Healing in Bitumen
Towards a pragmatic approach to quantify healing ability in asphalt binders
G.A. Leegwater (TU Delft - Civil Engineering & Geosciences)
S.M.J.G. Erkens – Promotor (TU Delft - Civil Engineering & Geosciences)
A. Varveri – Promotor (TU Delft - Civil Engineering & Geosciences)
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Abstract
The primary objective of this research was therefore to improve the fundamental understanding of healing in bitumen and to develop a reliable test method capable of assessing the healing performance of both traditional and innovative asphalt binders.
The research includes a comprehensive review of the state of the art, revealing that healing in asphalt is difficult to quantify due to ongoing debate regarding the definition and measurement of damage. Many existing tests focus on damage restoration without a well-defined initial damage state, contributing to highly test-dependent and inconsistent healing results. The literature also indicates that there is a conceptual healing model derived from polymer science which is widely accepted. However, it level of detail and validation is limited for bituminous materials.
To address these shortcomings, a novel two-piece healing test was developed using a Dynamic Shear Rheometer in tensile mode. This setup enables precise control of parameters governing contact development like normal force at the healing interface, temperature and healing duration. The use of an artificial damage at the start of a healing period ensures a known damage level, allowing for a clearer view on the healed performance. The set-up can address wide range of healing periods, ranging from 12 seconds to more than a week, allowing for detailed investigation of both short- and long-term healing behaviour.
Experimental results demonstrate that at least half of strength restoration originates from adhesion at the healing interface, emphasizing the importance of contact formation and interfacial stresses during healing. Healing was found to occur rapidly, with a substantial portion of recovery taking place within the first few minutes. This observation, combined with the short rest periods typical of heavily trafficked road networks, suggests that short-duration healing tests may capture the most relevant aspects of healing in practice. However, the finding that toughness recovers more slowly than strength highlights potential implications for fatigue performance and warrants further investigation.
The existing healing model was extended by incorporating normal force, surface roughness, and surface energy as governing parameters. Overall, this research advances the understanding of binder healing, underscores the dominant role of adhesion, and provides a foundation for developing practical, predictive healing tests that support sustainable pavement design.