Segregation and bleeding; control during placing

Segregation is the separation of the constituents of a fresh concrete mix, so that the mix is no longer homogeneous: the coarse aggregate separates from the mortar, or the paste separates from the aggregate. It results in honeycombing, weak and non-uniform concrete.

Bleeding is a form of segregation in which part of the mixing water rises to the surface of freshly placed concrete as the heavier solids settle. It forms a water layer (and laitance) on the surface, and water pockets under aggregate and bars, weakening the concrete.

Controlling segregation during placing

1. Limit the free fall of concrete to 1.5 m; use chutes, drop pipes (elephant trunk) or a tremie for deeper placing.
2. Place concrete as near to its final position as possible and do not use a vibrator to move it over long distances.
3. Place in layers of 300-450 mm and compact each layer promptly; avoid over-vibration.
4. Use a well-graded aggregate, a suitable cohesive mix with enough fines, and not too high a slump (use superplasticiser instead of water); consider air entrainment.
5. Keep chutes at proper slope (and use baffles at the end) and avoid discharging against formwork or reinforcement.
6. Use tight formwork that does not leak the paste; make a barrier or hopper at the discharge end to receive concrete.
7. Re-mix if segregation has occurred before placing.