Browsing by Department "Department of Civil Engineering"
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- ItemOpen AccessA Comparative Study on Shear Strength Testing of Single and Multi-layer Interfaces using Large Direct Shear Apparatus(2021) Muluti, Shade; Kalumba, Denis; Sobhee-Beetul, LaxmeeGeotechnical structures such as composite liner systems in landfills consist of multiple interfaces, which include a broad range of geosynthetics in conjunction with soil, rocks and any other related materials. This results in the introduction of many interface planes into the structure, which can potentially create instability especially along the slope and ultimately result in failure. To date, many laboratories use single interface testing instead of multi-layer interface testing to determine geosynthetic shear design characteristic values that are used in the design of structures such as landfill liners. A topic of discussion remains the preferred interface testing configuration and only a few studies have substantiated and quantified the significance of varying the different interface shear testing configurations. This study, therefore, aimed to evaluate and compare the effects of the use of the two interface test configurations on the shear strength of soil/geosynthetic and geosynthetic/geosynthetic interfaces. Furthermore, it was intended to identify the test configuration that provides the most critical shear strength results, while also understanding the fundamental mechanisms responsible for the shear strength observed. In this study, three geosynthetics were used: geotextile (GTX), geomembrane (GMB) and geosynthetic clay liner (GCL), which generally constitute the critical interface components of a lining system in a modern South African landfill liner. Two soils were utilised as a part of the materials required for the investigation and they were: river sand and red clay. The laboratory tests were conducted under saturated conditions in accordance with the ASTM D5321 and ASTM D6243 standards, using a 305 mm x 305 mm large direct shear box. The tests were carried out over a range of applied normal pressures of 50, 100, 200 and 400 kPa. A constant shear rate of 1.0 mm/min was used in the interface tests that did not involve GCLs or clay specimens and therefore no excess pore pressure was anticipated at the interface. On the other hand, for all other interface tests involving either clay or GCLs samples, a shearing rate of 0.1 mm/min was utilized. The results showed that nonlinear behaviour of the shear stress versus shear displacement responses was exhibited in both the single and multi-layer interface tests, regardless of the normal stress applied. However, it was noted that with an increase in normal stress applied, the deviation in mobilized shear stress between the two test configurations increased, with single interface tests yielding higher shear stress values compared to multi-layer interface tests. In single interface tests, the high shear stresses could be related to the clamping that confined each of the test specimens during shearing to one end of the shear block. On the other hand, only the top and bottom test specimens were clamped in multi-layer interface tests, thus allowing failure to have occurred at the weakest of the available interfaces. Moreover, for single interface tests, peak strengths were generally 9% lower for the range of normal stresses considered, whereas Large Displacement (LD) strengths were generally 24 % lower for the single interface tests, compared to the peak and LD strength values for multi-layer interface tests. This was particularly observed at low normal stresses between 50 and 200 kPa, and it could probably have been caused by the rigid clamping of the geosynthetics which results in some tensile strains in the geosynthetics. In addition, it was observed in multi-layer interface tests that a transfer of shear stresses within the system could have occurred, which could have led to higher overall shear resistance of the composite. As a result, single interface tests yielded a conservative estimate of the peak and LD shear strengths for the tested interfaces compared to multi-layer interface tests. This may be attributed to higher displacement along with the critical interface in single interface tests than in multi-layer interface tests. To allow the investigator to observe the displacement, as well as the possible transfer of shear stresses within the system during the shearing of the various geosynthetics, it was recommended that real-time monitoring of the displacement mobilization should be carried out in multi-layer interface tests during shearing.
- ItemOpen AccessA comparative study on the structural behavior of concrete arch dams subjected to swelling due to aggregate sllica reactions(2018) Stehle, Hermann Theodor; Moyo, PilateSouth Africa is considered a water-scarce country and this fact alone stresses the absolute need to preserve its water resources. As time goes by, the ageing of dams in South Africa is becoming an increasingly important factor to consider from a dam safety perspective. When considering concrete dams, Alkali Aggregate Reactions (AAR) which is the collective term referring to the potential chemical reactions between the cement and the coarse aggregate in the concrete, are a major cause of ageing. AAR causes internal swelling of concrete leading to stresses that eventually manifest on a macroscopic level as inter alia cracks, deformation and opening of horizontal construction joints. Although the effect of AAR expansion on arch dams is complex, certain behavioural phenomena have been identified as typical indicators of swelling concrete. These are well covered by literature. This thesis aims to compare the structural behaviour of concrete arch dams in South Africa that are subjected to swelling due to AAR. Three arch dams, namely Hartebeeskuil Dam, Poortjieskloof Dam and Thabina Dam (all located in different climatic regions), were identified and their behavioural patterns were investigated by using visual techniques along with the interpretation of instrumentation results. The typical instrumentation results that were used for interpretation purposes included geodetic surveying results, crack width gauge results, in situ stress measurement results and trivec measurement results. Poortjieskloof Dam, the oldest of the three dams, showed permanent upstream displacement trends of both flanks, but the centre of the arch showed a downstream displacement trend. Both flanks show swelling towards the abutments and rising crest levels are evident throughout the length of the dam wall. The dam wall was cracked quite severely on the downstream face and the horizontal joints showed clear separation. The most recent displacement trends suggest that the rate of AAR is decreasing. Hartebeeskuil Dam, the second oldest of the three dams, showed permanent upstream displacement trends throughout the length of the dam wall. Both flanks show swelling towards the abutments and crest levels at both flanks show some settlement. The central section of the arch show rising crest levels. The results of in situ stress measurements carried out in 1999 showed that the downstream section of the arch is experiencing tensile stresses while the upstream section of the arch is mostly experiencing compressive stresses. The cracking patterns on both the upstream and downstream faces seem to agree with these findings. The results generally seem to suggest that the AAR mainly occurs on the upstream side of the arch and that the effective arch has become thinner due to the tension zone on the downstream side. The most recent displacement trends suggest that the rate of AAR is not showing any signs of decreasing. Thabina Dam, the youngest of the three dams, showed permanent upstream trends of the right flank while the central region and left flank of the arch showed downstream trends. The flanks have moved permanently towards each other and the crest levels have increased throughout the length of the arch section. The most recent trends show increasing rates of strain especially in the vertical (z) and tangential (y) directions. More recently the entire arch has started showing upstream displacement trends. These may indicate the onset of a swelling mechanism in the concrete, most likely AAR, but extensive testing is required to prove this.
