Experimental study on pullout behaviour of basalt fiber-reinforced polymers minibar embedded in ultra-high performance seawater sea-sand concrete

Pullout tests were conducted on 150 specimens to characterize the bond behaviours of eco-friendly basalt fiber-reinforced polymer minibars (MB) embedded in ultra-high performance seawater sea-sand concrete (UHP-SSC). The experimental variables comprised surface textures of MB (straight and twisted), constituent contents of crushed seashell (SH), and coarse aggregate (CA) in UHP-SSC. The microstructures and surface morphologies of the UHP-SSC matrix and MB were characterized by low-field nuclear magnetic resonance (LF-NMR) and scanning electron microscopy (SEM). A constitutive model for the pullout load-slip relationships was proposed. Experimental results showed that the increments in the bond strength and energy absorption capability provided by twisted MB were more than twice those provided by straight MB, resulting from the additional mechanical anchorage. The acceptable content of SH in sea sand is less than 5% since it could enhance bond strength by 6%, while significant contents show opposite effects. Incorporating CA decreased the interfacial bond behaviours between MB and UHP-SSC, especially with a high volume fraction exceeding 10%. The microscopic observations and pore structures consisted of the evolution of bond behaviours and clarified the corresponding mechanism. The proposed constitutive model considering SH and CA content fits well with the experimental data with relevant regression coefficients over 0.89.