Q.: We plan to pour a 4-foot-square slab that's surrounded by a perimeter wall on three sides and an existing slab on the fourth. We plan to match the grade of our new slab with that of the existing slab. Strips of material for isolation (expansion) joints have been placed along the entire perimeter of the slab to be poured, including along the walls and existing slab. My first question: Should we drill holes, fill them with epoxy and insert reinforcing bars into the existing walls and slab, or should we stop the bars short of the walls and slab to allow the new slab to move independently (float)? My second question concerns the footing for the perimeter walls, which is so high the new slab will rest directly on it. Should we place isolation strips on top of the footing, underneath the slab, to control cracking?
A.: We advise floating the slab, making sure to first thoroughly compact the subgrade, base course or both. Most details we've seen recommend not placing the slab directly on the footing, but if you've got enough vertical clearance, we think a ¼-inch layer of sand would work better than isolation-joint material. If you don't have the clearance, a layer of 30-pound roofing felt should work. The idea is to reduce restraint so your slab doesn't crack.
That's why you shouldn't dowel into the perimeter walls and glue the bars with epoxy. Restraint increases the chance that your slab will crack. However, the concrete may crack anyway because the bars also restrain shrinkage. But if the slab does crack, the bars will keep the crack from opening too wide and help maintain aggregate interlock so you don't get faulting at the crack.