Water is an essential ingredient in concrete, but uncontrolled excessive moisture can create a whole host of problems with concrete floor slabs. Some of the modes of distress include:

  • Adhesive breakdown of adhered finish floor coverings
  • Debonding of coatings
  • Osmotic blisters of epoxy systems including coatings and epoxy terrazzo
  • High pH (alkali) attack of floor finishes
  • Microbial growths
  • Flooring expansion, such as cupping of wood strips or planks
  • Reactions between incompatible floor patching/leveling materials

Although the interaction of moisture and floor finishes (adhered floor coverings and coatings) has gained much attention in recent years, moisture issues also affect bare concrete floors through the following modes:

  • Staining
  • Efflorescence
  • Expansion of contaminants and popouts leading to further cracking
  • Condensation on slab surfaces making hazardous conditions for traffic
  • Curling

Despite the development of many excellent, longstanding practices, we continue to see a large number of moisture-related problems and hundreds of millions of dollars are still spent annually in the U.S. to correct such problems. Some contributing factors include: design professionals still specify a blotter/cushion/subbase layer over a vapor retarder—six years after ACI Committee 302 reinstated its original recommendation for slabs to be placed directly on the vapor retarder; vapor retarders are not carefully protected during construction; concrete floor slab mixes often are made with smaller than desirable aggregate and mixes are not optimized for minimum shrinkage; inappropriate moisture tests still are specified; moisture testing is not always done correctly; some unreasonably moisture-sensitive adhesives are still on the market; there is no performance specification or test method for moisture resistance of adhesives or floor coatings; and failures of some moisture-suppression products occur because there are no performance specifications for moisture suppression products. Let's look more closely at some of these continuing causes of moisture problems and how to avoid them.

Vapor Retarders

This author firmly believes that a floor slab is part of the building envelope and that every slab on ground should have a vapor retarder meeting ASTM F1745 installed directly beneath the concrete. Just as we do not tolerate water leaks in roofs or walls, we should not accept structures built so that moisture can infiltrate floor slabs. Highly moisture-resistant floors can be constructed following the principles in ACI 302.1R-06, “Guide for Concrete Floor and Slab Construction,” and PCA EB119, “Concrete Floors and Moisture.” A properly selected and installed vapor retarder is essential for long-term moisture resistance.

Vapor retarder material selection and placement was the subject of an extraordinary amount of debate during the 1990s. The original ACI 302-69, “Recommended Practice for Concrete Floor and Slab Construction,” indicated vapor barriers are used under the slab when floor coverings, household goods, or equipment must be protected from damage by moist floor conditions.

In the years following, some field experience suggested that concrete placed directly on a low permeability vapor retarder was more prone to cracking and curling.

In the subsequent ACI 302.1R-89, “Guide for Concrete Floor and Slab Construction,” the use of a blotter or cushion layer on top of the vapor retarder was first described. Vapor barriers were said to aggravate the problem of plastic and drying shrinkage cracking and should be avoided if ground moisture conditions permit. A granular, self-draining fill was recommended to act as a blotter over the vapor barrier.

In the ensuing decade, many floors built with this blotter/cushion layer construction detail suffered moisture-related failures. The many reasons for these failures are now quite well understood.

  • A 6-mil polyethylene sheet placed over granular subbase and covered with compacted granular material often sustains many punctures, rendering the vapor retarder ineffective.
  • Dampening the blotter/cushion layer to compact it adds significant water to the floor system. For example, 6% moisture in a 3-inch-thick cushion layer adds roughly 1½ pounds of water below every square foot of concrete. A concrete mix typically contains about 3 pounds of water per square foot of floor for a 4-inch-thick slab at 0.5 w/c ratio with 517 pounds of cementitious material (neglecting water in aggregate); thus, the minimal water in the cushion layer required for compaction increases the free water in the floor system by 50%, providing a long-term source of unwanted moisture.
  • The blotter/cushion layer can be exposed to precipitation and become exceedingly wet prior to concrete placement.
  • Compacted granular subbase fill typically has 10% to 20% void space. This space on top of the vapor retarder can act as a “plenum” for unwanted moisture intrusion over long distances. For example, water infiltration at the perimeter of the structure can travel with little resistance in this layer.
  • If the vapor retarder is installed intact, then it functions as a “bathtub” beneath the granular blotter/cushion layer. Liquid water or moisture vapor that gets into this layer after construction (e.g., pipe leaks, perimeter infiltration) cannot easily drain outward through the vapor retarder and therefore infiltrates and recharges the slab from below.

The commonly cited advantages of a blotter/cushion layer (less bleeding, shorter time to trowel, less plastic shrinkage cracking, reduced curling) can be achieved just as well by using well-designed mixes with properly graded aggregates and following ACI recommended hot weather concreting practices when necessary.

In April 2001, ACI Committee 302 published an update to this guideline, reverting to early ACI and PCA practice, recommending that concrete slabs be placed directly on vapor retarders if the slab will receive vapor sensitive floor covering. It is this author's opinion that all floor slabs should be constructed directly on an appropriate vapor retarder for the following reasons: