Simplified Viscoelastic Continuum Damage Model Research Articles

Impact of asphalt concrete fatigue endurance limit definition on pavement performance prediction

Many well-constructed Hot Mix Asphalt pavements have been in service for 40 or more years without any evidence of fatigue cracking. This field experience suggests that there exists a strain level, known as the fatigue endurance limit (FEL), below which an asphalt concrete pavement will not exhibit fatigue cracks. Several studies have been conducted to define and verify this limit. Each of these methods is associated with certain assumptions regarding the nature of the FEL and heretofore a comprehensive comparison of each has not been made using a consistent set of mixtures. Likewise, the impact of any observed differences in FEL on the predicted pavement performance has not been made. This paper investigates and compares six different methods for identifying the FEL: NCHRP 9–44A approach, simplified viscoelastic continuum damage model, smeared-healing with continuum damage model, plateau value approach, pseudo-strain analysis method, and reduced cycles method. Each method is found to yield different values ranges from approximately 30–170 microstrains at 21.1 °C. The predicted FEL from each of the six methods are then used with the mechanistic empirical design algorithm to evaluate their effects on predicted pavement performance. Simulation outputs show different pavement performance and perpetual pavement structural design thicknesses from each of the methods. The study outcomes are expected to benefit future field verification research of FEL as it provides comprehensive analyses using six different methods. This future verification research may indicate the method that best represents actual perpetual pavement design and performance.

Reliability Analysis of Fatigue Life Prediction from the Viscoelastic Continuum Damage Model

The fatigue resistance of asphalt concrete materials plays an important role in the service life of a pavement. Several empirical and mechanistic models and efforts are under way toward improving these models for accurate prediction of fatigue life. The reliability of predictions with these models has not received substantial attention. This study used the simplified viscoelastic continuum damage model to analyze the reliability of fatigue predictions. Modulus and fatigue tests were conducted on a standard dense graded asphalt mixture to characterize the model and create deterministic fatigue life predictions. Monte Carlo simulations were then used to calculate the reliability of the fatigue predictions, given the variation in input parameters. The analysis was conducted for combinations of three experimental failure criteria and two model failure criteria at two strain levels for a total of 12 study cases. Differences in reliability between the combinations of failure conditions are identified and discussed. There are three major findings of this study. First, depending on the selection of failure criteria, the prediction errors could range from ±22% to ±52% at a 50% level of reliability, depending on the exact combination of failure criterion and strain level. Second, with an experimental failure criterion of 50%, modulus reduction shows better reliability regardless of the model failure criterion chosen to predict fatigue life. Third, the model failure criterion based on constant pseudostiffness at failure was found to be more reliable than was the pseudostrain energy release rate criterion when characterization was performed with three tests.

Effect of Silo Storage Time on the Characteristics of Virgin and Reclaimed Asphalt Pavement Mixtures

Many hot-mix asphalt plants store material in heated silos before it is ready to be transported to construction sites. The time that material is stored in the silo is not controlled and varies widely, depending on several factors. As the material is exposed to elevated temperatures, short-term aging of the binder may occur. Another important consideration is the interaction between reclaimed asphalt pavement (RAP) and virgin binders, as blending or diffusion could occur between the binders. In this study, a virgin and 25% RAP mixture were sampled at incremental silo storage times up to 10 h. Characterization testing included performance grading, rheological indexes, Glover–Rowe parameter evaluation, rolling thin film oven aging on the binders, complex modulus, a simplified viscoelastic continuum damage model (S-VECD) for fatigue, and thermal stress restrained specimen testing of the mixtures. Simulations that used layered viscoelastic critical distresses pavement analysis to predict fatigue behavior from the S-VECD model were used to show the potential effects of silo storage time on pavement life. Results from all tests indicated that mixtures aged with an increase in silo storage time. RAP materials experienced a greater effect; this effect may be a function of the air void content or indication of blending–diffusion in the silo. Rolling thin film oven aging showed that current laboratory conditioning methods do not necessarily simulate asphalt plant production. Production parameters, such as silo storage time, have a significant impact on mixture performance.

