Browsing by Author "Beushausen, Hans-Dieter"

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    A Critical Evaluation of the Use of Crack Width Requirements in the Durability Design of Marine Reinforced Concrete Structures
    (2023) Elias, Nicholas; Beushausen, Hans-Dieter
    Crack width requirements (CWRs), which aim to limit cracks in reinforced concrete (RC) to maximum prescribed values, play a major – and often dominant – role in the design of marine RC structures. However, there are several issues with the current CWRs, chief among which is the fact that, despite decades of research, no clear relationship between crack width and steel reinforcement corrosion rate in concrete has been found. Instead, there exist two opposing schools of thought in the literature – one which says that there is a relationship between crack width and reinforcement corrosion rate, and one which argues that no such relationship exists – with good evidence to support both schools of thought. Recent research has shown that even small cracks, with widths below the required values, may lead to extensive corrosion. It is therefore uncertain whether designing for the CWRs actually improves durability and extends the service life of marine RC structures. Furthermore, the use of the CWRs, which frequently results in large increases in the required amount of reinforcing steel, may lead to significant increases in the cost and environmental impact of marine RC structures. Yet, to date, these impacts have not been quantified. In order to address these issues, this study was aimed at evaluating the effect of designing to meet the current CWRs on the durability, cost, and environmental sustainability of marine RC structures. This was done by designing two sets – one with, and one without the CWRs – of typical marine RC structural elements. Based on real industry projects, two different types of elements were designed – a crane rail beam for a coal export jetty in Matola, Mozambique, and a precast crown wall unit for a breakwater in Rupert's Bay, St. Helena Island. The designs were carried out using a combination of BS 6349 and EN 1992-1-1:2004, as these are the codes of practice typically used in the South African coastal engineering industry. The effects of designing for the current CWRs on durability, cost, and environmental sustainability were then quantified by carrying out service life modelling, life cycle cost assessments (LCCAs) and estimating embodied carbon (EC) values for the designed members. The results of the service life modelling show that, for the range of crack widths likely to occur in practice, the use of the current CWRs does not improve durability, and may even reduce service life, as they encourage the use of more, smaller diameter reinforcement bars, which has the effect of increasing corrosion rate and reducing the time taken for a critical amount of the reinforcement to be lost due to corrosion. Furthermore, the results of the LCCA and EC estimates imply that the current CWRs are not the most cost-effective method for durability design and may result in significant increases in cost and environmental impact. Taken together, these results suggest that, even if a relationship is assumed to exist between crack width and corrosion rate, the current CWRs are neither the most effective, nor efficient way of addressing the effects of cracking on the durability of marine RC structures. It is therefore recommended that the current crack width requirements should be removed from the durability design codes of practice and replaced with either a limitation on steel stress, a more lenient crack width requirement (for example, of 0.5 mm rather than 0.3 mm), or a performance-based crack width requirement, which takes better account of the complexity of cracking and its effect on durability. It is also recommended that engineers be given the option to use other methods of providing durability, such as the use of crack-sealing and waterproofing admixtures, or hydrophobic treatments, instead of the current CWRs. However, before any of these recommendations can be implemented, the results of this study need to be confirmed with further research. Owing to the limitations of both service life modelling and accelerated laboratory corrosion, it is recommended that this further research should take the form of an extensive evaluation of the durability performance of existing marine RC structures.
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    A Practical Carbonation Model for Service Life Design of Reinforced Concrete Structures
    (2021) Harold, Romuald Fotso Lele; Beushausen, Hans-Dieter; Alexander, Mark
    The increase in atmospheric carbon dioxide concentration due to global warming has a direct impact on the amount of carbonating concrete structures. For the past years, numerous studies have been done in South Africa on the subject and models developed to predict carbonation in concrete structures. Despite the large amount of resources and research effort put into developing these models, the translation from theory to practice represents a great challenge for design engineers in the field of durability design. This study presents a design tool based on existing models for use in practical applications. The proposed design tool assists in computing the service life of carbonating concrete structures and provides reliability values associated with the service life. It accounts for different binder compositions and binder types, as well as different locations and environmental land uses in South Africa. The validation of the design tool was done by comparing the service life prediction results to existing models, which generally showed good agreement. The developed design tool can be applied for predicting the long-term performance of new RC structures as well as improving the basis for quality assessment of existing, newly built RC structures. For the design of new structures, the designer is required to make certain assumptions concerning the information to be used for the simulation. These include values for the binder type, binder content, OPI, cover depth, land use and exposure parameters. For the quality control of new structures, the way in which the model parameters are obtained differs from that of new structures. As the structure already exists, both the concrete quality, cover depth and environmental loading can be measured directly on the structure with appropriate testing procedures. The outcome of applying the design tool for the analysis of concrete produced for the Gauteng Freeway Improvement project (GFIP) is also presented, with a case study of precast and in-situ structures chosen for the analysis.
