United States Department of Transportation
i
Application of titanium alloy bars for strengthening reinforced concrete bridge girders (part a: shear) : final report.
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2017-07-04
[PDF-41.06 MB]
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TRIS Online Accession Number:1643652
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Abstract:Large numbers of conventionally reinforced concrete bridges (RC) were constructed during the interstate highway expansion of the 1950’s and remain in the national inventory. Coincidently, deformed steel reinforcing bars were standardized. The standardized deformation requirements dramatically changed bond provisions in all relevant design codes and designers began to use straight-bar terminations of the flexural reinforcing bars where they were no longer required by calculation. This produced terminations in flexural tension zones without special detailing provisions. At the same time, design codes overestimated the concrete contribution to shear resistance and thus designs provided less transverse steel than permissible by modern standards. These poor details combined with heavier loads have resulted in diagonal cracking of the girders. Application of modern design provisions to assess vintage RC members typically results in low predicted capacity. Load restrictions or replacement of bridges is expensive and thus structurally effective and economical strengthening approaches are required. Near-surface mounting (NSM) of supplemental reinforcing bars has emerged as a common retrofit method for strengthening RC structures. In this method, bars are bonded with an adhesive within grooves that are cut into the surface of the member. The most common reinforcing material used in the NSM application is fiber reinforced polymer (FRP). FRP materials are elastic until fracture thus providing no ductility. In addition, they can debond prematurely limiting the effectiveness of the repair. For this reason, new titanium alloy bars (TiABs) were developed as a potential alternative for FRP bars in NSM applications. Small diameter (1/4 in.) NSM TiABS were applied to full-size bridge girder specimens that were shear deficient and tested to failure in the laboratory. Two different epoxy materials were considered, as well as two different configurations of TiAB bars (single leg and double leg stirrups). The combined effects of high-cycle fatigue loading and repeated freeze-thaw exposure were investigated to assess long-term durability of the proposed approach and materials. Results showed that NSM TiABs provided significant increases in the strength when compared to similar unstrengthened specimens. The TiABs were observed to achieve the yield stress prior to/at ultimate. Anchorage of the TiABs was sufficient to produce rupture over the main diagonal cracks. The double leg TiABs were easier to construct and provide additional confinement across the beam soffit. It was determined that some reduction in strength was observed for one of the epoxy materials due to the combined fatigue and environmental effects. Three methods were used to analyze the experimental results which could conservatively predict strength. Strength reduction factors were developed for each method to be used in design.
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