Q.: I need to pour a 9-inch-thick paving slab atop a structural slab, but there is a 3-inch cement stabilized sand bond breaker required between the structural slab and the paving slab. How do I keep my formwork in place while I pour the paving slab? What sort of anchoring system can I use on this cement stabilized sand layer?
A.: A clear, succinct answer was provided by Ramon Cook. He says:
- Don’t form atop sand. Period.
- Place the forms on the base slab, level them, place sand inside the forms, level the sand, then put the rebar on sand chairs. If your hardware dealer doesn’t know what that means, get another dealer, but don’t scrimp. Sand chairs cost more than plain ones but so does doing the work over.
- Use poly only if specified; otherwise, don’t bring it up.
- Shoot fasteners through 12-inch-high steel edge forms (can usually be rented) into the base slab at 36 inches on center. But if you are planning to use form-edge screeding equipment, the overturning forces on the side forms will be high, so drill in 5/8-inch diameter bolts on 4-foot centers. There are patented bolt systems with recoverable bolt bodies versus systems where you will lose all of the inserts. Other anchoring systems use wedges or other techniques, and most are recoverable. Some of them can be rented.
- Leveling is the most important step and the biggest problem: Set your form edge to a laser line and fasten it well. All other surfaces can be finished to string or wooden screed. It saves a ton of time to level once. Also remember to properly cure the concrete surface.