Y. Richard Kim

A viscoelastic continuum damage model and its application to uniaxial behavior of asphalt concrete

Abstract An existing viscoelastic constitutive model which accounts for the effects of rate-dependent damage growth is described and applied successfully to characterize the uniaxial stress, constant strain rate behavior of asphalt concrete. The special case of an elastic continuum damage model with multiaxial loading, which is based upon thermodynamics of irreversible processes with internal state variables, is first reviewed and then it is shown how this model has been extended to a corresponding viscoelastic damage model through the use of an elastic-viscoelastic correspondence principle. The general mathematical model is next specialized to uniaxial loading. A rate-type evolution law, similar in form to a crack growth law for a viscoelastic medium, is adopted for describing the damage growth within the body. Results from laboratory tests of uniaxial specimens under axial tension at different strain rates are then shown to be consistent with the theory. The discussion of data analysis describes the specific procedure used here to obtain the material parameters in the constitutive model for uniaxial loading and how the method may be generalized for multiaxial loading.

Improved calculation method of damage parameter in viscoelastic continuum damage model

Modelling the performance of asphalt concrete using continuum damage theories is an approach that has gained international attention in recent years. These types of models are advantageous because they ignore many of the complicated physical interactions at the microscale level and instead characterise a material using macroscale observations. One such model, the viscoelastic continuum damage model, is used in this study to examine the fatigue performance of asphalt concrete mixtures. A mathematically rigorous exploration is undertaken to specialise the model for easy prediction and characterisation using cyclic fatigue tests on cylindrical specimens. This process reveals that certain theoretical shortcomings are evident in other similar models and corrects them with a newly developed model. The resulting model is capable of capturing the underlying material property, i.e. the damage characteristic curve, which is responsible for the performance of controlled stress, controlled crosshead strain and constant crosshead rate monotonic tension until failure tests.

Fatigue Cracking Resistance of Fiber-Reinforced Asphalt Concrete

The influence of fibers on the fatigue cracking resistance of asphalt concrete is investigated using fracture energy. Nylon, a popular facing yarn of carpets, is used for the actual recycled carpet fibers in asphalt pavement. The experimental program is designed with two phases: the single fiber pull-out test and the indirect tension strength test. Through pull-out tests of 15-denier single nylon fibers, the critical fiber embedded length is determined to be 9.2 mm. As for indirect tension strength tests, samples of asphalt concrete mixed with nylon fibers of two lengths, 6 and 12 mm, based on results of the pull-out tests (critical embedded length) and three volume fractions, 0.25, 0.5, and 1%, are prepared and tested. Asphalt concrete samples fabricated with fibers of 1% and 12 mm results in 85% higher fracture energy than non-reinforced specimens, showing improved fatigue cracking resistance. Although an optimized asphalt mix design with fibers has not been developed for this study, the increased fracture energy represents a potential for improving asphalt fatigue life, which may be facilitated through the use of recycled carpet fibers.

Development of a Failure Criterion for Asphalt Mixtures Under Different Modes of Fatigue Loading

In the study reported in this paper, the previously developed failure criterion for the viscoelastic continuum damage (VECD) model (referred to here as GOR, where O indicates old and GR is the rate of release of the pseudostrain energy) was applied to different modes of fatigue loading. The research team found that this criterion was mode-of-loading dependent and therefore considered insufficient. To mitigate this limitation, the GOR criterion was refined to become a new failure criterion, the GR method, which resolved the mode-of-load dependency issue. A characteristic relationship, which was found to exist in recycled asphalt pavement (RAP) and non-RAP mixtures between the rate of change of the averaged released pseudostrain energy during fatigue testing and the final fatigue life was derived in this study. This relationship is independent of mode of loading, strain amplitude, and temperature. The proposed failure criterion combines the advantages of the VECD model and this characteristic relationship, which originate from fundamental mixture properties. This proposed method can predict the fatigue life of asphalt concrete mixtures across different modes of loading, temperatures, and strain amplitudes within typical sample-to-sample variability that is observed in fatigue testing.

Determination of Time-Domain Viscoelastic Functions Using Optimized Interconversion Techniques

ABSTRACT Several viscoelastic response functions are available to characterize the LVE behavior of asphalt concrete, some in time domain such as relaxation modulus E(t) and creep compliance D(t) and other such as complex modulus E* in frequency domain. The use of the complex modulus test has risen sharply after it has been incorporated in the M-E Pavement Design Guide and in the Superpave Simple Performance Test. With the availability of E* data it becomes advantageous to use mathematical interconversion techniques to obtain time-domain functions E(t) and D(t) which are typically used for constitutive modeling and other applications. This paper addresses the steps involved in conducting the interconversion between frequency-domain and time-domain functions. Issues considered include: a) presmoothing of raw data, b) refinement of phase angle data, c) Prony series representation of the fitted data including determination and sign-control of the Prony series coefficients, and d) interconversion techniques: approximate vs. exact. Finally, interconversion methods are evaluated by comparing D(t) data converted from E* to that measured in the lab.

