Zhong, Hui; (2023) Static and Dynamic Mechanical Properties of Sustainable Engineered Geopolymer Composites. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Engineered geopolymer composites (EGCs) exhibiting superior tensile strain-hardening and multiple cracking behaviour are sustainable alternatives to traditional ductile cementitious composites whereas the extremely high cost and potential environmental impact of the widely used oil-coated polyvinyl alcohol (PVA) or polyethylene fibres in EGCs would limit their large-scale application. Partial replacement of these fibres with recycled fibres can reduce the material cost and improve the sustainability of EGCs. In recent years, the fresh properties and static mechanical properties of EGCs made from different precursors including fly ash and ground granulated blast-furnace slag have been increasingly studied, while the dynamic mechanical properties of EGCs cured at ambient temperature have been rarely explored. This thesis aims to develop a novel sustainable fly ash-slag based EGC with recycled tyre polymer (RTP) fibre as partial substitute for PVA fibre based on micromechanics design theory and systematically investigate the effect of hybrid PVA and RTP fibre content on the quasi-static and dynamic mechanical properties of EGCs. Firstly, a series of tests were conducted to study the engineering properties of EGCs with different PVA and RTP fibre dosages including workability, setting time, drying shrinkage, compressive strength, elastic modulus, splitting tensile strength, uniaxial tensile behaviour as well as the micromechanical characteristics in terms of fibre bridging stress-crack opening and strain-hardening indices. Afterwards, the dynamic compressive and splitting tensile behaviour were explored using the split Hopkinson pressure bar apparatus in terms of failure pattern, stress-strain (or displacement) response, dynamic compressive and splitting tensile strengths, dynamic increase factor (DIF) and energy absorption capacity. Some empirical equations were proposed to estimate the relationships between DIF and strain rate. Afterwards, the microstructural characterisation using scanning electron microscopy (SEM), backscattered electron microscopy (BSE) and X-ray computed tomography (XCT) techniques was carried out to understand the fibre bridging mechanism of the hybrid PVA-RTP fibre reinforced EGC. Lastly, the optimal mixtures of EGC were proposed considering the material cost, environmental impact, and acceptable engineering properties for civil infrastructure applications. Results indicate that the developed EGCs can meet the strength-based and energy-based criteria for robust tensile strain-hardening behaviour. Replacing a small amount of PVA fibre with RTP fibre can lead to better dynamic mechanical properties and drying shrinkage resistance, lower material cost and higher sustainability as well as retaining acceptable static mechanical properties. This thesis can provide new insights into the effect of hybrid fibre reinforcement on the dynamic mechanical behaviour of EGCs under a wide range of strain rates (10⁰ s¯¹ to 10³ s¯¹) as well as promote the development and application of low-carbon construction materials in the future.

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