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Asphalt composites are used to construct 90% of roads in the United States. These composites consist of asphalt binder, which is a product of the refinery process of oil, aggregates, and air voids. Fatigue cracking is one of the most important distresses that causes damage in asphalt pavements. However, there is still a gap in the understanding of the fatigue process of asphalt composites, such as the influence of material properties on this phenomenon and how the material microstructure changes as a result of fatigue damage. This study focuses on the results of two experiments that were performed on asphalt composites to better understand phenomena related to fatigue cracking: nano-mechanical characterization of the properties of the asphalt composite material and X-ray Computed Tomography nondestructive imaging of damage in the microstructure. These experimental measurements were performed on specimens that are first damaged in the Dynamic Mechanical Analyzer (DMA). The DMA is a tool commonly used for the characterization of fatigue cracking. This test method applies cyclic loads on asphalt composites, damaging them, and in the process determines the viscoelastic properties of the composite throughout the test. The nano-mechanical characterization experiment gives valuable results of the elastic modulus and hardness of the aggregate, binder, and the aggregate-binder interface that can be used to characterize different binder and aggregate combinations. The nanoindentation experiment successfully measured interface properties in the mix. The interface has elastic modulus and hardness values greater than the binder but smaller than the aggregate. This demonstrates that an interaction between these two phases creates a dissimilar phase between the two. The second experiment using X-ray CT gives measurements that are indicative of the influences of fatigue damage on micro-level changes in the material microstructure. The results of this experiment revealed important changes regarding the nature of fatigue damage and its relationship to changes in the geometry of air voids and cracks in asphalt composites. The X-ray CT experiment measured size and shape parameters of air voids at 20 microns/pixel resolution at different damage levels. These results illustrated that reduction in bonding strength in the binder is involved in failure in the mix and thus fatigue cracking is not solely responsible for failure. This conclusion is made based on the results not showing a statistically significant change in air void shape and size parameters with increased damage. This is illustrated by viewing changes in the air void structure within the mix, there is no evidence of crack propagation, or drastic changes in the shape, size, or volume of air voids within the mix. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/149446
The study of asphalt chemo-mechanics requires a basic understanding of the physical properties and chemical composition of asphalt and how these properties are linked to changes in performance induced by chemical modifications. This work uniquely implements the framework of chemo-mechanics by investigating two types of chemical modification processes, natural (oxidative aging) and synthetic (chemical doping) as they relate not only to macro-scale properties of asphalt binder but also to the asphalt microstructure and nanorheology. Furthermore, this study demonstrates the application of atomic force microscopy (AFM) imaging and the extraction of nano-scale engineering properties, i.e. elastic modulus, relaxation modulus, and surface energy, as a method to predict performance related to the fatigue characteristics of asphalt binders by modeling intrinsic material flaws present amongst phase interfaces. It was revealed that oxidative aging induces substantial microstructural changes in asphalt, including variations in phase structure, phase properties, and phase distribution. It has also been shown that certain asphalt chemical parameters have a consistent and measureable effect on the asphalt microstructure that is observed with AFM. In fact, particular phases that emerged via chemical doping revealed a surprising correlation between oxidative aging and the saturates chemical parameter of asphalt in terms of how they explicitly impact durability and performance of asphalt. By implementing a crack initiation model -- which requires measureable microstructural characteristics as an input parameter -- it was found that microstructural flaws (depending on the extremity) can have a more profound impact on asphalt performance than the properties of the material located between the flaws. It was also discovered by comparing the findings to performance data in the Strategic Highway Research Program's (SHRP's) Materials Reference Library (MRL), that the crack initiation model predicts very similar performance as the SHRP's distress resistance indicators. Overall, this body of work yields improved input values for asphalt prediction models and serves as the basis for ongoing studies in the areas of asphalt chemical mapping, modeling of nano-damage, and nano-modification using AFM. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/149372
An important new state-of-the-art report prepared by RILEM Technical Committee 108 ICC. It has been written by a team of leading international experts from the UK, USA, Canada, Israel, Germany, Denmark, South Africa, Italy and France. Research studies over recent years in the field of cement science have focused on the behaviour of the interfaces between the components of cement-based materials. The techniques used in other areas of materials science are being applied to the complex materials found in cements and concretes, and this book provides a significant survey of the present state of the art.
