A tunnel or bridge structure exposed to salt water can expect corrosion of the embedded steel during its service life. Cathodic Protection (CP) has proven itself as the only permanent repair of existing corroded steel reinforced concrete.
Many underwater tunnel structures have been experiencing water leakages worldwide. Tunnel structures experiencing water leakages are not only old, but also new in some case. The concrete tunnels structures located underwater are generally protected by waterproof membranes as the first defence to prevent water leakages and rebar corrosion. However, once water leakage occurs, the corrosion mechanism is quite different from other concrete structures which are exposed to marine or de-icing salt environments. When rebars corrode in concrete, the accumulating corrosion products develop expansive force and crack the concrete. When the concrete cracks grow, the concrete spalls and falls to the roadway. For the based slab, concrete spalls create potholes on the driveway. Therefore, it is important to clearly understand the corrosion condition of the rebars in the tunnel caused by salt water leakage.
Loss of Durability
Why does the durability of bridges, multi-level car parks, supporting walls, tunnels and sea water structures decrease?
The main problem is the de-icing salt on the streets. These salts contain chlorides which penetrate into the constructions and destroy the protective layer of the rebar - the consequence: corrosion.
These factors together with a too thin concrete cover and too low density as well as changing weather conditions and humidity lead to an increased risk of corrosion. Corrosion of the rebar reduces the steel cross section and as a consequence the support safety. Furthermore, it cause cracks due to the increased volume of the rust.
Factors of influence on the corrosion risk of the rebar.
Concrete Remediation Works
Certain methodology must be developed to remediate the corroding steel and mitigate further corrosion on mild steel components and reinforcement. This would allow the tunnel to achieve its required life with minimal ongoing maintenance. This involved the repair of damaged concrete, encapsulation of mild steel bolts and application of cathodic protection.
Design Options for Cathodic Protection System
There are various design options to be considered to provide cathodic protection for tunnel reinforcement. The three main options were:
a) Ribbon/discrete anodes in slots/ drilled holes in the concrete.
b) A distributed anode system along the full length of the tunnel.
c) Installation of remote anode groundbeds at the two ends of the tunnel