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September 12 - September 18, 2011

What is an acceptable level of moisture in concrete walls before painting with an acrylic system, and how is the moisture level measured?

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Selected Answers

From William Slama of International Paint/Ceilcote Products on September 21, 2011:
This is a qualification/addition to my submitted answer. My answer was appropriate for catalyzed or solvent polymer systems. Acrylic systems may have more lattitude for water content.

From William Slama of International Paint/Ceilcote Products on September 21, 2011:
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 may contain 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. Like the primer, the plastic sheet stops the water vapor transmission; and the presence of water at the surface is easily diagnosed by either the dark color of the concrete surface or the condensation of water on the underside of the plastic sheet. It should be stressed that the test should be done on the as-prepared surface. That is important since the condition can change by the time the concrete is prepared. In the absence of accidental exposure to water such as rain or spills, the change is normally for the better. The sandblasting or blast tracking surface preparation tends to “open” the surface, usually helping to dry it. We have experienced extreme lack of correlation between the ASTM plastic sheet test and the Calcium Chloride test in both directions. That being the case, the only test requirement we have is for the plastic sheet test to show no moisture.

From Lee Edelman of Independant on September 15, 2011:
     Moisture content depends on the relative humidity. Cure time will depend on water-cement ratio. An 18-inch by 18-inch clear plastic sheet is placed over the concrete and sealed on all four sides. After 16 hours, if any moisture or darkening of the concrete surface is found, the concrete is too wet for coating. Make sure the sheet does not come in contact with direct sunlight. If a quantitative method is specified, a moisture meter can be used. Keep in mind that moisture meters measure the top inch and concrete dries from the top down. Depending on the coating manufacturer's and owner's specification, moisture levels can be required to be less than 2.5%.

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