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    Alternative patch repair materials for rebar corrosion damage
    (2017) Jassa, Primesh; Beushausen, Hans-Dieter; Ngassam, Ines Tchetgnia
    Reinforced concrete (RC) is extensively used in the construction industry. It is particularly used to guarantee that infrastructure assets around the world last for multiple years whilst ensuring that the structural integrity and serviceability of the structure is maintained. However, in practice countless RC constructions are failing prematurely due to a large number of factors of which the corrosion of steel embedded within concrete is the most significant (Matthews et al., 2003). Steel corrosion is particularly pernicious to concrete due to the expansive nature of the corrosion by-products formed, which commonly leads to cracking and spalling. One of the most common methods adopted in the rehabilitation of corrosion damaged concrete is the patch repair procedure. However, in practice this method has shown to often be unreliable as a consequence of the widespread occurrence of shrinkage cracking and poor substrate-patch adhesion leading to debonding of the patch repair. From a practical point of view, such failed repair systems essentially restore the repaired concrete back to a deteriorated state. The underlying cause of poor durability in patch repairs is attributed to a range of reasons including, the lack of understanding of the substrate-patch composite system and the limited availability of appropriate design standards. Furthermore, there is a lack of understanding in the repair industry on the critical material properties actually required for durable patch repairs. There is a common belief that repairing concrete with specialised proprietary repair materials would guarantee durability. However the widespread premature failure of patch repairs conducted using such materials has proven the contrary. A proper patch repair process includes treatment of the corroded steel, adequate substrate surface preparation, installing sacrificial anodes (at least for chloride contaminated concrete) and surface coating. In principle, if this process is correctly followed then the material requirements for a durable, non-structural repair would be to fill in the cavity created by removing contaminated concrete, resist shrinkage induced cracking and/or debonding and provide protection against chloride ingress (in chloride environments). The material used for patch repairs could be any appropriate repair material and it does not specifically need to be a specialised cementitious repair mortar. This dissertation presents an understanding of the materials and issues concerning the durability and serviceability of patch repairs, with the aim of identifying alternative non-structural patch repair materials for the durable repair of corrosion-damaged concrete structures. The potential patch repair materials studied in this dissertation were rubberised waterproofing bitumen, polymer (copolymer of vinyl acetate and ethylene) with 5% cement replacement and 60%, 80% and 100% fly ash (FA) mortar. Patch repairs were conducted on substrate moulds to test application and observe cracking/debonding occurrence. Furthermore, compressive strength, durability index, accelerated drying shrinkage, restrained shrinkage, workability and SEM tests were conducted. It was concluded that the 60% FA repair material had the best overall performance with the polymer-cement concrete exhibiting good bonding and crack resistance properties. This research established that innovative alternative repair materials such as a 60% FA or polymer-cement concrete material, can be developed for non-structural patch repairs with improved long-term performance relative to conventional materials. The research has further provided a foundation for the development and design of durable repair mortars by identifying the principal material performance properties required of such materials.
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    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.
