Segregation & Bleeding in the Concrete | Causes, Effects And Remedies
Segregation in concrete
Segregation is the ( Separation of the constituent materials in concrete ) In the concrete technology, segregation is of 3 types.
- Separation of the Coarse aggregate from the concrete mixture,
- Separation of the Cement pastes from the concrete during its plastic stage.
- Separation of the water from the concrete mix (Bleeding in concrete)
Concrete is a mixture of the Cement, fine & coarse aggregates. A good concrete is 1 which all the constituents are properly categorized to form homogeneous mixture. The primary cause of the Segregation in concrete is the differences in specific gravities of constituents, Specific gravity of the Cement is in between 3.1-3.6g / cc, & for aggregate it lies between 2.6 – 2.7g / cc due to this differences, the aggregate separates from the matrix & causes segregation in concrete.
Some other factors causing Segregation in the concrete.
- Transporting Concrete mixes for the long distances.
- Poorly proportioned mix where sufficient matrix is not there to bind aggregates.
- Dropping concrete from more than one-m.
- Vibrating concrete for long time.
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How to Minimize Segregation in the concrete.
- Segregation can be controlled by maintaining proper proportioning mix.
- By peculiar handling, placing, transporting, compacting & finishing of the concrete.
- Adding air entraining agents, admixtures & pozzolanic materials in the mix segregation controlled to some extent.
Bleeding in Concrete.
Bleeding is a form of the segregation in which water present in concrete mix is pushed upwards due to the settlement of the cement & aggregate. The specific gravity of the water is low, due to this water tends to move upwards. Bleeding ordinarily occurs in wet mix of the concrete.
The Prime factor for bleeding in the concrete is the high dosage of the Water cement ratio. Higher water-cement ratio weakens concrete & leads to excessive bleeding.
The bleeding in the concrete is not harmful if the rate of the evaporation of water is equal to the rate of bleeding. Normal bleeding is quite good & it enhances the workability of the concrete.
When the concrete is fully plastic, bleeding may not cause much harm. However concrete is still in the plastic stage later it is subsidized & compacted.
How to bleeding effects Concrete Stability
- As mentioned above water move to upwards in the bleeding, Sometimes with this water certain amount of the cement moved along with water to concrete surface. When the top surface is worked up with the trowel, the aggregate goes down & cement paste forms at the top surface this is called Laitance in the concrete. Due to formation of the Laitance, structures may lose its wearing capacity & decreases its life.
- Water while moving from the bottom to the top forms continuous channels. Due to this channels the concrete becomes permeable & allow water to move, which forms water voids in the matrix & reduces the bond between aggregate & the cement paste.
- Forming of the water at the top surface of the concrete results in delaying the surface finishing.
- Concrete becomes permeable & loses its homogeneity.
- Excessive bleeding breaks the bond between the reinforcement & concrete.
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How the bleeding in the Concrete is controlled
- Bleeding in the concrete is controlled by Adding minimum water content in the concrete mix.
- Encouraging use of air en-training admixtures in the mix.
- By adding more cement in mix.
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