You can use the Materials Paintbrush to quickly apply a different material such as poured concrete to the foundation floor.
To apply a material to the floor:
Display your model in 3D view and make sure the floor is visible in the view.
Select Edit
> Materials Paintbrush, or click the Materials Paintbrush button
on any tabbed toolbar.
In the catalog panel, select the material you want to apply.
Click on the floor surface. The material is immediately applied.
Right-click and select Finish.