Engineering

Publication Search Results

Now showing 1 - 10 of 21
  • (2022) Kia, Layla
    Thesis
    Timber construction is rapidly evolving towards high-rise and high-tech. The demand for mid- to high-rise buildings using engineered timber can be attributed to its high structural efficiency and mitigation of carbon dioxide emissions. However, due to the relatively low mass density and stiffness characteristics of timber, lateral load resistance is often the governing criteria for design. Restrictive design regulations, limited education and lack of innovation has seen the development of multi-storey timber structures mimic that of traditional steel and concrete buildings. This has led to the full potentials of timber as a construction material, and timber structures as an architectural form, not to be realised. In this research, an efficient structural system consisting of a timber-steel hybrid exoskeleton is proposed and tested. Specifically, composite timber-steel encased columns and steel-timber buckling restrained braces (STBRB). Experimental testing of composite timber-steel encased columns subjected to concentric and eccentric loading indicates significant stiffness and load carrying enhancement (over 100% in some cases) compared to bare timber columns with intermediate to stocky slenderness. Analytical models based on the principles of structural mechanics and simplified bilinear elastoplastic relationship accurately predicted the load carrying capacity and offers a simple method of analysis which can be used in practice. Detailed nonlinear 3D finite element (FE) simulations of the columns are developed and verified against the experimental results using ABAQUS software. Based on the experimental, analytical, and numerical results, the behaviour of composite timber-steel encased columns is found to be significantly influenced by the ratio of steel strength to timber strength (Asfsy/Atfcm) as well as knots/imperfections, particularly in low-grade wood. Cyclic tests representative of seismic actions on steel-timber buckling restrained braces (STBRB) have demonstrated stable hysteretic energy dissipation and a cumulative inelastic ductility capacity (CID) beyond the requirements prescribed in AISC 341. STBRBs with steel collars placed at the critical ends of the casings demonstrated the highest ductility and energy dissipation. The results from the study showcased the proposed system as a feasible and sustainable alternative to conventional concrete/steel BRBs.

  • (2022) Chen, Yuhui
    Thesis
    Ride-sourcing services are rapidly spreading around the world. The ride-sourcing service refers to a point-to-point on-demand ride service operated by various companies, which organize and coordinate drivers using their vehicles to provide passengers with ride services. How ride-sourcing services and public transport are interacting with each other and thus yielding system-wide impacts have not received sufficient attention. This thesis extends the literature by proposing multi-class, multi-modal traffic assignment models to optimize the transport system with the presence of ride-sourcing and public transport services. The first part of the thesis develops a stylized model with a simple network with single origin-destination pair in order to analytically examine the mode choice behavior of travelers and the operation strategies of a public transport operator and a ride-sourcing operator. In such a multi-modal system, users may travel by bus, train, or ride-sourcing service. In particular, we develop a tractable bi-level model that quantifies the user equilibrium travel choices in the lower-level, where the travel choice equilibrium can be formulated as a variational inequality problem, and optimizes the operation strategies of the public transport operator that aims to minimize total system cost and the ride-sourcing operator that aims to maximize its profit in the upper-level. The existence and uniqueness of the multi-modal travel choice equilibrium are also analyzed. How the operation decision variables might affect users' mode choices and system performance is investigated both analytically and numerically. The second part of the thesis extends the stylized model to a general network model, which includes also solo-driving, and multiple OD pairs to depict a more realistic problem setting. The general network model is applied on a case study in the context of Sydney. The existence and uniqueness are also investigated for the general network model. The method of Frank-Wolfe combined with diagonalization is applied to generate numerical solutions, and illustrate the analytical observations and generate further understanding. The results show that the total system cost can be reduced while the profit of the ride-sourcing company can be increased under appropriate operating strategies of the public transport operator and the ride-sourcing operator.

