Influence of substrate moisture on the interfacial bonding between calcium silicate hydrate and epoxy

The adhesion at concrete-epoxy interface is crucial to concrete repairing. The multi-scale mechanism, particularly the influence of water to the adhesion yet remains unclear due to the limitation of experimental probes. This study combines molecular dynamics (MD) and experiments to investigate the influence of substrate moisture on the bonding properties between concrete and epoxy. Calcium silicate hydrate (C-S-H) and epoxy interfacial MD models were constructed with various moisture conditions inside C-S-H and on C-S-H/epoxy interface, whose equilibrium configuration, dynamic property, bond connection, interfacial interaction energy and mechanical properties were quantified. C-S-H/epoxy samples under different moisture conditions were synthesized for macro splitting tensile tests. MD results suggested the C-S-H/epoxy interface is stable under different moist conditions and is often stronger than the epoxy matrix. The interfacial Ca ions are crucial to adhesion by forming Ca-OC-S-H and Ca-OE bond. However, these bonds were reduced in the presence of interfacial water molecules. Our experiment provides the first tensile strength data between epoxy and pure C-S-H of different moisture content, which is one order of magnitude larger than reported data of concrete-epoxy interfacial bonding. It also reveals the change of interfacial fracture mode from brittle to ductile as C-S-H changes from completely dry to partially or fully saturated. This work provides new insight for understanding the bonding mechanism on C-S-H/epoxy interface, and may benefit the engineering application of concrete infrastructures reparation.