Rheological properties and stability of ethylene vinyl acetate polymer modified bitumen

To improve bitumen (BIT) properties as binder for asphalt, polymers are commonly used. The polymers improve the temperature susceptibility of bitumen by increasing its stiffness at high service temperatures and reducing stiffness at low service temperatures. The addition of polymers as bitumen modifier contributes to BIT viscoelasticity. The semi-crystalline copolymer, ethylene vinyl acetate (EVA) is one of the plastomers used as polymer modifiers for BIT. EVA polymers modify bitumen by forming a tough and rigid network that resists deformation. One of the most important properties of asphalt pavements is stability on ageing. It is very important to follow the ageing processes because the paving bitumen and polymer modified bitumen (PmB) are submitted to two main ageing steps. These are, firstly, the mixing process with aggregates at high temperature and laying process where chemical structure of BIT and PmB may be disrupted rather quickly and, secondly, the service life in the pavement where the structure changes more slowly but steadily. The aim of this work was to study the effect of the addition of EVA copolymer on the morphology, rheological and physical properties before and after ageing. The influence of EVA contents on rheological and physical properties which are involved in conventional tests before and after ageing were investigated. The rheological properties of BIT and PmB, i.e. complex shear modulus (G*), complex viscosity (ni*) and phase angle (delta) were measured with a dynamic shear rheometer (DSR) in the linear viscoelastic range (LVN) and in a broad temperature range at traffic frequency, before and after ageing. The critical temperature, i.e. resistance on permanent deformation is done according to SHRP (Strategic Highway Research Program). The resistance of BIT and PmB on permanent deformation was determined from the equation G*/sin delta. The morphology of BIT and PmB was determined by means of fluorescent light optical microscopy. Conventional tests were determined against norms. The ageing of materials, thermooxidative stability, was determined by Rolling Thin Film Oven Test (RTFOT). The results indicated that the rheological properties, morphology as well as physical properties of the PmB were dependent on the polymer content. The rheological properties of road bitumen were improved by means of EVA polymer modification as proven by conventional tests, rheological parameters G*, ni*, delta and resistance to permanent deformation. With the addition of EVA, G* and ni* were increased as well as the elastic response. PmB has a higher temperature resistance on permanent deformation under traffic frequencies than BIT which means better properties in use in road construction. At low polymer contents, the samples showed the existence of dispersed polymer particles in a continuous bitumen phase, whereas at high polymer content (5%) a continuous polymer phase was observed. The penetration and softening point were increased with the EVA addition. After ageing the penetration values and elastic recovery decreased, while the softening point increased. It indicated that thermooxidative degradation was present and the crosslinking processes after primary degradation were involved. The phase angle (delta) of BIT and PmBs decreased and the plateau on the curve delta/T was destroyed after ageing which indicated that the physical polymer network was degraded as a consequence of polymer and bitumen ageing.