Investigation on bond durability of GFRP bar/engineered cementitious composite under alkaline-saline environments

Abstract

Integration of GFRP bars and Engineered Cementitious Composites (ECC) as construction materials for a structure could significantly improve its long-term durability against harsh environments, while maintaining high loading bearing capacity, striving for overcoming corrosion-induced cracking and premature failure existed in conventional steel reinforced concrete structures. Although significant research has been conducted on the bond durability between GFRP bars and various types of concrete, very few studies specifically addressing the bond durability between GFRP bars and ECC could be found in the literature and their damage mechanisms are not clear, especially from the perspectives of multi-factorial consideration at both the micro and macro levels. As a result, the conclusions drawn from the existing studies cannot be entirely applied to these two materials. Hence, this research investigates the impact of alkaline-saline environments on the interfacial bond properties of GFRP bars embedded in ECC from macro and micro perspectives. To examine the degradation of bond properties under the effect of different factors, a total of 45 pullout specimens were prepared. The experimental variables included different immersion durations (0, 30, 60, and 90 days), surface treatments of GFRP bars, concrete types, and matrix compressive strengths. The specimens were subjected to accelerated aging tests with increased concentration of the alkaline-saline solution before conducting pullout tests. The analysis of bond degradation included the assessment of weight loss, failure mode, bond mechanism, bond stress, Scanning Electron Microscopy (SEM) imaging, and specimen composition. The results demonstrated that failure modes were highly dependent on concrete types and bond mechanisms. Untreated bar specimens exhibited higher bond strength when combined with ECC, while ribbed bar specimens showed little difference in peak strength between E30 and N30 specimens. The bond strength of ribbed bar/ECC specimens decreased by 5.88% and 8.55%, respectively, after 60 days and 90 days immersion, compared with that of the specimens without immersion. Furthermore, an improvement in the compressive strength of ECC resulted in enhanced bond strength. Specifically, the average bond strength of E50 specimens increased by approximately 14.5% compared to E30 specimens after 60 days of immersion. SEM analysis revealed an increase in calcium silicate hydrate gel (C–S–H) at the ECC interface after immersion, as NaOH and NaCl solutions penetrated the concrete and bond interface. The bond-slip relationships of the specimens were affected by immersion durations. The matrix-added fibers increased the ductility of specimens, providing benefits in the residual stage. The bond-slip curves were fitted with the mature models, and mBPE model demonstrated a strong agreement with the ascending and descending branches. Lastly, referring to fib Bulletin 40, the bond strength retention of ribbed bar/E30 specimens was predicted to be 69.05% and 67.25% after 50 and 100 years of expected service life, respectively.