Segregation and bleeding with causes

Segregation is the separation of the constituents of fresh concrete so that the mix is no longer uniform: coarse aggregate separates from the mortar (or, in a wet mix, cement paste separates from aggregate). It gives honeycombing, weak and porous patches and a loss of strength and durability.

Causes of segregation

• Too wet or too dry (harsh) mix, high slump with poor cohesion.
• Poorly graded aggregate: excess of coarse or very large size, lack of fines.
• Dropping concrete from more than about 1.0 to 1.5 m, or letting it slide down a chute.
• Over-vibration, or vibrating a very wet mix; striking against reinforcement or form.
• Long transport on rough ground, or discharging a heap that rolls to the edges.
• Large difference between density of aggregate and mortar (e.g. light-weight aggregate).

Control: well-graded aggregate, adequate cement paste, correct W/C, short free fall, trunk or tremie pipes, proper vibration.

Bleeding (water gain) is the upward movement of mixing water to the top surface of freshly placed concrete as the heavier solids settle. The water collects as a layer on the surface or under coarse aggregate and reinforcement bars.

Causes of bleeding

• High water content / high W/C ratio and high slump.
• Lack of fines in the aggregate, or coarse and poorly graded sand.
• Low fineness of cement or low cement content (less surface area to hold water).
• Prolonged vibration, or deep placing in one lift.
• Low temperature (slower setting gives more time for settlement).

Effects: a weak, porous, dusty surface (laitance) with poor wear resistance; weak bond with the next lift; water pockets under bars and aggregate, which lower bond and make the concrete more permeable. Control: lower W/C, more fines, finer cement, air entrainment, correct vibration, and removal of laitance before the next lift.