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    Applying the DER rating system for the visual assessment of defects on concrete dams
    (2021) Muhenje, Mariura; Beushausen, Hans-Dieter; Beushausen, Hans; Ndawula, Joanitta
    NamWater is responsible for the management and maintenance of Namibia's main dams, amongst the many other infrastructure assets under their ownership. They have, as a result, devised asset management policies and practices with the aim of reaping the benefits of asset management. Additionally, they have aligned themselves with the current approaches to dam safety management in managing the dams. These current approaches do however not provide for a guided and standard approach when it comes to the visual assessment of surface defects on the dam elements. This may result in varying assessment outcomes from different individuals based on their differences in training and experience. The DER rating system used for the rating of defects on road structures, as input into the STRUMAN BMS used by SANRAL and other entities, provides for a standard approach to visually assessing the defects on these road structures. A study by Gombele (2017:79) has also demonstrated the possibility of using the DER rating system for the assessment of defects on cooling towers in a power generation environment. Additionally, the rating of defects can also play a role in dam risk determination, as demonstrated in the CIRIA 1 Project RP568 risk assessment methodology (Morris, Hewlett and Elliott, 2000:15). In quest for a standard and guided approach for visually assessing surface defects on dam elements, this study applied the DER rating on selected elements of three NamWater dams. The approach was by initially identifying dam elements that are deemed equivalent to the bridge items in the TMH19: Part A. There are variations in the design and functions of the dam elements versus those of bridge elements and thus the relevance of the defects may also vary for the two structure types. Thus, the focus of this study was on using the DER rating system to only assess the surface defects on the dam elements. This may be useful for the initial phase of a condition assessment for dams whereby it can provide a quantifiable indication of durability issues. While the study was able to demonstrate the possibility of using the DER rating system to assess defects such as cracks, spalling and erosion on the surfaces of dam elements, gaps where identified in its applicability. Only 33% of the bridge items in the TMH19: Part A were deemed relevant for the visual assessment of the dam items, coupled with the significant amount of U (unable to inspect) ratings given during the assessment. Furthermore, of the identified defects, a significant amount was given a low Relevancy rating (R) meaning that they are of a low relevance to the structural integrity of the dam structure. This may be due to the fact that the guiding tables used are originally for bridge items and thus not entirely suitable for dams. For the DER rating system to be applied extensively to dam items, defects that are specific to dam elements will therefore need to be incorporated into the guiding tables. Additionally, the weighting of the ratings for certain defects would need to be revised to specifically align with the consequence of the defect on the dam item. This process may require the compilation of a database of historical defects, guided by expert engineering judgement, to provide for guiding tables that are specific to dams. Assessment of more dams that vary in age, type, and performance may also be required to get a more diversified outcome of the applicability of the DER rating system on dams.
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    Assessing the age at cracking of concrete repair mortars
    (2012) Chilwesa, Masuzyo; Beushausen, Hans-Dieter; Moyo, P
    The amount of concrete infrastructure needing repair and rehabilitation is increasing worldwide. The bonded overlay technique, which involves removal- of a damaged concrete layer on an existing concrete base (substrate) and replacing it with a new layer is one of the most widely used techniques. Due to thermal and hygral differences in the two composites, differential shrinkage occurs. This leads to overlay shrinkage restraint by the relatively mature substrate. Restrained shrinkage in bonded overlays can cause stress build up and may result in cracking. Cracking due to restrained deformation is a major problem as it may lead to durability concerns. Overlay resistance to crack initiation, development and propagation depends on a number of time-dependent properties of the concrete. To be able to predict the onset of cracking requires knowledge of the different material properties and how they interact with each other. In this study, an investigation was carried out on whether the performance with respect to cracking of concrete overlays can be adequately predicted from tests such as ring test, free shrinkage strain, tensile strength test, tensile relaxation and elastic modulus. Five concrete mortar types i.e. three commercially available mortars and two laboratory mixed mortars with water-tocement ratio (w/c) = 0.45 and w/c = 0.6 were used in the ring test and material property tests. The influence of curing on the crack resistance of overlays was also investigated. An analytical madel for predicting age at cracking in bonded overlays based upon time-development of overlay material properties and the superposition principle was developed. Results from the model were compared with results from the ring test. Results indicate that crack resistance of repair mortars depends upon the combined influence of the different material properties. In particular tensile stress relaxation appears to have a large influence. Curing was observed to delay the onset of cracking. Results also indicate that although the ring test will give the correct order of cracking, it will not give the actual age at cracking when assessing materials for crack resistance.
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    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.
