A residual strength prediction model of 3D-printed hybrid fiber-reinforced concrete at elevated temperature

Evaluating the mechanical performance of 3D printed concrete (3DPC) under elevated temperatures is crucial for fire safety. However, existing residual performance models predominantly rely on data-driven approaches for traditional concrete. Meanwhile, 3DPC studies are largely confined to fresh-state buildability, leaving its hardened residual properties practically unexplored. To address this gap, this study proposes a first-generation micromechanical model, utilizing the surface exposure temperature ( T ) as the primary independent variable, to predict the residual compressive strength of basalt and polypropylene (PP) hybrid fiber-reinforced 3DPC exposed to 20–800 °C. By coupling X-ray diffraction (XRD) analysis, thermogravimetric (TG) analysis, mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM), the model deconstructs high-temperature damage into matrix thermochemical degradation, porosity evolution, and basalt fiber deterioration. Incorporating the Weibull distribution, the model innovatively decouples porosity evolution into a dual-mechanism process driven by physical phase transitions (PP fiber melting) and chemical decomposition. Furthermore, a dynamic thermal attenuation function for basalt fibers was derived, identifying a critical failure threshold of 549.7 °C, which was strongly corroborated by SEM evidence. Addressing structural anisotropy, the model applies macroscopic equivalent homogenization to phenomenologically capture quasi-static interlayer defects using a directional strength reduction factor. Validation shows the model accurately captures macroscopic residual strength, with relative errors strictly controlled within 8%, providing a robust mechanistic framework for assessing 3D printed hybrid fiber-reinforced concrete structural integrity in fire.