What Does Moisture Vapor Emission Rate Mean?
The moisture vapor emission rate is a measurement of the speed and amount of water that is released from concrete slabs. Water is a necessary component of the concrete mixing and pouring process; however, if excessive water is used, then a high moisture vapor emission rate may occur, resulting in delamination on coated concrete structures.
Corrosionpedia Explains Moisture Vapor Emission Rate
All concrete structures have a moisture vapor emission rate. If the moisture vapor emission rate is at an acceptable level then it is unlikely that the moisture emission will cause problems. However, if the concrete structure is coated and the water vapor emission rate is too high then issues may arise.
The main problem with high moisture vapor emission rates for concrete structures is delamination of the coating from the concrete structure. This is cause by the pressure that the moisture exerts under the coating as it attempts to leave the concrete structure. Delamination can ultimately lead to complete coating failure. Preventing this type of failure requires using the appropriate amount of water during the concrete mixing process and allowing sufficient time for moisture emission prior to coating.
Possibly the most popular way to measure the moisture vapor emission rate is through the use of calcium chloride. For this test, calcium chloride with a known initial weight is placed on a concrete test sample, both of which are then put in a plastic container to isolate them from the atmosphere. The concrete test sample and calcium chloride are left in the sealed environment for a predetermined time and then are removed and weighed again. The weight gain of the calcium chloride is used to determine the moisture vapor emission rate of the concrete. Specifications from societies such as ASTM determine the criteria for this test.