The builder I used to haul materials for used an "L" shaped galvanized bracket to secure posts when the posts were not dug into the ground. On most of the job sights I observed, the anchor bolts were placed in specific locations, then a treated mud sill was laid down then the post was set with brackets on either side. The skirt board was then nailed to both the sill plate and the posts.
That was a few years ago and I have no idea what those brackets cost. I would spend some time searching on the net first.
As discussed here I believe and on other forums, ACQ treated wood is corrosive to fasteners. If you go with drilling and using expansion anchors, find something in stainless if you can (more bucks).
Perhaps you could either find some heavy angle or weld up some plate steel yourself or have a shop do it. Except for the reaction with the ACQ, as long as you eliminate dampness, fairly thick brackets with a good paint job should last fairly long.
There is a material available similar to ice dam for building decks with ACQ. Even with stainless or galvanized hardware, at least one builder that I read about puts a piece of that membrane between all of his fasteners and the wood, the idea to negate deterioration. Standard stick construction on the slab would maybe be more cost effective as someone suggested.
Setting the posts outside of the existing slab would cost you in wasted materials if the slab is laid out based on center to center dimensions. A lot of figuring would be required when ordering too. I agree that going larger may be the key to using the existing pad. That way, you can stay with center to center measurements.
