Accelerated Long-Term Aging Protocol for Arizona Asphalt Mixes
📨Principal Investigator:Hasan Ozer
🤝 Sponsor:  Southwest Pavement Technology Consortium (SWPT)
📅 Timeline: 2023 – Ongoing
Highlights
Introduction
Asphalt pavements in desert environments experience significantly accelerated aging because of prolonged exposure to high temperatures, solar radiation, and oxidation. In Arizona, pavement surface temperatures routinely exceed conditions considered in many national laboratories aging standards, leading to premature stiffening of asphalt binders and increased susceptibility to thermal and fatigue cracking. Existing long-term aging procedures often require extended laboratory conditioning durations that are impractical for routine balanced mix design implementation and may not fully represent the behavior of heavily polymer-modified asphalt mixtures used in the Southwest region. Laboratory mix design evaluates asphalt pavements in their freshly compacted state, but pavements age in the field for decades. As asphalt binder oxidizes over time, it becomes stiffer and more brittle, dramatically changing the cracking performance of the mix. A pavement that looks excellent at the time of construction may develop severe cracking problems years later as the binder ages to a point where it can no longer accommodate thermal and traffic-induced stresses. To accurately evaluate long-term cracking performance in the laboratory, mix designers need a way to simulate field-aged material.
The current standard for laboratory LTA in the United States is NCHRP 09-54, which calls for aging loose mix in an oven at 95°C for approximately 17-19 days for Arizona climatic conditions. This protocol was calibrated based on field aging data from a range of U.S. climates, but Arizona presents a unique challenge. With ambient temperatures exceeding 110°F (43°C) for over 100 days per year, and pavement surface temperatures regularly surpassing 160°F (71°C), Arizona binders age dramatically faster than in cooler regions. Applying the national standard to Arizona mixes significantly underestimates the degree of aging that actually occurs in the field. This SWPT study was initiated to develop an Arizona-appropriate aging protocol that gives realistic predictions without requiring impractically long lab conditioning times.Â
This project focused on developing an accelerated laboratory long-term aging protocol representative of Arizona’s climatic conditions while maintaining realistic binder and mixture performance characteristics. The study evaluated multiple modified and unmodified asphalt binders using rheological, chemical, and fracture-based performance characterization techniques. By integrating climatic aging models, laboratory conditioning procedures, and mixture-level cracking experiments, the research established a practical framework for simulating field aging within significantly shorter laboratory durations.
Methodology and Framework
The project developed a comprehensive experimental program to investigate the long-term aging behavior of asphalt mixtures under Arizona climatic conditions. Five asphalt binder systems, including multiple polymer-modified binders and unmodified binders commonly used in the region, were evaluated through controlled loose-mixture oven aging procedures. The study compared the conventional NCHRP 09-54 long-term aging protocol at 95°C with an accelerated protocol at 135°C designed to reduce laboratory conditioning duration while maintaining representative aging mechanisms. Â
The research team approached LTA protocol development in two steps: first, establishing what degree of aging occurs in Arizona pavements through both climate-based analysis and field core extraction; second, finding an accelerated laboratory conditioning combination (temperature and time) that replicates that state. Binder aging was characterized using the Dynamic Shear Rheometer (DSR) to measure rheological changes, FTIR (Fourier Transform Infrared) spectroscopy to quantify chemical oxidation indices (carbonyl and sulfoxide area ratios), and the Glover-Rowe parameter (a rheology-based indicator of binder embrittlement strongly predictive of field cracking). Field test sections on S. 74th Street in Mesa, AZ provided core samples at known pavement ages, creating a field calibration dataset of aged binder properties at different service lives.
Laboratory aging trials were conducted at multiple temperatures (85°C, 95°C, and 135°C) and durations. At each aging combination, extracted binders and compacted mixture specimens were tested to determine which lab condition best matched the field-aged material state. Mixture performance was evaluated using IDEAL-CT for intermediate-temperature cracking index and SCB-CMOD for fracture energy at low and intermediate temperatures. The study paid careful attention to ensuring that high-temperature aging did not induce binder degradation artifacts.