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.
Composites materials have aroused a great interest over the last few decades. Several applications of fibrous composites, functionally graded materials, laminated composites, nano-structured reinforcements, morphing structures, can be found in many engineering fields, such as aerospace, mechanical, naval and civil engineering. The necessity of lightweight structures, smart and adaptive systems, high-level strength, have led both the academic research and the manufacturing development to a recurring employment of these materials. Many journal papers and technical notes have been published extensively over the last seventy years in international scientific journals of different engineering fields. For this reason, the establishment of this second edition of Mechanics of Composites International Conference has appeared appropriate to continue what has been begun during the first edition occurred in 2014 at Stony Brook University (USA). MECHCOMP wants to be an occasion for many researchers from each part of the globe to meet and discuss about the recent advancements regarding the use of composite structures. As a proof of this event, which has taken place in Porto (Portugal), selected plenary and key-note lectures have been collected in the present book.
The focus of this work lies on the microstructure-based modeling and characterization of a discontinuous fiber-reinforced thermoset in the form of sheet molding compound (SMC). A microstructure-based parameter identification scheme for SMC with an inhomogeneous fiber orientation distribution is introduced. Different cruciform specimen designs, including two concepts to reinforce the specimens' arms are evaluated. Additionally, a micromechanical mean-field damage model for the SMC is introduced.
Internationally, much attention is given to causes, prevention, and rehabilitation of cracking in concrete, flexible, and composite pavements. The Sixth RILEMInternational Conference on Cracking in Pavements (Chicago, June 16-18, 2008) provided a forum for discussion of recent developments and research results.This book is a collection of papers fr
This research is concerned with quantifying damage in asphalt mixtures at the micro level. X ray Computer Tomography (CT) a non destructive technique along with image analysis has been used to study the internal microstructural properties of asphalt. During laboratory testing of asphalt mixtures, it has been observed that specimens lose strength without any visible cracks. UK asphalt mixtures have been tested in uniaxial compression and tension compression fatigue tests and scanned in X ray CT. In the uniaxial compression test, specimens have been tested at three different strain rates. Both monotonic and cyclic tests have been conducted at three different temperatures. Testing has been carried out both continuously and with rest periods at selected stages. The specimens were scanned in X ray before starting the tests and also during the testing at on selected stages until failure. X ray machine operation was optimized to achieve good quality of images of different types of asphalt samples. The 2D images of the specimens were collected from the X ray CT and were stacked to regenerate into 3D images of the asphalt samples. Techniques for adjusting the threshold grey values of the images and analysing the X ray images for different parameters have been developed. The images have been analysed to evaluate the microstructure of the asphalt specimens internally and non destructively. Air voids content is considered as the parameter that represents the change in microdamage during the application of loading cycles. Moisture damage in asphalt mixtures was studied from X ray CT. Two types of mixture were investigated, one with acid aggregate and one with basic aggregate, with three different ranges of void content. Dry specimens and specimens saturated in the laboratory were scanned in X ray CT to study the internal connected air voids which cause the permeability to moisture in an asphalt mixture and result in moisture damage. Damage due to combined moisture and ageing was studied from X ray images. From the analysis of X ray images, it was observed that a non uniform increase in air voids occurred both along the height and across the diameter of the specimens tested in monotonic compression and tension compression fatigue. This may perhaps be due to the heterogeneous nature of asphalt. New voids developed along with a size increase and joining together of existing voids. Using continuum damage mechanics, the data from both the mechanical testing and from X ray computer tomography was compared. For specimens tested in fatigue, damage parameters were determined for a damage model. The dissipated pseudo strain energy approach was applied to the test data and the parameters for the damage model were determined. A modified model with a new parameter of adhesion between binder and aggregate was used for data analysis. Results from X ray computer tomography and from the fatigue damage model were compared. In the case of specimens tested for moisture damage and ageing, the retained saturation was determined from X ray image analysis and was related to the stiffness of asphalt mixtures. Asphalt mixtures containing basic aggregate were found to have a high retained stiffness value after moisture and ageing tests compare to mixtures containing acidic aggregate. The stiffness values for the retained saturation were determined and it is observed that in the case of mixtures containing acidic aggregate, the retained stiffness decrease with the increase in retained saturation.