• The construction industry has long been judged on its quality of work and ability to complete projects on schedule. Construction managers, general contractors, and subcontractors who must meet tight deadlines 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 drying 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 subcontractors cannot proceed, and manufacturer’s warrantees won’t be honored.

    Of the various building materials, concrete can be one of the most vexing sources of moisture. Although a slab may appear to be dry, the appearance can be misleading. Under average ambient conditions, a concrete slab poured within an enclosed building will dry at a rate of about 1 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 and humidity.

    Unless the excess moisture is somehow removed, 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 a flooring materials installation vary based on the product being applied. Some materials readily absorb or wick water. Others are applied using adhesives that are degraded 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 cement, giving concrete its great strength. But the mix always contains more water than the hydration reaction requires. This is exacerbated by lightweight aggregate, which tends to retain moisture.

    In typical commercial floor slabs, there’s likely to be about 15 gallons of extra water in every cubic yard of the pour. This surplus must dry through evaporation after the concrete has cured or it can interfere with flooring adhesion, warp wooden floors, or help grow mold in the leveling compound or in drywall.

    Snow and rain contribute additional moisture in slabs that are exposed and wet curing adds additional water. Also, the materials used to construct the building may retain moisture. Fire roofing, laden with water when applied, dries slowly inside enclosed areas. Large amounts of water can be trapped in concrete block walls. Joint compound and paint emit large volumes of moisture 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 condensation that drips onto the slab. In addition to slowing the construction schedule, excess moisture represents another major threat—the potential for mold.

    An inadequate HVAC system

    Optimum indoor drying conditions require low relative humidity, warm temperature, and 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 not moisture removal.

  • Running the system during construction can spread dust and mold spores throughout the ventilation system and even cause damage to the HVAC equipment, coils, or filters.
  • Openings between the structure and the outdoors during construciton, as well as between various areas of the building, make controlling temperature and humidity nearly impossible.
  • Running the system prior to commissioning the building can lead to warranty issues and concerns.
  • Desiccant dehumidification

    The more efficient, reliable, and faster method of moisture abatement is aggressive drying through desiccant dehumidification. Desiccant dehumidifiers will lower the air’s relative humidity and dew points even when drying air at moderate relative humidity and cold temperatures.

    This approach uses portable, inflatable plastic ducts as part of the airflow system, precluding reliance on the HVAC system. The temporary ducts can be moved easily as work progresses into other areas of the construction site.

    Control of the ambient environment at construction sites is typically limited to heaters during cold weather. But these 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 sometimes be used to make the space more comfortable during warm periods, but cannot provide significant concrete drying results.

    The dying process

    To remove excess moisture one must have a low relative humidity in the air above the slab. Desiccant dehumidifiers 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 mechanically created in the air above the slab (ambient condition). The moisture vapor will be desorbed from the concrete into the air and will be removed 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. The required air change rates can fluctuate based on ceiling height, thickness of slab, tightness of envelope, type of vapor barrier (or lack thereof), outside weather conditions, and a host of other variables.

    One added benefit to using desiccant dehumidifiers: materials other than the slab will also 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 then weeks. And mold growth on construction materials will remain in check.

    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 flooring or adhesive 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 this method, popular in Europe, which measures relative humidity by drilling a hole in the concrete and inserting a probe.

  • Calcium chloride (ASTM F 1869). This widely used quantitative test measures moisture in only a thin upper layer of the slab (about 1/2 inch). The test is based on the rate of absorption of moisture by calcium chloride cystals in a sealed environment on top of the slab. The results are subject to temperature and humidity levels the day before the test is started.
  • 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 1/2- to 3/4-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, be sure to employ a qualified testing technician, such as one certified by teh International Concrete Repair Institute. Always defer to the flooring manufacturer’s recommendations for required moisture levels and the testing method to be employed.

In summary

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, leading to delays in the schedule for applying flooring materials to the slab. Moving ahead without adhering to the flooring manufacturer’s specifications for moisture content will compromise warrantees and excess moisture creates the risk that mold will form.

One solution is to establish and maintain optimum indoor environment conditions using a desiccant dehumidification system. The drying process compresses the time for achieving acceptable moisture levels in the slab and other building materials.

David Simkins (david.simkins@polygongroup.com) 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 800-686-8377.