OpenUCT :: Browsing by Author "Alexander, Mark Gavin"

Browsing by Author "Alexander, Mark Gavin"

Now showing 1 - 20 of 47

Open Access
Acid resistance of sewer pipe concrete
(2012) Motsieloa, Ntseuoa; Alexander, Mark Gavin
The corrosion of sewer pipe concrete is a global problem, especially in warm climates in which biogenically generated sulphuric acid attacks the concrete and lowers its structural integrity. The Virginia Experimental Sewer is a ‘live sewer’ in South Africa for investigating acid resistance of concrete made with different cement/aggregate combinations...
Open Access
Acid resistance of sewer pipe concrete
(2007) Fourie, Clyde William; Alexander, Mark Gavin
A hydrochloric acid solution laboratory test was developed to assess the resistance of concrete used in the manufacture of pipes to sulphuric acid environments encountered in sewers. A hydrochloric acid solution (pH of 1,00) simulates the attack on concrete sewer pipes at the average daily sewage level region and in severely corrosive sulphuric acid environments (pH of less than 1,00) commonly encountered in South African sewers. Calcium sulphate, which precipitates in a sulphuric acid solution and masks further attack, is eliminated simulating the constant dissolution of the precipitate at the average daily sewage level region and loosely attached precipitate in the severely corrosive sewer environments. A straightforward procedure is used to determine the acid resistance of dolomite aggregate concrete used in the manufacture of sewer pipes for exposure to sulphuric acid sewer environments. Acid resistance and neutralisation capacity of the concrete are measured as functions of mass loss and hydrogen ion consumption with time (over a period of approximately 96 hours or less). A procedure to determine the acid resistance of siliceous aggregate concrete requires a longer test period (approximately 200 hours or more). A concrete containing calcium aluminate cement and a synthetic alag™ aggregate made from the calcium aluminate cement clinker gave the best resistance to the hydrochloric acid solution (pH of 1,00) and a severely corrosive biologically-induced sulphuric acid environment (pH of less than 1,00). A similar concrete containing dolomite aggregate gave poorer resistance to the respective solutions. A dolomite aggregate concrete containing calcium aluminate cement gave better resistance in the biologically-induced sulphuric acid environment than a similar concrete containing ordinary portland cement but both concretes were of similar resistance in the hydrochloric acid solution. A poor quality dolomite stone (low strength and high porosity), which fractures during pipe manufacture gave poorer resistance in the hydrochloric solution than a concrete containing a sound dolomite stone but better resistance in the biologically-induced sulphuric acid environment. Concrete containing siliceous aggregate gave the poorest resistance to both the acidic environments. Assessing acid resistance using the hydrochloric acid solution test method illustrates the manner in which the physical and chemical properties influences the attack and dissolution of concrete in an acidic environment, which is crucial in the development of a sewer pipe concrete to resist and neutralise a biologically-induced sulphuric acid sewer environment.
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 influence of crack width on the durability potential of cracked concrete using the durability index approach
(2015) Kanjee, Janina Prakash; Beushausen, Hans-Dieter; Alexander, Mark Gavin; Otieno, Mike Benjamin
Durability is a major concern for reinforced concrete (RC) structures. RC structures both in service and new, are subject to cracking. Irrespective of the cause of the cracking, cracks can increase the rate of penetration of aggressive species into concrete and modify the transport properties. Consequently, the service life of corrosionaffected RC structures may be drastically reduced in the presence of cracks. However, no modifications are made for the influence of cracking on the penetration of aggressive species into concrete when analysing durability test results or making service life predictions, even through concrete is very often in a cracked state. This study focused on the influence of cracks on the ingress of aggressive species (carbon dioxide and chlorides) into cracked concrete in comparison to uncracked concrete. The aim was to establish any correlations between the transport properties in uncracked and cracked concrete. Furthermore, in a broader context, the aim was to assess to what extent the modified cracked concrete parameters used in service life predictions affect the service life outputs, when compared with service life outputs obtained using the uncracked concrete parameters. Six concretes mixes were investigated comprising two water/binder (w/b) ratios (0.40 and 0.55) and three binder types (100% CEM I 52.5N (PC), 70/30 PC/FA and 50/50 PC/GGBS). 