Q.: Our firm was hired to repair leaks in the concrete roof of a college building in North Carolina. The roof doesn't have many cracks, but most of the joints are leaking where PVC (polyvinyl chloride) waterstops placed in the joints have separated from the concrete. We're seating the waterstops in epoxy, rebuilding the joint faces, placing foam backup rods in the joints and then sealing them with an elastomeric sealant. Because the roof deck is regularly used by pedestrians, we were also asked to restore the deteriorated concrete surface. With two scabblers we spent a week removing the top 1/4 inch of concrete, but the deck still produced a hollow sound when we pulled a chain across it afterwards. We think it would take too long to remove the entire 4-inch surface slab with a scabbler or a planer. With a jackhammer, we're afraid we'd damage the waterproofing membrane between the bottom structural slab and the top surface slab. Are there any other ways of removing concrete that would be practical?

A.: Instead of removing the old concrete we're also considering placing an unbonded concrete overlay reinforced with polypropylene fibers. We can increase the elevation 1 to 2 inches without problems, but can an unbonded overlay be placed this thin? Should we construct joints in the overlay closer together than the 24-foot joint spacing of the existing slab, so that flexural cracks don't occur? What materials can we use to break bond?

Scarifiers or water blasting equipment are other tools that might be used to remove the surface of a concrete slab to a specified depth. Manufacturers of such equipment are listed under the headings, "Scarifying Machines" and "Scarifying Tools," in either the Concrete SourceBook or the December Buyers' Guide of CONCRETE CONSTRUCTION.

According to ACI 302.1R-80, Guide for Concrete Floor and Slab Construction, "[an unbonded] topping must be a minimum of 2 1/2 inches thick, but thicker toppings are recommended." Thus you ought to devise some way of accommodating a topping 2 1/2 inches thick. Plastic sheet, felt paper, a layer of sand or a chemical bond breaker can be used to prevent bond to the base slab. Manufacturers of chemical bond breakers are listed in the Concrete SourceBook and the December Buyers' Guide of CONCRETE CONSTRUCTION under "Bond Breakers." To control cracking in slabs, American Concrete Institute Committee 302 has recently agreed to adopt a new rule of thumb in place of fixed joint spacings. This rule is that the joint spacing should be twice the distance in feet of the thickness in inches. Thus a 2 1/2-inch topping would require joints every 5 feet in both directions. Since 5-foot spacings wouldn't fit well into the previous 24-foot spacing pattern, you would probably want to reduce the spacings to 4 feet. There is a current controversy about the effectiveness of polypropylene fibers in preventing cracking. Whether joint spacings can be increased if polypropylene fibers are used depends on what credence you give to the two sides of the controversy.