Influence of Hybrid Fibers on Bond Characteristics of the Glass Fiber–Reinforced Polymer Bars with Standard and High-Strength Geopolymer Concrete

This study examines the influence of hybrid fibers (HFs, polypropylene and brass-coated steel fibers) on the bond mechanism of glass fiber–reinforced polymer bars in geopolymer concrete. Additionally, the effects of bar diameter, embedment length, and concrete compressive strength are also investigated. The results indicate that incorporating HFs improved pullout load capacity, bond stress versus slip curves’ initial stiffness, and adhesive bond strength. HF is very effective in controlling the crack width (reduces it substantially) observed on the specimen surfaces. As a result, HF altered the failure mode; the specimens that failed owing to splitting of the concrete in plain concrete (PC) altered their failure mode to pullout. Energy-based bond toughness metric was employed to assess the post-peak response, and it was found that the inclusion of fibers improved it significantly. However, when embedment length and bar diameter increased, bond stress decreased faster in HF specimens than in PC specimens. Raising the concrete compressive strength enhanced the bond strength (up to 33.41 %), initial stiffness, and adhesive bond strength. The experimental data were compared to analytical models (Cosenza-Manfredi-Realfonzo [CMR] and modified Eligehausen, Popov, and Bertero [mBPE]), and the CMR model had a slightly greater bond strength than the mBPE model and performed better in conformity with the experimental results.