Enhancing the flexural toughness of UHPC through flexible layer-modified aggregates: A novel interfacial toughening strategy

Enhancing the interfacial deformability of UHPC positively impacts its toughness and durability. In this work, a novel interfacial toughening strategy was proposed and employed for UHPC, in which the aggregates were treated with polyacrylate emulsion (PL) or PL modified by silica fume or carbon nanotubes to form an interfacial flexible layer (FL). The flexural characteristics of the prepared UHPC were comprehensively investigated, with attention to the damage evolution based on acoustic emission. Meanwhile, the corresponding toughening mechanism was discussed. The results showed that the FL modified by carbon nanotubes effectively enhanced the flexural deformation capacity, energy absorption capacity, and toughness of UHPC, while maintaining flexural strength. Introducing FL reduced ringing count and acoustic emission energy and mitigated damage rate of UHPC. The FL altered the flexural damage mode of UHPC by alleviating stress concentration to prevent sudden matrix cracking and fiber debonding. During the elastic stage, FL and the UHPC matrix jointly sustained tensile cracks, enhancing the matrix's energy absorption capacity, which correlated positively with the percentage of tensile cracks. In the softening stage, this capacity correlated positively with the percentage of shear cracks. Moreover, FL reduced the probability of microcracks at the interface. Although the FL reduced the average microhardness at the interface, it stabilized the performance of hydration products and increased their maximum microhardness. The FL promoted interfacial energy dissipation and synergistically bridged microcracks with steel fibers, ultimately enhancing the flexural toughness of UHPC.