  • (2023) Wei, Bangyang
    Thesis
    This thesis mainly focuses on modeling and managing a transportation system with shared mobility services. Shared mobility services such as ridesharing (or carpooling) and ride-sourcing services have attracted growing attention. However, the impacts of ridesharing on dynamic traffic are not fully clear. Besides, how to propose effective measures to manage city traffic considering shared mobility services is also an urgent issue to be addressed. Furthermore, with the growing number of electric vehicles in ride-sourcing markets, it is important to study how many charging piles to be installed to satisfy the charging demand and how to reduce the impacts of the cruising of electric ride-sourcing vehicles (ERVs) when waiting for trip orders. The research purpose of the thesis is threefold. (i) In order to investigate the impacts of ridesharing on travelers' choices in a multi-modal transportation system, this thesis examines the ridesharing problem within a doubly dynamical framework with both day-to-day traffic variation and within-day traffic dynamics. And congestion pricing schemes are proposed to reduce network congestion and improve traffic efficiency. (ii) In terms of management strategies for stimulating carpooling to improve transportation system efficiency under the limited road space, this study investigates the road capacity allocation scheme and step tolling-rewarding scheme for carpooling. (iii) From the perspectives of planning and operation, this thesis investigates the provisions of parking spaces and charging piles for electric vehicles in ride-sourcing markets. Some interesting findings are given. Ridesharing may not necessarily reduce traffic congestion in some cases and the proposed dynamic congestion pricing schemes could reduce congestion. Under the proposed joint temporal capacity allocation and step tolling-rewarding scheme, carpooling should not be rewarded on some occasions, and instead should be penalized to avoid overcrowded carpooling flows in order to improve system efficiency. In addition, by providing parking spaces for ERVs, ride-sourcing demand increases, charging demand reduces, and profit and social welfare increase. Overall, this thesis increases the understanding of shared mobility services in a transportation system, and the insights from the proposed strategies can guide transportation agencies in traffic management and planning as well as transportation companies in fleet operation.

  • (2023) Afroz, Sumaiya
    Thesis
    This research investigated shrinkage of low carbon cement concrete to assess its subsequent effects on cracking and corrosion affecting the overall durability. The low carbon binder compositions considered were 40% and 60% slag, 30% fly ash and 20% to 65% limestone calcined clay cement (LC3). Shrinkage was monitored for both paste and concrete. Furthermore, microstructural properties such as phase assemblage, rate of hydration, and pore structure of the paste samples were studied. The cracking potentials of concrete mixes were evaluated using ring test. The self-healing potential of the binder compositions was examined by laser microscopy and Micro-CT techniques. To study the corrosion of steel reinforcement in a marine environment, the corrosion potential of embedded reinforcement in artificially cracked samples was investigated using the linear polarization method. The location of corrosion onset in relation to the presence of cracks was also identified using the Micro-CT technique. The deciding factor governing the extent of autogenous shrinkage for any given low-carbon mix was the pore structure refinement due to hydration at any given age. Complementing the shrinkage results, LC3 cracked the earliest compared to plain, fly ash based and slag-based concrete. The early-age cracking of concrete depended on the rate of shrinkage development rather than the magnitude of shrinkage. In spite of the crack healing, corrosion initiated and propagated for all steel embedded in paste sample. Therefore, autogenic self-healing showed negligible influence on durability related to corrosion. Low carbon cement with higher replacement levels showed general corrosion along the length of the mild steel due to weak steel paste interface in addition to the pitting corrosion. Therefore, this thesis highlights the secondary effect of binder composition in corrosion propagation. The overall shrinkage-related durability was significantly dependent on the hydration of the binders. Furthermore, the shrinkage caused by numerous factors including hydration, microstructure evolution, and drying was related to subsequent features of concrete such as the formation of cracks and the potential of corrosion propagation. Therefore, shrinkage, cracking and corrosion were investigated in a holistic and systematic manner and the overall shrinkage-related durability of low carbon cement and concrete was assessed.

  • (2022) Liu, Shuang
    Thesis
    Small, constructed waterbodies, including wetlands, ponds and lakes, are designed to attenuate floods and enhance water quality. Although guidelines inform the design of these waterbodies, many waterbodies still experience harmful algal blooms (HABs) and poor water quality. To date there have been limited studies on understanding HABs in small waterbodies and there is an ongoing reliance on models and methods derived from large lakes for monitoring and controlling HABs. Considering the increasing number of small waterbodies being constructed worldwide, and the increasing HAB risk with anthropogenic climate change, it is important to improve our knowledge of how small waterbodies respond to HABs now and under a variable climate. This thesis initially reviewed stormwater waterbody design guidelines from around the world and identified design criteria related to HAB control. Gaps in current guidelines were highlighted and key design factors were identified by testing the existing design criteria on a case study of over 200 waterbodies in the greater Melbourne region. A clear limitation in understanding HAB risk is the sparse in situ data. To this point, remote sensing is increasingly being used as a promising method to supplement data for ungauged or sparsely gauged waterbodies. However, accurate HAB detection for small-medium waterbodies remains challenging. Based on these findings, the second part of this thesis investigated effective monitoring strategies for HABs in small waterbodies using remotely sensed data. The HAB detection skill of three satellite products was tested using a novel pseudo-waterbody approach. Interestingly, higher spatial resolution did not improve detection performance and available spectral bands were more important. A major challenge in utilizing remote sensing is the baseline water color variations across small waterbodies. Therefore, the final part of this thesis focused the water color of small waterbodies using remote sensing imagery as a proxy for spatial and temporal variations. Overall, this thesis shows that there is room for improvement in HAB design criteria. Indeed, carefully planned studies are required to develop better guidelines and to integrate field sampling and remote sensing data. The sensitivity of remotely sensed water color to climate variability in some waterbodies, may also provide important insights into how these waterbodies respond to a changing climate.

  • (2022) Hammad, Mahmoud
    Thesis
    Since sustainability in construction has become a major headline, the use of timber composite structures has grown in popularity. Apart from the lower carbon and energy footprint, the high (tensile) strength over density of timber compared to concrete (and even mild steel) is regarded as one of the driving factors for the increase in popularity of the timber and timber composite structures which can be easily assembled, altered, or dismantled with minimum number of trades on the construction site, less noise, and interruption to the surrounding environment. The use of timber beams/joists or plates in conjunction with reinforced concrete (RC) slabs, or so-called timber concrete composite (TCC) floors/decks, has also grown rapidly in the past few decades. Replacing reinforced concrete or steel beams with timber beams/joists, considerably reduce the self-weight, energy- and carbon- footprint of the structure that in turn has contributed to popularity of the timber and timber composite structures. Considering all advantages of timber composite floors, a large body of research has been devoted to structural behaviour of the timber composite floors subjected to sagging moment. But less attention has been paid to behaviour of timber composite floors subjected to hogging moment. The structural behaviour of timber precast concrete composite (TCC) beam to column subassemblies subjected to hogging bending moments was investigated by full-scale laboratory testing, analytical and numerical modelling. Push-out and push-down tests were performed on timber precast concrete composite joints and beams with coach (lag) screw shear connectors embedded in the small grout pockets, respectively. The laboratory tests were used to establish and discuss the influence of size and type of timber beams (i.e., laminated veneer lumber (LVL) or glued laminated timber (GLT)), size of screw shear connectors and amount of reinforcement in the slabs on the failure modes, load-displacement, stiffness, load carrying capacity and ductility of the TCC beam to column connections under hogging bending moments. An analytical model for estimating the peak load and stiffness of the TCC beams under hogging bending moment was proposed by modifying the -method to account for the effect of concrete cracking in a simplified manner. The analytical model was validated against the experimental results. Full scale timber-timber composite (TTC) beam to column subassemblies were fabricated and subjected to push-down loads to assess structural performance of the TTC floors subjected to hogging moment. A total of thirty-one TTC and six bare timber beam-to-column subassemblies (including fourteen replicates) were fabricated and tested to failure subject to hogging bending moment. The TTC beams were fabricated by connecting the cross laminated timber (CLT) slab to the top edge of a pair of laminated veneer lumber (LVL) or glued laminated timber (GLT) beams/joists. The effect of the CLT slab thickness, width, and orientation (i.e., loaded lengthwise or crosswise), column penetration in the CLT slab, bending moment over shear force ratio (span length), degree of shear interaction between the slabs and beams (controlled by size of shear connectors) and type of beams (LVL or GLT) on the structural performance of the TTC subassemblies were investigated experimentally. Moreover, an analytical model for the composite Timoshenko beams was adopted and modified to predict the stiffness and load carrying capacity of the TTC beams under hogging moment. Lastly, finite element (FE) models of the TCC and TTC subassemblies were built and analysed using ABAQUS software. The results of FE simulations were compared with the experimental results to validate and demonstrate the accuracy of the FE models for predicting stiffness, load carrying capacity and failure mode of the TCC and TTC beam to column subassemblies subjected to hogging moment. It was shown that FE models can accurately capture complex modes of failure in TTC and TCC beams subjected to hogging moments. The validated FE models were utilised to conduct a parametric study and assess effect of reinforcing proportion and level of composite action on the stiffness and load carrying capacity of the TCC and TTC beam to column connections.

