RHEOLOGICAL ANALYSIS OF BITUMENS BY DYNAMIC SHEAR RHEOMETER: EFFECT OF THE THERMAL HISTORY ON THE RESULTS

In order to prevent binder-related pavement failures, it is very important to be able to characterize the performances of bituminous binders that play an essential role in the construction and reinforcement of pavements. However, because of its specific and complex composition, bitumen is a visco-elastic material whose intrinsic rheological properties depend upon the temperature and the loading time. One way to characterize this particular behaviour is to measure the response of the material under dynamic loading conditions, for instance in shear mode. The visco-elastic nature at high and intermediate pavement temperatures can then be described by Dynamic Oscillatory Shear Rheometer (DSR) measurements. But, the applicability of the preparation of test samples usually carried out on neat bitumens is now questioned when analyzing, in particular, modified binders: test standardisation becomes more and more necessary with a wide agreement on the required specifications. This paper points out the effect of the test sample preparation on the rheological results obtained by DSR measurements. In this study, two different sample preparations were used to explore the thermal effect: loading the binder into moulds, 24 hours prior to testing in the rheometer, or loading of the sample directly into the plates gap. In order to cover a variety of sampling procedures, a 70/100 penetration grade bitumen was chosen and two Polymer modified Bitumens were manufactured by adding 3% of saturated plastomer (Ethylene/Vinyl Acetate) or 3% of unsaturated elastomer (Styrene/Butadiene/Styrene) to this base bitumen. These three binders were then tested before and after ageing. Moreover, the effect of temperature rising up or going down during testing in the rheometer was determined on a 6% saturated plastomer modified bitumen. Because of this high degree of modification (phase inversion), the temperature effect was exalted. This effect was then studied against both sample preparations previously analysed. In conclusion, the determination of the complex modulus and the phase angle using a Dynamic Shear Rheometer appears to be sensitive to the sample preparation and especially to the thermal history of the binders. Consequently, in order to make relative comparison between samples, it seems very important to have the same sample thermal history. This one appears to be more critical in the case of plastomer modified bitumen. For the covering abstract see ITRD E121480.