Bleeding in concrete

Bleeding (water gain) is the rise of mixing water to the surface of freshly placed concrete, caused by the settlement of the heavier solid particles. It is a form of segregation.

Why and how it occurs

1. The solid particles (cement, aggregate) are heavier than water, and the mix cannot hold all the mixing water in suspension after placing and compaction.
2. The solids settle under gravity, and the displaced water moves upward through channels (and along aggregate and steel surfaces).
3. The water collects at the top surface (or as pockets underneath aggregate and reinforcement). Bleeding is greater with a high w/c, coarse cement, a poorly graded mix, a lack of fines, thick sections and over-vibration.

Influence of bleeding rate

High bleeding

• A weak, porous, dusty top layer (laitance) with poor wear resistance, and a poor bond to the next lift.
• Water gaps below aggregate and bars: weak bond, lower strength, and permeable paths.
• Higher w/c at the top, plus plastic settlement cracking.
• Scaling and low durability.

Low bleeding

• Generally good, but if the surface dries too fast (hot, windy weather), plastic shrinkage cracks occur because bleed water cannot replace the evaporated water.
• Concrete with very low bleeding is hard to finish if finishing is delayed.

Control

Use a lower w/c, finer cement, more fines, air entrainment, well-graded aggregates, proper compaction and pozzolanic admixtures, and delay finishing until the bleed water has gone.