This layer also serves as a pressure field extender for a soil gas ventilation system, if one is installed. This layer helps to further prevent bulk soil moisture from wicking up to the slab and allows for that moisture to be drained out if a drainage system is installed (PATH 2006). A capillary break layer consisting of three to four inches of clean gravel (no fines) should be installed below the vapor retarder.It is recommended that the vapor retarder be in direct contact with the concrete slab and that no sand or gravel be placed in between (Lstiburek 2008). The vapor retarder will prevent moisture in the ground from diffusing through the slab and into the building. A vapor retarder such as a 6 mil thick polyethylene sheet should be placed directly below the concrete slab (DOE 2009).Manage exterior ground and rain water by using gutters and downspouts and by grading the ground around the perimeter at least six inches of fall over ten feet of run.These two slab-on-grade configurations are illustrated in Figures 4-2 and 4-3. These guidelines and recommendations apply to thickened edge/monolithic slabs and stem wall foundations with independent above grade slab configurations (PATH 2006). This is done by using adequate drainage and by the use of vapor retarders. The following construction practices will prevent excess water in the form of liquid water and vapor from creating problems. Improper moisture management can lead to structural damage, damage to floor finishes, and mold growth, which can be very costly to repair and hazardous to one's health. Techniques for controlling the build-up and movement of moisture in the foundation are an essential component of the overall construction. Drainage Techniques for Slab-on-Grade Foundations Capillary flow through subsurface foundation assembliesįigure 4-3.Second, liquid water must be kept from accumulating around and under the foundation. First, since the soil in contact with the foundation and floor slab is always at 100% relative humidity, foundations must deal with water vapor that will tend to migrate toward the interior under most conditions. In general, moisture management schemes must control water in two states. In these cases, consultation with local building officials and a structural engineer is recommended. Where expansive soils are present or in areas of high seismic activity, special foundation construction techniques may be necessary. For this reason, footings must be placed beneath the maximum frost penetration depth unless founded on bedrock or proven non-frost susceptible soil or insulated to prevent frost penetration. Freezing water beneath footings can heave, causing cracking and other structural problems. Footings must be designed with adequate size to distribute the load to the soil. Similarly, grade beams at the edge of the foundation support the superstructure above. Concrete spread footings must provide support beneath foundation walls and columns. Foundation walls must be designed to resist vertical loads from the structure above and transfer these loads to the footing. The proper use of welded wire fabric and concrete with a low water/cement ratio can reduce shrinkage cracking, which is an important concern for appearance and can also aid radon infiltration control strategies.įoundation walls are typically constructed of cast-in-place concrete or concrete masonry units. Concrete slab-on-grade floors are generally designed to have sufficient strength to support floor loads without reinforcing when poured on undisturbed or compacted soil. In some cases additional footings (often a thickened slab) are necessary under bearing walls or columns in the center of the slab. The major structural components of a slab-on-grade foundation are the floor slab itself and either grade beams or foundation walls with footings at the perimeter of the slab (see Figures 4-2 and 4-3). Slab-on-Grade Foundation with Exterior Insulation 4.1 Recommended Design and Construction Details
Grade beam with brick veneer/ exterior and under slab insulationįigure 4-1.Concrete wall with brick veneer/ under slab insulation.
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