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2 1.1 Background Prestressing concrete bridge beams with carbon fiber reinforced polymer (CFRP) tendons has been a research topic for the last three decades. Several researchers have shown that CFRP can be a viable alternative to steel strands in prestressed concrete bridge applications, especially in aggres- sive environments where steel corrosion is a concern. The advantages of prestressing CFRP result from the inherent material properties, which include resistance to electrochemical corrosion, high strength-to-weight, and high stiffness-to-weight ratios. A review of the literature indicated that prestressing CFRP has been used in only about 80 demonstration bridges worldwide since 1988. Thirty-four responses to a survey of state departments of transportation (DOTs) and the District of Columbia revealed that a major contributing factor to the limited use of this technology in the United States is the lack of well-defined and established design specifications. Apart from a limited number of guides, manuals, and commentaries, there are currently no comprehensive guidelines available for the design of concrete structures prestressed with CFRP systems in the United States. The design of concrete bridges prestressed with CFRP systems depends on several factors includ- ing material properties of CFRP system components, load transfer mechanisms, anchorage properties, sustained loads and environmental conditions. The serviceability and strength design of bonded and unbonded CFRP prestressed concrete beams is addressed in this report in terms of stress limits for CFRP tendons. This includes harped/draped configurations, prestress losses, flexure and shear capacity, serviceability limit states, durability, fatigue, bond, and devel- opment and transfer lengths. The design of CFRP prestressed concrete beams requires special consideration of the requirements associated with the use of a high-strength, elastic, brittle, and orthotropic composite material, and its complex behavior. 1.2 Research Objective The objective of this project was to develop design and material guide specifications in the AASHTO Load and Resistance Factor Design (LRFD) format for the design of concrete bridge beams prestressed with CFRP systems using either pretensioning or post-tensioning. In pursu- ing this objective, the research team considered the following: â¢ Establishing guidelines for the maximum allowable jacking forces for prestressing CFRP cables or bars with different configurations; â¢ Providing recommendations on the limitations of the use of anchorage systems; â¢ Measuring the prestress losses due to the relaxation of prestressing CFRP cables and bars and their anchorage system as well as thermal fluctuations; C H A P T E R 1 Introduction
Introduction 3 â¢ Validating the prestress losses due to concrete creep and shrinkage, elastic shortening, and seating; â¢ Quantifying the transfer length, camber, and long-term deflection of beams prestressed with CFRP cables and bars; â¢ Characterizing the flexural behavior of the full-scale bridge girders; and â¢ Calibrating the strength resistance factors for CFRP prestressed beams according to the LRFD philosophy. 1.3 Research Plan and Methodology The following tasks were performed to achieve the project objective: â¢ Review of relevant practice, data, specifications, and research findings from both foreign and domestic sources on the prestressing of concrete girders using CFRP systems; â¢ Identification of parameters that influence the design of CFRP prestressed beams, and the development of a work plan for the development of the design methods; â¢ Execution of an experimental program involving testing of materials, small-scale beams and prisms, and full-scale prestressed concrete bridge beams; â¢ Conduct of finite element simulations, sectional analyses, and reliability analyses to further understand the behavior at the material level and member level; â¢ Preparation of proposed specifications and commentary for the design of CFRP prestressed concrete girders together with the design examples to illustrate the application of the recom- mended design methods and specifications; and â¢ Preparation of a report that documents the entire research. Figure 1.1 shows the process used to determine the critical issues that were addressed in the project and Figure 1.2 illustrates the process used to develop the design and material guide specifications. 1.4 Organization of the Report This chapter presents the background, objectives, methodology, and scope of the project. Chapter 2 summarizes earlier experimental and analytical investigations. Chapter 3 presents the results from the experimental and analytical investigations performed in this project. Chapter 4 summarizes major research findings and discusses the applications of the proposed guide specifi- cations. Chapter 5 presents a summary of the research and recommendations for future research studies. In addition, Attachment A presents the Proposed AASHTO LRFD Bridge Design Guide Specifications and Material Specifications for Concrete Bridge Beams Prestressed with CFRP Systems and Attachment B presents design examples illustrating the use of the proposed design approach. Appendices A through F are available online and provide further details on the different aspects of the research as follows: â¢ Appendix A: Review of Previous Work â¢ Appendix B: Parameters Influencing the Design â¢ Appendix C: Experimental Testing Program â¢ Appendix D: Test Results and Discussions â¢ Appendix E: Finite Element and Numerical Simulations â¢ Appendix F: Reliability Analysis Study
4 Design of Concrete Bridge Beams Prestressed with CFRP Systems Compilation of Database Comparison and Evaluation of Existing Design Models Identify Parameters Affecting the Design of CFRP Prestressed Beams Deficiency of Existing Design Models Annotated Outline of the Proposed LRFD Bridge Design Specifications Recommend Design Method and Prepare Work Plan Issues to be addressed in this project Research Required to Develop Design Methods Beam Detailing: 1. Anchorage types 2. Straight vs. harped/draped 3. Cables vs. bars 4. Pretensioned vs. post-tensioned 5. Bonded vs. unbonded a. Parameters Well Documented in the Database 1. Anchorage and elastic losses 2. Transfer and development length 3. Pretensioned applications 4. External posttensioning applications b. Parameters not Well Documented in the Database 1. Friction and stress relaxation losses 2. Long-term performance (relaxation, deformations) 3. Thermal effect 4. Internal post-tensioning 5. Strength resistance factors Research Program: 1. Conduct experimental tests 2. Develop finite element models 3. Conduct sectional analysis 4. Conduct reliability analysis Literature Review Survey Results Existing Experimental Studies Field Applications Existing Analytical Models Figure 1.1. Process for determining potential critical issues.
Introduction 5 Parameters that Influence the Behavior of CFRP Prestressed Beams Parameters not Covered by Previous Experimental Studies Remaining Parameters Experimental Program Numerical and Analytical Program Identify critical issues for tests and provide details of test girders Provide experimental data for the calibration of FE Models Experimental and Parametric Study Results Develop Design Methods Development of Bridge Design and Material Guide Specifications 1. Identify Existing Analytical Models 2. Compiled Database 3. Consider Results from this Study Reliability Analysis Figure 1.2. Process for developing design and material guide specifications.