Linbing Wang

Modeling Mode I Cracking Failure in Asphalt Binder by Using Nonconserved Phase-Field Model

AbstractCracking failure in asphalt binder in winter has always been one of the most serious problems in pavement structures. Classical fracture mechanics is the most widely used method to analyze the initiation and propagation of cracks. In this paper, a new modeling and computational tool—namely, the phase-field method—is proposed for modeling the Mode I cracking failure in asphalt binder. This method describes the microstructure using a phase-field variable that assumes 1 in the intact solid and −1 in the crack region. The fracture toughness is modeled as the surface energy stored in the diffuse interface between the intact solid and crack void. To account for the growth of cracks, a nonconserved Allen-Cahn equation is adopted to evolve the phase-field variable. The energy-based formulation of the phase-field method handles the competition between the growth of surface energy and release of elastic energy in a natural way: the crack propagation is a result of the energy minimization in the direction of...

Investigation of the Asphalt Self-Healing Mechanism Using a Phase-Field Model

The self-healing mechanism of asphalt has always been a challenging issue for pavement engineers; up to now there is no general agreement on the fundamental mechanism. In this paper, combined with Atomic Force Microscopy (AFM) technology, the self-healing mechanism of asphalt is simulated by using the Phase-field Model in two ways: thermodynamic approach and mechanical approach. In the thermodynamic approach, self-healing is considered as a material-phase-rearrangement process based on the Cahn-Hilliard dynamics. Micro-cracks will form and disappear in the stress concentration zone near the phase interfaces because of phase separation that is demonstrated by AFM results; in the mechanical approach, the micro structure is described using a phase-field variable which assumes positive one in the intact solid and negative one in the existing crack void. Allen-Cahn dynamics is adopted to evolve the phase-field variable that the surface energy will result in the “wetting” of two surfaces and finally leads to self-healing. By using both approaches, pavement engineers are able to better understand the asphalt self-healing mechanism.

Image Analysis Technique for Aggregate Morphology Analysis with Two-Dimensional Fourier Transform Method

This paper presents some results of the NCHRP 4–34 Laser Detection and Ranging project. A high-resolution Fourier transform interferometry (FTI) system was developed for aggregate morphology characterization with the use of a fiber-optic coupler to form a Youngs double-pinhole interferometer. The FTI system used a charge-coupled device camera to capture fringe images of aggregates with a size ranging from ¾ in. to No. 50 and to reconstruct three-dimensional surfaces of aggregates on the basis of fringe distortions from the captured digital images. A program was developed in MATLAB to quantify morphologic characteristics of aggregates from a variety of sources. The output results include sphericity, flatness ratio, elongation ratio, angularity, and texture for each individual aggregate particle. In this paper, only the morphologic characteristics of seven types of coarse aggregate with sizes ranging from ¾ in. to No. 4 are presented. These aggregates were also analyzed with the Aggregate Image Measurement System (AIMS) II and the University of Illinois aggregate image analyzer (UIAIA) for further comparison. The FTI system is easy to operate and can rapidly quantify morphologic characteristics of aggregates. The FTI results of these aggregates are generally consistent with the results of AIMS II and UIAIA with regard to roughness rankings despite minor differences for several aggregates.

Noninvasive Measurement of Three-Dimensional Permanent Strains in Asphalt Concrete with X-Ray Tomography Imaging

Because of the stiffness differences between aggregates and mastics, permanent deformation of asphalt concrete is localized mainly in the soft mastics. Therefore, studies on the microresponse of the aggregates and mastics could provide a better understanding of the macrobehavior of asphalt concrete. A method is presented to measure the deformations in the mastics; this was correlated with the permanent deformation resulting from the asphalt pavement analyzer test. An automated procedure with tomography images to reconstruct three-dimensional particles was developed. The translations of the particles can be obtained from the coordinate differences of the mass centers before and after testing. The microstrains in the mastics and macrostrains in the mixture can be calculated on the basis of particle translations. The procedures described have significance for future study on how the aggregated shape, size, and arrangement affect the macroresponse of the mixture in three dimensions and how the relative stiffness of aggregates and mastics affects rutting potential.

Evaluation of Aggregate Imaging Techniques for Quantification of Morphological Characteristics

Aggregate morphological characteristics, including shape, angularity, and surface texture, have a significant impact on the engineering properties of construction materials such as hot-mix asphalt and hydraulic cement concrete. Consequently, the quantification of morphological characteristics of aggregates is essential for quality control of both aggregate production and pavement construction. Imaging techniques provide a cost-effective means for measuring the aggregate morphological characteristics conveniently without tedious work. However, these imaging techniques adopt various mathematical methods with different instrument setups and result in different definitions of morphological descriptors that are usually incomparable with each other. This paper evaluates prevalent imaging techniques used for aggregate morphological characteristics analysis, including equipment cost, repeatability, reliability, accuracy, and measured morphological parameters. Three imaging techniques (second-generation Aggregate Imaging Measurement System, first-generation University of Illinois aggregate image analyzer, and Fourier transform interferometer system), are further evaluated by comparing the analysis results of seven types of aggregates passing a ¾-in. sieve and retained on a ½-in. sieve with manual measurements and visual rankings. Analysis of variance between measurements using different methods is also conducted to evaluate the accuracy of each aggregate imaging system. From the data analysis, recommendations that depend on morphological characteristics of most interest to engineers are made for the selection of appropriate imaging-analysis techniques.

