Concrete out gassing is one of the primary reasons for pinholes in self leveling cements and bubbles or fish-eyes in resinous flooring systems. Concrete is a porous substrate which breathes and absorbs liquids when open to the environment. Out gassing is caused by a difference in the temperature and humidity of the air versus the concrete. When the air temperature is warmer than the slab, especially in a low humidity, vapor will leave the concrete slab. This problem can be avoided by applying a good quality penetrating primer prior to the application of self leveling concrete or resinous flooring systems. The most common types of primers are based on acrylic or epoxy based. A primer not only prevents the concrete substrate from out-gassing but also helps in improving the adhesion of the flooring system.
For self leveling cements usually acrylic / styrene acrylic based primers are used. Sometimes epoxy based primers are also recommended. If you are using a acrylic / styrene acryclic based primer, please follow the application instructions given below –
a. Shake well to ensure primer is thoroughly mixed and there is no sediment in the container bottom. Add 1 part primer to 1 part water in a clean container.
b. Apply primer to substrate and work into the concrete surface with a stiff bristle push broom. Spread evenly to thoroughly coat surface and avoid any puddling. Maintain material, surface and ambient temperatures above 50F.
c. For excessively porous and absorbent concrete surfaces like gypsum sub floors, double prime the surface with an initial application of primer diluted 3 parts water to 1 part primer, wait 3 hours, then prime normally.
d. Let the primed surface dry to tacky, transparent thin film (min 3 hours to 24 hours max.). Lower temperatures and/or higher humidity will extend required minimum dry times. When the primer dries completely, it is time to pour the self leveling cement.
e. If you are polishing self-leveling concrete toppings, then it is a good idea to primer with epoxy primers and broadcast 30 mesh clean dry sand before installing self-leveling concrete topping.
You can watch the entire installation process HERE.