100 x 100 x 500 mm beams were cast and cracks were induced after seven days in the mid-span of each beam using three-point loading. Two crack width ranges were investigated; 0.1-0.4 mm (wcr1) and 0.5-0.8 mm (wcr2). The central section of the beam that contained the crack was sawn from the rest of the beam and used for either accelerated carbonation or bulk chloride diffusion testing. Cores were drilled from the outer sections of the beam and used as specimens for the Durability Index tests. The cracked specimens were monitored for carbonation (accelerated carbonation) and chloride ingress (bulk diffusion), while the uncracked ones were monitored for durability parameters (OPI, WSI & CCI) after 8 and 16 weeks of exposure. Firstly, it can be concluded that the presence of cracks modifies the transport properties of concrete by promoting rapid increase of ingress of aggressive species (CO₂ & Cl-) into the concrete matrix. It was found that the degree to which the transport properties were modified increased as the crack width increased. This was primarily attributed to the increase in surface area created by the crack, which allowed increased amounts of species (CO₂ & Cl-) to penetrate into the concrete matrix. In the case of carbon dioxide ingress, the presence of cracks significantly increased the rate of carbonation (up to 50 %) in the concrete specimens that contained blended cements PC/FA and PC/GGBS when compared to the PC concrete specimens. However, in the case of chloride ingress the effects of cracks in the PC mix resulted in the highest presence of chlorides (up to 78 %) in the concrete specimens in comparison to the chlorides present in the PC/FA and PC/GGBS concrete specimens. Secondly, when the sound (DI) and cracked durability parameters (carbonation and diffusion coefficient) where used in carbonation and chloride ingress service predictions, it was found that the DI service life prediction outputs were more conservative in relation to service life outputs from the durability parameters obtained from cracked concrete specimens. These results highlighted the degree of influence which the presence of cracks had on modifying transport properties in concrete. Furthermore, it also highlights the impact of the presence of cracks on the service life of RC structures and the prediction of long-term carbonation- and chloride- induced corrosion. Due to the significant influence that cracks have on modifying the transport properties of concrete, the results show that some reduction factors need to be applied to the results from the DI approach to reflect a more realistic durability potential of the concrete. Further research into understanding how other crack parameters (crack depth, frequency etc.) modify transport properties in concrete will lead to a more accurate insight into dealing with and accounting for the presence of cracks in RC structures.
Open Access
Carbonation of concrete bridge structures in three South African localities
(2004) Yam, Wood Kuen; Alexander, Mark Gavin; Mackechnie, James R
The rate of carbonation for the localities of the Cape Peninsula, Durban (i.e. Durban - KwaZulu Natal South Coast) and Johannesburg (i.e. the motorway system and between Heidelberg Road and Geldenhuis interchanges on the N3 freeway) were studied in order to derive carbonation prediction models for each of these localities. The derivation of the prediction models was based on field carbonation data measured from approximately 30 in-service bridges in each locality. One of the uses of the derived models was to allow the preparation of maintenance plans so as to avoid carbonation-induced corrosion for structures in these localities. Since the rate of carbonation depends strongly on material and environmental factors, the carbonation data from each locality were analysed separately on the grounds that these localities have different climatic conditions. The data within each locality represent different material and exposure conditions, and the data were therefore grouped according to the concrete strength grade (as a measure of concrete quality) and exposure conditions, prior to statistical analysis. Based on the method of least squares, as well as integration of the understanding of the process of carbonation and knowledge of climatic conditions of each locality, carbonation prediction models for a variety of concretes for each locality were derived. Results show that bridge structures in the Johannesburg locality have the highest carbonation rate due to the relatively dry environment throughout the year. Bridges in Durban locality exhibit a lower carbonation rate than Johannesburg bridges, but higher than Cape Peninsula bridges owing to shorter rainfall duration and higher temperature. In addition, the carbonation rates of both exposed and sheltered elements with similar concrete strength grades for bridges in Durban are very similar, i.e. exposure condition has little influence on carbonation rate for these elements. The same is true for bridges in the Johannesburg locality. It is surmised that short precipitation times and high relative humidity in Durban locality make the near surface moisture content of exposed and sheltered elements very similar. Likewise, it is surmised that short rainfall duration and low relative humidity in Johannesburg locality result in essentially the same near surface moisture content of concrete elements throughout the exposure time. The data in Durban locality show that old concretes have a slower carbonation rate than modem concretes with the same concrete strength grade. This is likely due to the changes in cement properties over the years, related to the need for fast track development for modem structures. This finding indicates that the prediction models are not suitable for carbonation predictions for future structures (produced by modem cements) as the rates of carbonation will be different. Oxygen Permeability Index (OPI) was investigated in an attempt to predict the rate of carbonation. According to the philosophy and testing procedures for OPI, it is considered that early age OPI may be superior to concrete strength grade for carbonation predictions because of better characterisation of the permeability of (cover) concrete. However, due to the lack of early age OPI information for the data, using OPI as a carbonation prediction tool was not entirely successful. Further research in this regard is worthwhile.
