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Externally bonded CFRP strips are ideal for rehabilitating existing members due to their high tensile strength-to-weight ratio, relatively low cost, formability, and expedited installation times. However, without proper anchorage, CFRP strips debond from the concrete surface before the strength of the CFRP can be mobilized. Consequently, CFRP spike anchors are used to delay debonding and develop the fracture strength of the CFRP strips. Previous research conducted on large-scale T-beams have shown that uni-directional (vertical) and bi-directional (vertical and horizontal) applications of CFRP strips and anchors increased the cracking load and ultimate shear capacity of members with transverse steel reinforcement, thin webs, and span-to-depth ratios above three (i.e., sectional behavior). However, the ability of the CFRP to strengthen wide-webbed members remained unknown. The objective of this research was to investigate the feasibility of strengthening wide-webbed reinforced concrete pile cap girders in shear using CFRP strips and CFRP anchors and to develop comprehensive design and detailing recommendations for CFRP shear strengthening. An experimental test program, consisting of nine tests, was created to investigate: 1) the effects of loading conditions, 2) retrofitting uncracked and cracked sections, 3) placing anchors in tension zones, and 4) uni- and bi-directional CFRP layouts on the shear performance of 32-in. deep by 32-in. wide reinforced concrete pile cap girders. The test results indicated that the uni-directional CFRP layouts were able to increase the shear capacity by as much as 56% while the bi-directional layouts increased the cracking shear load by as much as 22%. Anchored CFRP layouts were found to be as efficient as fully wrapped layouts despite anchors being placed in tension zones. Moreover, at the onset of the loss of shear strength, the measured lower bound average CFRP fracture strain was 0.007, which is significantly larger than the permitted effective strain in current ACI 440.2R-08 design guidelines (i.e., 0.004).
This book comprises the proceedings of the Annual Conference of the Canadian Society of Civil Engineering 2022. The contents of this volume focus on specialty conferences in construction, environmental, hydrotechnical, materials, structures, transportation engineering, etc. This volume will prove a valuable resource for those in academia and industry.
The objective of this research is the evaluation of shear behavior of full-scale reinforced concrete T-beams strengthened with carbon fiber reinforced polymer (CFRP) sheets and CFRP anchors. Although the CRFP material has high tensile strength, premature failure due to debonding CFRP sheets prevents utilizing that strength. The use of CFRP anchors prevents this failure, so the CFRP sheets are able to reach ultimate strain. The current shear design is based on plasticity, which assumes that all steel (ductile material) stirrups, across the critical section yield at ultimate. However the strain in the CFRP (brittle material), is essential to estimate the shear contribution of CFRP. To evaluate the validity of CFRP strengthening for shear, 24 tests were conducted with several parameters including shear-span-to-depth ratio, depth of beams, different transverse reinforcement ratios, and the layout of CFRP strips. In addition, a simple shear behavior model was developed to explain the differences between ductile and brittle material. From test observation, the use of CFRP anchors resulted in U-wrap application to perform like continuous wrapping which implies that a CFRP strip reached rupture strain because the anchors prevented debonding failure. However, all FRP strips did not rupture simultaneously because the strain distribution across a critical crack was not uniform. The average strain across the critical crack was about 0.005. Therefore a conservative value of effective strain (0.004) was selected for design purposes. In addition, when a beam is strengthened with CFRP, interactions between the contributions of the CFRP, steel or concrete must be taken into account. Factors ka, ks, and kf were introduced in the proposed shear design equations. Factor ka reflects the change in the material contributions as the shear span to depth ratio (a/d ratio) changes in deep beams. Factors ks and kf account for the change in steel or CFRP shear contribution due to the change in the critical crack angle as well as the interactions between the steel and FRP transverse reinforcement. As the amount of either steel or FRP material increase, the efficiency of the other material decreases.
During the interstate expansion of the 1950s, many conventionally reinforced concrete deck girder bridges were built throughout the country. These now vintage bridges commonly exhibit diagonal cracking and rate inadequately for shear, thus they are candidates for shear strengthening to extend their useful life. Near-surface mounted (NSM) retrofitting is a promising new strengthening technique, but limited test data are available for carbon fiber reinforced polymer (CFRP) in shear strengthening making the long-term durability of NSM-CFRP unknown. This paper provides experimental results from realistic full-scale specimens strengthened with NSM-CFRP. Specimens were tested for shear strength and subjected to environmental exposures to assess long-term durability. Small cylinder specimens were tested to investigate relative performance of different adhesives on bond strength under different environmental exposures. Test results provide a better understanding of the NSM-CFRP shear behavior and strength. Recommendations for shear strength design with NSM-CFRP are made.
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.
Fifteen tests were conducted to evaluate the shear performance of beams with carbon fiber reinforced polymer (CFRP) laminates and CFRP anchors. The specimens consisted of 24-in. deep T-beams. The specimens were strengthened in shear with CFRP laminates that were anchored using several different CFRP end anchorage details. Load was applied to the reinforced concrete members at three different shear span-to-depth ratios. Observations of the behavior and data from the tests were used to evaluate the performance of the CFRP laminates and CFRP anchors. Overall, a 30-40% increase in shear strength was observed when anchored CFRP laminates were installed on members loaded at a shear span-to-depth ratio greater than two. The CFRP strengthening system performed well when properly detailed CFRP anchors were installed. Design recommendations regarding the installation of the CFRP anchors were developed. The CFRP anchorage detail developed in this study provided additional CFRP material in critical locations to reinforce the anchor and prevent premature failures from occurring due to anchor rupture. Theoretical calculations predicting the shear strength of the retrofitted concrete members were carried out and compared to the measured strengths of the members. Based on this analysis, a design equation was developed that produced conservative results for all of the specimens tested.
