The construction industry has long been judged on its quality of work and ability to complete projects on schedule. Construction managers, general contractors and sub-contractors who must meet tight timetables and warranty the work feel the most heat. One challenge often faced: a concrete slab with elevated moisture levels.
Frequently, the construction schedule won’t allow the luxury of waiting for the natural hydration process. As a result, the long time it takes for concrete slabs to dry can create delays. Unless the specifications for moisture content in the concrete are met, sealers can’t be applied, flooring sub-contractors cannot proceed and, certainly, manufacturer’s warrantees won’t be honored.
Of the various building materials, concrete can be one of the most vexing sources of moisture. While a slab may appear to be dry, even to the point of walking on it soon after it was poured, the 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 even that is a generalization. The actual rate will differ based on the concrete batch ingredients (the mix design), curing procedures and ambient conditions -- indoor and outdoor temperature, humidity and weather.
Unless the excess moisture is dried, several problems may result:
- Moisture migration to the surface, which can result in failure of adhesives, discoloration of flooring materials and blisters in coatings.
- Growth of mold in other materials as high ambient moisture remains.
- Poor initial adhesion of flooring installed on the slab.
Why flooring fails
The reasons for failure of an installation of flooring materials onto a slab vary based on the specifications of the product being applied. Some materials readily absorb or wick water. Others contain adhesive compounds that are damaged by moisture. For example, low volatile organic compounds (VOC) adhesives for carpet and resilient flooring are environmentally friendly, but can be compromised by excess moisture and high pH. Some examples of how various materials react:
- Hardwood flooring and millwork will absorb moisture, resulting in warping and swelling of the wood or wood composite materials.
- Moisture-sensitive adhesives used to install vinyl composition tile (VCT), rubberized high performance sports floors and fiber-backed carpet often are compromised when the slab is too moist.
- Moisture can stain and discolor resilient flooring and coatings.
- If the water-based adhesive on rubber-backed carpet tiles becomes wet, it often will wick upward in the joints and stain the carpet.
Amount of excess moisture
Concrete cures by hydration when water reacts with the cement powder, giving concrete its great strength. But the mix always contains more water than the hydration reaction requires. This has become even more so prevalent with the use of light weight aggregate which tends to retain moisture.
In typical commercial floor slabs, there's likely to be about 50 gallons of extra water in every cubic yard of the pour. This surplus must be removed after the concrete has cured, or it will interfere with flooring adhesion, warp wooden floors, or help grow mold in the leveling compound or in gypsum board.
Relative humidity and weather, in the form of snow and rain, are major contributors of additional moisture. Also, the materials used to construct the building retain moisture. Fire roofing, laden with water when applied, dries slowly inside enclosed areas. Large amounts of water can be trapped in concrete wall blocks. Joint compound and paint emit large volumes of water as they dry.
A combination of these factors can create exceptionally high humidity levels inside a building. If the conditions are right, the interior atmosphere can even create fog and condensation that drips onto the slab.
In addition to slowing the construction schedule, excess moisture represents another major threat – the potential for mold. In order to grow, mold requires a food source, suitable temperatures and moisture. Of those, moisture is the only factor that is controllable.
A facility’s HVAC system is inadequate
Optimum indoor drying conditions require low relative humidity, regardless of temperature, with constant airflow over the slab surface. The use of a building’s HVAC system to establish low relative humidity is inadequate for several reasons:
- HVAC 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 even cause damage to the HVAC equipment, coils or filters.
- Many openings between the structure and the outdoors, as well as between various areas of the building, make controlling temperature and humidity uniformly nearly impossible.
- Running the system prior to commissioning the building can lead to warranty issues and concerns.
The more efficient, reliable and faster method of moisture abatement is aggressive drying through desiccant dehumidification. Desiccant dehumidifiers will create low relative humidity and dew points when drying air at a condition far from saturation or at low temperatures.
This approach uses portable, inflatable plastic ducts as part of the airflow system, precluding any reliance upon the HVAC distribution system. Also, the temporary ducts can be moved easily as work progresses into other areas of the construction site.
