| Author: | Weng, Kefan |
| Title: | Shear behavior of high-performance reinforced concrete beams enabled by FRP and UHPC |
| Advisors: | Dai, Jian-guo (CEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Reinforced concrete construction Concrete beams Fiber-reinforced concrete Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxxiii, 313 pages : color illustrations |
| Language: | English |
| Abstract: | Strain-Hardening Ultra-High Performance Concrete (UHPC) has been engineered to exhibit exceptional compressive strengths ranging from 150 to 250 MPa, coupled with high tensile strength, tensile strain hardening, and enhanced tensile multi-cracking capabilities. Compared to conventional concrete, UHPC demonstrates markedly reduced water permeability and increased resistance to chloride penetration positioning it an ideal material for integration with corrosion-free fiber-reinforced polymer (FRP) to produce high performance and sustainable marine infrastructure. The application of UHPC offers dual benefits in this context. Firstly, the high strength of UHPC enhances the utility of FRP. Secondly, the strain hardening behavior significantly improves the effective strain capacity of FRP, thereby enhancing its structural application potential. This study is dedicated to investigating the shear behavior of reinforced concrete (RC) beams that incorporate the FRP/UHPC composite system. This system is explored both as a method for repairing and strengthening existing structures and as an innovative permanent formwork in prefabricated construction. A comprehensive research program was carried out to clarify the fiber hybridization and matrix design effect of strain hardening UHPC, interface bond behavior of UHPC to cast-in-place concrete, shear behavior of FRP-UHPC permanent formwork enhanced RC beams and FRP-UHPC composite layer strengthened RC beams. The first part of this dissertation is devoted to producing the Strain-Hardening Ultra-High-Performance Concrete (SH-UHPC) through micromechanics design and use of hybrid fibers. This study explored the advanced properties of SH-UHPC by employing a novel hybrid design strategy for reinforcing fibers and matrices. This approach yielded SH-UHPC with exceptional mechanical characteristics, including compressive strengths surpassing 210 MPa, tensile strengths of 7 to 15 MPa, and a tensile ductility range of 0.4–5.6%, alongside a crack width between 41–86 μm at the peak tensile strain. The effects of fiber content (1%-3% 20 mm steel fiber), fiber hybridization (combining PE and steel fibers), and fiber aspect ratios (100 for 20 mm steel fiber and 750 for 18 mm PE fiber) on the mechanical properties of SH-UHPC was comprehensively investigated. The study utilized the digital image correlation (DIC) method to analyze the intricate multiple cracking behavior of SH-UHPC. Additionally, the overall performance including compressive strength (f'c), tensile strength (ft), tensile strain capacity (εt), crack width (w), and crack width standard deviation (sw) was evaluated through a quintuple-dimensional (5D) analysis framework and compared against SH-UHPC in existing literature. Furthermore, the cost analysis considered the overall performance was conducted for an extensive understanding of the material's adoption and its potential for widespread application in the construction industry. The second part of this dissertation examines the interface shear behavior between prefabricated UHPC and cast-in-place concrete by employing the push-out test. The failure process and mechanisms of the UHPC/concrete interface are analyzed with the DIC method. The main influencing factors related to the teeth-shaped UHPC groove include their number, height, geometry and strength, as well as the loading direction relative to the shear key. Three typical failure modes were identified from the test: pure interface failure, combined interface and cast-in-place concrete side failure, and concrete splitting failure. It was found that the UHPC teeth's dimensions critically influence the shear interface capacity, with an increase in tooth height correlating to enhanced resilience against cracking and ultimate failure. The geometric design of the teeth, particularly the triangular shape, was observed to optimize stress distribution and increase shear-compression area. Furthermore, the study identifies a strategic inter-tooth spacing that maximizes shear capacity, underscoring a design optimization potential. The analysis also proposes that a 1% volume fraction of steel fiber within UHPC facilitates sufficient interface shear strength, indicating potential for cost-effective material utilization in formwork applications. The third part of this dissertation investigates the shear behavior of FRP-UHPC permanent formwork-enhanced concrete beams. A total of twelve beams were prepared and tested to examine the impact of various factors, including the span-to-depth ratio (a/d = 1.57 vs. 2.52), the matrix fiber type (steel vs. polyethylene fiber), and the enhancement effect of CFRP within the UHPC formwork. The failure process and crack propagation are analyzed with the DIC method. Key findings demonstrate that the FRPUHPC formwork with grooved geometry exhibited an excellent bond with the concrete member. Compared to control beams, a substantial increase in load capacity of RC beams with UHPC formwork was identified, ranging from 40% to 65%, as well as notable improvements in initial and post-crack stiffness. The incorporation of CFRP bars into the UHPC formwork significantly restrains shear crack development, thereby enhancing the shear capacity of the beams. Particularly, the ductile shear failure mode of the composite beams was observed for the first time. In addition, decreasing the shear span-to-depth ratio led to a distinct shear failure mode with increased load capacity. A theoretical analysis method was proposed for predicting the shear capacity of the FRP-UHPC permanent formwork-enhanced RC beams. The last part of this dissertation presents a comprehensive investigation into the shear strengthening of RC beams using FRP-UHPC composite layer. A total of ten RC beams were prepared and tested to evaluate the efficacy of the FRP-UHPC layer in enhancing shear capacity, including eight FRP-UHPC strengthened beams and two control beams. Key variables on the strengthening performance were the type of fiber in the strengthening layer (steel fiber vs. polyethylene fiber), the bonding methods (epoxy vs. surface treatment), and the shear span-to-depth ratios (2.52 vs. 1.57). Experimental findings revealed that the FRP-UHPC strengthened beams exhibited substantially increased stiffness and load capacity compared to control beams. A consistent shear-compression failure mode was observed in all beams, with the cracking behavior and failure mechanisms further analyzed through the DIC technique. It was found that fiber-reinforced FRP-UHPC composite layer significantly suppressed the formation of shear cracks and reduced the crack width. Micro-scale analyses demonstrated a robust bond existing between UHPC and the concrete matrix. A finite element (FE) model was constructed to simulate the experimental tests for a comprehensive understanding of the mechanical behaviors and validated its effectiveness. The findings of this study provided useful knowledge for the design and application of the proposed FRP-UHPC composite layer in strengthening RC structures. This dissertation work has created a significant body of new knowledge on the FRP-UHPC composite system and their potential applications, in terms of material development, interface shear force transfer mechanisms, shear strengthening mechanisms, and permanent formwork system. The proposed system has a huge potential for applications particularly in sustainable marine concrete structures, with aims to improve the durability, structural efficiency and constructability. |
| Rights: | All rights reserved |
| Access: | open access |
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