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    Research on bond behavior between steel rebar and self-compacting geopolymer concrete (SCGC) containing recycled aggregate by large-scale beams: The role of different hybrid activator content and precursor materials
    (Elsevier Sci Ltd, 2025) Utu, Rumeysa; Katlav, Metin; Donmez, Izzeddin; Kina, Ceren; Turk, Kazim
    This paper aims to experimentally evaluate, for the first time in the literature, the bond strength between steel rebar and self-compacting geopolymer concrete (SCGC) containing 100 % recycled aggregates, considering the effects of different hybrid activator ratios and precursor material combinations, using large-scale reinforced concrete (RC) beams. With this aim, a total of twelve full-scale SCGC beams, each with dimensions of 200 x 300 x 2000 mm, were produced with different hybrid activator ratios ((Na2SiO3 / (Ca(OH)2 + Na2SiO3)= 0.15, 0.20, 0.25) and precursor material combinations (single, binary and ternary) and tested under four-point bending loading after a 90-day curing period. Test outcomes were compared and evaluated based on main structural performance parameters, including crack patterns and propagation, failure modes, load-midspan displacement curves, load-strain behavior, and bond strength. Moreover, the predictive performance of some existing mechanics-based models for predicting bond strength was evaluated for spliced steel rebar in the SCGC beams. According to the experimental outcomes, both the hybrid activator ratio and the precursor material combinations had remarkable effects on the bond behavior of SCGC beams. In general, lower hybrid activator ratios primarily induced flexural cracks concentrated within the pure bending region, while increasing the hybrid activator content led to a greater number of cracks, particularly transforming into inclined (shear) cracks in the shear region. As for the influence of precursor materials, binary blends-the combination of silica fume (SF) and ground granulated blast furnace slag (BS)-consistently provided superior structural performance, characterized by improved crack control, enhanced load-carrying capacity, and higher bond strength. Notably, the 0.50SF+ 0.50BS_0.15 N specimen with a 0.15 hybrid activator ratio achieved the highest peak load of 96.58 kN and the maximum bond strength of 4.31 MPa among all tested specimens. Furthermore, while existing mechanical bond strength models offered moderately accurate predictions for SCGC, they failed to comprehensively account for the unique interaction mechanisms inherent to geopolymer systems. Therefore, this study underscores the importance of optimizing both activator dosage and precursor synergy to ensure reliable predicted bond performance in SCGC. All in all, these results are expected to provide valuable guidance for structural engineers seeking to implement environmentally friendly, durable, and structurally efficient SCGC members in real-world construction applications.