Assessment of Thermal and Durability Cracks in Asphalt Pavements in the Southwest Region
📨 Principal Investigator: Hasan Ozer
🤝 Sponsor: Southwest Pavement Technology Consortium (SWPT)
📅 Timeline: 2023 – Ongoing
Highlights
Introduction
Wide transverse thermal cracks are increasingly common in asphalt pavements throughout the Southwest region, particularly in Arizona’s urban streets, highways, and parking facilities. Unlike traditional low-temperature cracking observed in colder climates, these cracks develop in regions where freezing temperatures are rare but large daily temperature swings and rapid cooling cycles are common. Over time, the cracks widen significantly, often reaching widths between 2 and 10 inches, creating severe maintenance challenges and accelerating pavement deterioration. For years, practitioners attributed these cracks to poor construction or inadequate mix design, but the underlying mechanism was not well understood, and without understanding the mechanism, it has been difficult to systematically prevent them.
Despite the widespread occurrence of these distresses, the mechanisms driving thermal fatigue cracking in desert climates remain poorly understood. This project investigates the interaction between climatic loading, pavement structure, binder aging, and asphalt mixture durability to better understand why certain pavements develop severe wide cracking while others do not. The study integrates field investigations, laboratory characterization, numerical heat-transfer modeling, finite element simulations, and performance-based asphalt mixture design strategies to develop practical approaches for mitigating thermal cracking in the Southwest regio The research combined field forensics at 12 cracked pavement sites, climate analysis of Phoenix’s unique temperature environment, finite element thermal stress modeling, and laboratory fracture testing of both field-extracted and laboratory-designed mixes. The hypothesis driving the work was that Arizona’s extreme daily temperature swings are the primary driver of wide crack formation, through a mechanism of repeated thermal fatigue rather than the single-cycle thermal cracking that dominates in cold climates.
Methodology and Framework
The project combined regional forensic investigations with mechanistic and laboratory-based performance evaluation techniques. Twelve pavement sections exhibiting severe wide cracking, block cracking, and control sections without visible distress were identified and cored across the Phoenix metropolitan area. Recovered asphalt binders and field core volumetrics were analyzed to quantify aging severity, mixture characteristics, and cracking susceptibility. Rheological testing included Dynamic Shear Rheometer (DSR) characterization, PG grading, Glover-Rowe parameter analysis, and master curve development to evaluate stiffness evolution and aging behavior of field-aged materials. This forensic dataset allowed a statistical comparison of material properties between cracked and uncracked sections, isolating the factors most strongly associated with wide crack development.
Simultaneously, a finite element thermal stress analysis framework was developed to simulate daily temperature cycling through the pavement cross-section. Using site-specific weather data, the FEM model calculated thermal stresses in the asphalt layer resulting from repeated cooling cycles. Phoenix’s temperature profile was compared against climate data from other U.S. cities, quantifying the city’s diurnal temperature range (DTR) and daily cooling rate relative to national norms. Laboratory mix design experiments then tested whether reducing compaction effort could produce mixes with higher fracture energy and better resistance to thermal fatigue cracking, without sacrificing rutting performance.
To develop more durable asphalt mixtures, the research introduced low-design-gyration (N50) mix strategies focused on improving compactability, effective binder volume, and fracture resistance. Laboratory testing included IDEAL-CT, SCB-CMOD fracture testing, IDT moisture susceptibility testing, Asphalt Pavement Analyzer (APA) rutting evaluation, and density characterization. Five full-scale pavement test sections were then constructed in Mesa, Arizona using different binder systems, RAP contents, and mixture designs to validate performance under real field conditions.
Key Findings
Arizona’s Extreme Thermal Environment
The climate analysis revealed that Phoenix is an outlier among U.S. cities in the parameters most relevant to thermal fatigue cracking. The diurnal temperature range (DTR) averages 15°C higher in Phoenix than the climate reference used in most pavement design standards. More significantly, Phoenix’s average daily cooling rate (the rate at which pavement temperature drops in the evening) is approximately 2.5 times faster than in other major U.S. cities. This rapid cooling generates large and rapid tensile stress pulses in the asphalt layer. Over the hundreds of repeated cycles that accumulate over a pavement’s service life, this thermal fatigue loading can propagate cracks upward from the bottom of the asphalt layer, ultimately creating the wide surface cracks observed in the field.
The FEM thermal stress modeling confirmed that stress magnitudes in Phoenix during a typical summer-to-night cooling cycle are sufficient to induce incremental crack propagation in aged, stiff binders, even at temperatures that would be considered mild elsewhere. The model also showed that the rate of crack propagation accelerates significantly as binders age and stiffen, creating a positive feedback loop: the more the binder ages, the more brittle it becomes, the faster cracks propagate under repeated thermal loading, and the wider those cracks eventually appear.
Field Forensics: Aged Binders and Low Binder Volume
The forensic comparison between cracked and uncracked sites revealed two material factors strongly associated with wide crack occurrence. First, wide-crack sites had severely aged binders: DSR testing of extracted binders showed effective performance grades of PG 88-106 at cracked sites, compared to significantly lower effective grades at uncracked comparison sections. Glover-Rowe parameters at cracked sites fell in the “high cracking risk” zone, indicating binder embrittlement well beyond what would be predicted based on pavement age alone. Second, wide-crack sites consistently exhibited lower VMA and lower effective binder volume than uncracked sites.
Mix Design Solution: Reducing Gyration Count
A promising practical solution emerged from the laboratory mix design experiments: reducing the number of design gyrations from N100 (current practice) to N50 or N70. Higher gyration counts produce denser, stiffer mixes with lower air voids and VMA. By designing to lower gyration counts, engineers can produce mixes with higher binder volume and more flexible aggregate skeletons, translating directly to improved cracking resistance. Laboratory SCB-CMOD fracture energy measurements showed that mixes designed at N50 had 50–100% higher fracture energy than their N100 counterparts at equivalent binder grades. Critically, Hamburg Wheel Tracking Tests confirmed that the rutting resistance of these lower-gyration mixes remained within acceptable limits, suggesting that the tradeoff is favorable for Arizona conditions.
Optimized Low-Gyration Mixtures Improved Cracking Resistance
The developed low-design-gyration asphalt mixtures demonstrated significantly improved fracture resistance while maintaining acceptable rutting performance. IDEAL-CT and SCB-CMOD testing showed that lower-gyration mixtures achieved substantially higher CT Index and fracture energy values due to slower crack propagation and improved toughness behavior. The findings demonstrated that improved aggregate packing and higher effective binder volume can significantly enhance pavement durability under thermal fatigue loading conditions.