National Academies Press: OpenBook

Design of FRP Systems for Strengthening Concrete Girders in Shear (2011)

Chapter: Chapter 4 - Summary of Findings and Recommendations for Future Research

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Page 39
Suggested Citation:"Chapter 4 - Summary of Findings and Recommendations for Future Research." National Academies of Sciences, Engineering, and Medicine. 2011. Design of FRP Systems for Strengthening Concrete Girders in Shear. Washington, DC: The National Academies Press. doi: 10.17226/14465.
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Page 39
Page 40
Suggested Citation:"Chapter 4 - Summary of Findings and Recommendations for Future Research." National Academies of Sciences, Engineering, and Medicine. 2011. Design of FRP Systems for Strengthening Concrete Girders in Shear. Washington, DC: The National Academies Press. doi: 10.17226/14465.
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Page 40

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39 This section summarizes the findings of this research effort and provides suggestions for future research. 4.1 Summary of Findings This research effort has produced recommended design methods and specifications for strengthening concrete girders in shear using externally bonded FRP systems. The findings of this research regarding use of FRP systems for strengthening concrete girders in shear are as follows: • Externally bonded FRP can be used to enhance the shear resistance of concrete girders. • Externally bonded FRP systems can be applied in a variety of configurations and molded to any geometrical shape to provide side bonding, U-wrap, or complete wrapping of the girder web and as continuous sheets or discrete strips. The fibers of the composite may be oriented in any direction. Fibers oriented perpendicular to the longitudinal axis of the girder provide the most practical application, but fibers ori- ented orthogonal to the shear crack provide the most effec- tive application. • There is an interaction between transverse steel reinforce- ment and externally bonded FRP shear reinforcement. The effectiveness of externally bonded FRP for shear strength- ening decreases as the transverse steel reinforcement ratio increases. • The shear span-to-depth ratio (a/d) influences the effec- tiveness of externally bonded FRP for shear. Shear span-to- depth ratios (a/d) less than 2.0 develop arch action resistance mechanisms (deep beam behavior) that reduce the effective- ness of FRP shear strengthening. • Tests on large scale RC T-beams showed that the size-effect has little influence on the effectiveness of externally bonded FRP (effective strains in the FRP of the large-scale specimens were found to be similar to those reported for small-scale tests). • Tests on beams with pre-existing cracks prior to strength- ening showed that stirrups yield at a lower shear force than for beams without cracks. However, the existence of cracks seemed not to change the failure modes of the beams sug- gesting that the existence of cracks does not adversely influence the effectiveness of FRP shear strengthening. • Beam continuity (negative moment) did not appear to influence the behavior of the beams strengthened with FRP (similar behavior to beams tested under positive moment conditions). • The effectiveness of externally bonded FRP for shear strengthening depends on the failure mode (i.e., FRP rupture or debonding). Debonding occurs when girders are strength- ened in shear by side bonding and occasionally in cases of U-wrapping due to insufficient bond length. FRP rupture can be reached through use of proper anchorage such as that provided by complete wrapping, adequate bond length, or by some form of mechanical anchorage. Girders strength- ened in shear with a U-wrapping configuration fail by either debonding or FRP rupture, depending on the bond characteristics. • The use of mechanical anchorage delays and, in some cases, prevents debonding of the FRP, resulting in a greater increase of the ultimate shear resistance. Use of horizontal strips of FRP as mechanical anchorage does not provide much addi- tional shear capacity. Continuous CFRP plates with anchor- age bolts are not very effective in anchoring the CFRP sheets. Discontinuous CFRP plates attached with concrete wedge anchors or bolts through the web provide the most effective mechanical anchorage. The effectiveness of this method can be further improved by the use of a sandwich application that prevents slippage of the FRP sheet from beneath the anchorage plate. For PC girders with very thin webs, the embedment length of anchor bolts may not be sufficient to avoid premature failure due to the anchor bolts pulling out. In such cases, the use of a thru-bolt systems are more practical and provide better performance. C H A P T E R 4 Summary of Findings and Recommendations for Future Research

• The effectiveness of FRP for shear strengthening is signifi- cantly affected by the cross-sectional shape of the girder. Debonding of FRP accompanied by peeling of the concrete cover can result in a web crushing failure. • If the stresses in the stirrups are less than the yield strength, the FRP strengthening can help delay yielding and prevent fatigue failure of the girder in shear. However, if the stirrups have already yielded, the FRP strengthening system would not reduce the stresses but could help restrain steel stress increases and prevent catastrophic failure of the girder. • In the absence of a proper anchorage system, the externally bonded FRP sheets for shear strengthening could become debonded and result in no shear strengthening. • Limiting the stress in the stirrups to the yield strength will assure that shear fatigue failure of the girder will not occur. • The effective FRP strain used in evaluating the FRP shear contribution can be expressed by two separate design expres- sions to consider the two predominant failure modes (i.e., debonding and FRP rupture). One expression is for mem- bers in which sufficient anchorage is provided (FRP rupture failure mode), and the other is for members in which insuf- ficient anchorage is provided (FRP debonding failure mode). 4.2 Suggestions for Future Research Based on the work performed in this research, the follow- ing research efforts are needed to enhance understanding of the mechanisms associated with the use of externally bonded FRP for shear. • An interaction exists between the internal transverse steel reinforcement and externally bonded FRP shear reinforce- ment, but there are insufficient data to quantify this inter- action. Further investigations are needed to better quantify the mechanisms involved in this interaction and incorpo- rate it into an enhanced model for the shear resistance of RC beams strengthened with externally-bonded FRP. • The use of mechanical anchorage involving discontinuous CFRP plates attached with steel concrete wedge anchors or bolts through the web was found to delay or, in some cases, prevent debonding of FRP. However, because these anchors and bolts are susceptible to corrosion, research is needed to explore alternative mechanical anchorage techniques that are not susceptible to such corrosion. • The cross-sectional geometry of PC girders influences the effectiveness of externally bonded FRP. Also thin web and stiff flange geometry reduce the effectiveness of the FRP shear strengthening. However, limited results are available to fully understand the mechanisms involved in such behav- ior. Further research is needed to examine the effect of cross- sectional geometry of PC girders. • The effective strain concept was adopted for design guide- lines and codes to provide a simple and practical method for estimating the shear contribution of FRP. Research is needed to investigate the non-uniform FRP distribution and develop more reliable design equations. • Research is needed to investigate the long-term fatigue per- formance of FRP systems for shear strengthening, particu- larly the effects of cracks on bond characteristics. 40

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 678: Design of FRP Systems for Strengthening Concrete Girders in Shear offers suggested design guidelines for concrete girders strengthened in shear using externally bonded Fiber-Reinforced Polymer (FRP) systems.

The guidelines address the strengthening schemes and application of the FRP systems and their contribution to shear capacity of reinforced and prestressed concrete girders. The guidelines are supplemented by design examples to illustrate their use for concrete beams strengthened with different FRP systems.

Appendix A of NCHRP Report 678, which contains the research agency’s final report, provides further elaboration on the work performed in this project. Appendix A: Research Description and Findings, is only available online.

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