Permanent Deformation In Idealised “Sand Asphalt” Bituminous Mixtures

Abstract

ABSTRACT This paper investigates the quasi-static permanent deformation (rutting) behaviour of two idealised bituminous mixtures (sand asphalts). The permanent deformation behaviour is assessed using simplified quasi-static uni-axial and tri-axial laboratory tests performed over a range of temperatures, stress levels and strain rates. The results are compared to results from a standard repeated load laboratory test and a more simulative laboratory scale wheel tracking test performed at different temperatures and load levels. Results from the quasi-static uni-axial testing show that the deformation behaviour of the idealised mixtures follows the same pattern as the deformation behaviour of the bitumen used in the idealised mixtures (previously measured). At high stress levels the response is non-linear with a power law creep exponent of approximately 2.4. At lower stress levels the effective power law exponent decreases and the behaviour becomes more linear. The effect of the aggregate is to reduce the strain rate corresponding to a particular applied stress level (i.e. stiffening effect). Results from the tri-axial testing show that this stiffening effect depends on the ratio of mean stress to deviator stress as well as mixture volumetrics and temperature. Analysis of results from a standard repeated load uni-axial test often used as a practical tool to assess the permanent deformation resistance of bituminous mixtures shows that, due to the lower applied stress levels, the non-linear behaviour of the mixture is reduced resulting in lower values of the effective creep exponent. Results from more realistic laboratory scale simulative wheel tracking tests show a similar value of creep exponent to that measured in the quasi-static uni-axial and tri-axial tests due to the higher applied stress levels under the tyre in this test.