Q. We recently placed a 3-inch-thick overlay for a small heated indoor parking area. We first routed existing cracks and filled them with an epoxy material. Next, we shotblasted the existing concrete surface, then broomed in a proprietary bonding agent. Then we placed 5000-psi pea gravel having a 4- to 5-inch slump. The owner observed our work very carefully and was pleased that no cracks had appeared a month after we did the work. We were called back a month after the work was done, however, because the edges of the overlay had debonded. Also, cracks had formed perpendicular to the edges over about half of the overlay perimeter. The cracks are about 2 feet long and are spaced 4 to 8 feet apart. We plan to remove and replace a 2-foot-wide section of the overlay around the perimeter, but we don't want to make the same mistake twice. What could have caused the loss of bond and cracking?

A. There are several possible causes for the debonding. If too much time passes between applying the bonding agent and placing the overlay, the bonding agent could act more like a bond breaker. Another possible cause is excessive drying shrinkage of the overlay concrete. When the heat came on in the garage, drying and shrinkage of the concrete was accelerated. Pea gravel concrete is more shrinkage-prone because it requires larger amounts of mixing water than concrete with 3/4-inch aggregate or larger. More water means more drying shrinkage. The shrinkage causes curling that can "unzip" the overlay at the edges. It also causes cracking.You didn't mention curing, but that's a critical factor affecting overlay performance. According to Bill Perenchio of Wiss, Janney, Elstner in Northbrook, Ill., concrete bond strength doesn't develop as fast as compressive strength. If curing is ineffective or is discontinued too soon, loss of bond is likely. Perenchio says the only sure way to cure overlays is with water applied externally, followed by wet burlap covered with plastic sheeting. He says a curing compound isn't enough to ensure proper curing.We wouldn't recommend removing and repairing concrete at the overlay edges until you see whether the rest of the concrete stays bonded. Use a chain drag or soundings with a hammer to find out how far back from the edge the concrete has debonded. Then check it again in a month or two. If debonding is progressive, you may need to replace all of the overlay.

Reader Response

I recently repaired a debonded, 3-inch-thick overlay that also was severely cracked due to high shrinkage. The debonded sections were identified with a chain drag. One of the most successful repair methods we used was to drill small holes into the debonded sections and pump a low-viscosity epoxy between the overlay and the underlying slab. This worked very well and resulted in a completely bonded overlay. The overlay was at least 1 year old, so almost all of the shrinkage had taken place. The cracks were filled with sand and epoxy afterwards. The above method was not cheap, but it was less expensive than removing the overlay, cleaning the floor, and recasting the concrete. Also, reuse of the slab was a matter of a couple days, not 28 days.

C. Kuilman, P.E.
K. C. Design & Construction Management