IntroductionAlkaline environment-induced (Basalt fiber reinforced polymer) BFRP bars degradation is the prominent issues to BFRP-reinforced seawater sea sand concrete (SSSC) durability. A new type of basalt-carbon hybrid FRP (HFRP) bars was developed to improve the mechanical properties and durability of BFRP bars in marine environments. Herein, the effect of simulated SSSC pore solution alkalinity on the durability of HFRP bars was studied. Macro- and micro-properties of HFRP samples were tested, i.e., tensile properties, interlayer shear properties, microstructure, chemical composition, and chemical stability. Meanwhile, the degradation mechanism of HFRP bars in simulated SSSC pore solutions with different alkalinity has also been summarized. The results show that compared with BFRP bars, the initial tensile strength of HFRP bars has been significantly improved, and its interfacial strength has not weakened. After 90 d of corrosion, the increase in alkalinity leads to a maximum decrease in tensile strength and interlayer shear strength degradation of 19.67% and 23.43%, respectively. The degradation of HFRP bars includes fiber etching, resin hydrolysis, degradation of interfaces on fiber/resin and BFRP/CFRP. The smaller diameter and excellent corrosion resistance of carbon fiber, and BFRP/CFRP interface barrier function delay the penetration and corrosion of moisture and existing ions, i.e., Cl^- and OH^-, improving the durability of HFRP bars in high alkaline SSSC environments.MethodsTensile properties, interlayer shear properties, microstructure, chemical composition, and chemical stability of HFRP bars in simulated SSSC pore solution with different alkalinity were systematically measured. The research mainly adopts a combination of macroscopic performance testing and microscopic testing analysis.Both epoxy resin based BFRP and HFRP bars with a nominal diameter d of 10 mm were prepared by pultrusion process. HFRP bars consists of inner and outer cores made of CFRP and BFRP materials, respectively. The volume fraction of carbon fiber replacing basalt fiber was 25%.Three alkaline salt solutions with different pH values were designed to study the effects of simulated SSSC pore solution alkalinity on HFRP bars. The ordinary SSSC pore solution (Ca(OH)₂ 2 g/L, KOH 19.9 g/L, NaOH 2.4 g/L) was prepared from seawater with a pH value of 13.2. Two lower alkalinity SSSC pore solutions with pH value of 12.3 and 10.1 were diluted with artificial seawater, respectively. All corrosion exposure experiments were conducted in a constant temperature water tank at 55 ℃.The tensile performance was evaluated according to ACI 40.3R-12. The total length of the sample was 1000 mm, and the testing section was 10 d. The interlayer shear performance was determined based on ASTM D4475. The length and span of the sample were 7d and 6d, respectively. All experiments were conducted using MTS testing. The testing rates for tension and interlayer shear are 2 mm/min and 1.3 mm/min, respectively.Digital microscopy (DM) and SEM-EDS were used to analyze the corrosion morphology and composition of BFRP and HFRP bars after corrosion. AFM was used to analyze the microstructure of the transition zone at the BFRP/CFRP interface.Results and discussionThe macroscopic morphology analysis results indicate that the carbon fiber hybrid did not change the tensile line elastic failure mode and interlayer interface failure modes of HFRP bars. Compared to BFRP bars, the ultimate load of HFRP bars was increased, and the explosive fibers during failure were significantly reduced with the extension of corrosion time and the increase in simulated SSSC pore solution alkalinity; Both BFRP and HFRP bars interlayer interface failures involve a main crack that runs through and deviates from the BFRP/CFRP interface.The carbon fiber hybrid improves the tensile properties and interlayer interface properties of HFRP bars, and enhances its corrosion resistance in alkaline simulated SSSC pore solution. Taking tensile strength as an example, compared with BFRP bars, the tensile strength of BFRP bars in simulated SSSC pore solutions with pH values of 10.1, 12.4, and 13.2 was 78.48%, 71.20%, and 26.20% of its original value, while the corresponding values of HFRP bars were 87.86%, 85.22%, and 55.26%, respectively. After 90 d of corrosion, the maximum decrease in tensile strength and interlayer shear strength due to the increase in alkalinity was 19.67% and 23.43%, respectively.The degradation of HFRP bars in alkaline SSSC pore solution includes fiber etching, resin hydrolysis, and degradation of fiber/ resin interfacial and BFRP/CFRP interfacial properties. The smaller diameter of carbon fiber, excellent corrosion resistance, and BFRP/CFRP interface barrier function delay the penetration and corrosion of moisture and existing ions, improving the durability of HFRP bars in high alkaline SSSC environments.