Comparative study on using thermoplastic polymers to improve asphalt mixtures characteristics

Abstract

Asphalt mixtures are composite building materials consisting of a mineral skeleton mixed with a bituminous binder, following a recipe which may also include fibres and/or polymers. The natural aggregates sustain the mixture structure, but adequate bitumen behaviour under various temperature and mechanically-induced stresses is also essential for the structural durability. Much research effort was directed towards improving the asphalt mixtures’ resistance to permanent deformation, implying an increase in mixture stiffness. At the same time, the mixture must exhibit enough low temperature cracking resistance. Six reference asphalt mixture samples were prepared and tested: mixtures M1 and M2 for base and binder courses respectively, as well as four mixtures for wearing courses (two asphalt concrete - AC1 and AC2, a stabilised mixture SMA containing fibres, and a porous mixture - PM). A 50/70 penetration grade bitumen was used to prepare all mixtures. In some cases, the obtained results did not meet the standard requirements. Bitumen or mixture modification is commonly performed by adding thermoplastic or elastomeric polymers, to improve the asphalt mixture behaviour. In this study, the effects of four thermoplastic polymers on the stiffness modulus, dynamic creep and fatigue resistance were studied. All tested polymers were introduced as grains during mixture preparation. Polymer addition led to a 31 % to 104 % increase in mixture stiffness modulus. A 220 % average increase in fatigue resistance was observed for mixtures M1 and M2. For the wearing course mixtures, creep resistance is expressed through a 99 % reduction in deformation speed and a 50 % to 80 % reduction in rut depth. The obtained results met the standard requirements. Using grain polymers is currently an effective alternative to polymer-modified bitumen, because of several technological and economical advantages. Polymer quality is essential to obtain adequate mixture characteristics.