Importance of Bleeding Concrete

Segregation

• The three distinct components of concrete have distinct and unique properties, including their weight. The aggregate component, which is usually sand, gravel and crushed stone, is the heaviest of the three, while water is the lightest. Properly mixed concrete will incorporate all three components into a homogeneous consistency, but the natural tendency of these components is to separate. The heaviest component, the aggregate, tends to sink toward the bottom, while the lightest component, water, rises toward the surface. This tendency to separate is called segregation.

Bleeding

• The form of segregation in concrete that involves water gain, or water accumulation, is sometimes referred as bleeding. During the bleeding process the water in the concrete mixture migrates upward. The water may emerge onto the surface, or it may become trapped below the surface by aggregate or reinforcement materials, where it is referred to as internal bleeding.

Complications

• Internal bleeding can create water voids beneath the aggregate where it accumulates. This may prevent the aggregate from forming a sturdy bond with the cement paste. The water involved in external bleeding may carry cementitious material to the surface, forming laitance, which is primarily a layer of cement and water with little aggregate. This laitance surface does not wear well and may break down under exposure, resulting in flaky dust during dry seasons and a muddy residue during wet seasons. Excess bleeding, whether internal or external, can result in weaker concrete.

Considerations

• All concrete will bleed to some extent, and bleeding that occurs at the same rate as water evaporation will do little harm to the finished concrete. You can reduce the effects of bleeding by using the proper proportions of water, aggregate and cement and by thoroughly mixing these components together.