The impact of Hisingerite on asphalt shear resistance
Two Marshall-designed asphalt mixtures were developed with common coarse aggregate, added filler (1% hydrated lime) and bituminous binder (5.8% acid-modified bitumen). The target aggregate grading and overall volumetrics were not significantly different. Different fine aggregate (dust sources) were used in each asphalt mixture. Dust A contained predominantly Nontronite clay minerals while 82% of the Dust B clay minerals were identified as Hisingerite. Hisingerite is a rarely encountered and poorly studied clay mineral. Specialist geotechnical interpretation of its unique properties indicated potentially adverse impact on mastic stability and asphalt shear response. This research assessed the impact of Hisingerite clay on otherwise identical asphalt mastic, as an indicator of the impact of Hisingerite-rich dust sources on asphalt performance. Six M1000 (acid-modified) bitumen samples were each used to manufacture two mastic samples, one containing each fine aggregate. The same hydrated lime was added, and all mastic samples were manufactured with constant proportions of bitumen, added filler and fine aggregate, representative of the asphalt mixture designs. Wheel tracking, resilient modulus, tensile strength and moisture resistance testing did not indicate any detrimental effects from Hisingerite-rich Dust B nor did mastic complex modulus master curves. Multiple stress creep recovery testing found the Dust B mastic to deform less than Dust A mastic under cyclic shear. This was concluded to likely reflect the lower density and higher absorption of the Dust B fine aggregate. Despite the concerning physical properties, it was concluded that the incorporation of Hisingerite-rich fine aggregate presented no risk to asphalt performance. It follows that there is no basis for additional routine testing of potential dust sources to identify Hisingerite content. This research also highlighted the importance of controlling mastic composition during testing, with further testing at a constant ‘effective’ binder volume required. Maintaining the filler dosage constant relative to the mass (or volume) of the dust (or the bitumen) must also be considered.
