An improved micromechanical model for multiscale fiber reinforced ultra-high performance concrete: From hydration, dehydration to thermal damage

Multiscale fiber-reinforced ultra-high performance concrete (MSFUHPC) was recently developed to obtain the desired thermal and mechanical properties for engineering structures suffering from fire accidents. Quantitative methods to characterize the evolution of its thermal damage are necessary to design MSFUHPC and remain to be presented. This study presents an improved micromechanical model focusing on the material's stiffness from hydration through dehydration to thermal damage. MSFUHPC is renowned for its exceptional mechanical properties and durability; however, its susceptibility to thermal degradation poses significant challenges, particularly in fire scenarios. The integration of multiscale fibers—comprising steel, polyethylene, and carbon fibers and carbon nanotubes—alongside lightweight aggregates such as fly ash cenospheres, enhances the material's performance subjected to elevated temperatures. The proposed micromechanical model captures the complex interactions between the fibers, sand and cement matrix at various scales. By considering the effects of hydration and dehydration, the model offers valuable insights into the mechanisms leading to thermal damage during thermal exposure. Moreover, two Weibull probabilistic models are introduced to characterize the evolution of thermal cracking and sand debonding. Experimental studies are carried out to estimate the thermal and mechanical properties of MSFUHPC, thereby validating the model's feasibility. The results indicate that the multiscale fiber reinforcement significantly mitigates thermal damage. These findings underscore the importance of optimizing fiber and aggregate combinations to achieve superior fire resistance. Moreover, the proposed micromechanical model can serve as input parameters for thermomechanically coupled analysis of structures and components.