Qingli Dai

SIMULATION OF ASPHALT MATERIALS USING FINITE ELEMENT MICROMECHANICAL MODEL WITH DAMAGE MECHANICS

A theoretical and numerical study of the micromechanical behavior of asphalt concrete was undertaken. Asphalt is a heterogeneous material composed of aggregates, binder cement, and air voids. The load-carrying behavior of such a material is strongly related to the local load transfer between aggregate particles, and this is taken as the microstructural response. Numerical simulation of this material behavior was accomplished by developing a special finite element model that incorporated the mechanical load-carrying response between the aggregates. The finite element scheme incorporated a network of special frame elements, each with a stiffness matrix developed from an approximate elasticity solution of the stress and displacement field in a cementation layer between particle pairs. A damage mechanics approach was then incorporated within this solution, and this approach led to the construction of a softening model capable of predicting typical global inelastic behavior found in asphalt materials. This theory was then implemented within the ABAQUS finite element code to conduct simulations of particular laboratory specimens. A series of model simulations of indirect tension (IDT) tests were conducted to investigate the effect of variation of specimen microstructure on the sample response. Simulation results of the overall sample behavior compared favorably with experimental results. Additional comparisons were made of the evolving damage behavior within the IDT test samples, and numerical results gave reasonable predictions.

Three-Dimensional Microstructural-Based Discrete Element Viscoelastic Modeling of Creep Compliance Tests for Asphalt Mixtures

Microstructural-based discrete element (DE) models have been used for a better understanding of asphalt pavement concrete since the late 1990s. Most current studies have been done with two-dimensional (2D) models. Moreover, elastic models are primarily employed for simulation of an asphalt matrix’s time-dependent behaviors. A 2D model is too simple to capture the complex microstructure of asphalt concrete, and an elastic model is not sufficient for simulating an asphalt matrix’s viscoelastic behaviors. Therefore, it is necessary to consider a three-dimensional (3D) viscoelastic model for microstructural-based DE simulation of asphalt mixture behaviors. Currently, it is easy to build such a 3D microstructural-based DE viscoelastic model using the existing techniques presented in the previous studies. A major challenge, however, is to reduce the computation time to run the 3D microstructural-based DE viscoelastic modeling process which is extremely time-consuming. The primary objective of this paper is to s...

PARAMETRIC MODEL STUDY OF MICROSTRUCTURE EFFECTS ON DAMAGE BEHAVIOR OF ASPHALT SAMPLES

This paper presents a computational modeling study of the microstructural influence on damage behavior of asphalt materials. Computer generated asphalt samples were created for numerical simulation in indirect tension and compression testing geometries. Our previously developed micromechanical finite element model was used in the simulations. This model uses a special purpose finite element that incorporates the mechanical load-carrying response between neighboring aggregates. The element was developed from an approximate elasticity solution of the stress and displacement field in a cementation layer between particle pairs. The computational model establishes a network of such elements to simulate an asphalt mass. Continuum damage mechanics was then incorporated within this scheme leading to the construction of a micro-damage model capable of predicting typical global inelastic behavior found in asphalt materials. A series of model asphalt samples have been generated and simulated with controllable microstructure variation in an effort to determine the effects of particular microstructural variables on the material response. These simulations explored the relationship between microstructure parameters and damage behavior of particular asphalt samples.

Prediction of Dynamic Modulus and Phase Angle of Stone-Based Composites Using a Micromechanical Finite-Element Approach

This paper presents a micromechanical finite-element (FE) model for predicting the viscoelastic properties (dynamic modulus and phase angle) of asphalt mixtures, typical stone-based composites. The two-dimensional (2D) microstructure of asphalt mixtures was captured by optically scanning the surface image of sectioned specimens. FE mesh of image samples was generated within each aggregate and asphalt mastic. Along the aggregate boundary, the FEs share the nodes to connect the deformation. The micromechanical FE model was accomplished by incorporating specimen microstructure and ingredient properties (viscoelastic asphalt mastic and elastic aggregates). The generalized Maxwell model was applied for viscoelastic asphalt mastic with calibrated parameters from nonlinear regression analysis of the mastic test data on dynamic modulus and phase angle. The displacement-based FE simulations were conducted on the numerical samples under sinusoidal cyclic loading. The predicted dynamic modulus and phase angle were c...

