Dehumidification Puts Concrete Slab drying On The Fast Track


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.


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[degrees] to 70[degrees] 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.

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 percent 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 establishing and maintaining 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


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.

To be effective, the dry air must be contained. If a building under construction is open to the exterior when a slab is curing, temporary enclosures may need to be erected to contain the dry air where needed.

Depending on the amount of moisture to be removed and the conditions present, the number of hourly air changes to be effective can vary greatly. Air change rates can fluctuate depending on ceiling height, thickness of slab, tightness of envelope, type of vapor barrier (or lack of one), outside weather conditions and a host of other variables.

Trades can continue to work in the spaces being dried, but they must not tamper with equipment, fans, or ducts. They must keep the "envelope" secure by closing doors and windows.


Cost considerations


Although the question of cost seems simple, all jobs are different as they relate to moisture targets, building size and the differing weather conditions. The difference between a 10,000-square-foot recreation room with a 12-foot ceiling and the same room with a 20-foot ceiling is significant. Additionally, if temporary generators are required the cost will be greater.

Good engineering and design can reduce the overall cost while resulting in an effective slab drying system.


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.

However, mechanically drying the floor might be a better long-term and cost-effective solution. Most sealers provide a barrier to trap moisture vapor inside the slab, limiting the amount of emissions that can affect adhesives installed on the concrete. The free moisture remains trapped or searches to find an avenue to escape, leading to the possibility of future flooring failures. Dehumidification systems remove the moisture rather than divert it, reducing the chance that excess moisture vapor will create future problems.


Accelerating Best Buy Construction


A new Best Buy store being constructed in Portland, Ore., was scheduled to be completed four weeks after the building was closed in. Sheetrock, painting and flooring were scheduled to go quickly, but high interior humidity and moisture trapped in the concrete slab was proving a difficult problem.

The contractor had more than 1 acre of VCT to lay down and would not guarantee the adhesion of the floor coverings if the concrete was not dry. On top of that, the owner wanted to shorten the construction schedule by a week. Conventional methods would obviously take too long.

The initial calcium chloride test readings for the slab measured at 8.6 pounds/1000 sq. ft./24 hours with a target of 5 pounds.

A dehumidification equipment specialist provided three industrial sized desiccant dehumidifiers to accelerate the drying, distributing the dry air with flexible ducts.

In three weeks, the equipment had dried the concrete slabs to 4.4 pounds/1000 sq. ft./24 hrs. Additionally the overall drying process was sped up, allowing the contractor to meet the tight deadline for completion.