Matthew W Witczak

Viscoelastic, Viscoplastic, and Damage Modeling of Asphalt Concrete in Unconfined Compression

A comprehensive constitutive model for asphalt concrete was calibrated that included viscoelasticity, viscoplasticity, and irreversible micro-structural damage in unconfined compression. Three different types of laboratory tests were designed and performed to calibrate each of these response components. Small-strain dynamic modulus tests were used to calibrate the undamaged linear viscoelastic properties. Cyclic creep and recovery tests to failure were performed to calibrate the viscoplastic properties. Constant-rate-of-strain tests to failure were used to calibrate the damage behavior. These tests were performed at a wide range of temperatures, loading rates, and stress levels. Upon calibration of each individual response, the model was validated by predicting the results of other constant-rate-of-strain tests at temperatures and strain rates different from those used in the calibrations. The predictions for these different conditions indicate that the comprehensive model can realistically simulate a wide range of asphalt concrete behavior.

Rational Modeling of Tertiary Flow for Asphalt Mixtures

The objective of this research study was to evaluate several mathematical models to be used in calculating the onset of tertiary flow [referred to as the flow number (FN) parameter] for asphalt mixtures. The FN indicates the onset of shear deformation in asphalt mixtures, which is a significant parameter in evaluating rutting in the field. The FN is obtained from the repeated load permanent deformation (RLPD) laboratory test. Current modeling techniques in determining the FN use a polynomial model fitting approach, which works well for most conventional asphalt mixtures. However, analysis and observations on the use of this polynomial model for rubber-modified asphalt mixtures showed problems in identifying the true FN values. The scope of the work included the collection and analysis of more than 300 RLPD test data files, which comprised more than 40 mixtures, a wide range of test temperatures, and several stress levels. A new comprehensive mathematical model was recommended to accurately determine the FN. The results and analysis were evaluated through manual calculations and found to be accurate, rational, and applicable to all mixture types, whether a tertiary stage was reached or not.

USE OF STIFFNESS OF HOT-MIX ASPHALT AS A SIMPLE PERFORMANCE TEST

The objective was to determine whether the stiffness of a mix could be used as a simple performance test (SPT) parameter to complement the Superpave® volumetric mix design. This was investigated by a statistical analysis of the strength of the correlation between different mixture stiffness parameters and field performance (rutting, thermal, and fatigue cracking). A total of 30 mixtures were tested with laboratory-fabricated specimens. The studied stiffness parameters were compressive dynamic (complex) modulus |E*|, simple shear tester (SST) shear (complex) modulus |G*|, and dynamic elastic modulus Ed, obtained from ultrasonic wave propagation. Also, computed stiffness factors |E*|/sin ϕ and |G*|/sin ϕ for rutting and |E*|sin ϕ for cracking were studied as analogous to the Superpave binder specification. Research indicated that the correlation to rutting varied based on test temperature and frequency; it peaked at 54.4°C and 5 Hz. At peak conditions, |E*|/sin ϕ had better statistical correlation to rutting than |E*|, but correlations reversed at lower frequencies. Although |E*| and |G*| had similar correlation to rutting, analysis of test data indicated that the SST shear testing gave lower stiffness values and higher phase-angle values than the compressive dynamic modulus testing, even when Poisson’s ratio effects were considered. This was especially true at high temperatures. Because of these and other reasons, the dynamic modulus |E*| was recommended as the SPT parameter for rutting as well as for fatigue cracking. None of the studied parameters turned out to be adequate performance indicators for thermal cracking.

Asphalt Mix Master Curve Construction Using Sigmoidal Fitting Function with Non-Linear Least Squares Optimization

This paper presents a new simplified method of constructing a master curve of asphalt mix using test data covering a large range of temperatures from –18°C to 55°C. It utilizes sigmoidal fitting function and compressive cyclic (complex) modulus test data obtained at matrix combination of different frequencies and test temperatures. In the master curve construction, the time temperature superposition was modeled two different ways. First, using known time-temperature superposition equations, and second, shilling test data experimentally, i.e., not assuming any functional form for the time-temperature relationship. The master curve construction was done using an Excel spreadsheet with the solver function, which is a tool for performing optimization with non-linear least squares regression technique. The analysis of over sixty mixtures indicated that the experimental approach agreed the best with the Arrhenius shifting equation, correlation coefficient R 2 being 0.922. Also, the experimental shifting was most flexible, producing the best fit among the studied shifting equations due to the fact that it has the most degree of freedom.

Harmonized resilient modulus test method for unbound pavement materials

A new laboratory test protocol for the measurement of resilient modulus of unbound pavement materials is described. This harmonized protocol combines the best features from four state-of-the-art resilient modulus test procedures in current use. The harmonized procedure most closely follows the recommended protocol proposed in NCHRP Project 1-28, Appendix E, with some exceptions. The main modifications deal with revised and more rational stress sequences and a more accurate stress-dependent resilient modulus predictive equation. Thirteen different predictive models and 25 sets of resilient modulus test data were evaluated as the basis for the recommended predictive equation.

