Many of the moisture problems common to slabs on the ground can be minimized or eliminated by using the correct grading, properly selecting fill or sub-base materials, and installing a vapor retarder.
How to Solve Moisture Problems with Vapor Correction
The first step to addressing the moisture problem is to determine whether a vapor retarder or vapor barrier is necessary. Vapor retarders are often incorrectly referred to as "vapor barriers." A vapor retarder is a material that will effectively minimize the transmission of water vapor from the soil support system through a slab, but it is not 100 percent effective in preventing the vapor's passage.
Protection against moisture is necessary for any slab on grade that will be covered by tile, wood, carpet, impermeable floor coatings (such as urethane, epoxy, or acrylic terrazzo), or where the floor will be in contact with any moisture-sensitive equipment or product. Concrete slabs 4 or more inches thick, of good quality and correctly consolidated, are impermeable to the passage of water from the ground, unless the water is under considerable pressure. However, concrete of the same thickness is not impermeable to the slow passage of water vapor. This water vapor needs to be eliminated in order to stop deterioration of floor coverings or damage to sensitive equipment.
Vapor retarders are rugged, multireinforced plastic sheets. ACI Committee 302 recommends that a polyethylene film with a thickness of no less than 10 mils (0.25 millimeters) should be used as an effective vapor retarder. This thickness offers increased resistance to moisture transmission while providing more durability during and after its installation.
The blotter layer is a common practice in many parts of the United States (Figure 1). The 4-inch layer of material serves as a blotter to help equalize moisture content and reduce plastic and shrinkage cracking. The fill takes water out of the bottom of the slab while evaporation and heat take the water out of the top of the slab. The blotter helps shorten bleed time, making it possible to start finishing earlier. This layer also serves as protection for polyethylene films from normal jobsite traffic and abuse.
Placement directly under a slab: Contractors should not place concrete directly on the vapor retarder unless the blotter layer has the potential of becoming a water reservoir (Figure 2). Water trapped in the blotter layer would have to come through the slab as water vapor. The only time concrete should be placed directly on a vapor retarder is when there is the possibility of the blotter layer becoming wet. Not using a blotter layer can increase the potential for curling, surface crusting, and longer bleeding periods.
Curling and surface cracking result from the bottom of a concrete slab holding moisture longer than the top of the slab. Concrete placed in direct contact with a vapor retarder has significantly larger dimensional change in the first hour after casting than concrete placed on a granular base. This change in dimension can cause shrinkage cracking. A vapor retarder used directly under a slab can also cause premature finishing, in which the finishing process starts before all of the bleed water reaches the surface. Sealing the bleed water in will cause scaling and other surface defects.
Concrete placed directly on a vapor retarder needs to have superior quality and low slump. Lower slumps will help with the longer bleeding period and decrease the waiting period before floating and troweling can start.
Lafarge Corporation is committed to providing the best possible technical services to our customers. Should you have any questions regarding the use of vapor barriers or vapor retarders with concrete, contact your local Lafarge technical sales engineer.