Flexural performance of U-shaped high-strength ECC permanent formwork-concrete composite beams

This paper presents the design of a U-shaped High-Strength ECC Permanent Formwork-Concrete Composite Beam that facilitates the construction of ECC-concrete composite beams. To ensure the reliability of the High-Strength ECC and concrete interface, two interface construction forms, point grooves and rectangular grooves, were developed. Bond properties of these interfaces were assessed through double-sided straight shear tests, revealing commendable bond performance, with shear strengths of 3.72 MPa for point grooves and 2.15 MPa for rectangular grooves. Subsequently, the U-shaped High-Strength ECC permanent formwork was designed to incorporate these interfacial forms and varying High-Strength ECC layer thicknesses, resulting in the fabrication of composite beams. Four-point flexural tests were performed on five composite beams alongside a reference ordinary reinforced concrete beam to investigate how the interface structure and the thickness of the High-Strength ECC layer affect flexural performance. Results indicate that both point and rectangular groove interfaces exhibited similar flexural performance, with no debonding observed during failure. The U-shaped High-Strength ECC permanent formwork significantly improved crack patterns in ordinary reinforced concrete beams, enhancing their crack loads and flexural capacity. When the thickness of both the web and bottom plate of the U-shaped High-Strength ECC layer ranged from 20 mm to 30 mm and 20 mm to 60 mm, respectively, the crack load increased by 22.9-63.8 %, and the flexural capacity improved by 8.1-19.9 %. Furthermore, a theoretical calculation method for predicting the flexural cracking load as well as the ultimate load of U-shaped High-Strength ECC-concrete composite beams is proposed, with comparisons between theoretical and experimental results validating its effectiveness.