Fatigue cracking remains a primary issue impacting the durability and longevity of asphalt pavements, driven by constant vehicular loads and environmental conditions. Self-healing capabilities in asphalt binders offer potential to mitigate fatigue, although knowledge gaps remain in fully harnessing this property. Research continues to advance, utilizing methods such as the Linear Amplitude Sweep (LAS) test and the Simplified Viscoelastic Continuum Damage (S-VECD) model to evaluate the fatigue and self-healing characteristics of asphalt. Recent studies further explore various modifications to asphalt binders, such as the inclusion of polymer additives, and develop new frameworks aimed at achieving consistent fatigue performance measurements. This paper reviews advancements in evaluating fatigue performance and self-healing capacities in asphalt, presenting findings that underscore the need for a comprehensive framework to assess bitumen fatigue performance consistently, ultimately guiding engineers toward optimal material selection.

Introduction

Fatigue cracking is a common form of degradation in asphalt pavements, triggered by repetitive vehicle loading and climatic stressors. Over time, these stresses generate cracks, compromise structural integrity, and reduce pavement longevity. Fatigue cracking originates in the bituminous phase of asphalt mixtures and propagates through the pavement structure, thus placing significant emphasis on the fatigue behavior of bitumen-based materials in understanding pavement distress. A promising avenue for mitigating fatigue cracking lies in the self-healing properties of asphalt binders, which can slow down crack development through molecular interdiffusion and capillary flow mechanisms. High temperature further promotes self-healing by increasing the movement of bitumen molecules.

Despite potential benefits, the mechanisms behind asphalt’s self-healing remain insufficiently understood, necessitating more precise assessment tools and frameworks. Current methods include the LAS test and the S-VECD model, which analyze parameters like complex shear modulus, shear strain, shear stress, and phase angle. This data enables prediction of fatigue life and self-healing capacity in asphalt. However, further refinement is needed to ensure consistency in measuring fatigue performance across different bitumen types and modifications. This paper reviews experimental approaches and recent research that assess the fatigue behavior and self-healing performance of various asphalt mixtures, highlighting the need for a new, comprehensive framework for enhanced material selection.

Mechanisms of fatigue cracking and self-healing in asphalt binders

Fatigue cracking in asphalt pavements occurs mainly at the later stages of pavement life, where the binder becomes rigid due to long-term aging. Cyclic loading, combined with temperature changes, leads to horizontal tensile strains that exceed the tensile strength at the base layer, inducing microcracks that expand over time. This ultimately results in pavement structural failure. The asphalt binder's long-term exposure to cyclic loading and environmental factors results in a brittle, less flexible material prone to cracking under stress.

Self-healing in asphalt binders is primarily governed by two mechanisms: molecular interdiffusion and capillary flow. The interdiffusion process, facilitated by elevated temperatures, enables bitumen molecules to move more freely, allowing cracks to close over time. Molecular characteristics such as binder density and glass transition temperature also play crucial roles in determining self-healing rates. These mechanisms underscore the importance of selecting binders with favorable self-healing properties to extend the pavement service life.

Assessment methods for fatigue performance and self-healing

The LAS test (AASHTO TP 101) and the S-VECD method are two widely used methods for evaluating the fatigue response and self-healing capability of asphalt binders. LAS testing measures parameters such as complex shear modulus, shear strain, shear stress, and phase angle. These values are then used in the S-VECD model to calculate parameters that represent the internal state of the binder, notably the integrity (C) and state condition (S) parameters. Plotting these values on a C vs. S curve produces the Damage Characteristic Curve (DCC), a key tool for predicting fatigue performance.

The S parameter, crucial in understanding binder fatigue behavior, aligns with Schapery's work potential theory of damage evolution. Consequently, these methods allow for a quantitative assessment of both fatigue resistance and self-healing potential. However, recent studies highlight inconsistencies in these metrics, suggesting that further improvements to the evaluation frameworks are needed to ensure reliable assessments.

