Doctoral Degrees (Civil Engineering)
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- ItemRivet and wire rope reinforcement in extrusion-based printed concrete.(Stellenbosch : Stellenbosch University, 2024-03) Bester, Frederick Albrecht; Van Zijl, Gideon P. A. G.; Kruger, Jacques; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: From its ancient origins, concrete stands as a testament to human ingenuity, evolving over millennia to become an indispensable material of modem construction_ Its journey, from rudimentary applications in ancient civilisations to the sophisticated techniques of today, mirrors the relentless pursuit of innovation in humanity. The historical trajectory of concrete, spanning from debated origins around 10 000 to 20 000 years ago to the architectural marvels of the Roman Empire, has been marked by continuous advancements The period between 1750 and 1910 witnessed a surge in further innovation, heralding the modem era of reinforced concrete This era is characterised by the advent of various reinforcement methods and the onset of scientific explorations into the intricate synergistic relationship between reinforcement and concrete. However, as we navigate the challenges of the 21st century, the construction industry faces a paradigm shift. The environmental implications of construction, coupled with the inefficiencies inherent in traditional methods, necessitate the exploration of innovative concrete construction techniques. Among these, additive manufacturing with concrete, specifically extension-based concrete printing, emerges as a promising avenue to revolutionize the contemporary construction landscape. However, its widespread adoption hinges on addressing critical challenges, especially the automated integration of reinforcement during the printing process. This dissemination delves deep into extrusion-based concrete printing, aiming to address the pressing issue of reinforcement integration. Drawing Inspiration from the historical evolution of concrete reinforcement, the research seeks to further the extant knowledge of reinforcement strategies for extension-based concrete printing. Through a systems approach review, twelve unique reinforcement strategies are evaluated by gauging their alignment with the advantages of this novel technology. The research highlights the potential of fibre and wire rope entrainment as longitudinally aligned reinforcement technologies, given their alignment with the construction automation potential of the overarching additive manufacturing process. Furthermore, by enhancing the functionality of existing fastening technology, the dissemination introduces and experimentally evaluates a novel blind rivet-based reinforcement strategy. The research shows the potential of this strategy to bridge the interlayer interfaces during printing and enhance the structural integrity of printed concrete objects. The study also delves into the composite action of printed concrete and longitudinal wire rope reinforcement, assessing the reliability of different anchorage methods, culminating in the establishment of analytical smeared-cracking models grounded in established standards These models provide deeper insights into the behaviour of printed concrete reinforced with longitudinally aligned wire ropes. In essence, this dissemination offers a comprehensive exploration of reinforcement strategies for extrusion-based concrete printing It aims to aid in bridging the gap between traditional construction techniques and the potential of additive manufacturing in construction Through rigorous experimentation and analysis, the research provides a roadmap for the futile of reinforced concrete printing, paving the way for more efficient and innovative construction methodologies. The findings presented herein are composed to shape the next chapter in the storied history of concrete, merging the lessons of the past with the possibilities of the future.
