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Reprinted from the Portland Cement Association Engineering Bulletin DESIGN AND CONTROL OF CONCRETE MIXTURES Thirteenth Edition by Steven H. Kosmatka and William C. Panarese Moisture Content in Concrete Page 172 The in-place moisture content or relative humidity of hardened concrete is sometimes useful to determine if concrete is dry enough for application of floor-covering materials and coatings. The moisture content must also be low enough to avoid spalling of concrete to be exposed to temperatures above the boiling point of water. The most direct method for. determining moisture content is to dry cut a specimen from the concrete element in question, place it in a moistureproof container, and transport it to a laboratory for test. After obtaining the specimen’s initial weight, dry the specimen in an oven at about 220T until constant weight is achieved. The difference between the two weights divided by the dry weight, times 100, yields the moisture content in percent. Moisture content or relative humidity meters are also available. Although it can require several months of air drying, a relative.humidity of about 80% or less is often required before floor covering materials can be placed on concrete floors. Another useful test is the polyethylene-sheet test. (LINK TO PHOTO DEMO) A 4-ft-square of clear polyethlene is taped to the floor: if no moisture condenses under it after 24 to 48 hours, the slab is considered dry enough for some coatings and flooring materials. |
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