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Large numbers of reinforced concrete deck girder (RCDG) bridges were built during the highway infrastructure boom of the 1950's. The advent of standardized deformed steel reinforcing bars during this time allowed for straight bar terminations in flexural tension regions. Designers of the time terminated reinforcing bars where they were no longer required by calculation and did not account for additional demands from the combination of shear and flexure. The design provisions of the time allowed higher shear stresses in the concrete than allowed in standards today which reduced the required quantity of transverse reinforcing steel. In addition, heavier trucks and higher traffic volumes on roadways today have greatly increased the service loading on these bridges. Engineers evaluating these older RCDG bridges often determine unsatisfactory load ratings due to flexural anchorage deficiencies in the girders, especially when the influence of shear is considered. These deficiencies result from inadequate capacity compared to current design standards due to poor cutoff details used in the initial design. Strengthening methods are necessary because comprehensive replacements of the large number of bridges are not economically feasible. Experimental research was conducted to evaluate the behavior of poorly detailed flexural anchorages and to develop methods to strengthen them. Realistic vintage girder specimens were constructed, retrofitted, instrumented, and tested to failure. The specimens reported in this thesis were full-scale inverted-T (IT) beams. Some of the specimens contained straight bar terminations crossing a preformed diagonal crack in the flexural tension region to investigate the influence of shear on the retrofit schemes. Instrumentation focused on measurement of the reinforcing steel stresses surrounding the diagonal crack and along the development length of the cutoff bars. Using results of past research to quantify the behavior of girders with straight-bar flexural anchorages in flexural tension regions, an innovative strengthening technique was developed using either near-surface mounted (NSM) stainless steel or titanium. Results from the NSM strengthening technique demonstrated the ability to delay or prevent flexural anchorage failures, with increased deformation capacities and increased strengths from 17% to 39% over baseline specimens. To show the success of this research and the immediate need for strengthened flexural anchorages, this research has already been implemented on a bridge in Mosier, Oregon. This groundbreaking research is described in detail in Appendix F.
Many older reinforced concrete deck girder (RCDG) bridges contain straight bar terminations of flexural reinforcement. Common bridge design practice of the 1950s did not consider the additional demands on the terminated bars from shear and flexure. Moreover, more stringent code specifications and heavier permit trucks contribute to the insufficient ratings and presence of diagonal cracks in RCDG bridges. Replacement of these bridges is not economically feasible and thus strengthening methods are necessary. The goal of this research was to investigate anchorage strengthening methods using full-scale vintage girder specimens. The specimens were constructed with a flexural anchorage deficiency by terminating two flexural bars that extended only one-third of their development length past a 45° preformed diagonal crack. A common flexural strengthening technique called near-surface mounting (NSM) was applied to T-specimens. Two metallic materials were selected for the NSM reinforcement: titanium and stainless steel. These materials were chosen because of their high strength, ductility, environmental durability, and ability to fabricate mechanical anchorages. This study found that the NSM strengthening technique with metallic materials increased the midspan displacement by at least 85%, and load capacity by at least 31% for each specimen. In addition, a case study was performed to simulate the positive moment anchorage strengthening of the Mosier Bridge in Oregon.
Externally bonded carbon fiber-reinforced polymer (CFRP) composites have been instrumental in the flexural strengthening of concrete structures because of their high strength-to-weight ratio, corrosion resistance, rapid and easy installation, and reduced cost compared to complete or partial component replacement. However, a governing failure mode in most externally bonded CFRP applications is debonding from the concrete substrate, which limits the composite strength utilization and deformability of a strengthened concrete member. Improved performance, in terms of both deformability and ultimate strength, may be achieved with the addition of transverse U-Wrap anchorage for longitudinally oriented CFRP tension reinforcement. Design guidance for U-Wrap anchorage is, however, lacking due to the limited experimental data. This work reports on flexural test results from structural-scale reinforced concrete girders strengthened in flexure with externally bonded CFRP anchored with U-Wraps. The parameter of interest is the ratio of the areas of U-Wrap anchorage to flexural CFRP, which is quantified by the anchorage ratio. Experiments employ a 3D digital image correlation (DIC) system to collect full-field displacement data that can accurately characterize the effects of U-Wrap anchorage on the strain in the flexural CFRP as well as the failure origin and progression. U-Wrap anchorage resulted in an increase in CFRP strain at ultimate flexural capacity of up to 57% with a corresponding increase in moment capacity and a change in failure mode. However, varying the ratio of ratio of the areas of U-Wrap anchorage to flexural CFRP had little apparent effect on the flexural capacity of the girder.
