Doctoral Degrees (Civil Engineering)

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    Rheo-mechanics, durability and microstructural characterisation of slag-nodified metakaolin-based geopolymer concrete for extrusion-based 3D printing applications
    (Stellenbosch : Stellenbosch University, 2023-11) Jaji, Mustapha Bamidele; Babafemi, Adewumi John; van Zijl, Gideon Pieter Adriaan Greeff; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering
    ENGLISH ABSTRACT: Extrusion-based 3D-printed geopolymer concrete (3DPGPC) is a potential alternative to Portland cement concrete (PCC). Research is sparse on the use of metakaolin (MK) for extrusion-based 3D concrete printing applications. The widespread adoption of 3DPGPC is limited due to the unknown durability properties and the long setting time of a two-part geopolymer system. To address the long setting time, this study modified MK-based 3DPGPC with slag up to 30% for 3D printing due to its high Ca2+ ion content. The printable mixture developed comprises M1 (100% MK-3DPGPC) and M2 (95% MK and 5% slag), beyond 5% slag inclusion; the mixtures stiffened with inadequate open time for printing. To address the stiffening, sodium phosphate is incorporated to achieve sufficient open time for constructability of the new mixtures and to improve structural build-up in the mixtures containing slag, M-S10 (90% MK and 10% slag), M-S20 (80% MK and 20% slag) and M-S30 (70% MK and 30% slag), while the mixture without slag, M-S0 (100% MK), is the control. The slump obtained using a mini-slump cone is in the range of 3–5.5 mm and the slump flow using a slump flow table is between 148–157 mm. The setting time using the Vicat apparatus depicts an open time of 6.8 hours for the control (M-S0), and 1.2–1.3 hours for slag-modified mixtures. Rheology tests using an ICAR rheometer reveal that the initial static yield shear stress (𝜏𝑠,𝑖 ) increased from 1898–1900 Pa and initial dynamic yield shear stress (𝜏𝐷,𝑖 ) evolve from 1452–1482 Pa due to 5% slag inclusion. Also, re-floccution (Rthix) and structuration (Athix) rates improved from 5.16 and 0.2 Pa/s to 5.2 and 0.4 Pa/s, respectively. After 28 days of curing age, 70 mm × 140 mm cored cylindrical-3DPGPC specimens exhibited compressive strength of 23.7–33.13 MPa and splitting tensile strength of 1.79–2.43 MPa. Saw-cut 40 mm × 40 mm × 160 mm beam specimens attained flexural strength of 5.48– 7.29 MPa and an interlayer bond strength of 5.40–6.90 MPa. The durability of 3DPGPC is investigated using the water absorption test, capillary and gel porosity test, oxygen permeability index (OPI), and drying shrinkage tests. After 90 days of curing, the drying shrinkages in the vertical direction are 2.98 and 2.86% for the control specimen (M1) and the slag-modified specimen (M2), respectively. In the horizontal direction, the drying shrinkages are 1.14 and 1.1%, respectively. The vertical strain obtained during drying includes plastic shrinkage, drying shrinkage, and vertical creep due to the sustained weight of the upper layers in the fresh state. Drying shrinkage varied along and across the layers of 3DPGPC, depicting anisotropic behaviour. After 90 days of curing, water absorption decreases to 7.33% and 5.2% in M1 and M2 specimens, respectively. The total porosity of 3DPGPC decreases from 20.5–14.5% after 90 days of curing, while mould cast decreasesfrom 15 to 10% in M1 specimens. Slag inclusion further reduce the porosity of 3DPGPC, and mould cast from 17–10.9% and 11.5–8%, respectively. After 90 days of curing, 3DPGPC specimens cored perpendicular to the printing direction (vertical) exhibits (OPI) of 11.07–11.86 kPa, and specimens cored perpendicular to the printing direction (horizontal) exhibits OPI in the range of 10.99–11.74 kPa, while mould cast specimens exhibit OPI of 11.23–11.92 kPa. CT-scan shows that mould-cast specimens have a total porosity of 4.07% and exhibit spherical pores, while 3DPGPC have a total porosity of 1.81% and exhibit elongated pores due to pumping. CT-scan also reveals that porosity is position-dependent in 3DPGPC due to the presence of voids between 0.1–1.7 mm at the interlayer, whereas mould-cast specimens exhibit randomly distributed voids in the range of 0.1–2.5 mm in diameter. Backscattered electron images show increasing C-S-H, N-A-S-H and C-A-S-H gel formation due to the presence of alumina, silica, sodium in MK and high Ca2+ ion as slag content increases. The BrunauerEmmett-Teller (BET) surface area increases with an increase in slag content from 5–23 m2 /g, resulting in the densified 3DPGPC matrix, thereby improving buildability from 27 layers to 42 layers and enhancing mechanical performance. Nitrogen physisorption test shows that the adsorption and desorption isotherms and the hysteresis loops are within the IUPAC Class IV and H3 types, indicating the presence of mesopores (2–50 nm) and macropores (>50 nm). This research demonstrates that MK-based 3DPGPC can be successfully 3D printed and modified with slag to improve the fresh properties, rheology, mechanical properties, microstructural morphology, pore characteristics, and long-term durability performance. It also reveals that 3DPGPC exhibits anisotropy in orthogonal directions. The results obtained from this study are recommended for numerical modelling strategies.
