Investigating the rheological properties of hot bituminous mastics made up using plastic waste materials as filler

The large amount of plastic waste is currently causing serious pollution to the environment mainly due to long decomposition times and their dispersion. In order to avoid that the pollution by plastics could worsen further, it is trying to encourage companies to recycle existing pallets to create new products. This study investigates the volumetric and mechanical properties of hot bituminous mastics made up using plastic waste materials (PW) as filler. The PW has a maximum size of 2 mm and a melting temperature between 120 and 260 °C. Scanning Electron Microscopy of the PW reveals a mesh structure of elongate elements marked by a much rougher surface than traditional limestone filler (LF). The first step of the study focused primarily on searching for the optimal blending time for the mastics; a total of nine mastics were made up, and no substantial differences were found for any of them on increasing mixing time from 10 up to 60 min in terms of softening point, penetration at 25 °C or dynamic viscosity at 100 °C. Thus, a period of 10 min was selected for mixing all the mastics. The second step focused on the investigation of rheological properties using a dynamic shear rheometer and carrying out a Frequency Sweep (FS) test at temperatures ranging from 0 °C to 50 °C with increments of 10 °C, and a Multiple Stress Creep and Recovery (MSCR) test under 0.1 and 3.2 kPa load levels at temperatures of 40 and 50 °C. The two resulting best solutions are made up of a) 20% PW (HP02), and b) 5% LF plus 15% PW (HLP02), both by the total weight of B5070. Specifically, the shear modulus G* for mastics containing PW showed a distinct increase in G* compared both with B5070 and traditional mastics made up of LF; in particular, the gap becomes more marked moving towards 50 °C, where the best performance of HP02 is obtained. Moving on to illustrate the main results of the MSCR analysis, it was observed that the HP02 and HLP02 solutions showed good recoverable deformation already from the end of the loading phase without waiting for the last step of the unloading cycles at two test temperatures and under two stress levels.