Shihui Shen

FATIGUE ENDURANCE LIMIT FOR HIGHWAY AND AIRPORT PAVEMENTS

The existence of a fatigue endurance limit has been postulated for a considerable time. With the increasing emphasis on extended-life hot-mix asphalt pavement, or perpetual pavement, verification of the existence of this endurance limit, a strain below which none or very little fatigue damage develops, has become a substantial consideration in the design of these new multilayered full-depth pavements. Fatigue data are presented that were collected on a surface mix and a binder mixture tested for an extended period from 5 million to 48 million load repetitions at strain levels down to 70 microstrain. The fatigue results are analyzed in the traditional manner and using the dissipated energy ratio. This analysis shows that there is a difference in the data at normal strain levels recommended for fatigue testing and at the low strain levels. This difference cannot substantiate an endurance limit using traditional analysis procedures, but the dissipated energy approach clearly shows a distinct change in material behavior at low flexural strain levels, which supports the fact that at low strain levels the damage accumulated from each load cycle is disproportionately less than what is predicted from extrapolations of fatigue testing at normal strain levels. This reduced damage may be attributed to the healing process. The conclusion of this study is that laboratory testing can verify the existence of a fatigue endurance limit in the range of 90 to 70 microstrain below which the fatigue life of the mixture is significantly extended relative to normal design considerations.

Dissipated Energy Approach to Study Hot-Mix Asphalt Healing in Fatigue

The healing phenomenon has been noted by pavement engineers for years, but its relation to hot-mix asphalt (HMA) fatigue behavior is still far from clear. This study conducted an analysis of healing and HMA fatigue behavior by introducing a specifically designed fatigue-healing test. These results help explain the differences in fatigue behavior at normal and low strain levels. An approach using the ratio of dissipated energy change, which is based on energy concepts, is used in this study. The results show that healing does exist, and its effect on fatigue life can be indicated by an energy recovery per second of rest period. The effect of healing is more prominent at low strain levels or in very long rest periods. At low strain conditions, the dominance of healing compared with the very low external load damage, considering the energy equilibrium, can result in full damage recovery. This full recovery of energy explains the existence of a fatigue endurance limit, below which HMA materials tend to have extraordinarily long fatigue lives that, as is shown, can be related to healing. The testing conducted clearly shows why polymer modification may extend the fatigue life in the field even though laboratory testing may show minimal differences compared with the neat binder test results.

Application of the Dissipated Energy Concept in Fatigue Endurance Limit Testing

A fatigue endurance limit has been postulated to exist in hot-mix asphalt pavement performance. It cannot be observed and studied with the use of traditional phenomenological approaches as seen by the totally different fatigue behavior at low strain-damage levels close to the fatigue endurance limit. The ratio of dissipated energy change succeeds in defining and investigating the existence of a fatigue endurance limit with a unique relationship between plateau value (PV) and fatigue life (Nf), regardless of strain-damage levels, mixture types, loading modes, and other testing conditions. To determine a fatigue endurance limit requires an extraordinarily long time to conduct testing. This paper applied the PV to the study of a fatigue endurance limit to validate a shortened laboratory testing procedure. Statistical analysis shows that the shortened test can predict the PV with sufficient accuracy. By applying the validated relationship between PV and Nf, the extremely long fatigue life under low strain-dam...

A Dissipated Energy Approach to Fatigue Evaluation

ABSTRACT The fatigue behaviour of bituminous binders and/or bitumen-filler mastics has been postulated as having a strong correlation with the fatigue behaviour of asphalt mixtures. The binder is one of the major factors controlling fatigue of the asphalt mixture and is considered as the leading media of energy dissipation. It is verified in this paper that the application of the Ratio of Dissipated Energy Change (RDEC) approach in terms of the fatigue characteristics of bituminous binders and mastics produces a unique energy parameter, known as the Plateau Value (PV), similar to the PV previously identified for asphalt mixtures. The relationship between PV and fatigue life (Nf) is found to be unique for asphalt mixtures and binders (mastics). This suggests the RDEC approach is a fundamental approach for fatigue analysis of HMA. Furthermore, the two PV-Nf curves for asphalt mixtures and binders are strongly related, which provides a new way to explain mixture fatigue behaviour from a binders rheological characteristics.

Validating the Fatigue Endurance Limit for Hot Mix Asphalt

This report presents the findings of research performed to investigate the existence of a fatigue endurance limit for hot mix asphalt (HMA) mixtures, the effect of HMA mixture characteristics on the endurance limit, and the potential for the limits incorporation in structural design methods for flexible pavements. The report describes the research performed and includes proposed standard practices using various experimental and analytical procedures for determining the endurance limit of HMA mixtures. Thus, the report will be of immediate interest to materials and structural design engineers in state highway agencies and engineers in the HMA construction industry.

Quantification of Cohesive Healing of Asphalt Binder and its Impact Factors Based on Dissipated Energy Analysis

ABSTRACT The effect of healing on asphalt material fatigue performance has been accepted as an important property which is, at least partially, responsible for the significant difference between the laboratory and field fatigue behavior. Although some studies have been conducted to characterize the asphalt healing and its mechanism, very few researches have clearly identified how different loading and climate conditions can influence the effect of healing in a quantitative way. In addition, little suggestion has been provided on the integration healing into fatigue life prediction and pavement design. Therefore, this study develops a research based on the fundamental dissipated energy analysis to quantify the cohesive healing in asphalt binder. The key impact factors on healing are evaluated in a systematic way under different loading and environmental conditions. A healing rate predictive model and fatigue life prediction model based on binder rheological properties and visco-elastic energy dissipation is developed, which provides a promising way to integrate healing into a more mechanistic based pavement design procedure. The methodology used in this study can also be applied to investigate and quantify the adhesive healing at the aggregate-asphalt interface when aggregates are introduced into the experimental testing.

