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Fatigue cracking is one of the most common distresses of asphalt pavements, whereas healing is a counter process to cracking which alleviates cracking damage and extends fatigue life of asphalt pavements. Most of existing methods to characterize fatigue cracking and healing are generally empirical or phenomenological in nature, which does not satisfy the need to develop mechanistic-based pavement design methods. The objective of this study is to characterize fatigue cracking and healing of asphalt mixtures using an energy-based mechanistic approach. A controlled-strain repeated direct tension (RDT) test is selected to generate both fatigue cracking and permanent deformation in an asphalt mixture specimen. Fatigue cracking is separated from permanent deformation from a mechanical viewpoint. The development of fatigue cracking is described by the evolution of the damage density and the increase of the average crack size with the increase of loading cycles. A creep and step-loading recovery (CSR) test is designed to measure the internal stress in the recovery phase of an asphalt mixture specimen. The internal stress and the strain measured in the recovery phase are used to conduct the mechanistic analysis of recovery and healing of the asphalt mixture specimen. Then healing is described using the decrease of the damage density and average crack size with time. Different types of asphalt mixtures produce distinctly different fatigue cracking and healing characteristics. The effect of mixture composition, temperature, and aging are evaluated using the approach above. The entire series of tests for fatigue, permanent deformation and healing can be completed in one day, with the healing part requiring only a matter of minutes. The methods proposed in this study characterize fatigue cracking and healing of asphalt mixtures using its essential cause and effect relationship.
Studies show that the microstructure of the fine aggregate matrix has a significant influence on the mechanical properties and evolution of damage in an asphalt mixture. However, very little work has been done to quantitatively characterize the microstructure of the asphalt binder within the fine aggregate matrix of asphalt mixtures. The first objective of this study was to quantitatively characterize the three dimensional microstructure of the asphalt binder within the fine aggregate matrix (FAM) of an asphalt mixture and compare the influence of binder content, coarse aggregate gradation, and fine aggregate gradation on this microstructure. Studies indicate that gradation of the fine aggregate has the most influence of the degree of anisotropy whereas gradation of the coarse aggregate has the most influence on the direction anisotropy of the asphalt mastic within the fine aggregate matrix. Addition of asphalt binder or adjustments to the fine aggregate gradation also resulted in a more uniform distribution of the asphalt mastic within the fine aggregate matrix. The second objective of this study was to compare the internal microstructure of the mortar within a full-scale asphalt mixture to the internal microstructure of the FAM specimen and also conduct a limited evaluation of the influence of mixture properties and methods of compaction on the engineering properties of the FAM specimens. Fatigue cracking is a significant form of pavement distress in flexible pavements. The properties of the sand-asphalt mortars or FAM can be used to characterize the evolution of fatigue crack growth and self-healing in full-scale asphalt mixtures. The results from this study, although limited in number, indicate that in most cases the SGC (Superpave Gyratory Compactor) compacted FAM specimen had a microstructure that most closely resembled the microstructure of the mortar within a full-scale asphalt mixture. Another finding from this study was that, at a given level of damage, the healing characteristic of the three different types of FAM mixes evaluated was not significantly different. This indicates that the healing rate is mostly dictated by the type of binder and not significantly influenced by the gradation or binder content, as long as the volumetric distribution of the mastic was the same.
Fatigue cracking is a significant form of pavement distress in flexible pavements. The properties of the sand-asphalt mortars or fine aggregate matrix (FAM) can be used to characterize the evolution of fatigue crack growth and self-healing in asphalt mixtures. This study compares the internal microstructure of the mortar within a full asphalt mixture to the internal microstructure of the FAM specimen. This study also conducts a limited evaluation of the influence of mixture properties and methods of compaction on the engineering properties of the FAM specimens. The results from this study, although limited in number, indicate that in most cases the SGC compacted FAM specimen had a microstructure that most closely resembled the microstructure of the mortar within a full asphalt mixture. Another finding from this study was that, at a given level of damage, the healing characteristic of the three different types of FAM mixes was not significantly different. This indicates that the healing rate is mostly dictated by the type of binder and not significantly influenced by the gradation or binder content, as long as the volumetric distribution of the mastic was the same. In other words, the inherent healing characteristics of the asphalt binder plays a more significant role relative to other properties (e.g. volumetrics) in the overall fatigue cracking resistance of the asphalt mixture.
