• Screening of Asphalt Extenders and Recycling Agents

    NCDOT Research Project Number: 2020-16

 Executive Summary

  • The objectives of this project are to: 1) develop recommendations for extender and recycling agent product approval and dosage selection; and 2) identify appropriate aging procedure(s) to simulate long-term oxidative aging coupled with test methods and parameters to characterize the cracking resistance of asphalt binders. 

    Three NCDOT approved surface asphalt mixtures were evaluated in this study, including a reclaimed asphalt pavement (RAP) mixture, recycled asphalt shingle (RAS) mixture, and RAP/RAS mixture. Three extenders and two recycling agents (RAs) were evaluated. The extenders were blended with PG 58-28 virgin binder whereas the RAs were blended with PG 64-22 virgin binder. Blends of recycled binder, virgin binder, and a RA or extender were prepared that achieved similar high-temperature AASHTO M 320 performance-graded properties to virgin PG 64-22 binders in North Carolina. These blends exhibit distinct but also potentially superior low- and intermediate-temperature characteristics compared to PG 64-22 virgin binders on the basis of performance-grade properties and several other rheological measures of durability. 

    In contrast, NCDOT’s current practice to use a PG 58-28 virgin binder in high recycled binder replacement mixtures resulted in similar intermediate- and low-temperature performance-graded properties to PG 64-22 virgin binders in North Carolina for the materials evaluated. However, the current practice resulted in high-temperature performance-graded properties that were distinct from PG 64-22 virgin binders. 


    (a) RAS lumps visible after blending in original state (b) No RAS lumps visible after RTFO aging

    The collective results highlight that neither the current practice to use a PG 58-28 virgin binder in high recycled binder replacement mixtures or the use of an extender or RA can fully restore rheological properties of recycled binder blends to those of PG 64-22 virgin binders. The inability of additives of fully restore rheological properties was also identified in some cases through inferior measures of the balance between stiffness and relaxation characteristics, including ΔTc and R values.

    Long-term aged asphalt mixtures prepared according to the current practice and with the additives displayed similar dynamic modulus and fatigue cracking resistance, which may have been a consequence of the harsh laboratory-induced age level. Short-term aged mixtures prepared according to the current practice and with the additives met recommended Hamburg Wheel Track Test rutting limits. The additives were found to be ineffective at increasing recycled binder contribution in asphalt mixtures. 

    Based on the collective results and findings, an additive dosage selection and product approval process was proposed. In addition, the results of this study suggest that the consideration of alternative rheological measures of binder durability that are readily obtained from standard Performance Graded (PG) test results may aid in screening of asphalt binders for cracking resistance

Cassie A. Castorena
Cassie A. Castorena
Andrew D. Wargo, Ph.D., PE
Mustan Kadibhai, PE, CPM
NC State University

 Report Period

  • August 1, 2019 - February 28, 2021


  • Complete


  • Pavement, Materials and Maintenance

 Sub Category

  • Asphalt Mix Design

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