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Title: Structural performance of precast reinforced geopolymer concrete sandwich panels enabled by FRP connectors
Authors: Huang, Junqi
Degree: Ph.D.
Issue Date: 2019
Abstract: Precast concrete sandwich panels (PCSPs), as a typical structural element used in the precast industry, have been widely used as the facade walls or load-bearing walls in engineering practice. They consist of inner and outer reinforced concrete wythes, core insulation and connectors penetrating through the insulation. Depending on the stiffness and strength performance, the PCSPs are classified into three categories: fully composite, partially composite and non-composite. Traditionally, concrete block and steel bent-up bar were used as the connectors, which could achieve a high degree of composite action (in terms of stiffness and strength). However, the thermal bridge effect may occur due to the higher thermal conductivity of steel and concrete. Consequently, the energy efficiency of the entire sandwich panel is reduced. Therefore, fiber-reinforced polymer (FRP) materials have been recently used to manufacture the connectors due to their high strength but low thermal conductivity. However, limited work has been conducted to investigate how the FRP connectors influence the structural performance of the formed PCSPs. In addition, most existing FRP connectors were designed to transfer one-directional shear force and the formed PCSPs were usually non-composite type due to the lower stiffness and capacity of the connectors. Against the above background, this thesis aims to develop an innovative PCSP system. In this system, environmentally geopolymer concrete is adopted to replace ordinary Portland cement (OPC) for use in the two wythes. The geopolymer concrete is produced by the alkaline activation of industrial by-products (i.e., blend of fly ash and slag) to realize ambient temperature curing and achieve comparable performance to the OPC. Meanwhile, FRP rebar is used to replace steel rebar to improve the durability and minimize the thickness of the entire panel. In addition, a tubular glass FRP (GFRP) connector was developed to enhance the composite action of the PCSP. As a result, the proposed PCSP system can retain the energy efficiency of all existing PCSPs but is entitled with low carbon-footprint, high durability and superior structural efficiency. The thesis is composed of extensive experimental and finite element (FE) analysis work covering three main parts: (1) the development of a new type of GFRP tubular connector and its performance characterization; (2) structural performance of steel and FRP-reinforced geopolymer concrete one-way slabs; and (3) the structural behaviour of the GFRP connector-enabled PCSP system.
In the first part of this thesis, twenty-five in-plane direct shear tests were conducted to evaluate the effect of GFRP laminate thickness, projected length, and shear force direction on the performance of three types of GFRP connectors (i.e., flat plate, corrugated plate and hexagonal tube). The shear force vs. relative slip relationships and failure modes of all types of connector were studied and discussed. Besides, three-dimensional FE analysis was conducted to reproduce the test results, aiming to facilitate an in-depth understanding of the failure mechanisms and the full-range performance of the connectors. In the second part of this thesis, the flexural performance of six steel reinforced geopolymer concrete one-way slabs and six OPC concrete counterparts were tested through four-point bending. The shear performance of six basalt FRP (BFRP) reinforced geopolymer concrete one-way slabs were also evaluated. The investigating parameters mainly included concrete strength and reinforcement ratio. Their load-deflection relationships, crack patterns, failure modes and load-strain relationships were studied and compared. Meanwhile, two dimensional (2D) FE analysis was conducted to reproduce the test results to fully understand the behaviour of the reinforced geopolymer concrete slabs as the two wythes in the PCSP system. In the third part of this thesis, an experimental study was carried out on the flexural performance of eight precast geopolymer concrete sandwich panels. Four parameters were investigated, including the connector type (i.e., plate-type and hexagonal tube connector), connector spacing, rebar type (i.e., steel and BFRP rebar) and reinforcement ratio. The load-deflection relationships, crack patterns, failure modes, load-strain relationships and degrees of composite action (in terms of both stiffness and strength) of the specimens were carefully investigated. In addition, 2D FE analysis incorporating the shear-slip constitutive laws of FRP connectors was conducted to reproduce the test results. A simplified but innovative approach (with closed-form solutions) based on the continuum method was developed for predicting the stiffness and serviceability of the PCSP under out-of-plane load and validated by the test results. Upon the completion of the thesis, an in-depth understanding of the shear transfer mechanism of the proposed tubular GFRP connectors, the failure mechanisms of steel and BFRP reinforced geopolymer concrete one-way slabs, and the mechanisms of full-range interaction between the tubular GFRP connectors and BFRP-reinforced geopolymer concrete wythes has achieved. These scientific findings have laid a solid foundation for the development of design guidelines for the proposed PCSP system, which is believed to have great potential for use in the prefabricated construction industry in Hong Kong, mainland China as well as the rest of the world.
Subjects: Hong Kong Polytechnic University -- Dissertations
Fiber reinforced concrete
Precast concrete construction
Structural analysis (Engineering)
Pages: xx, 203 pages : color illustrations
Appears in Collections:Thesis

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