  • (2022) Wang, Xin
    Thesis
    Soil minerals that can participate in the redox cycling of Fe(II)/Fe(III) have received a significant amount of attention over recent decades in terms of their capability to reduce contaminants. Although evidence has demonstrated that the biogeochemical coupling of Fe and N redox cycles has potentially important capabilities, the relevance of abiotic Fe and N redox transformations remain unclear. Therefore, in this thesis, aspects around abiotic nitrate (NO3-) reduction under environmentally relevant conditions has been undertaken to shed further insight into NO3- chemodenitrification driven by Fe(II)/Fe(III)-bearing minerals. The kinetics of NO3- chemodenitrification in Fe(II)/Fe(III) (oxy)(hydr)oxide heterogeneous systems were investigated at neutral pH (i.e. pH 6.5 to 7.5). Of the suite of minerals examined, NO3- chemodenitrification was only relevant in the presence of green rust (GR), a layered double hydroxide Fe(II)/Fe(III) mineral. High anion (SO42-/Cl-) concentrations could arrest chemodenitrification, most likely through competition for sorption, and hence electron transfer, sites. Stable N and O isotope studies indicated that 15ε and 18ε values produced from NO3- chemodenitrification, principally to ammonium (NH4+), by GR(SO42-) (37.9‰ and 14.4‰) and GR(Cl-) (12.9‰ and 4.5‰) were unique, suggesting the possibility of distinguishing between NO3- chemodenitrification induced by these two minerals as well as from microbial NO3- denitrification. Electron transfer from aqueous Fe(II) to structural Fe(III) in montmorillonite has recently been discovered to be a pathway to GR formation at circumneutral pH. In contrast to the observations described above, the kinetics of NO3- chemodenitrification by these GR minerals were found to be extremely slow. It was demonstrated that the reduction potentials induced by the GR minerals formed through this unique pathway (~ -110 to -210 mV) were much higher than those obtained for GR minerals prepared in the absence of montmorillonite (~ -300 mV). As such, the driving force for electron transfer was decreased to the point that GRs formed through this unique pathway will no longer provide a competitive process to microbial NO3- denitrification reactions. Although iron sulfides are known to facilitate NO3- chemodenitrification, there are still fundamental knowledge gaps on reaction mechanisms and stable N and O isotope dynamics. Here, studies were conducted to examine aspects around NO3- chemodenitrification by mackinawite (FeS). Although it was observed that NO3- chemodenitrification kinetics were faster at acid pH values, in disagreement with thermodynamics, this observation most likely resulted from the higher reactivity of a soluble reductant or possibly FeS oxidation products precipitating at higher pH values passivating redox-active surfaces. These oxidation products included greigite (Fe3S4), elemental sulfur (S0), thiosulfate (S2O32-) and sulfate (SO42-), suggesting that both Fe and S participated in the reduction of NO3-. In this case, the major reduced N species was N2(g), and not NH4+. No N and O stable isotope fractionation was observed for the first electron transfer step in the reduction of NO3- indicating that it is a reversible reaction and, as a result, this reaction will introduce variability to the kinetic isotope effects produced by other concurrent NO3- (chemo)denitrification reactions. Overall, this thesis has significantly advanced knowledge on NO3- chemodenitrification reactions, mechanisms and methods for distinguishing chemical and microbial denitrification pathways.

