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S U M M A R Y Background Fiber-reinforced polymer (FRP) systems have been used for more than 20 years and are becoming a widely accepted method of strengthening concrete structures. The use of FRP composites in rehabilitating structures has grown in popularity due to its advantages over conventional materials and wide range of structural applications. FRP systems for strengthening reinforced or prestressed concrete girders consist of externally bonded lam- inates or near-surface mounted bars. These systems may contain either carbon or glass fibers. Because of their light-weight and exceptional formability, FRP reinforcements can be quickly and easily bonded to even the most curved and irregular surfaces. The high strength-to-weight ratio of FRP composites makes them more structurally efficient than traditional strengthening materials. In addition, FRP composites are noncorrosive, non- magnetic, nonconductive, and generally resistant to chemicals. Externally bonded FRP systems composites are generally used for flexural strengthening, confinement and improvement of ductility in columns, or shear strengthening. Although flexure is typically the limiting mode of failure in bridge girder design, shear failure may dominate in cases where the transverse reinforcement has severely corroded or the flexural strength has been increased due to flexural strengthening. In such cases, the shear capacity should be enhanced to avoid catastrophic failures. A significant amount of research has been conducted on flexural and axial strengthening but limited investigations have been con- ducted on the use of externally bonded FRP for shear strengthening. Nevertheless, several models have been proposed for predicting the shear contribution of externally bonded FRP. These models are diverse in their approach and in many cases contradictory in their esti- mates of strength increase. FRP reinforcement configurations include the selection of sur- faces to be bonded (side bonding, U-wrap, complete wrap), continuous reinforcement or a series of discrete strips, and orientation of the primary direction of fibers. The bond charac- teristics between the FRP and concrete substrate add to the complexity in understanding the FRP shear contribution. The effectiveness of the strengthening method has been found to depend on the mode of failure. The use of FRPs for external strengthening of concrete structures has been hindered by the lack of comprehensive design provisions. Design of FRP strengthening systems has been based on system- or project-specific research. The AASHTO LRFD Bridge Design Specifi- cations (AASHTO, 2008) include no provisions for the design of externally bonded FRP sys- tems. NCHRP Project 12-75 was initiated to develop a recommended design method for shear strengthening of concrete girders using FRP systems. Design of FRP Systems for Strengthening Concrete Girders in Shear 1
2Project Objectives and Scope The objective of this project was to develop design methods, specifications, and examples for the design of FRP systems for strengthening concrete girders in shear. The proposed specifications are intended for incorporation into the AASHTO LRFD Bridge Design Spec- ifications (AASHTO, 2008). Such specifications will provide design engineers with the information needed for considering externally bonded FRP systems for shear strengthening of existing structures. To accomplish this objective, the research involved the following tasks: ⢠Study and review relevant practices, existing models and specifications, and research find- ings from both foreign and domestic sources regarding the use of externally bonded FRP for shear strengthening of concrete girders. ⢠Identify and evaluate criteria that influence the performance of FRP shear strengthening systems based on review of the literature (including the development of a database of tests related to FRP shear strengthening). ⢠Evaluate the performance of existing design methods. ⢠Investigate in full-scale tests, the key parameters affecting the shear performance of exter- nally bonded FRP for both reinforced concrete (RC) and prestressed concrete (PC) girders. ⢠Recommended provisions and specifications for incorporation into the AASHTO LRFD Bridge Design Specifications. Overview To develop design provisions for shear strengthening with externally bonded FRP sys- tems, the parameters affecting the behavior of such systems were identified first through review of the existing literature. Also, a database of the reported test results on the use of externally bonded FRP for shear strengthening was compiled. An experimental program was then developed to further study parameters that were considered to have not been suffi- ciently investigated in earlier tests, including the effects of pre-cracking, continuity (nega- tive moment), long-term conditioning (such as fatigue loading and corrosion of internal steel reinforcement), and prestressing. The experimental program included full-scale tests on RC T-beams and AASHTO type PC I-girders because most current design equations used in design specifications are based on small-scale test results. An assessment of the existing design methods found significant differences in the magnitude of the FRP shear contribu- tion calculated by various design methods. This assessment revealed the deficiencies of the existing design methods in predicting the shear resistance of a wide range of girder and FRP reinforcement characteristics. Therefore, new design equations for predicting the shear con- tribution of externally bonded FRP systems were developed and calibrated. Research Findings The major findings of this research effort are summarized as follows: ⢠Externally bonded FRP can be used to enhance the shear resistance of concrete girders. ⢠The effectiveness of externally bonded FRP for shear strengthening depends on the failure mode (i.e., FRP rupture or debonding). ⢠The effectiveness of FRP shear strengthening is significantly affected by the cross-sectional shape of the girder. ⢠The use of a properly designed mechanical anchorage system delays and in some cases prevents debonding of the FRP, resulting in a more effective strengthening scheme.
⢠An interaction exists between the transverse steel reinforcement and externally bonded FRP; the effectiveness of externally bonded FRP for shear decreases as the transverse steel reinforcement ratio increases. ⢠The shear span-to-depth ratio (a/d) has a significant influence on the effectiveness of externally bonded FRP for shear. ⢠The size-effect has little influence on the effectiveness of externally bonded FRP and thus empirical design expressions calibrated from small-scale test results should provide rea- sonable accuracy. ⢠The presence of pre-existing cracks and slight damage due to corrosion of the internal transverse steel reinforcement prior to strengthening does not seem to impair the effec- tiveness of the external FRP shear reinforcement. ⢠Beam continuity does not seem to influence the effectiveness of the FRP strengthening system. ⢠The effective FRP strain used in evaluating the FRP shear contribution can be expressed by two separate design expressions to consider the two predominant failure modes (i.e., debonding and FRP rupture). ⢠Under severe fatigue loading conditions (e.g., stirrups yielding), externally bonded FRP shear reinforcement may experience debonding if proper anchorage is not provided. If stress in the stirrups can be maintained below the yield strength, the externally bonded FRP shear strengthening can help delay fatigue yielding of the stirrups and extend the fatigue life of the girder. If stirrups have already yielded prior to FRP application, the FRP may still help contain the stresses and prevent catastrophic failure but not necessarily extending the service life of the girder. 3