Other methods such as heating and cooling are commonly utilized at construction sites to control the ambient environment. These processes, however, are not effective at significant moisture removal. In fact, heating the space with standard direct fired construction heaters often will add moisture through combustion. Cooling equipment will make the space more comfortable during warm periods, but cannot provide the conditions necessary to provide significant concrete drying results.
The drying process
The process to remove excess moisture is dependent upon lowering the moisture vapor pressure between the slab and the ambient conditions above it. The desiccant dehumidifier will provide the dry air capable of both reducing the threat of condensation on the surface (liquid moisture), and decreasing the overall vapor pressure in the space. Moisture will travel from areas of high vapor pressure (within the slab) to the areas of lower vapor pressure which are being mechanically created (ambient condition). The moisture vapor will be desorbed from the concrete into the air and will be pushed out of the space by air movement.
The number of hourly air changes to be effective can vary greatly depending on the amount of moisture to be removed and the conditions present. Air change rates can fluctuate based 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.
One added benefit to using desiccant dehumidifiers: materials other than the slab will dry more quickly. Drywall compound can be ready for sanding the day after dehumidification begins in any climate. Fireproofing has been dried to industry standards in a matter of a few days rather than weeks. And, mold growth on construction materials will remain in check.
The use of sealers
Although the use of dehumidifiers for drying concrete slabs is growing quickly, it still may be some time before it surpasses the popularity of the common concrete sealer. The decision to use a sealer or a dehumidification system to prepare the slab for installing floor coverings is based on several factors including budget, deadlines, severity of the problem, warranties and more. If one expects to install flooring in a few days, the use of a sealer is an option, as the dehumidification process requires more time.
However, the mechanical drying of the floor might be a better long-term and cost-effective solution. Most sealers provide a barrier to trap the moisture vapor inside the slab, limiting the amount of emissions that can affect the 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 are designed to remove the moisture rather then divert it. This technique will reduce the chance of excess moisture vapor to permeate the adhesive and create future problems.
Measuring moisture in a slab
Once a dehumidification program is implemented, it’s important to measure the moisture content in the slab to confirm when flooring installation can proceed, based on the manufacturer’s specifications. Several qualitative and quantitative testing methods determine moisture content during the drying process. The most commonly used techniques are:
- Relative Humidity Probe Test (ASTM F-2170) – The U.S. construction market is just beginning to be comfortable with the quantitative method, popular in Europe, which measures relative humidity by drilling a hole in the concrete and inserting a probe into it for three days.
- Calcium Chloride (ASTM F-1869) – This widely used quantitative test measures moisture in only a thin upper layer of the slab (about ½-inch). The test is based on the rate of absorption of moisture by the calcium chloride. Results are subject to temperature and humidity levels the day before the test results are viewed, which can produce different readings.
- Electronic Meters – A variety of meters, including some based on radio frequencies and others that require pins to be inserted into the concrete, measure conductivity. The wood industry developed the method, which measures to depths of about ½- to ¾-inch. It is best used to obtain qualitative readings over an entire floor, followed up with a quantitative test.
In every use of measurement methods, the guiding principle is to refer to the manufacturer’s recommendations for expected moisture levels and the desired methods for monitoring. To obtain the most accurate results, two or more methods should be employed to cross check readings. Relying on the drying contractor’s experience with testing procedures is the best guarantee of reliable results.
Water is an essential element in concrete. But excess moisture, in the form of water in the concrete or humidity in the air, will slow the drying process. In turn, schedules for applying flooring materials to the slab will be delayed. Moving ahead without adhering to the flooring manufacturer’s specifications for moisture content will compromise warrantees. Further, excess moisture creates the risk that mold will form.
The solution is to establish and maintain optimum indoor environment conditions through a desiccant dehumidification system. The drying process compresses the time for achieving acceptable moisture levels in the slab and other building materials.
David Simkins is director of industrial services with Polygon, a leader in water and fire damage restoration and remediation, and for temporary climate control in construction and industrial applications, with 23 offices in North America. Simkins can be reached at firstname.lastname@example.org or 800-686-8377.