Hello Cristina
When you say "strength of a 25 MPa concrete", which material property of Concrete are you changing in Discovery Live? You can specify Youngs Modulus whose default value is 2.5e+10 Pa. See below image. Other material properties like Ultimate Tensile Strength are not exposed in Discovery Live. You can use Discovery AIM which can give you access to additional material properties.
If you still have questions, please share the model to look into the issue.
You are navigating away from the AIS Discovery experience