Rheological characteristics of polymer modified and aged bitumens
The demands on asphalt pavements, as a result of the growth in traffic volumes, traffic loads and tyre contact pressures, has resulted in an increased interest in the use of modified bitumens, particularly over the last ten years. Of the various types of modified and specialised binders that are available worldwide, polymer modified bitumens (PMB’s) have tended to be the most popular. Polymer modification significantly alters the rheological characteristics of the binder, thereby requiring the use of fundamental rheological testing methods rather than empirical methods, to provide an indication of the performance of the binder and subsequently the asphalt mixture. This thesis is concerned with the use of a Dynamic Shear Rheometer (DSR) to quantify the fundamental rheological characteristics of various unaged and aged PMB’s. The parallel plate testing methodology used with the Bohlin Model DSR50 Dynamic Shear Rheometer requires accurate temperature control by means of a circulating fluid bath, a combination of different plate diameters and sample geometries, and the use of small strains in order to measure the linear viscoelastic rheological characteristics of a bitumen specimen. Conventional and dynamic shear rheometry testing of various penetration grade bitumens, semi-crystalline ethylene vinyl acetate (EVA) PMB’s and thermoplastic rubber styrene butadiene styrene (SBS) PMB’s have indicated that the rheological characteristics differ considerably between the unmodified and polymer modified bitumens as well as between the plastomeric EVA and elastomeric SBS PMB’s. The DSR dynamic rheological parameters of complex modulus and phase angle indicate that the modification mechanism of EVA PMB’s consists of the crystallisation of rigid three dimensional networks within the bitumen. These rigid crystalline structures increase the stiffness and elastic component of the viscoelastic balance of the PMB up to the temperature associated with the melting of the copolymer. The modification mechanism of SBS PMB’s consists of the establishment of a highly elastic network within the bitumen that increases the elasticity and stiffness, particularly at high temperatures. The higher melting temperature of the SBS copolymer allows the rheological character of the SBS PMB to extend to temperatures greater than those found for EVA PMB’s. DSR measurements of the rheological changes associated with laboratory ageing of EVA PMB’s indicate that the ageing mechanism is linked to a chemical change of the copolymer due to fusion of the crystallites. This chemical change leads to a degradation of the polymer and, therefore, a transition of the rheological behaviour towards that of an unmodified bitumen. The rheological changes associated with the ageing of SBS PMB’s is linked to a breakdown of the molecular structure of the SBS copolymer to form a lower molecular weight polymer substructure. This results in an increased viscous behaviour after ageing compared to the increased elastic behaviour found for unmodified bitumens.