Efficacy and efficiency of dual electrochemical strategies for remedying rebar corrosion in heterogeneous concrete

Steel reinforced concrete is highly susceptible to chemical attacks in many field conditions. These chemical attacks include (but not limited to): sulphate attacks, alkali silica reactions and alkali carbonate reactions, that can considerably reduce the service life of structures if not addressed early enough. Corrosion of embedded steel bars has the potential to grow in a self-accelerated mode to cause premature deterioration of concrete structures. Specifically, corrosion of rebars undermines concrete structures by causing volumetric expansion of steel and consequentially cracking of the concrete cover. Chlorides are particularly notorious for rebar corrosion as they can come from a variety of environments. Ingress of such corrosive ions into concrete involves three mechanisms: i.e. natural diffusion, electromigration and hydraulic migration; where the first two are most predominant in water saturated concrete. Existing literature has revealed that there are two promising approaches to address this: i.e. Electrical injection of corrosion inhibitors (EICI) and electrical removal of chlorides (ERC). These two techniques involve the application of an electrical field either to inject beneficial corrosion inhibitor species into concrete or extract chloride ions from concrete to protect embedded steel rebars against corrosion, and thus both techniques can be applied concurrently.

Unfortunately, reputable literature on EICI and ERC is limited and the findings on efficacy and effectiveness of both electrochemical treatments are far from conclusive. Therefore, considering the widespread existence of reinforced concrete structures, adequate long-term protection of concrete against deterioration by rebar corrosion remains a priority. In light of this, Professor Tongyan Pan from the University of Northwestern together with Dr. Junying Rao at the Guizhou University developed and solved a 3D mathematical model using finite element analysis that could be used to model the transport of species in concrete. Their goal was to distinguish the microstructure and transport properties of the different phases of concrete in order to attain high accuracy. Their work is currently published in the research journal, Cement and Concrete Composites.

In their approach, a mathematical model in the form of a Multiphysics, non-linear differential equation was established for evaluating the efficacy and efficiency of a parallel EICI and ERC scheme, aided by three accelerated electrochemical experiments developed for parametric determination and for model validation. Based on the method of three-dimensional microstructure reconstruction, a finite element analysis was applied to solve the mathematical model that couples three key mechanisms of mass transport in the matrix of heterogeneous concrete, including natural diffusion, hydraulic migration and electromigration.

The authors reported that chloride, the primary cause of rebar corrosion, was found to be effectively removed under an electrical current density large than 3 A/m² after one year of treatment. Remarkably, the numerical model of the concrete specimen gave results that were in good agreement with the experimental measurements, that is, within a 6% difference.

In summary, the study presented a mathematical model and its finite element analysis that were developed to predict the evolution of concentrations of corrosion inhibitor and chlorides in heterogeneous concrete under an externally applied electrical field and the effects of other physical fields. The study showed that that ERC aided with EICI could be an economic remedial strategy for reinforced concrete susceptible of rebar corrosion. In a statement to Advances in Engineering, Professor Tongyan Pan said their new mathematical model and its finite element solution developed in their study can be used as a convenient tool to evaluate generic strategies for concrete remediation.