Greg White

Most of Australia's runways are surfaced with Marshall designed, dense graded, hot mix asphalt. There surfaces are typically replaced by asphalt overlay every 10-15 years. Traditionally, conventional C320 bitumen was used as the binder. To address a number of perceived performance issues, since the start of the 21st century, airport asphalt in Australia has often included premium binders. There is a perception of a reduction in performance of binders in Australia and overseas. Aircraft and their operation at major airports have also become ever more severe with continued growth in wheel loads and tyre pressures. The field performance of airport asphalt has been questioned with a number of failures being well documented. Binder comprises only 5-6% of the mass of airport-quality asphalt. However, its influence on the performance of the mixture cannot be disputed. This critical element of this mix is, however, one of the least transparently monitored elements. Aggregates are subject to an array of independent testing and petrographic analysis, quarry faces are inspected and source rock approved ahead of time. Bitumen, by contrast, is required only to pass through empirical 'gates' for viscosity and other parameters, which vary for different grades and binder types. In an effort to improve airport asphalt's field performance a new proprietary binder was developed called JetBind. JetBind was developed to meet the specific demands of airport pavements in Australia's generally hot climate. The high temperature/high shear performance of the JetBind was compared to that of commonly specified products. Comparisons were made using the USA's Performance Grading test known as the Multiple Stress Creep Recovery protocol for the Dynamic Shear Rheometer. Comparisons were also made for Marshall and other properties. The test results show the benefits of JetBind in comparison to the current grades of binder for Australia airports.

Aircraft braking forces have increased and will continue to increase in the future. This has resulted in increased shear stresses at the interface between the surface layer and the underlying pavement. An increase in reports of delamination failures has followed. Shear stresses induced by an extreme aircraft braking event were compared to measured interface shear strengths. Interface shear strength was measured from cores recovered from runway surfaces typical of major Australian airports. The runways were exposed to aircraft operations also typical of major Australian airports. The resulting stress-strength ratios identified an area 50 mm in front of the leading edge of the tyre as the critical location for delamination failure where stress-strength ratios approached 1.0. Interface cohesion is provided by tack coat adhesion and contributes to interface shear strength. It is recommended that a premium tack coat material, such as JetBondTM, be specified for future airport overlay projects.