Molecular insights into the weakening effect of water on cement/epoxy interface

The strong interfacial integrity of FRP/concrete bonded system is shown to be significantly weakened in wet environment, with the interfacial debonding between concrete and epoxy adhesive as a major failure mode. However, the fundamental mechanism is still largely unknown. Here a molecular model of calcium silicate hydrate (C-S-H)/epoxy interface is constructed to analyze interfacial degradation mechanism as affected by water molecules, by relating with variations in the interfacial behavior and the failure mode. The simulation results show that water molecules in local C-S-H/epoxy interface degrade the interface and lead to a shift in failure mode from cohesive failure of C-S-H gel to interfacial debonding between C-S-H and epoxy, in accord with experimental observation. By simultaneously capturing interfacial degradation and failure mode transition, it is shown that water molecules significantly weaken the interfacial integrity of cement/epoxy interface, and hence the long-term durability of FRP/concrete bonded system.