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
