Q. Coating on concrete – complete im-permeability to moisture?
In designing protective coating on concrete structures, stoppage of water ingress by the coating is generally required. Since chloride ions frequently diffuse into concrete in solution and cause deterioration of concrete structures, prevention of water transmission into coating certainly helps to protect concrete structure. Though if water gets behind the coating from some means and becomes trapped, its effect may not be desirable. Firstly, vapour pressure will be developed behind surface treatment and this leads to loss of adhesion and eventual peeling off of coating. Furthermorewater creates a suitable environment for mould growth on concrete surface.
As a matter of fact surface treatment must be so selected that it's impermeable to liquid water though it is permeable to water vapour. This 'breathing' function enhances the concrete to lose moisture through evaporation and reject uptake of water at the time of wet periods.