- ItemOpen AccessA comparison between conventional brick and timber-frame brick veneer housing(1978) Mostert, J J JTimber-frame brick veneer construction originated in countries in which houses were traditionally constructed with loadbearing timber frames which were clad with non-loadbearing weather boarding or similar light-weight materials. Timber-frame house construction is extensively used in countries such as Canada, Scandinavia, Australia and North America. With the Canadian system, buildings of up to three storeys can be erected with loadbearing timber frames. This particular system was also adopted in France - the Igny project~ and in England - the Harlow project. In most of these cases, basements or crawl-spaces underneath the house were incorporated in the design. In these countries there have a trend towards the use of a brick cladding (veneer) for timber-frame structures. This is a comparatively recent introduction in these countries, but it has already been extensively used. This trend is primarily related to the appearance and other advantages (for example durability and ease of maintenance in the case of a face-brick clauding) of a masonry exterior, while largely retaining the low cost benefits of timber-frame construction.
- ItemOpen AccessA Comparison of Wastewater Process Modelling Tools: Case Study of Potsdam WWTW(2022) Govender, Neeren; Ikumi, DavidA wastewater process model can either be based on a steady-state or dynamic mathematical modelling approach. The characterisation of the influent wastewater and specific wastewater parameters are critical input parameters to the mathematical model, whether for design, optimisation or to measure the expected performance of a wastewater treatment works (WWTW). A wastewater treatment process is inherently dynamic because of the changes in wastewater characteristics and flow rates, impacting the plant's process capacity and performance. Mathematical models have the capability to model such changes with ease and predict the expected performance of the plant. This dissertation involves a theoretical modelling approach to compare steady-state and dynamic-state modelling tools on an existing full-scale WWTW, namely the Potsdam WWTW located in Cape Town (South Africa). The models adopted for this research included: i) A plant-wide steady-state model developed by the Author as part of this dissertation (Author's PWM); ii) Plant-Wide Steady-State Design (PWSSD) developed by Wu (2014); iii) BioWin 5.0 (developed by Envirosim Associates Ltd; Barker and Dold, 1997; Envirosim 2007); and iv) UCTPHO dynamic model (Wentzel et al., 1992). The goal was to compare the output and results of the various models adopted for this research under steady-state and dynamic-state conditions where the models have been calibrated on historical field data and to examine the impact that specific wastewater characteristics have on the process capacity and the overall performance of the WWTW. To accomplish this goal, the following objectives were achieved: 1. The characterisation of the influent wastewater is critical in modelling. For a new plant (green field site), the wastewater characterisation will be based on typical norms as published in the relevant research literature. For an existing plant, the wastewater characterisation is typically based on historical sampling data. There can be challenges with historical sampling data, as data is often not measured, missing or not credible. In this case, designers are required to make assumptions to fill the gaps in the wastewater characterisation, adopting typical norms as published in the research literature and using sound process engineering judgement which was the scenario for the case study WWTW. 2. The documented design capacity of the Potsdam WWTW is 47 Mℓ/day (47,000 kgCOD/day) whereas the total theoretical process capacity estimated as part of this dissertation is 60.75 Mℓ/day (64,660 kgCOD/day), 37.5% higher than the documented process capacity. 3. The various models correlated well in terms of output results for both steady-state and dynamic-state conditions when compared across all process units for both the liquid and sludge streams, using the same input process parameters (flow rates, load patterns, wastewater characterisation, fractions and design assumptions). The steady-state models (Author's PWM and PWSSD) were almost identical in output results except for the aerobic digester. The output results of the dynamic-state models (BioWin and UCTPHO) were similar to the steady-state models but did differ for a few variables, attributed to the fact that the dynamic models use dynamic kinetic equations under constant or dynamic flow and load conditions, and this will produce different results than the less complex steady-state models which are based on constant flow and load conditions. 4. An evaluation of the impact of selected influent characteristic parameters on the system performance variables for biological nutrient (nitrogen and phosphorus) removal was performed for each of the steady-state and dynamicstate models. The parameters selected included the influent TKN (TKN/COD ratio), the maximum specific growth rate of nitrifiers (µMax) and readily biodegradable COD (RBCOD), and during the analysis, all other input parameters to the models were kept constant. The influent TKN and RBCOD are specific wastewater characteristics that can vary during the lifetime of a WWTW having a strong impact on its N and P removal performance. µMax is also a critical wastewater parameter and at sludge ages close to the minimum sludge age for nitrification, this parameter impacts severely on the performance of biological nitrogen removal systems. It was concluded that substantial changes in the influent TKN, µMax and RBCOD will significantly impact a WWTW, specifically concerning N and P removal, therefore, impacting effluent and sludge quality. The various models followed similar trends; however, the following discrepancies were noted: ▪ When the TKN was increased, only the BioWin model considered the impact that the pH outside the range of 7.2 – 8.0 would have on the µMax and nitrification capacity of the bioreactor. This is a shortfall in the other models ▪ When the µMax was varied, the different models followed the same trend, but nitrification problems occurred at different µMax values. The BioWin model showed partial nitrification at a higher µMax threshold value than the other models, which occurred under minimum temperature conditions only ▪ When the RBCOD was varied, the only difference in the models is the RBCOD fraction at which complete biological phosphate removal took place. The steady-state models