Segregation is the “Separation of constituent materials in concrete.” 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)
Concrete is a mixture of cement, fine and coarse aggregates. Good concrete is one in 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.
Some other factors causing segregation in concrete:-
- Transporting concrete mixes for long distances.
- Poorly proportioned mix, where sufficient matrix is not there to bind the aggregates
- Dropping concrete from more than 1m.
- Vibrating concrete for a long time.
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.
In 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; the design mix must be such that the 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
.Honeycombs are hollow spaces and cavities left in concrete mass on the surface or inside the concrete mass where concrete could not reach. These look like honey bees nest.
Honeycombs which are on sides are visible to naked eyes and can be detected easily as soon shuttering is removed. Honeycombs which are inside the mass of concrete can only be detected by advanced techniques like ultrasonic testing etc.
Honeycomb is due to non-reaching of concrete to all places due to to which cavities and hallow pockets are created,
main reasons are:
Honeycombs are hollow spaces and cavities left in concrete mass on the surface or inside the concrete mass where concrete could not reach. These look like honey bees nest.
Honeycombs which are on sides are visible to naked eyes and can be detected easily as soon shuttering is removed. Honeycombs which are inside the mass of concrete can only be detected by advanced techniques like ultrasonic testing etc.
Honeycomb is due to non-reaching of concrete to all places due to which cavities and hallow pockets are created, main reasons are:
1) Improper vibration during concrete.
2) Less cover to reinforcement bars
3) Use of very stiff concrete (this can be avoided by controlling water as per slump test).
4) Places like the junction of the beam to beam to column and to one or more beams are the typical spots where honeycombs are observed. This is due to jumbling of reinforcement of beams and column rods at one place; special attention is required at such place during concreting and vibrating.
5) Presence of more percentage of the bigger size of aggregate in concrete also prevents concrete to fill narrow spaces between the reinforcement rods.
