Experimental evaluation of mechanical properties and microstructure for recycled aggregate concrete collaboratively modified with nano-silica and mixed fibers

The present study was conducted to meet the demand of the construction schedule for emergency repair and construction in special projects by using the recycled aggregates produced by mechanically crushing the demolished materials on-site. The study aims to modify recycled aggregate concrete (RAC) by incorporating silica nanoparticles (NS) and mixed fibers together to meet the needs of the above special project. Here, the properties of 21 kinds of concrete with different NS contents and mixed proportions of fibers were tested, including their fresh properties, hardening properties and microstructure. Their working properties in the fresh state were studied by measuring slump and unit weight; while the mechanical properties after hardening were mainly evaluated in terms of compressive strength, splitting tensile strength, modulus of elasticity and flexural strength, and here the flexural toughness as well as the residual strength of the specimens were calculated using two methods. In addition, the effects of different NS contents, steel fibers and polyvinyl alcohol (PVA) fibers on crack development and water absorption were investigated in the microstructure. The pores between the cement matrix, interfacial transition zone (ITZ) and fibers and how they are alteredby NS were characterized by SEM and 3D-CT scans. All these results indicate that NS content has a significant effect on both the workability and mechanical properties of RAC, that steel fibers are beneficial to improve the overall mechanical index of RAC, while PVA fibers have a limited enhancement effect, and that NS particles in the microstructure help to fill the pores of 0.001-0.01 mm3 volume in the cement matrix and improve the performance of the ITZ.