Francois Engelbrecht, WC Dept of Transport
PAPER
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Abstract
The parameters of the Bending Beam Rheometer (BBR) test give an indication of an asphalt binder’s ability to resist low temperature cracking. The standard test procedure of the ASTM D 6648-08 (2012) was used to determine the flexural creep stiffness of bituminous binders using the BBR. The method specifies two test specimen moulds fabricated from either aluminium/stainless steel or silicone for low-temperature testing.. Bituminous binders were evaluated at various ages to analyse their changes to rheological behaviour.
This research included several degrees of laboratory simulated ageing: Unaged, Rolling Thin Film Oven (RTFO) and with the Pressure Ageing Vessel (PAV) at 20 hr, 40 hr and 80 hr. The unaged and aged binders were tested with the BBR ranging from -36°C to 0°C, using rubber and steel moulds, and Dynamic Shear Rheometer (DSR) at temperatures ranging from 10°C to 70°C.
Due to noticeable differences resulting from each mould type, the dimensions of the BBR beams were measured before and after testing. The rubber moulds appeared to be at the lower limit of the specification tolerance and the steel moulds at the upper limit. In some cases the rubber-moulded beams did not meet the ASTM requirements, and the results were unusable. The tests done with the steel moulds resulted in less fluctuation in the results of bituminous binder stiffness.
The research concludes that the steel moulds are more accurate during the BBR testing to evaluate the binder’s behaviour in low temperatures. The standard test method of the ASTM prescribes that both moulds may be used.
About the author
Francois Engelbrecht obtained his BEng degree (2015) and MEng degree (2017) in Pavement Engineering at Stellenbosch University. He gained specialised experience in the rheology of bituminous binders while undertaking his Masters research on “Age related performance of typical seal binders in South Africa” and presented a paper on this research at CAPSA 2019. Francois works for the Western Cape Government Department of Transport and Public Works.
