- Sponsor
- Department of Civil and Environmental Engineering
Optimization of Polymer-Softener-Modified Binder for Resilient and
Sustainable Asphalt Concrete
Advisor: Professor Imad L. Al-Qadi
Flexible pavements are increasingly expected to carry heavier traffic under wider
temperature extremes while incorporating higher recycled binder contents. These
combined demands increase susceptibility to distresses and have accelerated the
adoption of modified binders (polymers and softeners). However, polymer–softenermodified
binders remain insufficiently understood, particularly with respect to
compatibility, stability, aging sensitivity, and whether potential performance gains translate
into measurable pavement and life-cycle benefits. A framework to design, evaluate, and
implement polymer–softener-modified binders and mixtures for resilient, sustainable,
flexible pavements was developed to address this gap.
Four connected tasks were developed and executed that span binder, mixture,
pavement, and life-cycle scales. Binders were modified to produce softened, polymermodified,
and polymer–softener-modified binders. Binder characterization included
Fourier transform infrared spectroscopy (FTIR) oxidation and polymer degradation
indices, storage stability (cigar tube test), thermal stability (mass loss test), softener
volatility (thermogravimetric analysis), together with cracking-and fatigue-related indices
(ΔTc and Δ|G*|peak τ). Balanced asphalt concrete (AC) mixture performance was evaluated
using the Illinois Flexibility Index Test and Hamburg Wheel-Tracking Test, complemented
by Asphalt Mixture Performance Tester (AMPT)-based mechanistic tests (dynamic
modulus, cyclic fatigue, and stress sweep rutting).
A predictive model, which included a risk-based failure framework, was developed
to estimate mixture cracking potential (Flexibility Index) from binder rheology and key
mixture variables. Finally, a cradle-to-grave Life Cycle Assessment (LCA) of AC with eight
binder alternatives was conducted over a 50-year analysis period. FlexPaveTM-predicted
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rutting and cracking were translated into roughness growth to trigger rehabilitation and to
quantify roughness-related excess fuel consumption.
Dual modification, polymer–softener-modified binder, is feasible and valuable
when used in AC; however, the modified binder performance depends on softener
chemistry, thermal exposure, and base binder source. Softeners can mitigate SBS
degradation during aging. While softener volatility may adversely affect intended lowtemperature
benefits, thermally stable softeners maintain properties and reduce long-term
cracking potential.
At the AC mixture scale, AC with softened binder exhibited less cracking potential
than unsoftened mixes. While polymer modification improved AC balanced performance
by increasing fatigue tolerance and reducing rutting susceptibility, AC with softened binder
reduced cracking potential but increased rutting potential when used without polymer.
Hence, AC with polymer–softener-modified binders provided the benefits of both, and
rutting and cracking potential could be controlled. It was found that polymer-softener
modification increased the AC fatigue life, as indicated by the fatigue indices DR and sapp,
substantially. In addition, the AC rutting strain index confirmed a reduction in rut potential.
Although the materials stage of cradle-to-gate life cycle assessment (LCA) is
dominated by binder impacts, the life-cycle is driven by performance. AC with polymer–
softener-modified binders reduced rehabilitation frequency, thereby lowering
maintenance and use-stage impacts, due to slowed roughness growth controlled by
rutting and cracking. It was concluded that well-designed polymer–softener blends can
improve flexible pavement performance, durability, and sustainability.