had the same RBCOD fraction, and the dynamic-state model (i.e., BioWin) had a higher RBCOD fraction at which complete biological phosphorous removal took place due to the fact that BioWin adopts two different maximum specific growth rates for the poly accumulating organisms (PAOs), namely a higher growth rate constant under phosphorous rich conditions which results in a higher uptake of phosphorous, and a lower growth rate constant under phosphorous limited conditions, where phosphorous uptake is limited. With a lower PAO growth in the BioWin model resulting in a lower PAO population, there is less potential for aerobic polyphosphate uptake, resulting in higher effluent Ortho-P concentrations. A WWTW is complex with many interactions of different processes (biological, chemical and physical) and products taking the form of various phases (aqueous, gas and solid), complicated further by variations in influent characteristics, concentrations and flows. To manage these complexities, wastewater process models have been developed over the last three decades from stand-alone models for individual process units to plant-wide computational steady-state and dynamic models, which cater for a broad spectrum of wastewater engineering objectives. Steady-state models are powerful as they comprise simple and explicit algebraic equations that easily allow the estimation of design requirements and operation requirements for a WWTW with much less input information than dynamic models. They are often pre-processers to the dynamic models. In contrast, dynamic models require detailed input formation and sophisticated mathematical solvers but are more accurate in predicting effluent quality, system responses to dynamic conditions and the inhibitory effects of pH, temperature and metabolic products. The decision of selecting a steady-state or dynamic-state model is influenced by several factors, such as available information and influent data, user competency and modelling experience, size and complexity of the plant, as well as the detail and accuracy required by the designer. Wastewater process models therefore serve as a valuable tool for design, optimisation and operational and control strategies.
- ItemOpen AccessA computer program for the analysis and plotting of ocean wave refraction diagrams.(1978) Wale, Anton Wessels
- ItemOpen AccessA Critical Evaluation of the Use of Crack Width Requirements in the Durability Design of Marine Reinforced Concrete Structures(2023) Elias, Nicholas; Beushausen, Hans-DieterCrack width requirements (CWRs), which aim to limit cracks in reinforced concrete (RC) to maximum prescribed values, play a major – and often dominant – role in the design of marine RC structures. However, there are several issues with the current CWRs, chief among which is the fact that, despite decades of research, no clear relationship between crack width and steel reinforcement corrosion rate in concrete has been found. Instead, there exist two opposing schools of thought in the literature – one which says that there is a relationship between crack width and reinforcement corrosion rate, and one which argues that no such relationship exists – with good evidence to support both schools of thought. Recent research has shown that even small cracks, with widths below the required values, may lead to extensive corrosion. It is therefore uncertain whether designing for the CWRs actually improves durability and extends the service life of marine RC structures. Furthermore, the use of the CWRs, which frequently results in large increases in the required amount of reinforcing steel, may lead to significant increases in the cost and environmental impact of marine RC structures. Yet, to date, these impacts have not been quantified. In order to address these issues, this study was aimed at evaluating the effect of designing to meet the current CWRs on the durability, cost, and environmental sustainability of marine RC structures. This was done by designing two sets – one with, and one without the CWRs – of typical marine RC structural elements. Based on real industry projects, two different types of elements were designed – a crane rail beam for a coal export jetty in Matola, Mozambique, and a precast crown wall unit for a breakwater in Rupert's Bay, St. Helena Island. The designs were carried out using a combination of BS 6349 and EN 1992-1-1:2004, as these are the codes of practice typically used in the South African coastal engineering industry. The effects of designing for the current CWRs on durability, cost, and environmental sustainability were then quantified by carrying out service life modelling, life cycle cost assessments (LCCAs) and estimating embodied carbon (EC) values for the designed members. The results of the service life modelling show that, for the range of crack widths likely to occur in practice, the use of the current CWRs does not improve durability, and may even reduce service life, as they encourage the use of more, smaller diameter reinforcement bars, which has the effect of increasing corrosion rate and reducing the time taken for a critical amount of the reinforcement to be lost due to corrosion. Furthermore, the results of the LCCA and EC estimates imply that the current CWRs are not the most cost-effective method for durability design and may result in significant increases in cost and environmental impact. Taken together, these results suggest that, even if a relationship is assumed to exist between crack width and corrosion rate, the current CWRs are neither the most effective, nor efficient way of addressing the effects of cracking on the durability of marine RC structures. It is therefore recommended that the current crack width requirements should be removed from the durability design codes of practice and replaced with either a limitation on steel stress, a more lenient crack width requirement (for example, of 0.5 mm rather than 0.3 mm), or a performance-based crack width requirement, which takes better account of the complexity of cracking and its effect on durability. It is also recommended that engineers be given the option to use other methods of providing durability, such as the use of crack-sealing and waterproofing admixtures, or hydrophobic treatments, instead of the current CWRs. However, before any of these recommendations can be implemented, the results of this study need to be confirmed with further research. Owing to the limitations of both service life modelling and accelerated laboratory corrosion, it is recommended that this further research should take the form of an extensive evaluation of the durability performance of existing marine RC structures.