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    Improved statistical flood frequency estimation approach for South Africa
    (Stellenbosch : Stellenbosch University, 2023-12) Van der Spuy, Danie; Du Plessis, Jakobus Andries; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The performance of the most frequently used flood frequency probability distributions in South Africa, using the current statistical approach, were reviewed and all tend to perform poorly when lower probability frequency events are estimated, especially where outliers are present in the dataset. This can, essentially, be attributed to the challenge to analyse very limited ‘samples’ of annual flood peak populations, which are an unknown. As a result, outliers were inadequately 'managed' by attempting to 'normalise' the flood peak dataset, which concealed the significance of the observed data. Thus, by considering the outliers properly, the research was undertaken with the aim to improve the current statistical approach and to develop a more stable and consistent probability distribution. The re-evaluation of the plotting position was considered as a precondition to the research. The plotting position concept was initially applied more than a century ago. Since then, many alternative plotting position techniques have been developed that, practically, do not differ much from each other. The ineffective management of outliers is regarded as the main reason for the inconsistency in flood frequency results. Exploiting a more pragmatic approach, managing outliers by using Z-scores, an alternative plotting position technique was developed, referred to as the 'Z-set' plotting position. The main advantages of the Z-set plotting position technique, distinguishing it from the other plotting position techniques, is its consistency, the more sensible inclusion of outliers and a more homogeneous appearance. The Z-set plotting position technique also proved to be less susceptible to different record lengths than the existing plotting position techniques, which may encourage more consistent choices of appropriate probability distributions. The methodology followed in the development of the proposed new probability distribution, named IPZA, might be considered as unconventional, but the multiple regression approach was used to accommodate the strongly skewed data, often associated with annual flood peak series. The main advantages of the IPZA distribution, is its consistency, the simplicity of application (only one set of frequency factors for every parameter, regardless of the skewness), the integrated handling of outliers and the use of conventional method of moments, eliminating the need to adjust any moments. The performance of the Z-set plotting position and the IPZA probability distribution exceeded initial expectations. The results of both are more consistent than and, by taking outliers into account, appear to be more sensible than existing plotting positions and probability distributions. It is concluded that the Z-set plotting technique and the IPZA probability distribution should be used as valuable additions to the existing set of decision-making tools for hydrologists/engineers performing flood frequency analyses. Evidently, this does not exclude the use of existing probability distributions. It is sound practice to use more than one probability distribution to assist in making a more informed scientific decision.
