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63 5.1 Summary of Findings This research effort produced recommended design and material guide specifications for the design of concrete beams prestressed with CFRP systems for bridge applications. These design and material guide specifications were provided to the AASHTO Committee on Bridges and Structures. The following are some of the highlights of the research findings: â¢ Anchorage: â All anchorage used in this project attained either the design tensile capacity (for anchorage that was an integral part of the prestressing system) or the jacking stress (for anchorage that was used only for stressing and removed at transfer) of the prestressing CFRP cables and bars during material testing and field applications. â¢ Prestress losses: â The measured anchorage seating loss for socket type anchors were less than 1.0% of the jacking stress. â The equations for calculating the elastic shortening losses for pretensioned and post- tensioned applications with steel tendons (AASHTO LRFD, 2017) are applicable for calcu- lating elastic shortening losses for beams prestressed with CFRP systems. â The stress relaxation losses of the prestressing CFRP cables and bars are independent of the length of the CFRP tendons. â A linear relationship exists between stress relaxation of prestressing CFRP tendons and the logarithm of time. â Stress relaxation equations were developed for CFRP systems in which the anchorage is a permanent, integral part of the prestressing system and for systems in which the anchorage is not a permanent part of the prestressing system. â The current AASHTO LRFD (2017) equations for estimating creep and shrinkage losses are appropriate for beams prestressed with CFRP tendons. â The transfer length of the prestressing CFRP increased by 16 in. and 24 in. for prestressed prisms with prestressing CFRP cables and bars, respectively. For prestressing CFRP cables, the bond deterioration did not cause further loss of prestressing force. However, the bond deterioration due to thermal fluctuation in the case of prestressing CFRP bars resulted in up to a 40% reduction of the prestressing force and slippage of the prestressing CFRP bars. â The current AASHTO equation for calculating the friction losses in post-tensioned beams with prestressing CFRP cables is appropriate. A wobble coefficient of 0.0004 (1/ft.) and coefficient of friction of 0.2 are proposed for CFRP cables in polypropylene ducts. â¢ Harping of prestressing CFRP: â Prestressing CFRP cables exhibited higher retention of tensile capacities than CFRP bars. â Premature failure (splitting) and significant reduction of the tensile capacity of the pre- stressing CFRP bars occurred with harping angles greater than 10Â°. C H A P T E R 5 Summary of Findings and Recommendations for Future Research
64 Design of Concrete Bridge Beams Prestressed with CFRP Systems â Harped prestressing CFRP bars retained a small portion of the ultimate tensile strength. â The 1 in. steel and 2 in. plastic deviators that are commonly used for prestressing with steel tendons were not suitable for prestressing CFRP cables. â When 20 in. and 40 in. diameter deviators were used, prestressing CFRP cables retained more than 90% and 100% of their design tensile strengths, respectively, for harping angles up to 20Â°. â¢ Transfer of CFRP cables and bars: â Transfer lengths for prestressing CFRP cables and bars ranged between 40 to 50 times the diameter of the prestressing CFRP. â Limiting the level of prestress immediately prior to transfer to 70% and 65% of the design tensile strength of the CFRP cables and bars, respectively, would help prevent creep rupture failure and provide sufficient reserved strain to resist flexural loads. â¢ Full-scale beam performance: â The 2.3 million loading cycles did not affect the stiffness or the strength of the CFRP prestressed beams. â The unbonded post-tensioned beams exhibited greater deformability compared to the bonded post-tensioned and pretensioned beams. â¢ Flexural design: â The behavior of pretensioned CFRP beams depended on the modulus of elasticity of prestressing CFRP, concrete strength of the deck, reinforcement ratio, and prestressing level. â The span-to-depth ratio, concrete strength, prestressing ratio, and cable profile were the primary parameters that affected the increase of the unbonded tendon stress. â A minimum amount of bonded reinforcement was required to control the crack width and distribution, and to avoid the tied-arch behavior of unbonded post-tensioned beams. 5.2 Recommendations for Future Research The following research is recommended to further enhance the findings reported within this project: â¢ Evaluating the application of prestressing CFRP in continuous prestressed beams. This project investigated the behavior and design procedures for simply supported beams; research to evaluate the application and design procedures for continuous beams prestressing CFRP systems, especially unbonded post-tensioned beams is needed. â¢ External applications for post-tensioned beams. Applicability of the design of unbonded post- tensioned beams to beams strengthened with external CFRP post-tensioning reinforcement has not been evaluated. Research is needed to evaluate the applicability of these procedures for CFRP systems that are used for external strengthening. â¢ Shear behavior of prestressed beams with prestressing CFRP systems. This project investigated the behavior of simply supported prestressed beams, reinforced in shear with steel reinforce- ment, under flexural monotonic and fatigue loading. Research is needed to evaluate the shear behavior of prestressed beams with prestressing CFRP systems and transverse reinforcement with FRP. â¢ Harping issues for prestressing CFRP. This project showed that the application of the pre- stressing CFRP in harped configurations requires use of large diameter harping devices with large contact surfaces between the deviators and the prestressing tendons. The feasibility of using deviators with large diameters (e.g., 20 and 40 in.) in precast plants needs to be inves- tigated. Also, the use of cushioning materials, such as Teflon or other polymers, between the prestressing CFRP and steel deviator is expected to enhance the harping tensile capacity retention of the prestressing CFRP systems. An experimental investigation of the cushioning
Summary of Findings and Recommendations for Future Research 65 material types and their use with harped CFRP cables or bars will be helpful. In addition, use of the prestressing CFRP systems with deviated or harped profiles in pre-tensioned or external post-tensioned beams needs to evaluated in experimental and analytical investigation of the flexural behavior of the CFRP prestressed beams with harped strand profiles. â¢ Stress Relaxation. Stress relaxation loss of prestressing CFRP systems was evaluated under laboratory temperature and humidity conditions; the effects of environmental conditions and the variety of anchorage details on stress relaxation of prestressing CFRP systems needs to be investigated. â¢ Thermal fluctuation effects. Thermal fluctuation cycles lead to deterioration of the bond between prestressing CFRP and concrete, resulting in reduction of the prestressing force. The effect of concrete cover, CFRP embedment length, and the effects of concrete compressive and tensile strength on such deteriorations needs to be investigated. Also, the bond performance between prestressing CFRP and concrete and its effect on development lengths needs to be evaluated on large specimens resembling bridge beams.