Why are 6 cubes cast for testing the Compressive Strength?

I agree with the reason given by CB Sowmya, but only partially. We don't assume concrete strength to be 2/3rd of the characteristic strength, but there's a logical reason behind it. She's only stated the shape factor with the correct 20% decrease in strength, but there is also a size factor. What haRead more

I agree with the reason given by CB Sowmya, **but only partially. **

**We don’t assume** concrete strength to be 2/3rd of the characteristic strength, but there’s a logical reason behind it. She’s only stated the shape factor with the correct 20% decrease in strength, but there is also a size factor.

What happens is, first of all, due to slenderness, concrete cylinders have shown to have 80% of the characteristic strength corresponding to 150mm cubes. But also, even concrete cubes of sizes exceeding 450mm show only 85% the strength as compared to the standard 150mm ones. Since the concrete members will neither be cubes and not just 450mm, **we need to take into account both the factors.**

So, the new compressive strength becomes 0.8*0.85*fck = 0.68 fck. Now, 0.68 fck is very close to 0.67 fck or fck/1.5. So, for a rounder figure, we take it as 0.67 fck.

Note: This 1.5 isn’t the factor of safety. (FoS). A factor of 1.5 is further applied for design and the design strength becomes 0.67 fck / 1.5 = 0.446 fck.

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## Kuldeep Singh

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 compressivRead more

Whenever testing for cube strength, it is always advised to make and test

three specimensfrom 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 daysfor 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.

See lessFor both the tests, 3 cubes each are employed. Hence, a total of6 cubesgo through the compression test.