A common cause of delamination of concrete slab is hard trowel finishing an air-entrained concrete slab.  In such situation, the following four distinct microstructural zonation form due to hard troweling:

• A dense, dark, hard, low water-cement ratio (w/c), low (to literally no) air surface zone to a depth of 1/8 to¼ in. that is enriched in mortar fraction and low in coarse aggregate fraction (the “densified surface layer” or “DSL”)

• A transition zone of gradually decreasing air and increasing paste content from the interior toward the surface; the zone has a depth-wise variation of air content, which is higher than the air in the DSL but lower than the interior air, and a w/c similar to that in the interior trowel-unaffected concrete (this transition zone is the “distorted layer” or “DT”; it extends to a depth of ¾ to 1 in., and contains four characteristic microstructural features – elongated or stringy voids; tears or short, discontinuous separations in the paste; aggregate-paste bond distortions; and very steep air and paste profiles, Figures 1, 3, 4, and 5);

• The plane of delamination, or a long, continuous, near-horizontal surface-parallel separation between the trowel-affected DSL plus DT zone and the trowel-unaffected interior concrete (Figures 1, 2, 5, and 6) that stays within the top 1 in. of the surface and is less than ¼ in width; and finally,

• The trowel-unaffected interior concrete with more or less uniform air, paste and w/c profiles (Figures 1 to 6; “as placed concrete” or “APC”)