The Substitution of Prestressing Steel by Carbon Fibre Composites

摘要

"...Some day, glass fibres or plastic will be used as tendons for prestressing. This idea was first mentioned by Freyssinet in 1938..." is a footnote remark of Prof. Dr.-Ing. Fritz Leonhardt in his book 'PRESTRESSED CONCRETE -^sDesign and Construction' in 1954 (Leonhardt 1954), almost half a century ago. He expressed an idea and a presentiment which has now been technically and economically solved. Due to their excellent strain- and strength properties in fibre directions, especially carbon fibre reinforced plastics (cfrp) offer an ideal alternative to conventional steel tendons. However, regarding the requirements given in the "European Technical Approval Guideline of Post-^sTensioning Kits for Prestressing of Structures" (ETAG 013 2001) the anchorage design of CFRP-tendons must follow new approaches to meet the CFRP's special characteristics (e.g. lack of isotropy and lack of plasticity) to prevent a premature anchorage failure prior to the ultimate strength of the tendon. The anchorage problem has been solved by combining a clamping and bonding anchorage device thus that the anchorage presented complies with the ETAG requirements for steel tendons and cables. The use of flat lamella-shaped CFRP tendons instead of bar-shaped members allows the tendon to be directly applied and anchored on the surface of the member to be strengthened with a minimum of eccentricity. Meanwhile the CFRP-monostrip-tendons proved its effectiveness in numerous strengthening projects some of which are described and illustrated in this paper. Based on the results of the monostrip research, a CFRP multiplayer tendon is under development, which is aimed to perform a breaking force up to 2.4 MN by using 6 layers of the monostrip type. In order to raise the performance of the tendons up to that level, the anchorage strain-adaption-capacitiy was therefore to be improved.