Open Access
Chloride transport through concrete and implications for rapid chloride testing
(2006) Gardner, Thomas J; Alexander, Mark Gavin; Stanish, K
Word processed copy. Includes bibliographical references (leaves 171-182).
Open Access
Composition and microstructure of concrete mixtures subjected to biogenic acid corrosion and their role in corrosion prediction of concrete outfall sewers
(2016) Kiliswa, Moses Wopicho; Alexander, Mark Gavin
Wastewater conveyance and treatment facilities, which include outfall sewers, manholes, and treatment works, are among the key constituents of a country's infrastructure. Most of these facilities are made of concrete due to its low production costs, versatility, inherent strength and durability under most conditions. However, under certain conditions, sewage that is conveyed through outfall sewers becomes septic and hydrogen sulphide (H2S) gas is generated. When this gas is released from the sewage and absorbed onto the moist concrete sewer pipe walls, it is microbially converted by sulphide-oxidising bacteria to sulphuric acid (biogenic H2SO4) which reacts with the acid-soluble components of concrete causing it to corrode. In principle, the biogenic H2SO4 concrete corrosion mechanism entails simultaneous destruction of the calcium hydroxide (CH) in the hydrated cement paste (HCP) and substituting a larger molecule of calcium sulphate into the concrete matrix thus causing pressure and spalling of the adjacent concrete and aggregate particles. In addition, the calcium sulphate precipitates as gypsum which reacts with various aluminates to form secondary ettringite. These mechanisms lead to the loss of stiffness and strength, accompanied by expansion and cracking, and eventually transformation of the affected concrete matrix into a soft and pulpy non-cohesive layer. The biogenic concrete corrosion rate depends, inter alia, on the chemical composition of binders (cement and supplementary cementitious materials (SCMs)) and microstructural characteristics of concrete mixtures used in the manufacture of sewer pipes. The needed properties of concrete mixtures for sewer pipe applications can be determined by biogenic corrosion prediction models, such as the widely used deterministic (mechanistic) Life Factor Method (LFM). The service life of wastewater treatment facilities made of concrete depends on the input parameters in corrosion prediction models. The motivation behind the current study was based on the need to improve the ability to predict the design life of concrete sewers by improving the input parameters in the LFM, which is used in South Africa. The design life of concrete sewers in South Africa has traditionally been 40 years. The main objective of the current study was to characterise the microstructure of both Portland cement (PC) and calcium aluminate cement (CAC) based concrete mixtures that had been subjected to biogenic corrosion mechanisms in an operational sewer environment for approximately 127 months (10½ years); further, based on the understanding of the underlying mechanisms of attack, proposals were made to improve the LFM, for which the corrosion rate-controlling input parameter, referred to as alkalinity (or equivalent CaCO3, as a summation for both binder and aggregate) is based on the characteristics of plain PC-based binder systems. In addition to the main objective above, a parallel study was undertaken to characterise parameters that influence biogenic concrete corrosion rates based on measurements taken in two sewer environments/sites in different geographical locations in South Africa. One of the study sites was the Virginia Experimental Sewer (VES) in Virginia, Free State Province, while the other site was a manhole within the Langa Pump Station in Cape Town, Western Cape Province. The VES consists of 900 mm diameter by 300 mm long concrete pipe samples made from both PC- and CAC-based (plain and blended) binder systems, the top 120° being cut to form 'lids', so that they are removable. The removable 'lids' enable scheduled observations and sample recovery to be undertaken. Moreover, the 'lids' also act as windows through which core samples can be placed in plastic baskets that are hung at certain sections in the sewer headspace, so that they can be accessed for monitoring.
Open Access
Corrosion propagation in cracked and uncracked concrete
(2008) Otieno, Mike; Alexander, Mark Gavin; Beushausen, Hans-Dieter
Thesis (M.Sc. (Civil Engineering))--University of Cape Town, 2008. Includes bibliographical references.
