| Author: | Zhu, Jixiang |
| Title: | Development of FRP-reinforced UHS-ECC composite system for performance enhancement of RC beams |
| Advisors: | Dai, Jian-guo (CEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Reinforced concrete Concrete beams Reinforced concrete construction Composite materials Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxiii, 186 pages : color illustrations |
| Language: | English |
| Abstract: | The latest innovations in structural engineering are often driven by the advent of advanced construction materials. Over the past three decades, concrete construction has made several landmark advances in new material technologies, including ultra-high-performance concrete (UHPC), high-performance fiber-reinforced cementitious composites (HPFRC) and fiber-reinforced polymer (FRP) composites. UHPC is characterized by a dense and discontinuous pore structure that reduces defects and mass transport in concrete, significantly improving its strength and durability. HPFRC, often referred to as Strain Hardening Cementitious Composites (SHCC) or Engineered Cementitious Composites (ECC), are characterized by multiple cracking, strain-hardening and ultra-high ductility. FRP composites have the advantages of having a high strength-to-weight ratio and being non-metallic, which solves the critical problem of steel corrosion in marine concrete structures. The use of these new materials enables lighter and more durable RC structures, ensures the structural safety and resilience under extreme mechanical and/or environmental loading, reduces the overall environmental impact, and mimeses the life-cycle costs. This study aims to develop a novel Ultra-High-Strength Engineered Cementitious Composites (UHS-ECC) reinforced with Fiber-Reinforced Polymer (FRP) for the performance enhancement of RC structures. The adoption of FRP and UHS-ECC system combines the advantages of FRP, UHPC, and ECC technologies and entitles concrete structures with enhanced durability, structural efficiency, and constructability in a cost-effective manner. This dissertation starts with the development and design of FRP-reinforced UHS-ECC panels to obtain a comprehensive understanding on the composite action of this system. After that, the size effect of UHS-ECC was also studied to facilitate better understanding of the material behavior and constitutive law for structural design and finite element analysis (FEA). Then, in-depth investigations on the structural performance of FRP-reinforced UHS-ECC, respectively, as permanent formwork and strengthening system, were carried out through flexural tests and FEA. It was revealed through the tests and analyses that the ultra-high strength and multiple cracking behavior of UHS-ECC significantly improved the material utilization efficiency of FRP and avoided local stress concentration, leading to much enhanced load capacity, stiffness and ductility of FRP-reinforced UHS-ECC panels. Due to the extremely high mechanical performance of such panel, reinforced concrete (RC) beams with FRP-reinforced UHS-ECC as the permanent formwork or the strengthening layer exhibited excellent strength and ductility performance, ensuring their structure efficiency and resilience under various loading conditions. The work in the thesis has led to the creation of an innovative and high-performance composite system that retains the benefits of traditional concrete structures while significantly improving the constructability, structural efficiency and durability of the members, as well as the cost-effective use of new materials. A comprehensive understanding of the composite action in RC members incorporating the FRP-reinforced UHS-ECC system was achieved. The developed FRP-reinforced UHS-ECC composite system has great potential for use in both prefabricated and cast-in-place concrete infrastructure applications. Detailed contents of main chapters are briefly summarized as follows: In Chapter 3, Ultra-High-Strength Engineered Cementitious Composites (UHS-ECC) panels with Fiber-Reinforced Polymer (FRP) reinforcement (i.e., FRP bars and girds) were proposed for the construction of sustainable marine structures. Based on four-pointed bending tests, the mechanical performance of FRP-reinforced UHS-ECC [with 2% polyethylene (PE) fibers] and FRP-reinforced ultra-high-strength concrete (UHSC, without fibers) panels were investigated and compared to understand the composite action between UHS-ECC and FRP. Compared with the FRP-reinforced UHSC panel, the FRP-reinforced UHS-ECC panel showed significantly higher ultimate load (139%–173% higher), stiffness, and deformation capacity. It was also found that the use of seawater as the raw material had almost no effect on the mechanical performance of FRP-reinforced UHS-ECC (UHSC) panels. For FRP-reinforced UHS-ECC (UHSC) panels, it was revealed that using UHS-ECC to replace