The duality of advantages and disadvantages of bleeding and the harmfulness of segregation

While bleeding occurs, the mixture itself is also settling and shrinking. As a result, the water content of the surface mixture will continue to increase before initial setting, and the water-cement ratio will increase, resulting in a decrease in the strength of the concrete surface and the formation of a layer of “dead skin” that is easy to peel off. During the bleeding process, pore channels are formed, which reduces the strength, impermeability and frost resistance of the concrete. At the bite interface of the formwork, the slurry will be taken away, leaving only sand and gravel, and the aggregate will be exposed. However, concrete bleeding has both advantages and disadvantages. A small amount of bleeding is beneficial. Controlling no agitation during bleeding and no large amount of evaporation can reduce the actual water-cement ratio of the mixture, prevent surface dryness, and prevent the occurrence of plastic cracks. It plays a moist and protective role.
Serious bleeding of concrete may lead to segregation, but the two cannot be confused. Segregation refers to a state in which the components of the mixture separate from each other, causing unevenness and loss of bonding and compactness. Concrete segregation often occurs in the following two situations: one is that the coarse aggregate easily slides down the chute or sinks in the steel formwork due to its specific gravity, gradation and other characteristics during self-unloading or transportation; the other is that the slurry The body is lost from the surface of the mixture, and the phenomenon of bottom grabbing occurs. Often occurs in pumped, high-flow concrete. As a result, the homogeneity of the concrete deteriorates, the surface water-cement ratio increases rapidly, the overall density, grip strength, and bite force are insufficient, and the strength decreases, posing structural safety risks. </p