Concrete is not unlike a human being in the sense that it breathes and gasses. Moisture is always present in concrete and as the substrate heats up, the moisture moves through the capillaries in the form of vapor. Due to the moisture in concrete, it is always looking to reach equilibrium with its environment and creates moisture vapor emissions (MVE).
Prior to the application of any coating the substrate should be tested for MVE. Most of the substrates that we deal with in water and wastewater are located outside and not in a controlled environment. The only test method that has any validity in these scenarios is ASTM D 4263 “Standard Test Method for Indicating Moisture in Concrete by Plastic Sheet Method” and this method is only qualitative, not quantitative. If the surface that you are coating is located in a climate controlled environment then a better choice would be ASTM F 1869 “Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride” or ASTM F 2170 “Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes”.
It is generally accepted in the industry that the MVE rate shall be 3lbs or less per 1,000 sq. ft. in a 24 hour period or a maximum of 85% relative humidity (but always consult the coating manufacturer for exact limitations).
A low viscosity primer should be applied to the substrate for improved penetration, surface wetting characteristics, and adhesion. The surface of the concrete should never be greater than a “Surface Saturated Dry” (free of any standing or glistening water) substrate. Application of the first coat should be made when the temperature of the substrate is falling to lessen the affects of outgassing of the substrate in the coating.
Other performance tips should be considered during application by reviewing the manufacturer’s application bulletin. These factors may include:
- Substrate Temperature – Can effect flow and leveling characteristics and could create a surface skin that would cause solvent entrapment in solvent rich coatings.
- Material Temperature – Will affect the pot life of the materials used.
- Humidity Levels – Can aid in creating Blush issues with certain products. Slow the dry times or accelerate them based on the chemistry of the product.
There is a relatively new standard for measuring the dry film thickness of coatings on concrete substrates, SSPC PA-9“Measurement of Dry Coating Thickness on Cementitious Substrates Using Ultrasonic Gages”. This standard is similar in nature to SSPC PA-2 used to measure dry film thickness of metallic substrates, with one major change. The standard requires that you specify a level (1-4) of minimum and maximum film thickness for acceptability.
- Level 1 – Minimum – As specified – Maximum – As Specified
- Level 2 – Minimum – As specified – Maximum – 125% of Maximum
- Level 3 – Minimum – 75% of Minimum – Maximum – 125% of Maximum
- Level 4 – Minimum – 75% of Minimum – Maximum – Unrestricted
These levels can greatly affect the amount of material required to complete a project and may place too restrictive of a window on the applicator due to the peaks and valleys found in a prepared concrete substrate.
The method of application of coatings on concrete can vary from brush and roll, spray, flat blade squeegee, notched squeegee, trowel, screed box, power trowel, plural component spray, etc. Always consult with the manufacturer for acceptable methods of application.
Next up in the series: “Inspection”