Optimization of bitumen bands in mastic asphalt for high-speed bus routes

The increasing frequency of pavement distresses at Bus Rapid Transit (BRT) stations in Tehran, including alligator cracking, rutting, and shoving, necessitates the application of high-performance asphalt mixtures. This study assesses the feasibility of utilizing Stone Matrix Asphalt (SMA) containing 0.3% cellulose fibers as a replacement for conventional Hot Mix Asphalt (HMA) at these stations. Following field investigations at three BRT lines in Tehran, mix designs for both asphalt types were prepared using identical aggregates and 60/70 penetration grade bitumen. Specimens were fabricated with five different bitumen percentages (three replicates each) and subjected to Marshall tests, resilient modulus testing (ASTM D4123), and rutting evaluation using the Hamburg Wheel Tracker (AASHTO T324). Statistical analysis of the results indicated that the SMA mixture, with an optimum bitumen content of 6.1%, exhibited a significantly higher resilient modulus (3250 MPa vs. 2100 MPa, P<0.001) and a substantially lower final rut depth (3.2 mm vs. 8.1 mm, P<0.0001) compared to the HMA mixture with an optimum bitumen content of 4.38%. This represents a 60% reduction in rutting potential. This superior performance is attributed to the SMA's gap-graded aggregate structure, higher filler content, and the formation of a mastic reinforced with bitumen-fiber mortar. Consequently, cellulose fiber-reinforced Stone Matrix Asphalt is recommended as a suitable option for enhancing the durability of pavements at BRT stations in Tehran.