Tailoring of steel fiber surface by coating cellulose nanocrystal for enhanced flexural properties of UHPC

Ultra-high-performance concrete (UHPC) is an advanced generation of cementitious composites with excellent compressive strength and durability. However, the relatively poor interfacial transition zone (ITZ) between steel fibers and UHPC matrix leads to insufficient flexural properties and hinders its further applications. This study proposed a cost-effective, sustainable, and highly reactive coating material, cellulose nanocrystals (CNCs), to densify the ITZ of steel fiber surfaces, thus enhancing the flexural properties of UHPC. Utilizing the Herschel-Bulkley model, the critical concentration of 1.0 % is determined for enabling uniform dispersion of CNCs, enhancing the coating performance and reliability. The performance of CNCs as coating materials was evaluated by flexural test of UHPC with CNCs-coated steel fibers, pull-off test, scanning electron microscope (SEM), atomic force microscope (AFM), energy-dispersive spectroscopy (EDS), and Fourier-transform infrared spectroscopy (FTIR). Results showed that compared to UHPC with pristine steel fibers, the flexural strength and toughness of UHPC with CNCs-coated steel fibers were increased by up to 14 % and 18 %, because the single fiber pull-off energy was increased by 50 %. It can be attributed to the densified ITZ which is validated by the increased UHD C-S-H and HD C-S-H on ITZ. However, when the concentration of CNCs suspension (i.e., 1.5 %) exceeds the critical value, the CNCs coating film would stick the uniformly dispersed steel fibers together and then affect their distribution, thus reducing the mechanical performance of UHPC. This work provides a green and effective approach for promoting flexural properties and an in-depth understanding of CNCs coating mechanism.