Baseline Cracking Characterization of Arizona Asphalt Mixes
📨 Contact: Hasan Ozer
🔖 Co-PI: Kamil Kaloush
🤝 Sponsor: Southwest Pavement Technology Consortium (SWPT)
📅 Timeline: 2021 – 2022
Highlights
Introduction
Cracking is the dominant pavement distress concern in Arizona. While extreme summer heat captures the public imagination, it is the state’s equally extreme temperature swings that create the fatigue conditions most damaging to asphalt pavements over time. Designing mixes that can withstand both the thermal demands of summer and the fracture demands of low-temperature and fatigue loading requires knowing, quantitatively, how well current mixes actually perform. Yet before this study, no systematic database of cracking resistance existed for the range of mixes used across Arizona’s diverse climatic regions.
The Southwest Pavement Technology (SWPT) consortium initiated this project to close that gap. Led by Principal Investigators Dr. Hasan Ozer and Dr. Kamil Kaloush at Arizona State University and published in July 2022, the study evaluated 13 plant-produced Arizona mixes alongside supplemental laboratory mixes covering a range of binder grades, SBS polymer modification levels, and mix design types. The goal was not only to characterize existing practice, but to establish the quantitative benchmarks that agencies will need when transitioning to a balanced mix design (BMD) approach, one that explicitly considers both rutting resistance and cracking resistance as co-equal design requirements.
Methodology and Framework
The testing program was built around four complementary fracture tests, each capturing a different dimension of cracking behavior at different temperatures and loading rates. The IDEAL-CT measures the cracking index at intermediate temperatures (25°C), reflecting fatigue cracking susceptibility. The I-FIT provides a flexibility index that correlates with field fatigue cracking performance. The Semi-Circular Bending (SCB) test measures fracture energy and tensile strength at intermediate and low temperatures. The Disc-Compact Tension (DCT) test specifically targets low-temperature fracture energy, probing the mix’s resistance to thermal cracking in cold conditions. Together, this test battery provided a multi-dimensional performance fingerprint for each mix.
A key innovation of the study was the development of the “critical cracking temperature” concept: for each mix, researchers identified the temperature at which the cracking performance index drops below an acceptable threshold. Arizona was divided into three climate zones based on pavement temperature data, and critical cracking temperatures were mapped to each zone, allowing mix designers to select binders and mix types appropriate for local thermal conditions. The study also proposed a phased BMD implementation roadmap connecting test thresholds to agency adoption milestones.
Key Findings
Effect of Binder Grade and Polymer Modification
The study found a clear and consistent relationship between binder stiffness, polymer modification, and cracking performance. Unmodified binders at higher performance grades (PG 76-16 and PG 82-16) showed notably lower cracking resistance at intermediate temperatures compared to softer or polymer-modified alternatives. SBS polymer modification significantly improved performance across all four test methods, with polymer-modified mixes showing 40–80% higher fracture energy in SCB tests and substantially improved IDEAL-CT cracking indices. This finding has direct implications for Arizona’s binder selection philosophy: optimizing exclusively for rutting resistance risks creating mixes that are brittle and vulnerable to fatigue and thermal cracking during the cooler months.
At low temperatures, the DCT results revealed that some unmodified stiff-binder mixes had critical cracking temperatures well within the range of actual Arizona winter pavement temperatures, particularly in northern Arizona, the high desert, and at high elevations. In those regions, thermal cracking driven by insufficient low-temperature fracture resistance may already be contributing to observed field distress, even though Arizona is not typically thought of as a “cold-climate” state.
SMA Mix Design Performance
Stone Matrix Asphalt (SMA) mixes emerged as the strongest performers in the cracking battery, outperforming conventional dense-graded mixes by a wide margin across all test temperatures and methods. SMA mixes benefited from their rich binder content, gap-graded aggregate structure, and the use of fiber stabilizers that retain binder in place during compaction. The SCB fracture energy and IDEAL-CT cracking index for SMA mixes were dramatically higher than for conventional Superpave mixes at equivalent binder grades, suggesting that mix type itself is a powerful lever for improving cracking resistance. These results support increasing SMA usage in Arizona for high-traffic surface courses where cracking durability is a priority.
Roadmap for Balanced Mix Design in Arizona
A key output of the study was a practical BMD implementation roadmap tailored to Arizona conditions. The roadmap proposed a phased approach: first, establishing the cracking performance database built in this study as a reference library; second, selecting candidate test thresholds for cracking performance criteria appropriate to each climate zone; and third, running parallel mix design programs that simultaneously satisfy HWTT rutting thresholds and cracking performance thresholds. The study recommended IDEAL-CT and SCB as the primary cracking tests for routine BMD implementation in Arizona, based on their repeatability, relatively simple specimen preparation, and demonstrated sensitivity to field cracking performance, while suggesting DCT for supplemental evaluation in northern Arizona and high-elevation applications.