Recent advances in fatigue and self-healing studies of asphalt mixtures

A substantial body of research has emerged, exploring various asphalt modifications to improve fatigue performance and self-healing capacity. These studies often use the LAS and S-VECD frameworks to establish correlations between fatigue life and strain levels across different bitumen types and formulations.

For example, Jiao et al. investigated the fatigue performance of asphalt mixtures containing reclaimed asphalt pavement (RAP). Their results demonstrated a strong correlation between LAS test data and fatigue life, validated by four-point beam fatigue tests. Muhammad et al. similarly explored relationships between LAS test results and other testing methods, finding that the LAS test effectively predicted results from the four-point bending beam test, offering a reliable measure of fatigue life.

In another study, Sabouri et al. established a link between LAS and four-point bending beam fatigue tests, concluding that the conventional fatigue index (G∗sinδ) was inadequate in predicting fatigue performance accurately. The Glover–Rowe parameter, as demonstrated by Ilyin and Yadykova, proved more effective in assessing polymer-modified asphalt, enabling rapid fatigue and rutting resistance evaluation under varied temperature conditions.

Modifications and additives in asphalt for enhanced self-healing

Several studies have investigated modifications to asphalt to boost self-healing, with mixed results regarding the impact of various additives on healing performance. Xie et al. and Wang et al. examined the influence of short- and long-term aging on the self-healing capabilities of neat asphalt (NA) and styrene-butadiene-styrene (SBS)-modified asphalt. Their findings showed that aging and SBS generally reduced healing efficiency, although certain molecular compositions helped retain self-healing potential.

Aurilio and Baaj tested self-healing polymer-modified bitumen (SPB) and found that while elastomeric additives enhanced bitumen properties, they did not significantly improve self-healing. In contrast, Lv et al. tested self-healing polymers (STPB and IPAB), finding that STPB promoted self-healing in asphalt at room temperature. They proposed a new framework based on stored potential cohesion (SPC) to address the inconsistencies observed in conventional fatigue metrics.

Challenges in existing frameworks and the need for a comprehensive model

The existing frameworks for fatigue performance assessment exhibit several limitations, often failing to provide consistent evaluations across various asphalt types and modifications. The reliance on the peak of stored pseudo-strain energy (PSE) as the failure criterion for assessing fatigue life (Nf) has proven inadequate, as discrepancies have been observed between Nf and fatigue performance as measured by the DCC. Lv et al. and Wang et al. have highlighted these inconsistencies, advocating for a new framework that incorporates SPC rather than Nf to provide a more reliable failure definition.

An ideal framework would address three main criteria: it would (1) ensure higher fatigue performance consistency in DCC analysis for bitumen with improved healing properties, (2) accurately define fatigue failure across varied bituminous materials, and (3) provide ranking consistency between failure definitions and fatigue performance. This comprehensive model would allow engineers to select the most suitable asphalt binder for specific applications with greater confidence in the material's durability and longevity.

Conclusion

The development of fatigue cracking in asphalt pavements poses a significant challenge to road infrastructure, driven by cyclic vehicular loads and environmental conditions. Self-healing properties in asphalt binders offer a promising solution, though their full potential remains underexplored. Existing frameworks, including the LAS and S-VECD models, provide essential insights into fatigue and self-healing performance but face limitations in delivering consistent evaluations across diverse asphalt formulations.

Recent research underscores the value of polymer modifications and additives in enhancing self-healing, though results remain mixed, and further studies are needed to establish a definitive correlation. The introduction of a new framework based on stored potential cohesion promises to address current inconsistencies, offering a more robust and consistent evaluation of fatigue performance across different asphalt binders.

In conclusion, advancing the methodologies and frameworks for evaluating fatigue and self-healing performance in asphalt pavements is crucial. A comprehensive framework incorporating these advancements would significantly enhance the selection process, guiding engineers toward optimal materials that balance durability, resilience, and self-healing potential, ultimately extending the service life of asphalt pavements.