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13 The research team developed and executed the following approach to achieve the project objectives listed in Chapter 1: ⢠Literature search: The research team searched the litera- ture looking for performance criteria and data of prestressed girders where end zone cracking was reported. The search included national and international resources such as jour- nal papers and reports published in the past 60 years. Also, the search tried to collect information on any acceptance or rejection measures developed for prestressed girders with end zone reinforcement. The research team found that most of the available measures are related to flexural cracks in conventionally reinforced beams. Very few publications that deal with end zone cracking of prestressed bridge girders were available. The literature search showed that there is no unified approach or set of criteria that is available and widely accepted by highway authorities in the United States. The majority of publications on end zone cracking agreed that crack width is the best measure that can be used to develop practical acceptance/rejection criteria. ⢠National survey: After searching the literature, and due to the lack of information on polices used by precast produc- ers and highway authorities regarding acceptance/refusal of girders with end zone cracking, the research team devel- oped a national survey to collect information on the expe- rience regarding longitudinal end zone cracking. The national survey was sent to all of the state DOTs, other owner agencies, selected bridge consultants, and precast concrete producers. It was also sent to about 150 PCI bridge product producers, the PCI Committee on Bridges, the PCI Bridge Producers Committee, and selected Canadian agen- cies. The questionnaire included questions on reinforcement details, strand release process, criteria for repair and rejection of cracked members, and repair methods. The national sur- vey and its results are provided in Appendix B. Also, a sum- mary of the survey results is presented in Chapter 3. ⢠Required tasks: Based on the information collected from the literature review and analysis of the national survey results, the research team identified the following set of issues that, if addressed, would help establish procedures for the acceptance, repair, or rejection of precast/prestressed concrete girders with end zone cracking. These issues are 1. Effect of end zone cracking on structural capacity, dura- bility and aesthetics of prestressed concrete girders. 2. Improvement of the current design of end zone reinforce- ment to reflect recent usage of high-strength concrete and high levels of prestress. 3. Methods and material of end zone crack repair, if required. ⢠Work plan: To investigate these issues, the research team developed the following work plan. The plan has four Task 6 subtasks of the project, as shown in Table 2.1. A set of questions were developed to be answered by each subtask. Details of work in each subtask including results and con- clusions are presented in Chapter 3. ⢠Project deliverables: After the subtasks listed in Table 2.1 were conducted and the results were analyzed, the research team developed 1. A userâs manual for acceptance criteria and repair materi- als and methods. 2. Improved crack control reinforcement details for use in new girders. 3. Proposed revisions to the AASHTO LRFD Bridge Design Specifications. C H A P T E R 2 Research Approach
14 Table 2.1. Questions and corresponding work-plan subtasks developed to reach project objectives. Questions Work-Plan Subtasks 1. Does end zone cracking negatively affect the flexural and shear capacities of prestressed girders? 2. Do variations of the end zone reinforcement details have significant effect on the number, width, and pattern of end zone cracks? 1. Structural Investigation and Full-Scale Girder Testing Analysis of previous work, identification of influencing parameters, and determination of potential structural effect of end zone cracking led to the design and testing of eight full-scale girders. Two girders were fabricated in each of four states: FL, TN, VA and WA. Shear capacity, flexural capacity, and variations of end zone reinforcement details were included. These details included LRFD recommendations, proposed detail, and, if available, local practices. 3. If epoxy injection is used to repair end zone cracking, can repair restore the tensile capacity of the cracked concrete? 4. Is epoxy injection capable of completely filling the crack through the width of the web? 2. Epoxy Injection Testing Two 12-ft long specimens were fabricated by Concrete Industries (CI), Lincoln, NE, as part of an NU 1350 (53-in. deep) bridge girder production. The first specimen was fabricated with only shear reinforcement and no additional end zone reinforcement. The second specimen was fabricated with a different method of end zone reinforcement at each end (LRFD method and the proposed method). The specimens were repaired using epoxy injection by CI staff using the procedure from PCIâs Manual for the Evaluation and Repair of Precast, Prestressed Concrete Bridge Products (11). Sections of the web at the ends were saw cut, visually examined, and structurally tested. 5. If repair is required, what repair method and material should be used? 6. Should the end zone surface be sealed with a surface sealant regardless of whether cracks are required to be filled with a patching material? 3. Durability Testing This testing was conducted in three stages. In Stage I, 60 4 8-in. concrete cylinders were fabricated and sealed using five commercial sealants. ASTM D6489-99 Specification, Water Absorption Test of Hardened Concrete Coated with Water Repellant. Sealant effectiveness was evaluated. The best performing sealants were selected for further testing. In Stage II, 49 prisms were fabricated with preformed cracks ranging in width from 0.007 to 0.054 in. The results of Stage II were verified through a series of tests on 69 prisms in Stage III. 7. Does the width of end zone cracking change with time? 8. If end zone cracking is detected at the precast plant and no repair was conducted, do these cracks lead to corrosion of the strands and bars, or delamination of the concrete? 4. Field Inspection of Bridges The research team developed criteria for bridges to be inspected. Two states (Nebraska and Virginia) were targeted. Several bridges were inspected in each state. The inspection process included Collection of reports of inspection conducted at the plant. Examination of the report and identification of repair method and material. Collection of inspection reports of the bridges in service. Visits by the research team of the bridges under study. The inspection included observation of crack growth since production and of signs of reinforcement corrosion and concrete delamination.