A Multi-scale Study of Enhancing Mechanical Property in Ultra-High Performance Concrete by Steel-fiber@Nano-silica

The interfacial property between steel fiber and matrix is critical factor affecting the improvement effect on mechanical property and crack prevention of Ultra-High Performance Concrete (UHPC). Multi-scale methods consisted of molecular dynamics, Raman, SEM-EDS, AFM and MIP were empolied to reveal the multi-scale mechanism of strength improvement via steel fiber@nano-silica on UHPC. Futhermore, the preparation of steel fiber surface modification was optimized. On the microscale, Ca2+ in the UHPC matrix form Ca-O bonds with O in nano-silica which improve fracture toughness during steel fiber pull out from matrix. On the mesoscopic scale, nano-silica on the surface of steel fibers provides nucleation sites for the generation of calcium silicate hydrates, which improves the surface roughness and increases the interlocking effect between the steel fiber and matrix. On the macroscale, the calcium silicate hydrate formed on the surface of steel fiber fills the macro capillary pores in the transition zone between fiber and matrix, so as to adjust the weak area on the microstructure of UHPC and improve the mechanical properties of UHPC. These multi-scale mechanism of strength improvement proposed in this investigation furnishes an approach to design UHPC with modified fibers.