stress strain curve for concrete and modulus of elasticity are the most important factors that affect the finite element modelling of concrete structures. the data needed for the FE modelling should simulate the actual behavior of the real structure that could be tested in labs. for most common codes and standards, the concrete strength for the structural elements is equal to 0.85 of the concrete cylinder compression strength obtained from the standard compression test. they argue that this strength is equivalent to the full strength of the cylinder due to the shape and size effects. I am wondering if we could reduce the concrete strength by the same amount when modelling the concrete using FEM or the FEM will take the effect of size and shape on the calculations. in most cases when trying to model the full strength it gives higher capacity than the experimental or design equations with the same amount (1/0.85).
The cylinder or cube strength are basically uniaxial test and concrete in practice is subjected to different conditions and also subjected to size effect.
Therefore one has to use the stress in concrete as per Codal provisions. In FEM modelling or simulation the stress in concrete has to be defined as per analysis or design requirement and model used for material
Thanks Om Prakash Chhangani
That is why I am asking why we take this effect into account when using design equations by different code and neglect this effect when modelling the concrete stress strain curve
Dear Essam
For beams, the stress profile is parabolic as shown in the attached figure. To make calculations easier, code of practice usually replace the parabolic curve by a rectangular block. Therefore, it is just an approximation to a complicated situation. Furthermore, we all know that codes of practice use too many partial safety factors, so reducing the concrete strength to 0.85 is just another partial safety factor. In design situations, such factors are necessary; however, when doing scientific research, all safety factors should be removed, including FEM
The figure is based on ACI code
Dear Kareem
Thanks for your feedbacks
But
The 0.85 reduction is well known that it deal with the change from cylinder strength to concrete strength in real structures not a safety factor
The concrete block height (a) is used to change the parabola to a rectangle with the same force C
Dear Essam
It is known that the compression strength of cylinder is about (0.8-0.85) of cubes for the same concrete. This is usually related to the shape and size effect as you stated. If codes use the 0.85 reduction to account for the size and shape effect, why can't it just use the cubes strength instead of the cylinder strength. However, concrete members are slender; therefore, cylinder strength better represent such elements. Hence, I have to stick with my convection
But thank you this is a very interesting question
The friction between the loading plates and the cube test specimen leads to more confining effect (especially with its short height to width ratio) comparing to the cylinder specimen. We use cylinder results in FEA as its axial strength reflects nearly zero confining stresses while the confining effects is separately taken into account using the appropriate yield criteria and hardening functions (like concrete damaged plasticity and smeared crack)
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