Browsing by Author "Leo, Emmanuel"

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    A review on the efficiency of different supplementary cementitious materials as a partial replacement for Portland cement in concrete
    (2022) Taiwo, Ridwan A; Alexander, Mark; Leo, Emmanuel
    The effects of global warming and climate change are important and have attracted the attention of many researchers. Global warming is a result of the presence of increasing amounts of greenhouse gases in the atmosphere. Carbon dioxide, which is largely emitted into the atmosphere during the manufacture of cement clinker, is one of the greenhouse gases. Hence, researchers have explored the use of some waste materials and naturally occurring minerals as a partial replacement for cement in concrete. These materials are often referred to as supplementary cementitious materials (SCMs). Apart from the potential benefits of these SCMs for the properties of concrete, they also bring about a reduction in the amount of waste in landfill sites, as these wastes can cause land, water, and air pollution, thereby posing threats to human health. However, despite the potential benefits of SCMs in the cement and construction industry, they have not been fully utilized especially in developing countries in Africa. This may be due to low awareness of the potential benefits of SCMs among the stakeholders in the construction industry, and also limited availability. Nevertheless, due to extensive research into the usability of different materials as SCM, various materials are available in the construction market as binder systems. Thus, selecting the appropriate binder system to get the desired result for a particular concrete might be difficult for construction personnel. Hence, this study presents a review of the effects of various SCMs on the mechanical and durability properties of concrete. Six SCMs are reviewed. These SCMs include fly ash, silica fume, which are industry by-products; metakaolin, limestone calcined clay, which are naturally occurring minerals; rice husk ash, which is an agricultural waste material; and limestone-fly ash, which is a combination of an industrial by-product and a naturally occurring material. Firstly, an overview of the mechanical and durability properties of concrete is presented. This includes the presentation of general factors affecting the mechanical and durability properties of concrete. Subsequently, the effect of the various SCMs on mechanical (such as strength, elastic modulus, creep, and shrinkage) and durability properties (freeze-thaw, acid attack, sulphate attack, chloride-induced corrosion, carbonation-induced corrosion, and alkali-silica reaction) of concrete are presented. The review shows that the inclusion of appropriate dosage of these SCMs in concrete or mortar enhances their properties. Certain limitations of these SCMs are also discussed. This study also identifies areas of further research in relation to the properties of concrete produced with the SCMs.
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    Dynamic performance of pre-cast prestressed beams – cast in-situ slab composite bridges
    (2013) Leo, Emmanuel; Moyo, P; Beushausen, H. D
    Most bridge management systems still rely on visual inspections for condition assessment of bridges; this means that damage in inaccessible parts of the structure such as shear connectors in concrete composite bridges remain undetected until catastrophic failure occurs. Localized non-destructive techniques such as ultrasonic techniques, radar method, impact testing, magnetic based methods and proof load tests are limited to small areas, time consuming and require prior knowledge of the damaged zone. These limitations can be overcome by using dynamics-based techniques. The main objective of this work is to investigate experimentally the effectiveness of dynamics-based techniques in assessing the condition of shear connectors in concrete composite bridges consisting of pre-cast prestressed beams and a cast in-situ slab based on measurements taken from the surface of the accessible deck slab. In this research, shear links of 8mm bars extended from beam to the slab are used to stimulate shear connectors in real bridges. The experimental work involved building five concrete composite beams each with different number of shear connectors. The testing procedure consisted of measuring the dynamic properties in both the undamaged and damaged beams. Damage was introduced by accelerating corrosion to a group of shear connectors near the supports in each composite beam. Push-off test was conducted in order to determine the shear capacity of the shear connectors in both undamaged and damaged state. The modal tests were successfully executed and from the modal analysis results it was observed that a beam with large number of shear connectors produce high frequencies and high amplitudes of frequency response functions (FRFs) compared to the one with less number of shear connectors. After the shear connectors were damaged all beams showed similar results. In the FRFs, the frequency peaks shifted to the left and the peaks amplitudes changed, the natural frequencies generally dropped indicating the existence of damage. In an attempt to locate regions with damaged shear connectors, the Coordinate Modal Assurance Criteria (COMAC), change of flexibility, change of curvature and strain energy method were used. All methods showed positive and negative results. The change of flexibility method showed minimum negative results compared to other methods in locating regions with damaged shear connectors. Generally, Results show that dynamics-based techniques can be used to detect and localize regions with damaged shear connectors in pre-cast prestressed beams - cast in-situ slab composite bridges by only taking vibration measurements from the surface of the accessible deck slab.