Since the first CP (Cathodic Protection) projects were carried out in the Netherlands nearly 30 years ago, this specialised concrete maintenance method has been developed further into a fully fledged and proven effective method for stopping rebar corrosion. This technology has become the basis for many other applications that halt all manner of rebar corrosion in every possible market segment
Causes of rebar corrosion and the application of CP.
In most cases rebar corrosion is often the actual cause of concrete damage. The other causes are usually derived from chemical influences, mixed-in impurities in the mixture or external mechanical causes.
As rust products have a much greater volume than the original steel rebar, internal stresses are created and the coverage is pushed off and concrete damage occurs. This rebar corrosion can arise as the passivation of the steel, the protective layer that is formed around the steel in the concrete, is broken. In principle this can happen in 2 ways – by carbonation (acidification as a result of carbon dioxide in the air) and by chlorides. The latter could be in the form of calcium chloride mixed into the mortar as a curing accelerator or as penetrated chloride resulting from road salts or from salt spray on objects situated on or near the coast. The application of chloride as a curing accelerator is generally found in objects and prefab concrete built before 1974. The concrete requirements of 1974 limited the application of chloride to a maximum of 0.4% of the weight of the cement. However, it is important to determine that this applies to new non-carbonated concrete, due to the reduction of the PH value (acidity) of the concrete. The lower the acidity, the quicker the concrete becomes carbonated by the penetrating carbon dioxide.
Penetrated chlorides occur in structures (particularly in joints, beams and abutments that are prone to leaks), floors of underground car parks and access walkways in residential buildings, because of the use of road salt. They also occur on facades and concrete objects in a broad band on the coast where the salt, dissolved in water or damp, slowly penetrates into the concrete.
By losing the passivation of the steel rebar and with sufficient presence of oxygen and damp, the steel rebar will start to corrode. It is also useful to know that corrosion is actually an electrochemical process. There are small, but certainly measurable currents, because a potential difference is created between the cathode side and the anode side of the reaction.
Why do we make a distinction between carbonation-induced and chloride-induced rebar corrosion? The most important difference is expressed in the way in which the steel rusts. In carbonation-induced rebar corrosion there is a more even pattern and the formation of rust products is accompanied by a greater volume increase. The coverage is pushed off and the rebar is exposed. With chlorides-induced corrosion you have a dangerous form of pitting corrosion. This is dangerous for several reasons. First of all, this mechanism does not give off any warning signals straightaway by pushing off the coverage, as the characteristic rust bloom only becomes visible after significant corrosion of the rebar and after the produced rust has seeped through the concrete layer to the outside. At that time there is already a significant pit in the rebar and that is where the constructive risk lies. As the rebar is intended to absorb the tensile forces in the concrete, a strong reduction in the rebar diameter immediately reduces the load-bearing capacity of the entire construction. Another additional problem in the presence of chlorides is that traditional concrete repairs accelerate corrosion alongside the repair and as a result the repair fails more quickly. This is known as the so-called patching effect. Another major difference between chloride-induced and carbonation-induced rebar corrosion is that the form caused by chlorides has much higher potential differences. In carbonation-induced corrosion we can measure potential differences of – 150 mV, in corrosion caused by chlorides this can be up to -350 mV and more.
In other words, rebar corrosion caused by chlorides is a dangerous mechanism that cannot be repaired or kept under control in a traditional manner. For those situations CP provides a 100% guaranteed solutions.
Cathodic protection exists in many forms. The principle is based on inverting the current by introducing another anode that takes on the role of the steel rebar in solution. This puts the steel on the protective side of the reaction, ensuring it can no longer rust.
The European standard NEN-EN 12696 and the Dutch CUR Recommendation 45 impose rules on applying CP systems. Those rules cover the design, the measurements and the testing during construction and prior to commissioning, and compulsory monitoring of the systems for 10 years following their installation.
There are 2 basis principles for CP:
- The galvanising cathodic protection where a metal that is less precious than the steel, usually active zinc, is used as anode material. Its operation is based on the galvanising effect of the metals. The base metal sacrifices itself for the more precious metal. In fact we do the same when we galvanise steel. As this system actually becomes its own battery, it does not need an external current source and enough protective current is supplied to protect the steel. After the installation, additional work will not be required. This form of CP is applied as a surface anode, for example as zinc foil on a conductive gel or as a discrete anode to be built in or repaired. The operation can be extremely local and the costs of incidental problems are much lower than for the 2nd version of the CP.
Cathodic protection on the basis of impressed current. In this case an anode material is applied that is supplied by an extremely low protective current from an external supply. There are several types of anode materials available that all have their own advantages and disadvantages. For example, there are conductive coatings, and the extremely durable titanium strips, netting and gauze anodes and various other types of drill-in anodes. Where galvanised systems fall short, for example in larger surfaces and higher rebar concentrations (in heavy-duty constructions), these systems that are designed specifically for the situation provide the solution for the corrosion problems of the concrete construction. As said, the construction of these types of systems requires more examination and an accurate and detailed design and implementation process. Work for specialists to be sure!
Cathodic Prevention – other applications of cathodic protection
Although the application of CP is usually linked to the presence of high concentrations of chlorides in concrete, CP has other interesting applications. They include:
- In strongly carbonated or porous concrete in an aggressive environment, with poor or costly access for example, applying a CP system on a preventive basis can provide a more economical alternative to maintenance.
- Incorporating it as a preventive measure in the new-build phase, you have a “dormant” system that is monitored. However, when higher current densities are measured the system is actually switched on when pitting corrosion becomes a real possibility. These types of solutions are also produced by a growing realisation that prevention is better than the cure, whilst they are also a strategic and economically justifiable option as part of a TCO approach.
The engineers and corrosion specialists of SealteQ study the problems and advise on the possibilities. They design and specify the solutions and implement the entire process in accordance with the state of the art, including maintenance, monitoring and reporting and it goes without saying that this is all in accordance with international regulations. By applying CP you are 100% sure that there will be no corrosion in the protected parts.
Our service, focused strongly on innovation and quality, offers our clients the certainty of a customised solution. With the application of Total Cost Management and the Total Cost of Ownership approach we offer added value to your strategic maintenance management.