no packing is to be used between the specimen and the auxiliary platens or between the compression machine platens and the auxiliary platens).
e. Apply the load without shock and increased continuously at a rate of approx 15 MN/m2 per minute until no greater load can be sustained. Record the maximum load.
Calculations:
Fig. 3.3.1 Typical device for "equivalent cube" test.
Data:
Area to be test: 150mm X 150mm
The maximum load: KN
Result:
stress = N/mm
Discussion:
From the above results, we can see that the reading in test 1 and 4 (cube compressive strength test and equivalent cube test) has a small different. The reading in equivalent cube test was a little bit higher than that of cube compressive strength test. It can be explained by energy is needed to break the bonding between the forced concrete and the around concrete.
To make the test more accurate, The test should be carried on immediately after the specimen was taken out from the curing tank to prevent the loss of moisture. On the other hand, the cube/beam should be placed such that the force baring area is parallel to the testing jig.
Because it uses the broken portion of beam from flexural strength test, extra material is not needed. Although there is little error for the reading obtained, the percentage error is not so significant. As a result, equivalent cube test is a good method to exam the compressive strength of the specimen.
Comparing the compressive strength to the tensile strength of concrete, the tensile strength is only 1/10 of its compressive strength. This reflects the fact that concrete is strong in compression but weak in tension.
Rate of loading is quite important to the test compressive strength. In general, the higher the loading rate the higher the measured strength. It is thought that under slow loading rates more sub critical cracking may occur or that slow loading allows more creep to occur which increase the amount of strain at a given load.
Conclusion:
The compressive strength of the specimen is kN/mm2. It’s about 10 times the tensile strength. And it is a little bit higher than the result obtained from the cube compressive strength test.