Open Access
Critical review of the South African Concrete Durability Index tests
(2014) Mukadam, Zaahir; Alexander, Mark Gavin; Beushausen, Hans-Dieter
The Durability Index (DI) approach is a performance-based approach used in South Africa for specifying the durability of reinforced concrete structures with regard to service life and concrete quality. The approach comprises three DI tests which are used to measure the durability-related transport processes of permeation, absorption and ionic diffusion. These tests are the oxygen permeability index (OPI), water sorptivity index (WSI), and chloride conductivity index (CCI) tests, respectively. Much work has been done over the past two decades in developing the DI tests to the present stage, where they are being, and have been, used in large projects such as the Gauteng Freeway Improvement Project, which made use of the OPI and WSI tests as a basis for checking the quality of the concrete used in the major road upgrade project. Non-compliance with target values can result in rejection of the material or penalties imposed on the concrete supplier or contractor. This highlights the need for the DI tests to be accurate and reliable so that engineers have confidence in their results. This dissertation presents a critical review of the DI tests. The overall objectives were to critically examine the present DI testing regimes and identify and address areas of concern with the tests in order to make them more reliable. One of the major areas of concern related to specimen preconditioning: the 7-day oven drying regime and specimen saturation techniques have been called into question. Consequently, this work investigated the influence of the drying method and saturation processes on the DI results.
Open Access
Designed for durability in reinforced concrete structures, and associated economics
(1997) Scott, Allan Nye; Alexander, Mark Gavin; Stevens, Alan J
This dissertation outlines a model developed to allow for the comparison of various design options on a life-cycle cost basis for reinforced concrete structures. The model consists of two interlinked components: the first part of the model is technical, and can be used to estimate the service life of a structure within a specified environment based on a set of prediction models; the second part of the model is used to determine the economic implications of the various design options over a specified evaluation period. The use of a particular predictive model is subject to the environment in which it was calibrated. In many cases the models which are presented were developed overseas and as such their values may not be directly applicable to South African environments. The approaches and development of the models are however useful and if calibrated to particular South African environments could be of considerable benefit. A survey of consulting engineers was also conducted to determine common perceptions of various durability related issues. The results of the survey are presented in this project. The size of the sample was relatively small and as such it would be inappropriate to apply the results categorically to all engineers or organisations. The survey is of value however in that it identifies some areas of potential opposition to the concept of designing for durability and highlights other areas where its acceptance and implementation may be more favourable.
Open Access
Deterioration of reinforced concrete in a marine environment : repair costs and maintenance strategies
(1994) Strohmeier, Jörg Harald; Alexander, Mark Gavin; Stevens, Alan J; Mackechnie, James R
This dissertation comprises an investigation into the rate at which reinforced concrete structures deteriorate in the Cape Peninsula due to reinforcement corrosion, the reasons for this deterioration, and the accompanying repair costs. The costing of repair work is calculated per m2 of reinforced concrete and is based on quoted labour rates and material rates of repair materials supplied by four major suppliers in the Western Cape. Formulas are included which enable a person using the data listed in tables and figures to calculate what repair costs will be in future, and also enable the calculation of monthly/annual deposit amounts in order to save sufficient money for future maintenance at a specified date. Life cycle costing and decision models for the maintenance of concrete structures are discussed and guidelines for the establishment of optimal maintenance cycles are included. Based on the results of the life cycle costing exercise the importance of planned preventative maintenance is highlighted. Finally, locally and internationally available maintenance management computer systems are reviewed.
Open Access
The development of a rapid chloride test for concrete and its use in engineering practice
(1997) Streicher, Pieter Emilius; Alexander, Mark Gavin
The test is of low cost, has a sound theoretical basis, and as far as can be ascertained, is the most rapid of all chloride tests developed to date. Due to the short test duration, it is possible to test concretes cured for very short periods. Samples can also be left immersed in chloride solution and re-tested at later ages to study time-dependent phenomena such as chloride binding. The chloride conductivity test involves the standardisation of the concrete pore solution by vacuum-saturating a concrete sample with a 5 M NaCl solution. A single conductivity measurement of the sample is then performed. Different concrete samples yield different conductivities primarily because of differences in their pore and micro-structure. The measured conductivity is related to the diffusibility ratio as well as to the theoretical steady state chloride diffusivity of concrete. Trial tests were performed to determine the optimum conditions for the test (sample conditioning. test procedure) and to verify the test relationships.