Effect of Binder Modification on the Performance of an Ultra-Thin Overlay Pavement Preservation Strategy

The objective of this study was to determine whether asphalt rubber (AR) binders will provide similar or better performance compared with a polymer modified asphalt (PMA) binder when used in a high-performance, ultra-thin lift overlay pavement preservation strategy. Current specifications for these types of overlays normally require the use of a PMA binder, because it has the ability to make these overlays more elastic under traffic loading and is less sensitive to temperature fluctuations. However, several state agencies are looking into the feasibility of incorporating sustainable and environmentally friendly technologies, such as AR binders and warm mix asphalt (WMA) technologies, into their asphalt mixtures, including those used for high-performance, ultra-thin lift overlays. This study examined the effect of binder modification type (AR or PMA) and the influence of the use of WMA in high-performance, ultra-thin lift overlays. In general, the mixtures that were tested provided comparable rutting, moisture damage, and low-temperature cracking performance in the high-performance, ultra-thin lift overlay. However, the use of the AR binders indicated reduced performance compared with the PMA binder in mixture fatigue cracking via the beam fatigue test, and mixture reflective cracking in the overlay tester. The results from the beam fatigue test were not always supported by the fatigue life predictions from the simplified viscoelastic continuum damage model. The only detriment to mixture performance for mixtures incorporating WMA was a reduction in fatigue cracking performance when used with AR binders.

Comparison of Fatigue Cracking Performance of Asphalt Pavements Predicted by Pavement ME and LVECD Programs

Mechanistic–empirical pavement design has received significant attention from the pavement community as the method for designing asphalt pavements in the future. Currently available software for mechanistic–empirical pavement design includes the AASHTOWare Pavement ME Design (Pavement ME) program. The Pavement ME program allows users to predict pavement distresses by applying layered elastic theory for the mechanical responses and using empirical models for the distress predictions. The layered viscoelastic pavement design for critical distresses (LVECD) program, which employs three-dimensional viscoelastic finite element analysis with moving loads, can also be used to predict the fatigue and rutting performance of pavements. The LVECD program employs the simplified viscoelastic continuum damage (S-VECD) model as the material model for the fatigue performance predictions of asphalt mixtures under complex loading and environmental conditions. This paper examines and compares the performance of 33 pavement sections from five research projects located in the United States, Canada, and South Korea by using both the Pavement ME and LVECD computer programs. To verify the results obtained from these two programs, the simulations were compared with the field performance data. In terms of ranking, the LVECD simulations provided better agreement with the field performance data than did the Pavement ME simulations. One of the main reasons for the better predictions obtained by the LVECD program is that its fatigue performance predictions depend on the mixture properties of all the layers, whereas the Pavement ME program considers the fatigue properties of only the bottom layer mixture.

Mechanistic evaluation of fatigue cracking in asphalt pavements

Over the last several decades, significant research has been conducted to predict the fatigue cracking performance of asphalt pavements. Recently, the simplified viscoelastic continuum damage (S-VECD) model was developed as an efficient method of characterising the fatigue performance of asphalt mixtures under a wide range of loading conditions. Two important material properties that can be determined from the S-VECD model are the damage characteristic curve that defines how damage evolves in a specimen and the energy-based failure criterion that defines when the specimen fails. These two material functions are unique for a given mixture regardless of temperature, mode of loading, stress/strain amplitude and loading history. This study presents the application of the Layered Viscoelastic Crirtical Distresses (LVECD) programme to predict the fatigue performance of 18 pavement sections from different locations in the United States and Canada. The capability of the LVECD programme to capture crack initiation, crack propagation and damage in the pavement sections is investigated by comparing the simulation results with field observations. This study found reasonable agreement in trends between the damage growth throughout the pavement cross sections as predicted by the LVECD programme and the surface crack growth as evidenced by field observations.

Fatigue Performance of Rubberized Stone Matrix Asphalt by a Simplified Viscoelastic Continuum Damage Model

AbstractThe study investigated the fatigue performance of rubberized stone matrix asphalt (SMA) in the dry process and compared its fatigue characterization with SMAs with other typical binders. Five SMA mixtures were used with the following asphalt binders: (1) virgin asphalt of PG 67-22, (2) crumb rubber modifier (CRM) modified in wet process, (3) CRM modified in dry process, (4) terminal-blend binder, and (5) styrene-butadiene-styrene (SBS) modified binder. Dynamic modulus and direct-tension fatigue tests were performed using the asphalt mixture performance tester (AMPT) system. In addition, improved linear amplitude sweep (LAS) tests were performed on the binders by a dynamic shear rheometer (DSR). The fatigue test data were analyzed by the simplified viscoelastic continuum damage (S-VECD) theory. The results showed that (1) dynamic modulus of the rubberized SMA in the dry process was similar to that of the wet process and higher than that of SMA with virgin asphalt, although a little lower than those...

