Your concrete slab is poured, but if that slab is going to have hard-wheel traffic on its surface, your concerns may be only starting. The edges of the slab at the joints are frequent places for deterioration with the spalling of these edges. Small hard, non-pneumatic wheels on forklift vehicles and other equipment often place great stress on any unsupported edges.
Bill Perenchio, a Northfield, Ill., concrete consultant and an expert in this area, has written a number of articles on the topic. Perenchio says that even if joint caulking has been injected, joints soon become unsupported due to the opening of the joint as the concrete dries and shrinks.
Perenchio has found one repair method to be particularly effective in dealing with spalled or broken slab edges at joints. The first step is to excavate 1½ inches back from the slab edge at the joint and of an equal depth. The resulting channel should then be cleared of any loose debris and dust before being filled with a mortar composed of low viscosity epoxy and sand. The maximum amount of sand should be used without decreasing the workability of the mortar to a point where it is not usable. This mortar should also be consolidated well to be sure that no air pockets exist.
Because the repair is made above a contraction joint, the joint must be respected. While this can be done by placing an insert into the center of the channel just above the lower part of the joint, contractors usually opt to fill the channel completely, and then return after the epoxy has sufficiently hardened to re-cut the joint with a diamond saw. Because the fresh epoxy is sticky and difficult to finish, the usual technique is to slightly overfill the channel and then return after the epoxy has set, heat it to its softening temperature with a weed burner just above the surface, and scrape off the excess epoxy with an ice scraper.
Due to the resilience of the epoxy over portland cement, the epoxy will deform slightly under point or line loading rather than spall. The newly sawed joint can be filled if desired, but it is not necessary, due to this resiliency. Also, because the mortar contains a large percentage of sand, it has good resistance to abrasion. This repair should last the lifetime of the concrete slab.
“I have used this repair method on several jobs across the country,” says Perenchio. “These resulted in great success.”
Harald Greve, president of Applied Engineering and Technology, Princeton, N.J., recommends doweling at a joint to keep the edges at the same elevation. “Improper curing of the slab can cause curling which makes the edges of the slab—at expansion joints—lift off the soil bearing, in which case wheel loads crossing over the joints will cause something of a diving-board effect,” says Greve. “This edge slab curling will cause one side to go down and up and then the other side to go down and up as the wheel load passes over the slab edges. In this case, the proper repair will be grouting beneath the slab to re-establish the slab bearing and doweling the slab edges together.”
Curled slab edges will lift off the soil bearing along the joint edges and should be grouted to re-establish the proper bearing. If grouting repairs are not made, then cracks will develop longitudinally, parallel to the joint. Cementitious grout of two parts sand to one part cement can be injected below the curled slab edges. This grout injection in combination with doweling will re-establish proper slab performance.
Dowels are set into pockets in the concrete, mid-height on both sides of the concrete slab so that one side is fixed and the other side is, in effect, contained in a sliding sleeve connection. The dowel should be stainless steel and smooth. One side of the dowel is set with a tube around it so that the concrete does not bond to that end of the dowel.
“Some contractors prefer to coat the expansion sleeve side of the dowel with grease to assure that no concrete bond occurs and free expansion contraction movement can take place,” says Greve. “Dowels are placed in the pockets so that the side of the dowel that's fixed can be totally bonded with a high-strength bonding material, like methyl methacrylate mortar, epoxy mortar, or plain polymer mortar. Concrete polymer mortar can be used as a patch. The other side should be free so that it can move laterally in response to changes in temperature.”