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reinforcedconcrete

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Cathodic Protection of Reinforced Concrete Diaphragm Walls

The Middle East with its high temperatures and extremely high saline content of its sea-water & ground water forms an extremely aggressive environment in terms of corrosion. When constructing structures that are exposed to this environment the challenge is how to ensure corrosion durability.

For diaphragm walls that are cast in situ, this is a special challenge for two reasons.

The first reason is that during construction, some methods employed involve the boring of the excavation, lowering the steel cage into the bored excavation and then casting. The excavation is typically inundated with bore -hole chemicals to ensure that the excavation does not collapse. During this period, it can be expected that chlorides from the surrounding groundwater permeate the excavation walls. Therefore it follows that the reinforcing steel cage will, once it is lowered, be immediately exposed to potentially high levels of chloride.  

The second reason that diaphragm walls are a challenge in terms of ensuring durability, is the exposure after construction. On one side the reinforcing steel is exposed to highly saline sea-water, while on the soil side, the wall is exposed to highly saline ground-water. The wall is seemingly sandwiched between to very corrosion environments.

By deploying a well-designed cathodic protection system using impressed current, the corrosion of the reinforcing steel can be stopped or significantly reduced to a negligible amount. Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded "sacrificial metal" to act as the anode. These systems (also known as sacrificial cathodic protection)  can be designed with a service of approaching 20 years.

For longer service life systems, the ‘sacrificial metal’ method above is not sufficient. Metals made from coated titanium cast into the concrete matrix. These provide a means to create electrochemical cell, by introducing direct current onto the steel to be protected, in this case the reinforcing steel. The source of the direct current is typically located close to the structure in an electrical room. These systems can be designed in excess of 50 years.

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Once deployed, these systems can be monitored with extreme ease remotely. Many asset owners fret about the upkeep of these systems. In fact, the upkeep is insignificant. With IoT devices, Ducorr has already developed systems that can be monitored from your smart phone or tablet, sending you updates at intervals you chose.

These systems are extremely powerful durability tools providing a hands-on and active approach for managing corrosion – almost unheard off a few decades ago.

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Problems of Durability and Reinforcement Measures for Concrete Tunnel Structures

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.

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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.

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

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