A comprehensive evaluation of the fatigue behaviour of plant-produced RAP mixtures

In this study, the fatigue performance of 12 plant-produced mixtures from New Hampshire and Vermont that contain reclaimed asphalt pavement (RAP) contents of 0–40% by total weight of mixture was evaluated. The mixture tests included dynamic modulus, uniaxial fatigue, beam fatigue, and overlay tests. Also, the simplified viscoelastic continuum damage (S-VECD) model failure criterion, called the GR method, was applied and input to the linear viscoelastic pavement analysis for critical distresses (LVECD) programme to predict the fatigue behaviour of the tested mixtures on thin and thick asphalt pavement structures. In order to explain the observed fatigue behaviour, the performance grades (PGs) of the binders that were extracted and recovered from the mixtures were determined. In general, the addition of RAP resulted in an increase in the stiffness of the materials. The magnitude of the impact of higher RAP percentages varied with each set of mixtures. The S-VECD model and beam fatigue test data showed a loss of fatigue resistance for high-percentage RAP mixtures in most of the cases. The overlay tester results showed clear drops in performance at higher RAP contents. The impact of lowering the PG of the virgin binder to compensate for higher levels of RAP also was studied. Lowering the PG led to improvement in the fatigue properties and was found to be a convenient practice. The changes in the measured properties also appeared to be a function of mix design variables that included the stiffness of the RAP, asphalt content, and production parameters such as silo storage times. In some cases, the effects of these factors outweighed the impact of the RAP level or PG of the virgin binder in the mixtures.

Ageing and rejuvenators: evaluating their impact on high RAP mixtures fatigue cracking characteristics using advanced mechanistic models and testing methods

Fatigue cracking of asphalt mixtures is highly dependent on ageing. Using larger amounts of reclaimed asphalt pavement (RAP) presents a concern that the resultant mixtures may be prone to fatigue cracking because of the aged binder in the RAP. Several studies have indicated that asphalt rejuvenators can allow more aged binder to be incorporated into asphalt mixtures. The four-point flexural beam fatigue test, HMA (hot-mix asphalt) fracture mechanics model, simplified viscoelastic continuum damage model, and the semi-circular bending test were used to evaluate the effect of ageing on the fatigue characteristics of high RAP mixtures modified with rejuvenators. The results from these tests were compared to see if they provided similar performance trends. The results indicated that the long-term ageing used in this study did not have a significant effect on the fatigue characteristics of the high RAP mixture with and without rejuvenators. Comparison of the fatigue tests did not show universal agreement.

Unified failure criterion for asphalt binder under cyclic fatigue loading

Defining failure and developing a unified failure criterion for the fatigue testing of asphalt materials remain a challenge. This study seeks to develop a failure criterion for the fatigue testing of asphalt binders under cyclic loading in the dynamic shear rheometer. Newly developed pseudo-strain energy (PSE)-based failure analysis is introduced for both the time sweep fatigue test and the accelerated linear amplitude sweep (LAS) test (AASHTO TP101). The presented methodology builds upon recent advances in the simplified viscoelastic continuum damage (S-VECD) modelling of asphalt mixtures. Trends in stored PSE have been proven to be effective in defining failure for the LAS tests of asphalt binders. This new proposed failure definition is material-dependent and, thus, is effective in capturing the benefits of asphalt modification for binder fatigue resistance. In addition, it is found that a unique relationship that is independent of loading history exists between the PSE release rate and fatigue life. The fatigue life predictions using this relationship and the S-VECD model are in reasonable agreement with the laboratory-measured fatigue life data and also generally relate well with the field fatigue performance measured in the FHWA-ALF (Federal Highway Administration – Accelerated Loading Facility) pooled fund study.

