Why is the characteristic compressive strength fck reduced to 0.67 fck?
Following points part of the answer to the question : IS 456 and IS 1343 are the code of practice you can use. The method of proportions aggregates for concrete is part design mix process. Size and Grading impacts the design mix greatly. larger the size of aggregate smaller is the cement requiremenRead more
Following points part of the answer to the question :
- IS 456 and IS 1343 are the code of practice you can use.
- The method of proportions aggregates for concrete is part design mix process.
- Size and Grading impacts the design mix greatly.
- larger the size of aggregate smaller is the cement requirement for a given W/C ratio
- Compressive strength of the mix tends to increase with decrease in aggregate size.
- perform gradation tests using sieves. Coarse aggregates should conform to table 2 of IS 383 and Fine aggregate should conform to Zone 1 of IS 383
There is ACI method available for the same exercise ACI 211.1is the code you can use.
Hope this helps.
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Kuldeep Singh
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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