OpenUCT :: Browsing by Author "Moyo, Pilate"

Browsing by Author "Moyo, Pilate"

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Open Access
A comparative study on the structural behavior of concrete arch dams subjected to swelling due to aggregate sllica reactions
(2018) Stehle, Hermann Theodor; Moyo, Pilate
South 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.
Open Access
A deep learning-based approach towards automating visual reinforced concrete bridge inspections
(2021) Dube, Bright N; Moyo, Pilate; Matongo, Kabani
Visual 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.
Open Access
A framework for the evaluation of the structural safety of existing concrete gravity dams
(2019) Dankers, Clyde Lleland; Moyo, Pilate
Addressing 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.
Open Access
A numerical investigation of the dynamic behaviour of continuous, multi-span railway bridges
(2018) Ludwig, Chad; Moyo, Pilate; Busatta, Fulvio
The 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.
Open Access
A step towards standardising accelerated corrosion tests on laboratory reinforced concrete specimens
(2012) Malumbela, Goitseone; Moyo, Pilate; Alexander, Mark
Natural 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.
Open Access
Ambient vibration testing of concrete dams
(2011) Bukenya, Patrick; Moyo, Pilate; Beushausen, Hans-Dieter
In this thesis, seven techniques namely; rational fractional polynomial, complex exponential, frequency domain decomposition (FDD) based techniques which include; frequency domain decomposition (FDD), enhanced frequency domain decomposition (EFDD), curve fitting frequency domain decomposition (CFDD) and stochastic subspace identification (SSI) methods namely; unweighted principal component (UPC), principal component (PC) and canonical variant analysis (CVA)) have been applied to data from ambient vibration testing of two concrete dams namely; Roode Elsberg and Kouga dams.
Open Access
An economic evaluation on FRP for bridge construction in South African coastal areas
(2022) Ndeutapo, Samuel; Moyo, Pilate
Fibre Reinforced Polymers (FRP) have been used for approximately 30 years as concrete strengthening materials in structural applications. This is primarily due to Steel Reinforced Concrete (SRC) being susceptible to the corrosion of reinforcing steel, especially in coastal environments. However, the use of FRP composites for bridge construction is faced with challenges. These challenges arise from uncertainties regarding the high material and initial construction costs. Additionally, there is limited research available on the costs involved when incorporating the fibres in bridge constriction. It then becomes critical to understand the life cycle costs of FRP in bridge construction. Furthermore, suitable economic and deterioration models can be used to predict these life cycle costs by employing a LCCA. The research aim was to investigate the economic viability of using FRP as reinforcing elements in bridge construction. This was done by conducting a life cycle cost analysis (LCCA) on two highway beam bridge superstructure designs: a SRC superstructure and a GFRP-RC superstructure to determine the preferred design, which was used to conclude on the economic viability of FRP in bridge construction. The deterministic approach to the LCCA was selected and the outcome was expressed in terms of Present Worth of Costs (PWOC). The initial construction costs were found to be the bulk, while the disposal costs were the least of the total LCC for both superstructure alternatives. Furthermore, initial construction costs of the SRC superstructure were found to be less than that of the GFRP-RC superstructure, by a margin of R 873 094. This was primarily due to the cost of E-glass reinforcement, approximately 2.1 to 2.9 times more expensive than the cost of steel reinforcement. Moreover, the cost of GFRP was seen to have decreased over the years. LCC savings were seen from the GFRP-RC superstructure over the SRC superstructure, by a margin of R4 627 830 in terms of maintenance costs. This was mainly due to the application of a corrosion inhibitor (concrete surface treatment) and the use of a cathodic protection system on the SRC bridge superstructure. Furthermore, the GFRP-RC superstructure was found to be the least cost-effective investment from approximately 34 years of a 75 year LCCA period. At the end of the analysis period, the SRC bridge superstructure was found to have cost savings of approximately R 753 921 in PWOC over the GFRP-RC superstructure. Furthermore, a sensitivity analysis of the various input costs and discount rates of the LCCA was also conducted. Initial construction costs were found to have the highest positive correlation on the outcome of the LCCA. The other costs and as well as the discount rate were all found to have an insignificant effect on the outcome of the LCCA. It was concluded that it was not economically viable to include FRP as reinforcing elements in bridge construction at the time. However, since the cost of FRP was seen to decrease over time, the inclusion of FRP rebars in bridge construction might be economically viable in the future.
