ETD Collection

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Subject: search.filters.subject.Concrete construction

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    Evaluation of selected repair methods for chloride-included corrosion damaged reinforced concrete railway bridges
    (2019) Jogiat, Mohamed
    Premature deterioration of reinforced concrete railway bridges before and after repair is a serious concern as it is costly and poses a major risk on safety and performance. Reinforced concrete railway bridges in aggressive environments (near the sea) face the risk of ingress of corrosion agents (oxygen, moisture and chlorides) into the concrete to the reinforcing steel. Although, corrosion of the reinforcing steel is not the only cause of structural deficiencies in railway bridges, it is a significant contributor to deterioration and therefore of major concern. In order to guide the selection of a suitable repair option, one repair material from each category (patch repair mortars, barrier systems, electrochemical methods and corrosion inhibitors) was investigated. The effectiveness of selection was assessed by employing electrochemical techniques to quantify the performance of each selected repair material in stifling chlorideinduced corrosion in reinforced concrete. This study focuses on the evaluation of selected repair materials for chloride-induced corrosion in reinforced concrete using 100 x 100 x 500 mm long beam specimens. The four selected repairs were applied to the reinforced concrete beams after a period of 200 days after casting. The beams had a constant concrete cover to reinforcing steel of 20mm. The beam specimens were casted using admixed chlorides into the mix and were subjected to a cycle of 3 days wetting (with 5% NaCl solution) and 4 days drying. The beam specimens were monitored for half-cell potential (Cu/CuSO4), corrosion rate (coulostatic technique) and concrete resistivity (Wenner probe technique). Results indicate that the selected repair materials in this study had varied influences on the halfcell potential and corrosion rate values. The patch repair material replaced the concrete cover with a more durable material, confirmed from the Durability Index (DI) tests conducted. The resistivity of the repair mortar was measured to be higher than the concrete. Due to the replacement of the concrete cover, the corrosion rate values reduced when compared to the control reinforced beam specimens. However, the half-cell potential values indicated the probability of corrosion is still high after application. The barrier method, applied a silane-based sealer on the reinforced concrete beams. The resistivity of the concrete increased after application of the barrier method. The corrosion rates after application of the barrier method was lower than the corrosion rates of the control reinforced concrete beams. The half-cell potential results indicated the corrosion risk is still high after application of the barrier method. The electrochemical repair was the only repair material that showed more negative potentials than the control beams and corrosion rates were significantly higher than all the other repair methods after application. The reason for this can be attributed to the zinc anode dominating the potential and corrosion rate values. Therefore, no conclusion can be made on the corrosion condition of the reinforcing steel. Alternative methods should be employed in determining the effectiveness of sacrificial anode repairs.
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    A critical appraisal of the developments in durability design, specification, construction and testing of concrete structures in South Africa
    (2019) Dlamini, Nonhle Nonkululeko
    The costs associated with the repair of deteriorating concrete structures have been reported to be substantial; however, the costs resulting from poorly designed or executed repairs may be even higher. Therefore, it is important to consider durability of concrete in the design and construction of concrete structures. Concrete durability is the capability of concrete to maintain serviceability over a specified period of time without showing significant deterioration. The aim of this research was to consolidate knowledge from work done in durability design, specification, construction and testing of concrete structures in South Africa; and investigate the application of this knowledge in practice. The method selected to investigate this was surveys in the form of structured questionnaires issued through electronic mails. The survey samples amongst stakeholders were selected by stratified random sampling which ensured a reduced likelihood of bias. Survey samples were selected as follows: Consulting Engineers (Sample size - 21, response rate – 86%); Tertiary Institutions (Sample size - 8, response rate – 100%); Contractors (Sample size - 19, response rate – 33%); Readymix Concrete Suppliers (Sample size - 9, response rate – 33%); Cement Suppliers (Sample size - 4, response rate – 75%); and Concrete Admixture Suppliers (Sample size - 4, response rate – 100%). From the data analysis, all stakeholders demonstrated awareness in the subject of durability of concrete as it was found that durability design concepts are incorporated through a careful selection of materials, consideration of the environment to which a structure will be exposed and careful planning of construction processes. The current methods in practice for design, specification, construction and testing of concrete were found to follow a ‘hybrid’ approach; which is a combination of prescriptive and performance-based approach. In general, the responses from the different stakeholders show similarities in terms of awareness of the need to have durable concrete, but this awareness is not being implemented by applying measureable durability-related properties of concrete to evaluate its penetrability. With the DI testing methods being incorporated into SANS standards, it is expected that the industry implement these tests in construction projects with the aim of achieving durable concrete. Also, learners in tertiary institutions should be introduced to these test methods as a step towards creating awareness in industry in which they eventually practice.
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    Statistical modelling of the shrinkage behaviour of South African concretes
    (2012-02-03) Gaylard, Petra Cornelia
    A hierarchical non-linear model was developed for the time-dependent shrinkage behaviour of South African concretes, from historical laboratory data. The fit of fifteen growth curve models to the shrinkage-time profiles was evaluated and MCDA was used to identify the best model. The three parameters of the chosen growth curve model were modelled in terms of covariates (relating to concrete raw materials, concrete composition and shrinkage testing conditions) by multivariate multiple regression to produce the WITS model. The model largely conformed to existing knowledge about the factors affecting concrete shrinkage. Published models for concrete shrinkage were compared to the WITS model regarding their predictive ability with respect to the South African data set. The WITS model performed the best across a variety of graphical and numerical goodness-of-fit measures. The importance of the study is two-fold:  The concept of hierarchical non-linear modelling has been applied for the first time to the modelling of the time-dependent properties of concrete.  This is the first comprehensive model to bring together laboratory data on the shrinkage of concrete generated in South Africa over a span of thirty years.