Bonding characteristics between existing concrete substrate and high ductility geopolymer overlays under direct shear loading.

High ductility geopolymer (HDG) looks promising as an exceptional repair material due to the faultless mechanical properties and environmental benefits. The interfacial bonding characteristics between existing normal concrete (NC) substrate and HDG overlay are the vital impact on the overall performance of repaired structures. This study presented the NC-HDG bonding behavior under the direct shear tests, and totally 45 specimens were designed with various concrete strength grades, interface roughness and elongations of HDG. To capture the fracture characteristics, the digital image correlation (DIC) technique was employed. Test results showed that the direct shear bonding strength (τ u) had an increasing trend as the NC strength dropped or the interface roughness and elongations of HDG were increased, among the interface roughness affected the τ u mostly, with the improvement amount of 185.07 %∼343.28 %. The direct shear strength ratio (defined as the ratio of τ u to the inherent shear strength of NC) was ranged as 0.110∼0.489, i.e., at least half of shear strength of NC was lost during repairing. Based on the DIC measured results, the improvement of NC strength, interface roughness and elongations of HDG all delayed the interfacial crack initiation under direct shear loading. Moreover, the upper-bound limit analysis was adopted, a simplified method for predicting the direct shear bonding strength of NC-HDG interface was proposed and showed a sufficient precision. • 45 NC-HDG bonded specimen are conducted under direct shear test. • Interface roughness affected the τ u mostly. • At least half of shear strength of NC was lost during repairing. • A method for predicting the bonding strength was proposed. [ABSTRACT FROM AUTHOR]