Thickness Design of Slab-on-Grade Construction: Review of Some Key Aspects

Abstract

Slab-on-grade construction is popular for heavily loaded ground floor slabs. For design, graphical or tabular methods are recommended by US standards. Scattered design formulations for various loadings, namely, wheel load, distributed load, line load, and concentrated load, are compiled in this article from the literature. The effects of the three pertinent factors, namely, modulus of subgrade reaction, allowable tensile stress in concrete, and load contact area, over the design thickness are examined with a parametric study. The design depth of a slab is more sensitive to the lower values of the modulus of subgrade reaction. The allowable stress in concrete affects the design thickness more for the lower modulus of subgrade reaction, and, therefore, the concrete strength and factor of safety should be carefully chosen in such cases. The contact area of loads (wheel/concentrated) generally do not affect the design depth much, except for a contact area radius that is higher than 75 mm in one formula for concentrated loads. Now, to control cracking due to thermal or shrinkage effects, nominal reinforcement is provided in unreinforced grade slabs according to national (Indian) requirements. Taking advantage of this reinforcement, thickness reduction can facilitate economical construction. A nomogram is available in the literature for that purpose. To avoid human error in the application of the nomogram, a generalized expression was developed in this study. Using this equation, the percentage reduction in the unreinforced thickness of the slab on grade on account of the given percentage reinforcement can be directly estimated, regardless of the system of units followed. Alternatively, the reinforcement provided in existing grade slabs may be used to enhance their load-carrying capacity, and tables and charts showing the (notional) increase in the depth of slabs on grade are provided as well.