A nanoscale adhesion mechanism of cement-epoxy interface under varying moisture conditions: A molecular dynamics study

The epoxy-concrete interface is a critical phase in a bonded system, such as FRP-bonded to a concrete structure. The effectiveness of this bond is generally affected by the presence of moisture at the interface. In this paper, a nanoscale adhesion mechanism of the epoxy-cement interface under moisture variations was studied using molecular dynamics (MD) modeling. The epoxy-cement interface was first modeled by representing calcium silicate hydrate (C–S–H), and diglycidyl ether of bisphenol-A (DGEBA) resin. The surface moisture was simulated by inserting different layers of water. In addition, the macroscopic pull-off adhesion test was performed to validate the MD findings, where the latter provided an explanation for the observed macroscopic behavior. The results indicated that the interatomic and intermolecular forces across the interface at dry and relatively low moist systems are mainly due to the van der Waals (vdW) interactions. However, at relatively higher moisture content, the electrostatic interaction becomes more significant, which is responsible for the degradation process in the adhesion energy. Further, the increase in the debonding work was mainly caused by an increase in the adhesion between the C–S–H and water molecules at the interface where the latter forms hydrogen bonds with the substrate (i.e., C–S–H).