  • (2021) Afroz, Rounak
    Thesis
    Drought is one of the most complex hydroclimatic extremes, with enormous effects on the environment, economy, and society. In many locations, climate change is expected to increase the severity and frequency of droughts. However, most climate change impacts to date focus on relatively short-duration droughts (e.g., 12 months) and do not consider the evolution of droughts in terms of time or space. This means that evaluation of climate models and projections based on those same climate models are limited to point-by-point analyses with no information on drought evolution and decay and the future changes in these attributes. The multi-scale nature of droughts which can last over multiple years is also not considered in many climate change impact assessments which use fixed temporal windows for analysis. This thesis proposes new methods to address these issues and applies the new methods to analyse future drought across Australia with a specific focus on the spatial and temporal evolution of drought historically and in the future in the Murray Darling Basin (MDB). The main methodological contribution of the thesis is a new multi-time scale drought index termed the Residual Mass Severity Index (RMSI). The RMSI improves on the existing drought indices because it does not require a temporal window to be specified to characterize extreme deficit periods and these periods can be automatically identified. The RMSI is then used to evaluate the performance of general circulation models (GCMs) in characterizing historical drought. There is a reasonable consistency in GCMs in terms of representing drought frequency but only a few GCMs exhibited acceptable skill in capturing drought recovery and peak magnitude of drought. Spatiotemporal drought evolution was then characterized in the MDB. The areas worst affected by drought tend to experience faster build-up and slower recession than the basin as a whole. Finally, spatiotemporal changes in drought characteristics in MDB represented by GCMs in the current and future climate were assessed. Overall, an increase in drought duration, severity, and time to recede are projected for the future.

  • (2022) Oudone, Phetdala
    Thesis
    Dissolved organic carbon is stored and processed in groundwater in three ways. It is stored on minerals by adsorption, it is biologically processed through biodegradation, and it also undergoes a process to return to groundwater called desorption. This biophysiochemical research shows that the groundwater system is therefore a vital part of the global carbon cycle and carbon sink. This research fills a gap in the existing understanding of how to calculate the global carbon budget, as does not yet include the dissolved organic carbon that is stored in groundwater. This thesis exclusively explores processes determining dissolved organic carbon character and concentration in groundwater in different geological environments. This is new, useful knowledge to describe the biophysiochemical process. This research did not examine human interference in adding carbon to groundwater. This research found how dissolved organic carbon is stored and processed in groundwater due to biodegradation and desorption, and how it is adsorbed onto sediment surface. This research explored the characteristics and concentration of Dissolved organic carbon in groundwater by using Liquid Chromatography-Organic Carbon Detection, and other techniques, to examine dissolved organic carbon in terms of its fractions: humic substances, hydrophobic organic carbon, biopolymers, building blocks (BB), low molecular weight neutrals and low molecular weight acids. There were several key findings. First, the results showed that both semi-arid inland low sedimentary organic carbon environments – i.e., Maules Creek and Wellington – were a carbon source; while the high rainfall temperate coastal peatland environment of Anna Bay was a carbon sink. Secondly, another key finding was that dissolved organic carbon was not processed as a whole chemical compound, but it was processed by its fractions where each fraction was processed distinctly. For example, humic substances were only adsorbed/desorbed in groundwater; while low molecular weight neutrals were only consumed by microbes in the biodegradation process in groundwater.

