Abaqus: How to lower the stiffness linear curve of load vs deflection of concrete slab

Dear all,

Iam modelling a simply supported RC slab with Abaqus using Concrete Damaged Plasticity subjected to a concentrated load at the top surface of the slab. I have used element type C3D8R for concrete and truss elements to represent steel bars.

I found the result of FEM showed that the linear part of load versus deflection, i.e. from zero load up to yield load shows a very stiff curve compare to the experimental test results.

My question is that how to lower down this curve so that it would match the experimental test result.

Any help is very much appreciated. Thank you.