Shear behaviour of precast steel-UHPC composite beams with high-performance demolishable concrete in connections

This study introduces a new type of demountable precast steel-UHPC composite beam that utilizes high-performance demolishable concrete (HPDC) in the post-pouring connections to address the issues of non-demountability and limited recyclability associated with traditional precast steel-UHPC composite beams. The proposed system achieves demountability by casting the HPDC in the connection between the studs and the precast UHPC slabs. The HPDC is cast by replacing a portion of fine aggregates in UHPC matrix with PMMA thermo-reversible properties, while retaining the conventional configuration and construction method of composite beams. To investigate the shear performance of stud shear connectors embedded in HPDC, a series of push-out tests were conducted. The experimental results revealed that: 1) the failure mode is primarily governed by shear stud fracture; 2) the ultimate shear capacity is comparable to that of conventional precast steel-UHPC composite beams; and 3) the PMMA fine aggregates in HPDC improve the ductility of the proposed demountable composite beams, as evidenced by a distinct yielding plateau in the descending branch of the load-slip curve. Based on the test results, existing design models for ultimate shear capacity in steel-concrete composite beam were assessed, and a new model for predicting the load-slip behaviour of stud connectors in HPDC connections was developed and evaluated. The solid-liquid phase transition of PMMA in the HPDC of the cast-in-place zone upon thermal activation converts the HPDC into a viscous flow state during demolition, enabling precise interfacial separation. No detectable damage is observed on the recycled precast UHPC slabs, thus verifying the demountability of the system.