Guidelines for Permitting Overloads; Part 1: Effect of Overloaded Vehicles on the Indiana Highway Network

Abstract

Closed form analysis is commonly used to analyze pavement structures. This type of analysis assumes linear elastic material properties and static loading conditions. In reality, pavement materials are not linear elastic materials. For example, asphalt mixtures are viscoelastic materials and cohesive soils are elastic-plastic materials. Also truck loads are moving loads. The difference between the closed form analysis assumptions and the actual pavement conditions leads to significant difference between measured and predicted pavement response. A study has been conducted at Purdue University to develop a procedure for permitting overloaded trucks in Indiana. This study was funded by the Indiana Department of Transportation (INDOT) and the Federal Highway Administration (FHWA). As a part of this study a three-dimensional, dynamic finite element program (3D-DFEM) was used to analyse flexible and rigid pavements and develop load equivalency factors. Truck loads moving at different speeds were included in the analysis and a number of material models were used to represent the actual pavement materials behaviour under moving loads. The 3D-DFEM was verified for flexible and rigid pavement analysis. Two verification studies were conducted for each pavement type: Static, linear elastic analysis and dynamic, nonlinear analysis. In the static verification studies, linear elastic material properties were assumed and the 3D-DFEM predictions were compared with the results of a multi-layer analysis (for flexible pavement) and Westergaard's equations (for rigid pavement). In the dynamic analysis verification studies, measured pavement deflections were compared with the 3D-DFEM predictions under similar conditions. All verification studies showed excellent agreement between field and predicted pavement response. Load equivalency factors (LEF) were developed for flexible and rigid pavements. The LEF of any load j and cross section i was defined as the number of the 18-kip single axle load (SAL) applications required to develop the same pavement response of one pass of load j on the same cross section i. Permanent deformation at the pavement surface which accumulates from different layers is used as the equivalency criteria for flexible pavement LEF's, while total surface deformation, elastic and plastic, is used for rigid pavement LEFs. Both LEF sets are based on nonlinear dynamic analysis and consider the effect of load repetitions. Comparisons between the developed LEFs and the appropriate AASHTO LEFs showed excellent agreement.