Experimental and Analytical Evaluation of FRP-Retrofitted Reinforced Concrete Diaphragms for In-Plane Shear Strengthening

Abstract

Diaphragms are a key component of the horizontal lateral force resisting system (hLFRS) in a reinforced concrete (RC) structure and are crucial for providing sufficient load path to the vertical lateral force resisting system (vLFRS). Diaphragms may require retrofit due to the relocation of vLFRS elements during renovation, penetrations cut into the slab, or in older RC structures that were designed according to outdated design codes. Multiple methods exist for retrofitting diaphragms, including a concrete overlay, infilling of penetrations, and the application of externally bonded fiber reinforced polymer (FRP). FRP is an attractive option because it is quick to install, non-corrosive, and increases the mass of the structure far less than other methods. Although FRP is commonly used to strengthen the in-plane shear capacity of reinforced concrete diaphragms, there is no guidance for this specific application in the current ACI PRC-440.2R-23 (2023) Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. A design methodology was developed by Hutton et al. (2023) along with design recommendations based on a smaller dataset, but there is a need for a broader dataset to validate and modify existing guidance. One part of this thesis describes an experimental program intended to broaden the depth of experimental data relevant to in-plane shear strengthening of RC diaphragms with FRP. The program contained eight cantilever RC diaphragm specimens subject to a displacement controlled reversed cyclic loading protocol at the free end. The diaphragm specimens were designed to represent the diaphragm shear zone adjacent to a shear wall, a typical location of high in-plane shear demand. Seven specimens were strengthened with FRP, while one served to establish a baseline. FRP performance was examined as a function of ply configuration, ply orientation, anchorage type, and anchorage scheme. A diaphragm specimen database was assembled, including the eight specimens described in this thesis, five specimens from Aryan et al. (2022), and six specimens from Hutton et al. (2023). It was shown that FRP strengthening was effective in increasing in-plane shear capacity of all 16 strengthened specimens. The typical failure mode of the strengthened specimens was FRP debonding followed by diagonal shear failure in the concrete, although anchorage failures, FRP rupture, and crushing of the compression strut also occurred. As FRP surface coverage increased, diaphragm ductility reduced while the shear strength contribution of the FRP increased. Configuring FRP in an orthogonal grid was more effective in increasing diaphragm shear strength relative to the same total quantity of FRP oriented parallel to applied shear. Variations in anchorage were also investigated and found to have little effect on diaphragm strength. The database was used to refine and expand design recommendations for shear strengthening of reinforced concrete diaphragms with externally bonded FRP. Recommendations relevant to the shear strength contribution for varying orientations of FRP, reinforcement limits, and anchorage schemes were proposed. Existing recommendations from Hutton et al. (2023) relevant to FRP effective design strains and anchorage design were corroborated. Additionally, the stringer-panel model (SPM) was investigated as an analytical technique for determining diaphragm demands. Direct comparisons between chord, collector, and diaphragm shear demands were made between the SPM and the equivalent beam model (EBM). In the one design example, the SPM demonstrated higher fidelity, capturing indirect load paths and more distributed demands in the diaphragm. Furthermore, the SPM offers a practical advantage as a single model can accommodate multiple load combinations while producing interpretable design demands for detailing steel reinforcement or externally bonded FRP retrofits.