Many studies conducted on the use of Fibre Reinforced Polymer (FRP) sheets for shear strengthening have been completed on beams that are small in relation to the effective bond length of the FRP. This study focuses on the use of FRP sheets for large-scale concrete members. Two series of large scale concrete beams were strengthened in shear to develop a better understanding of the behaviour of FRP shear strengthening on large specimens and to develop a means of predicting the shear capacity for all sizes of members. The first series of three specimens were obtained from a full sized I-section prestressed concrete bridge girder. One of these specimens was tested as a control, with the remaining two strengthened for shear with different configurations of Carbon Fibre Reinforced Polymer (CFRP) sheets. To develop a better understanding of bond behaviour of large-scale specimens, three reinforced concrete T-section beams were fabricated with a web height that was larger than those found in the literature. One of these beams was strengthened in shear using Glass Fibre Reinforced Polymer (GFRP) sheets and two others were strengthened using CFRP sheets. These beams were tested under static loading conditions. (Abstract shortened by UMI.).
Carbon fiber reinforced polymer (CFRP) materials are emerging as an effective means of strengthening and rehabilitating bridges. Near surface mounting (NSM) is a newer technique for application of CFRP for retrofitting of bridge members that provides advantages over conventional strengthening techniques. The technique is still new and uncertainties remain regarding design including the influence of member proportions, flexural reinforcing steel, and CFRP spacing. Further, retrofitted girders may also be exposed to millions of cycles of loading after rehabilitation. It is not known if the effects of fatigue loading will affect the service life of the retrofitted member. To address these issues, laboratory tests were performed on eight full-size reinforced concrete girders, representative of in-situ bridge girders, to determine the performance of NSM-CFRP retrofitting for shear. Two of the specimens were exposed to fatigue loading. Results indicated that NSM-CFRP retrofitting provides significant shear capacity increases and exposure to fatigue cycling did not affect the strength or behavior of the specimens.
During the interstate expansion of the 1950s, many conventionally reinforced concrete deck girder bridges were built throughout the country. These aging bridges commonly exhibit diagonal cracking and rate inadequately for shear, thus they are candidates for shear strengthening to extend their useful life. Carbon fiber reinforced polymers (CFRP) are emerging as effective materials for strengthening and rehabilitating such bridges. Near surface mounting (NSM) is a newer technique for application of CFRP for retrofitting bridge members that provides advantages over other strengthening techniques. The technique is still new and uncertainties remain regarding strength, long-term durability, and design including the influence of member proportions, flexural reinforcing steel, and CFRP spacing. Bridge girders retrofitted with NSM-CFRP may be exposed to millions of load cycles and environmental conditions and the influence of these exposures on performance are not established. To address these issues, laboratory tests were performed on ten full-size reinforced concrete girders, representative of in-situ bridge members, to determine the performance of NSM-CFRP retrofitting for shear strengthening. One of the specimens was exposed to fatigue loading, two were subjected to environmental exposures, and one was subjected to combined environmental exposure and fatigue loading. Results indicated that NSM-CFRP retrofitting provided significant shear capacity increases, and the high-cycle fatigue and environmental exposures considered did not adversely affect the strength or behavior of the girders. Environmental exposures of some of the adhesives considered did show somewhat reduced performance; therefore, careful selection of materials is important to ensure performance over the expected lifetime. Recommendations for shear strength design with NSM-CFRP are made.
During the interstate expansion of the 1950s, many conventionally reinforced concrete deck girder bridges were built throughout the country. These aging bridges commonly exhibit diagonal cracking and rate inadequately for shear, thus they are candidates for shear strengthening to extend their useful life. Carbon fiber reinforced polymers (CFRP) are emerging as effective materials for strengthening and rehabilitating such bridges. Near surface mounting (NSM) is a newer technique for application of CFRP for retrofitting bridge members that provides advantages over other strengthening techniques. The technique is still new and uncertainties remain regarding strength, long-term durability, and design including the influence of member proportions, flexural reinforcing steel, and CFRP spacing. Bridge girders retrofitted with NSM-CFRP may be exposed to millions of load cycles and environmental conditions and the influence of these exposures on performance are not established. To address these issues, laboratory tests were performed on ten full-size reinforced concrete girders, representative of in-situ bridge members, to determine the performance of NSM-CFRP retrofitting for shear strengthening. One of the specimens was exposed to fatigue loading, two were subjected to environmental exposures, and one was subjected to combined environmental exposure and fatigue loading. Results indicated that NSM-CFRP retrofitting provided significant shear capacity increases, and the high-cycle fatigue and environmental exposures considered did not adversely affect the strength or behavior of the girders. Environmental exposures of some of the adhesives considered did show somewhat reduced performance; therefore, careful selection of materials is important to ensure performance over the expected lifetime. Recommendations for shear strength design with NSM-CFRP are made.