Degradation of mechanical properties and microstructure evolution of basalt-carbon based hybrid FRP bars in real seawater and sea-sand concrete

The degradation of basalt fibre-reinforced polymer (BFRP) bars induced by alkaline environments hinders their application in seawater and sea-sand concrete (SWSC). The use of carbon fibres (CFs) to partially replace basalt fibres in the preparation of hybrid-FRP (HFRP) bars is an effective method for enhancing the durability of BFRP-bar. To evaluate the effectiveness of using CFs to replace part of the basalt fibres in preparing HFRP bars to enhance the durability of BFRP-bar in real SWSC. The tensile and interlayer interface properties of HFRP bars with different CFs hybrid contents (VCF, %) and arrangements in the SWSC were tested after exposure to seawater at different temperatures. Digital microscopy, scanning electron microscopy, X-ray computed tomography and matrix digestion analysis were used to investigate the microstructural evolution of HFRP bars and SWSC after conditioning. The results showed that the VCF increasing to 10% or 25% significantly alleviated the interlaminar shear strength (ILSS) and tensile strength (TS) losses. The maximum enhancements in the ILSS and TS retention for the SWSC-embedded BFRP bar were 19.12% and 17.55%, respectively. The hybrid CFs changed the failure model of the BFRP-bar. Compared with HFRP bars with CFs in the core, HFRP bars are more likely to deteriorate when the CFs are dispersed circumferentially. The hybrid CFs improved the environmental reduction coefficients (ERC) of TS and ILSS of the HFRP bars. Compared with the SWSC, the ERC of the HFRP bars in the normal concrete was significantly improved.