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    Assessment and prediction of chloride ingress and carbonation in patch repair mortars
    (2016) Arito, Ezekiel; Beushausen, Hans-Dieter
    The specifications for concrete patch repair mortars usually entail mechanical properties such as compressive strength and tensile strength. However, these properties may not directly relate to the desired performance in relation to durability and prevention of reinforcement corrosion. In addition, the literature does not show any direct relationship between compressive strength and durability properties of concrete and mortars in natural exposure conditions. Relevant performance requirements, such as carbonation resistance and chloride resistance, are usually not considered despite the fact that they have a direct influence on the durability performance of concrete repair mortars. The widespread premature failure of patch repairs which meet the existing compressive strength criteria implies that the use of compressive strength as a performance indicator may not provide a reliable measure of the durability performance. Therefore it can be argued that modern concrete repairs should be based on durability considerations, rather than compressive strength. In this study, an experimental investigation was conducted to determine the durability performance of patch repair mortars. Experimental results were analysed to investigate the correlations that exist between (i) electrical conductivity (Chloride Conductivity Index test) and rate of chloride-ion diffusion (bulk diffusion test), and (ii) gas permeability (Oxygen Permeability Index test) and rate of carbonation (accelerated carbonation test) in patch repair mortars. Eight mortar mixes were used in the investigation, including four commercially available repair mortars and four laboratory-made mortar mixes. To vary the pore structure of the laboratory mixes, different water/binder ratios (0.45 and 0.60) and binder types (100% Portland cement and 50/50 blend of Portland cement/blast furnace slag) were used to make the mortar specimens. Two curing conditions (dry and moist) were adopted with the aim of investigating the influence of curing on durability performance of patch repair mortars. Test results indicate good correlations between electrical conductivity and rate of chloride diffusion (correlation coefficient of 0.9112), and between oxygen permeability and rate of carbonation (correlation coefficient of 0.6751). This correlation was mainly attributed to the fact that these material properties largely depend on the pore structure (specifically the size, connectivity and tortuosity of pores). The good correlation further implies that electrical conductivity and oxygen permeability of repair mortars as evaluated by the CCI and OPI tests may provide a reasonable measure of chloride resistance and carbonation resistance respectively. However, the prediction of chloride ingress and carbonation depth from the electrical and gas permeability properties respectively, ought to be implemented within the range where reasonable correlation can be established. The results also showed that the durability performance of repair mortars in terms of chloride and carbonation resistance is sensitive to material factors, such as w/b ratio, curing type and binder type, which directly influence penetrability. Service life models for predicting chloride ingress and carbonation in the patch repair mortars used in this study were developed based on modified Fickian equations. The prediction profiles for chloride penetration were developed from a modified solution to Fick's second law of diffusion, while the carbonation depth prediction profiles were developed from the square-root-of-time law. Chloride penetration and carbonation depth could be predicted using the developed profiles. Though several assumptions that should be verified and/or modified in future work were made, the modelling results of this study serve as useful framework for evaluating the resistance to chloride ingress and carbonation in patch repair mortars.
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    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.
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    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.
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    Concrete surface coatings and the influence of substrate moisture condition on bond strength
    (2016) Kay, Sean; Beushausen, Hans-Dieter
    Concrete structures, in particular reinforced concrete structures, have been designed and built for many years. Many previously built structures are now being compromised with regard to their condition and structural integrity. There has developed a need to maintain these structures and protect them in order to protect the users and minimise the expenses associated with repair and maintenance. Instead of resorting to demolishing and rebuilding, engineers are becoming increasingly capable of restoring and enhancing existing structures in order to maximise the original structures lifespan. One of the ways in which this is achieved is through the implementation of various repair and maintenance strategies. These strategies can range from cathodic protection of the steel reinforcement to increasing concrete cover of the section. The easiest and often most commonly used method, although often not effective when used on its own, is to coat the concrete surface with a decorative or protective coating. There are many coatings available, ranging from simple to complex formulations of inorganic and/or organic materials. These coatings each perform a unique function and will often be designed to combat a specific problem which the structure is exposed to. The formulation of the coatings are almost always designed correctly when they are manufactured, however, premature bond failure due to poor substrate surface preparation and various substrate moisture conditions still seem to occur after coating application. This investigation will give insight into the effects a sound, clean and profiled concrete substrate that is subjected to different moisture conditions will have on the bond strength achieved when using cementitious based coatings. The reason for differing the moisture conditions of the substrate is that often on site the moisture condition of an existing substrate is not known prior to application of the coating. Through this investigation, epoxy modified cementitious coatings have been shown to provide an acceptable bond strength. In some cases, failure within the coating instead of the acceptable failure within the substrate has resulted, due only to the level of dryness of the substrate. A thorough investigation on the theory behind surface coatings, their properties and performance requirements was done and subsequently followed by a detailed experimental programme that was performed and analysed to provide insight to the said coating behaviour.
