Browsing by Author "Alexander, Mark G."

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    An international perspective on performance-based specifications for concrete durability, with a suggested framework for implementation
    (2025) Mosoy, Florah; Alexander, Mark G.
    Reinforced concrete (RC) is one of the world's most durable building materials. However, outdated and insufficient durability specifications often lead to the early deterioration of RC structures, escalating global maintenance costs. Prescriptive durability specifications for reinforced concrete (RC) often include mix design parameters such as minimum cement content and maximum water/binder ratio, which are intended to contribute to durability. However, achieving a specific compressive strength alone does not guarantee durability. These rigid regulations stifle innovation in design and construction and often fail to capture the durability characteristics of modern concrete. Even if a mixed design complies with specifications, it may not achieve its intended service life under varying exposure conditions. This study establishes an international perspective on using performance-based specifications for concrete durability, contrasting them with the commonly used prescriptive based specifications. Performance-based approaches allow for customizing concrete mixtures to meet specific performance goals by focusing on measurable properties that ensure performance under particular conditions. These requirements can be applied to various stages, including design, service life modelling, specification, pre-qualification, and conformance evaluation. The study involved a review of current durability provisions in selected standards from the USA, Canada, Australia, Europe, India and South Africa, followed by an analysis of performance-based specifications implementation in these countries. Key factors influencing the adoption of performance-based specifications, such as regulatory frameworks, industry practices, and environmental exposure classification systems, were examined. The study proposes a practical framework for adopting performance-based specifications for concrete durability, covering aspects such as verifying environmental exposure conditions, concrete cover requirements, material constituents, alternative cementitious materials, testing methods for performance specification and also regulatory framework. This framework aims to educate professionals on the practical implications of performance-based specifications and promote its adoption for enhancing the durability and sustainability of concrete structures. By addressing these elements, this study provides a framework to establish a structured, adaptable approach that ensures concrete structures meet desired durability and service life requirements under specific environmental conditions. This framework prioritizes measurable performance criteria over rigid prescriptive measures, enabling tailored concrete mix designs that address exposure conditions, material quality, and maintenance expectations. By doing so, the proposed framework promotes innovation, sustainability, and cost-effectiveness in concrete construction, ultimately enhancing structural resilience and reducing long-term repair and maintenance costs.
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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.