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The indirect tensile test is commonly used to evaluate crack and fatigue resistance of asphalt mixtures. However, laboratory tests are time-consuming and laborious in general. Numerical simulation provides a technical way for studying the mechanical behavior of asphalt mixtures. In this research, a laboratory test and discrete element method were used to explore the effects of temperature, air void content, loading rate, and the homogeneity of a mixture on the splitting strength of asphalt mixtures. For asphalt mixtures, a three-dimensional random modeling method and two-dimensional modeling method based on X-ray computed tomography and digital image processing were developed. Also, a homogeneity evaluation index based on ring segmentation was proposed. The results show that an increase in loading rate and decrease in temperature resulted in a significant increase in splitting strength. With an increase in voids, the splitting strength of an asphalt mixture decreased. The Pearson correlation coefficient indicates that there seems to be no clear connection between splitting strength and homogeneity, but there is a significant correlation between the homogeneity and differences in the splitting strength.
The discrete element method (DEM) has recently been widely used to analyze the micromechanics behavior of asphalt mixtures. The objective of this study is to present a more accurate image-processing technique to perform an indirect tensile test of asphalt mixture. Cross-sectional images of the specimen were obtained by an X-ray computed tomography (CT) technique. These CT images were processed by annular segmentation combined with bimodal threshold. The image information was transformed to coordinate information by the user-defined program in Matrix Laboratory (MATLAB). Then, a two-dimensional discrete element model of the indirect tensile test specimen was reconstructed. The indirect tensile test was simulated by the DEM and verified in the laboratory. It was found that this image-processing technology could separate connected or overlapping aggregates and better avoid defects among the aggregates. During the simulation test, the distributed internal forces were gradually concentrated along the loading axis. Four-stage crack initially appeared directly under the loading area and quickly developed along the interface between the aggregate and mortar, mainly along the loading axis. In view of the consistency of the load-displacement curves and crack distribution characteristics in both the simulation and laboratory tests, this numerical method is able to simulate the indirect tensile test.
In this paper, the shear resistance of asphalt mixtures, which accounts for the permanent deformation characteristics of flexible pavements to a large extent, is analyzed based on the discrete element (DE) method from a microscopic perspective. This study first considered the processes used to obtain the microscopic parameters for the DE model, which typically simulated an asphalt mixture based on its three components. Then the study employed Burger's model to simulate the rheological behavior of asphalt sand mastics (fine aggregates, fines, and asphalt binder). A random generation algorithm was also developed to generate coarse aggregate elements in the DE model complying with the realistic gradations of asphalt mixtures. So as to more precisely model the rheological characteristics of asphalt sand mastics, the microscopic parameters of Burger's model were calibrated via simulations of uniaxial tests in the DE model. Finally, meaningful conclusions were achieved by analyzing the simulation result and the laboratory result. The simulation result was consistent with the laboratory test result, so the use of the established DE model to evaluate the shear resistance property of asphalt mixtures is feasible.
Introduction and Research Approach -- Findings -- Interpretation, Appraisal, and Applications -- Conclusions and Recommendations -- References -- Appendixes.
To evaluate the generation method of a digital asphalt-mixture specimen based on the discrete-element method, an algorithm for generating three-dimensional coarse aggregates is presented in this study. The simulation result shows that this algorithm can reflect the actual geometry (e.g., shape, size, fracture surface, and angularity) of aggregate particles. The digital three-dimensional specimen generated using this algorithm can model the three-phase system of coarse aggregates, air voids, and asphalt mastic for asphalt mixtures well. To estimate the distribution of coarse aggregates, both in the digital specimen and real asphalt mixture, the position and quantity of the coarse aggregates within a two-dimensional section of the asphalt mixture were adopted as evaluation indices. The results showed that the digital asphalt-mixture specimen was in good agreement with the real asphalt mixture, and the evaluation indices could be used to quantitatively analyze whether the digital specimen could reflect the real asphalt mixture. The proposed approach based on the discrete-element method can be used as a supplemental tool to evaluate the uniformity of asphalt mixtures for micromechanical analysis.
The discrete element method is now increasingly used in the micro-mechanical analysis of asphalt mixtures. The digital image-based method is usually used to prepare the 2D discrete element sample. However, this method is costly and time-consuming. In this study, a series of algorithms were developed to generate the 2D discrete element sample of asphalt mixtures based on the probability analysis. Firstly, the shapes and sizes of the 2D aggregates cut from 3D aggregates were analyzed and their probabilities were computed. Based on this, an algorithm was proposed to generate the 2D aggregates. Secondly, a method was developed to calculate the number of 2D aggregates in the 2D asphalt mixture sample. Lastly, the algorithms for generating the 2D discrete element sample were summarized and discussed. With these algorithms, the 2D discrete element sample of asphalt mixtures can be generated directly and rapidly.
The design and construction of “long and deep” tunnels, i.e. tunnels under mountains, characterised by either considerable length and/or overburden, represent a considerable challenge. The scope of this book is not to instruct how to design and construct such tunnels but to share a method to identify the potential hazards related to the process of designing and constructing long and deep tunnels, to produce a relevant comprehensive analysis and listing, to quantify the probability and consequences, and to design proper mitigation measures and countermeasures. The design, developed using probabilistic methods, is verified during execution by means of the so called Plan for Advance of the Tunnel (PAT) method, which allows adapting the design and control parameters of the future stretches of the tunnel to the results of the stretches already finished, using the monitoring data base. Numerous criteria are given to identify the key parameters, necessary for the PAT procedure. Best practices of excavation management with the help of real time monitoring and control are also provided. Furthermore cost and time evaluation systems are analysed. Finally, contractual aspects related to construction by contract are investigated, for best development and application of models more appropriate for tunnelling-construction contracts. The work will be of interest to practising engineers, designers, consultants and students in mining, underground, tunnelling, transportation and construction engineering, as well as to foundation and geological engineers, urban planners/developers and architects.
X-ray Computed Tomography (CT) scanning has been widely used for medical diagnosis. This technique is now attracting increasing interest as a tool in non-destructive testing in engineering. This book reports the early results of research into this application, with particular reference to deformation and failure of geomaterials. Presenting papers of the International Workshop on X-CT for Geomaterials at Kumamoto, Japan in 2003. The book is intended for researchers and professionals in the fields of geotechnical engineering, soil, rock and concrete engineering, and geology.
Functional Pavement Design is a collections of 186 papers from 27 different countries, which were presented at the 4th Chinese-European Workshops (CEW) on Functional Pavement Design (Delft, the Netherlands, 29 June-1 July 2016). The focus of the CEW series is on field tests, laboratory test methods and advanced analysis techniques, and cover analysis, material development and production, experimental characterization, design and construction of pavements. The main areas covered by the book include: - Flexible pavements - Pavement and bitumen - Pavement performance and LCCA - Pavement structures - Pavements and environment - Pavements and innovation - Rigid pavements - Safety - Traffic engineering Functional Pavement Design is for contributing to the establishment of a new generation of pavement design methodologies in which rational mechanics principles, advanced constitutive models and advanced material characterization techniques shall constitute the backbone of the design process. The book will be much of interest to professionals and academics in pavement engineering and related disciplines.