Key Findings
Proposed Accelerated Aging Protocol: 135°C for 30 Hours
Analysis of binder rheological properties and FTIR oxidation indices showed that aging loose mix specimens at 135°C for 30 hours closely matched the material state measured in field cores representing approximately 8–12 years of service in the Phoenix metropolitan area. By comparison, the NCHRP 09-54 protocol at 95°C would require approximately 19 days to reach the same aging state. The proposed Arizona protocol reduces conditioning time from 19 days to 30 hours while achieving an equivalent binder aging state. The DSR phase angle and dynamic modulus of extracted binders from 135°C/30-hour aged specimens aligned closely with field-core binders in both magnitude and temperature sensitivity, providing confidence that the aging mechanism is representative.Â
The FTIR carbonyl and sulfoxide indices also corroborated the equivalence: the chemical signature of oxidation in the lab-aged binders matched the field-aged signatures when the 135°C/30-hour condition was used. The protocol proved consistent across the different binder grades and modification types tested, suggesting it is broadly applicable to the mix types currently used across the SWPT region.
Practical Oven Aging Protocol: 135°C for 6 and 15 Hours
A practical laboratory long-term aging (LTA) protocol was developed to evaluate the cracking performance of asphalt mixtures under the extreme climatic conditions of Arizona. The study aimed to establish efficient laboratory aging durations while maintaining reliable characterization of mixture behavior and oxidative aging. Subsequently, asphalt mixtures produced with five different binders, including both modified and unmodified materials, were evaluated using test-specific aging durations. Based on the sensitivity of the performance tests, 6 hours at 135 °C was selected for intermediate-temperature cracking evaluation using IDEAL-CT, while 15 hours at 135 °C was adopted for low-temperature fracture characterization using the SCB-CMOD test.
Mixture Cracking Performance After LTA
IDEAL-CT cracking indices and SCB fracture energy values dropped substantially after long-term aging, since oxidized binders are stiffer and more brittle. Critically, some mixes that appeared to have adequate cracking resistance in the short-term (as-compacted) state fell below acceptable thresholds after the 30-hour LTA protocol. This finding has direct design implications: evaluating mixes only in their fresh state may result in approving designs that will exhibit premature cracking in the field within 5–10 years of service. By incorporating the proposed LTA protocol into routine mix design evaluation, engineers can screen out brittle-prone mixes before they are placed on Arizona roads.
The proposed LTA protocol was further validated using plant-produced mixtures and field-aged pavement sections. Test sections were constructed to monitor oxidative aging under in-service conditions, and pavement cores were collected periodically for up to 18 months. Recovered binders from laboratory-aged mixtures were compared with binders extracted from field cores. Results demonstrated that the proposed test-based aging durations consistently differentiated stiffness and cracking resistance across binder types while significantly reducing conditioning time compared with conventional laboratory aging procedures. Strong agreement between binder-level rheological characteristics, FTIR oxidation indices, and mixture-level cracking performance confirmed the applicability and reliability of the proposed methodology. Overall, the study provides a practical, efficient, and scalable approach for evaluating the long-term cracking performance of asphalt mixtures in hot-climate regions such as Arizona.
Field Validation at Mesa Test Section
The field test section on S. 74th St. in Mesa, AZ served as the ground-truth dataset for protocol calibration. A full-scale pavement test section was constructed in Mesa, Arizona, with two in-place density levels (93% and 95%) and three surface conditions: uncoated, fog seal, and polymer-modified master seal (PMM). Field cores were collected after 1, 6, and 12 months of exposure and sectioned to evaluate aging gradients through the pavement depth. Recovered binders were characterized using rheological and chemical analyses, including Dynamic Shear Rheometer (DSR), Glover–Rowe parameter, and FTIR oxidation indices, and compared with Pressure Aging Vessel (PAV)-aged laboratory binders. Phoenix’s climate resulted in binder embrittlement occurring roughly 2–3 times faster than in comparable northern U.S. pavement sections, consistent with climate analysis showing that Phoenix receives substantially more “aging degree-days” annually than most U.S. locations. Results demonstrated that oxidative aging was most severe at the pavement surface of low-density, uncoated sections, leading to significant increases in binder stiffness and changes in viscoelastic behavior. The low-density control section reached an aging condition approximately equivalent to one PAV cycle after 12 months of field exposure, while high-density pavements treated with PMM significantly reduced oxidative aging and preserved polymer behavior. Minimal oxidation was observed in deeper pavement layers regardless of treatment type or compaction level, confirming that both improved in-place density and timely surface treatments effectively mitigate oxidative aging in hot climates.