The Neutral Axis Concept in Asphalt Pavement Analysis

The neutral axis concept is a fundamental and useful entity in elastic beamlike and platelike structures (where it is called the neutral surface). It distinguishes between tensile and compressive zones, governs how moments are redistributed when entering an inelastic range, and functions as a damage-sensitive attribute in condition monitoring applications. The objective of this study was to investigate the existence and characteristics of the neutral axis concept for layered elasticity - which is the most prevalent theory in the design and analysis of asphalt pavements. The investigation was done in silico by first examining the concept for an isotropic half-space, and then for two- and three-layered half-spaces. The main findings from these synthetic investigations were (1) more than one neutral surface can exist; (2) stress and strain neutral surfaces do not overlap; and (3) locations of the neutral surfaces are affected by the loading configuration. An actual case study involving Interstate 475 was examined to demonstrate the practical implications of these findings. It is concluded that all familiar/expected neutral axis characteristics deviate considerably when carried over to layered elastic theory. It is thus recommended to avoid utilizing intuition obtained from the classic neutral axis concept to interpret the mechanistic behavior of asphalt pavements.