Evaluation of Recycled Components in Stone Matrix Asphalt Mixes

Currently, Missouri DOT (MoDOT) does not allow use of recycled materials in SMAs although the HMAs are routinely incorporated with them. MoDOT commissioned this study to investigate the effect of incorporation of recycled materials such as RAP and GTR in SMAs. In Phase 1 of the study, plant-produced mixtures were collected, and recreated in the lab under BMD methodology to determine acceptable upper-limit on RAP for initial implementation. In Phase 2, friction properties of mixtures were obtained by testing aggregate stockpiles, lab and plant produced mixtures, and a field section containing GTR. Based on the results, the following conclusions were drawn: 1) The current use of conglomerate (non-homogeneous), unfractionated RAP stockpiles poses practical limitations in achieving passing BMD thresholds at higher levels of RAP, 2) Suitable frictional characteristics were achieved at lower levels of RAP but results indicated that the softer aggregates present in current RAP stockpiles tended to reduce skid resistance, and 3) Use of GTR appears to be promising option for incorporating recycled materials into Missouri pavements, based on lab BMD and skid resistance results in the lab and field. Based on the findings, the following recommendations were made :1) MoDOT should consider allowing RAP in SMA mixtures but, an upper limit of 15% asphalt binder replacement (ABR) from RAP is recommended at this time, 2) If greater than 15% ABR by RAP is considered, a value engineering proposal should be provided, accompanied with BMD tests, 3) The research supports the continued and increased usage of GTR as a possible approach towards achieving good mixture durability, skid resistance, and mixture sustainability.

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Report number: cmr 23-016
Published: December 2023

Project number: TR202020
Authors: William G. Buttlar, Magdy Addelrahman, Punyaslok Rath, Jim Meister, Nandita Gettu, and Ahmed S. El-Ashwah

Performing organization: University of Missouri-Columbia and Missouri University of Science and Technology