Enhancing interface shear performance and bending resistance of UHPC permanent formwork for composite short beams by different interface treatment

The study assessed interfacial shear properties of precast UHPC (ultra-high performance concrete)-cast-in-place NC (normal concrete) composite specimens using six interface treatments: smoothing, epoxy resin, grid chiseling, sand blasting, grooving, and steel stud connectors. Shear loads and slip were recorded for analysis. Optimal treatment guided fabrication of U-shaped UHPC permanent formwork with varying thicknesses, forming UHPC permanent formwork-NC/RC composite short beams for flexural tests. Results showed interface roughness treatment significantly enhanced interface shear performance. Notably, 1.8 cm SWM grid chiseling produced the highest shear strength, 4.7 times smoother interface. Increasing UHPC formwork thickness increased ultimate load capacity; 20 mm thickness was 81.4% higher than 10 mm. Treating U-shaped UHPC permanent formwork bolstered bending resistance; chiseling and grooving enhanced ultimate load capacity by 30.1% and 19.5%, respectively. UHPC permanent formwork outperformed RC short beams, modifying failure modes and enhancing bending capacity. The findings offer practical insights for UHPC formwork application in engineering.