Investigation of Fracture Behavior of Heterogeneous Infrastructure Materials with Extended-Finite-Element Method and Image Analysis

Infrastructure materials are essential components of the nation’s infrastructure and transportation systems. Deteriorating infrastructures require the development of computational tools to predict fracture behavior. The extended-finite-element method (XFEM) has been recently developed to eliminate remesh efforts by allowing crack propagation within continuous elements. The object of this study is to employ XFEM and image analysis techniques to numerically investigate fracture behavior within infrastructure materials. The XFEM was addressed with a discontinuous crack and inclusion enrichment function with the level-set method. The crack growth and stress intensity factors were also formulated. An extended-finite-element fracture model (XFE-FM) was developed with the MATLAB program for predicting fracture behavior with single-edge-notched beam (SEB) and split tensile (ST) tests. The developed XFE-FM was first validated with SEB testing on a homogeneous sample. In order to capture the real material microstru...

Review of advances in understanding impacts of mix composition characteristics on asphalt concrete (AC) mechanics

The overall performance of an asphalt concrete (AC) mixture is dependent on its composition including the properties, proportions and distributions of the ingredients. For existing asphalt mix design methods, however, there is a missing link between the mix composition and the overall properties and performance. To improve the fundamental understanding of AC mixtures, this paper presents a comprehensive review of the advances in understanding the mix composition characteristics and their impact on AC mechanics. This review focused on the links between the mix composition and the overall properties. The review contains a brief background of this study, followed by a discussion of four typical mixture composition characteristics, namely aggregate size, aggregate morphology, asphalt matrix time-dependent properties and anisotropic AC microstructural characteristics. Furthermore, three types of methods were reviewed for understanding the links between the composition characteristics and the AC behaviours: the experiment-based methods, multiphase micromechanical models and numerical models. The numerical models include discrete element models and finite element models. Finally, a brief summary and further research needs are provided.

Integrated Experimental-Numerical Approach for Estimating Asphalt Mixture Induction Healing Level through Discrete Element Modeling of a Single-Edge Notched Beam Test

AbstractThe induction healing effects have been experimentally investigated through the cyclic beam fracture and healing testing of asphalt mixture samples with conductive steel wool fibers. This study aims to investigate the healing level of asphalt mixture through the integrated work of laboratory experiment and numerical simulation. Nine single-edge notched beam (SENB) samples containing steel fibers were prepared and used for the laboratory fracture test. Afterward, three divided groups of fractured samples were subjected to the induction healing at controlled temperatures of 60°C, 80°C, and 100°C, respectively. Then the reloading fracture test was conducted with three groups of samples to measure the fracture strength recovery ratio (FSRR) after induction-healing process, which is defined as the ratio of the recovered peak load to the original peak load. The discrete element method (DEM) was utilized to simulate the beam fracture process. A fracture toughness-based bond healing model was used to simu...

Air void effect on an idealised asphalt mixture using two-dimensional and three-dimensional discrete element modelling approach

In this study, an idealised asphalt mixture was modelled with the discrete element method for both two-dimensional (2D) and three-dimensional (3D) cases. The air voids were randomly generated and counted within the models to reach a specific air void level (e.g. 6%). The 2D models were used to compute the strain and stress responses when the specimens were subjected to a compressive load. Then, the moduli of the specimens were computed from the stress–strain curves. The 3D idealised model was generated using a number of layered 2D models. The air void distribution patterns were also studied with 2D and 3D randomly generated models at specific air void levels. The results showed that the modulus deviation increases when the air void level increases. In addition, the modulus deviations of the 3D models were found to be much lower than those of the 2D models. When comparing the modulus predictions from the 2D models with those from the 3D models, the research proved that the 3D models yielded higher moduli than the 2D models. The average of the predicted modulus difference between 2D and 3D models was 26% at 10% air voids, and 7% at 4% air voids. When the air void increased from 0 to 10%, the modulus decreased by 30% in the 3D models, when compared with 48% in the 2D models. The 2D and 3D models predicted the same modulus for 0% of air voids. However, the 2D models under-predicted the mixture modulus, especially when the air void level was higher. In the 2D modelling of the asphalt mixtures, a large number of models were needed to achieve a reasonable prediction due to larger deviation, even at lower air void levels. At higher air void levels, the 3D models yielded a much higher prediction than 2D simulations.

