Dallas N. Little

Moisture susceptibility of asphalt mixtures, Part 1: mechanisms

The detrimental effects of water in asphalt mixtures and its manifestation as distresses in asphalt pavements were first recognised in the 1930s and have been studied extensively during the last 35 years. This deterioration process, referred to as moisture damage, is generally defined as the degradation of the mechanical properties of the material due to the presence of moisture in its microstructure. Moisture damage is a complex phenomenon that involves thermodynamic, chemical, physical and mechanical processes. This paper describes the processes by which moisture damage affects asphalt mixtures. A critique of various moisture damage mechanisms is presented, followed by a review of recent work on modes of moisture transport (i.e. water permeability, capillary rise and vapour diffusion) and their relationship to moisture damage. Special attention is given to the characterisation of void structures of asphalt mixtures, which is an important factor that influences moisture transport. Finally, the paper presents a review of existing theories on the adhesive bond between aggregates and asphalt binders and the effect of the presence of moisture at the interface. The mechanisms described in the paper are complemented by a second paper that presents recent advances in moisture damage characterisation using experimental methods, analytical-based approaches (i.e. fracture mechanics, continuum mechanics, thermodynamics and micromechanics), and numerical modelling.

Limits on Adhesive Bond Energy for Improved Resistance of Hot-Mix Asphalt to Moisture Damage

The loss of physical adhesion between the aggregate and the asphalt binder is one of the important mechanisms that accelerate moisture damage in hot-mix asphalt pavements. In this study, two parameters related to bond energy—adhesive bond energy between the aggregate and the asphalt and reduction of free energy when asphalt debonds from the aggregate surface in the presence of moisture—were quantified with surface energies of both materials. Threshold values of these parameters to identify asphalt-aggregate combinations susceptible to premature moisture damage were derived by comparison of the values of these parameters with observed field performance for several mixes. Results show significant differences in bond energies developed between various aggregates and a given binder. This finding illustrates the importance of binder-aggregate compatibility and the sensitivity of calculated bond strength to surface energy measurements. Asphalt binders from different sources with the same performance grade were ...

Adhesion in Bitumen-Aggregate Systems and Quantification of the Effects of Water on the Adhesive Bond

This research is intended to contribute toward the understanding, development, and implementation of a more fundamental design process for bituminous pavement materials, utilizing thermodynamic properties of the materials involved. The theory developed by van Oss, Chaudhury and Good forms the basis of this research. Optimization of techniques to characterize surface energy, as well as consideration and evaluation of additional factors that influence adhesion in the presence of water, are pursued. A synthesis of theories and mechanisms of bitumen-aggregate adhesion is presented, and existing and potential techniques for surface energy characterization are reviewed to establish firm background knowledge on this subject. The Wilhelmy plate technique was scrutinized and improved methodologies and analysis procedures are proposed. Inverse gas chromatography (IGC) is introduced as an alternative technique. A reasonable comparison of total surface energy values from these techniques with mechanical surface tension values was found. Results suggest that bitumen surface energies do not vary substantially. Inability of these techniques to detect the effect of a liquid additive is rationalized by the ‘potential’ surface energy concept. Suggestions for a more realistic characterization of bitumen polar surface energy components are presented. A static gravimetric sorption technique was employed to characterize aggregate surface energies. Dynamic vapor sorption was identified as a candidate alternative technique for aggregate surface energy characterization. A study on the effect of pH on surface energy components of water revealed that this effect is practically negligible. Calculation of the free energy of electrostatic interaction (ΔG supra EL) indicated that this term contributes less than 1% to the total free energy of adhesion. Despite this finding, it is shown that ΔG supra EL alone is able to distinguish moisture sensitive mixtures. The significance of electrical phenomena at the interface is elucidated through another mechanism following the work of M.E. Labib. The relationship between pH and electron donor-acceptor properties of aggregate surfaces is presented. The Labib approach potentially offers the solution to quantify the effect of pH on adhesion. In addition, it should be possible to resolve issues with the acid-base scale proposed by the founders of the current theory, by replacing it with a more absolute donor-acceptor scale.

MOISTURE DAMAGE EVALUATION OF ASPHALT MIXTURES BY CONSIDERING BOTH MOISTURE DIFFUSION AND REPEATED-LOAD CONDITIONS

Two moisture damage models based on major moisture failure mechanisms are proposed. The adhesion failure model was developed to analyze the adhesive fracture between asphalt and aggregate in the presence of water. Cohesive and adhesive fractures in an asphalt-aggregate system are directly related to the surface energy characteristics of asphalt and aggregate. The surface energy of adhesion with or without the presence of water can be calculated from the surface energies of asphalt and aggregate. A moisture diffusion model was developed based on the one-dimensional consolidation of soil and a gas adsorption model. The moisture diffusion model was used to obtain the moisture diffusion characteristics of asphalt binders, including the amount of moisture that can permeate a binder and the diffusivity of the binder. The amount of moisture that permeates a binder is identified as a key factor in the moisture damage. Finally, mechanics-based experiments conducted on asphalt mixtures validated the results from the adhesion failure and diffusion models.

