We are involved in a warehouse renovation project. The new warehouse tenants will be moving heavy machinery and forklifts into the facility, and the structural engineer has determined that the 6-inch-thick concrete floor is not thick enough to support the additional weight. Otherwise, the floor is in good condition. The structural engineer recommends placement of a 2-inch-thick bonded overlay on the floor. Do you have any recommendations on mix proportions for the overlay, especially aggregate sizes? Should the overlay be reinforced? What about joint spacing?
We spoke to Jerry Holland of Lockwood-Greene Engineers and he had several recommendations. As far as mix proportions, it's very important to keep the water content of the overlay as low as possible to minimize shrinkage. Shrinkage of cementitious toppings is a major concern for two reasons. First, thin concrete sections tend to shrink more than thicker sections. Second, the topping will be susceptible to curling because the underlying concrete will not absorb as much water from the overlay as an aggregate base. As a result, the overlay surface will dry faster than the bottom portion of the overlay. As the surface dries, it shrinks, and the overlay curls upward at the edges. This may cause debonding of the overlay. Holland stressed that the mix should contain uniformly graded aggregates rather than gap-graded aggregates. He said that he has had success with similar overlays using a combination of #67 and #89 aggregates (ASTM D 448). Because welded wire fabric would be difficult to handle and position in a 2-inch-thick overlay, use fiber reinforcement. Holland recommended using steel fibers instead of synthetic fibers for these service conditions because they provide more structural reinforcement and would help hold the concrete together in areas where the overlay does not achieve a good bond to the substrate. The overlay joints must be sawed directly over the underlying floor joints, and Holland recommends sawing them full depth. According to Holland, an overlay joint and an underlying joint may begin and end at the same place, but they often are not aligned perfectly along the entire length of the joint. Sawing the overlay joint to its full depth reduces the chances of reflective cracking in the overlay in areas where the joints are not perfectly aligned.