  • (2021) Song, Zhao
    Thesis
    Due to the high toxicity and ubiquitous presence of inorganic arsenic (As) in groundwaters, there have been a number of studies investigating the removal of inorganic arsenic though most of the existing technologies are constrained by low energy efficiency and ineffectiveness in As(III) removal under circumneutral pH conditions. In this work we present proof-of-concept of a modified double potential step chronoamperometry (DPSC) method which achieves in-situ As(III) oxidation and concomitant electro-sorption of As(V) onto the electrode. The use of both carbon cloth and redox active polyvinylferrocene (PVF) functionalized carbon nanotube electrodes are investigated in this work. Our work with the carbon cloth electrode showed that in-situ anodic As(III) oxidation and concomitant sorption of As(V) formed was achieved at an applied voltage of 1.2 V. Our results further showed that the sorbed As(V) was effectively electro-desorbed by reversing the polarity thereby regenerating the electrode. The in-situ anodic As(III) oxidation, sorption of As(V) and desorption of As(V) are affected by aqueous pH with high oxidation and sorption/desorption rates observed at elevated pH. The increase in As(III) oxidation and concomitant As(V) adsorption at higher pHs is related to (i) the rapid oxidation of the deprotonated species compared to the protonated species and (ii) stronger electrochemical interaction between the multi-charged As(V) species and the electrodes. At 1.2 V and an electrical energy consumption of 0.06 kWh m−3, the total As concentration can be reduced from 150 to 15 µg L−1 using an electrochemical cell with electrode area of 10 cm × 8 cm and electro-sorption time of 120-min. Based on the experimental results, we have developed a mathematical model to describe the kinetics and mechanism of arsenic removal by the modified DPSC method using the carbon cloth electrode with this model of use in predicting, and potentially optimising, process performance under various conditions. While the carbon cloth electrode was effective in As removal in the absence of competing anions, in the presence of anions such as Cl−, SO42− and NO3−, the As(III) removal efficiency by the carbon cloth electrode was very low. In order to improve the selectivity of As over competing anions that are typically present in groundwaters, we investigated the sorption of arsenic from simulated groundwaters by a redox active PVF functionalized electrode. Our results show that effective and sustainable As(III) removal was achieved even at 0 V once the electrode is activated via anodic polarization. During activation, ferrocene (Fc) in PVF is oxidized to the ferrocenium ion (Fc+) with the latter facilitating As(III) sorption and subsequent oxidation as well as As(V) sorption. The high affinity of Fc+ to As and weak attraction to competing anions at 0 V ensure high selectivity of As over Cl−, SO42− and NO3− at concentrations typical of groundwaters. Following the removal process, efficient regeneration of the electrode is achieved at −1.2 V wherein Fc+ is reduced to Fc thereby facilitating As desorption from the electrode surface. Our results further show that O2 and associated generation of hydrogen peroxide (H2O2) during the electrode regeneration step drives the oxidation of Fc to Fc+, thereby maintaining the constant generation of Fc+ required to achieve As(III) removal in subsequent cycles. Our results show that 91.8%  0.6% of As(III) could be selectively removed from a simulated groundwater over 10 cycles at an ultralow energy consumption of 0.12 kWh m−3. We also investigated the influence of divalent cations (i.e. Ca2+and Mg2+) on the removal of As(III) by a PVF functionalized electrode. Our results show that in the absence of divalent cations, nearly 90% ± 0.9% of As(III) removal is achieved over ten continuous cycles by single-pass DPSC, even in the presence of competing anions (5 HCO3−, 3 mM Cl−, 0.5 mM NO3−, 2 mM SO42−) however the presence of divalent cations (Ca2+ and Mg2+) significantly inhibits electrode regeneration. While the presence of Ca2+ and Mg2+ facilitates As(III) removal in the 1st step of the DPSC, the regeneration of the electrode in the 2nd step of DPSC is significantly inhibited, thereby decreasing the As(III) removal in the subsequent cycles. Based on the results from various control experiments and surface characterization, it appears that Ca2+/Mg2+ either acts as a bridge between the electrode surface and As anions or the sorption of Ca2+/Mg2+ increases the positive charge on the electrode surface thereby facilitating the sorption of As. The Ca2+/Mg2+ assisted sorption occurs even during the electrode regeneration process, thereby decreasing the electrode regeneration efficiency and As(III) removal in subsequent cycles. Overall, the results show that the DPSC method with either carbon cloth or PVF functionalized electrodes achieves in-situ As(III) oxidation, concomitant sorption of As(V) and desorption of As(V). However, the adverse impact of the presence of divalent cations (Ca2+ and Mg2+) on electrode regeneration and thus the efficacy of the As removal process must be taken into account. Future work should focus on i) optimization of the configuration of the system in order to minimize the influence of divalent cations on regeneration of the electrodes and ii) scale up of the unit to a size that is meaningful for practical application. Additionally, development of more stable PVF electrodes and/or alternate electrode materials for better removal efficiency is also required.