Open Access
Analysis and comparison of the South African and Eurocode live load models for railway bridges
(2018) Paulse, Sheryl Dawn; Moyo, Pilate
This dissertation is an analytical study that compares the South African Transport Services (SATS) and Eurocode (EC) live load models for railway bridges. The study is specifically concerned with the critical load effects of shear and bending moment. The load models are simulated as moving loads over the full length of simply supported and continuous railway systems with speeds not exceeding 180km/h. The study is limited to short to medium spans ranging from 5m – 40m analysed in increments of 5m. The position of the maximum load effects for simply supported systems was determined using the frame analysis module in Prokon. Maximum load effects were determined using the influence line method. Maximum load effects for the continuous systems were determined using the moving load option in STRAP. It was found that SATS live load models imposed on single span railway bridges, produce conservative load effects for short span bridges but become over conservative with an increase in span, when compared with characteristic values of the EC load model 71 (LM71). For heavy loads (α = 1,10) in LM71, there is a good comparison with that of the EC for static and design moment (for a track with standard maintenance) with values of 5% lower at 10m but become moderately conservative (2% - 5%) with an increase in span. In the case of design bending moment (for a carefully maintained track) the SATS code is moderately conservative (6% - 8%) over the full range of spans for a carefully maintained track. For heavy loads (α = 1,10) in LM71, there is a good comparison with that of the Eurocode for static and design shear (for a carefully maintained track) with values of 4% lower at 10m but becoming moderately conservative (1% - 5%) with an increase in span. In the case of design shear (for a track with standard maintenance) the SATS code compares well with that of the EC, with values of 5% lower at 10m but becoming moderately conservative (4% - 13%) with an increase in span. Live traffic loads imposed on equal span (limited to 2) continuous railway bridges, produce conservative static and design shear load effects (for a carefully maintained track) in the mid-range of spans but become moderately conservative with increase in span for heavy loads (α = 1,10) for load model SW/0. There is a good comparison with that of the EC for design shear force (for a carefully maintained track) with moderately conservative (1% - 9%) for short span and long span systems for heavy loads (α = 1,10) for load model SW/0. A similar comparison occurs for heavy loads (α = 1,21) for SW/0 for static and design shear for a carefully maintained track. Live traffic loads imposed on equal span (limited to 2) continuous railway bridges produce over conservative static bending moment load effects for short span and long span bridges (2 x 30m – 2 x 40m) for characteristic values and heavy loads (α = 1,10 and α = 1,21) for load model SW/0. Generally, there is not a good comparison with that of the EC for static and design bending moment, for two span continuous railway bridges. Live traffic loads imposed on equal span (limited to 3) continuous railway bridges produce moderately conservative static shear force effects for heavy loads (α = 1,10 and α = 1,21) for load model SW/0. The only significant value is at the 3 x 5m span (21% higher) and the 3 x 15 – 3 x 20m range of spans (9% - 10% lower) for heavy loads (α = 1,10) and (α = 1,21) respectively. A similar comparison is observed for design shear effects for both types of track for heavy loads (α = 1,10) and (α = 1,21) for a carefully maintained track. Generally, there is not a good comparison with that of the Eurocode for static and design bending moment, for three span continuous railway bridges.
Open Access
Analytical behaviour of FRP strengthened reinforced concrete beams under low velocity impact load incorporating rate dependant material constitutive models
(2014) Ruiters, Alton; Moyo, Pilate
Since the 1980s, the use of fibre reinforced polymer (FRP) composites in strengthening and rehabilitation of existing reinforced and pre-stressed structures has gained popularity. Versatility, high strength to weight ratio, corrosion resistance, excellent creep and fatigue behaviour, and ease of installation are amongst some of the advantages offered by externally bonded FRP systems over traditional strengthening methods. In addition to strengthening for static loading, there are many scenarios where strengthening is required to elements subjected to dynamic loads. The static behaviour of FRP strengthened RC beams has been the subject of extensive research. However, the dynamic behaviour of FRP strengthened RC beam elements remains unclear. Limited experimental studies are available that are focused on the response of FRP strengthened RC beams subjected to low velocity impact events. Furthermore, many of the Finite Element (FE) analysis models developed in these studies yielded results that were inconsistent with the test data. Key shortcomings of these models relate to a lack of definition of the FRP-concrete bond interface and considering rate dependent material behaviour.
