Analysis of Equivalence between Loading Rate and Stress Level of Fatigue Characteristics of Asphalt Mixture
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The accurate characterization of fatigue life affects the durability and reliability of asphalt pavement for the asphalt mixtures. This paper proposed a new fatigue test method and analysis method which, in addition to increasing the accuracy of fatigue characterization, reduces the time and cost consumed in laboratory tests of asphalt mixtures. According to the loading speed corresponding to different loading frequency and stress levels, the corresponding strength value is selected to determine the stress ratio in the fatigue equation. Therefore, the stress ratio can truly reflect the fatigue loading stress conditions and correspond to the fatigue loading speed one by one, avoiding the unscientific fatigue resistance design problem caused by the traditional S-N fatigue equation stress ratio, which takes a single strength value at a constant loading speed as the standard. Since the stress ratio takes into account the influence of loading speed under different loading frequencies and stress levels, a unified representation model of the fatigue equation of the asphalt mixture under different loading frequencies can be established; that is, the fatigue performance of the asphalt mixture under different loading frequencies can be normalized. The results showed that the loading rate in the strength test differed from the loading rate in the traditional fatigue test, which means that the fatigue curve could not be extended to point (1,1) when the fatigue equation was extended to a fatigue life Nf = 1. The fatigue test related to the velocity-dependent stress ratio in multiple experimental works, and data discreteness had a remarkable influence on fatigue characterization. The fatigue curve obtained by the fatigue equation under a constant loading rate was consistent with the fatigue curve under different loading rates. This method can reduce the duration of strength tests and the impact of the strength results of discreteness on fatigue characterization. The fatigue characterization model under constant loading rates considered the viscoelasticity of asphalt mixtures and guaranteed the correspondence of test conditions among stress level t, standard strength St, and fatigue life Nf. The stress levels and loading rates of the asphalt mixtures were confirmed to be equivalent. Furthermore, this paper established the relationship between strength and fatigue and obtained the fatigue life curve of the strength values. The fatigue performance of the asphalt mixtures is evaluated comprehensively.
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This work presents a new method for evaluating the time-temperature equivalence and unified characterization of the fatigue performance for asphalt mixtures. For this purpose, direct tensile strength tests and direct tensile fatigue tests were carried out on asphalt mixtures at different temperatures and loading rates. A time-temperature-dependent stress ratio was established. By exploring the correlation between stress ratio and fatigue life, a new fatigue model considering the effect of temperature and loading time was built. The time-temperature equivalent of fatigue performance was confirmed by analysis of the fatigue dates under several temperatures and loading frequencies. The results showed that the direct tensile strength of the asphalt mixture changed non-linearly with the variation of the loading rate or temperature. The stress ratio was also related to temperature and loading rate. The time-temperature stress ratio was linearly related to fatigue life in a double logarithmic coordinate system, and a new SN fatigue curve was formed. The fatigue life under several temperatures and various loading frequencies can be described in one fatigue model and the time-temperature equivalent can be explored. The master curve of fatigue life or stress ratio was built. The time-temperature superposition principle was applicable for fatigue life estimation and stress ratio calculation. The fatigue life under different loading frequencies and various temperatures of the pavement in service can be completely estimated. The relative error between the test results and the calculated values is less than 30%.
In view of shortcomings for the traditional fatigue theory in addressing fatigue cracking distress of asphalt pavement, this paper attempts to adopt fatigue damage theory, which is more consistent with the essence of pavement fatigue distress, to study the fatigue damage characteristic of asphalt mixture, and mainly focuses on its pivotal content—damage evolution model. Considering the important influence of loading frequency on the fatigue life and damage evolution characteristics of asphalt mixture, a new damage evolution model is constructed by introducing the influence of loading frequency into the classical Chaboche damage evolution model, as well as the theoretical analysis and laboratory tests are used to analyze and verify the model proposed in the paper. The results show that the calculated results of the model agree well with the measured, which can not only reflect the basic law of damage evolution and accumulation for asphalt mixture, but also reflect the influence of loading frequency on fatigue damage characteristics of mixture. Consequently, it is worthy of further application and promotion in the research of fatigue damage of asphalt pavement.
