ABSTRACT
Conventional flexible pavement road surfacings are failing due to increasing automobile traffic and heavy axle loading. In order to overcome the challenge, researchers and road authorities have used various materials as modifiers or reinforcements to improve on the properties. This research presents the rheological properties of High Density Polypropylene (HDPP) waste in wet and dry hot mixed asphalt (HMA). Asphalt mixes were constituted using 0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0% HDPP by total weight of mix. Following Asphalt Institute heavy traffic requirements, Marshall tests were conducted on specimens to characterize the stability, flow and void requirement specified in codes. Simple Performance Tests (SPT), ageing process and morphological assessments were carried out, using dynamic modulus, indirect tensile strength, fatigue rutting, creep strain, Scan Electron Micrograph (SEM) and thermal resistivity tests to assess field performance and durability of mixes. The results of Marshall tests showed optimum improvements at 2% HDPP for wet process and 0.5% HDPP for dry process over the control (0% HDPP). The wet and dry processes dynamic moduli increased by 50% and 14% respectively. Indirect Tensile Strength (ITS) improved by about 14% for wet process and 8% for dry process. All the rutting test results for wet process were largely below the maximum threshold of 8mm specified by National Cooperative Highway Research Programme (NCHRP) and 12.5mm by American Association of State Highway Transport and Officials (AASHTO); while for the dry process, only 0.5% HDPP satisfied the recommendations of the two standards. Creep strain reduction of about 20% was obtained for wet process and about 10% reduction for dry process. The temperature profile conducted using Thermo-Gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) and Scan Electron Micrograph (SEM) for wet process showed increased performance and better enhancement at optimum value of 2.0% HDPP than the control. In terms of strength, SPT, morphology, ageing process and temperature resistivity, 2.0% HDPP asphalt mixes were optimally better than the control while the general performances of 0.5% HDPP dry mixes were equally enhanced and slightly better than control. The study observed that polypropylene modified specimens under heavy traffic conditions could improve physical, mechanical and rheological properties of HMA. Optimal modifications of mixes at 2.0% and 0.5% HDPP were recommended for wet and dry processes respectively. These optimal contents of HDPP could mitigate failures in both structural and serviceability conditions thereby impacting on durability and cost effectiveness of pavement surfacing.