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- Volume 55, Issue 3, 2013
Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese - Volume 55, Issue 3, 2013
Volumes & issues
Volume 55, Issue 3, 2013
Modelling of the fracture process zone to improve the crack propagation criterion in concrete : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 2 –9 (2013)More Less
Modelling of tension cracking in quasi-brittle materials, such as concrete, plays an important role in improving the reliability and load-bearing capacity of the structure. In this study fracture mechanics is used to model tensile cracks with strain softening behaviour in concrete. An interface element, which considers the softening zone in terms of a stiffness matrix, is applied to simulate the cohesive zone model (CZM) as well as the stress-free region. To estimate the nodal force caused by shear stress, a new constitutive model is proposed based on previous experimental results. An improved Griffith-type energy approach is employed such that it can model the propagation of a discrete crack based on an accurate stiffness matrix. This model improves the analysis of discrete crack propagation and is more accurate than other existing models. To validate the model, three benchmark beams are simulated, namely a plain concrete beam with initial notch, a notched reinforced concrete beam and a beam with simple supports. The simulation results are admissible compared to the results reported recently in the literature.
Author S. BouharounSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 10 –17 (2013)More Less
This study seeks to understand the mechanisms that occur at the concrete/formwork interface when fresh concrete is poured into the formwork. Four ordinary concretes with paste volumes of 28, 30, 32 and 34%, and four concretes at different dosages of superplasticiser were studied. The rheological properties of fresh concrete were measured using the ICAR rheometer. A detailed study of the friction at the concrete/formwork interfaces was then performed using a plane/plane tribometer. The results highlight the role of the volume of paste and the superplasticiser dosage on the tribological behaviour of fresh concrete. The evolution of friction stress is linear (Coulomb's Law) regardless of the formulations tested in this study. In addition, these friction stresses at the concrete/formwork interface depend on the paste volume and the dosage of superplasticiser in the concrete.
The integration of railway asset management information to ensure maintenance effectiveness : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 18 –29 (2013)More Less
The challenge to sustain railways depends on the ability to optimise the utilisation of the asset base. It demands the establishment of a continuous asset improvement process which requires maintenance personnel continuously to improve their understanding of the infrastructure's performance and the relevance of the configuration to this performance. A successful asset management process incorporates these elements and results in the optimisation of the infrastructure life cycle by extending useful life while minimising the operational interference, thereby increasing capacity. The effect of condition-based maintenance is illustrated, signifying how this maintenance strategy increases maintenance effectiveness (doing the right things at the right places), thus decreasing maintenance costs. It is apparent that with an increase in maintenance effectiveness, less time will be spent on maintaining assets, resulting in an increase of asset availability adding to the business objectives, allowing for an increase in operations. The paper demonstrates how condition-based and condition performance-based maintenance can enable railway organisations to save up to roughly 30% on their maintenance costs, while increasing capacity to approximately the same value.
Laser-based assessment of road aggregate particle shape and texture properties with the aim of deriving comparative models : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 30 –35 (2013)More Less
Research was undertaken using an innovative three-dimensional (3D) laser scanning tool to study the shape and texture characteristics of road aggregate particles. Aggregate materials used for road construction, including G1 crushed rocks of different geological origins, recycled aggregate and alluvial gravel (not used as aggregate) were used for this study. Representative samples were scanned using the laser system to collect 3D aggregate data for analyses and, subsequently, develop comparative models. The objective was to arrange the aggregate particles in a sequence based on their surface texture. Two models were proposed and key aspects evaluated against each other. Ultimately, one model was selected that may be improved and used for further research. The study found that, while it is possible to use the 3D aggregate scan data to produce comparative models, distinguishing between particle shape and texture proved a daunting task. It was also concluded that particle elongation must be considered as a major influencing factor.
Author J.G. NdirituSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 36 –46 (2013)More Less
A framework for incorporating areal rainfall uncertainties into catchment modelling is presented and demonstrated through the daily streamflow simulation of the Mooi River catchment using the Australian Water Balance Model (AWBM). The framework is an extension of the typical hybrid manual-automatic model calibration-validation in which uncertainties are imposed as perturbations (disturbances) on the rainfall time series. The differences in areal rainfall obtained from different rain-gauge densities are used to generate the perturbations, and their variability is found to reduce as areal rainfall magnitude increases. The applied probability distributions of perturbations are therefore obtained for specified ranges of rainfall magnitude. The effect of incorporating uncertainties is assessed by finding out the impact of imposing perturbations on the validation performance of the rainfall-runoff modelling. This is done by carrying out 100 rainfall-runoff calibration-validation runs with perturbed and with unperturbed rainfall and comparing the 100 generated validation period runoff time series with the observed (historical) runoff series. With perturbations applied, the 5-95 percentile bounds from the resulting 100 streamflow ensembles contain 52% of the observed time series for the validation period. Without perturbations, only 25% of the observed flows fall within the bounds. The framework has the potential for practical use, but this would require a more rigorous identification of appropriate distributions of the perturbations.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 47 –56 (2013)More Less
This paper presents a study which investigates the feasibility of a concept to reduce the size of shear wall foundations for earthquake forces in regions of moderate seismicity. The approach is to allow rocking of the shear wall foundation and to include the contribution of a shear wall and reinforced concrete frame to assist as a lateral force-resisting system. A simplified multi degree-of-freedom model with non-linear material properties was used to investigate this lateral-force-resisting system subjected to base accelerations from recorded ground motions. An example building was studied with the shear wall foundation designed to resist 0%, 20%, 40%, 60%, 80% and 100% of the design overturning moment from the seismic event. Non-linear time-history analyses were performed with input from seven scaled ground-motion records. It is shown that the concept warrants more detailed studies and that a significantly reduced shear wall foundation size is possible without failure of the lateral force-resisting system.