- ItemInvestigation of novel deflector shapes for uncontrolled spillways(Stellenbosch : Stellenbosch University, 2024-02) Wright, Henry John; Bosman, Adèle ; Brink, Isobel ; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: The hydraulics of stepped spillways are generally well understood, although numerous fundamental hydraulic aspects remain inadequately explored. Critical knowledge gaps persist, including aerated flow hydraulics, hydraulics of embankment flows, hydraulics of stepped spillways for steep gravity dams, environmental hydraulics as well as turbulent interactions between cavity flow and skimming flow. Notably, the elusive safe unit discharge limits for stepped spillways remain undefined, with conflicting findings in the literature. The majority of stepped spillways have been designed for a maximum unit discharge of 25 to 30m3/s/m due to the risk of cavitation damage. It has further been reported that the critical velocity of approximately 20 m/s for the inception of cavitation on stepped spillways is obtained at a unit discharge of 25 m3/s/m. Further research in the field revealed that a bottom aerator becomes imperative for discharges greater than 30m3/s/m. However, discrepancies persist, with other researchers suggesting that the safe unit discharge is lower, quoting 11.5 m3/s/m to 14 m3/s/m for step heights of 0.6 m to 1.2m, respectively. Therefore, the exact limits of stepped spillways remain unquantified when water flows on the downstream slope. In China, the Flared Gate Pier (FGP) has been used on stepped spillways, particularly the X-type and Y-type piers. These piers support the crest gates and have been customised to contract the flow rapidly into a high-velocity jet. These piers have been used at, amongst others, the Dachaoshan Dam, a 111 m high Roller Compacted Concrete (RCC) gravity dam, with a maximum unit discharge designed of 193m3/s/m. These piers redirect flow into high-velocity jets, achieving efficient energy dissipation without relying on the stepped spillway face. Although historically utilised exclusively with gated spillways, FGPs hold potential as deflector-type energy dissipaters and were used as the basis for the novel deflector investigations in this research. To date, a variety of aerators have been fitted to improve spillway performance. Other aeration methods, such as the use of Roberts splitters, rectangular protrusions and triangular protrusions have been proposed, with some of these designs being successfully implemented. However, research has noted that these methods yield only marginal increases in the safe unit discharge of stepped spillways. The main concern regarding stepped spillways is the cavitation risk during high discharges, with a critical cavitation parameter of 0.5 compared to 0.2 for smooth chutes. This limits the maximum allowable unit discharge. While cavitation pitting has not been reported on prototype spillways, the exact conditions under which cavitation on stepped spillways may occur remain uncertain. The current research investigated the feasibility of a novel deflector form aimed at increasing the safe discharge capacity of spillways by deflecting the flow away from the spillway slope. The research incorporated a comprehensive approach, comprising a series of numerical models to simulate the hydrodynamic environment as well as four physical models. Numerical model simulations were undertaken with FLOW-3D HYDRO® and ANSYS FLUENT® computational fluid dynamics (CFD) software to optimise the deflector geometries before being tested with a physical model. A 1:50 scale physical model was constructed to investigate the influence of different deflector shapes. The investigation spans a range of prototype unit discharges ranging from 50 to 200m3/s/m and evaluates factors such as water surface profiles created by the deflector and pressure distribution on the deflector. A regression analysis was performed on the collected physical model data to develop equations that predict the jet's inner and outer trajectory and jet breakup length. The proposed novel deflectors developed in this study proved to be effective at various flow rates when the flow trajectory and threshold pressures were considered. These deflectors could be used for dams higher than 150 m and unit discharges ranging between 100 and 200 m3/s/m. Further research is required to improve, amongst others, deflector geometries, to study variables and to undertake additional measurements to conform and improve the efficiency of the novel deflectors, using this research as a basis.
