Bleeding

Summary

Bleeding - is a form of segregation where water rises to the top of freshly placed, curing concrete.

Technical Information

Bleeding occurs when the water in a concrete mix rises to the surface due to the fact it has a lower density than the cement and aggregates in the mix.

The longer the concrete takes to set, the more time there is for bleeding to take place.

Bleeding can be reduced by using very fine aggregates and fine cement replacing materials, reducing the water content in the mix or by using an air entraining agent.

Bleeding can be beneficial for the following reasons:

Lowers the water/cement ratio in the concrete and densifies it.
Hydrates dry-shake toppings.
Reduces the volume of water required for curing.

Bleeding nearly always occurs in concrete floor slabs, it is important to adequately control the rate of bleed to ensure it is neither too fast nor slow. Bleeding may not be physically seen in hotter countries as the water will evaporate at the rate it’s expelled.

Excessive bleeding can cause channels to form at the coarse aggregate surface which can lead to a weaker paste-aggregate bond and if the surface of the concrete is finished before bleeding is complete, delamination can occur.