Segregation in concrete
Segregation can be defined as the separation of the constituent materials of concrete. A good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture. There are considerable differences in the sizes and specific gravities of the constituent ingredients of concrete. Therefore, it is natural that the materials show a tendency to fall apart.
Segregation may be of three types
- Coarse aggregate separating out or settling down from the rest of the matrix.
- Paste separating away from coarse aggregate.
- Water separating out from the rest of the material being a material of lowest specific gravity.
A well made concrete, taking into consideration various parameters such as grading, size, shape and surface texture of aggregate with optimum quantity of waters makes a cohesive mix. Such concrete will not exhibit any tendency for segregation. The cohesive and fatty characteristics of matrix do not allow the aggregate to fall apart, at the same time; the matrix itself is sufficiently contained by the aggregate. Similarly, water also does not find it easy to move out freely from the rest of the ingredients.
The conditions favorable for segregation are:
- Badly proportioned mix where sufficient matrix is not there to bind and contain the aggregates
- Insufficiently mixed concrete with excess water content
- Dropping of concrete from heights as in the case of placing concrete in column concreting
- When concrete is discharged from a badly designed mixer, or from a mixer with worn out blades
- Conveyance of concrete by conveyor belts, wheel barrow, long distance haul by dumper, long lift by skip and hoist are the other situations promoting segregation of concrete
Vibration of concrete is one of the important methods of compaction. It should be remembered that only comparatively dry mix should be vibrated. It too wet a mix is excessively vibrated; it is likely that the concrete gets segregated. It should also be remembered that vibration is continued just for required time for optimum results. If the vibration is continued for a long time, particularly, in too wet a mix, it is likely to result in segregation of concrete due to settlement of coarse aggregate in matrix.
Prevention of segregation:-
- The concrete mix should be properly designed with optimum quantity of water i.e. not too wet nor too dry.
- Make sure the concrete is properly mixed at the correct speed in a transit mixture for at least two minutes. Regularly check the performance of mixer with respect to adequate uniformity of distribution of constituents in each batch.
- Transport the concrete mix correctly. Choose the shortest route for transportation of concrete mix.
- Place the concrete in its final position as soon as possible. Never place a concrete fromlarge heights.
- Formwork should be water tight so that paste should leakage from the forms. Do not vibrate formwork.
- Do not allow concrete to flow.
- Use the vibrator correctly and never use the vibrator to spread a heap of concrete over a large area.
- Vibrate the concrete for just the right time-not too long, not too less.
- Use chemical admixtures such as air entraining agent in the mix. Entrained air reduces the danger of segregation.
- If any segregation is observed in concrete, remixing should be done so to make it homogeneous again.
Bleeding in Concrete
Bleeding in concrete is sometimes referred as water gain. It is a particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete, being of the lowest specific gravity among all the ingredients of concrete. Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete. In thin members like roof slab or road slabs and when concrete is placed in sunny weather show excessive bleeding.
Due to bleeding, water comes up and accumulates at the surface. Sometimes, along with this water, certain quantity of cement also comes to the surface. When the surface is worked up with the trowel, the aggregate goes down and the cement and water come up to the top surface.
Prevention of Bleeding in concrete
- Bleeding can be reduced by proper proportioning and uniform and complete mixing.
- Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for the water to traverse.
- Air-entraining agent is very effective in reducing the bleeding.
- Bleeding can be reduced by the use of finer cement or cement with low alkali content. Rich mixes are less susceptible to bleeding than lean mixes.