Introduction to Bleeding of Concrete | types and factor affecting | how to reduce bleeding of concrete |

What is Bleeding of Concrete

Concrete is a mixture of cement, fine aggregate and course aggregate. A number of problems can occur in concrete due to improper mixing, curing and placing, one such problem is "bleeding" in concrete.

Bleeding in concrete is a phenomenon in which free water in the mixture rises to the surface and forms a paste of cement on the surface known as "laitance".

Bleeding occurs in concrete when the course settles and the free water rises to the surface. This upward movement of water moves continuously from the bottom to the top, forming a continuous channel. If a water cement ratio of 0.6 or more is used, the bleeding channel will continue. These continuous bleeding channels are often responsible for permeability in the structure. In the process of upward motion, water accumulates under the aggregates and creates water deflections and reduces the bond between the aggregates and the paste. Similarly, water that accumulates under reinforcing bars reduces the bond between reinforcement and concrete. The bleeding process is a normal phenomenon if it is at normal rate but can make the ligament weaker if it is at a higher rate. Bleeding is a type of correlation, in which water comes out of the concrete.

The largest factor in bleeding water rates is the water-to-cement ratio. High proportions can lead to excessive bleeding. Cement rate and fine aggregate can play a role in determining the bleed rate. The lower the penalty in your mixture, the greater the bleeding. Factors also include the height and pressure of the concrete. The relationship between bleed water and concrete heights begins as linear, but eventually becomes unavoidable at elevated heights.

There are different types of bleeding in Concrete

-Normal bleeding refers to the uniform leakage of water over the entire surface of the structure.

-Channel bleeding refers to the water coming from specific paths.

Not all bleed water will reach the concrete surface. Some bleed water may rise and be trapped under aggregates and reinforcements. This weakens the bond between the paste and those elements.

This goal is not necessarily to eliminate bleed water, but to manage it to ensure the quality of concrete. By allowing free water to drain to a surface and evaporate, the water / cement ratio of the structure is reduced, thus decreasing the capillary pore and increasing its density and durability. It can also be useful to aid in finishing tasks and reduce plastic shrinkage cracks.

In addition, it is important that most bleeds do not begin finishing work before evaporation occurs. Working the water back into the mixture will increase the ratio of water / cement to the top surface. This may also increase permeability. If this surface is exposed to traffic or aggressive environments, it can cause premature delimiting, blistering, and cracking.

Bleeding rates may be reduced by the use of complementary cemented materials, especially when finer mixtures are used. Fly ash can be effective in reducing bleed rate. Silica fume has the greatest effect on reducing bleeding.

Micro fibers used in concrete have also been shown to slow bleed rates as they control the speed of water migration to the surface, preventing the disposal of solid particles.

Factors affecting bleeding of concrete

The bleeding rate will be influenced by the drying conditions (windy and cold days). The settlement of concrete is very fast in summer and slightly slower during winter season.

The slowing rate of concrete leads to a prolonged period of concrete bleeding, increasing the risk of plastic cracking.

Concrete Mix Design - Mix design in concrete can be adjusted to pumping concrete to reduce segregation or bleeding.

Reduction in fines increases the bleeding characteristics of concrete.Sands deficient in fines also tend to increase the bleeding characteristics of the concrete.

Careful proportions are required to reduce bleeding, leaving behind penetration into the concrete. Due to the greater initial setting period, the concrete remains fresh and may increase bleeding.

Flowing concrete is susceptible to isolation and bleeding, requiring that the mix design and proportions take into account the use of superplastic.

Plastic disposal cracks and plastic shrinkage cracks are related to the rate and extend that are fresh concrete bleeds.

How to reduce bleeding of concrete.

1) Add a minimum amount of water to the concrete mix, use a chemical mixture to reduce the water demand for the required workability.

2) Design the concrete mix properly.if the mixing of concrete is not proper it also trends to bleeding of concrete therefore the mixing of concrete should be proper.

3) Use fly ash or other supplementary cemented materials.using of fly ash or other supplementary cemented material can reduce the bleeding of concrete.

4) It is very effective to use air entrants to reduce blood.air entrants are very effective for drying of bleeding water.

5) Add more cement to the mixture.addition of cement can be absorb the bleeding water.

that's why it is very effective.

6) If the sand coarser (fine modulus of 2.5 to 2.8 best compatibility) increase the amount of fine collected in the mixture and decrease proportionally.

Appropriate measures should be taken during the preparation of the mixture and method of mixing to reduce bleeding as this may cause weak structure and delay in the manufacturing cycle.