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39 CHAPTER 4 Summary of Findings and Recommendations for Future Research This section summarizes the findings of this research effort Tests on beams with pre-existing cracks prior to strength- and provides suggestions for future research. 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- 4.1 Summary of Findings gesting that the existence of cracks does not adversely This research effort has produced recommended design influence the effectiveness of FRP shear strengthening. methods and specifications for strengthening concrete girders Beam continuity (negative moment) did not appear to in shear using externally bonded FRP systems. The findings of influence the behavior of the beams strengthened with FRP this research regarding use of FRP systems for strengthening (similar behavior to beams tested under positive moment concrete girders in shear are as follows: conditions). The effectiveness of externally bonded FRP for shear Externally bonded FRP can be used to enhance the shear strengthening depends on the failure mode (i.e., FRP rupture resistance of concrete girders. or debonding). Debonding occurs when girders are strength- Externally bonded FRP systems can be applied in a variety ened in shear by side bonding and occasionally in cases of of configurations and molded to any geometrical shape to U-wrapping due to insufficient bond length. FRP rupture provide side bonding, U-wrap, or complete wrapping of the can be reached through use of proper anchorage such as that girder web and as continuous sheets or discrete strips. The provided by complete wrapping, adequate bond length, fibers of the composite may be oriented in any direction. or by some form of mechanical anchorage. Girders strength- Fibers oriented perpendicular to the longitudinal axis of the ened in shear with a U-wrapping configuration fail by girder provide the most practical application, but fibers ori- either debonding or FRP rupture, depending on the bond ented orthogonal to the shear crack provide the most effec- characteristics. tive application. The use of mechanical anchorage delays and, in some cases, There is an interaction between transverse steel reinforce- prevents debonding of the FRP, resulting in a greater increase ment and externally bonded FRP shear reinforcement. The of the ultimate shear resistance. Use of horizontal strips of effectiveness of externally bonded FRP for shear strength- FRP as mechanical anchorage does not provide much addi- ening decreases as the transverse steel reinforcement ratio tional shear capacity. Continuous CFRP plates with anchor- increases. age bolts are not very effective in anchoring the CFRP sheets. The shear span-to-depth ratio (a/d) influences the effec- Discontinuous CFRP plates attached with concrete wedge tiveness of externally bonded FRP for shear. Shear span-to- anchors or bolts through the web provide the most effective depth ratios (a/d) less than 2.0 develop arch action resistance mechanical anchorage. The effectiveness of this method can mechanisms (deep beam behavior) that reduce the effective- be further improved by the use of a sandwich application ness of FRP shear strengthening. that prevents slippage of the FRP sheet from beneath the Tests on large scale RC T-beams showed that the size-effect anchorage plate. For PC girders with very thin webs, the has little influence on the effectiveness of externally bonded embedment length of anchor bolts may not be sufficient to FRP (effective strains in the FRP of the large-scale specimens avoid premature failure due to the anchor bolts pulling were found to be similar to those reported for small-scale out. In such cases, the use of a thru-bolt systems are more tests). practical and provide better performance.