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    Condition assessment methods for prestressed concrete railway sleepers: feasibility for South African applications
    (2025) Lambrechs, Astrid; Beushausen, Hans-Dieter
    Concrete sleepers are a key component of railway systems, with estimates showing between 20 million to 35 million currently installed within the South African railway network. The condition assessment of concrete sleepers in South Africa poses a challenge for two main reasons: the scale at which condition measurements need to be conducted, and the poor access to sleepers within the ballast structure. To assess feasibility of concrete sleeper assessment methods, the quality of technical results was reviewed to ensure the information required for long-term lifecycle needs of concrete sleepers can be produced and that the risk associated with failures of critical defects are mitigated. The work established in the literature review was consolidated through a three-phase methodology to provide results for the assessment. This involved a Failure Mode and Effect Analysis (FMEA) that measured how well each of the methods can detect the failure modes, and then benchmarked the condition assessment methods against similar technology currently in use within the South African network. A test for defect relevance checked that the defects identified in the literature were relevant for South African applications and allowed for further categorisation of criticality. Longitudinal cracking and vertical cracks at the rail seat were found to be the most critical defects required to be detected by concrete sleeper condition assessment methods. Vehicle-mounted ultrasonic echo and vehicle-mounted imagining technology methods both showed good technical results from the FMEA, showing better quality information on sleeper condition than current methods in use. But only the vehicle-mounted imaging scored a residual profile value below the recommended technical threshold, and showed to benchmark cost, lead time and operational factors in line with current methods. Consequently, vehicle-mounted imaging is the most feasible option for concrete sleeper condition assessment for South African applications. Sleeper body abrasion cannot be well detected through any methods reviewed and will require further study of methods before visual inspections of this defect can be improved upon. Failure modes of concrete sleepers due to prestressing steel corrosion were unclear and further testing should be done to define what early-stage cracking displays to accurately predict the end of the useful life of a sleeper.
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    Condition assessment methods for prestressed concrete railway sleepers: feasibility for South African applications
    (2025) Lambrechs, Astrid; Beushausen, Hans-Dieter
    Concrete sleepers are a key component of railway systems, with estimates showing between 20 million to 35 million currently installed within the South African railway network. The condition assessment of concrete sleepers in South Africa poses a challenge for two main reasons: the scale at which condition measurements need to be conducted, and the poor access to sleepers within the ballast structure. To assess feasibility of concrete sleeper assessment methods, the quality of technical results was reviewed to ensure the information required for long-term lifecycle needs of concrete sleepers can be produced and that the risk associated with failures of critical defects are mitigated. The work established in the literature review was consolidated through a three-phase methodology to provide results for the assessment. This involved a Failure Mode and Effect Analysis (FMEA) that measured how well each of the methods can detect the failure modes, and then benchmarked the condition assessment methods against similar technology currently in use within the South African network. A test for defect relevance checked that the defects identified in the literature were relevant for South African applications and allowed for further categorisation of criticality. Longitudinal cracking and vertical cracks at the rail seat were found to be the most critical defects required to be detected by concrete sleeper condition assessment methods. Vehicle-mounted ultrasonic echo and vehicle-mounted imagining technology methods both showed good technical results from the FMEA, showing better quality information on sleeper condition than current methods in use. But only the vehicle-mounted imaging scored a residual profile value below the recommended technical threshold, and showed to benchmark cost, lead time and operational factors in line with current methods. Consequently, vehicle-mounted imaging is the most feasible option for concrete sleeper condition assessment for South African applications. Sleeper body abrasion cannot be well detected through any methods reviewed and will require further study of methods before visual inspections of this defect can be improved upon. Failure modes of concrete sleepers due to prestressing steel corrosion were unclear and further testing should be done to define what early-stage cracking displays to accurately predict the end of the useful life of a sleeper.
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    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.
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    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.
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    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.