Open Access
Application of vibration-based damage detection techniques to civil infrastructure : incorporating uncertainty quantification
(2008) Dzvukamanja, Setonam Komla; Moyo, Pilate; Alexander, Mark Gavin
Literature was reviewed with an aim to identify research needs in vibration based damage detection techniques and the quantification of the uncertainty in these techniques. It was discovered that the literature lacked examples of the explicit propagation of measurement uncertainty through damage detection algorithms. Instrumentation errors and variable environmental and operational conditions were identified as sources of uncertainties. It was established that in order to ensure reliability of the damage detection techniques and to assess their robustness, a damage detection framework which, accounting for sources of error in the measurements, propagates the uncertainty through the algorithms of the damage detection techniques. Standard methods of uncertainty quantification and propagation were reviewed and summarized, thus identifying the tools available for developing the desired framework for the inclusion of uncertainty quantification in damage detection techniques. Frameworks for the application of non-model-based vibration-based damage detection techniques, incorporating uncertainties, were developed. The frameworks consisted of data collection, feature extraction, feature discrimination and damage diagnosis with a quantitative measure of confidence in the diagnosis. The adopted feature extraction technique consisted of an algorithm that compared the residual errors of an ARX model fitted to a reference dynamic system with the residual errors of the same model fitted to a potentially damaged dynamic system. The damage-sensitive feature was chosen as the ratio between the standard deviation of the residual errors for the ARX model applied to the reference data and the standard deviation of the residual errors when the same model is fitted to data from an unknown structural state. Two feature discrimination techniques were investigated, namely a probability density approach and an outlier analysis approach. These feature discrimination techniques were statistical models that involved Monte Carlo simulations for uncertainty quantification. The frameworks for the application of non-model-based vibration-based damage detection techniques, incorporating uncertainties, were tested using experimental data. The test structures were steel-reinforced concrete beams. Damage was gradually introduced into the beams by the accelerated corrosion of their steel reinforcement. Vibration tests were conducted on the beams at various degrees of corrosion and different core temperatures of the beams. The results of the application of the proposed damage detection frameworks to the test data revealed a high correlation between the degree of corrosion and the probability that the structure was damaged. The chosen damagesensitive feature proved to be insensitive to changes in the core temperature of the beams. It was concluded that the ARX damage detection technique was capable of detecting the damage brought about by corrosion of the longitudinal steel reinforcement in concrete beams. By including uncertainty quantification, the damage detection frameworks proposed in this thesis were able to output quantitative measures of the certainty in their diagnoses. The frameworks accounted for instrumentation errors and errors due to changes in temperature. They can however be generalised to account for other environmental and operational effects by developing a comprehensive reference database of the adopted damage sensitive features. Civil infrastructure suffers from subtle and complex forms of damage, such as the deterioration brought about by steel reinforcement corrosion in concrete structures and due to the problems brought about by uncontrollable environmental and operational conditions. The frameworks developed in this thesis for the detection of damage address these complexities and are therefore applicable to the structural health monitoring of civil infrastructure.
Open Access
Assessing the economic value of using structural health monitoring systems on South African bridges by studying the Ermelo-Richards Bay Freight Railway line
(2017) Mmekwa, Keamogetswe Antoinette; Moyo, Pilate
There is a need for appropriate tools and techniques to undertake the vast task of sound repair, maintenance and rehabilitation of concrete infrastructure which is deemed to be deteriorating at unacceptable rates. Low economic growth predictions lead to limited budgets and a deferring of maintenance. The use of technology could be used to extend the useful life of concrete structures. Structural Health Monitoring Systems (SHMS) can be used to monitor structural integrity and the information obtained from these systems can be used in detecting overloading (on bridges for instance) and to alert asset managers of any due maintenance. Büyüköztürk (2007) argues that conventional methods of inspecting the condition of bridges are generally subjective and that this does not give a true reflection of the state of the structure. The objective of this study is to determine the economic value of using SHMS on South African bridges as opposed to conventional bridge inspection methods. The detailed study was conducted on railway bridges on the Transnet Freight Rail (TFR) Ermelo - Richards Bay coal route to assess the contribution that a commodities line such as this one makes to the South African economy. This study makes use of data from Transnet to establish economic value. It is recommended that the results and recommendations be used for a more detailed study into the value of SHMS to be extrapolated for use on other bridges (e.g. road bridges).
