Flexural behavior of composite beams with textile reinforced concrete (TRC) permanent formwork considering interface characteristics

The interface performance between composite reinforced concrete (RC) beams and TRC permanent formwork significantly influences the load-bearing capacity of composite beams. Based on bending tests of composite beams with varying dimensions, a numerical model considering the interface characteristics between concrete and formwork was established. The effects of member size, interface treatment, formwork shape, and the number of textile layers on the flexural behavior of composite beams were analyzed. Results indicate that as the size of composite beams increases, the interface transitions from localized microcracking to extensive debonding, altering the failure mode from bending to interface failure. For composite beams with U-shaped formwork and interfaces treated with cement paste, the load-bearing capacity increased by 16.78 %, 15.29 %, and 11.97 % for small-, medium-, and large-sized beams, respectively, compared to RC beams. This increase was approximately 10 % higher than that of composite beams with P-shaped formwork. Roughening the interface increased load-bearing capacity of composite beams by 5%- 8% compared to using cement paste at interface, while the insertion of shear pins into the roughened interface had a minimal further improvement. Each additional layer of textile in formwork boosted load-bearing capacity of the composite beams by approximately 3 %, with no significant rise beyond four layers. The interface slip fluctuated along the length of beams, with peak values concentrated in shear span. Ensuring effective bonding in this region is critical for enhancing the performance of composite beams. This study provides a foundation for the design and application of composite beams with permanent formwork.