New Predictive Models for Viscosity and Complex Shear Modulus of Asphalt Binders: For Use with Mechanistic–Empirical Pavement Design Guide

The main purpose of this paper is to present the development of a set of predictive models for the viscosity and complex shear modulus of asphalt binders. The model development was aimed at overcoming the limitations of current models used in the Mechanistic-Empirical Pavement Design Guide (MEPDG), which has been developed under NCHRP Project 1-37A and refined under NCHRP Project 1-40D. A comprehensive study was completed at Arizona State University to conduct a number of asphalt binder tests and to finalize a large binder characterization database containing 8,940 data points from 41 different virgin and modified binders. This database was used to develop the new models. The first of the models developed in this study is a fully revised version of the widely known ASTM Ai-VTSi viscosity model. This new, revised model takes into account the loading frequency applied on the binder while accurately predicting the viscosity at a specific temperature and loading frequency from the Ai and VTSi values of a binder. The other two new models developed are capable of accurately predicting the dynamic shear modulus (|G*b|) and associated phase angle (δb) of the binder at a specific temperature and loading frequency from the Ai and VTSi values of the binder. The models have been found to be rational, unbiased, accurate, and statistically sound. It is hypothesized that because of mathematical structures similar to those used in the MEPDG, the models developed in this study can be incorporated easily into a future revision of the guide.

Viscoplasticity Modeling of Asphalt Concrete Behavior

A constitutive model based on an extended form of the Schapery continuum damage formulation is currently being evaluated and developed as a comprehensive material model for asphalt concrete. This model considers the viscoelastic, damage, and viscoplastic components of asphalt concrete behavior over the full range conditions of interest for the mechanistic prediction of flexible pavement distresses. The focus of the present paper is limited to the viscoplastic response component at intermediate and high temperatures. The results confirm earlier findings that asphalt concrete in compression is a thermorheologically simple material well into the large strain viscoplastic regime at elevated temperature. The study demonstrates that the proposed viscoplastic model component provides a good representation of the viscoplastic response of asphalt concrete in uniaxial unconfined compression. Given the validity of time-temperature superposition for viscoplastic response, the viscoplastic material parameters can be calibrated from a limited number of uniform time and uniform load creep and recovery tests. Typical viscoplastic material parameters are derived for a representative asphalt concrete mixture.

Application of digital image correlation method to mechanical testing of asphalt-aggregate mixtures

The digital image correlation (DIC) method, a noncontact, full-field displacement measurement technique, has been applied to mechanical testing of asphalt concrete. A single couple charged device camera acquires images of an area of interest from a specimen in the undeformed and deformed states. These images are correlated to determine deformations, and advanced mathematical procedures are applied to these deformations to calculate strains. To verify the DIC measurements, vertical displacements for the middle and bottom sections of a specimen subjected to monotonic tension are compared with conventional linear variable differential transformer measurements. A series of DIC images captured during the monotonic and cyclic tests visualizes the evolution of the failure zone (i.e., the fracture process zone) at the crack tip. Also, it is demonstrated that the full-field measurement and post-processing nature of DIC allows a more accurate determination of the stress-strain behavior of the fracture process zone. The applicability of this method to a cylindrical specimen with a curved surface is also investigated by testing a 75-mm-diameter cylindrical specimen. Finally, the DIC method is extended to cyclic testing of asphalt mixtures with the aid of a synchronized image acquisition technique.

AYMA: Mechanistic Probabilistic System To Evaluate Flexible Pavement Performance

A new rational mechanistic model for analysis and design of flexible pavement systems has been developed. Furthermore, a fundamental probabilistic approach was incorporated into this system to account for the uncertainty of material and environmental conditions. The system was integrated in a user-friendly Windows program with a variety of user-selected options that include widely used models and those recently developed in the Strategic Highway Research Program project. Three basic types of distress can be investigated separately or all together, including fatigue cracking, permanent deformation, and low-temperature cracking. The mechanistic approach makes use of the JULEA layered elastic analysis program to obtain pavement response. The system provides optional deterministic and probabilistic solutions, accounts for aging and temperature effects over the asphalt materials, variable interface friction, multiple wheel loads, and user-selected locations for analysis. Tabular and graphical results provide expected distress values for each month as well as their variability, probability of failure, and assessment of the overall reliability of the pavement relative to each type of distress for a user-selected failure criterion. Only the load-associated module of AYMA is presented; a separate work describes the low-temperature cracking analysis.

Calibration of alligator fatigue cracking model for 2002 design guide

In AASHTOs 2002 design guide, the classic fatigue cracking mechanism, which normally initiates at the bottom of the asphalt layer and propagates to the surface (bottom-up cracking), was studied. The prediction of bottom-up alligator fatigue cracking was based on a mechanistic approach to calculate stress and strain. An empirical approach then related these strains to fatigue damage in pavement caused by traffic loads. To provide credibility to the new design procedure, the theoretically predicted distress models must be calibrated to “real-world” performance. In fact, calibration of these distress models is considered to be the most important activity to facilitate implementation, acceptance, and adoption of the design procedure and to establish confidence in the entire procedure. The procedure followed for the national calibration of the alligator fatigue–cracking model used in the AASHTO 2002 design guide is discussed. This calibration study used data from all over the United States, with different environments, material, and traffic. A total of 82 pavement sections from 24 different states were used in the calibration. Tensile strain at the bottom of each asphalt layer was calculated using a linear layer elastic analysis procedure. The initial (base) reference model used in the calibration was the Asphalt Institute MS-1 model. This model was used to compute the damage caused by traffic loads and pavement structure. Predicted damage was then correlated to the measured fatigue cracking in the field, and a transfer function was obtained for the alligator fatigue–cracking distress.