Fatigue Performance Evaluation of Rubberized Porous European Mixture by Simplified Viscoelastic Continuum Damage Model

The study investigated the fatigue performance of rubberized porous European mix (PEM) and control PEM and the influence introducing methods of crumb rubber modifier (CRM) on the fatigue performance of PEM. Four PEM mixtures were used: rubberized PEM in dry process, rubberized PEM in wet process, rubberized PEM with terminal blend binder, and control styrene–butadiene–styrene (SBS) modified PEM. Dynamic modulus and direct tension fatigue tests were performed with an asphalt mixture performance tester system. The fatigue test data were analyzed by the simplified viscoelastic continuum damage theory. There were four main results. First, the rutting resistance of the rubberized PEM in the dry process was similar to that of the wet process but slightly lower than that of the rubberized PEM with the terminal blend binder and SBS modified PEM. Second, the fatigue cracking resistance of the rubberized PEM in the dry process was similar to that of the wet process but significantly lower than those of the rubberized PEM with the terminal blend binder and SBS modified PEM. Third, the use of smaller size CRM and transpolyoctenamer (TOR) in the dry process may have made the properties of the dry process PEM close to those of the wet process. However, the 10% CRM or 4.5% TOR used in the study could not make rubberized PEMs produce longer fatigue life like the control SBS modified PEM. Fourth, the introduction of SBS into rubberized PEM by the terminal blend method may have significantly improved the fatigue performance of PEM.

Use of Mechanistic Models to Investigate Fatigue Performance of Asphalt Mixtures

Effects of design air void contents, design voids in mineral aggregate (VMA), and in-place air voids on the fatigue performance of asphalt mixtures were investigated with mechanistic analyses based on the viscoelastic continuum damage (VECD) analyses and the mechanistic–empirical pavement analysis using the AASHTOWare Pavement ME Design program. The VECD analyses included the simplified viscoelastic continuum damage model at the material level and two structural models: ( a) layered viscoelastic analysis and ( b) layered viscoelastic pavement analysis for critical distresses. The mix design of a 2013 accelerated loading facility test lane was selected to develop the volumetric mix designs with the design air voids of 3%, 4%, and 5%, design VMAs of 13%, 14%, and 15%, and in-place air void contents of 5%, 7%, and 9% with the Bailey method. Dynamic modulus and direct tension cyclic fatigue tests were performed in accordance with the AASHTO TP 107 procedure. The test results showed that the linear viscoelastic property was affected by the design VMA, design air void content, and in-place air void content in order of sensitivity. Also, the damage states at failure determined from the damage characteristic curves and the mechanistic fatigue predictions had consistent trends as observed for the design VMA, in-place air void, and design air void content in rank of sensitivity. Finally, the design VMA, in-place air void, and design air void parameters were found to be sensitive in the mechanistic analyses, whereas the parameter that was most sensitive in the pavement mechanistic–empirical analysis was the in-place air void content.

Fatigue and Rutting Evaluation of Laboratory-Produced Asphalt Mixtures Containing Reclaimed Asphalt Pavement

This study evaluated the performance of reclaimed asphalt pavement (RAP) mixtures for (a) RAP percentage, (b) asphalt content, and (c) various base binders. All RAP mixtures were produced in the laboratory for better control of the plant production variables. A simplified viscoelastic continuum damage model was used to evaluate the fatigue properties of the mixtures, and the triaxial stress sweep (TSS) test was used to evaluate rutting. In addition, layered viscoelastic critical distresses (LVECD) pavement analysis was used to predict the fatigue resistance of the mixtures. To explain the observed fatigue and rutting behavior, binder testing was performed on binders extracted and recovered from the mixtures. The test parameters included performance grade, low-temperature critical cracking temperature, and multiple stress creep and recovery parameters tested at a high temperature. Incorporating soft binder was found to be a promising strategy because the binder test data and LVECD predictions indicated a noticeable improvement in the fatigue resistance of the mixes. The TSS test data did not show a significant reduction in rutting resistance. The predictions clearly showed that increasing the asphalt layer thickness could lead to improved pavement performance. Mixtures with low asphalt binder content (0.5% below the optimum binder content) turned out to be susceptible to fatigue. The best strategy for incorporating high percentages of RAP seemed to be the use of a soft base binder while maintaining the optimum asphalt binder content or increasing the asphalt layer thickness.

