This article, the first in a series on delamination of troweled concrete surfaces, discusses how bleeding, surface setting and delamination are interrelated. Delaminations are separations in a slab, parallel to and generally near the upper surface. On floors, they're caused by a buildup of water and air beneath a dense layer of surface mortar. Because the water and air create a weakened zone, traffic causes the surface layer to break away from the base concrete.
Premature troweling often is cited as the cause of delaminations. But although premature troweling can, indeed, produce a dense layer of surface mortar by sealing the surface so air and water can't escape, similar effects can be produced by atmospheric conditions that affect setting and bleeding. When concrete bleeds, the paste and aggregates settle at different rates. Generally, jobsite factors that increase bleeding duration and surface setting rate in slabs increase the probability of delamination.