THICKNESS DESIGN FOR FLEXIBLE PAVEMENT: A PROBABILISTIC APPROACH

The thickness design procedure presented in this paper makes use of the concepts of limiting subgrade strain to control permanent deformation and limiting tensile strain in the asphalt layer (or limiting tensile stress in the cement-treated layer, if applicable) to control fatigue cracking. The input variables such as traffic load, ambient temperature, and subgrade resilient modulus are considered stochastic. The design nomographs incorporate reliability in design (50, 65, 80, and 95 percent), which is a unique feature of the method adopted here. Design nomographs are prepared for structural sections that consist of an asphalt concrete surface and a base of the designer's choice (asphalt treated, dense-graded aggregate, or cement treated) placed directly on the subgrade. A rational method for selecting asphalt grade is an integral part of the design procedure. The asphalt selection criteria dictate the use of relatively low-stiffness bituminous mixtures in cold climates and high-stiffness mixtures in hot climates. The region-to-region modulus variation, however, is accounted for by the judicious use of a multiplying factor that would transform the nomograph thickness to the "true" design value. To assess the reasonableness of the proposed procedure, the design thickness has been compared with that of the revised AASHO guide and with the Thickness Design Manual (MS-1) of the Asphalt Institute.