Assessment of Moisture-Tolerant Coatings for Decreasing Open Top Construction Time

Abstract

Open top construction is a practice commonly used in the construction of large structures, such as nuclear power plants, as it allows large equipment to be easily placed by lowering it into position from above. Doing so, however, requires the concrete floor to be finished and coated prior to placement. Current coating manufacturer recommendations state that concrete should be allowed to dry for a minimum of 28 days prior to coating application to avoid compromising the bond between the coating and the concrete or between coating layers that could result from excessive moisture. This requirement delays the construction process, adding significant costs. The ability to apply coatings without damage prior to 28 days would greatly reduce construction time and cost. Ten coating systems were evaluated in this study. The coatings were applied 7, 14, 21, 28, and 45 days after the end of wet curing. Coating adhesion was evaluated using the Standard Test Method for Pull-Off Strength of Coatings on Concrete Using Portable Adhesion Testers and the Standard Test Method for Evaluating Adhesion by Knife 7, 21, 28, and 56 days after application of the final top layer of the coating systems. Moisture vapor emission rate (MVER) and concrete relative humidity (RH) were monitored throughout the tests. Most but not all of the coatings investigated in this study may be applied to concrete as early as 7 days after completion of wet curing, at MVER values over 10 lb/1000 ft2/day (565 μg/m2/s) and internal relative humidity (RH) above 80%, without significant adverse effects on coating adhesion, offering the potential to speed open top construction of nuclear power plants. The thickness of concrete does not affect the value or rate of change in MVER or RH. Thicker coatings exhibit relatively poor performance in the knife test compared to thinner coatings. Coating systems should be evaluated to ensure that they can be successfully applied at early ages. Larger-scale prototype early-age applications should be performed and subjected to the full range of required testing for the appropriate Service Level prior to wide-scale application of these findings.