Problem Solving Forum
January 9 - January 13, 2023
How can I determine when concrete has cured sufficiently to be coated, besides waiting the 28 days typically specified?
Bill Slama of International Paint/Ceilcote Products on
January 11, 2023:
The two primary factors that are important for ade ...read more The two primary factors that are important for adequate concrete cure before coating are strength (particularly surface tensile strength) and dryness. Those specific requirements may vary based on the coating manufacturers requirements, but the following recommendations are commonly used: STRENGTH The amount of strength required depends upon the properties of the polymer system applied, such as rigidity and thickness, in addition to the thermal and physical service parameters. Thin film coatings may perform well over substrates with at least 150 psi (1 MPa) surface tensile strength, but most thicker systems require stronger foundations. at least 300 psi (2 MPa) surface tensile strength of the prepared substrate for optimum performance of most systems. DRYNESS Total dryness is preferred and is usually specified for optimum system performance. This is particularly true for polyester and vinyl ester systems, but some epoxy systems have good tolerance to moisture. The ASTM D 4263 Plastic Sheet Test is usually the best method to ensure that liquid phase water does not occur at the polymer/substrate interface and reduce the adhesion that has been developed or inhibit the cure of the primer used. Following is a better explanation: Concrete substrates, at the time of applying coating, lining or flooring systems, can contain various amounts of water due to a number of factors including residual excess water in the original (recently placed) floor; residual water from previous flooding or environmental exposure; and transmitted water from underneath on-grade slabs. The most important consideration is the presence of liquid phase water at the surface during the application and for the next 16 hours or so. That is a requirement to assure proper wetting of the concrete surface by the primer and ability for the primer to establish its initial cure without contact by water (liquid). If water is present or reaches the concrete surface within 16 hours, the adhesion and hence product performance may suffer by later disbonding or blistering. Basically, even “dry” concrete usually contains some water well below the surface. Assuming less than 100% relative humidity above the concrete, some water vapor is always leaving the concrete and at some depth, the water exists in liquid form (wet/dry line). The depth depends on the amount of water in the concrete, the density of the concrete and the surface drying (relative humidity and velocity). When the primer is applied, it is effectively a barrier to water vapor transmission. As a result, the wet/dry line moves upward toward the surface. It is important to ensure that the primer on the concrete does not see (touch) water for at least 16 hours so that optimum adhesion is obtained. Clearly, with this goal, the plastic sheet test (ASTM D4263) is the most direct way to check the concrete surface. Test several areas.