Impact of External Loading on the Time-Dependent Evolution of 3D Printed Concrete with Recycled Sand in the Green State (2025-03)¶

External loads are critical to the structural stability and construction precision of 3D printed concrete (3DPC) in the green state. However, in-depth research on this issue remains limited. This study prepared 3D printed concrete with recycled sand (3DPCRS) at replacement ratios of 0 %, 50 %, and 100 % and investigated the time-dependent evolution of 3DPCRS structures in the green state under stepwise loading. Using digital image analysis, the displacement and deformation characteristics of 3DPCRS structures were examined. Additionally, static rheological tests and unconfined uniaxial compression tests were conducted to evaluate the rheological properties and green strength of 3DPCRS. The results indicated that as the recycled sand (RS) replacement ratio increases, the plastic zone area of 3DPCRS structures in the green state under loading gradually decreases. Two displacement modes were identified under loading: instantaneous sudden displacement and static creep displacement, both of which decrease with increasing RS replacement ratio. Furthermore, the yield stress and development rate of the green strength of 3DPCRS accelerate as the RS replacement ratio increases. This study elucidates the mechanisms governing the time-dependent evolution of 3DPCRS structures under loading and proposes a bearing capacity calculation model for vertical loading on the basis of structural parameters. These findings provide valuable guidance for the construction and design of 3DPC structures under loading conditions.