๐What are bleeding & segregation of concrete?
๐ทANSWER
โ BLEEDING :
โบIt is the tendency of the water to rise to the surface of freshly laid concrete.
This results from inability of the solid material of concrete to hold the all the water mixed fro preparation of concrete and during the process of material downward settling.
If we resort to surface finish in presence of bleeding it would weaken the top surface. Type of cement used also has influence on bleeding process.
โโ Effects of bleeding :
โ Concrete loses its homogeneity.
โ Bleeding is responsible for causing permeability in concrete.
โ It reduces the bond between the reinforcement and concrete when water accumulates below reinforcing bars.
โ In the process of bleeding the accumulation of water creates a water voids and reduces bond between the aggregate and cement paste.
โ Due to bleeding pumping ability of concrete is reduced.
โ Increase in the water-cement ratio at the top.
โโ Ways to reduce the bleeding :
โ Reduce water content. Use lower slump mix.
โ Use finer cement.
โ Increase amount of fines in the sand.
โ Use supplementary cementitious materials.
โ Use air-entraining admixtures.
โ Bleeding lowers the cement strength.
โ SEGREGATION :
โบSegregation is the โSeparation of constituent materials in concrete.โ In concrete technology, segregation is of three types:-
1. Separation of Coarse aggregate from the concrete mixture.
2. Separation of Cement paste from the concrete during its plastic stage.
3. Separation of water from the concrete mix (Bleeding in concrete)
โบConcrete is a mixture of Cement, fine and coarse aggregates.
A good concrete is one which all the constituents are properly categorized to form a homogeneous mixture.
The primary cause of Segregation in concrete is the differences in specific gravities of the constituents, Specific gravity of Cement is in between 3.1-3.6g/cc, and for aggregate it lies between 2.6-2.7g/cc due to these differences, the aggregate separates from the matrix and causes segregation in concrete.
โโ Some other factors causing segregation in concrete:-
โ Transporting concrete mixes for long distances.
โ Poorly proportioned mix, where sufficient matrix is not there to bind the aggregates.
โ Dropping concrete from more than 1m.
โ Vibrating concrete for a long time.
โโ How to minimize segregation in concrete:-
1. Segregation can be controlled by maintaining proper proportioning the mix.
2. By peculiar handling, placing, transporting, compacting and finishing,
3. By using air entraining agents, admixtures and pozzolanic materials in the mix segregation controlled to some extent.
โบIn case of segregation, the heavy aggregate particles settle down leaving a sand cement mix on top affecting the quality adversely.
Fine aggregate could be used to overcome the problems.
However; design mix must be such that required strength could be achieved.
Similarly water ratio may be catered for in design mix to avoid bleeding keeping in view the local material specifications.
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