The influence of moisture and epoxy bonding agents on interfacial behavior between normal concrete substrate and ultrahigh performance concrete as a repair material: Experimental and molecular dynamics study

In this paper, moisture conditions and epoxy bonding agents on interfacial behavior between normal concrete (NC) and ultrahigh performance concrete (UHPC) were investigated by both experiment and molecular dynamics (MD) simulation methods. The NC was exposed to different moisture conditions and bonding agents prior to UHPC reparation. The bonding strength was studied by slant shear and splitting tensile tests. A quantitative analysis of backscattered electron images as well as X-ray diffraction were applied to study the microstructure of the overlay transition zone (OTZ) between NC-UHPC. In addition, molecular dynamics (MD) simulation was combined with the macro experiments to explore the bonding mechanism between calcium silicate hydrate (C-S -H) and epoxy adhesive agents. The results show that UHPC is a promising repair material for concrete structure reinforcement/reparation with excellent bonding strength. The splitting tensile strength increases with a higher area ratio of NC to UHPC under the same substrate conditions, but varies when different adhesive agents are applied. Waterborne epoxy resin (WEP) can improve the bonding property of an imperfect NC substrate, while this improvement effect decrease with a smooth NC substrate. On the other hand, bisphenol-A-type epoxy resin (E51) has a negative effect on the bonding property. Finally, NC substrate moisture has no significant influence on the microstructure of OTZ and bonding strength. The MD results show, the growth of bonding strength be-tween C-S-H and epoxy by adhesive agent is negligible while toughness can be improved to a certain extent. This work provides multi-scale new insight for understanding the bonding mechanism on NC-UHPC.