Guidelines to minimize bleeding in concrete
To form concrete, cement, fine aggregates and rolling aggregates are mixed with specific ratios. If the mixing, polymerization and placement method remain imperfect, various issues will happen. Bleeding is one of these crucial problems. Bleeding means a type of segregation in which water flows out of the concrete.
Concrete bleeding is happened when the course of aggregates likely to set and the free water in the mixture will surge to the surface and create cement paste on the surface known as "milt". The movement of water to ascending direction at the time of bottom-up infiltration produces continuous channels. If the applicable water / cement ratio is equivalent to or over and above 0.6, the purge channels become continuous.
The infiltration into the structure occurs because of these continuous bleeding channels. During the upward movement, water is accumulated under the aggregates and develops water voids as well as reduces the bond among aggregates and pulp.
Similarly, the water is accumulated under the reinforcement bars to reduce the bond among the reinforcement and the concrete. If the bleeding persists at a normal rate, it is not damaging, but a high rate of bleeding results in damaging the bond.
Causes of bleeding:
The bleeding in the concrete mix mainly occurs because of segregation. For segregation, the heavy aggregate particles are settled and water is surged to the surface and formed a layer. This upward movement of water also causes fine cement particles. The upper surface of slabs and pavements will not display greater quality of wear.
Bleeding happens on the concrete surface specifically when the water / cement ratio remains high. The bleeding rate depends on the type of cement used and the amount of fine aggregate.
Influence of bleeding : Bleeding offers the following adverse effects -
1. Uniformity of the concrete is hampered.
2. Penetrability happens in the concrete.
3. The water accumulated under the reinforcement bars reduces the bond among reinforcement and concrete.
4. At the time of bleeding, the storage of water develops voids and reduces the bond among the aggregate and the cement paste.
5. Due to the bleeding, the pumping strength of the concrete is decreased.
6. Bleeding raises the water-cement ratio at the top.
7. The storage of water at the top leads to delay in finishing the surface.
Steps to reduce bleeding:
1) Add minimum water content in the concrete mix, use chemical admixtures to reduce the amount of water required to attain the appropriate workability.
2) Design the concrete mix perfectly.
3) Use fly ash or other cementitious materials.
4) Airborne blends are suitable for reducing bleeding.
5) Add extra cement to the mixture.
6) The amount of fine aggregate should be raised when the sand is coarser (a modulus of fineness of 2.5 to 2.8 is perfect) in the mixture and proportionally reduce the aggregate.
Perfect precautions should be undertaken during the development of the mixture and the mixing method to minimize the bleeding since it causes a breakable structure and an interruption of the construction cycle.