This paper deals with an extensive research program into theupgrading of the fatigue transfer functions of asphalt mixes inuse in South Africa. Parts of the program have recently been completedand the aim of the paper is to summarise and discuss the resultsobtained to date. Two stages of the program are discussed in thispaper: a comparison of stiffnesses of asphalt mixes obtained fromdifferent sources, and the evaluation of the fatigue life of laboratoryasphalt mixes and development of laboratory fatigue regressionmodels.

In the first of the above-mentioned stages, stiffnesses ofasphalt mixes were determined in the laboratory (using the IndirectTensile Test) and from two theoretical stiffness prediction models(Shell method and Asphalt Institute method). Attempts were alsomade to backcalculate field stiffness using the Multi-Depth Deflectometer(MDD) results of four Heavy Vehicle Simulator (HVS) field tests.In the second stage fatigue tests were undertaken in the third-point beam fatigue test apparatus in the constant strain modeat various strain levels. Regression models were developed forthe following independent variables: strain, temperature, initialflexural stiffness, load frequency, aggregate grading and bindertype.

The findings indicate that the Shell BANDS- program and theAsphalt Institute's equation (AI) can be used for an approximatedetermination of asphalt stiffness, provided that the methodsare used correctly. There is also some indication that there isa correlation between the resilient modulus obtained in the IndirectTensile Test and the actual effective field stiffness, but thishas to be verified by further studies. In the second part of theinvestigation it was found that the fatigue regression modelsobtained from this study gave a higher life than other modelssuch as those of the Asphalt Institute, TRRL and NCHRP. The coefficientsof determination obtained were, however, very good.