360R-06 Design of Slabs-on-Ground
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| 3.6.5 Stabilization of base and subbase—Base and
subbase materials are often densified by mechanical compaction to improve the k value. The relative cost of possible alternatives, such as chemical stabilization of the subgrade, use of high-quality base courses, or providing a thicker slab, should be considered. The mechanical compaction of clay and silt is measured as a percent of standard Proctor density (ASTM D 698) or modified Proctor density (ASTM D 1557). Minimum dry densities typically specified for these materials are from 90 to 95% of the maximum dry densities of the standard and modified tests, respectively. 3.6.6 Grading tolerance—Usually, compliance with the initial rough- and fine-grading tolerance is based on level surveys using a grid pattern of no more than 20 ft (6.1 m). Grading tolerances specified for a project should be consis- tent with the recommendations of ACI 302.1R, Chapter 4. 3.6.7 Vapor retarder/barrier — Because all concrete is permeable to some degree, water and water vapor can move through slabs-on-ground (Brewer 1965; Neville 1996). This can adversely affect the storage of moisture-sensitive products on the slab, humidity control within the building, and a variety of flooring materials from coatings to carpets. Manu- facturers of these coverings specify a maximum moisture emission rate from the slab surface, generally in the range of 3 to 5 lb/1000 ft 2 (12 to 21 N/100 m 2 )/24 hours or a maximum relative humidity, generally 75 to 80% at a depth of 40% of the slab thickness. The use and the location of vapor retarders/barriers require careful consideration. Figure 3.7 provides guidance. Excess water in the slab not taken up by chemical action will evaporate through the top of the slab until equilibrium is reached with ambient humidity. Additionally, moisture can transpire from the subgrade and through the slab. If the base material under the slab is saturated and subjected to a hydro- static head, as for a basement slab below a water table, liquid water may flow through cracks or joints in the concrete. If hydrostatic forces can occur, they must be included in the slab design considerations. The amount of flow will depend on the amount of head and the width, length, and frequency of the joints and cracks in the concrete. If the base material is saturated or near saturation and there is no head, moisture can still be transmitted into the slab by capillary action of the interconnected voids in the concrete. Positive subgrade drainage is necessary where water would otherwise reach the slab base. Further, an open-graded stone is frequently used as a base course to form a break against capillary rise of moisture in the subgrade. Although vapor retarders/barriers can substantially reduce vapor transmission through slabs, some water vapor will transpire through the slab if the vapor pressure above the slab is less than that below the slab. Climate-control systems may lower the relative humidity above the slab and result in water vapor movement through the slab. The vapor pressure is a function of temperature and relative humidity. The vapor drive is from high to low humidity and from warm to cold temperatures. The temperature of the soil subgrade is usually lower than that of the space above the slab. The relative humidity of the subgrade is typically 100%. Water in the subgrade under slabs-on-ground can change due to seasonal fluctuations of shallow water tables, capillary rise in the subgrade soils, poor subsurface drainage, ponding of storm water adjacent to the slab-on-ground, overwatering of plants and lawns adjacent to the slabs-on-ground, or from Fig. 3.6—Effect of subbase thickness on design modulus of subgrade reaction. (Note: 1 pci = 0.2714 MN/m 3 ; 1 in. = 25.4 mm.) |