Preventing and Mitigating Moisture Intrusion into Concrete Slabs

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Expertise Includes:

    • Building Damage Assessment
    • Construction Defect Evaluation
    • Building Foundation Issues
    • Building Envelope/Water Intrusion
    • Building Codes and Standards
    • Exterior Wall & Roofing Systems
    • Structural Design - Collapse/Failure Analysis

In a previous blog, I discussed issues with moisture intrusion into concrete slabs-on-grade.  In this blog, I will examine methods to address issues with moisture intrusion into concrete slabs, including preventative measures and mitigation techniques.

Preventative measures include:

  • Use of a low-permeance vapor barrier, which effectively keeps moisture from reaching the underside of a slab-on-grade.
  • Providing adequate time for the natural drying of the concrete. Slabs-on-grade should always be moisture tested before the installation of finished flooring materials such as tile, wood laminate, coatings, or paints.
  • Using accelerated slab drying techniques or using a topical moisture and pH suppression system.
  • Using a low water-to-cement ratio in the concrete mix.
  • Keeping below-grade excavations free of water to prevent reservoirs of moisture which can migrate upwards into and through slabs-on-grade. In addition, the existence of excessive moisture below a slab-on-grade can cause structural degradation of the allowable soil bearing pressure and swelling or shrinkage of the underlying soil.
  • Using sealants/coatings/water repellants, i.e., types of paints such as epoxy, to protect the exterior surfaces of bare concrete and prevent moisture from entering control joints and natural cracks. Such coatings can also seal gaps between adjoining surfaces, and around flashings and penetration openings, which would otherwise allow unimpeded pathways for moisture to enter.
  • Use of a crack healing, crystalline technology or other type of moisture suppression admixture.

Mitigation techniques to avoid moisture problems after they have occurred include:

  • Installation of sub-drains at a structure’s perimeter to carry water away.
  • Removal, cleaning, and drying affected areas of finished flooring installed on slab-on-grade and placing a vapor barrier on top of the slab and under the finished flooring prior to its reinstallation to prevent the migration of moisture.
  • Use of vapor-permeable finished flooring materials such as carpeting.

Vapor barrier being installed under a yet to be poured slab-on-grade.

Generally, vapor barriers beneath slabs-on-grade should be a minimum of (15) mils thick, meet the requirements of ASTM E1745 Class A, and include the manufacturer’s recommended adhesive or pressure-sensitive tape.  Vapor barriers meeting this spec are virtually guaranteed to be effective and long-lasting, i.e., they will not break down over time.  However, for residential slabs-on-grades, i.e., houses, garages, and basement floors, slabs-on-grade use of typical builder-grade (6)-mil polyethylene plastic sheeting as a vapor barrier is deemed acceptable.  This is the case due to the fact that residential slabs are generally only subjected to foot traffic and/or light furniture/fixture loading.  Note that for exterior slabs such as equipment pads, patios, driveways, etc., the use of a vapor barrier is not recommended.

With regards to slabs-on-metal-deck, the multitude of concerns above, minus the issues with bearing on soil, are all applicable.  Slabs-on-metal-deck are elevated slabs supported by galvanized corrugated metal decking panels.  This decking is in turn supported by and welded or fastened via screws to framing consisting of steel bar joists or structural steel shapes.  They are typically comprised of a light-weight, low aggregate concrete mix, and are reinforced with welded wire fabric, or small #3 or #4 bars.  Slabs-on-metal-deck are found in many commercial office buildings, warehouses, and retail establishments. See photograph below of a warehouse floor being installed.

Slab-on-metal-deck floor installation.  Image Credit: ConstructionPros.com

In addition to elevated floor slabs, some commercial buildings also have flat roofs consisting of a slab-on-metal-deck.  To protect the concrete from moisture intrusion due to the weather, a membrane such as felt, then tar and gravel may be deposited over the slab.  This is one type of roof commonly referred to as “built-up” roofing, although there are a number of different construction types which use this definition.  Another term used is “ballast” roofing.  As this name implies, these roofs also act as ballast weight to resist uplift forces induced by high winds the building may be subjected to.  An additional advantage of this type of roof is superior fire resistance.  See photograph below of tar and gravel roofing being installed.

Flat built-up roofing does have some drawbacks, however.  Ponding (or pooling) of water can occur, which if frequent and significant enough, can lead to breakdown of the tar.  This results in moisture intrusion onto and through the slab, and consequently, roof leaks.  Also, exceptionally strong sun exposure can cause the hot tar to damage the underlying membrane, thereby subjecting the slab to moisture.  Some simple methods of prevention of these issues include:

  • Only using a tar and gravel system in areas where recommended
  • Performing periodic maintenance on the roof system
  • Incorporating a slight pitch on the surface of the roof to promote positive drainage
  • Sealing the slab with an epoxy paint or similar system

Mitigation techniques to avoid moisture intrusion problems after they have occurred on slab-on-metal- deck tar and gravel roofs include:

  • Performing maintenance involving patching leaking spots with fresh tar and new gravel
  • Resurfacing portions of the roof which have degraded
  • Resurfacing the entire roof if its lifespan has been reached or exceeded

In summary, regardless of whether moisture intrusion is a concern with slabs-on-grade or slabs-on-metal-deck, there are a multitude of ways to prevent and/or mitigate this issue.  As an added benefit, protecting the concrete from long-term exposure to moisture will increase its service life as well.

George Sanford, PE, holds a Bachelor of Science in Mechanical Engineering from North Carolina State University in Raleigh, North Carolina. George has more than 20 years of applied structural engineering experience specializing in residential, commercial, and industrial structures and foundations. Throughout his career, George has designed and analyzed structures, supervised engineers, prepared construction documents (drawings and specifications).  He has an in-depth knowledge of many building codes, standards, rules, and regulations including the agencies that govern and provide guidance to building designers such as the International Code Council (ICC) American Society of Civil Engineers (ASCE), Steel Joist Institute (SJI) and the American Iron and Steel Institute (AISI).

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