Precast Inc. - July/August 2008 - (Page 9) At that time, our production staff had more than 12 years of experience in building bridge girders and provided many ideas and solutions for casting CFRP girders efficiently. Were there any problems in obtaining special equipment to work with CFRP reinforcing? We had to buy zip guns and zip ties to tie the CFRP reinforcing bars together nice and tight. Also, Mitsubishi supplied us with the ramming equipment used to crimp aluminum jackets around the ends of the CFRP strands and prestressing chucks. What specific problems did you experience? First, the chucks and wedges used in the prestressing of the CFRP strand presented a hurdle. We needed to effectively prestress these strands by developing a specialized chuck that would be able to couple with a standard steel prestressing chuck. We came up with a coupling system that worked by threading the outside of the CFRP chuck to a coupler that was then attached to a standard chuck for regular steel prestressing strands. Secondly, CFRP material is quite sensitive to heat and sunlight. During the setup of one of the double tees, an 8-foot-diameter roll of CFRP strand was delivered to the form area in an enclosed crate. We removed the lid of the crate and exposed only a small section of the material to direct sunlight. In less than two hours the entire roll snapped because of differential heating. From that point forward we had to ensure that the CFRP strands were not exposed to sunlight or heat, either directly or indirectly, during any part of the setup process. Lastly, as many precasters would imagine, we had to overcome issues with our new steel forms and pinch points in the casting, but we were able to remedy these issues with lessons learned from the prototype double tee. How is CFRP different from conventional steel reinforcing? I can give you several examples. We needed to ensure that the pretensioning strand did not rub against bulkheads, inserts or any other material prior to prestressing. We could not lay out the CFRP strands on the ground, so we used tarps beneath the material. And, as I just mentioned, it was critical to keep the strands protected from exposure to heat or sunlight and ensure that nothing was ever dropped on the CFRP strands. During the casting of the concrete, we had to be careful not to drop the concrete directly onto these strands, JULY/AUGUST 2008 | WWW.PRECAST.ORG 9 http://www.precast.org
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