Fatigue cracking is one of the primary modes of distress in asphalt pavements that has an important economic impact. Fatigue resistance characterization of an asphalt mixture is a complex issue due to: (i) composite nature of the material, (ii) gradation of aggregate particles, (iii) variation of asphalt film thickness, (iv) air voids distributions, (v) asphalt binder nonlinear viscoelastic behavior, (vi) effects of binder oxidative aging as a function of time, and (vii) micro crack healing during rest periods. Different methods to assess fatigue cracking in asphalt materials are available in the literature. However, there is no methodology to characterize fatigue cracking behavior of asphalt materials that is independent of the mode of loading (controlled-strain or controlled-stress). The objective of this research is to develop a new methodology to characterize fatigue cracking of the fine aggregate matrix (FAM) portion of asphalt mixtures using dynamic mechanical analyses (DMA). This is accomplished through different, but related, approaches. The first approach relies on identifying the various mechanisms of energy dissipation during fatigue cracking that are manifested in: (i) nonlinear viscoelastic deformation, (ii) fracture, and (iii) permanent deformation. Energy indices were derived to quantify each of these energy dissipation mechanisms and to quantify fatigue cracking irrespective of the mode of loading. The first outcome of the approach is a fatigue damage parameter (crack growth index) that provides comparable results for a given material even when tested under different modes of loading and different load (strain or stress) amplitudes. The developed fatigue characterization method has a lower coefficient of variation when compared to conventional parameters (number of load cycles to failure or cumulative dissipated energy). The crack growth index parameter was also qualitatively and quantitatively compared to three dissipated energy methods available in the literature. The second outcome of this research is a constitutive model that can describe both asphalt mixtures' nonlinear viscoelastic response and fatigue damage in one formulation. Nonlinear viscoelastic as well as damage parameters were obtained for both modes of loading. This second approach has the advantage that the constitutive model can be implemented in a numerical framework to describe the response of asphalt mixtures under various boundary conditions.
Asphalt pavement cracking is the most prevalent distress in pavements. In flexible pavements, fatigue cracking is a major cause of deterioration and can significantly reduce the service life of pavements [1]. Fatigue cracking is caused by traffic loading and can be accelerated by aging of the asphalt, freeze-thaw cycles, and poorly designed asphalt concrete mixture. Fatigue resistance of asphalt mixes could be improved by adding Polymer Modified Asphalt Cement (PMAC) [2]. In particular, the use of Styrene-Butadiene-Styrene (SBS) was found to be an efficient way to increase the fatigue life of mixes [3]. However, the primary issue is the lack of consistent performance testing methods to determine fatigue performance. In addition, the relationship between the PMAC properties and mixture performance is not fully understood. This thesis will focus on the evaluation of asphalt mixes with PMAC using the 4 point-bending beam (4PB) test to determine the fatigue performance of asphalt mixtures. The classical fatigue “WÖHLER'' curve and “DGCB” damage rate method, which was developed at Département Génie Civil et Bâtiment in Lyon, have been used to evaluate and characterize the fatigue of the asphalt mixes in this study. In general, it was found that the fatigue life (Nf50%) was improved when Polymer Modified Asphalt Cement was used, and the polymer content increased. Both fatigue analysis methods, by WÖHLER curve and the DGCB method, showed that the addition of SBS polymer improved the fatigue life and reduced the damage from fatigue loading. Finally, some recommendations were made with regards to fatigue testing.
In the recent past, new materials, laboratory and in-situ testing methods and construction techniques have been introduced. In addition, modern computational techniques such as the finite element method enable the utilization of sophisticated constitutive models for realistic model-based predictions of the response of pavements. The 7th RILEM International Conference on Cracking of Pavements provided an international forum for the exchange of ideas, information and knowledge amongst experts involved in computational analysis, material production, experimental characterization, design and construction of pavements. All submitted contributions were subjected to an exhaustive refereed peer review procedure by the Scientific Committee, the Editors and a large group of international experts in the topic. On the basis of their recommendations, 129 contributions which best suited the goals and the objectives of the Conference were chosen for presentation and inclusion in the Proceedings. The strong message that emanates from the accepted contributions is that, by accounting for the idiosyncrasies of the response of pavement engineering materials, modern sophisticated constitutive models in combination with new experimental material characterization and construction techniques provide a powerful arsenal for understanding and designing against the mechanisms and the processes causing cracking and pavement response deterioration. As such they enable the adoption of truly "mechanistic" design methodologies. The papers represent the following topics: Laboratory evaluation of asphalt concrete cracking potential; Pavement cracking detection; Field investigation of pavement cracking; Pavement cracking modeling response, crack analysis and damage prediction; Performance of concrete pavements and white toppings; Fatigue cracking and damage characterization of asphalt concrete; Evaluation of the effectiveness of asphalt concrete modification; Crack growth parameters and mechanisms; Evaluation, quantification and modeling of asphalt healing properties; Reinforcement and interlayer systems for crack mitigation; Thermal and low temperature cracking of pavements; and Cracking propensity of WMA and recycled asphalts.