Open Access
Assessment of local water distribution infrastructure management and maintenance challenges
(2018) Mothetho, Motlatsi; Moyo, Pilate
Central to the South African government’s vision of providing services to all is on-going maintenance of public infrastructure. Since 1994 the government focused on addressing backlogs in the provision of water services through new infrastructure investment; however it failed to make sufficient investment in the maintenance and renewal of this infrastructure (SAICE, 2006). Older infrastructure is not being renewed or refurbished as required and planned preventative maintenance on new infrastructure is inadequate (SAICE, 2006). This has been generally attributed to poor management strategies that are exacerbated by lack of skills in water services utilities and low levels of funding provisions (Mescht & Jaarsveld, 2012; FFC, 2013). The continuing poor maintenance of water distribution networks has contributed to high leakage rates in South Africa (FFC, 2013; DBSA, 2012). To address challenges of maintenance of water distribution infrastructure a regulatory framework to guide municipalities is critical. The government approved the National Infrastructure Management Strategy (NIMS) in 2006 to support simultaneous infrastructure investment and maintenance (CIDB, 2008). One of the key thrusts of the strategy is the strengthening of the regulatory framework that governs planning and budgeting for maintenance. The literature survey of this study found that initiatives associated with the NIMS were very slow in gaining traction. The study reviews water services infrastructure management frameworks that are based on present legislative instruments and standards for two study areas; City of Capetown and City of Johannesburg. Challenges associated with effective management of water distribution infrastructure are assessed based on established infrastructure management policies, strategies and asset management plans for each entity. For each study area leakage control strategies are the key maintenance strategy outputs associated with the implementation of the management frameworks; therefore the study reviewed sector plans and annual reports to assess challenges associated with carrying out effective maintenance. The findings of the study show a correlation between the adoption of maintenance management strategies and the improvement of the performance of water distribution networks for both Cape Town and Johannesburg. The strategies are driven at the highest level of decision making in the municipalities as budgeting requirements are supported by the Integrated Development Plans of each study area. The maintenance allocations however remain below the international benchmark to enable the municipal entities to carry out satisfactory maintenance of their distribution infrastructure.
Open Access
Behavior of RC beams patch repaired and strengthened with FRP composites : a numerical study
(2014) Mundeli, Salathiel; Moyo, Pilate
Reinforced concrete (RC) beams get deteriorated and become deficient mainly due to corrosion of steel reinforcements, poor maintenance and design, earthquakes and aging. Patch repair and structural strengthening using fiber reinforced polymers (FRP) have been increasingly adopted all over the world as an economical solution to upgrade the load carrying capacity of such beams. However, the failure modes of such repaired and strengthened RC beams are governed by brittle and sudden premature debonding which involves separation of external reinforcement; i.e. FRP and RC beam. Different researchers have used different approaches including experimental, analytical and numerical to investigate the behavior of patch repaired and FRP strengthened RC beams. It is noteworthy that there are no such numerical studies that investigated the effect of patch repair. In this study, a numerical investigation was carried out using the commercial finite elements analysis software ABAQUS with the aim of investigating the overall behavior of RC beams patch repaired and strengthened with FRP plates including the failure mechanisms.
Open Access
Behaviour of FRP strengthened RC Beams with concrete patch repairs subjected to impact loading
(2017) Habimana, Philbert; Moyo, Pilate; Matongo, Kabani
The acceptable performance levels and serviceability of reinforced concrete (RC) structures are always the priorities of asset managers, engineers and researchers in any country. RC structures in service may fail to adequately perform due to changes in functionality, corrosion attack on the reinforcing bars, lack of proper and timely maintenance, and loading and standards updating, among other reasons. Impact loading is an extreme form of loading that can damage RC structures such as bridges, interchanges and flyovers during their life span. The repair and strengthening of deteriorating RC structures in service, by using concrete patch repairs and fibre reinforced polymers (FRP) respectively, has attracted a lot of attention from researchers and engineers. Nevertheless, these rehabilitated RC structures in service are susceptible to future deterioration with adverse effects. Inspection and periodic maintenance of strategic RC structures in use are essential for their safe serviceability and to avoid or mitigate economic loss. This experimental study was conducted on fifteen RC beams with the size of 155 x 254 x 2000 mm, in order to study their behaviour under impact loading testing. Twelve out of these fifteen RC beams were intentionally damaged by uniformly reducing 14 % of the cross-section of their main reinforcing bars, as this simulated the effects of corrosion on RC structures. The drop test, with the impactor applied from varying drop heights, was selected from the different types of impact loading testing methods and used in this research. Each tested RC beam was subjected to eight consecutive drop tests. During this experimental study 120 tests were performed and, from these tests, dynamic responses were recorded for analysis. Two transducers, a load cell and high-speed camera (HSC), were used to record data. In general the captured and stored dynamic responses led to the extraction of contact forces and deflections results. In addition, the HSC recorded video footage of the impact scenarios of the RC beams. The combined use of software such as Photron FASTCAM Analysis (PFA) and Matlab R2014a enables the acquisition of deflection results and, on the basis of these results, residual deflection
