Forensic Investigation to Determine the Reasons for Premature Failure in an Asphalt Surface Layer

ABSTRACT Premature failure in the form of cracking, stripping and brittleness of a hot mix asphalt surfacing on a major road in South Africa was investigated. Anecdotal evidence as well as pavement structural evaluation with the FWD confirmed that the distress was restricted to the surface layer alone. Testing on the recovered binder indicated that excessive filler material was present in the mix. Testing of the filler included (laser) grading of filler, X-ray diffraction (XRD) tests, X-ray fluorescence spectrometry (XRFS) tests and an isotope dating test. It was found that prescribed addition of one percent lime was largely absent, which raised questions regarding the origin of the filler material. The filler was found to be super fine (average size 10 micrometer) and of mostly a tillite material origin. Rigden voids, viscosity, and softening point tests on the filler-binder mastic helped to quantify the stiffening effect of the filler. Binder film thicknesses were significantly reduced due to the surface area of the super-fine filler, which is linked to the observed brittleness, early cracking and stripping of the asphalt layer. KEYWORDS: Premature FailureAsphaltFillerSuperfinesMastic