Evaluation of Fatigue Models of Hot-Mix Asphalt Through Laboratory Testing

Many studies have investigated the fatigue resistance of hot-mix asphalt (HMA) mixtures on the basis of the material properties and structural responses. In this study, a four-point bending-beam fatigue apparatus was used to measure the fatigue life of a typical Michigan HMA mixture under various frequencies (1, 5, and 10 Hz) and temperatures (4°C, 13°C, and 21.3°C). The objective of this study was to evaluate laboratory models for predicting fatigue over wide ranges of testing conditions to rank their fatigue performance on the HMA mixtures. In addition, this paper aimed to correlate the flexural stiffness (modulus) with dynamic modulus, because the dynamic moduli of HMA are key input parameters in the Mechanistic–Empirical Pavement Design Guide (MEPDG). The correlation results showed a strong linear correlation between the flexural stiffness and dynamic modulus, with the flexural stiffness being 30% lower than the dynamic modulus. By providing a linkage between dynamic modulus and flexural stiffness, the study helped to substantiate the concept of using dynamic modulus in the MEPDG or to evaluate rutting. In addition, Rowe and Bouldins stiffness degradation concept was used to compare the ability of different models to predict fatigue life. The results of this study showed that the energy-based fatigue prediction model, which considered rest periods, various testing temperatures, and loading frequencies, correlated well with the laboratory-determined fatigue life.

Energy Based Laboratory Fatigue Failure Criteria for Asphalt Materials

The definition of fatigue failure in the laboratory is not only an important but also a controversial issue. Researchers have developed a number of fatigue failure criteria, including the most traditional one, which defines failure at the cycle to 50 % initial modulus reduction. However, this definition is always challenged due to its lack of physical background. Recent studies showed that the dissipated energy ratio approach appears to be a favorable concept, which takes into account the fundamental dissipated energy evolution behavior of asphalt materials during a cyclic fatigue test. This paper conducted a review of three different energy based fatigue failure criteria and evaluated their applicability for fatigue data from asphalt binders and mixtures and under both stress and strain controlled loading modes. A macroscopic failure criterion is recommended, which is defined as the sudden change of the dissipated energy evolution curve and is consistently related to the beginning of macrocrack propagation. In addition, by comparing different failure criteria, the traditional 50 % initial modulus reduction criterion was found to have a strong correlation with energy based macroscopic fatigue failure for both mixtures and binders. It is thus suggested that the 50 % initial modulus reduction failure can be used as a simple but reasonable fatigue criterion, which indicates the transition from microcrack to macrocrack.

Modeling the Effects of Temperature and Loading Rate on Fatigue Property of Asphalt Binder

Fatigue cracking is one of three major distresses for asphalt pavement. Asphalt binder is a very important component of asphaltic mixture. The fatigue performance of asphalt binder also greatly affects that of asphaltic mixture. Asphalt binder’s fatigue characteristics are affected by external conditions, such as temperature and loading rate. It is warranted to study the effects of these external conditions on fatigue of asphalt binder and take them into account. New fatigue tests and properties have been proposed to characterize the fatigue behavior of asphalt binders, such as monotonic constant shear-rate tests. This study investigated the effects of temperature and shear rate on fatigue properties of asphalt binder. Two asphalt binders were tested with a range of temperature and shear rates. It was found that time-temperature superposition principle also applies to critical strain energy density and shear strength of asphalt binders. The values of shift factors for building the master curves of critical strain energy and shear strength are comparable.

Evaluation of the Effects of Asphalt Binder on the Properties of Hot Mix Asphalt at Intermediate Temperatures

Asphalt binder is an essential component of asphaltic mixtures. The performance of asphaltic mixture is directly related to mechanical characteristics of the binder. With the development of new material characterization methods for both asphalt binder and asphaltic mixture, there is a need to re-evaluate the relationship between the properties of binder and asphaltic mixture such that a proper understanding and selection of asphalt binder can be made to improve the performance of asphaltic mixture. In this study, the effects of asphalt binder properties on asphaltic mixture at intermediate temperature were evaluated using new materials characterization methods. Five asphalt binders and five asphalt mixtures containing these binders with same aggregates sources were tested. Four of the binders were modified with different techniques. Complex shear modulus and monotonic constant shear-rates tests were conducted on asphalt binders whereas dynamic modulus and indirect tensile strength tests were conducted on the corresponding mixtures. The effects of modification techniques on the properties of asphalt binders and relationship between properties of binder and asphaltic mixture were evaluated in this study. The results indicated that the modulus of asphalt binder directly affects that of asphalt mixture, as expected. The failure strain of asphalt binder controls that of asphalt mixture. However, the shear strength and fracture resistance, critical strain energy density in this case, of asphalt binder are not directly correlated with those of asphaltic mixtures. It is believed that the interaction between asphalt binder and aggregates, instead of asphalt binder alone, plays a significant role in the fracture of asphaltic mixture.