Testing concrete slabs for excess moisture has become a common construction requirement, particularly where flooring or impermeable membranes are to be installed on top of the slab. But, while several standard moisture test methods are available, no single test reveals everything that should be considered in deciding when flooring can be installed or a coating applied. Before looking at the tests themselves, let’s review the source of the moisture. Concrete is mixed and slabs are placed with more water than will be needed for hydration of the cement. Given the right thermodynamic conditions, slabs dry over time as the excess water evaporates. How rapidly and forcefully this occurs is determined largely by the difference between the vapor pressure in the slab and the vapor pressure in the air over the slab, as well as conditions below the slab. Vapor pressure, in turn, is affected by temperature. Problems with floor coverings, such as bubbles, blisters, and delamination, occur when an impermeable floor covering or sealer traps excess moisture remaining in the slab. The key is to wait until the moisture level reaches an equilibrium point or is acceptably close to it before sealing. Pinpointing that specific equilibrium point sounds relatively simple in theory, but difficult to measure. Moisture is not evenly distributed throughout a slab, and changing environmental conditions cause moisture to move into or out of the slab. Fortunately, experience and some simple tests can help determine if the moisture content in a slab is within an acceptable range for various impermeable coverings. But first we need to know what we are measuring and what the results mean. Water leaving concrete. Both fresh concrete and slabs that have cured and hardened have excess water. Immediately after concrete is placed and throughout the curing period, efforts are made to prevent water from evaporating from the slab so that the concrete can properly hydrate. After the initial curing period, however, we want the concrete to dry out, and large amounts of water are given off by a newly placed slab. Concrete with a higher water-to-cementitious material ratio gives off proportionately more moisture simply because there is more free moisture in the slab and the differential vapor pressure between the concrete and the air remains high. But if the concrete is simply air drying, the rate finally decreases, which means the differential vapor pressure is greatly reduced. When the water vapor approaches equilibrium, flooring can be applied. However, even after it has hardened, most concrete remains porous, so its moisture increases or decreases with changing temperature and humidity. Concrete’s permeability, the rate at which it will allow moisture to pass, depends on the size and distribution of the pores in the concrete matrix. Generally speaking, the lower the water-to-cementitious-materials ratio of a concrete mixture, the lower its permeability will be after it has cured. Concrete’s permeability can also be reduced by adding any of a number of products to fill in the voids in the concrete matrix. Moisture given off by a concrete slab in an enclosed space is a concern.