Causes and Prevention of Segregation
separation of the constituents of a heterogeneous mixture so that their distribution is no longer uniform.
In the case of concrete, it is the differences in the size of particles and in the specific gravity of the mix constituents that are the primary causes of segregation, but its extent can be controlled by the choice of suitable grading and by care in handling.
It is worth noting that a higher viscosity of the fresh cement paste component militates against the downward movement of the heavier aggregate particles; consequently, mixes with low water/ cement ratios are less prone to segregation.
There are two forms of segregation.
In the first, the coarser particles tend to separate out because they tend to travel further along a slope or to settle more than finer particles.
The second form of segregation, occurring particularly in wet mixes, is manifested by the separation of grout (cement plus water) from the mix.
With some gradings, when a lean mix is used, the first type of segregation may occur if the mix is too dry; addition of water would improve the cohesion of the mix,but when the mix becomes too wet the second type of segregation would take place.
The actual extent of segregation depends on the method of handling and placing of concrete.
If the concrete does not have far to travel and is transferred directly from the skip or bucket to the final position in the form, the danger of segregation is small.
On the other hand, dropping concrete from a considerable height, passing along a chute, particularly with changes of direction, and discharging against an obstacle – all these encourage segregation so that under such circumstances a particularly cohesive mix should be used.