Open Access
The development of empirical chloride-induced corrosion rate prediction models for cracked and uncracked steel reinforced concrete structures in the marine tidal zone
(2014) Otieno, Mike Benjamin; Beushausen, Hans-Dieter; Alexander, Mark Gavin
Empirical chloride-induced corrosion rate prediction models for cracked and uncracked reinforced concrete (RC) structures in the marine tidal exposure zone are proposed in this study. The data used to develop the models were obtained from parallel corrosion experiments carried out by exposing half of 210 beam specimens to accelerated laboratory corrosion (cyclic 3 days wetting with 5% NaCl solution followed by 4 days air-drying) while the other half were left to undergo natural corrosion in a marine tidal zone in Cape Town (Table Bay). The main experimental variables were pre-corrosion flexural cover cracking, cover depth and concrete quality (binder type and w/b ratio). Corrosion rate, half-cell potential and concrete resistivity were monitored bi-weekly throughout the experimental period. The experimental results show that even though each of the variables investigated affects corrosion rate in a certain manner, their combined influence is complex. In general, regardless of the exposure environment (laboratory or marine tidal zone), for a given concrete quality and cover depth, pre-corrosion cover cracking was found to result in higher corrosion rates than in uncracked concrete, but with the field corrosion rates being much lower than the corresponding laboratory ones. Even though corrosion rates in both the field and laboratory specimens increased with an increase in crack width, the influence of concrete quality and cover depth was still evident. However, the effect of cover cracking on corrosion rate diminished with increasing concrete quality. In the blended cement concretes, the effect of concrete quality is further diminished by the inherent high resistivities of these concretes. The increase in corrosion rate due to increase in crack width, regardless of w/b ratio and cover depth, was generally higher in the 100% CEM I 42.5N concrete specimens than in the blended ones. A framework is proposed that can be used to objectively compare predicted corrosion rates for specimens with similar concrete quality (influenced by binder type and w/b ratio) but different cover depths and crack widths. The framework, which incorporates the combined influence of cover depth, crack width and concrete quality (quantified using chloride diffusion coefficient) on corrosion rate, is the basis of the proposed corrosion rate prediction models for cracked concrete. Sensitivity analyses on the proposed models show that if any two of the three input parameters (cover depth, crack width and concrete quality) are simultaneously varied, their effect on corrosion rate is dependent on the value of the third (unchanged) parameter. Furthermore, (i) the initial cover depth was found to have no effect on the extent to which a change in cover depth affects corrosion rate; a similar trend was found in the case of sensitivity of corrosion rate to change in crack width , and (ii) the extent to which a change in either crack width or cover depth affects corrosion rate is dependent mainly on the concrete quality. In general, the sensitivity analyses showed that corrosion rate is more sensitive to change in concrete quality than crack width and cover depth. The proposed models can be used to (i) quantify the propagation phase with respect to a given performance limit using relevant corrosion-induced damage prediction models, and (ii) select suitable design combinations of cover depth, concrete quality and crack width to meet the desired durability performance of a given RC structure in the marine environment.
Open Access
The durability of precast concrete elements
(2000) Ronné, Phillip Dean; Mackechnie, James R; Alexander, Mark Gavin
Modern fast track construction methods increasingly favour the use of precast concrete elements. Precast box culverts are structurally significant units, subject to an important combination of bridge loadings. Culverts occasionally in contact with water pose a high durability risk. Despite this, the current specifications allow a reduction in cover to reinforcing steel for precast culverts to only 20 mm from at least 40 mm for cast-in-place culverts.
