Lateral forces generated by wind, earthquakes, and other horizontal loads are trans-mitted from the floor diaphragms to the columns and walls that comprise the vertical lateral force resisting system in a building. Strengthening of the diaphragms in older reinforced concrete buildings may be necessary for several reasons, including to enhance seismic performance, address inadequate strength or stiffness, provide missing or incomplete load paths, improve inadequate shear transfer/connection capacity, and to accommodate changes in the use and occupancy of the structure. Engineers are currently using externally bonded fiber reinforced polymer (FRP) composites to retrofit deficient diaphragms. However, this application is beyond the scope of current FRP-related design documents, including ACI PRC-440.2R-17 "Guide for the Design and Construction of Externally bonded FRP Systems for Strengthening Concrete Structures". The lack of consensus around design recommendations for FRP strengthening of diaphragms is problematic and creates uncertainty about which approaches are proven and what are best practice. This thesis summarizes the results from an experimental research program designed to investigate the shear behavior of reinforced concrete diaphragms strengthened using external-ly bonded FRP. Six one-half scale reinforced concrete cantilever diaphragms were tested in shear to evaluate the influence of FRP material, density, spacing, orientation, and intermediate anchorage configuration on the performance of diaphragm strengthening. The specimens were designed to represent the diaphragm shear zone adjacent to a shear wall in a concrete building. The tests were performed using a reverse cyclic displacement protocol representative of earthquake actions. The tests included a baseline unretrofitted concrete specimen, followed by five retrofitted specimens with different configurations of externally bonded FRP. Each retrofitted specimen was designed to maintain a similar FRP axial stiffness while varying the FRP retrofit parameters. The results demonstrated that externally bonded FRP retrofitting improved both the shear strength and stiffness of the strengthened test specimens. All the retrofitted specimens experienced an FRP debonding failure initiated by intermediate shear cracks with the field of the diaphragm, occurring after yielding of the internal steel rebar. The results highlighted that the overall behavior of the specimens was influenced by the way the retrofit schemes were proportioned and detailed. For example, the application of FRP parallel to the direction of applied shear was found to be most effective at increasing the diaphragm strength. Conversely, the application of FRP perpendicular to the applied shear was found to increase the diaphragm ductility. In addition, the shear strength contribution of externally bonded FRP was significantly influenced by the retrofit surface coverage. Compared with narrow strips of high-density fabric, retrofits detailed with less dense fabric spread uniformly over the surface exhibited superior performance due to better control of the shear cracks. Furthermore, no meaningful difference in performance was observed between diaphragms strengthened with glass and carbon FRP composites, provided the retrofits were proportioned to achieve com-parable levels of stiffness. This finding suggests that either type of fabric may be suitable for diaphragm strengthening. Finally, the use of overstrength intermediate FRP anchors did not noticeably affect the FRP shear strength contribution. However, the presence of intermediate anchors led to localized failures that concentrated inelastic diaphragm response between anchor locations, resulting in a significant reduction in diaphragm deformation capacity. The test results were used to develop design recommendations for shear strengthening existing concrete diaphragms using externally bonded FRP. The recommendations included guidance on how to establish the effective FRP design strain and the nominal shear strength contribution of the FRP, both of which tended to be conservative and underestimated the actual behavior observed during the experiments. The recommendations also address the use of intermediate and end FRP anchors, limitations on the clear spacing between sheets, and other factors pertinent to retrofit design. / Master of Science / The floor diaphragm in a reinforced concrete building transmits lateral forces generated by wind, earthquakes, and other horizontal loads to the building's vertical lateral force resisting system. Diaphragms in older reinforced concrete buildings are often retrofitted to meet seismic demands. Retrofitting deficient diaphragms increases infrastructure sustainability by promoting reuse and reconfiguration of existing buildings while mitigating structural deficiencies. Using externally bonded fiber reinforced polymer (FRP) composites is a com-mon strengthening technique often used without supporting guidance or test data. An industry need for diaphragm retrofit provisions, coupled with a substantial lack of data clearly indicates a need for experimental testing of diaphragm elements strengthened with externally bonded FRP. This thesis summarizes the results from an experimental research program designed to investigate the shear behavior of reinforced concrete diaphragms strengthened using externally bonded FRP. Six reinforced concrete diaphragm specimens were tested to study how variations in FRP material, density, spacing, orientation, and anchorage configuration impacted the performance of the retrofit. One specimen served as a control while the five other specimens were retrofitted with various configurations of FRP. The control specimen experienced a diagonal tension shear failure while each FRP strengthened specimen exhibited an FRP debonding failure, which was initiated by intermediate shear cracks occurring within the field of the diaphragm. The experimental results were analyzed to understand how the FRP retro-fits affected the strength, stiffness, ductility, and energy dissipation of each specimen. It was concluded that externally bonded FRP improves the seismic performance of a building by increasing the in-plane shear strength of the diaphragm. Existing design provisions were evaluated and compared to the experimental findings. Design recommendations were formed based on the observed affect of the test variables.