Multiphysics Modeling of Rebar and Concrete Corrosion

The complexity of concrete’s microstructure makes the theoretical and experimental investigation of its transport properties a great challenge. Transport of fluids and materials depend on a large number of factors, such as porosity, pore size distribution, connectivity, and tortuosity. The range of the random microstructure of concrete, from nanometers (C-S-H) to micrometers (cement paste) to millimeters (mortar and concrete) to meters (final end use) covers nine orders of magnitude in size. It is thus a challenge to try to theoretically relate the microstructure and the properties of concrete from single scale modeling method. This has led to the use of multiscale modeling technology, coupled with modern experimental investigations, which have provided some insight into the microstructure.

Quantum Chemistry-Based Study of Iron Oxidation at the Iron-Water Interface

Illustrative Structure of Iron Oxidation at the surface

water molecules, calcium and chloride atoms are put on top of the oxidated iron layer

Each layer has a particular crystal structure which represent a specific iron oxidation state

Reaxff forcefield modeling steel corrosion

Pan, T., and Lu, Y., Quantum-Chemistry Based Study of Rebar Passivation in Alkaline Concrete Environment. International Journal of Electrochemical Science, 6 (2011) 4967 - 4983, 2011.

Multiphysics Modeling by Coupling FEA

Rebar Corrosion – multiphysics modeling application

Step I: Multiscale Modeling of Cl- Ingress

Step II: Rebar Corrosion & Rust Expansion

Step III: Concrete Cracking – Discrete Cracking Model

Pan, T., and Lu, Y., Stochastic Modeling of Reinforced Concrete Cracking Due to Non-Uniform Corrosion: An FEM-Based Cross-Scale Analysis, ASCE's Journal of Materials in Civil Engineering, 2011 (in press)

Corrosion initiation, propagation and self-healing properties in cracked concrete

The major degradation mechanism in civil engineering concrete structures is corrosion of reinforcement due to chloride penetration. Corrosion reduces serviceability and safety due to cracking and spalling of concrete and loss of steel cross section. Recently, service life design has moved from prescriptive performance based. The current approach aims at postponing initiation of corrosion until the end of the required service life with a predetermined reliability, based on simplified modelling of transport in uncracked concrete and testing of laboratory samples for chloride diffusion. Real structures under service load contain cracks and execution defects. Cracks are fast transport routes for chloride, but the effect is mitigated by poorly known mechanisms such as self-healing and crack blocking. Current models do not cover the effect of cracks, voids and compaction defects in concrete on chloride transport and corrosion initiation, rendering them less robust than desired. A project is carried out aimed at modelling the influence of cracks on the initiation and propagation of reinforcement corrosion.

Cathodic Protection of Rebar in Concrete

The deterioration of reinforced concrete structures such as bridges, buildings and infrastructure is a major nationwide concern because of the expenses incurred in the repair and replacement of the affected structures. A primary cause of this deterioration is corrosion of the reinforcing steel in the structure. We have presented characterization and diagnostics of deterioration mechanisms from Cross-scale/Multiphysics approaches. To propose applicable remediation and rehabilitation methods, many corrosion inhibiting methodologies and protection technologies are proposed.

One of the most commonly used methods of retarding corrosion and extending the life of the structure is cathodic protection (CP). However, CP of concrete structures is poorly understood at a fundamental level. CP systems are operated according to largely empirical protocols that may not be optimal treated. Modeling and numerical simulation can help determine better operating conditions, as well as elucidate the fundamental processes that are dominant in field systems, and provide guidance as to what measurements should be made in the lab.

The following example models cathodic protection of a steel reinforcing bar in concrete. Three different electrochemical reactions are considered on the steel surface. Charge and oxygen transport are modeled in the concrete domain, where the electrolyte conductivity and oxygen diffusivity depend on the moisture content. The impact of different moisture levels on the corrosion currents is investigated too.

PS(5)=0.4 Surface: Electrolyte potential (V) Contour: Electrolyte potential (V)

PS(13)=0.8 Surface: Concentration (mol/m3) Contour: Concentration (mol/m3)