You can use the Pads tool to insert a concrete slab directly on your terrain. By drawing the outline of the pad, you control the precise size and shape of the slab.
To create a slab on grade:
Select Insert
> Terrain > Pads, or click the Pads button on the Terrain
toolbar.
In the catalog panel, select the type of slab you want to insert.
Select a start point for the slab.
Continue selecting points to define the outline
of the slab. Note that the last point picked always closes back to the
start point, so you don’t have to pick the start point again.
Right-click and select Finish.