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    Hydraulic impacts and management of intermittent water supply
    (Stellenbosch : Stellenbosch University, 2023-03) Loubser, Carlo; Jacobs, Heinz; Wium, Jan; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: Various challenges, such as limited freshwater resources, climate change impacts, rapid population growth, urbanisation and underinvestment in water supply infrastructure, have led to intermittent water supply (IWS) in potable water distribution systems. Earlier research has confirmed that IWS negatively impacts the consumers, the infrastructure and the water supply authorities. In potable water distribution systems, water quality is of the utmost importance. In systems subjected to IWS, water quality is often compromised. However, there is a large body of literature addressing issues related to water quality in IWS systems. In contrast, a number of more elementary and hydraulic aspects related to IWS systems have not been researched in depth. This research sets out to answer the following questions that are unrelated to water quality, but instead focusses on the hydraulics and infrastructure elements of the IWS systems. Is IWS prevalent in South African water supply systems? Can IWS be avoided in urban areas faced with water scarcity and which mechanisms are available to water services providers to avoid having to implement IWS? Can a model be developed to crudely assess the pipe infrastructure in order to evaluate whether IWS is a threat based on the potential maximum capacity of the network? How can systems subjected to IWS be compared in a quantifiable manner? And, how can the reliability of supply be assessed in systems subjected to IWS and what are the potential consequences of unreliable supply schedules? Earlier research on the global prevalence of IWS indicated that South Africa practises continuous water supply. Data on IWS in South Africa was collated by considering four different source types. The data were spatially and temporally analysed to determine the prevalence of IWS in South Africa. The population affected by IWS increased by ~26% between 2008 and 2017, which exceeds the population increase of ~12% over the same period. Moreover, 22 million people in South Africa were affected by IWS in 2017. Results from this research confirm an increased prevalence of IWS over time and show that 65 of the 231 municipalities in South Africa supplied water intermittently; 32 had continuous water supply and no data was available for the remainder. The outcomes highlight the widespread occurrence of IWS in South Africa. Between the years 2015 and 2018, the City of Cape Town, South Africa, experienced the worst drought on record, which placed tremendous pressure on the city’s bulk water resources. Despite many of the major dams nearly running dry in the summer of 2017/2018, and the available water diminishing to only a few months of supply, the City of Cape Town managed to avoid implementing IWS. This research presents the various strategies adopted and implemented by the City of Cape Town in order to avoid IWS. The successes achieved led to the City of Cape Town reputedly becoming the number one water saving city in the world in October 2018, saving as much as 500 ML per day, which equates to about half the normal demand before the drought. Given the well-known and widely published negative impacts associated with IWS, it would be useful for water services providers in water scarce regions to take cognisance of how the City of Cape Town managed to avoid implementing IWS. Planners are often faced with the challenge to provide crude estimates of water distribution system infrastructure capacity and associated cost in the early phases of greenfield developments. This research investigated the relationship between physical and hydraulic characteristics of a water distribution system and the corresponding serviced area. A model was compiled linking the total pipeline length of a water distribution system to the peak flow rate. The model enables prediction of the total length of water distribution system pipes required to service a future development area as a function of the peak flow rate. Alternatively, the model can estimate the potential maximum peak flow rate that can be supplied if the total pipeline length is known. IWS can result from several potential causes, including inadequate overall pipeline length or a lack of network reinforcements when new developments are added on the fringes of developed areas. Systems subjected to IWS due to these causes could be expected to have insufficient overall pipe length or different diameter distributions when compared to the model results. Thus, the model would allow a user to ascertain whether an existing water distribution system has potentially been stretched beyond its design capacity. Water supply authorities need tools to help understand IWS and the associated implications. A new indexing framework involving the causes and impacts associated with IWS has been developed. In addition, a novel approach allows for quantification of the severity of IWS based on knowledge of a few readily available inputs. The indexing framework and quantification tool could lead to improved understanding of IWS and could assist water supply authorities faced with IWS to make informed decisions. Two towns located in the Mpumalanga Province of South Africa, had IWS implemented for a number of years prior to this study. While the general causes and impacts of IWS are fairly well researched and documented, the causal-consequential pathways of IWS as experienced in one of the study areas, offer a new perspective on consumer acceptance and conduct related to various forms of IWS. Logging of flow rates recorded by meters supplying water intermittently, also offers new information on supply availability, supply durations and reliability of supply when supply schedules are managed by manually opening and closing of zone valves. Unreliable and unavailable supply led to an increased number of illegal connections. Once water theft and leakage rates exceeded certain levels, the water services provider struggled to maintain control over the water supply infrastructure and a spiral of decline was observed.
