Deterioration Mechanism And Performance Enhancement Of FRP/Concrete Interface Under Aggressive Ion Environments:A Molecular Dynamics Study

Fiber-reinforced polymers（FRP）are used to strengthen concrete structures,which is one of the effective measures to improve the performance of conventional concrete structures.At present,the external adhesion of FRP sheets to concrete structures is the focus of research.In the actual application,FRP sheets are generally adhered to the concrete structures with the adhesives（usually epoxy resin）.Given the natural differences in the constituent materials and the environmental sensitivity of the adhesives,the FRP/concrete interface inevitably becomes the weakest area of the FRP-reinforced concrete structures.In particular,the interfacial issues are critical for FRP-reinforced concrete structures serving in aggressive environments.Interfacial deterioration often leads to the entire failure of FRP-reinforced concrete structures.Thus,for the optimal design and long-life maintenance of FRP-reinforced concrete structures,it is of great theoretical and practical significance to study the deterioration mechanisms and enhancement methods of FRP/concrete interface under aggressive environments.The findings and results of this paper are as follows.1.Molecular modeling and validation of FRP/concrete interface.Based on the multi-scale mapping relationship of"FRP/concrete interface-epoxy/concrete interface-epoxy/C-S-H interface",the molecular models of cross-linked epoxy resin and C-S-H gel are established and verified at the nano-scale.After that,a model of the epoxy/C-S-H interface is built to reveal the bonding mechanism between the epoxy resin and C-S-H from the perspective of chemical bonds.It is found that（1）the established molecular models of epoxy and C-S-H match well with the previous experimental and simulation data.（2）The interatomic interaction between epoxy and C-S-H is proportional to the size of epoxy molecules,and the size effect can be eliminated by averaging the interatomic interaction energy to the unit bonding contact area.（3）The chemical bonding interaction at the epoxy/C-S-H interface consists of a combination of ionic(Ca _CSH-O_epoxy)and H-bonds(H_CSH-O_epoxy and O_CSH-H_epoxy),and the Ca _CSH-O_epoxy bond is the main source of bonding interaction at the epoxy/C-S-H interface.2.Simulation study of the deterioration mechanism of epoxy/C-S-H interface under aggressive environment.Based on the molecular dynamics method,a molecular model of epoxy/C-S-H interface,with highly cross-linked epoxy resin,is designed and placed in five simulated environments（Dry,Water,Na₂SO₄,Na Cl,Na₂SO₄+Na Cl environments）.By applying peeling and shearing forces to the epoxy resin,the deterioration mechanisms of the epoxy/C-S-H interface under the coupling effect of environmental factors and external loading are explored.It is found that（1）the aggressive environments are unfavorable to the bonding and mechanical properties of the epoxy/C-S-H interface,and the environmental factors inducing the interfacial degradation are ranked as Na₂SO₄+Na Cl>Na Cl>Na₂SO₄>Water>Dry.（2）Water molecules are the primary factor inducing the deterioration of the epoxy/C-S-H interface.Water molecules invade the interfacial void region and seize the reactive sites of ionic and hydrogen bonds on the C-S-H surface,which significantly destroy the chemical bonding interactions between epoxy resin and C-S-H.（3）Aggressive ions are another important factor to accelerate the deterioration of epoxy/C-S-H interface.Aggressive ions can form hydrated ion clusters with water molecules,which accelerates the aggregation of water molecules in the interfacial region and further weakens the bonding performance of the epoxy/C-S-H interface.3.Simulation study of the strengthened performance of graphene oxide modified epoxy/C-S-H interface.Graphene oxide（GO）covered with carboxyl（-COOH）,hydroxyl（-OH）,and epoxide（-O-）groups is selected to nano-modify the epoxy resin molecules,and the effects of this manipulation on the mechanical and thermal properties of the epoxy/C-S-H interface are investigated at the molecular level.Research shows that（1）the involvement of GO in the curing cross-linking reaction of epoxy resin produces the restraining effect on the movement of epoxy resin,resulting in a significant improvement of the interfacial integrity and a noticeable increase of the load resistance of molecular systems.（2）The diffusing capacity of epoxy resin is suppressed,and the glass transition temperature of epoxy resin is raised to 434.5 K.（3）GO enhances the chemical bonding performance of the interfacial system.As the"Bridge"between epoxy and C-S-H,GO strengthens the ionic and hydrogen bonds by forming Ca CSH-OGO-Hepoxy,OCSH-HGO-Oepoxy（Nepoxy）and HCSH-OGO-Hepoxy connections in interfacial region.Expectantly,this study will present a new perspective for the durability study of FRP-reinforced concrete structures,and provide theoretical support for the micro-optimized design of FRP/concrete interface under aggressive environments.