The paper describes an integrated monitoring system installed in a reinforced concrete transit tunnel In Milan (Italy) where corrosion due to DC stray current is expected. The tunnel, about 9 km long, is divided into insulated sections, approximately 300 m long, obtained by reinforcement interruption and insulating membranes. Each section is monitored by means of reference electrodes to measure the potential and current probes to measure the current flowing within the concrete. All readings are collected in a centralized acquisition data system for interpretation and storing. The aim of the monitoring system is to control corrosion and to follow any modification of the reinforcement condition in order to anticipate corrosion attacks. The results obtained during commissioning and first period of working are presented and discussed.
Stray currents spread out by the DC transit systems in the soil can cause severe corrosion attacks on buried metallic structures [1-12], Concrete structures, especially reinforced tunnels, experienced some corrosion attacks in presence of stray currents [13,14], which are usually unexpected, because concrete structures are generally considered not affected by the stray currents.
The main reason of this believe is based on the fact that steel reinforcements in concrete are normally in passive conditions, hence corrosion is unlikely to occur, However, when particular conditions are established, such as a high chloride concentration or very strong electrical field, corrosion takes place, In Figure 1is shown the classical mechanism of stray current interference,
To monitor any possible corrosion occurrence due to stray currents, an experimental monitoring system has been adopted for a concrete transit tunnel structures [15],
Corrosion conditions
Stray currents can produce corrosion attacks on steel reinforcements in active conditions (the passivity being disrupted by chlorides or carbonation) and also in passive conditions when the interference field is strong and persistent, Furthermore, on cathodic zones (where current enters the interfered structure) hydrogen embrittlement on high strength steel can occur.
Conditions for corrosion occurrence on steel embedded in the concrete in presence of stray currents are rather different than steel exposed to neutral electrolytes, such as soil and waters, since the alkalinity of the concrete leads to the passivation of the steel. An interfering current affecting the reinforcements produces different anodic reactions depending upon the state of the steel. In active conditions, when the passivity has been disrupted by chlorides or carbonation, the reaction is the iron dissolution:
Fe= Fez++ 2e-;
for passive steel, that is in sound concrete, the reaction is the oxygen evolution:
2Hz0 = Oz+ 4H++ 4e-
Then, on passive steel, the reaction is not the iron dissolution; however, the oxygen evolution leads to the production of acidity (H+)which can lead to the disruption of the steel passivity. In presence of a non-stationary stray current, the produced acidity is usually neutralized by the concrete bulk alkalinity during the period of their absence. In the case of strong and persistent interfering conditions, the concrete is not able to neutralize the acidity, then the passivity breaks down and corrosion takes place.
