Investigation of bond shear behaviour of UHPC-NSC and damage prediction model under salt freeze-thaw environment

Ultra-high performance concrete-normal strength concrete (UHPC-NSC) repair interface properties after the salt freeze-thaw (S-F-T) cycle have great significance for concrete repair. The bond shear behaviour of the UHPC-NSC interface under the S-F-T environment was investigated. Single-side shear test method was used to evaluate bond shear strength. The substrate with compressive strength of 28 MPa, 36 MPa, and 42 MPa was used. The caste surface and high-pressure water-treated surface were considered. The findings revealed that as the strength of the substrate decreases, the total mass loss of the repair specimens increases, which reflects specimen surface erosion. The weight loss rate of all groups was lower than 1.53 % at the S-F-T cycle number of 300. S-F-T cycles can increase the chloride ion erosion area by more than 70.1 %. The action of S-F-T adversely impacted the failure behaviour of UHPC-NSC specimens. The lower the substrate strength and the smoother the interface, the bond strength reduced significantly. S-F-T cycles degrade the UHPC-NSC bond and decrease bond strength by 42 %-74 %, which decreases the durability of concrete repair. The lower the substrate strength and the smoother the interface, the bond strength reduced significantly. Finally, a UHPC-NSC interface damage prediction model was established to estimate the extent of damage to the UHPC-NSC interface caused by S-F-T cycles. The damage prediction model provides the basis for the durability design of concrete repair.