- ItemOpen AccessA deep learning-based approach towards automating visual reinforced concrete bridge inspections(2021) Dube, Bright N; Moyo, Pilate; Matongo, KabaniVisual inspections are fundamental to the maintenance of RC bridge infrastructure. However, their highly subjective nature often compromises the accuracy of inspection results and ultimately leads to inaccurate prioritisation of repair and rehabilitation activities. Visual inspections are also known to expose inspectors to height and trafficrelated hazards, and sometimes require the use of costly access equipment. Therefore, the present study investigated state-of-the-art Unmanned Aerial Vehicles (UAVs) and algorithms capable of automating visual RC bridge inspections in order to reduce inspector subjectivity, minimise inspection costs and enhance inspector safety. Convolutional neural network (CNN) algorithms are state-of-the-art in relation to the automatic detection of RC bridge defects. However, much of the prior research in this area focused on detecting the presence of defects and gave little to no attention to characterizing them according to defect type and degree (D) or extent (E) ratings. Four proof-of-concept CNN models were therefore developed, namely a defect-type detector, crack-type detector, exposed-rebar detector and a shrinkage crack D-rating model. Each model was built by first compiling defect images, labelling them according to defect/crack type and creating training and test sets at a 90-10% split. The training sets were then used to train the CNN models through transfer learning and fine-tuning using the fastai deep learning python library. The performance of each model was ultimately evaluated based on prediction accuracies on the test sets and their robustness to noise. Test accuracies ≥ 87% were attained by the trained models. This result shows that CNNs are capable of accurately identifying RC bridge corrosion, spalling, ASR, cracking and efflorescence, and assigning appropriate D ratings to shrinkage cracks. It was concluded that CNN models can be built to identify and allocate D and E ratings to any visible defect type, provided the requisite training data that sufficiently represents noisy real-world inspection conditions can be acquired. This formed the basis upon which a practical framework for UAV-enabled and deep learning-based RC bridge inspections was developed.
- ItemOpen AccessA finite element program for the static analysis of branched, thin shells of revolution under axisymmetric loading(1974) Griffin, T B; Doyle, WA finite element computer program which uses the conical, frustrum element is presented for the Linear elastic, static analysis of variable thickness, branched, thin shells of revolution, composed of straight sections and subject to general, axisymmetric mechanical, 1,oading. The thin she1,1, theory and finite element theory forming the basis of the analysis are described, with particular attention being given to the closing of the she l, 1, at the axis of symmetry, and shell branching. Numerous problems embodying al,1, relevant features of the program are analysed, and their solutions are discussed. A user's manual, for the program is appended, and guidelines for the efficient use of the program are given.
- ItemOpen AccessA framework for the evaluation of the structural safety of existing concrete gravity dams(2019) Dankers, Clyde Lleland; Moyo, PilateAddressing the aging and deterioration of dams is a relatively new challenge in the dam engineering field in South Africa. The average life expectancy of a dam is approximately 50 years, but they can normally be used for much longer periods than this. Maintenance and rehabilitation are vital to ensure that they achieve their design service lives and for the extension of their service lives. The rehabilitation of dams to extend their service lives and/or to ensure that they comply with modern stability criteria can be an extremely lengthy and costly exercise. It would therefore be valuable to do some research into the process of evaluating the structural safety of dams. The focus of this research study is specifically on the evaluation of the structural safety of large concrete gravity dams. The purpose of the research is to investigate the most commonly used and accepted methods to evaluate the structural safety of concrete gravity dams and develop a framework that can be used for the evaluation of the structural safety of concrete gravity dams. For the purpose of this research study, an existing large concrete gravity dam was evaluated as a case study. The dam is approximately 93 years old and provides water to the nearby local municipality for domestic purposes. According to the first and second Dam Safety Evaluation (DSE) reports the dam does not comply with modern stability criteria for concrete gravity dams. The findings of these dam safety evaluation reports led to the dam being labelled as essentially “unsafe” in the case of the occurrence of a large flood. Typically, this would mean that the dam must be rehabilitated to improve its safety. However, some engineers believe that this is not the case and that major rehabilitation is not necessary. In this research study a framework for the evaluation of the structural safety of existing concrete gravity dams was developed based on lessons learnt from the literature review and the case study. This recommended framework can be useful as a guide for future safety evaluations of existing concrete gravity dams. The potential benefits of using this recommended framework includes avoiding unnecessary rehabilitation work, as well as significant time and cost savings.