Portable Image Analysis System for Characterizing Aggregate Morphology

In the past decade, the application of image-based evaluation of particle shape, angularity, and texture has been widely researched to characterize aggregate morphology. Yet the lack of rapid, objective, and quantitative methods for assessment has inhibited its application in the engineering process. However, recent advances in technology have produced pocket computers with as much processing power as some desktop computers. This project takes advantage of these advances to develop an inexpensive portable image analysis system for characterizing aggregate morphology. The system uses an integral pocket computer–high resolution camera but can employ individual components consisting of a digital camera and a laptop or desktop computer. Digital images of coarse aggregate particles are captured with the camera and analyzed within the Matlab software program environment with a macro developed and written for this project to characterize particle morphology with respect to three parameters (shape, angularity, and texture) on the basis of the particle perimeter (outline or edge). For this purpose, several coarse aggregate types from 10 various Virginia sources were analyzed, and the reliability of the image processing was statistically assessed. Asymptotic analysis was performed to determine the number of images needed to obtain a statistically stable value for each aggregate parameter. It was determined that images acquired at close range (2 or 3 in.) were needed to provide sufficient resolution to adequately characterize the aggregates. Also, it was found that statistically valid values for the three aggregate parameters could be obtained from 15 particle images of random but stable orientation, thus making the system efficient in characterizing coarse aggregate morphology.

A multi-scale approach of Mode I Crack in ettringite

Ettringite, as one of the hydration products of cement paste, has attracted the attention of pavement engineers recently. In this paper, a concise and comprehensive multi-scale modelling approach is presented to study the Mode I fracture based on the continuum level tool Phase-field Theory (PFT) and the atomistic level tool Molecular Dynamics (MD) Simulation. The PFT is a new modelling and computational tool used to predict the crack propagation using a phase-field variable, which assumes to be positive one in the intact solid and negative one in the crack region. Realising the fact that at the crack tip, the PFT is unable to deal with the singularity of stress and strain, and thus we use the MD Simulation to further investigate mechanical performances at a finer level. By the energy coupling the two different approaches at the crack tip, we present the two-way multi-scale coupling simulation of ettringite to simulate Mode I cracking fracture. The comparison between our multi-scale results with results from the traditional fracture mechanics proves that our multi-scale model is reasonable.

Conversion of Testing Frequency to Loading Time Applied to the Mechanistic-Empirical Pavement Design Guide

This paper investigates the issue of converting testing frequency in a dynamic modulus test of hot-mix asphalt (HMA) to loading time for implementation in the proposed mechanistic-empirical pavement design guide. Two methods have been proposed in the literature. The first is to convert the test frequency (f; in hertz) to loading time (t; in seconds) by using t = 1/f. The second method is to convert the test angular frequency (to) to the loading time (in seconds) by using t = 1/ω = 1/2πf. An exact interconversion based on the representation of dynamic modulus results by using a generalized Kelvin model and a generalized Maxwell model is presented. The exact interconversion is compared with the two debated methods of converting the dynamic modulus or the storage modulus to a stiffness (inverse of creep compliance) and relaxation modulus. It is shown that both methods result in error in determining either the relaxation modulus or stiffness. On the other hand, it is shown that the resilient modulus can be adequately approximated by the dynamic modulus taken at a frequency of 1/t.

Diffusion of asphaltene, resin, aromatic and saturate components of asphalt on mineral aggregates surface: molecular dynamics simulation

In this research, the models of four asphalt components (asphaltene, resin, aromatics and saturate) and five minerals (SiO2, Al2O3, CaO, MgO and Fe2O3) were constructed individually, and then the interface models were constructed by adding the asphalt components and minerals together. The interfacial behaviour at molecular scale was simulated by setting boundary conditions, optimising the structure and canonical ensemble. The mean square displacement and diffusion coefficient of particles were selected to study the diffusion of asphalt components on the surface of different minerals. The results show that increasing the temperature can accelerate the diffusion of asphalt components. The diffusion speed of asphalt components on the surface of Al2O3 is faster than other mineral crystals. The temperature sensitivity of diffusion coefficient of asphalt components on the surface of CaO is the maximum. The diffusion speed of asphalt components ranked roughly as their molecular weight: saturate > aromatics > resin > asphaltene.

Evaluation of pavement response and performance under different scales of APT facilities

This paper focuses on comparative evaluation of pavement responses and performance under accelerated loading facility (ALF), Model Mobile Load Simulator 3(MMLS 3) and Asphalt Pavement Analyzer (APA) tests, in terms of rut depth, strain response, seismic stiffness and contact stress. Test slabs extracted from the ALF test lanes were trafficked with the MMLS3 under comparable environmental conditions at a laboratory in Virginia Tech. Some small specimens were cut from the slab for APA tests at VTRC. It is found that the monitored parameters yielded by the MMLS 3 test were comparable to the related full-scale ALF test results in terms of intrinsic material characteristics and pavement performance. It is concluded that MMLS 3 is an effective, economic and reliable trafficking tool to characterise rutting and fatigue performance of pavement materials with due regard to the relative structures. The APA test is not suitable to assess the fatigue resistance of pavement material, but is an efficient and effective method to rank rutting resistance of asphalt mixtures. The MMLS 3 test can be employed as the screen testing for establishing full-scale testing protocols as desired or required, which will significantly enhance economics of APT testing.