Bleeding in Concrete

Bleeding in Concrete

Bleeding is a form of segregation where some of the water in the concrete tends to rise to the surface of the freshly placed material. This arises due to the inability of the solid components of the concrete to hold all of the mixing water when they settle downwards (water being the lightest of all the mix constituents). Bleeding of the water continues until the cement paste has stiffened enough to end the sedimentation process.

Remixing of the bleed water during the finishing of the top surface must be avoided as this will result into a weak top surface. To avoid this, the finishing operations can be delayed until the bleed water has evaporated. Conversely, if evaporation of the surface water is faster than the rate of bleed then plastic shrinkage cracking may occur.

Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete. In thin members like roof slab and when concrete is placed in sunny weather excessive bleeding is bound to occur.

Due to bleeding, water comes up and accumulates at the surface. Sometimes, along with this water, certain quantity of cement also comes to the surface. When the surface is worked up with the trowel, the aggregate goes down and the cement and water come up to the top surface. This formation of cement paste at the surface is known as “Laitance” and this causes the top surface of slabs not to have good wearing quality.

Water while traversing from bottom to top, makes continuous channels. If the water cement ratio used is more than 0.7, the bleeding channels will remain continuous and un- segmented. These continuous bleeding channels are often responsible for causing permeability of the concrete structures. While the mixing water is in the process of coming up, it may be intercepted by aggregates. The bleeding water is likely to accumulate below the aggregate. This accumulation of water creates water voids and reduces the bond between the aggregates and the paste.

The above aspect is more pronounced in the case of flaky aggregate. Similarly, the water that accumulates below the reinforcing bars reduces the bond between the reinforcement and the concrete. The poor bond between the aggregate and the paste or the reinforcement and the paste due to bleeding can be remedied by re vibration of concrete. The formation of laitance and the consequent bad effect can be reduced by delayed finishing operations.

Bleeding rate increases with time up to about one hour or so and thereafter the rate decreases but continues more or less till the final setting time of cement.
  

 Prevention of Bleeding in concrete

• Bleeding can be reduced by proper proportioning and uniform and complete mixing. 

• Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for the water to traverse. 

• Bleeding can be reduced by the use of finer cement or cement with low alkali content. Rich mixes are less susceptible to bleeding than lean mixes.

The bleeding is not completely harmful if the rate of evaporation of water from the surface is equal to the rate of bleeding. Removal of water ( after it has played its role in providing workability) from the body of concrete by way of bleeding will do good to the concrete.

Early bleeding when the concrete mass is fully plastic may not cause much harm because concrete being in a fully plastic condition at that stage will get subsided and compacted. It is in the delayed bleeding state and when the concrete has lost its plasticity that undue harm is caused to the concrete. Controlled re vibration may be adopted to overcome the bad effect of bleeding.