- ItemOpen AccessA geospatial investigation of destination choice modelling. The case of the MYCITI integrated rapid transit bus system, Cape Town, South Africa(2021) Smith, Joanet; Zuidgeest, MarcusThe transport sector plays an integral role in a country's development and economy. Optimised transport networks and infrastructure can lead to increased economic development. Effective transport networks and public transportation systems are, therefore, essential to growing the South African economy. With an increasing demand for transportation services required by the South African population, the need exists to expand the capacity of local public transport networks. With this need declared, and grants released by the government, a high demand exists for the estimation, analysis, optimisation and forecast of public transport systems in South Africa. Public transportation studies are directly related to commuter demand as a result of commuter choices. Therefore, a key component for understanding the operational functionality of a public transport system lies in the accurate modelling of commuter choices. Although the spatial separation of activities forms the essence of travel demand, incorporating the effects of geospatial properties in travel behaviour modelling has only been formally studied in recent years. These recent studies noted a trend proposing that geospatial properties can influence travel behaviour. In the stated research, the need to investigate the effect of geospatial properties on travel behaviour was highlighted. With travel behaviour being the result of commuter choices, a multinomial logit choice modelling study was conducted to investigate the effect of geospatial properties on commuter destination choice for the case of the MyCiTi Integrated Rapid Transit system in Cape Town, South Africa.
- ItemOpen AccessA Location - Allocation Model For Pedestrian Footbridge infrastructure: A Case Study on the South Africa's National Roads Agency Limited (SANRAL) Network in Gauteng(2020) Mabe, Malesiba Millicent; Zuidgeest, MarcusRoad fatalities are one of the leading causes of unnatural deaths in developing countries, such as South Africa. According to the Road Traffic Management Corporation (2017), the number of road traffic fatalities were recorded to be 11 676 in the year 2016 and 42.6% of these road traffic fatalities involved pedestrian. According to the National Road Safety Strategy 2016 – 2030 most pedestrian crashes are due to illegal pedestrian crossings mainly across high speed roads (highways) (Department of Transport, 2011). The research study aims to develop a geo-spatial model to determine the most suitable location to allocate pedestrian footbridge infrastructure along the South African National Road Agency Limited (SANRAL) network in Gauteng Province. Previously road planning and design in South Africa followed international best practices, with development of the highway road system mainly orientated to serve motorised transport by primarily providing infrastructure for vehicular traffic along the highways , as pedestrians and highways were not expected to coexist in any proximity to each other. The presence of pedestrians along highways can no longer be ignored and there is a great need to overcome the highly fragmented spatial planning formed by the apartheid policies. For most non-motorised transport (NMT) users in South Africa, highways offer the shortest and most efficient direct routes, but also represent a major obstacle for many pedestrians who are separated by them. Limited studies on the location and allocation of the pedestrian footbridge for pedestrians are reported in the literature, further research in this field should be considered, especially along the SANRAL network where there are high number of residential areas along high-speed roads, this behaviour is unique and common in South Africa. Based on a literature review the factors affecting the location and allocation of pedestrian footbridges were identified, the following were factors identified: population density (serviced by the pedestrian footbridge), land use activities, socio-economic characteristics of the population (e.g. Gender, employment, race, and income level), crash hotspots/statistics (along the route), distance to access bridge and vehicle traffic volume along the route. From these factors spatial data was gathered to carry out spatial multi-criteria analyses (SMCA). The results from the spatial analysis showed that high population densities adjacent to highway, accessibility and land use are key in the location of pedestrian footbridge. The results also show that there is a strong correlation between: land use diversity, socio-economic, crash hotspots/statistics (along the route), vehicle traffic volume and population density (serviced by the pedestrian footbridge). The higher an area has these factors, the Higher the Pedestrian Footbridge location index (high pedestrian footbridge location demand), hence the recommendation to allocate in those locations.
- ItemOpen AccessA numerical investigation of the dynamic behaviour of continuous, multi-span railway bridges(2018) Ludwig, Chad; Moyo, Pilate; Busatta, FulvioThe dynamic behaviour of railway bridges has been investigated for over a century [Ichikawa et al., 2000]. With the introduction of high speed trains in recent history, a host of complex problems regarding resonance have been observed and studied. These studies, which include Bjorklund [2004]; Gabald´on et al. [2009]; Goicolea et al. [2002]; Rigueiro et al. [2010]; Kumaran et al. [2003]; Kwark et al. [2004] and Xia and Zhang [2005], have focused on resonance in relatively short-spanned simply supported railway bridges. New design methods were incorporated in design codes such as Eurocode (EN 1991-2) [2003] to address these problems in practice. In the past, railway bridges were designed for static effects, while dynamic effects were accounted for by the application of an amplification factor. It has become increasingly necessary to perform a full dynamic analysis, especially with regard to high speed trains. In the case of continuous, multi-span railway bridges carrying heavy haul trains, such an analysis is not explicitly specified. Design codes, even as recent as the modern Eurocode (EN 1991-2) [2003], do not address scenarios where axle loads are higher than 30 tonnes/axle, or trains become very long. Previous work on the dynamic behaviour of continuous bridges is limited. The dynamic properties of continuous beams was studied as early as Lin [1962], and more recently by Saeedi and Bhat [2011]. The response of continuous beams or bridges subjected to moving forces or masses was studied by Cheung et al. [1999], Johansson et al. [2013] and Ichikawa et al. [2000]. These investigations were limited to analytical methods to determine the dynamic properties (natural frequencies and mode shapes) and response of beams or bridges. In this research, the response of multi-span, continuous bridges trafficked by heavy haul trains travelling at low to moderate speeds was investigated. The study comprises an investigation of bridges with spans ranging from one to ten, and span lengths of 40 m, 45 m and 50 m modelled using the Finite Element Method in SOFiSTiK. Loading is based on heavy haul trains, which were modelled using the moving forces load model. Natural frequencies and mode shapes were obtained, and displacements and accelerations were calculated for train speeds varying from 20 km/h to 100 km/h. A case-study of the Olifants River Viaduct (ORV), the longest continuous railway bridge in South Africa, is also carried out. From the study it is evident that as the number of bridge spans increase, the envelope of natural frequencies in the concentrated zone increase but the frequencies become very closely spaced, indicating that the modes might be difficult to determine experimentally. Displacement and accelerations were generally higher in the first and last span of the multi-span models. A difference in maximum displacements was only noticeable when comparing models with the number of spans ranging from 1 – 4, thereafter maximum displacements were not affected by the number of spans in the model. Accelerations increased as the speed increased. At low speeds, the number of spans did not significantly influence the peak deck acceleration, however, at higher speeds models with the greater number of spans generally had lower maximum accelerations.
