RISAFoundation was originally created as mat slab design software, but with some tweaks in the latest version (v6.0) it is now useful for slab on grade design as well. To get a design for slab on grade simply draw a slab element which has the appropriate thickness:
When defining the Design Rule for the Design Strips, be sure to use the Mid-Depth Only rebar option. This is as opposed to having two layers of rebar (top and bottom) within the slab. When choosing this option RISAFoundation will place a single layer of rebar at the middle of the slab.
Be sure to use a reasonable bar size, and the software will automatically determine the required spacing to meet strength requirements for the slab.