Why Does Delamination Occur?
Delamination occurs when the fresh concrete surface is sealed by troweling while the underlying concrete is plastic and bleeding or able to release air. Delaminations form fairly late in the finishing process after floating and after the first troweling. Rapid evaporation of bleed water due to surface drying (wind, sun, or low humidity) makes the surface appear ready to trowel while the underlying concrete is plastic and can still bleed or release air. Vapor barriers under slabs force water to rise and compound the problem. The use of fly ash and chemical retarders will delay initial set of the underlying concrete and allow bleed water and air to move upward after the surface is sealed. Entrained air reduces bleeding and promotes early finishing which will produce a dense impermeable surface layer. A cool sub-grade delays set in the bottom relative to the top. Air and water collect under the dense surface layer during finishing. Delamination is more likely to form if: • The underlying
