Browsing by Author "Mackechnie, James R"

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    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.
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    Corrosion of reinforcement in concrete : the effectiveness of organic corrosion inhibitors
    (1999) Rylands, Thaabit; Mackechnie, James R
    Reinforcement corrosion in concrete has presented engineers with the challenge of finding ways of prolonging the service life of structures built in aggressive environments. One method of increasing the durability of concrete in aggressive environments is the use of corrosion inhibitors. In this work, two organic corrosion inhibitors were tested to observe their effectiveness in decreasing the rate of corrosion or delaying the onset of corrosion. One of the inhibitors was a migrating corrosion inhibitor while the other was an admixed inhibitor. The corrosion rate of reinforcement in concrete specimens used in this evaluation, was measured using the Linear Polarisation Resistance method. The performance of the admixed inhibitor was also measured in aqueous phase tests. Results of the tests conducted indicate that the admixed inhibitor does delay the onset of corrosion. The Mel caused short to medium term inhibition when the chloride concentration was less than 1.5%.
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    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.
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    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.
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    Performance and properties of structural concrete made with corex slag
    (2001) Jaufeerally, Hassen; Alexander, Mark Gavin; Mackechnie, James R
    Slag has been used in concrete as a cementitious extender for about 50 years in South Africa. Traditionally, blastfurnace slag has been used in concrete but recently a new product called Ground Granulated Corex Slag (GGCS) has become available on the market in the Western Cape Province. The objective of this research is to characterise the properties of this new product in concrete. In this study, the properties of corex slag concrete were compared to normal blastfurnace slag and plain CEM I concretes. A series of mixes was undertaken, with water:binder ratios varying between 0.4 and 0.3, and slag replacement levels between 30 and 70%. The physical characterisation of corex slag revealed that the material is finer than blastfurnace slag, having more ultra-fine particles. The oxide analysis showed that the higher proportions of the calcium and magnesium oxides present in corex slag increase the potential for hydraulic activity of the binder compared with blastfurnace slag. The investigation of the fresh concrete properties revealed that the consistence of concrete increases with the use of slag, especially at higher water:binder ratios. Blastfurnace slag performed marginally better than corex slag. The use of slag generally increased the setting time of the paste, with pastes containing corex slag having a shorter final setting time than those containing blastfurnace slag. The setting time was also found to increase with increasing stag replacement level. The increase in setting time caused the bleed time of slag concrete to increase but the total bleed volume was significantly reduced. The bleed properties of concrete were improved when corex slag was used. It was found that the compressive strength of the new cementitious material lagged behind that of CEM I controls during early ages but after seven days, corex slag concrete showed higher strength. The strength was observed to decrease with increasing water:binder ratio. From the compressive strength results, it was deduced that the optimum corex slag substitution rate varied with water:binder ratio, ranging from 45 to 60 % The elastic modulus of corex slag concrete was found to be equal to or higher than CEM I concrete, especially at low water:binder ratios (wtb = 0.4). Deformation experiments showed that corex slag concrete had the lowest creep and shrinkage strains of all the concretes at low water:binder ratios. At higher water:binder ratios, however, the shrinkage strains were in the same range as the other two materials. Prediction models were also assessed and it was found that no model was able to accurately predict both creep and shrinkage. Generally, the shrinkage predictions were more accurate. Durability index tests showed that corex slag concrete has good to excellent potential durability, based on historical data. Marine exposure testing indicated that corex slag binds a substantial amount of chlorides, hence reducing the risk of corrosion in marine environments. Furthermore, the risk of carbonation-induced corrosion is also decreased. Expansion associated with alkali-silica reaction is minimised with the use of corex slag and the efficiency of the material increases with replacement level. It is concluded that corex slag is an excellent extender for use in concrete. Finally, recommendations are made on the need to further characterise the properties of the material.
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    Performance of a penetrating corrosion inhibitor in controlling carbonation induced corrosion in reinforced concrete
    (2001) Heiyantuduwa, Rukshani; Alexander, Mark Gavin; Mackechnie, James R
    Reinforcement corrosion in concrete remains the most serious cause of premature deterioration of concrete structures world-wide, and many methods have been proposed to combat this problem. One method of improving the durability of concrete in aggressive environments is the use of corrosion inhibitors. In this work the effectiveness of an organic penetrating corrosion inhibitor in reducing the rate of corrosion and delaying the onset of corrosion in carbonated concrete is discussed, with reference to corrosion rates.