Assessment of Thermal and Durability Cracks in Asphalt Pavements in the Southwest Region
📨 Contact: Hasan Ozer
🔖 Researchers: Saed Aker, Awais Zahid, Masih Beheshti, Samuel Castro Brockman
🤝 Sponsor: Southwest Pavement Technology
📅 Timeline: July 2023 – Ongoing
Thermal and Durability Cracking
Thermal and durability cracking in asphalt pavements is a major deterioration mechanism for asphalt concrete (AC) pavements in areas especially experiencing temperature and precipitation extremes. Asphalt mixture durability is defined as the ability of compacted asphalt concrete to maintain its structural integrity throughout its expected service life when exposed to the damaging effects of the environment and traffic loading. Due to complex environmental mechanisms coupled with deteriorating AC characteristics on the pavement surface during service life, representative laboratory characterization and optimization of asphalt pavements against thermal and durability cracking is a major challenge. Despite the mechanistic-empirical models for thermal cracking, the root causes and deterioration mechanisms of thermal fatigue are not well understood. In this study, we develop a case study example of thermal and durability cracks widely observed in the Southwest of United States. Significant factors and root causes influencing occurrences of thermal durability cracks are presented using data compiled from forensic investigation of sites, field core characterization and mechanistic assessment of the conventional flexible pavement structures.
Root Cause of Wide Thermal Crack
Wide thermal cracks are commonly observed in asphalt concrete (AC) pavements in primary state and county highways, urban residential streets, and parking lots in the Southwest’s climatic regions. These cracks, categorized as transverse thermal cracks, can severely further deteriorate and widen to widths ranging from 2 to 10 inches. Maintaining pavement sections with these cracks is both challenging and costly. The substantial daily temperature variations and the absence of cold events in the study area suggest that thermal fatigue cracking is likely the predominant mechanism causing the initiation of wide thermal cracks. Despite its significance and widespread occurrences, there is very little known about the root causes of these cracks. Assessing the root causes of wide thermal cracks in the unique Southwestern climate and developing effective design, material, and maintenance strategies are necessary to mitigate this problem.
Investigation of Thermal Fatigue Cracking
The factors influencing the initiation and widening of thermal fatigue cracking are investigated in the Southwest region. The extent of the wide cracking issue is evaluated within a designated study area. Cores samples from cracked and control sites are analyzed for binder aging, rheological properties, density, and volumetric characteristics. Climatic variables such as diurnal temperature swings and cooling rates are identified as key contributors. Finite element models are used to quantify thermal stresses in pavement structures. Mix designs are developed to enhance durability and cracking resistance by increasing field density, optimizing aggregate packing, and reducing design gyrations to create more easily compactable mixes.
Field cores from 12 sites with severe wide cracking and control sections show no clear structural features linked to wide cracking. Sites with wide cracking have lower VMA and binder volume, and their binders are unmodified and stiffer due to aging, with PG grades ranging from 88 to 106. Climatic factors, particularly the diurnal temperature range and cooling rate, play a significant role in cracking. Arizona’s asphalt concrete experiences greater and faster temperature fluctuations, increasing thermal stresses. Finite Element Method (FEM) is used to develop a thermo-mechanical model. The hourly pavement temperature profiles from the FD model are used as inputs to the FE model in Abaqus software. Finite element models quantify these climatic factors. Mix designs were developed to improve asphalt concrete durability and cracking resistance by increasing density and effective binder volume through reduced design gyrations and optimized aggregate packing. Preliminary testing shows that mixes with lower design gyrations (N50, N70) offer better cracking resistance, but higher rut depth compared to N100 mixes. However, the lower gyration mixes achieve higher field densities, improving cracking resistance. Fracture experiments show a 50% increase in fracture energy, up to 100% with higher density compaction. Overall, wide cracks in Arizona pavements are strongly associated with repeated thermal fatigue stresses on severely aged surface mixes. The mixes with low VMA and effective binder volume along with inadequate in-place densities are compounding factors playing a role in the initiation and widening of transverse cracks. A better understanding of the mechanisms of thermal cracks is achieved in this research project. The tools and strategies that are being developed in the research project will allow developing mix designs and designing pavement structures to delay or permanently avoid occurrences of wide cracks in the region.
Example of wide cracking in Arizona