Asphaltic material is a kind of viscoelastic material, and its performance varies with test and materials conditions, such as stress states, temperatures, aging degree, gradations, and binders. The anti fatigue design of asphalt pavement has been stifled by these interferences. The objectives of this study are to reveal and compare the effects of loading frequency and aging degree on the fatigue performance of asphalt concretes. A series of tests for tensile fatigue on aged and unaged asphalt concretes were conducted at different loading frequencies. The theory of stress ratio considering different test conditions was proposed based on the conclusion that the stress ratio varies with stress loading rates. Then, the S-N fatigue equation was modified based on the theory. The fatigue test results were analyzed with the modified S-N fatigue equation to characterize the fatigue performance of asphalt concretes aged to different degrees under different loading frequencies. The different effects of these two factors on the fatigue characteristics of asphalt concretes were analyzed and compared. It could be found from the results that the influences of loading frequencies and aging degrees on fatigue characteristics of asphalt concrete were different. The fatigue test results of asphalt concrete under different loading frequencies could be characterized by one fatigue curve fitted by the modified S-N fatigue equation. It meant that the effects of loading frequency on fatigue were equivalent to the stress level. The fatigue test results of asphalt concrete with different aging degrees were fitted into different fatigue curves, respectively. The asphalt concretes aged 1 day had better fatigue life and more stable stress sensitivity than unaged asphalt concrete. When the aging degree was over 1 day, the fatigue performance deteriorated with the increase of aging degree. Moreover, it could be observed that the strength increased with the aging degrees when it was measured by the standard test loading rates. However, when the loading rates increased to a certain range, the contrary conclusion could be obtained because the viscoelasticity of asphalt concretes changed due to aging.
This study aims to evaluate the evolution of reliability with number of cycles for asphalt mixture under repeated fatigue loading, i.e. degradation reliability. A novel methodological framework was developed to analyze the reliability, which integrates four theories (reliability theory, probability distribution function transformation, fatigue damage and Weibull distribution). Based on the developed framework, a novel and simple method was established to calculate the degradation reliability through a probabilistic simulation technique. Flexural fatigue performance of asphalt mixture was then analyzed statistically by Weibull distribution function. The five numerical methods were adopted and compared to estimate Weibull parameters. Kolmogorov-Smirnov test was conducted for further confirming the validity of this distribution. The reliability of asphalt mixture under fatigue loading was assessed using the established method. Results show that the randomness evolution behaviors of asphalt mixture under fatigue loading can be described by the developed framework. The degradation reliability under non-fatigue damage behaviors may be able to be calculated by the established method. The two-parameters Weibull distribution allows to describe fatigue life data of asphalt mixture. The evolutions of reliability with number of cycles show a decrease trend with three-stage. The degradation reliability of asphalt mixture in practical project should be larger than the value corresponding to mean fatigue life as far as possible. Method of moments should be adopted to estimate distribution parameters for the evaluation of degradation reliability of asphalt mixture, the health state of asphalt mixture is thus early warned timely.
Improving fatigue cracking resistance of asphalt mixtures is an important issue that concerns roadway agencies, public and industry. There are various, well-established methods for improving the material characteristics, such as polymer modification methods. Recycling/reusing scrap tire rubber in asphalt pavements has also been an interest of engineers. As such, many different methods of scrap tire rubber (herein called crumb rubber) modification alternatives are being developed. This study focused on a comparative evaluation of the fatigue cracking resistance of various base and recycled tire rubber modified asphalt mixtures, including the relatively new and innovative devulcanized rubber (DVR), crumb rubber terminally blend (CRTB), crumb rubber wet process (CRWET), original/base PG58-28 and softer PG58-34 binders, by using Push-Pull Viscoelastic Continuum Damage (PP-VECD) analysis and AASHTOWare Pavement Mechanistic Empirical Design (Pavement ME) software. The results of the study revealed that rubberized asphalt mixtures provided better fatigue lives than the base and softer binders did. Especially, DVR mixtures outperformed at high strain levels, implying they can be used for heavy traffic roads to enhance the fatigue life of asphalt pavements. Both PP-VECD and Pavement ME produced results that showed similar trends.
Fatigue-cracking resistance of asphaltic materials and flexible pavements is strongly related to the properties of the binder under aging and its interaction with aggregate particles, which are small-scale physical-mechanical characteristics of components. Currently, there are binder test methods including the AASHTO TP 101 that are widely used to evaluate fatigue characteristics of asphalt binder. Despite the scientific leap for predicting and characterizing binder fatigue behavior in pavements, there are opportunities to better mimic the true state of the binder subjected to fatigue loading in pavements. This study attempted fatigue damage characterization using sand asphalt mortar (SAM) specimens, which are anticipated to better represent the realistic geometry (such as micrometer thick film) of binder in mixtures than typical binder fatigue tests that use 2-mm thick parallel plate specimens, while testing repeatability-efficiency can still be met due to the use of standard Ottawa sands as a load carrier between binder films. Multiple fatigue-related tests (i.e., time sweep and amplitude sweep) were conducted on both 2-mm thick binder specimens and SAM specimens by varying the level of aging to investigate the effects of binder length scale and aging on fatigue cracking behavior. The laboratory procedure to fabricate SAM specimens was presented with the testing repeatability, which supports the validity of the method. It was observed from the fatigue testing results that SAM testing could capture the microcracking and macrocracking phases more distinctively and sensitively than conventional binder fatigue testing, particularly with the influence of aging. These test results and findings imply that the SAM can be considered a proper solid phase to evaluate the properties and damage characteristics of binders and mastics under various conditions.
