In the first part, a new analytical approach, within the extended finite element (XFEM) framework, is proposed to compute Strain Energy Release Rates (SERRs) directly from Irwin's integral. Crack tip enrichment functions in XFEM allow for evaluation of integral quantities in closed form (for some crack configurations studied) and therefore results in an accurate and efficient method. The effects of high order enrichments, mesh refinement and the integration limits of Irwin's integral are examined in benchmark numerical examples. The results indicate that high order enrichment functions have significant effect on the convergence, in particular when the integral limits are finite. When the integral limits tend to zero, simpler SERR expressions are obtained and high order terms vanish. Nonetheless, these terms contribute indirectly via coefficients of first order terms. The analytical formulation is then extended to cracks in arbitrary orientations. Several benchmark examples are investigated including off-center cracks, inclined cracks and crack growth problems. On all these problems, the method is shown to work well, giving accurate results. Moreover, due to its analytical nature, no special postprocessing is required which leads to a fast approach to obtain Strain Energy Release Rates. Thus it is concluded that this method may provide a good alternative to the popular J-integral method. In the second part of the thesis, the stress-strain behavior of short single walled carbon nanotube (SWCNT) aggregates is investigated by a novel incremental constrained minimization approach. An AIREBO potential is used to model the interactions within and between CNTs. The idea is to homogenously disperse SWCNTs in the computational cell at random positions and orientations following spherical uniform distributions, and incrementally deform the cell while restraining the movement of atoms at the ends of nanotubes. The stress-strain response of the system is obtained in each loading direction and it is shown to converge to an isotropic behavior (a similar response in all directions) as the number of CNTs in the system increases. In addition, it is shown that the Young's modulus of the system increases linearly with the CNT aggregates density and the method agrees well with results obtained from molecular dynamics simulations running at near zero degrees kelvin, which are obtained at only a fraction of the CPU time required for MD methods.

The safety of suspension bridges depends on its main cables which are constructed of thousands of high strength steel wires radially clamped together at certain locations along the cable. After many years of service, these cables are showing signs of serious distress with many wires corroded and even broken inside. A new methodology to determine the reliability and safety of this structure is suggested in this research work. A three dimensional random field simulation is used to determine the remaining tensile strength in the cable. The key idea is to determine how an individual wire break affects the load transfer to the surrounding wires. This local damage eventually causes a global reduction in the load carrying capacity of the cable, up to a complete failure. A Monte Carlo technique is used to generate realizations of the wires' strength within a finite element model. Among the major contributions of the thesis is a novel technique for modeling the contact-friction mechanism between thousands of wires that account for load recovery in broken wires due to friction induced by radial clamps. The idea is to place elasto-plastic springs at the contact points between wires. These springs have varying parameters depending on their proximity to the clamping loads and are assigned according to the Boussinesq's solution to a point load in half space. While traditional contact algorithms have difficulties converging on this problem, this technique converges in few iterations. Moreover, parallelization of the problem enables a full stochastic analysis to determine the effect of corrosion uncertainty on the cable's failure load. This method represents a dramatic improvement compared to the current inspection methods that are unreliable and expensive.