- ItemOpen AccessA Practical Carbonation Model for Service Life Design of Reinforced Concrete Structures(2021) Harold, Romuald Fotso Lele; Beushausen, Hans-Dieter; Alexander, MarkThe increase in atmospheric carbon dioxide concentration due to global warming has a direct impact on the amount of carbonating concrete structures. For the past years, numerous studies have been done in South Africa on the subject and models developed to predict carbonation in concrete structures. Despite the large amount of resources and research effort put into developing these models, the translation from theory to practice represents a great challenge for design engineers in the field of durability design. This study presents a design tool based on existing models for use in practical applications. The proposed design tool assists in computing the service life of carbonating concrete structures and provides reliability values associated with the service life. It accounts for different binder compositions and binder types, as well as different locations and environmental land uses in South Africa. The validation of the design tool was done by comparing the service life prediction results to existing models, which generally showed good agreement. The developed design tool can be applied for predicting the long-term performance of new RC structures as well as improving the basis for quality assessment of existing, newly built RC structures. For the design of new structures, the designer is required to make certain assumptions concerning the information to be used for the simulation. These include values for the binder type, binder content, OPI, cover depth, land use and exposure parameters. For the quality control of new structures, the way in which the model parameters are obtained differs from that of new structures. As the structure already exists, both the concrete quality, cover depth and environmental loading can be measured directly on the structure with appropriate testing procedures. The outcome of applying the design tool for the analysis of concrete produced for the Gauteng Freeway Improvement project (GFIP) is also presented, with a case study of precast and in-situ structures chosen for the analysis.
- ItemOpen AccessA programming approach to the numerical analysis of elasto-plastic continua(1978) Dittmer, Colin Thomas; Martin, J BThe application of a kinematic minimum principle involving a continuous functional subject to inequality constraints is described for the incremental analysis of elasto-plastic continua. A simple algorithm is used for solution of the resulting mathematical programming problem. The formulation is presented for problems in plane stress, plane strain or axial symmetry, using triangular constant strain finite elements, and is extended to the use of cubic quadrilateral isoparametric elements for which a numerical integration technique is employed to account for elasto-plastic interfaces within elements. The material is assumed to obey the von Mises yield condition, and be either elastic-perfectly plastic or linear kinematic hardening. Computational details and solution techniques are described, and numerical examples compared with experimental and numerical results in the literature. Some assessment is made of the relative computational efficiency of the method.
