Flexural performance of UHPC sandwich panels with UHPC-filled stainless steel pipe shear connectors

A type of ultra-high performance concrete (UHPC) sandwich panel with UHPC-filled stainless steel pipe connectors (USPSPC) has been developed, demonstrating exceptional flexural capacity and thermal insulation performance. Four-point flexural tests were conducted on two UHPC sandwich panels to investigate their failure modes, load-deflection responses, and relative slips between the top and bottom slabs. Finite element analysis (FEA) was performed to examine the effects of panel height, top and bottom slab thickness, longitudinal reinforcement ratio of the bottom slab, and the diameter of the stainless steel pipes on the flexural performance. Additionally, the anti-slip stiffness and degree of composite action (DCA) of the sandwich panels were analyzed through numerical modeling. The results indicated that the flexural failure process can be divided into three stages: elastic, cracking, and yielding. The cracks of the sandwich panel with a higher DCA (specimen SP1) initiated in the pure bending section, and the failure mode was similar to that of solid concrete panels. In contrast, the sandwich panel with a lower DCA (specimen SP2) exhibited concentrated cracks in the shear-span section and failed due to excessive deflection. FEA results showed that both the panel height and the diameter of the steel pipe connectors significantly influenced the flexural performance. Increasing the panel height from 15 cm to 20 cm resulted in a 53.6 % and 39.6 % increase in cracking and ultimate loads, respectively. Increasing the diameter of the steel pipe connectors from 42 mm to 48 mm led to a 24.6 % and 21.3 % increase in cracking and ultimate loads, respectively. The stiffness of all sandwich panels met the deflection limit requirements under normal service load. Additionally, numerical analysis results indicated that reducing the height of the sandwich panel and increasing the diameter of the stainless steel pipes positively affected both DCA and anti-slip stiffness of the sandwich panels. The findings of this study provide valuable recommendations for the flexural design of these innovative sandwich panels.