Open Access
Durability specifications for structural concrete: an international comparison
(2013) Kessy, Justine George; Alexander, Mark Gavin; Beushausen, Hans-Dieter
Premature deterioration of reinforced concrete (RC) structures has become an issue of global concern. As a result, many upgrades and improvements have recently been made in design standards and specifications, to include requirements that account for durable RC structures. This dissertation examines and compares such durability requirements in design standards and specifications developed in the United States of America, Australia, Canada, Europe, India, and South Africa. It discusses issues relating to exposure conditions, limiting values of material compositions and proportions, and cover depth to the reinforcing steel. Both prescriptive and performance requirements for concrete durability are described. In general terms, this dissertation concludes that most design standards are based on prescriptive requirements with a few having some elements of performance requirements for durability design. The prescriptive approach that outlines requirements for material compositions and proportions, procedures, and test methods, is commonly used in most design standards and specifications for durability purposes. Though such approaches may encompass requirements for, inter alia, minimum compressive strength, maximum water-to-cementitious material (w/cm) ratio and cover depth, the desired concrete performance is not generally described. Material and construction variability are not taken into account, and even if intensive construction supervision is carried out, it is difficult to ensure all specified parameters are achieved. Moreover, requirements such as maximum w/cm and minimum water content are impractical or costly to measure or verify in practice. Generally, it should be acknowledged that this approach has limited applications and often stifles innovations.In an attempt to move away from the prescriptive approach, research has focused on performance approaches, which measure relevant properties of the concrete, in particular transport-related properties that account for durability. Performance approaches impose few or no restrictions on the concrete composition, proportioning, or construction methods, but rather promote innovations. Worldwide there is a consensus that in order to extend the service life of RC structure, performance approaches are imperative. This dissertation gives an overview of the international efforts in the implementation of performance approaches, either in design standards or in project specifications.
Open Access
The effect of cyclic wetting and drying on the corrosion rate of steel in reinforced concrete
(2015) Golden, Gavin; Alexander, Mark Gavin; Beushausen, Hans-Dieter
Preventing chloride-induced reinforcement corrosion in marine concrete structures remains a concern for structural engineers. Marine structures are typically exposed to high chloride concentrations through direct exposure to sea water, which provides the primary conditions required for reinforcement corrosion. The progress of corrosion can be controlled through anodic, cathodic or resistivity processes. However, high chloride concentrations tend to prevent anodic control from governing the corrosion rate, while direct exposure to moisture generally prevents resistivity control. Consequently, cathodic control remains an important process in the marine environment to restrict the progress of corrosion. The primary cathodic reaction in reinforcement corrosion is the reduction of oxygen, and as a result the availability of oxygen is a key factor when considering reinforcement corrosion in the marine tidal zone. The corrosion rate may be effectively reduced if the drying time of the concrete during tidal cycles is sufficiently short, thereby reducing the oxygen supply at the level of the embedded steel.
Open Access
The effects of partitions on the vibration serviceability of concrete floors
(2012) Mutombo, Christian Kabongo; Moyo, Pilate; Alexander, Mark Gavin
The growing demand to construct long, slender floors with minimum supports for aesthetic and economic reasons especially in modern building developments has resulted in increased floor slenderness leading to vibration problems. As a result, vibration serviceability has become the governing design criterion for many of these new civil engineering structures. It is known that long span, slender floors possess lower natural frequencies and reduced damping leading to vibration serviceability problems. As vibration serviceability becomes a major concern in the design of concrete floors, investigations of the beneficial effects of non-structural elements to the vibration serviceability of floors are becoming increasingly important. The vibration serviceability of long span, slender concrete floors may be improved through the installation of non-structural elements such as partition walls and raised access floors. Little research exists into the quantification of the effects of various types of partitions on the vibration serviceability of concrete floors that support them. There are no guidelines available to designers which take into account the effects of partitions in the design of concrete floors for vibration serviceability besides the different damping ratios that are recommended in different codes for bare floors as well as floors with half- or full-height partitions. Therefore a research project was initiated with the objectives of investigating the effects of non-structural partitions on the vibration serviceability of concrete floors as well as proposing guidelines on how to account for partitions in the design of concrete floors for vibration serviceability. The research project consisted of modal testing on a prestressed pre-tensioned concrete floor slab. The excitation of the floor was generated by an electrodynamic shaker and instrumented impulse hammer. The electrodynamic shaker excitation was used mainly to extract the natural frequencies and mode shapes of the test floor. The instrumented impulse hammer excitation was used mainly for damping studies. Force-balanced QA 700 accelerometers with a sensitivity of 8 V/g mounted to base plates were used to measure the responses of the test floor. The Data Physics Signal Calc Mobilyser was used to acquire the data from the accelerometers. A personal computer was used to store, analyse and present the data. The curve fitting method in Vibrant Technologies ME'Scope was used to estimate the natural frequencies and mode shapes of the test floor. Modal 1.20 by Brownjohn (2009) was used to estimate the damping ratios of the test floor. Modal 1.20 uses the logarithmic decrement method to estimate damping. The tests investigated the effects of partitions on the vibration serviceability of concrete floors.
