Mechanistic design optimisation of thin asphalt surfacings using measured non-uniform tyre-road contact and incorporating rectangular contact shapes and layer cross-anisotropy

Abstract

Mechanistic design and construction of flexible pavements with relatively thin (< 50 mm) asphalt road surfacings still remain an art today and are aimed especially towards first-class user experiences. Risks for surfacing failures are difficult to assess mechanistically, but poor design and construction will undoubtedly lead to dissatisfied road users. Traditional mechanistic pavement design commonly uses a simple single circular tyre model with uniform vertical contact stress. It is popular because of its relative simplicity (mathematical) and fast speed of solution. Although relatively thin asphalt surfacings are still regarded as a “functional” layer designed and built by experienced designers and contractors, research has pointed towards noticeable effects when designing these layers as “structural” layers with improved tyre-road contact models, such as rectangular shapes, and/or non-uniform contact stresses. Assessment of tyre-road contact shapes from field data (52 895 truck tyres) shows that only 10% of tyre-road contact patches can be classified as circular, with 66% are single rectangular, and 24% are of a triple-rectangular shape. This information points towards the need for an improved mechanistic design of thin surfacings that focuses on optimisation and longevity. Research and associated analysis have led to improved closed-form analysis that is optimised for speed and contact shape. In this paper, improved multi-layer linear elastic analysis that incorporates an option to divide the tyre-road contact into three rectangular shapes (in addition to the conventional circular shape) is demonstrated. The software used in the study was developed for mechanistic-empirical (me) CRoss-anisotropic Analysis of Multi-layer Elastic Systems (meCRAMES). It was designed to include options for single/multiple circular and/or rectangular tyre-loading contact shapes, lateral loading, layer cross-anisotropy and interlayer slip. Analyses that were presented indicate potential failure zones inside these thin asphalt surfacings, which are dependent on tyre-road contact shapes, layer cross-anisotropy and/or interlayer slip. It is recommended that these aspects be incorporated during the design stage.