- ItemOpen AccessA prototype dynamic model for the co-treatment of a high strength simple-organic industrial effluent and coal-mine drainage(2020) Harding, Theodor; Ekama, George A; Ikumi, DavidThis research study's the use of biological sulfate reduction technologies for the treatment of Sasol Secunda's coal-mine drainage (CMD) using Fischer-Tropsch Reaction Water (FTRW) as a cost-efficient carbon source. The research aims to develop a prototype dynamic model that describes this co-treatment of FTRW and CMD in both a continuously stirred tank reactor (CSTR) biological sulfate reduction (BSR) system and a BSR gas-lift (BSR-GL) integrated system. The BSR-GL system recovers elemental sulfur (S0 ) from the H2S produced and stripped from the BSR unit. Furthermore, this study aims to use the prototype model for a quantitative comparison of the CSTR-BSR and BSR-GL systems. Two bench-scale 5-litre CSTR-BSR and a 20-litre BSR-GL system were operated, under varying feed COD concentrations and hydraulic retention times (HRTs), to generate datasets for use in verification and a rudimentary validation of the prototype model. The BSR-GL integrated system includes 1) a 1-litre H2S gas reactive absorption (ABS) unit utilising an aqueous ferric solution for the recovery of elemental sulfur (S0 ) from sulfide and 2) ferrous biological oxidation reactor to regenerate ferric from the ferrous for re-supply to the ABS unit. The datasets generated in the experimental study allowed for the identification, mathematical modelling and reaction verification of 32 components that interact as reactants and products in 23 reactions observed in the two BSR systems. The prototype model is presented in a mass and charge balanced Gujer matrix that includes, i) 5 SRB mediated processes, ii) 2 liquid-gas mass transfer processes, iii) 3 processes describing the ABS and Fe2+ bio-oxidation units, iv) 4 processes describing sulfide and elemental sulfur oxidation and v) the S0 and poly-sulfide aqueous equilibrium and vi) 9 processes describing death regeneration and BPO hydrolysis. This prototype model was implemented in the DHI WEST® software for initial stage simulation trials. The experimental datasets allowed for the first-stage estimation of the best-fit reaction rate equations and the calibration of the kinetic parameters related to the 23 reactions, using MATLAB® curve fitting toolbox. A pre-processor that describe the pH and equilibrium chemistry of the components of the artificially prepared FTRW+CMD feed mixture batches under varying total concentrations have also been developed in this research. This was done to generated influent file to the DHI WEST® simulations that incorporated the dynamics related to the FTRW+CMD feed mixtures. The sulfate utilisation rate (gSO4 -2 .l-1 .d-1 ) of the GL-BSR and CSTR-BSR systems were compared to determine which system had the best sulfate removal. The results were found to be as follows; a. On comparison it was found that the sulfate substrate utilisation rate for the CSTR_BSR system is 39.28% of that of the BSR-GL_N2 system, where both systems were fed at feed mixture of COD of 2500mgCOD/l, where the COD:SO4 2- was 0.7, b. For the same systems fed a feed mixture of COD at 5000mgCOD/l (COD:SO4 2- = 0.7), the sulfate substrate utilisation rate for the CSTR_BSR system was found to be 17.86% less than that of the BSR_GLN2 system. c. Finally, it was also found that the substrate utilisation rate for the CSTR_BSR system was 30.06% less than that of the BSR_GLN2 system at Se of 4gCOD/l, for both systems fed substrate at 5000mgCOD/l. Thus, it can be concluded that the sulfate substrate utilisation rate for the BSR-GL system is higher than that of the CSTR_BSR system, for systems fed COD feed mixtures at 2.5 or 5gCOD/l where both systems have the same effluent substrate concentrations. However, the difference in the comparative substrate utilisation rate is less at higher feed substrate concentrations. This is the influence of substrate inhibition on the active SRB biomass, which increases with higher effluent substrate concentrations. Finally, this research found that the use of gas-lift reactor technologies is superior to CSTR technologies in the treatment of coal-mine drainage utilising biological sulfate reduction (BSR). The CSTR-BSR system, fed sulfate between 1.6 to 14gSO4 2- /l, produced effluent with high dissolved H2S concentrations, on average 285mgS/l and maximum at >600mgS/l. Releasing this effluent to the environment would be hazardous to aquatic and human health and corrosive to infrastructure. As such, the effluent from the CSTR-BSR system requires further treatment to stabilise the water for any use. The BSR-GL technology allows for the conversion of the H2S produced during BSR reactions to form elemental sulfur, which is a resource recovered from this process, thus complying to the circular economy aim of this study.
- ItemOpen AccessA review on the efficiency of different supplementary cementitious materials as a partial replacement for Portland cement in concrete(2022) Taiwo, Ridwan A; Alexander, Mark; Leo, EmmanuelThe effects of global warming and climate change are important and have attracted the attention of many researchers. Global warming is a result of the presence of increasing amounts of greenhouse gases in the atmosphere. Carbon dioxide, which is largely emitted into the atmosphere during the manufacture of cement clinker, is one of the greenhouse gases. Hence, researchers have explored the use of some waste materials and naturally occurring minerals as a partial replacement for cement in concrete. These materials are often referred to as supplementary cementitious materials (SCMs). Apart from the potential benefits of these SCMs for the properties of concrete, they also bring about a reduction in the amount of waste in landfill sites, as these wastes can cause land, water, and air pollution, thereby posing threats to human health. However, despite the potential benefits of SCMs in the cement and construction industry, they have not been fully utilized especially in developing countries in Africa. This may be due to low awareness of the potential benefits of SCMs among the stakeholders in the construction industry, and also limited availability. Nevertheless, due to extensive research into the usability of different materials as SCM, various materials are available in the construction market as binder systems. Thus, selecting the appropriate binder system to get the desired result for a particular concrete might be difficult for construction personnel. Hence, this study presents a review of the effects of various SCMs on the mechanical and durability properties of concrete. Six SCMs are reviewed. These SCMs include fly ash, silica fume, which are industry by-products; metakaolin, limestone calcined clay, which are naturally occurring minerals; rice husk ash, which is an agricultural waste material; and limestone-fly ash, which is a combination of an industrial by-product and a naturally occurring material. Firstly, an overview of the mechanical and durability properties of concrete is presented. This includes the presentation of general factors affecting the mechanical and durability properties of concrete. Subsequently, the effect of the various SCMs on mechanical (such as strength, elastic modulus, creep, and shrinkage) and durability properties (freeze-thaw, acid attack, sulphate attack, chloride-induced corrosion, carbonation-induced corrosion, and alkali-silica reaction) of concrete are presented. The review shows that the inclusion of appropriate dosage of these SCMs in concrete or mortar enhances their properties. Certain limitations of these SCMs are also discussed. This study also identifies areas of further research in relation to the properties of concrete produced with the SCMs.
