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PhD Student, University of Adelaide

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Physiochemical and mechanical properties of cement mortar composites containing GO and reduced-GO

Cementitious materials are the most widely used construction materials throughout the world. Although this type of material has relatively good compressive strength, brittleness, very low tensile strength and strain, and low flexural strength are their weaknesses. Graphene is usually prepared by exfoliating the graphite in water using sonication and with oxidation using strong oxidizing agents. Graphite oxide is separated into several single layers, as graphene oxide (GO), which is easily dispersible in water. Another form of graphene is reduced graphene oxide (rGO) prepared by reducing oxygen groups of pristine graphene, which is obtained directly from graphite.

In recent years, several studies have proposed the significant potential of GO for enhancing mechanical properties of cementitious materials and designing new composites for specific applications. However, although significant progress has been made in previous studies, there is considerable inconsistency in these reports showing different effects of GO on the mechanical properties of composite due to the use of different GO materials and preparation conditions, neither properly characterized. These studies were mainly focused on conducting mechanical tests to show the effect of GO on the mechanical properties of composite without presenting or exploring how properties of used GO materials including the concentration, size, number of layers, defects, and density of oxygen groups could influence these properties. In this research, we present the first in series of studies, with the aim of investigating the influence of GO contents on the physiochemical and mechanical properties of cement mortar composites.

The results show that the optimal percentage (i.e. 0.1%) of GO in the composite led to a 37.5% and 77.7% increase in the 28 days tensile and compressive strengths of GO–cement mortar composites compared to the plain cement mortar composite. Study revealed that GO not only prevent the crack propagation from the nanoscale to microscale and increase the hydration degree of the cement mortar composites, but also improve accessibility of the water to the GO oxygen functional groups and cement C–S–H component, indicating the importance of having appropriate GO concentrations. These observations with obtained mechanical properties that were validated by SEM, TGA, XRD, and FTIR analyses provide an essential link between structural, chemical, and mechanical properties.

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Modeling the behavior of FRP-confined concrete using dynamic harmony search algorithm

Abstract: The accurate prediction of ultimate conditions for fiber reinforced polymer (FRP)-confined concrete is essential for the reliable structural analysis and design of resulting structural members. Nonlinear mathematical models can be used for accurate calibration of strength and strain enhancement ratios of FRP-confined concrete. In this paper, a new procedure is proposed to calibrate the nonlinear mathematical functions, which involved the use of a dynamic harmony search (DHS) algorithm. The harmony memory is dynamically adjusted based on a novel pitch generation scheme using a dynamic bandwidth and random number with normal standard distribution in DHS. A new design-oriented confinement model is proposed based on three influential factors of FRP area ratio ( \( \rho_{a} \) ), lateral confinement stiffness ratio ( \( \rho_{E} \) ), and strain ratio ( \( \rho_{\varepsilon } \) ). Five nonlinear mathematical design-oriented models are regressed on approximately 1000 axial compression tests of FRP-confined concrete in circular sections based on the proposed DHS algorithm. The proposed models for the prediction of the ultimate axial stress and strain of FRP-confined concrete are compared with the existing models. It has been shown that the DHS algorithm offers the best performance in terms of both accuracy and fast convergence rate in comparison with the other modified versions of harmony search algorithms for optimization problems. The proposed design-oriented model provides improved accuracy over the existing models. The accurate prediction of ultimate conditions for fiber reinforced polymer (FRP)-confined concrete is essential for the reliable structural analysis and design of resulting structural members. Nonlinear mathematical models can be used for accurate calibration of strength and strain enhancement ratios of FRP-confined concrete. In this paper, a new procedure is proposed to calibrate the nonlinear mathematical functions, which involved the use of a dynamic harmony search (DHS) algorithm. The harmony memory is dynamically adjusted based on a novel pitch generation scheme using a dynamic bandwidth and random number with normal standard distribution in DHS. A new design-oriented confinement model is proposed based on three influential factors of FRP area ratio ( \( \rho_{a} \) ), lateral confinement stiffness ratio ( \( \rho_{E} \) ), and strain ratio ( \( \rho_{\varepsilon } \) ). Five nonlinear mathematical design-oriented models are regressed on approximately 1000 axial compression tests of FRP-confined concrete in circular sections based on the proposed DHS algorithm. The proposed models for the prediction of the ultimate axial stress and strain of FRP-confined concrete are compared with the existing models. It has been shown that the DHS algorithm offers the best performance in terms of both accuracy and fast convergence rate in comparison with the other modified versions of harmony search algorithms for optimization problems. The proposed design-oriented model provides improved accuracy over the existing models. \( \rho_{a} \) \( \rho_{a} \) \( \rho_{E} \) \( \rho_{E} \) \( \rho_{\varepsilon } \) \( \rho_{\varepsilon } \)

Pub.: 13 Sep '16, Pinned: 25 Aug '17