Question: We are concrete contractors who normally install concrete floors. But we’ve been invited to provide a price for installing a polyaspartic polyurea coating on a floor we recently completed without a vapor retarding membrane under the slab. We would like to install the surface finish, but wonder about correct timing and preparation. How dry should the floor be and how should we proceed?
Answer: The fact that you placed and finished the floor provides you with helpful information about how to proceed. For instance, you know there is no vapor retarding membrane under the slab, so you know there will be continuous vapor transmission through the slab from the ground below.
The first thing you must do is measure the moisture level in the concrete. The two primary ways to accomplish this are measuring relative humidity (RH) of the slab, or the evaporation rate from the surface using the calcium chloride test.
Measuring the percentage RH in a slab is becoming the popular method because it provides information about the amount of moisture in the entire slab as opposed to the amount leaving the surface region. The test involves drilling holes 40% of the thickness of a slab and inserting a probe to measure RH. Each coating or flooring manufacturer provides information about the minimum RH percentage needed. Generally speaking, this is 80% or less.
The calcium chloride test involves sealing a known weight of calcium chloride under a plastic dish taped to the floor and leaving it for 60 to 72 hours. The moisture vapor emission rate (MVER) is calculated by weighing the calcium chloride and the number is reported as pounds/1000 square feet/24 hours. Again, each manufacturer of flooring or coatings has their own requirement, but it’s generally 3 to 5 pounds/1000 square feet/24 hours or less.
Concrete surfaces must be profiled to provide proper bond and the depth of the profiling relates to the thickness of the coating. The International Concrete Repair Institute (ICRI) CSP profile is usually referred to for the correct profile. Polyaspartic coatings don’t have much mil thickness, so the CSP 2 or 3 level are the usual requirement. Most contractors use diamond grinders now for this work—30 to 100 grit diamond tooling, the trend being more toward 30 grit profiles.
Because there is no vapor retarder under your work, the question remains as to whether there will be moisture problems at some future date that could delaminate the coating. To be safe you should install a surface moisture control system. Several manufacturers sell epoxy products for this purpose. They can withstand 12 pounds/1000 square feet/24 hours or more. You apply them to the profiled concrete surface before applying the polyaspartic finish coats.