Estimating Service Life of Prestressed Concrete Systems Exposed to Chlorides

In general, pretensioned concrete (PTC) structures are being designed with a target service life of about 100 years. However, cases of premature/catastrophic corrosion-induced strand failures have been reported. This necessitates a re-examination of the current practices for service life-based design (SLD) as well as corrosion assessment of new and existing PTC structures respectively, especially the choice of limit states and the parameters considered. The work presented in this paper is a component of the research carried out to develop tools to assist SLD and assessment of PTC structures, and focusses on the estimation of time required for Passive-to-Active (P-to-A) transition, which is the first step of chloride-induced corrosion of prestressed steel strands. For this, ordinary Portland cement-based reinforced mortar specimens (both unstressed and stressed categories) were prepared using prestressing (PS) steel king wires extracted from 7-wire strands. Totally, 10 specimens (5 + 5) were cast, cured for 28 days and then subjected to dry-wet (5-day drying followed by 2-day wetting) cycles in simulated concrete pore solution containing sodium chloride. The P-to-A transition was detected by continuous monitoring of the open circuit potential and impedance measurements at the end of every wet exposure period. The specimens were then autopsied and the concentration of chlorides in the mortar at the level of steel was determined and defined as Clth. It was found that the Clth of PS steel can reduce by half in OPC system when prestress is applied. A case study was performed to understand the implications of using overestimated values of Clth on the service life. An overestimation in service life of about 40% was observed when the Clth of unstressed PS steel was used - emphasizing the importance of determining the Clth of stressed steel and using it for service life estimation.