Steady-state constitutive relationship for idealised asphalt mixes V.S. Deshpande, D. Cebon * Department of Engineering, Cambridge University, Trumpington Street, Cambridge CB2 1PZ, UK Received 28 October 1998 Abstract A constitutive model for the steady-state deformation of idealised asphalt mixes is developed with the long term view to understanding the permanent deformation (rutting) of flexible pavements under vehicle loads. Triaxial compression tests are conducted on idealised asphalt mixes. Both volumetric and deviatoric strains are measured. The specimens are observed to dilate under compressive stresses and the deformation behaviour is seen to be dependent on the mean as well as the deviatoric stresses. A simple model for the overall steady-state behaviour of idealised mixes is developed using a ‘‘shear box’’ analogy from soil mechanics and theories for the deformation of nonlinear viscous composites. This constitutive relationship is dependent on the mean as well as deviatoric stresses and accounts for the dilation of the mixes under compressive loading. Predictions of the model are found to agree well with experimental measure- ments. Ó 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction The purpose of a pavement is to carry trac safely, conveniently and economically throughout its design life. There are several reasons why a pavement may cease to fulfil its function. The most common forms of structural failure are permanent deformation (rutting) and fatigue (cracking). The development of a model which explains the rutting of flexible pavements relies, at least in part, on understanding the constitutive behaviour of asphalt or bituminous mixes which are used in the upper layers of pavements. The majority of con- stitutive models for bituminous mixes are empiri- cal and consider only uniaxial stress states (usually compressive stresses) (Deshpande and Cebon, 1998). However, in order to study the response of a pavement under a wheel load, an understanding of the overall constitutive behaviour of asphalt is needed. Nijboer (1948) characterised bituminous mixes using soil mechanics theories. He distinguished between the behaviour of various mixes under triaxial loading using the angle of friction u and the cohesion c. Huschek (1985) extended the work of Nijboer (1948) and proposed a model which separated the total deformation resistance of the mix into three components: initial resistance, in- ternal friction due to the aggregate and the viscous resistance due to the bitumen. Each mechanism was modelled separately and then assembled to give an overall model describing the axial defor- mation behaviour of a mix under triaxial loading. However, his model needed to be calibrated by Mechanics of Materials 31 (1999) 271–287 * Corresponding author. E-mail: [email protected] 0167-6636/99/$ – see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 6 3 6 ( 9 9 ) 0 0 0 0 4 - 6
Steady-state constitutive relationship for idealised asphalt mixes
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