Bleeding of Concrete

Bleeding is the upward migration of mix water to the surface of freshly placed concrete due to gravitational settlement of solids (cement and aggregate). All concrete bleeds to some extent — this is normal and harmless. Excessive bleeding (>5% of mix water reaching the surface) is problematic — it leaves a weak, porous top layer of cement laitance, reduces cover-zone strength, and causes shrinkage cracks. Per IS 456:2000, bleeding is controlled through mix design (proper aggregate grading, adequate fines, appropriate cement content) rather than via additives.

Causes of excessive bleeding: (a) high w/c ratio (>0.55) — the most common cause; excess water has no other place to go; (b) low fine aggregate content — too few fines to retain water; (c) gap-graded aggregate — voids cause channels for upward water migration; (d) over-vibration during compaction; (e) high superplasticiser dosage — over-dispersion releases water; (f) freshly washed (over-saturated) aggregates without moisture correction; (g) low cement content. Effects: (a) weak surface layer (laitance) that must be removed before tile, paint, or further concrete is applied; (b) reduced cover-zone strength leading to early carbonation; (c) plastic shrinkage cracks if surface evaporation is rapid; (d) segregation of aggregate from cement.

Mitigation: (1) Proper mix design with adequate fines (Zone II sand) and cement content. (2) Correct moisture in aggregate stockpiles (not freshly washed). (3) Avoid over-vibration; vibrate just until cement paste rises to surface. (4) Cover bleed water — typically wait 30-60 minutes after pour for bleed water to surface, then trowel finish. (5) Cover with polyethylene sheet to prevent rapid surface evaporation in hot/dry conditions. The most-overlooked aspect: trowel finishing too early (before bleed water surfaces) traps the water in the surface layer, creating a porous, easy-to-crack finish. Always wait for bleed water to evaporate or be brushed off before final trowelling.