Reinforced concrete is one of the most widely used engineering materials as final lining of tunnels. One of the most important functions of the final lining is its durability in long-term to insure a safe and reliable service life for tunnels. According to different surrounding environments, common durability problems in tunnels which have significant effects on tunnel safety and serviceability are Chloride Penetration and Carbonation. Like other civil structures, the service life of tunnel linings can be divided into two distinct phases. The first phase is the initiation of corrosion in which no reinforcement corrosion occurs in the concrete and the second one is propagation of corrosion. Duration of the initiation phase depends mainly on the cover depth (thickness of the concrete cover above embedded reinforcing steels) and the penetration rate of the aggressive agents as well as on the concentration necessary to depassivate the reinforcing steels. By defining the end of the service life (lifetime) of final linings as the end of the initiation phase i.e. no corrosion allowed during the lifetime of the tunnel lining, and using the principles of Serviceability Limit State (SLS) concept used in structural design codes , the thickness of the concrete cover is calculated in this paper. In the current study an approach is presented to calculate the concrete cover thickness of tunnel final linings that can assure a durable, safe and reliable lifetime with the failure probability of 5% in 100 years.
