Evaluation of healing in asphalt binders using dynamic shear rheometer and molecular modeling techniques

A self-healing material has the inherent ability to partially reverse damage such as crack formation that might have occurred during its service. Significant evidence exists in the literature to indicate that asphalt binder is a self-healing material. It is also well known that healing has a substantial affect on the performance of asphalt mixtures and consequently on the serviceable life of asphalt pavements. For example, shift factors from laboratory experimental data to field observed data show that laboratory data underpredict field observations. There is a need to understand the mechanisms that are responsible for healing in asphalt binders as well as to develop test methods that can be used to determine properties related to these mechanisms. This thesis presents details and findings from a two-part study that addresses each one of these two aspects. In the first part of this study, a test method based on the use of a Dynamic Shear Rheometer (DSR) was developed to determine the parameters of characteristic healing function of asphalt binders. In the second part of this study, Molecular Modeling (MM) techniques were used to determine the interrelationship between molecular structure, surface free energy, self diffusivity, and other healing properties of asphalt binders.The healing characteristic equation parameter (Ro) which represents the instantaneous healing nature of the asphalt binders is analogous to surface energy in terms of effect on healing in asphalt binders. Ro values for three asphalt binders AAM, AAD and ABD are calculated and compared with the surface energy values available from the literature. It was observed that the Ro values are proportional to surface energy values. Surface energy values for five asphalt binders AAM, AAD, AAB, AAG and AAF are calculated using MM method based on SHRP representative molecules. These values were observed to be proportional to the surface energy values from literature. Bulk and surface diffusion coefficients of asphalt molecules are calculated using MM method. Parametric analysis was done to determine the effect of chemical structure of asphalt on its diffusion properties. It was observed that the higher percentage of saturates in the chemical structure results into higher diffusion coefficients

Use of molecular dynamics to investigate self-healing mechanisms in asphalt binders

The fatigue-cracking life of an asphalt mixture measured in the laboratory is generally a small fraction of the fatigue-cracking life observed in the field. One of the reasons for this large difference is the self-healing property of asphalt binders. Self-healing is a process that reverses the growth of fatigue cracks during rest periods between load applications. A thorough understanding of the healing mechanism is required to accurately model and predict the influence of healing on the fatigue-cracking life of asphalt mixtures. Previous studies have used experimental evidence to demonstrate a correlation between chemistry of asphalt functional groups, such as chain length and branching, and healing measured in asphalt binders. One of the mechanisms of healing is the self-diffusion of molecules across the crack interface. This paper demonstrates the use of molecular simulation techniques to investigate the correlation of chain length and chain branching to self-diffusivity of binder molecules. The findings reported in this paper are consistent with observations reported in previous studies and expand on the understanding of the relationship between molecular architecture, self-diffusivity, and self-healing properties of asphalt binders. © 2011 American Society of Civil Engineers

Evaluation of self healing properties of bituminous binders taking into account steric hardening effects

Steric hardening is a time-dependent phenomenon that can affect the healing properties of bituminous binders. In the experimental investigation reported in this paper, the rheological test method proposed for the evaluation of healing involved the use of a prolonged rest period included between two continuous oscillatory loading phases. A simple model was developed to subtract the contribution of steric hardening to the total stiffness gain attained during rest and thus to evaluate the recovery in mechanical properties due to self healing only. Analysis of the results indicated that cumulated damage of binders is composed by a reversible and a non reversible part. True self healing potential of the materials, as evaluated by means of the relative reversible damage index (RRD) and the relative index of fatigue life increase (RFIHEALING), was found to be a function of imposed damage, binder source and polymer modificatio

Investigation of self healing behaviour of asphalt mixes using beam on elastic foundation setup

Self healing of asphalt mixes is known for more than four decades. However, it is a complex phenomenon which depends on the duration of the rest period, temperature, crack size, etc. In order to quantify the self healing behaviour of asphalt mixes, a test setup was proposed in this research using an asphalt beam on an elastic foundation. Within this setup, a notched asphalt beam was glued on a low modulus rubber foundation, and a symmetric monotonic load was applied with loading–unloading–healing–reloading cycles. The rubber foundation was used to avoid permanent deformation and to ensure a controllable healing process. Experimental results indicate that the beam on elastic foundation (BOEF) setup is capable for self healing investigations of asphalt mixes. The healing process was quantified by the recovery of the strength and the recovery of the crack opening displacement. The time, temperature and crack size dependency of the self healing behaviour were observed over the healing periods. Moreover, a self healing model was proposed to decompose the self healing phenomenon observed in the BOEF healing tests. It is shown that at the beginning of the healing period the delayed visco-elastic healing is the main reason of the recovery of the crack opening displacement and the viscous healing is important for healing after longer time/higher temperature.Transport and PlanningCivil Engineering and Geoscience