Study on bonding strength, chloride penetration resistance, and microstructure of adhesive interface for the old concrete repaired by alkali activated materials

This work studied the effect of alkali activated slag-fly ash (AASFA) material on the adhesive interface mechanical properties and chloride permeability of repaired ordinary concrete. The splitting tensile strength and chloride diffusion coefficient of the bonding surface repaired using three methods were compared: direct repair of ordinary concrete (method I), surface agents of cementitious slurry and alkaline activated slag-fly ash slurry followed by repair with ordinary concrete (method II), and direct repair with alkaline activated slag-fly ash concrete (method III). The microstructure including pore size distribution and micro-morphology of adhesive interface were investigated systematically. The results showed that the effect of direct repairing with AASFA concrete was the best among the three repairing methods. In this case, the reasonable modulus of the alkali activator was 1.3, and the adoptable slag content was 50%. The splitting tensile strength of the new-to-old concrete prepared by method II increased first with the increase in slag content and then decreased. There was a good linear correlation between the pore tortuosity and the chloride diffusion coefficient, and the chloride penetration resistance was mainly affected by pore tortuosity. For AASFA as the repairing material, the bonding strength of new-to-old concrete was not only dominated by pore structure but also affected by chemical reaction. The large amount of C-(A)-S-H gel generated at the adhesive interface not only filled the pores of the bonding surface but also provided additional bonding strength.