Reinforced concrete structures are generally considered to be very durable. Environmental factors and exposure to substances that cause (electro-)chemical reactions in concrete however, may significantly reduce their designed life span and functionality.

Lochkorrosion an Stahlbetonbewehrung.

Damage to reinforcing steel is mostly caused by carbonation or infiltration of chloride into the concrete.
Carbonation is a chemical reaction of the cement matrix taking place in the presence of atmospheric carbon-dioxide. The cement matrix is transformed into calcium-carbonate. This reduces the alkalinity in the concrete which is protecting the steel against corrosion. At low pH-values the steel is left without corrosion protection.
Chloride from sea water, de-icing salt or salty dust for example can penetrate into the concrete and, after some time, reach the reinforcement. There it may trigger corrosion even in a highly alkaline environment.

Usually the corrosion process causes an increase in volume of the steel. Subsequently the concrete surface cracks, spalls and defects become visible. In the presence of chloride however, the corrosion may proceed such that the steel is dissolved without any increase in volume. In such cases the steel may suffer significant damage or may get completely dissolve without visible evidence like cracks or flaking.

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The duromac CP-System

duromac CP250 The duromac CP corrosion protection system usually is used on existing concrete structures, after defects from corroding reinforcement have been identified. duromac CP has been specifically designed for retrofit installation. Since it is a galvanic protection system, it requires little maintenance and is safe against faulty operation. Because it works without power supply, it can be operated in remote areas.
The installation should always be preceded by a diligent corrosion survey by qualified corrosion specialists. A correct root cause analysis and corrsion protection design ensure proper performance of the system and can significantly reduce the effort for such a system. Previous projects showed, that it is not unlikely, that with a proper survey the area that actually needs protection, can be reduced to 10 to 15% of the area that was under suspicion of being at risk for reinforcement corrosion. The cost for the system is reduced at a similar ratio.

Installation duromac CP250 We recommend to do the cabling such, that the connection between anodes and reinforcing steel to be protected can be interrupted at any time. This provides the opportunity to perform numerous different measurements to check proper performance of the system. Besides direct measurement of voltage produced by the anodes and protection current, it is also possible to carry out depolarisation tests and with anodes being disconnected it is possible to identify corroding areas using potential measurement.
By looping back the end of the anode ring wire to the connection box it is possible to check the integrity of the cabling. This is important, because the cables that are usually close to the surface of the concrete, are easily damaged, e. g. by drilling, grinding, driving nails and the like.

Data logger In addition, the CP-installation may be equipped with a monitoring system. At selected critical locations corrosion potentials may be recorded via reference electrodes embedded in the concrete. This provides the opportunity to take corrective action if the installation for some reason does not perform as planned.
Battery-operated customized data loggers, configured for the purpose by us, can record corrosion potentials and other values during long periods of time.

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Reinforcement Corrosion

Corrosion of steel in concrete is usually caused by either carbonation of the concrete or by contamination of the concrete with chlorides.
Under normal circumstances the concrete protects the steel by its high alkalinity of approx. pH 12.5. At such pH-values the steel is covered with a very thin, but dense layer of corroded steel, the so called passive layer. Even in the presence of water and oxygen the passive layer reduces further corrosion to a negligible rate.

Carbonation however, causes the pH-value to drop to a range (<10.5) where large areas of the passive layer get dissolved and steel starts to corrode. Carbonation is a chemical reaction of the pore solution in the concrete with crabon dioxyde from the air. In the course of carbonation the cement stone is transferred into lime stone.

In the presence of chlorides steel may corrode also without carbonation. In such case corrosion is usually confined to small areas. Such areas then corrode quickly. The process is called pitting corrosion. Chlorides may penetrate the concrete as salt. Sources for the salt may be, for example, de-icing salt, sea water, salty ground or cable fires.

It is commonly assumed, that two thirds of all damages on concrete bridges in Germany are caused by chloride, approximately five percent are caused by carbonation.

Stahlbetonbewehrung mit Flugrost (1).

