Long-term properties evolution and life prediction of glass fiber reinforced thermoplastic bending bars exposed in concrete alkaline environment

Glass fiber reinforced polypropylene (GFRPP) bending bars have many advantages, such as high toughness, high durability, repeated molding and recycling, which can be used in concrete structures to replace steel stirrup for solving the problems of corrosion and service life. However, the changes in the properties of GFRPP bending bars during long-term use in alkaline concrete environments require further investigation. In the present study, GFRPP bending bars were immersed in alkaline solution at 20 degrees C, 40 degrees C, and 60 degrees C for as long as 180 d. The water absorption, hydrolysis, mechanical properties, and microscopic properties of the bending bars were evaluated to understand the long-term performance and mechanism of degradation. The bending bars had a higher saturated water absorption percentage and time along with a lower diffusion rate than the straight bars due to the presence of pores and micro-cracks formed during the bending process. The dissolution of the glass fibers in alkaline solution led to the partial mass loss of the GFRPP bars. Compared to the control, the tensile strength retention of the bending bars was 20.6%-56.0 %. The tensile failure mode changed from fiber bursting before immersion to shear fracture after immersion due to the decreased interfacial bonding strength and increased glass fiber brittleness. The primary degradation mechanisms were the etching of glass fibers, interface debonding, and increased void volume (similar to 23.7 %). The bending bars were more seriously eroded than the straight bars due to the twisting of the fibers and imperfect interfaces. The predicted degradation times of the GFRPP bending bars ranged from 1.7 years (Haikou) to 3.9 years (Harbin), corresponding to a stable tensile strength retention of 22.13 %. The results provide a basis for evaluating the durability and service life of GFRPP bending bars and promote the application of GFRPP composite stirrups in concrete structures.