The condition monitoring of concrete in a railway tunnel presents a series of challenges in sensor design especially where the sensors are to be installed post construction. There is a need to minimise the intrusion into the tunnel lining whilst maintaining an ability to detect corrosion conditions on both interior and exterior layers of reinforcement. The severe time and physical constraints on the installation process and the overriding need to maintain electrical safety and electromagnetic compatibility with the railway environment further constrain the design. This paper describes the development and validation of sensors to meet these constraints and the integration of these into networked systems. The fundamental principles of the system and practical aspects of the work are detailed. Expected results from the system are discussed for future comparison with field results and a parallel laboratory study program.
The design codes of practice for reinforced concrete infrastructure generally require a minimum 120-year life to be achieved1. The design of a bored tunnel from pre-cast reinforced concrete segments is no different. In designing the components the engineers need to take account of the local environment around the concrete and to assess how this may change over the life of the structure. In the context of a bored tunnel, the environment is significantly different on the outside and inside of the tunnel. The two environments will themselves vary along the route and will suffer different degrees of change in response to seasonal and longer term variations in water table, tidal flow and land use. From the outside of the tunnel, conditions can be fairly aggressive, the lining may be exposed to highly corrosive soil environments containing both chloride and sulfate and other aggressive species in conjunction with high pressure from ground water. From the inside, the movement of trains causes a piston like action. Air compressed before an advancing train can escape through the cracks in the tunnel lining segments and between the segments forcing the linings to leak. This may allow contaminants to enter the tunnel and be concentrated as a result of evaporation. The fluctuating air pressure on the internal surface may promote drying of the internal surface and wick action2. Carbonation of the internal surface is also possible. Furthermore, leakage of the DC current into the tunnel lining could induce stray current corrosion3,4. Understanding what these changes are, and the impact they will have on the structures, is a complex task. Concrete is a ?living? material ? its chemistry is slowly changing in response to the cocktail of contaminants and pollutants presented to it. As its chemistry changes, the corrosion protection it offers to the steel reinforcing also changes. Whilst the design codes can account for this and provide a reasonable assurance that the structure, if properly constructed and maintained, will achieve its required life they cannot guarantee this. By the time that visual inspections indicate problems it maybe too late to undertake economic repair and the costs of maintaining the asset can become punitive. Traditional inspection programs require costly destructive sampling campaigns ? using coring and drilling to obtain through-wall samples for laboratory analysis ? with the need for subsequent repair. Whilst the cores can produce quantitative data the trending of this over the life of the structure is made difficult by the need to extend the periods between sampling to minimize the risk of structural damage, and by the necessity that the second core has to be taken in a different place. These difficulties can be overcome by the installation of permanent c