Open Access
The behaviour of patch repaired & RFP strengthened reinforced concrete beams: an experimental investigation
(2014) Dladla, Thabiso; Moyo, Pilate; Beushausen, Hans-Dieter
The aim of this study was to investigate the effect of extent of simulated uniform corrosion on reinforced concrete beams that have been patch repaired and strengthened in flexure. The most widespread cause of deterioration of reinforced concrete structures is due to corrosion of reinforcement (Bentur et al., 1997). According to Taljsten (2006) some reasons for the corrosion of structures can be attributed to incorrect design and poor construction methods, just to mention a few. Hollaway (2006) suggests that corrosion reduces the area of steel at the corrosion sites therefore decreasing load carrying capacity. The reduced steel area due to corrosion of steel in reinforced concrete beams is considered critical when it can lead to premature failure of a structure. Elgarf (1999) argues that large reductions in flexural capacity, strength and rigidity, which render a beam inadequate for serviceability loads are most likely to occur when localized pitting has extended to many sites resulting in extensive and relatively uniform levels of corrosion.
Open Access
Calibration of an arch dam model by the finite element method using ambient vibration testing
(2012) Makha, Ramonate; Moyo, Pilate; Beushausen, Hans-Dieter
The finite element model of an arch dam was calibrated for dynamic behaviour using the measured natural frequencies and mode shapes as benchmarks. The properties were extracted from the structure using ambient vibration testing techniques. Besides the geometry and general material properties of the dam wall concrete and foundation rock, the measured frequencies and mode shapes depend on the conditions at the dam site namely water level, temperature and the interactions between the several components of the dam. This study, however investigated the effects of the water level and to some extent, the effect of dissimilar foundation abutment material properties on the natural frequencies of the dam. A dam is continuously in harmonic motion due to some environmental factors such as wind. Either due to this movement of the dam itself or the internal movement of the reservoir water, a dynamic interaction occurs between the water and the dam wall where the movement of the one medium affects the other. A study conducted in the early twentieth century deduced that it is only part of the reservoir water that can be assumed to interact with the dam. It is from the same study that the Westergaard added mass concept was born which says that the interacting water mass can simply be added to the dam wall mass, a procedure from which the extraction of the dynamic properties can ensue as normal. This added mass formulation was derived on a basis of some assumptions which include a rigid and vertical dam wall and the incompressible water body. The added mass concept was extended to account for flexibility and curvature of the upstream dam wall in more recent studies. The extended version of the Westergaard method is normally referred to as the generalized Westergaard method. The original Westergaard added mass formulation was used to account for the dam wall- water interaction in the double curved Roode Elsberg dam model. This proved to be problematic as this dam is highly asymmetrical and has diverging reservoir walls, the characteristics of which are not catered for in the original Westergaard added mass method. The combined effect of using the original Westergaard method and these deficiencies in the formulation resulted in the model's natural frequencies being lower than the field ones, for the same ambient conditions. On the basis of literature, a factor of 0.8 on the added masses was applied on all the original Westergaard added masses to account for the effect of the diverging reservoir walls. The remaining masses were then reduced until a good correlation of the field frequencies and model frequencies was achieved. This was done to account for the effects of the flexibility of the dam and the curved upstream dam wall. All in all, a factor of 0.25 on the masses calculated using the original Westergaard added masses was applied to account for all the above-mentioned effects. This factor compares favourably with literature even though in literature it is rarely mentioned what effects are being accounted for when this factor is introduced. This work hence raises awareness about the shortcomings of the Westergaard method when used for model calibration and how those shortcomings can be accounted for. In summary, these shortcomings are brought about by assuming a prismatic and infinite reservoir, while in reality this is not always the case. It appears that these shortcomings affect the results of the added mass approach when used as a tool to represent the dam-water dynamic interaction in arch dams.
