Permanent deformation of asphalts

The simplest test used to study the deformation behaviour of asphalts was the static unconfined uniaxial compression test, termed the creep test, developed in the 1970s by Shell Bitumen (Hill 1973). The specimen was subjected to static axial compressive load over a long period (1 h). The test procedure was very simple and required low-cost equipment. In addition. Shell Bitumen developed a rut prediction procedure based on results of the creep test but soon realised that it underestimated rut depths measured in trial pavements (Hill et al. 1974). This was attributed to the effects of dynamic loading producing higher deformation in the wheel-tracking test (Van de Loo 1974). 

The inability of the static creep test to simulate the real loading conditions on site and to reflect better the improved performance of binder modifiers led to the establishment of the dynamic (cyclic) loading test at the beginning of the 1980s (Finn et al. 1983 Valkering et al. 1990). 

Although the repeated load axial test is a potential performance test to assess the deformation resistance of asphalt (BSI 1996), various researchers (Monismith and Taybali 1988 Nunn et al. 1999) proved that the application of confining (lateral) pressure had a significantly positive effect on asphalt performance when it was tested under axial loading. 

The use of the static creep test without confining lateral pressure, which was the first to be introduced, is currently almost obsolete. 

Better simulation in the laboratory of the stress conditions that exist in a pavement related to rut formation provides the loading of the specimen by a moving wheel. Hence, the wheeltracking test was developed to determine the performance of the asphalt in permanent deformation, particularly its susceptibility to deform under load. 

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The contribution of the asphalt binder to permanent deformation of asphalt mixtures is considered in terms of the minimum value for the stiffness parameter, G /sln5, at a maximum pavement design temperature (i. e., 1000 Pa 10 rad/s). This parameter correlates to that portion of the accumulated, non-recoverable deformation occurring In a pavement that Is attributable to the asphalt binder. The higher the value, the less deformable Is the pavement. Isochronal plots of GVsin 5 reveal distinct differences in the contribution of the additives to the mixture stiffness. In Figure 5, It is clear that the base asphalt would qualify for a maximum use... 

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