Fatigue cracking in asphalt concrete (AC) is of immense importance to pavement design and analysis because it is one of the most important forms of distress that can lead to structural failure in pavement. Once started, these types of cracks can be combined with other environmental factors leading to detrimental effects such as faster rates of pavement deterioration and shortened pavement life and functionality. Currently AASHTO TP101, also known as linear amplitude sweep (LAS) specification, is being widely used to evaluate the ability of an asphalt binder to resist fatigue. The LAS method, although mechanistic in its approach, has certain drawbacks. First, the test is performed on an aged 2-mm thick binder sample, which in reality may never exist in the AC where there is a varying non-uniform thickness of the binder across the components of the AC. Secondly, the test methodology predicts an increased fatigue resistance at lower strain levels of load when the binder ages. This is in contrast to the general belief among researchers that aging is one of the primary contributors to the acceleration of pavement cracking. This study aims to evaluate fatigue resistance in a more realistic approach that is more likely to exist in AC by incorporating sand asphalt mixtures. First, the linear viscoelastic properties of binder and sand asphalt mixture samples were evaluated to obtain the material properties under the influence of aging. Later, the fatigue tests on the sand asphalt mixture were investigated to understand the influence of a thin film of binder on the fatigue resistance. It was observed that based energy dissipation criterion for the binder evaluated a reasonable estimate for fatigue damage at relatively lower temperatures, but was limited to capture the influence of aging. Moreover, it was observed that fatigue testing on a binder at an intermediate temperature of 25 °C could cause edge effects to dominate as seen in the plateau regime for the phase angle in the time sweep tests. In order to overcome the edge effects in the binder LAS tests, the sand asphalt mixture testing was used for analyzing the binder fatigue resistance. Sand asphalt mixture testing could capture the microcracking and macrocracking phases more distinctively when compared to binder testing. In the case of pressure aging vessel (PAV) aged samples, it was observed that the macrocracking phase disappeared and was replaced by sudden changes in the material properties, indicating that the PAV aged mixture was more susceptible to fatigue cracking. By using the simplified viscoelastic continuum damage approach, the fatigue resistance of the binder and sand asphalt mixture was evaluated. The sand asphalt mixture testing was better to capture the influence of aging and changes in the microstructure during fatigue in comparison to binder fatigue tests..
Fatigue cracking is one of the primary modes of failure in asphalt pavements. Cracking typically occurs within the asphalt binder phase of asphalt mixtures. Thus, asphalt binder fatigue resistance is critical in determining overall pavement fatigue performance. One procedure commonly used to characterize asphalt binder fatigue resistance is the time sweep test, which consists of repeated torsional loading of a cylindrical specimen in the Dynamic Shear Rheometer (DSR). Generally, apparent changes in material properties with respect to number of cycles of loading are used to define fatigue failure of the asphalt binder. Results of this test have been shown to correlate well with asphalt mixture fatigue performance. However, the mechanisms responsible for changes in material properties during fatigue testing in the DSR were previously not well understood. Results in this study demonstrate that fracture can account for changes in loading resistance of asphalt binders during time sweep testing. Under cyclic torsional loading of cylindrical specimens, macro fracture is shown to manifest in the form of edge fracture. Edge fracture is a circumferential crack starting at the periphery of a cylindrical sample that propagates inward as loading is applied, reducing the effective sample size. Digital visualization of the fractured specimens allowed for determination of the fractured and intact sample area. Predictions of fracture propagation based on measurements of loading resistance and fracture mechanics concepts agreed favorably with actual measurements based on visualization. Furthermore, the fracture morphology and progression of crack growth of asphalt binders matched those observed for other materials under similar loading conditions. Based on these results, fatigue damage characterization of asphalt binders can be improved by incorporating fracture mechanics into an analysis framework for DSR fatigue test results. An analysis framework based on fracture principles is presented. The proposed model allows predicting fatigue life at any loading amplitude using the results of a single fatigue test. Additionally, it is demonstrated that time-temperature superposition is applicable to fatigue crack propagation of asphalt binders, allowing for efficient prediction of fatigue performance at multiple temperatures. The model is validated using a comparison between asphalt mixture and binder fatigue test results.
Several different types of modifiers are increasingly being used to improve the performance of asphalt binders or to achieve desired mixture production characteristics (e.g., Warm Mix Asphalt). However, current Superpave performance specifications do not accurately reflect the performance characteristics of these modified binders. The main objective of this study was to evaluate the inherent fatigue cracking resistance of asphalt binders in the form of a matrix with rigid particle inclusions. The underlying rationale for this approach was to subject the binders to a state of stress that is similar to the one in a full asphalt mixture. This was achieved by fabricating and testing composite specimens of the asphalt binders and glass beads with a specified gradation. Four asphalt binders with similar true temperature grades but different modifiers were used in this study. The viscoelastic and fatigue cracking characteristics of the binders were measured using the glass bead-binder composite specimens in a dynamic shear rheometer at an intermediate temperature. The results demonstrate that the four asphalt binders modified using different methods had different damage characteristics despite the fact that these four binders were rated to have a similar performance grade based on the Superpave specifications. Fatigue cracking characteristics of the glass bead-binder test specimens used in this study were qualitatively very similar to the fatigue cracking characteristics of full asphalt mixtures using the same binders. The rank order of fatigue cracking resistance for the four glass bead-binder mixtures compared reasonably well to the rank order of fatigue cracking resistance for the full asphalt mixtures that incorporated these asphalt binders.