Modelling Diffusive Transport Phenomena in Carbonated Concrete
For civil engineering structures the interaction between concrete, its reinforcing steel bars and various environmental agents like humidity, CO2, O2 , and chloride is extremely important. Generally, the concrete provides appropriate conditions to prevent steel reinforcement bars from environmental corrosion, because of a passive layer on the steel surface. Concrete carbonation [1] and consequent reduction of the pH in proximity of the steel bar may contribute to the destruction of this passive layer resulting in suitable electrochemical conditions for generating steel corrosion. If oxygen is present, then the steel bars might corrode and the strength of the whole structure would be compromised.
In a previous study [2], a computational model of the carbonation process in a porous concrete was validated against experimental results of concrete samples placed inside an accelerated carbonation chamber, by measuring the advance of the carbonation front caused by CO2 diffusion and its chemical interaction with concrete constituents. Now in this work we improve our model by introducing the change of porosity of the concrete, as a consequence of the carbonation reaction, and by modelling the resulting coupled diffusive transport phenomena of CO2 and O2 in the system.
In the computational work we use the Chemical Reaction Engineering Module in COMSOL Multiphysics® 5.4 to model the time dependent carbonation process of concrete and diffusive transport mechanisms of CO2 and O2. The carbonation reaction is set up in the Reaction Engineering interface, then we use the Generate Space-Dependent Model feature tool to export the properties to the Transport of Diluted Species interface. The diffusion of the species in the concrete is carried out by considering the concrete as a partially saturated porous media. The porosity, modified by the chemical processes, is also computed with COMSOL Multiphysics® using the ODEs and DAEs interfaces.
The results obtained with this work allow us to know the concentration of chemical species inside the concrete and the variation of the pH level due to the reaction with CO2, as well as the decrease in pore size. The presence of O2 in proximity of the steel bar is also quantified in order to analyse limiting conditions for the steel corrosion.
[1] V.G. Papadakis, C.G. Vayenas and M.N. Fardis, Fundamental modeling and experimental investigation of concrete carbonation, ACI Materials Journal, 88, N.4, 363-373 (1991).
[2] B. Chiné, R. Cuevas H., R. Jimenez S., A. Rodriguez R., A model of concrete carbonation using Comsol Multiphysics®, Comsol Conference 2018, October 21-23, 2018, Lausanne (Switzerland).
