Mathematical Modeling Applied to Control of Corrosion of Reinforcement Bars in Concrete

The most commonly recognized causes of de-passivation of reinforcing steel in concrete structures in industrial environments are chloride penetration and loss of alkalinity due to carbonization, but in addition stray current may be a cause of localized accelerated corrosion. In several cases including roadwork structures, bridges, retaining walls and marine structures, cathodic protection has proven to be an effective means to control corrosion. Cathodic protection of reinforcement bars in concrete may be provided by sacrificial, impressed current, or combined systems. The performance of a CP system will be critically affected by the level of contamination with chlorides- which affects both the conductivity of the concrete and the potentials required to provide protection. Performance of a system will also be critically affected by stray currents from nearby power supplies, which in the worst case can promote corrosion. However, the design of effective CP systems in new builds or retrofits is sometimes difficult to achieve due to lack of practical experience and the sometimes significant complexity of the structures. In this field, computational modelling can show important advantages.

The aim of this paper is to present a modelling approach for cathodic protection systems applied to steel reinforced concrete structures. The approach uses boundary element techniques, which firstly allow simplified representation of the effects of contamination by using a layered representation of the concrete, and which secondly allow simplified determination of the effects of possible stray current from nearby power sources.

Given appropriate polarization data for steel in concrete, together with polarization data for any sacrificial anodes and/or details of the ICCP output, anode ribbon, and cabling, the simulation can predict potentials and current density throughout the electrolyte. This provides potentials on the surfaces of the steel and anodes, as well as normal current density and corrosion rate. When combined with an optimization based reversed modelling technique, the simulation tool can be iteratively used to more efficiently exploit field measurements, i.e. to determine the polarization status and corrosion rate of the embedded steel provided that a number of potential measurements are given at accessible points of the structure.

The paper describes the methodologies used, and gives examples of application to structures including a metro tunnel, and a bridge deck. An example of use of reverse modelling will be provided.