Application of Thixotropy to Analyze Fatigue and Healing Characteristics of Asphalt Binder

The fatigue performance of asphalt binder is critical to understanding the fatigue performance of asphalt mixtures, especially the effects of healing. Although research into the fatigue and healing characteristics of asphalt binder is found in numerous references, an efficient technique to evaluate these characteristics still does not exist. Thixotropy is a concept that may help explain the material behavior and provide an efficient evaluation technique. This property is related to the breakdown and buildup of microstructure that may cause the changes observed during fatigue and healing tests. Thus, tracking the thixotropy of asphalt binders may be a good method to study fatigue and healing. For this study, experiments were performed to characterize the fatigue and healing characteristics of three typical asphalt binders. Then a common thixotropic model was characterized with a relatively simple stepped-flow test and oscillation experiment. The resulting model shows good correlation with the measured fatigue and healing tests. This finding, though based on a limited number of binders, suggests that thixotropy may play a role in the fatigue and healing characteristics of asphalt binder.

Development of a failure criterion for asphalt mixtures under fatigue loading

The failure criterion defines the applicable region associated with the continuum damage model and is important in characterising the service life of asphalt mixtures. A proper failure criterion should consistently predict the failure of the material that reaches macro-fracture. A previously developed criterion that uses the viscoelastic continuum damage (VECD) model exhibits high variability and is considered to be inefficient because it requires calibration tests at different temperatures. In this paper, a new concept that involves released pseudo strain energy is introduced. This released pseudo strain energy concept focuses on the dissipated energy that is related to stiffness reduction only and is fully compatible and predictable using the VECD model. A characteristic relationship is found between the stable rate of pseudo energy release during testing and the final fatigue life of the same mixture, independent of strain amplitude and temperature. Based on these observations, a new failure criterion is proposed. The proposed failure criterion combines the advantages of the VECD model and this characteristic relationship, which both originate from fundamental mixture properties, and is able to predict the fatigue life of asphalt concrete mixtures across different temperatures and strain amplitudes.

New relationships between falling weight deflectometer deflections and asphalt pavement layer condition indicators

New relationships have been identified between the layer condition indicators of flexible pavements and falling weight deflectometer (FWD) deflections. Synthetic databases were generated using dynamic finite element analysis with nonlinear material models. The sensitivity of various deflection basin parameters (DBPs) to layer conditions was comprehensively examined on the basis of the developed databases. Three types of layer condition indicators were identified in the study, including DBPs, effective layer moduli, and stresses and strains. The DBPs identified from the sensitivity study were used in developing new relationships between the selected condition indicators and FWD deflections by applying regression and artificial neural network techniques. Even though these relationships include the complicated dynamic effect of FWD loading and nonlinear behavior of unbound materials, the time to obtain results from these procedures is insignificant, thus making the procedures suitable for field implementation.

Implications of warm-mix asphalt on long-term oxidative ageing and fatigue performance of asphalt binders and mixtures

The use of warm mix asphalt (WMA) has been increasing in recent years due to its ability to reduce the production temperatures of asphalt concrete. The long-term implications of reduced production temperatures and, hence, reduced short-term ageing on long-term performance remain largely unknown. This study evaluates the effect of age hardening in WMA binders and mixtures compared with hot mix asphalt (HMA) binders and mixtures with respect to fatigue damage. Two WMA technologies are considered: foaming by water injection and Evotherm modification. For this study, the asphalt mixtures were subjected to laboratory conditioning in a forced air convection oven to simulate long-term field ageing according to AASHTO R30. The asphalt mixtures and extracted binders were subjected to linear viscoelastic and fatigue characterisation following ageing. Because oxidative ageing occurs within the asphalt binder phase of asphalt concrete, this paper focuses on the relative performance of WMA and HMA binders at various ageing levels and compares this binder performance to the respective mixture performance. Cyclic direct tension tests were used to measure the fatigue resistance of the asphalt mixtures, and the linear amplitude sweep (LAS) test was used to measure the fatigue resistance of the binders. Simplified viscoelastic continuum damage (S-VECD) analysis was performed to interpret the fatigue test results and predict the fatigue performance of the binders and mixtures using a pavement structural model. The results demonstrate that after substantial long-term ageing, differences between the fatigue performance of WMA and HMA become insignificant. The results also demonstrate good agreement between the binder and mixture results, indicating that the LAS test coupled with S-VECD analysis is able to capture the binders contribution to mixture fatigue.

Microstructural investigation of asphalt concrete for performing multiscale experimental studies

In this paper, a microstructural hypothesis for asphalt concrete (AC) is developed in order to provide a basis for a multiscale experimental investigation. The hypothesis is consistent with the belief that AC can be considered as a four-scale assemblage of components with different characteristic length scale, binder, mastic, fine aggregate matrix (FAM) and finally AC. The hypothesis is supported with a series of direct microstructural experiments including morphological observations with digital and scanning electron microscopy as well as quantitative evaluation using a novel meso-gravimetric test method developed specifically for this research. Morphological evaluation shows that asphalt mastic effectively exists as a basic building block for AC. Meso-gravimetric analysis finds that the volumetric composition of this mastic is equal to that found when assuming that mastic contains all of the effective asphalt binder and the filler-sized particles. Other key volumetric properties including FAM gradation and mastic concentration within the FAM and mixture are presented as well.