Laitance

Laitance is the weak, porous, fine-particle layer that forms on the surface of concrete due to bleeding (upward migration of mix water carrying cement fines). The laitance layer typically 1-3 mm thick is composed mainly of cement paste, fine aggregate, and excess water — and has very low strength (often < 5 MPa) and high permeability. Per IS 456:2000 and standard construction practice, laitance must be removed before any concrete is poured on top (e.g., at construction joints or before topping), or before any finish is applied (tile, paint, screed).

Formation: laitance forms naturally when concrete bleeds. The surfacing water carries cement fines (smaller than 75 μm) to the surface. As the water evaporates, the fines settle as a weak layer. Heavy bleeding (high w/c, gap-graded aggregate) produces thicker laitance; controlled bleeding produces minimal laitance. Site conditions affecting laitance thickness: (a) ambient temperature and humidity (low humidity = faster evaporation = thinner laitance); (b) wind speed (higher wind = faster evaporation); (c) trowel finishing (early trowelling traps water, increases laitance; late trowelling removes some laitance). Practical Indian sites: 0.5-2 mm laitance is typical; >3 mm indicates excessive bleeding requiring mix design review.

Removal methods: (a) Mechanical — wire brushing, sand-blasting, pneumatic chipping. Required for construction joints where new concrete is to be poured on top — minimum 1.5 mm of laitance must be removed to expose sound concrete. (b) Chemical — surface retarders applied before pour cause laitance to remain workable for 12-24 hours, then washed off with water spray. Used for architectural concrete and bridge deck surfaces. (c) Pressure washing — high-pressure water (15-25 MPa) effectively removes laitance from large areas. Critical site practice: at construction joints (e.g., between column kicker and column body), laitance removal is non-negotiable; otherwise the fresh concrete poor bond compromises shear capacity at the joint. The most-overlooked aspect: laitance removal in tight spaces — beam-column joints, between rebar bars — where mechanical access is limited; chemical retarder + water wash is the typical solution.