It is important for engineers and designers to be able to accurately estimate the damping within a structure; however, this is not a trivial task. Simplifications are often made in an effort to make damping estimation easier, but these simplifications rely on assumptions that may not be universally true. One important assumption is that the excitation input for a structure may be modeled as broad-band noise, but traffic loading on a bridge likely violates that assumption. Traffic loads are characterized by the velocities of the vehicles and trains crossing the bridge, which gives the input specific frequency content. This added complexity increases the difficulty in accurately estimating the damping. The problem of traffic crossing a bridge was studied by creating a finite element model of a bridge using a beam system that consisted of a series of stringers resting on top of a larger girder. Traffic loads were then simulated using moving point loads and moving masses to represent cars and trains crossing the bridge. In addition to the traffic loading case, an ambient loading case was conducted using uniform broad-band noise as a means of comparison. The accelerations at several locations along the bridge span were recorded and used as input for a variety of operational modal analysis (OMA) methods. The OMA methods included both frequency domain techniques, such as Frequency Domain Decomposition (FDD), and time domain based identification, such as blind source separation (BSS). The results from the various OMA methods demonstrated how traffic loading creates distortion in the frequency response spectra of the bridge. This distortion had adverse effects for damping ratio estimation and in certain cases led to extreme errors. The mode shape estimates were not found to be affected by the distortion, but that meant that mode shape estimates could not be used to identify potentially erroneous damping estimates. The cause for the distortion was later identified as the driving frequencies produced by the vehicle-bridge interactions. The term ``driving frequency'' refers to the frequency created by a car traveling over a bridge or, by analogy, by a moving load traveling over a beam. This frequency is directly correlated with the speed of the vehicle and the length of the bridge. By considering a single moving point load traveling across the bridge, the responses of the stringers and girder were studied and the effects of the driving frequencies were better quantified in both the time and frequency domains. It was found that peaks in frequency domain appear at the even multiples of each car's driving frequency, and as more cars travel across the bridge the peaks of closely spaced driving frequency multiples begin to merge. As the number of cars increases to a full hour-long simulation and the car velocities become uniformly distributed over a given interval, numerous peaks merge together to form sustained regions of elevated energy in the frequency domain. These regions distort the frequency response spectra of the bridge and obscure the modal information. In order to deal with these distorted regions, a new approach to modal identification was proposed that focused on using partial information from the modal peaks. The peaks in the frequency domain were divided into left- and right-side spectra in order to take advantage of any undistorted portions of the modal peaks. These side spectra were analyzed using a curve-fitting approach based on combining optimization methods with clustering analysis. The presence of distortion presented certain challenges to traditional curve-fitting approaches, such as polynomial least squares, but the optimization algorithm was able to overcome these issues while also adding efficiency to the curve-fitting process. The clustering analysis was used to quickly find the optimal subsets within the optimization-based curve-fitting results. By performing curve-fitting to side spectra, different sets of modal parameters were produced that fit each side. It was found that the modal parameters for the intact or undistorted side compared favorably with the true modal parameters. While this optimization and clustering methodology could not account for all types of distortion, it demonstrated large improvements as compared to traditional OMA approaches for the modes most severely impacted by the distortion. Another potential benefit of this method is that the distributions within the final clusters could be used to provide ranges of possible values for the damping ratios instead of only a single value.

In recent years, health monitoring of structure and infrastructure systems has become a valuable source of information for evaluating structural integrity, durability and reliability throughout the lifecycle of structures as well as ensuring optimal maintenance planning and operation. Important advances in sensor and computer technologies made possible to process a large amount of data, to extract the characteristic features of the signals, and to link those to the current structural conditions. In general, the process of data feature extraction relates to solving an inverse problem, in either a data-driven or a model-based type setting. This dissertation explores state-of-the-art hierarchical optimization-based computational algorithms for solving multiscale model-based inverse problems such as system identification and damage detection. The basic idea is to apply optimization tools to quantify an established model or system, characterized by a set of unknown governing parameters, via minimizing the discrepancy between the predicted system response and the measured data. We herein propose hierarchical optimization algorithms such as the improved artificial bee colony algorithms integrated with local search operators to accomplish this task. In this dissertation, developments in multiscale damage detection are presented in two parts. In the first part, efficient hybrid bee algorithms in both serial and parallel schemes are proposed for time domain input-output and output-only identification of macro-scale linear/nonlinear systems such as buildings and bridges. Solution updating strategies of the artificial bee colony algorithm are improved for faster convergence, meanwhile, the simplex method and gradient-based optimization techniques are employed as local search operators for accurate solution tuning. In the case of output-only measurements, both system parameters and the time history of input excitations can be simultaneously identified using a modified Newmark integration scheme. The synergy between the proposed method and Bayesian inference are proposed to quantify uncertainties of a system. Numerical and experimental applications are investigated and presented for macro-scale system identification, finite element model updating and damage detection. In the second part, a framework combining the eXtended Finite Element Method (XFEM) and the proposed optimization algorithms is investigated, for nondestructive detection of multiple flaws/defects embedded in meso-scale systems such as critical structural components like plates. The measurements are either static strains or displacements. The number of flaws as well as their locations and sizes can be identified. XFEM with circular and/or elliptical void enrichments is employed to solve the forward problem and alleviates the costly re-meshing along with the update of flaw boundaries in the identification process. Numerical investigations are presented to validate the proposed method in application to detection of multiple flaws and damage regions. Overall, the proposed multiscale methodologies show a great potential in assessing the structural integrity of building and bridge systems, critical structural components, etc., leading to a smart structure and infrastructure management system.

