Yong-Rak Kim

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.

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.

Micromechanical Model for Heterogeneous Asphalt Concrete Mixtures Subjected to Fracture Failure

Cracking is a main source of structural distress in asphalt materials and asphaltic pavements. To predict crack-associated fracture damage in asphalt mixtures, this study presents a model using the finite-element method and a cohesive zone fracture model. The approach allows advanced characterization of the microstructural damage evolution in a more realistic length scale, the mixture heterogeneity, the inelastic material behavior, and the interactions among mixture constituents. The model presented herein accounts for (1) actual mixture heterogeneity by using digital image techniques; (2) inelastic material behavior based on elastic-viscoelastic constitutive relations; and (3) microscale fracture damage represented by the cohesive zone fracture model. A computational modeling framework is presented, and the applicability of the model is demonstrated through simulations. Model simulations are further analyzed by comparing numerical predictions to laboratory test results and by conducting parametric analyses of fracture properties. It is expected that the successfully developed computational model can provide better insights into the effect of mixture constituents on overall mixture performance, while minimizing modeling efforts and producing more accurate simulations than traditional approaches, with significant savings in experimental costs and time.

Experimental Testing and Finite-Element Modeling to Evaluate the Effects of Aggregate Angularity on Bituminous Mixture Performance

This study evaluates the effects of aggregate angularity in bituminous mixtures. Previous studies have predominantly focused on the effects of aggregate angularity on the resistance to permanent deformation, while little work has investigated the role of aggregate angularity related to mixture volumetrics and fatigue performance. To investigate the effect of aggregate angularity on mixture performance and characteristics, five mixes with different combinations of coarse and fine aggregate angularity are evaluated by performing the uniaxial static creep test and the indirect tensile fracture energy test. The asphalt pavement analyzer test is also performed with five-year field project mixtures. Fracture energy test results are then incorporated with finite element simulations of virtual specimens produced to explore the detailed mechanisms of cracking related to the aggregate angularity. Rutting performance test results indicate that higher angularity in the mixture improves rut resistance due to better aggregate interlocking. The overall effect of angularity on the mixturesi¯ resistance to fatigue damage is positive because aggregate blends with higher angularity require more binder to meet mix design criteria, which mitigates cracking due to increased viscoelastic energy dissipation from the binder, while angular particles produce a higher stress concentration that results in potential cracks. Finite element simulations of virtual specimens support findings from experimental tests.

Computational Model to Predict Fatigue Damage Behavior of Asphalt Mixtures Under Cyclic Loading

Fatigue cracking and failure of inelastic heterogeneous asphalt concrete mixtures were modeled computationally with the finite element method. The model incorporates elastic behavior of the aggregate particles, visco-elastic behavior of the asphalt matrix, and time-dependent fracture both within the asphalt matrix and along boundaries between matrix and aggregate particles. Rate-dependent progressive cracking up to failure was implemented by incorporation of a cohesive zone fracture model. The resulting model was used to simulate comprehensive fatigue damage-associated mechanical behavior including microcracking, macrocracking, and eventual sample failure of several asphalt mixtures composed of different mixture constituents, which results in different damage evolution characteristics. Simulation results were compared with real fatigue testing data in both load-controlled and displacement-controlled modes and demonstrated good correlations to laboratory data with model calibrations. The approach proposed ...

Damage-Induced Modeling of Elastic-Viscoelastic Randomly Oriented Particulate Composites

This paper presents a model for predicting the damage-induced mechanical response of particle-reinforced composites. The modeling includes the effects of matrix viscoelasticity and fracture, both within the matrix and along the boundaries between matrix and rigid particles. Because of these inhomogeneities, the analysis is performed using the finite element method. Interface fracture is predicted by using a nonlinear viscoelastic cohesive zone model. Rate-dependent viscoelastic behavior of the matrix material and cohesive zone is incorporated by utilizing a numerical time-incrementalized algorithm. The proposed modeling approach can be successfully employed for numerous types of solid media that exhibit matrix viscoelasticity and complex damage evolution characteristics within the matrix as well as along the matrix-particle boundaries. Computational results are given for various asphalt concrete mixtures. Simulation results demonstrate that each model parameter and design variable significantly influences the mechanical behavior of the mixture.