Hardening, the most important cause of aging, can lead to different pavement failures. The first goal of the present research was to appraise the impact of different aging conditions and strain levels on fatigue behavior of asphalt binders and mixtures using frequency sweep, linear amplitude sweep (LAS), and four-point bending (4PB) fatigue tests. The second aim of this study was to assess the impact of aging on the correlation between fatigue life of asphalt binders and mixtures. The outcomes of the LAS test demonstrated that increasing the aging severity at low strain levels improved the asphalt binder fatigue life. However, aging reduced binders fatigue life when subjected under higher strain levels. The rheological master curves showed that aging had a remarkable influence on the viscoelastic characteristics of the binders at low-frequency ranges, and the aging influence on rheological characteristics of the binders decreased at high-frequency ranges. The polymer modified binders (PMB) performed better in terms of fatigue performance under higher loading frequencies and lower strain. However, by reducing the loading frequency and increasing the strain level, the degradation of the polymeric network led to a reduction in the fatigue performance of the PMB. The outcomes of the 4PB fatigue test indicated that aging reduced the fatigue life of neat and modified mixtures at high strain levels, while it had not any remarkable impact on the fatigue behavior of modified mixtures at low strains. The ANOVA analysis showed that aging and strain levels had a substantial influence on the fatigue life of asphalt mixtures with a significance level of 0.05. The correlation between the fatigue life of binders and mixtures demonstrated that applying aging on mixtures increases the accuracy and correlation coefficient between the fatigue life of binders and mixtures.
Fatigue damage is one of the major distress modes that have a critical impact on the performance and serviceability of asphalt pavements. High variability in testing results poses a challenge for predicting the fatigue life of asphalt mixtures using conventional deter- ministic modeling approaches. This study examined the efficacy of using a probabilistic approach for the analysis of the viscoelastic continuum damage (VECD) and fatigue life of fine aggregate mixtures (FAM) that were prepared using one hot-mix asphalt (HMA) and three types of warm-mix asphalt (WMA). The FAM mixtures were subjected to oscillating stress-controlled fatigue tests. The results highlighted the differ- ences between the probabilistic and deterministic VECD approaches in representing and predicting fatigue life. In addition, the results dem- onstrated the advantage of the probabilistic VECD approach in providing more reliable predictions of fatigue life that account for uncertainty in determining the model parameters. The probabilistic analysis results showed that the WMA mixtures tested in this study had less variability but shorter fatigue life than the control HMA mixture.
The fatigue test methods under different test conditions have strong impact on the test results, which obstructs a precise evaluation of fatigue characteristic for asphalt mixtures. Different test conditions means different stress states. Fatigue characteristic of a material is an objective attribute, and it should not vary with test methods and conditions. In order to accurately and objectively evaluate the fatigue properties of asphalt mixtures, a normalization model of fatigue characteristics for asphalt mixtures under different stress states was established based on the yield criterion in 3 dimensional stress states and the patterns of variation between the strength and the loading rate. The compressive, tensile and indirect tensile strength and fatigue tests were carried out respectively in different loading rates. The yield surface of strength for asphalt mixture was put forward based on the strength tests results and the yield criterion. Then, the normalization model of fatigue characteristics for asphalt mixtures under different stress states was established based on the yield criterion in 3 dimensional stress states. The results show that the differences of the fatigue tests results of asphalt mixtures under different test condition are large. It is difficult to evaluate the fatigue performance in different stress states of asphalt mixtures by using the conventional S-N fatigue equation. However, these differences can be reduced or even eliminated by using the normalization model, which considered the influence of the stress state and the strength corresponding to the fatigue loading frequency and stress level. The unified expression of fatigue characteristics under different test conditions for asphalt mixtures was realized. The fatigue behavior of asphalt mixture under different stress conditions can be objectively evaluated by using the normalized model.