Evaluating the Mechanical Performance of Cold Mix Reclaimed Asphalt Pavement Incorporating Crumb Rubber
Keywords:
crumb rubber, reclaimed asphalt pavement, rutting, moisture susceptibility, indirect tensile strength, Cold mix asphaltAbstract
Cold mix asphalt (CMA), produced at ambient temperature, is less energy-intensive, cleaner and safer than hot mix asphalt, making it well-suited to rural low-volume roads. Durability is nevertheless limited by incomplete binder coating, high air voids and residual moisture, which promote moisture damage and rutting, while surface-treated crumb rubber has yet to be combined with cold-mix reclaimed asphalt pavement (RAP). This study therefore evaluated cold mix RAP incorporating NaOH-treated crumb rubber. Materials were characterised by physical, XRF, XRD and binder-recovery tests, and crumb rubber (2.00-0.075 mm) was NaOH-treated at 70 °C for 2 h, then oven-dried at 60 °C for 24 h. Three dense-graded mixes within the TRRL envelope were designed: virgin aggregate, 50:50 RAP, and RAP with rubber replacing the 0/6 mm fines at 0.5-3.0%. Initial emulsion and premix water contents followed the MS-14 formula and coating tests, while optimum contents came from second-order polynomial regression of dry density, indirect tensile strength (ITS) and air voids. Marshall specimens (50 blows, cured at 40 ± 1 °C for 72 h) were tested for dry and soaked ITS to determine the tensile strength ratio (TSR) and for rutting using the Hamburg wheel tracking test (50 °C, 705 N, 10,000 passes), and the results were compared using a t-test (? = 0.05). The RAP mix required 8.2% fluid and 6.2% emulsion, compared with 11.0% and 8.9% for the virgin mix, a 30% saving, with air voids of 9.6% versus 11.5% and TSR of 0.83 versus 0.86. At the optimum 1.8% rubber, air voids fell to 8.0%, TSR rose to 1.15 and mean rut depth fell from 2.74 ± 0.43 to 2.12 ± 0.22 mm, a significant 22.63% improvement (p = 0.043). Treated crumb rubber therefore mitigates both moisture damage and rutting, and 1.8% is recommended for cold mix RAP surfacing of low-volume rural roads carrying up to one million equivalent standard axles.
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