Cohesive zone model to predict fracture in bituminous materials and asphaltic pavements: state-of-the-art review

Cohesive zone (CZ) modelling has been receiving increasing attention from the asphaltic materials and pavement mechanics community as a mechanistic approach to model crack initiation and propagation in materials and structures. The CZ model provides a powerful and efficient tool that can be easily implemented in existing computational methods for brittle, quasi-brittle and ductile failure as well as interfacial fracture, all of which are frequently observed in asphaltic materials. Accordingly, this paper introduces the CZ modelling approach in the form of a state-of-the-art review addressing the concept of CZ modelling, CZ constitutive relations, their implementation into computational methods and up-to-date applications of CZ modelling to bituminous mixtures and pavement structures. This paper also includes a brief discussion on the current challenges that researchers face and the future directions to the modelling of fracture in bituminous materials and pavements. CZ modelling is not a topic that can be possibly discussed in a single article; therefore, it should be clearly noted that this review primarily attempts to deliver some of the core aspects of CZ modelling in the area of bituminous composites.

Multiscale Modeling to Predict Mechanical Behavior of Asphalt Mixtures

This study presents a multiscale computational model for predicting the mechanical behavior of asphalt mixtures. The model can account for mixture heterogeneities by considering individual mixture constituents through the scale-linking technique: a local scale in a form of the heterogeneous representative volume element and a global scale that has been homogenized from local scale responses. The model is implemented with a finite element formulation, so that geometric complexities, material inelasticity, and the growth of time-dependent damage can be properly handled. Damage is in the form of cracks modeled with nonlinear viscoelastic cohesive zones. The primary purpose of this paper is to present the multiscale modeling framework developed and to evaluate the applicability of the multiscale modeling technique to determine the performance of asphalt mixtures and structures when damaged. This is accomplished by employing only material properties at the constituent level (local scale) as model inputs. The indirect tensile test of fine-aggregate matrix mixture is simulated as an example, and the simulation results are compared with experimental results to evaluate the applicability of the model. Predictive power of the model and the benefits related to the reduction of computational efforts and laboratory tests are further discussed.

Rate- and Temperature-Dependent Fracture Characteristics of Asphaltic Paving Mixtures

Cracking in asphaltic pavement layers causes primary failure of the roadway structure, and the fracture resistance and characteristics of asphalt mixtures significantly influence the service life of asphaltic roadways. A better understanding of the fracture process is considered a necessary step to the proper development of design-analysis procedures for asphaltic mixtures and pavement structures. However, such effort involves many challenges because of the complex nature of asphaltic materials. In this study, experiments were conducted using uniaxial compressive specimens to characterize the linear viscoelastic properties and semi-circular bending (SCB) specimens to characterize fracture behavior of a typical dense-graded asphalt paving mixture subjected to various loading rates and at different temperatures. The SCB fracture test was also incorporated with a digital image correlation (DIC) system and finite-element model simulations including material viscoelasticity and cohesive-zone fracture to effectively capture local fracture processes and resulting fracture properties. The test results and model simulations clearly demonstrate that: (1) the rate- and temperature-dependent fracture characteristics need to be identified at the local fracture process zone, and (2) the rate- and temperature-dependent fracture properties are necessary in the structural design of asphaltic pavements with which a wide range of strain rates and service temperatures is usually associated.

Geometrical evaluation and experimental verification to determine representative volume elements of heterogeneous asphalt mixtures

This paper presents an experimental verification of geometrically defined representative volume elements (RVEs) of heterogeneous asphalt concrete mixtures before any significant damage is initiated. A typical dense-graded Superpave mixture (12.5 mm nominal maximum aggregate size) is selected as a representative roadway paving mixture and used in this study to accomplish two parallel approaches: Geometrical analysis of mixture heterogeneity using two-dimensional actual images of asphalt concrete inner structures and experimental evaluation through uniaxial tensile tests of asphalt concrete mixtures incorporated with digital image correlation (DIC) technique. To properly address the significant heterogeneity of asphalt concrete mixtures in defining the RVE, several geometrical factors such as area fraction, gradation, orientation, and the distribution of aggregate particles in asphalt concrete mixtures are considered altogether. For the uniaxial tensile test with the DIC, the mean strains and their standard deviations captured by DIC are analyzed to confirm statistical homogeneity of RVEs evaluated from the geometrical analyses. The two approaches present similar results, indicating that typical dense-graded asphalt mixtures can be characterized for their material properties with an approximate RVE size of 60 mm. Findings from this study further imply that the simple geometrical analysis can be an efficient tool to reasonably determine the RVE of asphalt mixtures and other granular composites where significant heterogeneity is involved.