Equivalent Axle Load Factors for Design of Rigid Pavements Derived from Fatigue Criteria

Józef Judycki

doi:10.3846/bjrbe.2011.28

Abstract

This paper presents a determination of equivalent axle load factors for rigid pavements on the basis of fatigue criteria used in mechanistic-empirical methods of pavement design. Fatigue criteria for selected methods of rigid pavement design and analysis are presented. These criteria were used to derive the formulas to calculate equivalent axle load factors. The authors found that formulas derived for rigid pavements are much more complex than the well-known “fourth power equation”. The equivalency factors depend on the slab thickness and bending strength of cement concrete and other pavement parameters. There are considerable differences between the equivalent axle load factors derived from different fatigue criteria used in various design methods. The presented analysis indicates that traffic analysis and fatigue criteria should be interrelated.

Highlights

The equivalent axle load factors are the basis for determining the design traffic and, as a consequence, for designing of road pavements
This paper presents analytical determination of axle load equivalency factors for rigid pavements based on fatigue criteria applied in modern mechanistic-empirical methods of pavement design
The equations for calculating the equivalent axle load factors Fj for rigid pavements derived from the analysed fatigue criteria are much more complex than “fourth power equation” which is often used in practice

Summary

Introduction

The equivalent axle load factors are the basis for determining the design traffic and, as a consequence, for designing of road pavements. The AASHTO Guide for Design of Pavement Structures (1993) uses standard axle load of 80.1 kN (18-kip) and presents a set of tables which define the equivalent axle load factors for rigid pavements in relation to actual axle load, axle configuration, concrete slab thickness and terminal value of pt – Present Serviceability Index. Kuo and Lin (2001) used finite element model and developed regression formulas for equivalent axle load factors for rigid pavements These factors differed significantly from the AASHTO 1993 method and were strongly related to concrete slab thickness, slab support and material strength. This paper presents analytical determination of axle load equivalency factors for rigid pavements based on fatigue criteria applied in modern mechanistic-empirical methods of pavement design. This assumption is justified by the Westergaard formula for stresses in concrete slabs (Eq (24))

Derivation of equivalent axle load factors for rigid pavements

Relation between stresses induced in concrete slab and axle load

Calculated values of equivalent axle load factors Fj

Conclusions