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    Discrete sacrificial anodes and their use in service life extension of chloride contaminated reinforced concrete structures
    (2012) Arito, Philemon; Beushausen, Hans-Dieter; Alexander, Mark G.; Otieno, Mike
    There is an urgent need for economic and technically sound concrete repair technologies in South Africa as the number of deteriorating RC structures needing repair is increasing. Currently, the South African concrete repair industry mainly relies on the application of patch repairs and corrosion inhibitors while other potentially successful methods such as sacrificial anodes are rarely applied. Sacrificial anode cathodic protection (SACP) systems use metals that are higher than steel in the electrochemical series. These metals corrode preferentially to steel thereby supplying electrons to the cathodic steel surface. Common SACP systems include discrete zinc anodes installed in slots or drilled holes, non -structural jacket encasements, and overlays made from expanded zinc mesh or any other convenient form. The effectiveness of discrete SACP systems in service life extension of chloride contaminated RC slabs has been investigated in this study. Similarly, the extent to which discrete sacrificial anodes can be used to extend the service life of corroding chloride contaminated RC structures has been assessed through an extensive review of literature a s well as laboratory experiments on corroding chloride contaminated RC slabs . This study evaluates the performance of discrete SACP systems with respect to binder type, corrosion rate and the level of chloride contamination with an objective of developing guidelines for rational concrete repair when using SACP systems to repair RC structures that are deteriorating from corrosion of reinforcing steel in South Africa. Eight slabs were investigated. Four slabs were cast using 100% PC CEM 1, 42.5N while the other four were cast using a blend of 70/30 PC CEM 1, 42.5N/FA. Each slab comprised two portions: one half of each slab was cast using concrete that was admixed with 0.6% chloride by mass of binder while the other half was cast using concrete that was admixed with 1.8% chloride by mass of binder. The corrosion in the slabs was induced and accelerated using direct anodic current. The SACP system that was used in this study comprised discrete zinc disks put in a cylinder of a proprietary high alkaline mortar. The se anodes were installed in 1 0 0 x 10 0 x 60 mm deep cavities that had been made in concrete. The anodes were connected to the reinforcement cage using tie wires and thereafter backfilled with a repair mortar. The cavities in which the anodes were installed were arranged in a square grid of 450 x 450 mm. The corrosion rate and half-cell potential of the steel as well as the resistivity of concrete in the slabs were monitored over a duration of five month s. The discrete SACP system that has been investigated in this study can increase the service life of chloride contaminated RC structures that are deteriorating as a result of rebar corrosion. The discrete anodes reduced the corrosion rate of steel in the test slabs. The percentage reduction in the average corrosion rate was higher within the slabs that were cast using 100% PC CEM 1, 42.5N than in the slabs that were cast using a blend of 70/30 PC CEM 1, 42.5N/FA. The reduction in average corrosion rate within the slabs cast using 100% PC CEM 1, 42.5N ranged between 45-95%. Similarly, the reduction in the average corrosion rate of within the slabs that were cast using a blend of 70/30 PC CEM, 42.5N 1/ FA ranged between 54-75%. Throughout the test specimens, the portions of the slabs that was admixed with 0.6% chloride by mass of binder exhibited superior performance. These portions (admixed with 0.6% chloride by mass of binder) experienced the greatest reduction in average corrosion rate as well as the greatest shift in average half -cell potential towards values that are more negative. Finally, in order to achieve the long-term objective of increasing the service life of RC structures that are deteriorating as a result of rebar corrosion, lo ng-term studies ought to be undertaken to ascertain, with surety, the effectiveness of discrete SACP systems in corroding structures.
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    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.
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    The effect of aggregate on the age at cracking of bonded concrete overlays subjected to restrained deformation
    (2013) Dittmer, Thomas James; Beushausen, Hans-Dieter; Moyo, Pilate
    The need for effective and comprehensive repair techniques is increasing throughout the world as existing concrete structures continue to age. The bonded concrete overlay method, where a new repair material layer is cast over an existing damaged substrate, is the most commonly utilized method of repair. However, this repair technique is prone to failure due to differential volume changes between the newly cast repair material and the existing substrate. These volume changes can be attributed to thermal and shrinkage differences, with drying shrinkage being identified as they key source of the volume change. When the resulting induced stress form the differential volume changes exceeds the intrinsic tensile strength of the repair material, cracking failure occurs. This can be detrimental to the effectiveness of the repair and its durability. Overlay crack resistance has been found to be dependent on specific material properties, namely: tensile strength, elastic modulus and shrinkage. The effect of tensile relaxation on overlay performance is also significant, with the resulting 'stress relief' cause by this relaxation prolonging the time to cracking failure. In this investigation, the influence of the inclusion of coarse aggregate in the repair material is investigated. The study focuses on the specific impact of coarse aggregate volume content and size on the performance of bonded overlay materials, and the material properties that influence this performance. Two laboratory concrete mixes, with a w:c = 0.45 and 0.6, and a commercial repair product were tested with varying coarse aggregate volume contents. Further testing was conducted with w:c = 0.6 laboratory mixes with different nominal sized coarse aggregates. The impact of coarse aggregate volume and size on individual material properties, which included tensile strength, tensile relaxation, free drying shrinkage and elastic modulus, were tested separately for the various mixes. In conjunction with this, direct restrained shrinkage tests were conducted using ring tests to measure the influence of coarse aggregate volume and size on the time to cracking failure and crack intensity of the repair materials.
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    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.