Full depth asphalt pavements are designed to have little flexibility and can suffer from fatigue damage. The main factors contributing to fatigue damage are design parameters, climate (temperature), and traffic speed. Balancing these factors and providing road users with excellent roads without over designing is our challenge. WARRIP sought to apply the latest research done for Queensland in conjunction with Austroads Guides to improve the current WA design procedures. Laboratory testing was done to investigate both stiffness and fatigue of our typical WA asphalt mixes.
The purpose of this project is to characterise the stiffness and establish fatigue modulus values and performance of typical Western Australian asphalt mixes and evaluate opportunities for improving current asphalt mix and structural design practices in the states north, where WMAPT’s are greater than 35 °C. The aim of this study is to improve the cost-effectiveness and fatigue parameters for design of full depth asphalt (FDA) pavement design outcomes in WA.
Asphalt is a viscoelastic material and the stiffness is dependent on both the rate of loading (speed of traffic) and temperature. At higher temperatures and lower traffic speeds, asphalt exhibits lower modulus values.
In the current design methodology, asphalt fatigue life is directly related to the asphalt modulus. In thick (generally > 150 mm) asphalt pavements, lower modulus values result in greater deflections and higher strains at the bottom of the asphalt layer, thus, leading to increased fatigue damage (i.e. cracking) and a reduction in design life. As a result, increased asphalt pavement layer design thicknesses are required for thick (generally > 150 mm) asphalt pavements where the pavement temperatures are high and/or traffic speeds are low.
However, this is not the failure mechanism observed at high weighted mean annual pavement temperatures (WMAPTs) in the field. Observations of asphalt fatigue damage in service indicate that most of the damage accumulates at low temperatures.
WMAPTs in Western Australia vary from 24 °C in Albany, to 42 °C in Kununurra. The hot climate in the northern areas of the state, with WMAPTs in excess of 30 °C, therefore, leads to very thick FDA pavement design outcomes using the current design procedure.
- A review of the current asphalt thickness design procedures, including asphalt stiffness characterisation
- A review of findings from related projects
- Asphalt stiffness and fatigue testing of four typical Western Australia mixes
- Development of flexural modulus master curves and mix specific asphalt fatigue models for the mixes tested in accordance with Austroads test method AGPT/T274. Test setup depicted overleaf
- Parametric analysis comparing asphalt pavement layer design thickness using the current design procedure with mix specific input
Laboratory Testing and Findings
Four WA asphalt mixes have been tested as part of this project and indicated that:
Futures Studies Recommendations
- Testing at 30 °C and higher pushed the limits of AGPT/T274. The impact of higher temperatures must be investigated.
- Implement interim design approach for the design of asphalt layers that accounts for the reduced fatigue damage expected to occur at elevated pavement temperatures.
- Investigate a WMAPT limit where asphalt fatigue is no longer the critical failure mode.