Can anyone kindly explain in case of Hognestad compressive stress-strain curve for concrete, they have given fc" =0.85fc', but why researchers are use fc"=fc' instead. Where fc" =ultimate compressive strength in column and fc' =cylinder strength. Why all researchers take fc" = fc', what about reduction faction due to size effect?
Pandi Mani
Concrete is a brittle material and it does not undergo any necking or reduced area beyond ultimate point. So, there is no size effect here. Many researchers theoretically equate ultimate and breaking strength in the case of concrete as they are close to each other:
So far, Hognested stress-strain is concerned, and if fc"=0.85fc' is rightly understood, fc' is the ultimate strength and fc" is the breaking strength, which is 15% lesser than fc' and that makes some sense while looking at the curve.
Dear sir,
15% reduction in strength is made on concert column compressive strength at ultimate (fc") compared to ultimate compressive strength of cylinder (fc'). The reduction in 15% strength at ultimate is due to size effect only as "E.Hognested" said, no doubt in that.
But in numerical or FE analysis, why researchers are directly consider the ultimate compressive strength of RC as fc' instead of 0.85fc'. since due to size effect they must consider 0.85 fc' right but all of them took fc' only.
I am talking about "E.HOGNESTAD " compressive stress-strain model for concrete. In his model he suggested to take fc"=0.85fc', but every one took fc"=fc' instead. Thats what I need to get clarified?
Here we have two different considerations one is for design in ultimate strength where we need to use the size effect factor. On another hand if you are trying to find out actual breaking load and compare this with experimental results you need to use fc'. By this you actually know the reserve strength even beyond ultimate load carrying capacity in design. We need more deliberation on this account. Hope some more experts to comment.
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