EDITOR'S NOTE: Here you can find examples of how specific products were put to use to solve a problem in the field. If you have used a product that helped you successfully complete a job, please send your story to us at concretesurfaces@hanleywood.com.

Often construction schedules won't allow the luxury of waiting for a slab to dry to manufacturer's specifications before proceeding with scheduled flooring work.

One solution is to establish and maintain optimum indoor environment conditions while the slab dries. Using a desiccant dehumidification system provides good moisture abatement and aggressive drying, compressing the time for achieving acceptable moisture levels in the slab and other building materials.

The moisture problem

The problem of moisture in concrete slabs is two-fold. Flooring installed before the concrete slab has dried sufficiently can experience a variety of problems, from adhesive failure to warping and discoloration of the flooring material. But while a slab may appear to be dry within days of being poured, that appearance can be misleading. Under average ambient conditions, a concrete slab poured within an enclosed building will dry at a rate of about one inch per month. But the actual rate varies based on the concrete mix design, curing procedures and ambient conditions—indoor and outdoor temperature, humidity, and weather.

Moisture moves from areas of greater vapor pressure, in this case the slab, to areas of lower vapor pressure.
Moisture moves from areas of greater vapor pressure, in this case the slab, to areas of lower vapor pressure.

Concrete always contains more water than is required for the hydration of the cement, simply to make it workable. Additional water sometimes is applied to the slab during curing, and rain and snow can further increase the moisture content.

Relative humidity can also contribute moisture, as can the drying of other building materials such as fire proofing, concrete wall blocks, joint compound, and paint.

The combination of factors can create high humidity levels inside a building. If conditions are right, the interior atmosphere can even create fog and condensation that drips onto the slab.

Solving the problem

Moisture travels a path of least resistance. So, if the moisture level in the air is high, water in the concrete evaporates very slowly, or not at all. This slows construction and increases the potential for mold.

Flexible ductwork distributes dry air while work continues uninterrupted.
Flexible ductwork distributes dry air while work continues uninterrupted.

Optimum indoor drying conditions occur at 30% relative humidity and 65° to 70° F with constant airflow over the slab surface. Contractors sometimes attempt to use HVAC systems to establish low relative humidity, but these systems are engineered for temperature control and not moisture removal.

Running the system can spread dust and mold spores throughout the ventilation system and cause damage to the HVAC equipment, coils or filters. Because of that, some building owners no longer allow the use of the building's installed HVAC system during construction.

Another approach, heating the space with standard direct fired construction heaters, may compound the problem by adding moisture to the space through combustion. Cooling equipment will make the space more comfortable during warm periods but cannot provide the conditions necessary to significantly hasten concrete drying.

A desiccant dehumidification system provides a more efficient, productive, reliable and faster method of moisture abatement. Unlike cooling-based dehumidifiers, which cool the air to condense moisture, desiccants attract moisture molecules directly from the air and release them into an exhaust air stream. Desiccants can attract and hold from 10% to more than 10,000% of their dry weight in water vapor. They are very effective in removing moisture from the air at low humidity levels and do not freeze when operated at low temperatures. The end result is an extremely dried air source capable of drying the most saturated materials.

Desiccant units used for drying are different from those used as permanent installations in commercial buildings. Portable units, delivered to the site on trailers, are designed to withstand the construction environment and to provide the drying required to establish and maintain proper dew point and humidity levels.

Portable, inflatable plastic ducts used as part of the airflow system, preclude any reliance upon the HVAC distribution system. The temporary ducts can be moved easily as work progresses.

The drying process

Portable desiccant dehumidification equipment is built to withstand the construction environment while drying large volumes of air.
Portable desiccant dehumidification equipment is built to withstand the construction environment while drying large volumes of air.

The process of removing excess moisture from the concrete slab depends upon the lowering of the moisture vapor pressure between the slab and the ambient conditions above it. The desiccant dehumidifier provides the dry air capable of not only reducing the threat of condensation on the surface (liquid moisture), but reduces the overall vapor pressure in the space. Moisture travels from areas of high vapor pressure (within the slab) to the areas of lower vapor pressure being mechanically created (ambient condition). The moisture vapor is desorbed from the concrete into the air and pushed out of the space by air movement.

Dehumidification or sealer

Although the use of dehumidifiers for drying concrete slabs is growing quickly, concrete sealers remain a popular alternative. Choosing between using a sealer and a dehumidification system to prepare the slab for installing floor coverings is based on factors including budget, deadlines, severity of the problem, and warranties. If one expects to install flooring in a few days, the use of a sealer would be a wiser choice; the dehumidification process cannot be completed in a matter of a few days.

This information was provided by Russ Brown and Howard Kanare. Brown is national accounts manager with Munters Moisture Control Services, 800-422-6379 orrbrown@munters.com.

Kanare is senior principal scientist with Construction Technology Laboratories (CTL), 847-522-2285 orhkanare@ctlgroup.com.