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.
Durability is the capacity of a structure to last in time, performing the service for which it has been designed [1[. During lifetime of different structures, surrounding environment affects the structure chemically and physically. In long-term, the structure will deteriorate gradually at different rates depending on the aggressiveness of the surrounding environment and resistance of the structure. Accordingly, the deterioration process affects the structure aesthetically and structurally. Consequences of the deterioration process might be so serious that may pose a significant threat to serviceability and safety of the whole structure. Among various types of structures, durability of concrete structures has been studied extensively due to their widespread use. Therefore, in these structures common durability phenomena have been relatively well defined including steel corrosion by chloride penetration, concrete cover carbonation, concrete damage by freeze-thaw cycles in cold climates, long-term leaching of concrete solid phase by soft water as well as chemical attack of concrete by aggressive ions . Considerable efforts have been made to understand the mechanisms and damages of these phenomena. Among them, steel corrosion process is better understood with identified mechanisms and suggested mathematical models . In order to design a durable structure and escape from serious consequences of the steel corrosion in long-term, concrete structures must be designed against corrosion. There are basically two approaches to perform durability design:
prescription method and
The prescription method imposes specific requirements on material constituents and structure details on the basis of the exposure environment, expected service life and intended service condition of structure.