Often construction schedules won't allow the luxury of waiting for a concrete slab to dry to manufacturer's specifications before proceeding with scheduled flooring work. But unless the specifications for moisture content in the concrete are met, flooring sub-contractors cannot proceed, and manufacturers' warranties won't be honored.

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 only 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.

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.

Above: Moisture moves from areas of greater vapor pressure, in this case the slab, to areas of lower vapor pressure. Right: Flexible plastic ductwork distributes the dry air while work continues uninterrupted. Far right: Portable dessicant dehumidification equipment is built to withstand the construction environment while drying large volumes of air.
Above: Moisture moves from areas of greater vapor pressure, in this case the slab, to areas of lower vapor pressure. Right: Flexible plastic ductwork distributes the dry air while work continues uninterrupted. Far right: Portable dessicant dehumidification equipment is built to withstand the construction environment while drying large volumes of air.

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

The combination of factors can create exceptionally 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.

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 capacity.

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.