Table of Contents

4.2.1 Background

A major objective of asphalt research and development programs in Australia and overseas, since the late 1980s, has been the development and implementation of simple fundamental and simulative tests for characterisation of asphalt mixes to supplement or replace empirical tests that did not directly relate to road performance. In Australia, this has resulted in the implementation of asphalt mix design procedures based on gyratory compaction as well as a series of performance‑related tests.

In the USA, research programs have resulted in the implementation of ‘Superpave’ (SUperior PERforming asphalt PAVEment). The volumetric design concepts used in Australia are similar to Superpave. The main difference in the Australian developed procedure is the adoption of locally developed, more affordable equipment for compaction and mechanical testing of asphalt.

The aim of the Australian program was the development of a mix design procedure that:

  • is performance-related
  • enables the in-service performance of mixes to be predicted
  • is relatively affordable (in terms of new equipment cost)
  • is rapid and easy to use.

Outcomes of the Australian research program were first published in 1997 as APRG Report No.18 Selection and Design of Asphalt Mixes: Australian Provisional Guide. This was subsequently revised in 1998 and 2002, republished as Austroads Technical Report AP-T20-03, and included as an appendix to the first edition of this Guide in 2007. Updates of the design procedure, sometimes referred to as the Austroads mix design procedure, are incorporated in this Guide.

The driving force in the adoption of gyratory compaction was the belief that it achieves alignment of aggregate particles that is more representative of field placement of asphalt and is preferred where performance properties are to be measured.

While a number of road agencies in Australia and New Zealand have adopted the use of gyratory compaction for sample preparation and volumetric mix design, others continue to use Marshall compaction for volumetric properties or allow optional use of Marshall or gyratory compaction. The Modified Hubbard-Field Method for compaction or design of asphalt mixes has, however, been almost totally discontinued.

The Marshall method of compaction (i.e. impact compaction) remains in wide use due to its simplicity. It is recognised that the mechanical tests (i.e. stability and flow) used in the Marshall method of asphalt mix design are not necessarily good performance predictors, especially for heavy duty applications, due to the lack of correlation between the tested properties and the in‑place performance. Procedures for measurement and interpretation of Marshall stability and flow are included in this publication as they form part of the optimisation of volumetric properties. Additional performance testing (Level 2) using the procedures described in this Guide may, however, be undertaken on mixes designed by the Marshall method.

The status of adoption of alternative mix design procedures by road agencies at the time of preparation of this Guide is described in Implementation of Austroads Mix Design Procedures (Austroads 2013b).