Modelling concrete durability using system dynamics

Abstract

Corrosion of reinforcement is the major deterioration mechanism of the durability of reinforced concrete. Chloride ingress and carbonation are the main contributors to the corrosion. This work was aimed to model the corrosion processes using system dynamics which has a closed-loop structure that brings results from past actions of the system back into the model to control future actions. Two separate models are built for chloride ingress and carbonation. In each case, after the model is built, it is validated, and then parametric studies carried out on the influence of water / binder ratio (varied from 0.3 to 0.8) and fly ash percentage of binder (varied from 0 to 50%). Models were in good agreement with the published experimental studies. The obtained results predict the long-term characteristics of corrosion, and the changes in corrosion parameters such as crack width (chloride ingress into concrete) and carbonation depth (carbonation) over a design period of 50 years. Chloride ingress process is a positive or reinforcing loop in the system dynamics understanding where such ingress leads to the onset of cracking that in turn increases the ingress. The carbonation process on the other hand is a negative or balancing loop, since the diffusion of CO2 causes a carbonated region with reduced porosity that in turn decreases the diffusion. The effects of fly ash differ in the two models. In both cases, fly ash replacement (up to an optimum of around 30%) reduces porosity and hence increases strength and reduces diffusion, whether of chlorides or CO2. However, in the chloride ingress model, the reduction of OH- ions by the fly ash further reduces the ingress; whereas in the carbonation model, the same reduction increases the CO2 diffusion.