Reinforcing steel (10mm) with light rust caused by exposure to the atmospere during prolonged storage.

Stahlbetonbewehrung mit Rost (2).

Reinforcing steel (16mm) retrieved from a concrete structure. The increased volume of the corrosion products had caused the concrete cover to spall off.

Stahlbetonbewehrung mit erheblicher Abrostung (3).

Remains of a 12mm reinforcing bar.

Chloridinduzierte Korrosion (4).

Reinforcing bar (16mm) with marks of slight corrosion attack. The rebar was removed from a structural member contaminated with chlorides. One can see a localized corrosion spot as is typical for chloride exposure. Outside of this spot the steel is completely intact.

Chloridinduzierte Korrosion (5).

Reinforcing bar (16mm) with pitting corrosion. The corrosion was caused by chloride (salt), which had migrated through the concrete surface up to the reinforcement. In this case the corrosion had not caused any damage (spalling) to the concrete surface. The defect was discovered by accident.

The fact that such damage often remains unnoticed because the concrete surface remains intact, makes the chloride-induced corrosion particularly dangerous.

Chloridinduzierte Korrosion (6).

Same pitting defect as above. The remaining cross-section area is less than 20%.
This example also shows clearly that random checks done by opening the concrete may easily miss such defects.
Therefore, in case there is an initial suspicion, a comprehensive full surface investigation should be carried out using suitable testing methods (e. g. potential measurements as described in ASTM C876 with associated tests).

Chloridinduzierte Korrosion (7).

Reinforcement (12mm) with pitting corrosion. It is obvious that pitting corrosion can cut through reinforcing bars in very small spaces.

Chloridinduzierte Korrosion (8).

End hook (16mm) with severe corrosion defect.

Chloridinduzierte Korrosion (9).

Remains of a completely destroyed reinforcing bar of 32mm nominal diameter.

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Cathodic Protection (CP)

Pitting corrosion. Simplified Representation of the Corrosion Reaction:

During the smelting process huge amounts of energy are spent to separate oxygen from iron ore. Hence the steel (iron) is in an unnatural energetic state and in the presence of oxygen tends to release this energy by reacting with oxygen from the atmosphere or oxygen that is dissolved in water. In doing so, iron atoms exit the iron grid losing electrons (which is the anodic part of the reaction = dissolution of iron) while the electrons released into the steel react with water and oxygen to form OH- in other areas of the steel surface (cathodic part of the reaction = oxygen reduction). A potential difference builds up between anodic areas and cathodic areas of the steel surface.

This natural chemical reaction (corrosion) is strongly restrained by a thin, but dense layer of corroded steel - the passive layer - which is formed in the highly alkaline environment usually found in concrete. As long as this passive layer remains faultless, the steel does not corrode. Steel will also not corrode - even if the passive layer is damaged - under insufficient supply of oxygen or moisture.

Usually there is sufficient oxygen and moisture in the concrete's pore solution. Oxygen and water can both penetrate through the surface of the concrete.

The electrons released from the anodic areas get in contact with water (H2O) and Oxygen (O2) at the cathodic areas to form hydroxyl ions (OH-). These hydroxyl ions migrate through the pore solution towards the anodic areas and connect with ion atoms from the ion grid to form Fe(OH)2 - a precursor of rust.

Sacrificial anode. Corrosion Protection Through Sacrificial Anodes:

Less noble metals are more prone to loose some of their electrons than more noble metals. The so called standard electrode potential describes how easily metals release their electrons. The standard electrode potential of zinc (Zn) is significantly more negative than that of iron (Fe; resp. steel). (The galvanic series is a ranking of such standard electrode potentials for each metal.)

When zinc is embedded in the concrete and electrically connected to the reinforcing steel, the zinc works as the anodic area, because zinc has a more negative rest potential than the previously anodic areas of the steel. The potential of the steel is shifted in negative direction, such that even at high chloride levels no significant corrosion takes place. If designed correctly sacrificial anodes make a durable cathodic protection for the structure at a very good value for money.

Duromac CP

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