Given unprecedented levels of urbanization and motorization in developing countries and deteriorating infrastructure in developed countries, cities around the world have been facing the enormous challenge of delivering sustainable forms of infrastructure with fewer resources. In the developing world, the challenges in urban infrastructure investment become even more daunting as manifested by the staggering size of infrastructure funding gap. $1 trillion per annum over the period by 2020 is required by developing countries to meet the demand of rapid urbanization and to address the backlogs and deficiencies for infrastructure facilities (World Bank 2013). Therefore, prioritizing projects at the system level based on transparent and evidenced-based decision-making processes has emerged as one of the most promising ways to bridge such enormous funding gaps, especially for developing countries. Nevertheless, effective prioritization of infrastructure projects is hindered by a series of constraints including institutionalized inefficiency, inadequate data obstructing decision making, insufficient coordination among various stakeholders, lack of public consultation, lack of technical capacity for project evaluation and prioritization, and lack of consideration of possible alternatives in the infrastructure planning. Although there has been considerable discussion regarding the shortcomings of contemporary metropolitan transportation planning, there has been little effort to develop a strategy for prioritizing urban transport projects in developing countries. This calls for a new approach to addressing the above-mentioned issues. This thesis first presents the current status and general characteristics of urban transport decision-making in developed and developing countries alike. It then provides a comprehensive literature review on the evolution and application of scenario planning and multi-criteria, specifically in the field of transportation projects prioritization and transportation planning. As the main contribution of this research effort to current research, a novel project prioritization framework - which incorporates scenario planning into multi-criteria decision analysis (MCDA) for prioritizing urban transport projects - is proposed to support sustainable urban transport development, through the efficient use of existing project evaluation information and emergent scenario of various stakeholder's input. Such integration of scenario planning and MCDA provides a balanced view of both the analytical and intuitive components of the decision-making process and allow comparisons between different roles of various stakeholders. The framework is then applied to set priorities for nine recent urban transport projects constructed within a two-year framework in the Tianjin Binhai New Area, China. In addition, a case study on World Bank's infrastructure investment portfolio in China is also conducted in which a selection of urban transport projects in eight different cities is ranked. The results show that the proposed framework could serve as a consistent, robust and comprehensive infrastructure project prioritization strategy that reconciles diverse perspectives among stakeholders while introducing sustainability in urban transport decision making and linking the prioritization process to the transportation planning that precedes it.

<p> The durability of a concrete repair is highly dependent on the shear strength of the interface between new and old concrete. Therefore, the engineer designing the repair makes every effort to maximize this strength. To that end, pretreatments, such as prewetting the substrate and/or applying bonding agents, are commonly specified. The efficacy of these pretreatments is often debated, and previous studies have produced contradictory results. This research was undertaken to determine the effects of prewetting the substrate and applying a bonding agent, both in combination and individually. The bond strength in tension and the shear strength of the bond were measured using a variety of methods, including in-place testing and testing of extracted specimens. The results indicate that both prewetting and the use of a bonding agent can be beneficial to the shear strength of bonded overlays.</p>

<p>The main objective of this research is to study the effect of using different additives on the compressive and flexural strength of rammed earth structures. Different ratios of fly ash, and/or cement were added to the soil to identify their influence on the compressive strength. Recycled fiber materials were used to wrap and reinforce the cement-soil specimens in order to enhance the flexural strength of beams and control the cracks and the mode of failure. The study finds that adding cement to soil has significant effect on the soil strength, and it causes a remarkable increase in the strength while adding fly ash does not increase the compressive strength and it results in elastic modulus reduction. Furthermore, it is concluded that wrapping and reinforcing the specimens by burlap cloth or fiber mesh do not improve the flexural strength due to the weak bond with the cement-soil material. However, when a beam is reinforced by glass fiber exhibited improvement in the flexural strength and it experienced a plastic behavior after the proportional limit and it was able to absorb a large amount of energy without failure.

<p> The design of concrete bridges is primarily focused on strength characteristics. However, it is often the case that durability characteristics, specifically early deterioration of the bridge deck, requires expensive repairs before the designed service life of the bridge can come to term. Lin et al. (2012) identified the development of microcracks as a possible source of this early deterioration. He proposed that these microcracks propagated due to high local compressive stress induced by overweight trucks. The resulting permeability increase caused by the propagation of these microcracks is not significant enough to cause the kind of early deterioration of bridge deck observed in the field. However, the combined effect of mechanical loading and F/T can cause much more severe microcrack development, and thereby, deterioration of concrete. </p><p> Currently there are very few efficient ways of measuring the deterioration of concrete bridge decks. Standard tests are available for concrete samples, including ASTM C215 (dynamic modulus), ASTM C1202: Rapid Chloride Ion Penetration (RCIP), and Electrical Surface Resistivity (ESR). Alternatively, water absorption, measured by ASTM C642, can be used to determine the deterioration of concrete. The benefits of using water absorption to measure concrete durability include; relatively quick test periods, and no requirements on sample dimensions; thus, lending itself more to field testing of concrete cores extracted from bridge deck. </p><p> There is precedence correlating water absorption to the durability of concrete. Lin et al. (2012) observed that water absorption was directly proportional to the amount of charge passed in RCIP tests. There were several concerns with this study, therefore, the claim that water absorption correlated to the durability of concrete required further validation. </p><p> In this study, mechanically loaded, F/T concrete cylinders were subjected to a variety of tests including; dynamic modulus, water absorption, and ESR. An inversely proportional relationship was found between absorption and ESR with a coefficient of determination (R2) of 63.5%. This strong relationship very clearly provides supporting evidence to help validate the original conclusion proposed by Lin et al. (2012), that water absorption directly relates the durability of concrete.</p>