Open Access
Characterisation of bridge-track interaction of a multi-span viaduct subjected to heavy haul loading
(2022) Mupwedi, Emilia Joyce; Moyo, Pilate; Matongo, Kabani
In many countries, railway transportation has been the primary mode of transportation, and engineers have been pushing boundaries to increase productivity and reduce costs for decades. The rail is the most important component of the railway infrastructure because it serves as the driving surface, direction guidance, and force transmission. Continuously Welded Rail (CWR), which is defined as rails that have been welded together, are now used in modern railways. When the rail is built on a bridge, the bridge's and rolling stock's behavior adds additional forces to CWR rails. As a result of the coupling effects of tracks and deformed superstructures, additional rail stresses are superimposed on other forces. These extra stresses are caused primarily by the longitudinal elongation of the superstructure as a result of temperature, braking, traction, and deck movement. The interaction of these forces between the rail and the bridge is therefore known as Track-Bridge-Interaction (TBI). Therefore, the horizontal forces must be precisely managed to prevent rail failure. This research presents a characterization of TBI for heavy haul railways and management of longitudinal forces to minimize the possibility of failure due to superimposed longitudinal forces. The Olifants River Viaduct (ORV), a 1 km long bridge with CWR and two continuous spans of 11 spans at each end and a drop span in the middle, was used as a case study in the research. The ORV has been equipped with monitoring systems to help manage the tracks. Thus, data from these systems were used to categorize the interaction forces. The research focused on categorizing the trains crossing the ORV into six (A-F) categories; the categorization was based on the train length and the commodities being hauled. The research also studied the speed variations of each train crossing the bridge. The speeds were analyzed using python and statistical tools in excel. Lastly, the impact of crossing trains on rail forces, rail temperature, ambient temperature, and deck movement was analyzed using python and statistical tools in excel. The study showed that the most frequent train to cross the bridge are category D trains with six locomotives and 342 wagons, while the train speed is dependent on the train length and the commodities hauled. Thus, the short trains in categories (A, B, and E) cross the bridge at higher constant speeds while the long trains in categories (C and D) cross the bridge at reduced speeds than the short trains but exhibit speed variations and sometimes cross the bridge at speeds exceeding the 50 km/h limit. Therefore, higher dynamic forces should be expected from short trains crossing the bridge at high constant speeds, but no additional forces should be expected on the rails from these trains as they experience no speed variations. At the same time, the long trains experience significant speed variations of both acceleration and decelerations, which imposes additional forces on the rails due to traction and braking. The imposed forces on the rails are predominantly due to crossing trains with significant speed variations of acceleration and deceleration, the acceleration change ranges from 5-30 km/h, and deceleration change ranges from 5-20 km/h. The braking and acceleration effect causes a change in the rail forces, rail temperature, and deck deflection, which in turn imposes additional forces on the rails. Therefore, high speed variation induces additional longitudinal forces on the rails. However, the imposed acceleration forces are higher than the braking forces, but the braking imposed forces are the most critical one as they tend to cause an increase in the tensile and compression forces when the forces are at their peaks, and there is a train present on the bridge, while acceleration causes a decrease in the rail forces at those times. The deck movement forces imposed on the rails were predominantly due to ambient temperature, which showed a positive linear relationship between the two. The deck expands with increasing ambient temperature and contracts with a decrease in ambient temperature. In contrast, the compression forces were within the given limits of 1100 kN, while the tension forces exceeded the rail force limit of 1400 kN when the rail temperature was between 0 − 20℃, and the deck deflection above 83 mm in the negative direction, and a present train on the bridge, making the rail more susceptible to failure during winter
Open Access
Comparative study on the behaviour of concrete reinforced with FRP and steel singly reinforced beams
(2022) Bakheit, Paul Charles Saki; Moyo, Pilate
