Why are 6 cubes cast for testing the Compressive Strength?
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INDER KUMAR YADAV
6 cubes are casted for checking the compressive strength of concrete as it is given in the code and as,
In Indian standard, there are two different IS code which is defining concrete sampling frequency, those are IS 456 and IS 4926.
As per is 456 Clause 15.2.2, Sampling frequency is
0–5m³ – 1 set
6–15m³- 2 set
16–30m³ 3 set
30–50 m³ 4 set and +1 set for every 50m³ concrete.
This cubes are to be tested in 28 days.
Additional samples should be casted if required for 1 day, 3 days, 7 days testing done.
As per IS 4926 Clause 6.3.2,
One set of samples to be casted in every 50m³ concrete and additional samples (1 day, 3 days,7 days etc) to be casted if required.
Here, 1 set sample means 3 nos of cubes.
Whatever, the sampling frequency and number of samples to be cast to be fixed mutually by both vendor and client
Three cubes are tested for seven days of strength. The cube should have achieved at least 70% of its strength on the 7th day.
The other three cubes are tested for 28 days strength. The cube should have achieved at least 99% of its strength on the 28th day.
6 cubes are casted for testing the compressive strength because first 3 cubes are tested for 7 days and another for 28 days. The cubes tested for 7 days should gain 70% of strength and cubes tested for 28 days should gain 99% of strength.
Whenever testing for cube strength, it is always advised to make and test three specimens from the same mix. This is done to reduce or average out the error due to manually introduced differences in the three cubes.
Now, concrete and cement are always tested at both 7 days and 28 days for compressive strength. The compressive strength at seven days is known to be 65% of the final (28 days) strength. This double testing is done so that if the seven days strength is found inappropriate, the cubes may be straight away discarded to save 21 days’ time because the 28-day strength will also probably be below par. Now, a time even earlier than seven days is not chosen because, at that age, strength gain is very steep, and there’s a higher margin of error. Also, seven days is a round figure.
For both the tests, 3 cubes each are employed. Hence, a total of 6 cubes go through the compression test.