- ItemExperimental and numerical investigation on the effect of gaps in mass timber connections in fire(Stellenbosch : Stellenbosch University, 2024-02) Du Plessis, Maria Catharina (Marika); Walls, Richard Shaun; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: The resurgence of timber as a structural material and increasing trend to leave mass timber members and connections exposed in buildings, for aesthetic reasons, makes the fire performance of mass timber connections an important research area to ensure structural fire safety. The influence of gaps in mass timber connections, and how they influence the fire resistance rating of a connection, has been the subject of limited research, and this information is paramount for reliable estimates of mass timber connections’ structural response in fire to prevent premature failure. This dissertation focuses on investigating the influence of gaps and the use of an intumescent fire sealant as a passive fire protection measure in mass timber connections. Two sets of furnace tests were conducted with the first being a simple timber sample configuration consisting of two glulam timber blocks with a steel component recessed in a gap between them. Gap sizes of 0 mm, 3 mm, 6 mm, and 10 mm were constructed with some gaps protected with an intumescent fire sealant and others left exposed. The temperatures in the timber around the gap and the steel component were measured and it was found that these temperatures were largely underpredicted when evaluated against the charring model of the forthcoming Eurocode 5 (3rd draft) and the requirements of the IBC 2021, with 75% of the temperatures underpredicted by prEN1995-1-2 at 60-minutes and 40 % of the temperatures failing the IBC 2021 connection temperature criteria. However, the intumescent sealant proved to be successful in limiting the temperatures when compared to the unprotected samples. A finite element model was created for two samples of each sample group to further quantify and investigate the heat transfer phenomena in the gaps. Convection in the gaps appears to be limited and not highly dependent on the gap width, especially deeper in the gaps. Radiation exposure is significantly underpredicted if it is based only on the calculated radiation dependent on the gap geometry. Radiation in the finite element models had to be calibrated to account for the combustion in the gaps, radiation between the faces of the gaps, the variable material properties and other complex heat transfer phenomena that occur during combustion in gaps. To obtain good agreement in the experimental and numerical models for the 3 mm gap samples, during the first 70-minutes of the test, the radiation onto the steel component had to be increased 5 times compared to the calculated radiation based on the geometry alone. The second set of furnace tests were conducted with a proprietary concealed beam hanger connection, with samples manufactured to include the same gap sizes as those previously tested. In these results it was clear that unprotected 6 mm and 10 mm gaps should be avoided. The unprotected 0 mm and 3 mm samples performed better but still showed larger variability in the experimental temperatures. The intumescent sealant typically performed well and limited the temperature development in the aluminium bracket significantly, with the increase in temperatures in the unprotected samples (on average at 60-minutes) ranging between 62 % and 258 %, and -4 % to 21 % for protected samples, when compared to the bracket temperatures in the 0 mm samples. In these connections the application of an intumescent fire sealant improved the predictability of the thermal development in the connections and in the 3 mm gap protected samples the lowest temperatures were recorded. Constructing concealed connections with no gaps is very challenging during construction, and therefore it is important to develop guidelines how to limit the influence of gaps on the thermal development in connections in a practical way.
- ItemPhysico-chemical interventions for improved interlayer adhesion in 3D printable concrete.(Stellenbosch : Stellenbosch University, 2024-03) Munemo, Ruvimbo Eustina; Van Zijl, Gideon P. A. G.; Kruger, Jacques; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: 3D concrete printing (3DCP), a revolutionary additive manufacturing (AM) technique, has the potential to transform the construction industry. It is an advantageous technique that offers more geometric freedom while simultaneously promising resource efficiency and enhanced productivity. However, due to the layer-based intrinsic nature of ANL the lack of interlayer adhesion (IA) is a prevalent issue that significantly impacts the mechanical performance of printed structures. Although the mechanism leading to lack of IA between filament layer in 3D printable concrete (3DPC) have been investigated in literature, the synergistic and antagonistic nature of these mechanism• is rarely considered Weak adhesion bet-ween filament layers undermines the inherently advantageous properties of 3DPC, as well as those of the fabrication process. Therefore, to address this issue, this research endeavors to ascertain physico-chemical interventions to bolster IA between 3DPC filaments which do not impede the fabrication process. An extensive review of methods that have been Implemented to enhance interlayer bond strength (IBS) is conducted to elucidate beneficial characteristics and strategies that yield the most desirable IA. Furthermore, strategies employed for modelling the interlayer region (IR) are analysed and used as the foundation for determining ruling parameters in simulation of IBS. Subsequently, a rubric for selection and assessment of effectiveness of an intervention strategy is proposed. The principal selection criterion is that an intervention must enhance IA by leveraging intrinsic properties of 3DPC without hampering the efficiency of the fabrication process. Each intervention IS mechanically and microstructurally characterized and the mechanisms leading to IA enhancement are investigated Two interventions are proposed and implemented Both strategies are novel to the 3DCP industry but show great promise and reliability with a low degree of complexity in both accessibility and implementation. The first intervention, characterised by induction of thermo-hydrokinetics to improve surface malleability and moisture content through the incorporation of a short-lived localised burst of steam on the surface of the preceding filament layer prior to placement of the subsequent filament layer, is presented and rigorously tested to assess the Improvement in IA. The IBS is bolstered by 78 0/0. The Intervention is then evaluated against the pre-set criteria to validate whether it is viable technique for enhancing IBS. The behavior induced by this intervention leading to better IA is subsequently empirically and computationally modelled and analysed to illuminate the Influencing factors and how they contribute to the enhancement of IA. The proposed empirical model appears to be extendable as it was able to accurately simulate the results of other studies conducted m literature yielding statistically significant findings. The second intervention characterized by silicate impregnation in the IR via interfacial surface treatment, IS also presented and evaluated. It bolstered IBS by 103 0/0, as well as the bonded area, as evidenced by the enlarged failure area. This research is a comprehensive investigation into intervention strategies to mitigate the lack of IA, with proposed interventions that not only yield more robust IRS but are also viable techniques that can be seamlessly Integrated into the fabrication process. The findings encourage a more synergistic approach to ensure strategies amalgamate to create resilient structures.
