Appendix G 7 Factors Affecting Fatigue Performance
Major factors influencing fatigue performance in laboratory tests include:
- air voids content
- mode of loading
- mixture variables (particularly volume of binder).
Mode of loading
Two modes of loading are generally used for laboratory characterisation of fatigue performance. One method uses controlled stress and the other uses controlled strain. The two methods may give different results. The controlled strain method is more applicable for thin asphalt layers (< 80 mm) where the addition of the asphalt layer will not appreciably affect the deflection of the pavement under traffic. The controlled stress mode is more applicable for thick asphalt layers (> 150 mm) where the stiffness of the asphalt layer will affect the pavement deflection. The controlled strain method of measurement is generally used in Australia since the majority of asphalt surfacings are relatively thin.
Effect of mixture variables on fatigue performance
The effect of mixture variables on fatigue performance depends on the mode of loading. Table G 2 shows indicative trends resulting from an increase in the mixture variable. The information is applicable to the controlled strain mode of testing as used in the Austroads method. Results may be different for some of the variables if a different mode of testing, e.g. controlled stress, is employed.
Flexural stiffness may be different to resilient modulus values even when measurements are made at the same temperature and loading time. In the flexural stiffness calculation there is no assumption made of Poisson’s ratio, as is the case for the resilient modulus test, and also the mode of loading is different.
Laboratory fatigue life results can be used to rank materials or can be used in pavement design. For ranking purposes, comparison of fatigue life at the same strain level is a suitable means of comparing relative fatigue performance. When large differences exist in the stiffness of materials, this approach may be incorrect due to different strain levels which pavement structures may undergo.
Laboratory fatigue data may be used in pavement design. A shift factor is normally applied to laboratory data to account for the higher fatigue life encountered in the field. This increase in field fatigue life is attributed to factors such as rest periods, traffic wander and periodic healing. The magnitude of the shift factor has to be determined from comparison of laboratory data with field performance. Values in the range 10 to 100 times have been reported.
A summary of factors affecting the fatigue life is given in Table G 2.
|Increasing mixture variable||Effect on fatigue life||Effect on flexural stiffness|
|Grading coarse to fine||Increase||Decrease|