Tailored Extended Finite-Element Model for Predicting Crack Propagation and Fracture Properties within Idealized and Digital Cementitious Material Samples

This paper presents a tailored extended finite-element model (XFEM) to predict crack propagation and fracture properties within idealized and digital cementitious material samples. The microstructure of the idealized cement-based materials includes the cement paste, particles, and interfacial boundaries. The tailored XFEM was developed to allow crack propagation within finite elements by using discontinuous enrichment functions and level-set methods. The Heaviside jump and the elastic asymptotic crack-tip enrichment functions were used to account for the displacement discontinuity across the crack-surface and around the crack-tip. The maximum fracture energy release rate was used as a criterion for determining the crack growth. The shielding effects within the interfacial zone were addressed with a numerical search scheme. The tailored XFEM was implemented with a MATLAB program to simulate the compact tension (CT) and the single-edge notched Beam (SEB) tests. For a homogeneous CT testing sample, the XFEM prediction on stress intensity factors was verified with the fracture mechanics analysis. The idealized samples of cement-based materials were generated with varied microstructures, including particle locations, orientations, and shape factors. The tailored XFEM was applied to investigate the effects of these microparameters on the fracture patterns of the idealized samples under CT loading. The XFEM simulation was also conducted on a homogeneous offset-notched SEB sample to predict the mixed-mode crack propagation. The predicted crack path matches well with refined cohesion fracture modeling from a recent study. Further validation of the tailored XFEM was conducted with fracture simulation of a digital SEB sample generated from the actual tested specimen. The predicted crack path was favorably compared with the fracture pattern of the tested concrete specimen with a middle notch. These simulation results indicated that the tailored XFEM has the ability to accurately predict the crack propagation and fracture properties within idealized and digital cementitious material samples.

Investigation of Internal Frost Damage in Concrete with Thermodynamic Analysis, Microdamage Modeling, and Time-Domain Reflectometry Sensor Measurements

This study investigates the internal-frost damage due to ice-crystallization pressure in the concrete pore system. The methodology integrates thermodynamic analysis and a microdamage model as well as a unique time-domain reflectometry (TDR) sensor. The crystallization pressure in the microscale pore system of concrete at subcooling temperatures was calculated based upon thermodynamic analysis. An extended finite-element method (XFEM) was applied to simulate the fracture development induced by internal frost, with the estimated internal crystallization pressure as the input. The XFEM fracture simulation was conducted on a digitized concrete sample obtained with imaging processing and ellipse-fitting techniques. The simulated crack development under the crystallization pressure was found to match the observed fracture patterns of the tested single-edge notched specimen. The XFEM simulation results were verified by the open-mode fracture behavior in both middle-notched single-edge notched beam bending test and freezing-damage tests. Furthermore, the crystallization-pressure analysis and freezing-damage simulation were conducted to demonstrate the freezing-damage process using cement samples with idealized pore structures. To provide direct estimation of the crystallization pressure, an innovative TDR tube sensor was developed to nondestructively monitor the extent of freezing in concrete specimens. The results show that this new sensor provides noninvasive measurement of freezing degree, which can be used to directly estimate the internal crystallization pressure for XFEM analyses. A volume-based damage criterion was also proposed based on the new TDR sensor. This work established a framework to integrate sensor and simulations to holistically predict the internal-frost damage process in concrete specimens.