A common misconception is that concrete is ready to receive coatings after the concrete has cured for 28 days. While it is true that concrete reaches 99% of its compressive strength after 28 days, that doesn’t address the most important factor in determining whether the concrete is ready for a coating application – moisture. There are numerous ways to test moisture in concrete, but there is no quick, easy way to verify this on new construction projects. It takes the experience of an expert, monitoring the project requirements to determine when to apply a coating.
The two most often used test methods for determining moisture content in concrete are ASTM – F1869 and ASTM – F2170. ASTM – F1869 measures moisture vapor coming out of the concrete. ASTM – F2170 measures relative humidity of a concrete slab at 40% of the slab’s thickness, if curing from one side (typical slab-on-grade construction). Unfortunately, both of these tests require the area being tested to be in their final functional operating condition. Both tests also state that the data is a snapshot of the moisture in concrete at the moment when the test is concluded and tabulated.
It is vitally important to understand concrete design, the inclusion or exclusion of a vapor barrier, documentation of water/cement ratio at time of placement, curing methods used, and proper joint spacing and depth. Misreading and/or misunderstanding any of the previously mentioned items can result in moisture-related failures and finger pointing.
Current specifications rarely require an independent testing agent be present to monitor the aforementioned variables properly. Instead, they often place in charge the flooring contractor who may not be capable of performing these duties effectively. Moisture-related floor coating failures reportedly cost more than the total sales revenue for all epoxy and urethane floor coatings each year. Moisture in concrete to be coated is a serious issue. If you are not a resinous flooring expert, with years of experience testing concrete for floor coatings, contact an expert. You will be glad you did!