abstract: A large portion of the United States is known to have problematic expansive clay soil. These expansive clay soils can cause damage to major infrastructures such as roads and lightly loaded residential buildings. The shrinking or swelling potential of unsaturated expansive clay soils requires an understanding of unsaturated soil mechanics, such as matric suction profile and the site’s environmental condition, such as climate. In unsaturated soil engineering, the most used climatic parameter is Thornthwaite Moisture Index (TMI). Since its inception, there have been several versions of TMI models in the literature. Historically, TMI is used to predict suction parameters such as edge moisture variation length, the depth to equilibrium suction, and equilibrium suction. Currently, TMI is used in Post-Tension Institute’s Slab-on-grade Design Manual (DC 10.1-08) to estimate edge moisture variation length and equilibrium suction, and Australian Standard Residential Slabs and Footing (AS2870-2011) to predict the climatic zone and the depth to suction change. However, there is no clear-cut guidance on which version of TMI models to use, how the variables within TMI should be collected, the length of the study period for determination of TMI, or assumptions and compromises associated with TMI estimation methods. In this thesis, broad-scale study and comparison of the original TMI (1948) to the newer TMI models for the contiguous United States are conducted as well as in-depth analysis of the variables within TMI, using National Oceanic and Atmospheric Administration’s (NOAA) dataset and Geographic Information System (GIS). The results of the study, the recommendations for the state of practice for TMI and further research are discussed. / Dissertation/Thesis / Masters Thesis Civil, Environmental and Sustainable Engineering 2018

abstract: With the growth of global population, the demand for sustainable infrastructure is significantly increasing. Substructures with appropriate materials are required to be built in or above soil that can support the massive volume of construction demand. However, increased structural requirements often require ground improvement to increase the soil capacity. Moreover, certain soils are prone to liquefaction during an earthquake, which results in significant structural damage and loss of lives. While various soil treatment methods have been developed in the past to improve the soil’s load carrying ability, most of these traditional treatment methods have been found either hazardous and may cause irreversible damage to natural environment, or too disruptive to use beneath or adjacent to existing structures. Thus, alternative techniques are required to provide a more natural and sustainable solution. Biomediated methods of strengthening soil through mineral precipitation, in particular through microbially induced carbonate precipitation (MICP), have recently emerged as a promising means of soil improvement. In MICP, the precipitation of carbonate (usually in the form of calcium carbonate) is mediated by microorganisms and the process is referred to as biomineralization. The precipitated carbonate coats soil particles, precipitates in the voids, and bridges between soil particles, thereby improving the mechanical properties (e.g., strength, stiffness, and dilatancy). Although it has been reported that the soil’s mechanical properties can be extensively enhanced through MICP, the micro-scale mechanisms that influence the macro-scale constitutive response remain to be clearly explained. The utilization of alternative techniques such as MICP requires an in-depth understanding of the particle-scale contact mechanisms and the ability to predict the improvement in soil properties resulting from calcite precipitation. For this purpose, the discrete element method (DEM), which is extensively used to investigate granular materials, is adopted in this dissertation. Three-dimensional discrete element method (DEM) based numerical models are developed to simulate the response of bio-cemented sand under static and dynamic loading conditions and the micro-scale mechanisms of MICP are numerically investigated. Special focus is paid to the understanding of the particle scale mechanisms that are dominant in the common laboratory scale experiments including undrained and drained triaxial compression when calcite bridges are present in the soil, that enhances its load capacity. The mechanisms behind improvement of liquefaction resistance in cemented sands are also elucidated through the use of DEM. The thesis thus aims to provide the fundamental link that is important in ensuring proper material design for granular materials to enhance their mechanical performance. / Dissertation/Thesis / undrained simulation with flexible membrane / cyclic direct simple shear simulation / Doctoral Dissertation Civil, Environmental and Sustainable Engineering 2018