Segregation: forms and control

Segregation is the separation of the constituents of fresh concrete so that it is no longer uniform: coarse aggregate separates from the mortar, or water separates from the mix. It causes honeycombing, weak and non-uniform concrete, poor durability and surface defects.

Forms of segregation

1. Coarse aggregate separating out: heavy coarse particles settle or roll away from the mortar (common in dry, harsh and gap-graded mixes).
2. Paste (grout) separating from the coarse aggregate: in wet, high-slump mixes, the cement paste flows away from the aggregate.
3. Water separating (bleeding): water rises to the surface of a wet mix.

Causes

Poor grading, a too-wet or too-dry mix, excess vibration, dropping from a height, long chutes, rough handling, and differences in the density of the materials.

Control during discharge from a hopper

• Keep the discharge gate close to the receiving surface (free fall not more than 1-1.5 m), with a funnel or baffle.
• Open the gate fully and discharge centrally, so the concrete falls vertically into the middle of the receiving vehicle, not against its side.
• Use a hopper with a bottom sloping at about 60 degrees and a central outlet.

Control during placing

• Place concrete as close as possible to its final position, in layers 300-450 mm thick; do not move it with a vibrator.
• Limit free fall to 1.5 m; use chutes (with baffles), a tremie or drop pipes (elephant trunk) for deeper sections.
• Use well-graded aggregates, a suitable w/c ratio and fines content, or admixtures (air entrainment, viscosity modifier).
• Avoid over-vibration; use the right vibrators; use clean, tight formwork.
• Keep chutes at a suitable slope (about 1 vertical to 2-3 horizontal) so that the concrete neither stops nor shoots; avoid long transport over rough roads, and re-mix if segregation has already occurred.