We supplied 4x24-foot hollowcore slabs for a building floor that received a 2-inch-thick concrete topping placed by the concrete contractor. Part of the building was left unfinished for later use, and, when it was built out, the owner noticed a crack that was visible on the topping surface but not on the underside of the hollowcore members, which are visible from the floor below. The relatively straight crack was parallel to the longitudinal dimension of the hollowcore slab and was up to 0.06 inch in width, up to 2 inches deep, and at least 20 feet long. It ran under a partition wall that had just been built, so I couldn't get a more accurate length estimate. The owner is concerned that the crack was caused by a structural overload during the build-out process and wants assurances that the building doesn't have a structural problem. What do I tell the owner?
This kind of crack may be caused by drying shrinkage of the topping, which often is placed at a fairly high slump and water content. The cracks normally appear over the joints between hollowcore slabs. Service loads are a less likely cause. Welded wire fabric or closely spaced transverse reinforcing bars 1 inch below the surface would have helped to control crack width, but it's difficult to keep this reinforcement in its proper position. If the topping is not reinforced, cracks up to 0.06 inch wide wouldn't be unusual. It's also possible that the crack was caused by differences in elevation of the adjacent hollowcore members. When a topping slab is used over precast members, ACI 117-90, "Standard Specifications for Tolerances for Concrete Construction and Materials," allows up to a 3/4-inch offset of top surfaces of adjacent elements in an erected position. If the topping thickness between elements varied within this acceptable tolerance, it might cause the cracking you describe. Alternatively, if there's an expansion joint in the wall and the crack coincides with the joint location, it might indicate that building movement caused the crack. Toppings serve to distribute concentrated loads and may also provide shear transfer across the hollowcore units. To allay the owner's concern about a structural problem, a structural engineer could determine the most likely cause and decide if the crack adversely affects the topping's ability to perform its function. Drying shrinkage, uneven elevation of hollowcore members, or building movement are likely causes of a crack in the topping on hollowcore members. A structural engineer could determine whether the crack indicates a structural problem.