Coating Concrete with High-Performance Protective Linings: <i>Understanding Mechanisms of Outgassing-induced Film Defects</i>

Abstract Ordinary portland cement concrete is the most predominately used construction material for the building of chemical containment and immersion service structures in the municipal water and wastewater treatment industry. Protecting concrete with high-performance protective linings is crucial for preventing the deterioration of water and wastewater treatment structures from chemical attack, chloride ingress, and carbonation. The application of high-performance protective linings to hardened concrete—which includes shotcrete and cementitious repair mortars—can be a challenge due to its inherently porous structure full of tiny capillaries and air pockets, as well as bugholes when casted. Concrete outgassing is the release of air and vapor pressure from these pores and can lead to defects such as craters, bubbles, and pinholes in the wet film of a protective lining. This adverse phenomenon is typically caused by a temperature change within or near the concrete surface during application of the protective lining. Rising temperatures cause air and water vapor to expand and emit from concrete passing through the wet lining before it cures, ultimately leading to film defects. This paper will describe the mechanisms causing concrete outgassing and explore effective application methods to mitigate its impact on high-performance linings when applied to concrete and other cementitious substrates.