A Framework to Quantify the Effect of Healing in Bituminous Materials using Material Properties

ABSTRACT Significant evidence exists in the literature that healing has a substantial effect on the performance of asphalt mixtures and therefore asphalt pavements. The incorporation of the healing mechanism in tandem with the crack growth mechanism is necessary for comprehensive modeling of the fatigue or fracture processes in asphalt mixtures. This paper presents a new framework that combines material and mechanical properties of the bitumen to predict the effect of healing on mechanical properties and performance of the asphalt mixture. This framework is based on an analytical approach that will allow it to be incorporated with future analytical models of crack growth and damage. The paper also presents a new test method using the dynamic shear rheometer to obtain some of the material properties related to the healing mechanism that are required in the proposed framework. Results from preliminary tests conducted on selected materials support the hypothesis used in the development of the framework and the DSR based test method.

EFFECT OF MINERAL FILLERS ON FATIGUE RESISTANCE AND FUNDAMENTAL MATERIAL CHARACTERISTICS: MECHANISTIC EVALUATION

Complex characteristics of fatigue behavior were evaluated on the basis of test results and their mechanical analyses. The dynamic shear rheometer was used to characterize fundamental linear viscoelastic properties of asphalt binders and mastics. Various dynamic mechanical tests using cylindrical sand–asphalt samples mixed with pure binders, mastics, or both were also performed to estimate viscoelastic characteristics and fatigue behavior. To assess the filler effect, two distinctly compositionally different asphalt binders, AAD-1 and AAM-1, and two fillers, limestone and hydrated lime, were selected. Test results were analyzed using viscoelastic theory, a fatigue prediction model based on continuum damage mechanics, and a rheological composite model. The role of fillers in fatigue resistance was quantified, and induced mechanisms due to filler addition were investigated. The effect of hydrated lime, which is highly binder specific, as a filler was further discussed by comparing test results from hydrated lime filler and test results from limestone filler.

Structural characterization of micromechanical properties in asphalt using atomic force microscopy

This paper semiquantitatively evaluates the microrheological properties of asphalt binder using atomic force microscopy. It also presents differences between these properties amongst the various microstructures within an asphalt binder, in addition to the influence of oxidative aging on these properties. Nano-indentation experiments performed within a microgrid of asphalt phases determined micromechanical properties such as stiffness, adhesion, and elastic/plastic behavior. The evaluated materials include asphalts AAB, AAD, and ABD from the Materials Reference Library of the Strategic Highway Research Program, chosen because of variations in crude source, chemical composition, and elemental analysis. Analysis of nano-indentation creep measurements corresponding to phase-separated regions revealed heterogeneous domains in asphalt with different mechanical properties, and oxidative aging induced substantial microstructural change within these domains, including variations in phase structure, properties, and distribution. The form and extent of these changes, however, were different for each asphalt. Interpretation of data collected from the atomic force microcopy experiments in this study advances understanding of the microstructural composition of asphalt binders and the response of the microstructural phases of the asphalt binder under load, in addition to how the mechanical responses in the phases change with aging.

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.

Effect of Moisture Damage on Material Properties and Fatigue Resistance of Asphalt Mixtures

Dynamic mechanical analysis (DMA) has been used successfully to evaluate complex characteristics of fatigue damage and fracture of asphalt binders and mastics by measuring fundamental viscoelastic properties and damage characteristics. DMA was used to define the effect of moisture on fatigue damage and to concentrate on the fatigue damage susceptibility of the sand and asphalt mixture mastic fraction. Dynamic frequency sweep and time sweep tests were performed on cylindrical sand-asphalt samples in a dry state and after being subjected to moisture saturation. Test results clearly indicate that moisture reduces viscoelastic stiffness, fatigue resistance, and eventually fatigue life of sand-asphalt. The mechanistic role of moisture in fatigue was analyzed and quantified by using nonlinear viscoelastic theory based on pseudovariable concepts and a continuum damage fatigue model. The effect of material surface energies, which is strongly related to fracture and damage, is further discussed by using DMA fatigue test results and varying surface energy characteristics of individual mixture constituents. The DMA experimental procedure and analysis is an efficient way to identify the influence of moisture and to compare sand-asphalt mixtures in terms of moisture susceptibility.

SURFACE ENERGY MEASUREMENT OF ASPHALT AND ITS APPLICATION TO PREDICTING FATIGUE AND HEALING IN ASPHALT MIXTURES

Cohesive and adhesive bonding within the asphalt—aggregate system are directly related to the surface energy of the asphalt. The thermodynamic changes in the surface energy of adhesion and cohesion are related to the de-bonding of the interface between asphalt and aggregate and to cracks that may occur within the mastic, respectively. However, it is also true that thermodynamic changes in the surface energy are required to heal a fracture between the surfaces of the asphalt and the aggregate or within the mastic. The methodology and testing protocol for measuring the surface energy of asphalt are presented. Both the surface energy of dewetting (fracture) and the surface energy of wetting (healing) can be obtained from the contact angle measurement with the Wilhelmy plate method. Ten asphalts were tested; surface energies varied substantially as a function of asphalt composition and the level of aging to which the asphalt was subjected. By using thermodynamic theory, the adhesion and cohesion bonding energy within the asphaltaggregate systems were further analyzed. This analysis has the potential to select the most compatible asphalt—aggregate combination for mixtures. The surface energy is also a very important parameter in the fatigue and healing analysis of the asphalt pavement.