I'd make two suggestions:
1) return the mixer you have and rent something like this:
2) if you are going to have a heavier load on the slab, consider digging a 12 or 18" grade beam all the way around the perimeter with a couple of #6 bars at the bottom.
1) return the mixer you have and rent something like this:
2) if you are going to have a heavier load on the slab, consider digging a 12 or 18" grade beam all the way around the perimeter with a couple of #6 bars at the bottom.