Innovative Evaluation of Crumb Rubber Asphalt and Recycled Asphalt Pavement
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Recycling asphalt pavement is consistent with the concept of sustainability and when engineered properly, it can potentially provide a more cost-effective alternative to conventional road practices. In 2011, the Centre for Pavement and Transportation Technology (CPATT) at the University of Waterloo, the Ontario Tire Stewardship (OTS), and the Ministry of Transportation of Ontario (MTO) partnered to conduct several demonstration studies on the innovative use of Crumb Rubber Modified (CRM) asphalt pavements and Reclaimed Asphalt Pavement (RAP). In a bid to better understand and resolve the technical challenges associated with recycled Hot Mix Asphalt (HMA) mixtures as well as to advance Ontario’s paving industry to a more sustainable and economically viable direction, this research involved a comprehensive laboratory testing program to characterize the behaviour and mechanistic properties and compare the overall performance of an array of typical Ontario Superpave HMA mixtures incorporating 0, 15, 20 and 40% RAP, and CRM mixtures with 20% RAP. The laboratory testing protocols selected to characterize these mixtures include: Binder rheological assessment tests to assess failure properties and grade asphalt binders; Thermal Stress Restraint Specimen Test (TSRST) to determine fracture susceptibility at low-temperatures; the Hamburg Wheel Tracking Device (HWTD) for assessing the combined effects of rutting, stripping potential and moisture susceptibility; and Dynamic Modulus tests to evaluate the viscoelastic properties of the experimental matrixes over a range of loading frequencies and temperature scenarios. The study also included forensic assessment of past CRM pavement sections, field monitoring of the 2011 CRM-RAP demonstration sections, and an overall cost and sustainability assessment. The main research findings are summarized as follows: • Rheological characterization of binders indicated that the influence of RAP variation is highly related to the performance grade of the base virgin asphalt cement; while CRM binder modification significantly improved both complex shear modulus, G* and phase angle, δ parameters regardless of the binder grade. This had a significant impact on the rutting and thermal cracking performance of the evaluated HMA mixtures. • With exceptions to the recovered binders from 20% and 40% RAP HMA mixtures with PG 52-40 and 52-34 asphalt cement, all other recovered binders were observed to be more flexible at low and intermediate temperatures suggesting that the potential for improved resistance to fatigue failure exists. • Assessment of dynamic modulus, |E*| and phase angle, δ data suggests that the observed mix stiffness is not exclusively a function of the improvements made by the improved characteristics of the binder, but in combination with other factors. The master curve construction using the rheological analysis software (RHEATM) confirmed these behavioural tendencies. The observed mix stiffness was further observed to correlate well with the mechanistic performance test results. • The wet-process rubber terminal-blend HMA mixture was noted to be distinctively different from the rubber field-blend mixtures in terms of performance, but no evidence within the concerns of this research suggest that the rubber field-blend method is not effective or feasible. • Forensic studies on extracted pavement cores indicated that the observed pavement distresses are related to aggregate segregation resulting from the effects of permeability possibly caused by poorly constructed or compacted longitudinal joints. • In-service pavement monitoring indicated that the Rubberized-RAP sections in Ontario are all performing very well in comparison to the control sections with RAP. • Study findings also demonstrated the potential to incorporate up to 40% RAP contents into rubberized pavements. However, such designs must take into consideration the consensus properties of the aggregates and volumetric properties of the binder. • The 40% RAP HMA mix was found to be the most environmentally friendly pavement design alternative. However, the 20:20% CRM-RAP HMA mix was judged the most innovative and optimal sustainable option having satisfied the functional performance criteria and being the most cost-effective. Based on these findings, the research recommends that CRM used in Ontario rubberized HMA mixtures be subjected to both cryogenic and ambient methods of grinding. This is a more effective way to ensure better or comparable performance with conventional HMA mixtures. The implication of this would be higher initial construction costs, but the many benefits associated with rubberized pavements including its prolonged service life would provide a trade-off over the pavement’s lifecycle; especially in terms of maintenance or the need to carry out major rehabilitation.