Interfacial mechanical properties of recycled aggregate concrete reinforced by nano-materials

•We tested the interfacial tensile and shear strengths between the old and new mortars of recycled aggregate concrete.•We investigated the changes of the microstructure of the interfacial zones by SEM scanning technique.•We quantified the strength improvement achieved by use of nano-SiO2 and nano-carbon tubes.•We established the interaction diagram of the interfacial shear and tensile strengths based on the test results. This paper evaluates the effect of Nano-SiO2 and multi-walled carbon nanotubes addition on the interfacial tensile and shear strengths of recycled aggregate concrete. Tension and shear tests were conducted for the interfacial strengths between the new and old mortars. Scanning electron microscopy was also conducted to identify the change of the microstructure of the nanomaterial modified interfacial transition zones. The results indicated that addition of the Nano-SiO2 and the carbon nanotubes can generally improve the interfacial tensile and shear strengths at dosages not exceeding 1.0 and 0.2 wt%, and the maximum tensile and shear strength improvements, 51% and 53%, respectively, were both obtained by use of the carbon nanotubes. Scanning electron microscopy results confirmed that the strength improvement came from the nucleation and filling effect and pozzolanic activity of the Nano-SiO2 and the nucleation, filling and bridging effect of the carbon nanotubes. An interaction diagram between the interfacial tensile and shear strengths were established following the Mohr-Coulomb strength theory. The internal friction angle of the modified recycled aggregate concrete was found to be in close range with the reference specimens (±8%), possibly due to the similar influencing mechanism of the nanomaterials on the shear and tensile strength of the interfaces.