Bleed water carries cement and fine particles to the top of fresh concrete, where they harden into a thin, weak, often dusty layer. Sealants and membranes bonded to laitance fail with it. Grinding, wire brushing, abrasive blasting or shot blasting removes it to expose sound concrete. Joint faces in precast and tilt-up are ground for the same reason.
Joint faces are all laitance, grind them back to hard concrete or the sealant will come off with a skin on it.
Also heard as
- cement skin
- weak surface layer
Related terms
CSP
Concrete surface profile, a 1 to 10 scale of how rough a prepared concrete surface is.
Substrate failure
The joint face itself breaking, so sealant comes away with a skin of concrete, coating or EIFS still attached.
Adhesive failure
A sealant pulling cleanly away from one joint face, leaving the substrate bare.
Two-rag wipe
Cleaning a joint face with a solvent-wet cloth followed immediately by a dry cloth, before the solvent evaporates.