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    Road network supply efficiency in Cape Town, South Africa and its link to the incidence of road traffic crashes: an evaluation of equity
    (Stellenbosch : Stellenbosch University, 2023-03) ter Huurne, Dominique Andrea; Sinclair, Marion; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The Road Safety Problem, including its devastating effects on human livelihoods and socioeconomic development, is well-documented at both the global and local scales. In South Africa, road safety (especially for vulnerable road users) is a short-term strategic priority. The focus is shifting from reactive to proactive approaches to crash-risk assessment. The disadvantage of proactive approaches is that they require extensive input data: road inventory data, exposure data, and environmental data, which is typically unavailable in low- and middle-income countries like South Africa. In its latest road safety strategy, the City of Cape Town set an objective to conduct road safety assessments on all class 1 to class 3 roads by 2018; this objective was not achieved. There is a need for a network-wide road safety measure that uses frugal and/or readily available data inputs. This study examined the relationship between crash frequency and transport demand in the City of Cape Town. Commercially available Origin-Destination Floating Car Data and trajectory clustering were used to determine the city’s transport demand corridors for various input parameters. The Cross K-function was employed to assess the spatial relationship between these corridors and fatal and serious injury road traffic crash locations (2015 to 2017) at the city and census-suburb scales. Furthermore, the transport demand corridors were used to evaluate road supply network efficiency (class 1 to 3 roads) as well as this efficiency measure’s potential as a road safety measure and an indicator of infrastructure vulnerability (transport equity). This was believed to be extremely relevant in the context of South Africa’s intention to focus the application of road safety assessments on mobility roads, as the majority of pedestrian crashes have been found to occur on access roads. Road network supply efficiency was quantified using the Collective Remoteness Indicator (CRI). A simple linear correlation test was conducted between 1) the CRI and crash frequency and density, and 2) the CRI and the South African Index of Multiple Deprivation 2011. For all alternatives analysed at the city scale, road traffic crash locations and transport demand corridors were found to be positively related at all 10-m intervals between 10 m and 200 m. The study also revealed positive spatial relationships for various census suburbs, particularly for pedestrian crashes during the morning peak period.Moderate to strong correlations between the CRI and crash density were discovered at the census-suburb scale, suggesting that road supply network efficiency has the potential to be applied as a measure of road safety. A subsequent qualitative analysis found potential for road supply network efficiency to be applied as an indicator of infrastructure vulnerability. This novel road safety measure is deemed to be of practical value to identify and prioritise sites or areas for road safety assessments.
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    The development of a daily stochastic streamflow model for probabilistic water resource management
    (Stellenbosch : Stellenbosch University, 2023-03) Hoffman, Jahannes Jacobus; Du Plessis, Jakobus Andries; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: An ever-increasing water demand with limited supply of water in South Africa means that the focus of resource management needs to shift from a macro-level to micro-level. Well-defined research methodology regarding the management of larger water resource systems does exist in models such as STOMSA (Stochastic model of South Africa) and WRYM (Water Resources Yield model), which use monthly timesteps. In analysing smaller catchments, these macromodels need to be adapted to daily timesteps to enhance applicability in the management of resource systems for smaller local authorities. This research focused on the development of a daily stochastic streamflow model to be used in small, single site catchments for resource management by local authorities in South Africa. Such catchments usually consist of abstraction weirs with off-channel storage dams that should deal with the effects of short runoff response time associated with small catchments, where the monthly timestep analysis typically cannot account for the short-term variability in daily streamflow. The methodology used in the current research focused on the generation of daily stochastic streamflow data by retaining the day-to-day relationship of the historical streamflow series without the reliance on disaggregation models to generate daily data from larger timesteps. This was achieved by implementing a Markov process, as the core element, to generate the stochastic data, based on the day-to-day relationship of the historical daily dataset. To address seasonality associated with daily datasets, the concept of daily duration curves was introduced, which served as both a normalisation process of the historical data, as well as a statistical distribution for the random selection of stochastic streamflow data. To ensure repeatability, a Pseudo-Random Number (PRNG) generator was used in the randomisation process of generating the stochastic datasets. The Daily Markovian Stochastic Streamflow model (DMASS) was developed consisting of four modules. The Pre-processing Module used primary streamflow data from the Department of Water and Sanitation (DWS) to generate the daily streamflow time series. The Analysis Module analysed the daily streamflow time series to create the Daily Duration Curves (DDC) and the Cumulative Transition Probability Matrix (CTPM). The Generation Module used the DDC and CTPM to generate the stochastic sequences. The Climate Change Module provided the option to adjust the DDC according to the selected adjustment parameters.