Maintenance and repair of concrete infrastructure account for a large amount of financial resources. This is due to the deterioration of reinforced concrete through the corrosion of steel reinforcement. The depassivation of the steel creates an environment prone to chloride ingress and carbonation. This leads to steel corrosion and internal stresses which further exasperates the problem. Fiber Reinforced polymers can be an alternative to steel reinforced because it is considered to be chemically inert, high tensile-strength-to-weight ratio, high tensile-modulus-to-weight ratio, and high fatigue strength. Since it is chemically inert it will not undergo corrosion. Hence FRP will perform better in corrosive areas. The study aimed to investigate the behaviour of concrete reinforced with FRP. Its failure modes, crack width and patterns, and deformation of the reinforcing material. The experimental work consisted of testing a total of 5 simply supported beams, each tested under four-point loading until failure. One beam consisted of steel reinforcement, three beams consisted of the Carbon Fiber Reinforced Polymer (CFRP) reinforcement, the other beam had CFRP with an experimental anchorage. All the beams had the same cross section of 160mm x 250mm x 3040mm and designed to the same moment capacity. The FRP reinforced concrete beams and steel reinforced concrete beam failed through concrete crushing. This was expected for the FRP reinforced concrete beams but not the steel reinforced concrete beam. The FRP reinforced concrete beams exhibited extensive cracking in comparison to the steel reinforced concrete beams and significantly more deflection before failure. The maximum deflection produced by the FRP reinforced concrete beams was around 20mm more than the steel reinforced concrete beam. The large difference in values is because of the weak bonding between the FRPs and the surrounding concrete, also, the lower stiffness of the CFRCs in comparison to the SRC. The strain of the reinforcement inside the concrete showed that permanent deformation did not occur in the CFRP or steel. This was further supported by visually inspecting the reinforcement after the beams were loaded to failure. The maximum crack widths measured on the FRP reinforced concrete beams and the steel reinforced concrete beam was 2.5mm and 0.5mm, respectively. Horizontal cracks were observed at the depth of the FRP reinforcement which is attributed to a week bond between the FRP and concrete.
Open Access
A critical review of the current design guidelines for footbridges: with emphasis on the design for jogging forces
(2013) Townshend, Tessa; Moyo, Pilate
New materials and the modern trend of designing slender, lightweight footbridges with longer spans have resulted in bridges with lower inherent structural damping and natural frequency in the range of pedestrian-induced dynamic loading from activities such as walking, running and jogging. This has lead to a number of recent footbridges suffering from excessive vertical or lateral vibrations necessitating retrofitting, usually at high additional costs ((Butz, 2008; Sun and Yuan, 2008)). One of the most important aspects of the design of modern footbridges for dynamic forces is the development of a reliable model of both the structure and the human induced loads applied to it. However, the current design codes and design procedures are either outdated or very limited with regards to the type of footbridge and pedestrian loading they consider. There is a “lack of commonly accepted models for walking, running and jumping” loads (Occhiuzzi et al., 2008), and the majority of the research that has been done and human-induced loading models that are available only look at the vertical component of the walking load. Research done by Keller et al. (1996) found that slow jogging can cause vertical forces up to 1.6 times greater than those caused by walking at the same speed or running at higher speeds. However, of all the codes of practice reviewed in this thesis only the Sétra Guide gives an indication of how to model the vertical component of a person running using either the Semi-Sinusoidal method or the Fourier method. Occhiuzzi et al. (2008) proposed a third method, the Analytical method, for modelling the vertical component of the jogging load. This thesis extends these three jogging load models to include the lateral and longitudinal component of the jogging force and to account for multiple people. The vertical forces obtained using these jogging load models were compared with those measured by a person running at various speeds on an instrumented treadmill. The comparison showed that for a contact period of 75 of the running period all three jogging load models give a reasonable approximation of the actual vertical jogging forces.
Open Access
Dynamic modelling of arch dams in the ambient state
(2014) Vezi, Mfundo; Moyo, Pilate
To date, dam failures have resulted in significant losses in the commercial economy and in human life. Raising awareness in the field of structural health monitoring is neccesary to develop contemporary structural analysis and monitoring methods to ensure the integrity of these structures. Hence, the research aims at developing an analytical formulation that can be used in the dynamic modelling of arch dams, for structural health monitoring purposes. A hypothetical arch dam model was created to investigate the influence of a reservoir’s orientation and geometry on the dam’s dynamic properties, and the discrepancies between the Westergaard and Fluid Structure Interaction (FSI) analysis methods. The two analysis methods were then utilized to develop an updatable finite element model, in a case study pertaining to the 72m high Roode Elsberg concrete-arch dam. Thereafter, ambient vibration tests were conducted on the Roode Elsberg dam to measure its dynamic properties and validate the finite element models. The excitation on the dam was provided by the wind and the reservoir flowing over the spillway. Vibrations of the dam were measured and recorded by accelerometers placed on the cantilevered arch blocks. Finally, the Frequency Domain Decomposition (FDD) algorithm was used to analyse the acquired data and identify the natural frequencies of the dam.