Browsing by Subject "Debonding"
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Item Behavior of carbon fiber reinforced polymer (CFRP) anchors strengthening reinforced concrete structures(2014-12) Sun, Wei, 1982-; Jirsa, J. O. (James Otis); Ghannoum, Wassim M.Carbon Fiber Reinforcement Polymer (CFRP) materials are widely used to strengthen reinforced concrete structures because they are light weight, have high strength, and are relatively easy to install. In strengthening applications, CFRP strips are typically attached to the concrete surface using epoxy resin with fibers oriented in the direction needing additional tensile strength. However, if CFRP strips rely exclusively on bond strength with concrete, only 40% to 50% of the CFRP tensile strength can be developed before debonding occurs. In order to fully develop the tensile strength of CFRP strips, some form of anchorage is needed. CFRP anchors can be applied with relative ease and have recently been shown to provide effective anchorage of CFRP strips to concrete members. In many cases, however, current anchorage details may resulting in fracture or failure of CFRP anchors prior to developing the full strength of CFRP strips. Many design parameters, the effects of which are not well understood, can affect the behavior and strength of CFRP anchors. Moreover, previous studies have demonstrated that the quality of installation can influence anchor strength substantially. The objectives of the study presented are to: 1) provide engineers with design guidelines for CFRP anchors, and 2) deliver a reliable test for controlling the quality of installation and materials of CFRP anchorage systems. In all, 39 tests on 6”×6”×24” rectangular concrete beams were conducted to study the influence of five parameters on CFRP anchor strength and effectiveness: 1) the width of the CFRP strip being developed, 2) the material ratio of CFRP anchor to CFRP strip, 3) the concrete strength, 4) the length/angle of anchor fan, and 5) the bond between CFRP strip and concrete (bonded/unbonded). The same tests also served to develop the test methodology for quality control of the CFRP anchorage system. Based on experimental results, guidelines for designing CFRP anchors are proposed. A test specimen and methodology are also proposed for qualifying CFRP materials and anchorage-system installations. A Finite Element (FE) formulation was selected to provide a computational tool that is suited for simulating the behavior of CFRP strips and CFRP anchors. The ability of the selected FE formulation to reproduce the effects on behavior of varying the anchor-material ratio, concrete strength, length of anchor fan, and bond conditions was investigated. Six FE simulations were built by adjusting simulation parameters and comparing results with six experimental tests. Comparisons between experimental and numerical results indicate that the proposed FE formulation and parameter selections reproduced load-deflection and local strain behaviors with high fidelity.Item Impact of AASHTO LRFD specifications on the design of precast, pretensioned u-beam bridges(Texas A&M University, 2006-04-12) Adnan, MohsinTexas Department of Transportation (TxDOT) is currently designing its highway bridge structures using the AASHTO Standard Specifications for Highway Bridges, and it is expected that TxDOT will make transition to the use of the AASHTO LRFD Bridge Design Specifications before 2007. The objectives of this portion of the study are to evaluate the current LRFD Specifications to assess the calibration of the code with respect to typical Texas U54 bridge girders, to perform a critical review of the major changes when transitioning to LRFD design, and to recommend guidelines to assist TxDOT in implementing the LRFD Specifications. This study focused only on the service and ultimate limit states and additional limit states were not evaluated. The available literature was reviewed to document the background research relevant to the development of the LRFD Specifications, such that it can aid in meeting the research objectives. Two detailed design examples, for Texas U54 beams using the LRFD and Standard Specifications, were developed as a reference for TxDOT bridge design engineers. A parametric study was conducted for Texas U54 beams to perform an in-depth analysis of the differences between designs using both specifications. Major parameters considered in the parametric study included span length, girder spacing, strand diameter and skew angle. Based on the parametric study supplemented by the literature review, several conclusions were drawn and recommendations were made. The most crucial design issues were significantly restrictive debonding percentages and the limitations of approximate method of load distribution.The current LRFD provisions of debonding percentage of 25 percent per section and 40 percent per row will pose serious restrictions on the design of Texas U54 bridges. This will limit the span capability for the designs incorporating normal strength concretes. Based on previous research and successful past practice by TxDOT, it was recommended that up to 75% of the strands may be debonded, if certain conditions are met. The provisions given in the LRFD Specifications for the approximate load distribution are subject to certain limitations of span length, edge distance parameter (de) and number of beams. If these limitations are violated, the actual load distribution should be determined by refined analysis methods. During the parametric study, several of these limitations were found to be restrictive for typical Texas U54 beam bridges. Two cases with span lengths of 140 ft. and 150 ft., and a 60 degree skew were investigated by grillage analysis method.