1 DETERMINING THE EFFECT OF SUPER FINE TILLITE FILLER IN AN ASPHALT LAYER

An investigation into the cause of premature failure of a premix asphalt layer on a road in South Africa identified the filler material in the mix as the main cause. The failure was characterised by signs of dryness and an increase in permeability within a few months of laying the asphalt; distress was evident even while the project was still in progress. Later phases of deterioration were stripping of binder from surface stone, pumping and crocodile cracking. The asphalt was within specification and no immediate cause was apparent. Specialised testing was conducted on the recovered binder, detailed filler grading, X-ray diffraction (XRD) tests, X-ray fluorescence spectrometry (XRF) tests and isotope dating. The ashing of the recovered binder indicated that a super fine filler material was present in the mix. It was determined that the origin of the super fine filler material was mostly tillite. The stiffening effect of the superfine filler material was quantified by the Rigden voids test, and by testing the filler binder mastic. Binder film thickness and mix stiffness calculations quantified the influence of the observed super-fine tillite filler size material in the asphalt mix, and were used to explain the mechanism of distress.