Amit Bhasin

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 ...

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.

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.

Surface Free Energy to Identify Moisture Sensitivity of Materials for Asphalt Mixes

Conventional methods to quantify the moisture sensitivity of asphalt mixtures are based on the comparison of mechanical properties of the mix before and after a moisture-conditioning process. Although this approach consolidates the effect of material and mixture properties on moisture sensitivity, it does not identify the causes responsible for the poor or good performance of the mixture. In this study, surface free energy of asphalt binders and aggregates was used to derive energy parameters that quantify the moisture sensitivity of various combinations of materials. The moisture sensitivity of 12 asphalt mixtures carefully designed to represent a wide range of asphalt-aggregate interactions was measured in the laboratory under controlled conditions. Test results indicate that the moisture sensitivity of these mixtures correlates well with the energy parameters, which are based on the surface energy properties of the constituent materials. Incorporating the specific surface area of the aggregate into the energy parameters improved this correlation. The proposed energy parameters have the potential to serve as an effective tool by which to select material combinations that result in asphalt mixtures that are more resistant to moisture-induced damage.

Moisture susceptibility of asphalt mixtures, Part 2: characterisation and modelling

Moisture damage in asphalt pavements is the degradation of the mechanical properties of the asphalt composite due to the action of water. In a companion paper the mechanisms of moisture damage were discussed. It was established that in order to characterise moisture damage in asphalt mixtures, it is important to comprehensively describe and model the effect of thermodynamic, chemical, physical, and mechanical processes. This paper discusses existing tests and analytical methods that can be used to assess and quantify moisture damage potential in asphalt mixtures. These methods range from visual qualification of asphalt binder stripped from the aggregate, to analytical-based models that include multiple material properties derived using fracture mechanics, continuum mechanics, thermodynamics, and/or micromechanics. In addition, this paper presents a new approach for classifying moisture damage, which emphasises recent analytical developments. Finally, advances in the mathematical modelling of moisture damage are summarised and future research efforts in this area are identified.

Influence of Warm-Mix Additives and Reduced Aging on the Rheology of Asphalt Binders with Different Natural Wax Contents

This paper presents the results of our study to investigate the influence of natural wax in asphalt binders, warm-mix asphalt (WMA) additives, and reduced short-term aging on viscosity, stiffness, susceptibility to permanent deformation, fracture resistance, and thermal cracking resistance of asphalt binders. We used two controls to differentiate between the influence of WMA additive and that of reduced aging on the rheology of asphalt binders. The asphalt binder used in a WMA undergoes reduced short-term aging and consequently has relatively reduced stiffness compared with the binder in a similar hot-mix asphalt (HMA). Results indicate that certain WMA additives compensate, whereas others aggravate the initial reduced stiffness of asphalt binders used in WMA. Short-term aged binders with high natural wax content demonstrated strong interactions with some of the WMA additives and increased susceptibility to permanent deformation. In most cases, pressure aging vessel (PAV) residues of binders with WMA addi...

Coupled Micromechanical Model of Moisture-Induced Damage in Asphalt Mixtures

The combined effect of moisture and mechanical loading on asphalt mixtures has been recognized as one of the main causes of premature deterioration of flexible pavements. This paper presents a micromechanical model of moisture-induced damage in asphalt mixtures. The model couples the effect of moisture diffusion and mechanical loading to quantify the level of damage within the mixtures. The mechanical properties of the materials are defined as a function of the amount of moisture content. The cohesive zone modeling technique is used to simulate adhesive damage at the aggregate-mastic interfaces. Damage is evaluated based on the location and time for crack initiation and propagation at the aggregate-mastic interfaces and on the level of strains and stresses within the bulk of the mastic. Results show that micromechanical models provide a better understanding of moisture damage mechanisms in asphalt mixtures and can guide the development of continuum damage models.

EVALUATION OF SIMPLE PERFORMANCE TESTS ON HOT-MIX ASPHALT MIXTURES FROM SOUTH CENTRAL UNITED STATES

Since development of the Superpaver mix design procedure under the Strategic Highway Research Program about a decade ago, there has been a need to develop some type of a simple physical test to complement the Level 1 volumetric mixture design procedure. NCHRP Project 9-19 recognized the dynamic modulus test along with the flow time and flow number tests as the top three candidates for a simple performance test that could identify mixtures susceptible to permanent deformation. A critical evaluation of these three tests is presented along with the Super-pave shear test-frequency sweep at constant height with the asphalt pavement analyzer (APA) as the torture test to identify mixes susceptible to permanent deformation. The Hamburg wheel-tracking device (HWTD) also was used on selected mixtures. Special laboratory hot-mix asphalt mixes designed to exhibit low dynamic modulus but high recovery of strains were included in this evaluation. Results indicated that flow number value and flow time slope correlated better with laboratory rutting (APA and HWTD) than dynamic modulus. Flow number value, flow time slope, E*/sin Φ at 1 Hz, flow number slope, and flow time value were among the best five correlations with both the APA and the HWTD rut depths.