Performance-Based Moisture Susceptibility Evaluation of Warm-Mix Asphalt Concrete through Laboratory Tests

This study used the cyclic direct tension test, indirect tensile strength test, and Hamburg wheel tracking device (HWTD) test to evaluate the moisture susceptibility of warm-mix asphalt (WMA) mixtures. The stripped areas that were quantified by digital imaging analysis were derived from the cyclic direct tension, indirect tensile strength, and HWTD tests and were compared with the mechanical properties of the mixtures to identify sensitive moisture susceptibility indicators. These methods were applied to a Superpave® 9.5-mm hot-mix asphalt mixture and five corresponding WMA mixtures that used the following technologies: (a) Evotherm 3G that contained a chemical additive, (b) foaming, (c) WMA-A that contained a chemical additive and was under development, (d) WMA-B that contained an organic additive and was under development, and (e) WMA-C that also contained an organic additive (different from that used in WMA-B) and was under development. Fatigue life ratios obtained from the simplified viscoelastic continuum damage model combined with layered viscoelastic analyses were determined to be the most sensitive indicators for moisture susceptibility. The stripping infection points derived from the HWTD tests also showed good sensitivity to moisture conditioning; however, each stripping infection point was affected by the permanent deformation characteristics of a given mixture as well as its moisture susceptibility; therefore, the observation was inconclusive.

Nonlinear Viscoelastic Behavior of Asphalt Concrete and Its Implication for Fatigue Modeling

Mechanistic models for asphalt concrete (AC) can consider many different physical mechanisms. However, as more mechanisms are considered, the complexity increases, and it becomes important to balance accuracy and complexity to create a model that can be used by the engineering community. In some cases, the material response is dominated by the effects of only some key processes, and smearing the effects of the minor ones is acceptable. In other cases, many processes are important and need to be considered. In this paper, the importance of modeling the nonlinear viscoelasticity (NLVE) of AC for fatigue response prediction is assessed. Two mechanistic hypotheses are considered for describing this phenomenon: linear viscoelastic with damage and NLVE with damage. The importance of explicitly considering NLVE effects under fatigue loading is evaluated with laboratory tests, the simplified viscoelastic continuum damage (S-VECD) model, and an NLVE form of the S-VECD model. These two models are characterized and used to simulate and compare AC fatigue response under constant and random controlled stress and strain conditions. It is found that while the NLVE-based formulation better represents the material response in random loading and suggests less overall damage accumulation during fatigue, the two models predict similar amounts of modulus reduction. The primary conclusion from this study is that because the goal for fatigue assessment is to find the change in modulus over a long period of time, fatigue response modeling of AC does not need to explicitly consider NLVE.

Fatigue Crack Assessment of Asphalt Concrete Pavement on a Single Span Highway Bridge Subjected to a Moving Truck

Abstract Characterization of fatigue crack in asphalt concrete (AC) pavement placed on the bridge is quite complex because it involves not only viscoelastic behavior of AC mixture but also dynamic interaction between bridge and traffic. In this study, a new approach that accounts for these material and structural aspects of the problem has been proposed to evaluate the fatigue crack potential of AC pavement on the bridge. A highway bridge surfaced with the 13-mm stone mastic asphalt (SMA) pavement that was subjected to a moving two-axle dump truck was selected as a simulation target and its equation of motion was derived using the Lagrange’s formulation. Given the range of testing variables, the maximum center deflections of the bridge were calculated and converted into the peak strains at the bottom of the SMA pavement. A simplified linear viscoelastic continuum damage model (S-VECD) was then implemented for the strains ranging from the largest peak strain to the specified allowable strain to predict the fatigue life of the SMA. The outputs showed that the fatigue life of the SMA got shorter as strain increases, and that hard SMA failed earlier than soft SMA. It was also found that SMA’s temperature was more influential than vehicle’s speed in causing fatigue cracks.

Simplified Viscoelastic Continuum Damage Model as Platform for Asphalt Concrete Fatigue Analysis

Cracking in asphalt concrete pavements is a major form of pavement distress in the United States. Because the cracking phenomenon is complex and cracking is often affected by both material and structural factors, field engineers have no quick and effective test and analysis protocols. A suite of fatigue analysis tools—as well as applications built around the simplified viscoelastic continuum damage (S-VECD) model—is presented. The S-VECD formulation is presented in a summarized form. Next, the characterization protocols, which are consistent with the capabilities of the asphalt mixture performance tester, are shown. Considerable attention is then given to S-VECD–based analysis tools for assessment of material- and pavement-level fatigue performance. Results show that the S-VECD model can be used to predict the number of cycles until fatigue failure for both constant stress and constant strain loading. The S-VECD model's sensitivity to mixture composition and external factors is shown through predictions of the endurance limit. Finally, pavement performance predictions are used to show how the S-VECD model can predict the field performance results of full-scale accelerated pavement tests, quantify the expected performance of pavement design alternatives, and identify factors that affect top-down cracking.