Q.: We recently installed a 26,000-square-foot slab-on-grade that had a colored dry-shake hardener applied at a rate of 2 pounds per square foot. We have experienced numerous spots of delamination. We are beyond discussing the cause and are trying to arrive at a fix other than total slab replacement. Do you know of a way to repair a delaminated dry-shake hardener?
A.: We spoke to Bob Gulyas of Master Builders Inc., who suggested four different ways to repair the delaminated areas, depending on their size.
If the delaminations are small blisters, it may be possible to rebond the dry shake with an epoxy adhesive. This has the advantage of maintaining the original surface finish and color, making the repair less noticeable. First make sure the substrate surface is free of moisture, laitance and carbonated paste. Then have an experienced epoxy-injection applicator inject a low-viscosity epoxy adhesive into the blister through a small inlet port. Drill an exit port at the opposite edge of the blister to allow air and water (if present) to escape when the epoxy is injected. Inject until clear epoxy comes out the exit port. Avoid high injection pressures, which may "pop" the blister, requiring replacement of the shake in that area. Done correctly, this method will fill the cavity in the blister and rebond the dry shake to the substrate.
Another option for very small delaminations or blisters is to remove the delaminated area to a depth of 1/4 inch (a coring machine works well) and replace it with dry-shake hardener mixed with water. After removing the delaminated area, remove any laitance or contaminants and blow dust off the surface. Coat the substrate with a paste-type epoxy bonding agent. Mix the dry shake until it forms a thick putty, and trowel it on the surface. Then apply a liquid-membrane curing compound. Cure the surface well because this type of material may craze. Areas larger than 12 inches across can be repaired by sawcutting around the perimeter of the delaminated area to a depth of about 1 inch and removing the concrete with light chipping hammers. After removing the concrete, sandblast or shotblast the surface, blow off all dust, then dry the surface with a blower or heat lamp to remove residual moisture. Apply a paste-type epoxy bonding agent, fill the area with a pea-gravel concrete mix and apply the dry shake. Gulyas says you may need to apply the dry shake in three passes instead of two, but there should be plenty of water in the pea-gravel mix to saturate the hardener.
Because drying-shrinkage cracking is a concern for thin toppings, Gulyas recommends applying a liquid-membrane curing compound and covering the repair with wet burlap for two or three days. You may have to predry the surface with a heat lamp to get a more uniform color after curing. If a large portion of a slab panel is affected, the best repair is probably to remove the entire panel, repour the floor and apply the dry shake according to the manufacturer's recommendations. This will minimize the color differences between the new and existing floor. Before placing the concrete, be sure to check the air content -- it should be less than 3%.