This very wet concrete won't dry out for a long time.

Most people, even many people in the concrete business, think concrete is water tight. After all, we make water tanks and dams out of concrete. But the truth is that although concrete does a good job of containing liquid water-at least when there are no cracks-water vapor moves readily through concrete at a rate that depends on the concrete's porosity and permeability.

All concrete starts out wet. If there wasn't water in the mix, you couldn't place it and it would never gain strength. At a water-cement ratio of 0.50, there is about 300 pounds of water and 600 pounds of cement in a cubic yard. As the concrete begins to set, some of that water (about half) combines with the portland cement (through hydration) and some rises to the surface as bleed water where it evaporates. The rest is in the pores of the concrete.

Concrete doesn't begin to dry until after finishing and curing.

After the curing period, the slab begins to dry. At this point there is a lot of liquid water in the concrete pores—in fact, the slab is saturated. This liquid water begins to evaporate from the surface and if no additional water gets into the concrete, within about 90 days for normal-weight, 0.5 w/c concrete, the slab will be dry enough so that most floor coatings won't delaminate.

Water vapor leaves the surface of a concrete slab at a rate that is called the Moisture Vapor Emission Rate (MVER). When you read in a sealer data sheet that the MVER needs to be 3 pounds or 5 pounds, what that means is the number of pounds of water vapor per 1000 square feet per 24 hours. Envision a 31.6 x 31.6 foot section of concrete (1000 square feet) and imagine 3 pounds of water evaporating from the surface each day. Three pounds of water is about three pints ("a pint's a pound the world around"), so that's not much.

But what if the slab is placed on the ground without a vapor barrier? Think about what happens when you dig a hole in moist ground. Long before you get to the water table (liquid water), you will encounter damp soil. That's how the soil beneath your slabs looks—damp. The ground beneath nearly all concrete slabs is damp—in fact, it nearly always has a relative humidity of 100%. That means it is a continuous source of water vapor into the slab and the slab will never dry out—especially if you put a coating on the surface that restricts the movement of water vapor. ACI 302.2R-06, Guide for Concrete Floors that Receive Moisture-Sensitive Flooring Materials, states that "A concrete slab-on-ground without a vapor retarder/barrier directly beneath it may have a final relative humidity profile that does not benefit from any initial drying."

Find Products & Manufacturers