Durability assessment of pre-stressed concrete elements under stray current interference

The new Dutch railway line the 'Betuweroute' crosses the main gas pipeline system of the 'Gasunie', the main Dutch gas distributor, at 7 locations. To ensure a safe operation of both the railway line and the pipeline system, it is essential to guarantee the mechanical stability of the gas pipes in the soft soil; therefore at several locations ground retaining walls (GRW) and U-shaped vaults (USV) had to be constructed. Some of these structures were implemented using pre-stressed concrete sheet piles. A potential source of failure of buried pre-stressed or reinforced concrete is the electrical current that is present in the soil. This stray current mainly stems from leakage of traction power into the ground, and from a corrosion protection technique used for the Dutch gas pipeline system, called Cathodic Protection (CP). The latter uses the ground to conduct current from buried anodes to the pipeline to protect it against corrosion. The steel in the sheet piles may constitute an electrical pathway with a relatively low resistance. This results in a current flow through the pre-stressed steel, a phenomenon called interference, which can be harmful for the integrity of the concrete elements. The Dutch government, represented by Prorail bv, required a demonstrated service life of 100 years for all structures related to the Betuweroute. At that moment the effect of stray currents on the durability of buried concrete was not known, so the implications for the service life required assessment. Therefore TNO Building and Construction Research together with the Projectorganisatie Betuweroute, Grontmij Verkeer & Infrastructuur, Spanbeton bv and Leggedoor Concrete Repair/CPM Systems, defined a research project to develop the necessary tools and expertise for such an assessment, and to apply the results on the pre-stressed sheet piles [1]. This paper presents the developed tools and expertise. The following topics are addressed; - monitoring systems to measure interference, - a numerical model to quantify the local level of interference in the concrete - a numerical model of ion transport due to interference currents and the consequences for the concrete. With this latter model the changes in time of the chemical composition and subsequently the pH of the concrete near the steel could be calculated, giving the time until corrosion is initiated as a function of the level of interference. The results of this model are compared to an empirical model for stray current induced corrosion initiation.