Viable retrofit schemes are necessary to delay or offset replacement of deteriorating concrete bridge members. Carbon fiber reinforced polymer (CFRP) pultruded plates can be especially effective when retrofitting bridge members where stiffness, fatigue resistance, ease of installation, and weathering characteristics are a concern. The research reported in Chapter 1 was undertaken to examine the influence of fatigue loading, prior cracking, and patch materials on flexural performance of reinforced concrete members retrofitted with externally bonded CFRP plates. Moreover, experimental data from the six reinforced concrete beams tested as part of this research are expected to further evaluate available design equations for external retrofitting of reinforced concrete structures. The test results do not suggest a significant effect of fatigue loads; show that existing cracks do not significantly impact the strength of retrofitted members; and indicate that patch materials can reduce the available bond strength, and require additional surface preparation. The research reported in Chapter 2 presents a novel design approach utilizing externally bonded CFRP plates developed in an attempt to overcome construction errors in a member removed from an adjacent box girder bridge. The design methodology was evaluated based on data from testing of a retrofitted girder along with previous tests on as-is girders. Test data suggest appreciable improvements in terms of load carrying capacity and stiffness of the retrofitted girder. The relatively simple retrofit plan developed could have been used to delay replacement of the deficient girders. The research reported in Chapter 3 is aimed at filling some of the gaps in the available test data through retrofitting and testing of a 18.3 m (60 ft) prestressed box girder retrofitted with CFRP composite plates with mechanical anchors. Prior research on the use of CFRPs for retrofitting of existing structures has predominantly focused on mildly reinforced concrete members, and application to prestressed members is rather limited. Moreover, data regarding performance of mechanical anchors for enhancing bond characteristics of CFRP composites are scant. After a description of the design procedure, the test data are used to evaluate the design method, current design recommendations, and performance of mechanical anchors.
The Minnesota Department of Transportation has typically used epoxy-coated, straight-legged stirrups anchored in the tension zone as transverse reinforcement in prestressed concrete bridge girders. This configuration is readily placed after stressing the prestressing strands. American Concrete Institute (ACI) and American Association of State Highway and Transportation Officials (AASHTO) specifications require stirrups with bent legs that encompass the longitudinal reinforcement to properly anchor the stirrups. Such a configuration is specified to provide mechanical anchorage to the stirrup, ensuring that it will be able to develop its yield strength with a short anchorage length to resist shear within the web of the girder. AASHTO specifications for anchoring transverse reinforcement are the same for reinforced and prestressed concrete; however, in the case of prestressed concrete bridge girders, there are a number of differences that serve to enhance the anchorage of the transverse reinforcement, thereby enabling the straight bar detail. These include the precompression in the bottom flange of the girder in regions of web-shear cracking. In addition, the stirrup legs are usually embedded within a bottom flange that contains longitudinal strands outside the stirrups. The increased concrete cover over the stirrups provided by the bottom flange and the resistance to vertical splitting cracks along the legs of the stirrups provided by the longitudinal prestressing reinforcement outside the stirrups help to enhance the straight-legged anchorage in both regions of web-shear cracking and flexure-shear cracking. A two-phase experimental program was conducted to investigate the anchorage of straight-legged, epoxy-coated stirrups, which included bar pullout tests performed on 13 subassemblage specimens that represented the bottom flanges of prestressed concrete girders, to determine the effectiveness of straight-legged stirrup anchorage in developing yield strains. Additionally, four girder ends were cast with straight-legged stirrup anchorage details and tested in flexure-shear and web-shear. The straight leg stirrup anchorage detail was determined to be acceptable for Minnesota Department of Transportation (MnDOT) M and MN shaped girders as nominal shear capacities were exceeded and yield strains were measured in the stirrups prior to failure during each of the tests.
The in situ rehabilitation or upgrading of reinforced concrete members using bonded steel plates is an effective, convenient and economic method of improving structural performance. However, disadvantages inherent in the use of steel have stimulated research into the possibility of using fibre reinforced polymer (FRP) materials in its place, providing a non-corrosive, more versatile strengthening system. This book presents a detailed study of the flexural strengthening of reinforced and prestressed concrete members using fibre reinforces polymer composite plates. It is based to a large extent on material developed or provided by the consortium which studied the technology of plate bonding to upgrade structural units using carbon fibre / polymer composite materials. The research and trial tests were undertaken as part of the ROBUST project, one of several ventures in the UK Government's DTI-LINK Structural Composites Programme. The book has been designed for practising structural and civil engineers seeking to understand the principles and design technology of plate bonding, and for final year undergraduate and postgraduate engineers studying the principles of highway and bridge engineering and structural engineering. Detailed study of the flexural strengthening of reinforced and prestressed concrete members using fibre reinforced polymer composites Contains in-depth case histories
Large numbers of reinforced concrete deck girder bridges that were constructed during the interstate system expansion of the 1950s have developed diagonal cracking in the stems. Compared to the present AASHTO-LRFD standards, the provisions of the 1950s allowed for higher shear stress in the concrete, thus reducing the amount of transverse steel required. Further, service loading has increased over time. When load-rating these structures, the current design specification check of tension reinforcement anchorage often controls the capacity of these bridges. This check compares the applied tensile force in the reinforcement to the tensile force available based on the reinforcement development length. The tensile force demand is controlled by the load-induced moment and shear, the number of stirrups, and the diagonal crack angle. However, the crack angle considered in the design specification is commonly flatter then the angle of the vertically-oriented cracks generally noted in field inspections. The tensile force that can be developed in the flexural reinforcing steel depends on the diameter of the bar and the embedded length, however, little information is currently available regarding bond stresses developed with largerdiameter bars for full-size specimens in the presence of diagonal cracks. Experimental data from realistic full-size specimens with anchorage of flexural bars interacting with diagonal cracks would enhance ratings methods for evaluation of existing bridges. Ultimately, improved understanding of the response of these bridge girders can help maintain the operational safety and freight mobility of the transportation system, thereby allowing optional use of available resources for repair or replacement of truly deficient bridges.