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effect of shear rate on bitumen viscosity measurements – relevance.docx

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effect of shear rate on bitumen viscosity measurements – relevance

EFFECT OF SHEAR RATE ON BITUMEN VISCOSITY MEASUREMENTS – RELEVANCE TO HIGH TEMPERATUREPROCESSING OF BITUMINOUS PRODUCTSG.A.J. Mturi1, S.E. Zoorob2 and J. O’Connell11CSIR Built Environment, Transport Infrastructure Engineering, Pretoria 0001, South Africa, E-mail: gmturi@csir.co.za, E-mail: joconnel@csir.co.za2Coventry University, Department of Built Environment, Coventry CV1 5FB, United Kingdom, E-mail: salah.zoorob@AbstractASTM D4402 is the current standard test method for viscosity determination of bitumen at elevated temperatures (38-260°C), using a rotational viscometer. It specifies measurements to be done at a resisting torque range of 10-98% of maximum torque at a non-specified speed. This makes the test method neither a controlled shear rate (CSR) test nor a controlled shear stress (CSS) test. This paper discusses the limitations of rotational viscometry, especially when used for the determination ofviscosity measurements for shear-thinning modified binders, as well asthe advantages of dynamic shear rheometry (DSR) as an alternative method of viscosity measurement. Although temperatures at the pumping of bitumen and at the mixing and compaction of asphalt mixes are based on set binder viscosities, shear rates remain uncontrolled as the methods were established for unmodified binders with Newtonian flow at high temperatures. The implication of using uncontrolled shear rate viscosities for shear-thinning modified binders is that compaction and mixing temperatures can be overestimated.The current high temperatures used when processing shear-thinning modified binders have a significant effect on such issues as economics (energy costs), environment (carbon footprint) and binder/mix stiffness (ageing).INTRODUCTIONIn CSR rotational tests, the shear rate or rotational speed is preset or controlled. These tests are useful for liquids without a yield point and those to be used at a defined shear rate or flow conditions such as processing a liquid through a pipe.In CSS