Open Access
Effects of reinforcement corrosion on the structural performance of reinforced concrete beams
(1994) Theron, Gavin De Vos; Alexander, Mark Gavin; Kratz, Rolf Dietmar
This dissertation is an investigation into the effect of reinforcement corrosion on the structural performance of reinforced concrete beams. Two types of specimens are investigated, the first without any stirrups and the second with stirrups. The specimens were corroded galvanostatically as well as by subjecting them to alternate cycles of wetting and drying with a saline water. An attempt is made at classifying the extent of corrosion of the reinforcing steel and its effects on the concrete. The effect of the corrosion on the structural performance is measured by establishing its effect on the maximum load carrying capacity, the deflections, energy requirements and ductility ratio. The main conclusions made in respect of the effect of reinforcement corrosion are that it causes: a decrease in the load carrying capacity; an increase in the deflections at the equivalent load level; a decrease in the energy requirements to reach the maximum load; and a smoothing of the load-deflection relationship. A limited literature review is also presented to provide background information of corrosion in concrete and general structural behaviour. Guidelines for the development of an analytical model to predict the load carrying capacity of corrosion affected reinforced concrete beams are also given.
Open Access
Evaluation of mechanical properties of textile concrete subjected to different environmental exposures
(2007) Mumenya, Siphila Wanjika; Tait, Robert B; Alexander, Mark Gavin
Within the last decade, a new class of civil engineering materials referred to as High Performance Fibre Reinforced Cementitious Composites (HPFRCC) has been developed. As a subset in this new field, additions of polypropylene (PP) fibres, in the form of woven textile fabrics, have proved most successful as reinforcement to cementitious matrices, leading to the development of a novel composite material: so called "Textile Concrete" (TC). Although the strength and modulus of PP are not very high, careful design through the appropriate placement and high fibre volume fraction of a textile format, has led to sufficient strength and stiffness, while at the same time high toughness has been achieved. In particular the development of a fibrillated core fibre to which is attached an outer layer of ultrasonically welded, or bonded "fluffy" PP fibres, has been most successful, and is a unique feature. These outer fluffy layers provide excellent physical bonding characteristics to the cement matrix, thus overcoming the intrinsic hydrophobic nature and otherwise weak matrix bonding of PP fibres. In South Africa where TC has found ready application, the composite is produced from a fine grained cementitious matrix and a PP textile commercially known as CemForce.
Open Access
Evaluation of surface treatments for RC structures failing to meet South African durability index requirements
(2012) Burmeister, Nicholas; Beushausen, Hans-Dieter; Alexander, Mark Gavin
In many countries the budget for repairing existing infrastructure already exceeds that of building new infrastructure. The main durability problem is corrosion of reinforcing steel caused by the ingress of aggressive agents and moisture. The South African durability index approach makes use of indices to evaluate the potential durability of concrete. Once limiting values on the as built structure meet specific requirements, the structure is considered to be inherently durable. However, a clear design methodology or guidelines for concrete structures that do not meet the specified minimum requirements needs to be established. The potential for using surface treatments to prevent the ingress of corrosion causing agents (carbon dioxide, chloride ions and moisture) is presented in literature but the extent of protection and durability of these products is largely unknown. A laboratory investigation was developed to evaluate the performance and longevity of surface treatments under accelerated weathering and exposure conditions. Poor quality concrete failing to meet durability requirements was treated with six products and exposed to accelerated weathering conditions. The carbon dioxide and chloride ion resistance was then evaluated to establish performance and durability characteristics for each of the products. Results indicate that surface treatments restrict the passage of deleterious agents and certain products can be used as an acceptable method of corrosion prevention. The pure silane coating was found to effectively reduce the ingress of chlorides while the acrylic dispersion, acrylic resin, cement based and cement based fibre reinforced coatings were effective in preventing carbonation. Furthermore, it was found that the increased service life due to the application of anti - carbonation coatings could be quantified with a specific number of years. Service life quantification due to improved chloride ion resistance was not possible since modified and custom tests differing from the standard bulk diffusion test were used to evaluate performance. Chloride resistance was improved due to the pure silane however service life improvements could not be quantified. Unexpected failures occurred during the laboratory investigation rendering a number of tests inconclusive. Some guidance based on the knowledge gained during this project is provided for future testing.