Article discusses jobsite factors that can have a pronounced effect on the bleeding characteristics that can lead to delaminations in concrete slabs. For years, the absence of a bleedwater sheen on the surface of a concrete slab has been used as an indicator that the slab is ready for floating and troweling. If premature finishing seals the surface before bleeding has ceased, delaminations may occur. But the absence of bleedwater on a surface isn't a foolproof indicator that bleeding has ceased. If the evaporation rate exceeds the bleeding rate, concrete still can be bleeding even though no water is on the surface.
Although mix proportions and materials control the basic bleeding behavior of a concrete slab, conditions at the top and bottom of the slab modify this behavior. Under some of these variable conditions, the probability of undesirable consequences, such as delaminations, increases dramatically because of changes in bleeding characteristics. Water loss at the top of a slab is affected by environmental conditions such as wind, relative humidity and temperature. It can also be affected by finishing operations. Water loss at the slab bottom is affected by temperature and absorption properties of the subbase. The middle of the concrete isn't exposed to these top or bottom effects, and thus bleeds normally. But layers of differing permeability can create problems as bleedwater moves through the concrete. (Part two of a three-part series. Part one appears in January 1998.)