- ItemOptimisation of sand trap and settler designs for efficient deposition of suspended sediment(Stellenbosch : Stellenbosch University, 2024-03) Mc Leod, Claudia; Bosman, Adèle; Smit, G.J.F; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Sand traps and settlers are used at river abstraction or diversion works for the control of sediment loads for potable, irrigation and hydropower usage. The performance of a sand trap and settler is judged by its capability to sufficiently deposit suspended sediment particles and its flushing capability of bed load sediment. The design properties of a canal type sediment trap, such as the depth of flow, total length, cross-section, slope and inlet position, mainly determine its hydraulic efficiency. One of the main objectives of this study was to develop hydraulic design guidelines for sand traps and settlers for river conditions that have high loads of fine non-cohesive sand. This study investigates design considerations (dimensions, slope, cross-section, type of intake locations, and sediment intake concentration as well as inlet designs) of sediment traps by using an existing fully three-dimensional Computational Fluid Dynamics (CFD) model coupled in terms of hydrodynamics and sediment transport developed by Sawadogo (2015). The hydraulic performance of an existing sand trap and settler in Southern Africa was also investigated to identify hydraulic design aspects that could be improved. This was achieved by performing field measurements, analysing the field results of sediment deposition and velocities within the traps, and numerically investigating the case studies to recommend possible design upgrades to improve the efficiency of the traps. This study also investigates the innovative “Split-and-Settle” sand trap concept, initially proposed by Støle in 1993, by means of a three-dimensional CFD model as a second main objective. The “split-and-settle” approach directly refers to dividing the flow in a sand trap into sediment-free and sediment-laden water and then removing the sediment from the water. As sediment-laden water traverses a canal, the suspended sediment concentration increases near the bottom along the length of the canal as sediment tends to deposit. The split-and-settle concept leverages this sediment concentration gradient by dividing the flow into upper and lower parts. The concept was investigated by conducting physical modelling to generate data in a controlled environment to calibrate a numerical model. To calibrate and evaluate the sensitivity of the numerical model, appropriate parameters were adjusted until a good agreement was reached between the physical and numerical model results for the suspended sediment concentration. The parameters included the convection-diffusion equation’s turbulent Schmidt number, mesh configuration for capturing the split plate boundary and turbulent intensities. The principal contribution from this work is the calibrated and validated numerical model which could be further used to propose design guidelines for the split-and-settle sand trap. Additional benefits of the Split-and-Settle sand trap design include that it reduces the required length of sand traps and therefore are more economical whilst being hydraulically efficient, sustained operational capacity for handling substantial volumes of sediment-laden water, and minimal maintenance demand due to continuous flushing. Moreover, it operates without the inherent risk of scour holes that can become blocked or clogged and reduce efficiency. Ultimately, this research underscores the potential of the Split-and-Settle sand trap as a valuable tool in sediment management and hydraulic engineering.