Key measures for developing structure-function integrated ultra-high performance concrete (UHPC): Hierarchical modification network of multi-scale fillers

Abstract

Employing multi-scale fillers to regulate multiphase composites hierarchically is conducive to compensating for the defect that single-scale fillers can only work in a limited dimension, thereby providing a key measure for preparation of structure-function integrated ultra-high performance concrete (UHPC). In this paper, nanoscale nickel-coated carbon nanotube (Ni-MWCNTs), microscale stainless steel wires (SSWs), and macroscale steel fibers (SFs) are incorporated into UHPC matrix to explore the hierarchical regulation function of multi-scale fillers on the mechanical and electrical/sensing properties of UHPC. Results show that at the nanometer level, the incorporation of multi-scale fillers increases hydration rate of UHPC by 8.78 %, decreases gel pore volume by 75.58 %, and reduces the average Ca/Si ratio in the interfacial transition zone (ITZ) of SFs/SSWs-matrix by 23.42 %/4.52 %. At the micrometer level, the interfacial bond strength between SFs/SSWs and matrix is increased by 5.6 %/7.65 %. Based on the above two-levels regulation effect, the resistance of UHPC with multiscale fillers to three-point flexural cracking is significantly enhanced, reflecting in that the flexural loads corresponding to crack widths of 100 nm/50 mu m are increased by 69.36 %/12.35 % compared to UHPC with SSWs and SFs, the initial cracking load, flexural-tensile modulus are improved by 11.99 %, and 6.63 %, respectively. Furthermore, multi-scale fillers reduce the resistivity of UHPC through long- and short-range conduction effects, and through the gradual disconnection of the hierarchical structure, it exhibits more stable and sensitive sensing performance under flexural loads. The hybrid coefficient of multi-scale fillers on flexural strength is greater than one, verifying that Ni-MWCNTs, SSWs, and SFs have a positive hierarchical regulation effect on UHPC, and this offers an explicit theoretical guidance for developing structure-function integrated UHPC.