- ItemOpen AccessA seeded ambient temperature ferrit process for the treatment of AMD waters: Magnetite formation in the presence and absence of calcium ions under steady state operation(2003) Morgan, B E; Lahav, O; Hearne, G; Loewenthal, R EAn ambient temperature ferrite process has been developed for the removal of iron and non-ferrous metals from AMD waters. The process involves the controlled formation of magnetite (Fe3 O4 ) that has the capacity to substitute divalent and trivalent cations as part of the lattice, thus forming a stable easy-to-separate ferrite. This paper reports on continuous operations of the process in the absence and presence of Ca2+, which is well known to impede ferrite formation. In the first instance, the process involves the precipitation of hydroxy-metals at pH 10.5 and their subsequent adsorption onto magnetite seed in a contact stabilisation reactor. Second, liquid-solid separation is effected and the solid fraction is subsequently treated in an oxidising reactor in which a fraction of the ferrous species is oxidised to an intermediate ferric precipitate. Finally, both ferrous and ferric species undergo crystalchemical processing and are incorporated into stable magnetite. Results indicate that Ca2+ interference can be overcome by maintaining a high ratio of precipitated ferrous species to dissolved Ca2+. It was found that in order to attain the required high Fe2+:Ca2+ ratio, the solid ferrous-hydroxy species concentration in the oxidation reactor should be maintained at above 1 200 mg Fe/l. Ferrous to calcium ratios greater than 3 were found to favour magnetite formation. In the absence of Ca2+, a solid ferroushydroxy species concentration of approximately 500 mg/l was sufficient for magnetite formation. Operating the process at ferroushydroxy concentrations of lower than 1 200 and 500 mg/l in the presence and absence of calcium respectively enhanced the formation of other iron oxides, primarily goethite. In all experiments the iron concentration in the effluent was less than 1 mg/l, the sludge volume index (SVI) extremely low (< 4 ml/g) and the percentage of ferrous-hydroxy species in the sludge can be reduced to about 1%. These features, together with the potential to incorporate heavy metals into a stable compound, make the process very promising for AMD treatment.
- ItemOpen AccessA step towards standardising accelerated corrosion tests on laboratory reinforced concrete specimens(2012) Malumbela, Goitseone; Moyo, Pilate; Alexander, MarkNatural steel corrosion of reinforced concrete (RC) structures is a slow process which researchers find necessary to accelerate in laboratory tests to obtain needed damage in a short time. Regrettably, there is no standard procedure for accelerating steel corrosion in RC specimens. Researchers therefore continue to use various techniques to accelerate it. Unfortunately, structural damage and rate of steel corrosion are dependent on the accelerated corrosion technique used. Despite that, results obtained by researchers are applied by structural engineers and asset managers to in-service structures. This paper reviews previous techniques used to accelerate steel corrosion. Where possible it proposes standard procedures to accelerate steel corrosion. In other instances it points out needed further research. One of the procedures recommended in the paper is to contaminate selected faces of RC specimens with chlorides, as opposed to immersing samples in NaCl solution or adding chlorides to concrete mixes. It is also recommended to allow specimens to sufficiently dry during steel corrosion so as to promote steel corrosion.
- ItemOpen AccessA study into Healthcare Service Location Problems, Location and Allocation in the Inanda area(2020) Naidoo, Krishantha; Zuidgeest, MarcusInanda is a predominantly rural area located on the northern coast of the province of KwaZulu Natal, South Africa. It is bordered by the areas of Phoenix, Verulam and Tongaat. In the context of healthcare accessibility in the Inanda area, the research aimed at investigating the problem in service location planning. This was done by investigating level of accessibility to existing healthcare facilities available to the residents of Inanda. Following the classification of accessibility problems, recommendations were made on where the facility locations can be improved or expanded to provide better accessibility in terms of location-allocation. Literature that has been reviewed focused on geographic location, GIS and accessibility measures, spatial accessibility, models used to test accessibility, service location planning and accessibility measures and metrics so as to provide a background and precedent for the service location planning carried out in the research. The research aimed to confirm that accessibility to the healthcare facilities is indeed a problem and to propose alternative strategies to overcome the accessibility problems identified. The access to healthcare service locations is dependent on a number of factors. Some of these factors include travel time and distance, available capacity at facilities, existing road network, and provision or lack thereof of an efficient public transport system. This accessibility to the health service locations was assessed by using available GIS information on healthcare facilities and using accessibility analysis to identify problems in terms of the services location as well as additional location-allocation of current and additional facilities. The analysis was based on the assumption that all service locations have unlimited capacity. Flowmap was used as the tool to analyse the GIS data and conduct various accessibility models. The different models were Expansion Model Analysis, Relocation Model Analysis, Catchment Area and Clinic Allocation Analysis, Catchment Profile, Market share of Supply Locations, Regular Proximity Count, Average Distance in Competition, Proximity Count in Competition, Lowest Mean Trip Cost Alternate, Second Best Catchment Distance and Pareto Cover Set. The results of the research showed that while the locations of the existing healthcare facilities are not ideal, most are accessible to the majority of the Inanda residents. The information on actual capacity available at each of the locations was not available at the time of the research being carried out and would be worthwhile to research in the future.