Browsing by Author "Abdelrahman, Khaled"

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    Performance of Eccentrically Loaded Reinforced Concrete Columns Confined with Shape Memory Alloy Wires
    (2017-12-15) Abdelrahman, Khaled; El-Hacha, Raafat; Duncan, Neil; Shrive, Nigel; Priest, Jeffrey; Federico, Salvatore; Al-Mahaidi, Riadh
    Smart materials such as Shape Memory Alloys (SMA) are recently utilized for strengthening and repairing of concrete structures. The most common form of SMA wires used for concrete confinement applications is Nickle Titanium (NiTi) because it possesses unique thermo-mechanical properties such as the Shape Memory Effect (SME) along with high recovery stress (up to 600 MPa) and strain (up to 8 %). Initial investigations reported in the literature show that actively confining concrete columns with SMA wires can enhance the strength and significantly increase the ductility of concrete. To date, the vast majority of experiments on SMA-confined concrete have considered short, unreinforced, small-scale concrete cylinders subjected to concentric axial loading. The objective of this study is to present a systematic study of medium-size reinforced concrete circular columns, internally reinforced with longitudinal steel bars and transverse steel stirrups and externally confined with SMA wires, subjected to eccentric loading. Test data are compared against identical unconfined reinforced concrete (RC) columns, and confined RC columns strengthened passively using Carbon Fibre Reinforced Polymer (CFRP) wrap sheets, in order to quantify and verify the effectiveness of the active SMA-confinement method. In addition, an analytical model is presented to describe the behaviour of SMA-confined RC columns subjected to combined axial loads and bending moments. A parametric study was conducted to evaluate the effects of certain parameters on the overall response and the axial-flexural diagrams of the SMA-confined RC columns. The results of this study show that the RC columns confined with active SMA spirals exhibited significant enhancement in the strength and ductility compared to the unconfined RC columns subjected to varying load eccentricity. Additionally, the SMA-confined RC columns demonstrated superior overall performance when compared to the conventional passive CFRP-confined RC columns. The results of this study also show that the proposed analytical model conservatively predicted the axial load-bending moment response of the SMA-confined RC columns. The RC column confinement parameters investigated, namely the concrete compressive strength, the volumetric ratio of the SMA spirals, and the internal longitudinal steel reinforcement ratio, significantly influenced the load-moment interaction response of the SMA-confined RC columns.
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    The Molecular Basis for Impaired Cerebral Myogenic Response in Type 2 Diabetes
    (2015-04-06) Abdelrahman, Khaled; Cole, William
    Cognition and brain function are dependent on appropriate control of blood flow within the cerebral circulation. Cerebral blood flow is controlled through the interplay of several physiological mechanisms that regulate the contractility of vascular smooth muscle cells (VSMCs) within the wall of cerebral arteries. The myogenic response of cerebral arteries is a crucial mechanism that is responsible for maintaining adequate brain blood flow. This fundamental mechanism is due to cellular processes intrinsic to VSMCs including: 1) Ca2+-calmodulin-dependent activation of myosin light chain kinase and phosphorylation of LC20, 2) Rho-associated kinase (ROK)-dependent phosphorylation of myosin phosphatase targeting subunit 1 (MYPT1) and suppression of myosin light chain phosphatase activity, and 3) dynamic reorganization of the actin cytoskeleton. Inappropriate regulation of one or more of these mechanisms may contribute to the dysfunctional control of cerebral diameter and flow, predisposing type 2 diabetic patients to ischemic and hemorrhagic stroke. Here, we employed Goto-Kakizaki (GK) rats, a type 2 diabetic rat model, to identify the molecular basis for the dysfunctional myogenic constriction in early and established type 2 diabetes. We detected an enhanced basal myogenic tone in prediabetic GK cerebral arteries at low intraluminal pressure that progressed with the severity of diabetes such that the myogenic response was lost in arteries of GK rats with established diabetes. Our biochemical evidence shows that there are parallel, progressive alterations in MYPT1 and LC20 phosphorylation, as well as actin polymerization downstream of ROK that are consistent with the evolution of dysfunctional myogenic response. These findings provide a better understanding of the underlying defects responsible for dysfunctional control of cerebral arterial diameter and blood flow in type 2 diabetes.