360R-06 Design of Slabs-on-Ground
R-14 ACI COMMITTEE REPORT
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ACI COMMITTEE REPORT Specifications are frequently adopted to control only the minimum field density, such as 95% of the standard Proctor maximum density or 90% of the modified Proctor maximum dry density. To achieve a more uniform subgrade modulus, however, a range of density should be specified. For example, 100 ± 5% of the standard Proctor maximum density, or 95 ± 5% of the modified Proctor maximum dry density. The range specified, however, should be compatible with the type of soil, uniformity of soil, contractor’s opera- tion, and project needs. Specifying a lower density range for clay soils having a plasticity index of 20 or higher, for example, 92 ± 4% of the standard Proctor maximum dry density, is often used to control volume changes. Frequently, a moisture content range is also specified, for example, within ±3% of the optimum moisture content of the appropriate test. Higher moisture contents, from optimum moisture content to 4% above it, are frequently used to minimize volume changes. 3.6.4 Subbase and base materials—For many slabs-on- ground, the existing subgrade will provide adequate support. Table 3.3—Soil stabilization with chemical admixtures Admixture Quantity, % by weight of stabilized soil Process Applicability Effect on soil properties Portland cement Varies from approximately 2-1/2 to 4% for cement treatment to 6 to 12% for soil cements. Cohesive soil is pulverized so that at least 80% will pass No. 4 (4.75 mm) sieve, mixed with cement, moistened to between optimum and 2% wet, compacted to at least 95% maximum density and cured for 7 or 8 days while moistened with light sprinkling or protected by surface cover. Forms stabilized subgrade or base course. Wearing surfaces should be added to provide abrasion resistance. Not applicable to plastic clays. Unconfined compressive strength increased up to approximately 1000 psi (6.9 MPa). Decreases soil plasticity. Increases resistance to freezing and thawing, but remains vulnerable to frost. Bitumen 3 to 5% bitumen in the form of cutback asphalt emulsion, or liquid tars for sandy soils. 6 to 8% asphalt emulsions and light tars for fine-grained materials. For coarse-grained soils, antistrip compounds are added to promote particle coating by bitumen. Soil is pulverized, mixed with bitumen, solvent is aerated, and mixture compacted. Before mixing, coarse- grained soils should have moisture content as low as 2 to 4%. Water content of fine-grained soils should be several percent below optimum. Forms wearing surface or construction stage, for emergency conditions, or for low-cost roads. Used to form working base in cohesionless sand subgrades, or for improving quality of base course. Not applicable to plastic clays. Provides a binder to improve strength and to waterproof stabilized mixture. Calcium chloride 1/2 to 1-1/2% Normally applied at a rate of approxi- mately 0.5 lb/yd 2 (0.27 kg/m 2 ) area. Dry chemical is blended with soil aggregate mixture, water added, and mixtured compacted at optimum moisture by conventional compaction procedures. Used as dust palliative. Stabilized mixture of gravel-soil binder calcium chloride forms wearing surface in some secondary roads. Retards rate of moisture evaporation from the stabilized mixture, tends to reduce soil plasticity. Greatest effect in sodium clays with capacity for base exchange. Lowers freezing point of soil water, decreasing loss in strength from freezing and thawing. Lime 4 to 8%. Fly ash, between 10 and 20%, may be added to increase pozzolanic reaction. Lime is spread dry, mixed with soil by pulvimixers or discs, moisture compacted at optimum moisture to ordinary compaction densities. Used for base course and subbase stabilization. Generally restricted to warm or moderate climates because the mixture is susceptible to breakup under freezing and thawing. Decreases plasticity of soil, producing a grainy structure. Greatest effect in sodium clays with capacity for base exchange. Increases compressive strength up to a maximum of approximately 500 psi (3.4 MPa). Fig. 3.4—Standard Proctor curves for various soils. (Note: 1 lb/ft 3 = 0.1571 kN/m 3 .) Fig. 3.5—Standard and modified Proctor curves. |