# Why the elastic modulus defined in the material data is not used for the creep calculation? What equation is used for the calculation for the elastic modulus from the compressive strength in the time dependent material?

**Question:**

Previously I have questioned about the creep deformation and the fictitious force calculation due to creep (Previous ticket no. 15854433). The answer was that the elastic modulus for the calculation of the creep fictitious force is different from the elastic modulus defined in the material properties. In other words, if I define the elastic modulus of 28110 MPa in the material data and define compressive strength of 50 MPa in the time dependent material (CEB-FIP 1990), then the elastic modulus for the creep deformation and fictitious force is automatically calculated from the compressive strength defined in the time dependent material (38600 MPa) (i.e. not using the elastic modulus defined in the material data). This made the difference from the predicted numbers.

I am wondering why the elastic modulus defined in the material data is not used for the creep calculation, what equation is used for the calculation for the elastic modulus from the compressive strength in the time dependent material, and how I can do if I want to use the elastic modulus defined in the material data for the creep calculation.

**Answer:**

Actually compressive strength and elasticity of concrete are interdependent. It is not possible to vary the compressive strength of concrete without changing the elasticity as well. For this purpose, the formula as provided in CEB-FIP 1990 are used.

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