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Damping and strain rate effect of geopolymer
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posted on 16.02.2017by Feng, Kenan
Geopolymers are produced by alkali activation of aluminosilicate raw materials (fly ash or metakaolin), which are transformed into reaction products by polymerisation in a high pH environment and hydrothermal conditions at relatively low temperatures (up to 120°C). In the past decade, much experimental work has focused on the mechanical properties of geopolymer based materials subjected to static loading, covering Young's modulus, Poisson's ratio, compressive strength, splitting tensile strength, fracture energy, fire resistance and sulphate resistance. It has been shown that geopolymers can provide comparable performance to traditional cementitious binders in a range of applications, with the added advantage of significantly reduced greenhouse emissions. However, there are few published studies of damping properties and strain rate effect on geopolymers under dynamic loadings. Structural engineers routinely need to make assumptions about the dynamic properties (damping and impact resistance) of a building to simulate its response to dynamic loads such as strong winds, earthquake ground motions, and even impacts. To address this gap, this thesis explores both the damping property and strain rate effect of geopolymers as construction materials. The critical damping value (ζ) is the main parameter in relation to vibration reduction. In this study, free vibration tests and the traditional logarithmic decrement technique were used to measure the ζ of geopolymers. Geopolymers were prepared by activating fly ash using alkali solutions with different Si02/Na20 ratios. The results show that the ζ of the geopolymers is similar to that of the Ordinary Portland Cement (OPC) counterpart. The effect of strain rate on the compressive behaviours, the critical strain and the splitting tensile strength of geopolymer concrete and mortar are presented under a wide range of strain rate loadings. A Shimadzu AG-X 300 kN testing machine was adopted to measure the compressive behaviours at a quasi-static strain rate and low strain rates, and split Hopkinson pressure bar (SHPB) techniques were used at high strain rates. The dynamic increase factors for compressive strength (DIFfc), the dynamic increase factors for splitting tensile strength (DIFft) and the critical strain (DIFec) were measured and compared with results of OPC concrete and its empirical equations. The results show that the coarse aggregates in geopolymer concrete mixes play an important role in the increase of compressive strength and that the viscous effect and crack inertia are responsible for the improvement of splitting tensile strength. Furthermore, the existing formula for OPC concrete underestimates the DIFft of geopolymer concrete and therefore new empirical equations are proposed. In the initial stage of the candidature, the topic of carbon nanotube reinforced OPC paste was explored. However, due to occupational health and safety issues, it was not permitted to conduct SHPB tests on OPC pastes containing carbon nanotubes (CNTs) in the laboratory conditions. Preliminary research on Molecular Dynamics (MD) simulations of the energy absorption properties of CNTs was conducted and the relevant research findings are presented in the Appendix.