Assessment of reparation by epoxy deposition

The goal of this study is to understand whether the injection of epoxy in concrete only prevents the corrosion of steel reinforcements, or whether it can also improve the stiffness and strength properties of damaged concrete. Interface fracture propagation between organic and inorganic materials has been studied in various research fields such as chemistry, mechanics, material science, and civil engineering. The Finite Element Method (FEM) alone cannot explain what creates the bond between the faces of a crack filled with epoxy. Here, we combine Molecular Dynamics models and the eXtended Finite Element Method (XFEM) to assess the mechanical properties of fractured concrete repaired epoxy.

Multiscale modeling of fracture propagation in concrete without reparation

We model fracture propagation in concrete before damage (and hence reparation) has occurred. We use XFEM cohesive segments with a linear softening traction separation law to represent fractures. We calibrate concrete damage parameters against three point bending tests (Mode I) and four point bending tests (Mode II) by using neural network optimization method. The Crack Mouth Displacement (CMOD) is measured with clip edges attached on the bottom-center of beam in the experiments. The CMOD is calculated as the lateral displacement of the enriched elements in the numerical model. The ascending slope of the numerical force-displacement curve matches experimental results. However, the softening portion of the curve departs from the experimental results. The bilinear traction separation law should better capture the softening behavior.

Pictures: Ji and Arson, 2018

Molecular Dynamics model of concrete/epoxy interface behavior

Molecular Dynamics (MD) models allow calculating the van der Waals forces at the interface between two materials in order to predict the intermolecular interactions between epoxy adhesives and concrete materials.

We first optimize the molecular structure and the potential energy function of the concrete/epoxy interface using MD software LAMMPS. We consider the dynamic crosslinking of the diglycidyl ether of bisphenol A (DGEBA) and amines. The concrete material is represented by crystallin silica (alpha-quartz). The molecular scale peel test and shear test are conducted through steered molecular dynamics (SMD) to obtain the free energy between epoxy and the silica surface. The interface fracture energy and strength are calibrated from the free energy of the epoxy/concrete interface.

Pictures: Ji and Arson, 2018

Multiscale modeling of fracture propagation in repaired concrete

The energy and strength of the epoxy/concrete interface fractures obtained from MD simulations are used as input parameters in the XFEM cohesive segment model. We then compare the force displacement curves and the fracture patterns obtained in experiments done on repaired concrete to the results obtained with the MD-XFEM model.

Pictures: Ji and Arson, 2018