UHSC improved the stress transfer and deformation compatibility with FRP because the multiple cracking behavior of UHS-ECC lowered the crack-induced shear stress concentration along the FRP reinforcement. The developed FRP-reinforced UHS-ECC system showed great potential in the construction of durable and sustainable marine infrastructure. In Chapter 4, as the tensile behavior of ECC could be significantly affected by the distribution of the fiber orientation, the effect of fiber orientation distribution on the tensile performance of ultra-high-strength ECC in cases of different specimen thicknesses (10/20/30 mm) and various fiber lengths (6/12/18 mm) was studied through direct tension test. Ultra-high-strength ECC with compressive strength over 150 MPa and tensile strength over 5 MPa was achieved in this study. For the fiber length series, as fiber length increased, the tensile strain capacity increased, while the effect of fiber length on the tensile strength was negligible. For the specimen thickness series, the tensile strength was reduced significantly in the thicker specimens and the decline of tensile strength of specimens with 18-mm fiber was more dramatic compared to the specimens with 12-mm fiber. In addition, micromechanical analysis was conducted to interpret the tensile results. It was found that higher percentage of pulled-out fibers occurred in the UHS-ECC with shorter fiber length based on the micromechanical modelling, which led to a reduced bridging stress. For the specimen thickness series, higher percentage of ruptured fibers were observed in thick specimens according to the modelling result, since the fibers with larger inclination angle were subjected to higher stress due to snubbing effect and therefore reduced the fiber-bridging efficiency and capacity. The PSH indices increased significantly as the fiber length increased but exhibited a minor variation as the specimen thickness increased, which is consistent with the observations from the experiment. In Chapter 5, the use of FRP-reinforced UHS-ECC/UHPC as permanent formwork for RC beams was proposed. The mechanical performance of beams with FRP-reinforced permanent formwork were investigated through four-point bending test and beams with same experimental design were prepared to ensure the accuracy of test data. For beams with FRP-reinforced UHS-ECC permanent formwork, it was found that multiple cracking of UHS-ECC alleviated stress concentration between the formwork layer and the concrete substrate. For beams with FRP-reinforced UHPC permanent formwork, the crack localization occurred in the UHPC layer caused deformation incompatibility at nearby interface, which resulted in reduced stiffness, load capacity and ductility compared with beams with FRP-reinforced UHS-ECC permanent formwork. In general, the stiffnesses, yield loads and ultimate loads of beams with FRP-reinforced UHS-ECC/UHPC permanent formwork were close to those of control beams, and thus demonstrating the feasibility of the proposed surface preparation method. In Chapter 6, a novel strengthening system using FRP-reinforced UHS-ECC was proposed for retrofitting existing reinforced concrete structures. A total of 8 beams were tested under four-point bending to investigate the effects of different strengthening levels (non-, UHS-ECC-, and FRP-reinforced UHS-ECC-strengthened) and the composite behavior of different constituents (FRP and UHS-ECC) on the structural performance of the composite beams. Two different bonding methods were explored, epoxy-bonding and UHS-ECC-bonding. Experimental results showed that UHS-ECC-strengthened beams exhibited a limited increase of 8% in the load capacity and a reduced maximum deflection compared to control beams. Remarkably, when FRP-reinforced UHS-ECC was bonded to the RC beam, the load capacity of the composite beam was improved by 44%, while the value reduced to 30.6% when epoxy bonding was used, due to some debonding at the UHS-ECC/concrete interface. All tested beams failed by concrete crushing, indicating that both bonding methods were effective. A three-dimensional finite element model was proposed for the strengthening system and demonstrated to accurately predict the load-deflection curves and reveal the cracking process. Simulation results showed that FRP bar achieved 75% of its tensile strength, rendering it more effective than FRP grid as the reinforcement in the strengthening layer. Parametric studies revealed that the UHS-ECC/concrete bond property had more significant influence on the composite beam behavior than the FRP/UHS-ECC bond did, implying that weaker in-organic matrix with low cost might be feasible for impregnating FRP bars, and the UHS-ECC/concrete bond deserved more attention. |
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| Access: | open access |
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