A field study of the in situ moisture regime during active hydraulic tailings deposition : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 57 –68 (2013)More Less
A common method of managing tailings in semi-arid environments is to self-impound the waste as it dries. This paper presents the results of research carried out on the in situ drying behaviour of platinum tailings. Monitoring of gravimetric water contents following deposition and the quantity of water released during sedimentation indicated that a tailings beach acts as a gravitational thickener. After sedimentation, water contents were observed to decrease at a rate correlated with reference evapotranspiration, reaching a steady-state condition. Field capacity values determined from laboratory experimentation and predictive modelling correlated closely with this steady state. Further near-surface moisture reduction is inhibited due to the relative abundance of moisture rising from below to replenish deficits. Liquidity indices demonstrated that, as a result, only the head of the beach dries to a steady-state condition which provides strength to impound the tailings.
The use of ground-penetrating radar to develop a track substructure characterisation model : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 69 –78 (2013)More Less
The characterisation of track substructure is an integral component of track maintenance and rehabilitation. Traditionally this is done by determining the geotechnical properties of the discrete track formation layers by excavating test pits and sampling the substructure layers. The development of geophysical investigation techniques such as ground-penetrating radar (GPR) allows continuous assessment of the condition of the track substructure. The research described in this paper uses GPR to develop a track substructure characterisation model and provides classifications for both the ballast and formation layers. The ballast and formation were classified into four classes, namely very good, good, moderate and poor. When applying the model to two sections of track (with generally good and poor quality) 82% and 100% of classes had only one class difference compared to the traditional characterisation tests for the formation and ballast layers respectively. The GPR track substructure characterisation model that was developed therefore compares well with traditional characterisation techniques and will result in significant cost and time reduction. The characterisation of the track using GPR provides a continuous classification and enhances the accuracy of the data on which a maintenance engineer can base decisions.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 79 –85 (2013)More Less
A study was conducted to identify the causes of delay in completing road construction projects in Malawi. A literature review was done which yielded 72 typical causes of delay, and a questionnaire was sent to client, contractor and consultant representatives in Malawi. The results were analysed using the Relative Importance Index (RII) and Spearman's Rank Correlation Coefficients, which indicated that the top ten causes of delay in Malawi are: shortage of fuel, insufficient contractor cash-flow, shortage of foreign currency for importation of materials and equipment, slow payment procedures adopted by the client in making progress payments, insufficient equipment, delay in relocating utilities, shortage of construction materials, delay in paying compensation to land owners, shortage of technical personnel, and delay in site mobilisation. The causes of delay are significant and should be given attention by client organisations, consultants and contractors to enable the timely completion of projects in future. It should also be noted that most of the causes of delay are not unique to Malawi, and have been observed in other southern African countries such as South Africa, Botswana and Swaziland. Recommendations are made to prevent similar causes of delay in future.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 86 –95 (2013)More Less
Different types of loading due to traffic may act on bridges. The focus of this paper is on normal traffic loading according to the South African specification TMH7 (Committee of State Road Authorities 1988). TMH7 represents the code of practice for the design of highway bridges and culverts in South Africa. The aim of the paper is to provide additional insight into the effect of normal traffic loading patterns on the flexural analysis of skew bridges. This is necessary since TMH7 does not explicitly specify application patterns for normal traffic loading. Only the intensity of normal traffic loading is specified and it should be applied to yield the most adverse effects. In this paper a set of so-called "standard application patterns" are investigated and compared with the application patterns that yield the most adverse effects. The patterns are compared for flexural effects in selected design regions of the bridge deck. Numerical experiments were performed and are presented for a typical single-span bridge deck. The results of the numerical experiments are compared as the angle of skew of the bridge deck increases.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 96 –102 (2013)More Less
Structural optimisation has been the subject of intense research for many years. However, the engineering profession seems to be oblivious to the important role of natural structures. Natural structures are shapes created by nature and possess geometrical properties that are resistant to environmental conditions. Engineers of antiquity recognised the importance of natural shapes and incorporated these designs into many political and religious structures. Many of these structures are catenary or funicular in form, and are characterised by a pure compressive stress distribution - an essential requirement for masonry structures. Catenary shapes are robust and economical. These principles are applied to the design and construction of a prototype shell - a domed vault - intended as an alternative structure for low-cost housing. Design principles are described, as well as the construction process.
Exploring the flow number (FN) index as a means to characterise the HMA permanent deformation response under FN testing : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 103 –112 (2013)More Less
Permanent deformation (PD), or rutting, is one of the common distresses occurring in hot-mix asphalt (HMA) pavements. As part of the HMA mix- and structural-design processes to optimise field performance, various laboratory tests, including the Hamburg wheel tracking test (HWTT) and flow number (FN), have been developed to characterise the HMA mix rutting resistance potential. With this background, this study was conducted to explore the potential of routinely using the FN test to characterise the PD response of Texas HMA mixes as a supplement to the HWTT. Towards this goal, a new PD parameter - the FN index - was developed to differentiate and screen the HMA mixes. The research methodology incorporated a two-phase approach, namely: (1) laboratory testing and (2) field correlations. Overall, the findings indicated that the FN index, computed from the FN test data, has the ability to statistically differentiate the HMA mixes evaluated, as well as promising potential to supplement the HWTT for routine HMA mix-design and screening. In addition, the FN test has a practically reasonable test time (about three hours per specimen) and is cost-effective. However, the inability to readily test thin field cores and the need for field validation with long-term performance data remain some of the key challenges to be addressed with the FN test method.