Industrialized countries face the continuous aging of their concrete infrastructures exposed to the corrosion of their rebars. This inevitable pathology affects nearly 8 out of 10 works [1].
Corrosion control has become imperative to ensure the durability of reinforced and prestressed concrete structures. Many works in France will soon reach the end of their life cycle, requiring significant maintenance and rehabilitation means.
Nowadays, the predominant trend is towards rehabilitation rather than demolition and reconstruction. This shift in approach is explained by the direct costs of reconstruction (raw materials, labour, etc.), the indirect costs related to the implementation of interim solutions during the work, but also by a desire to preserve the planet’s resources and minimize the carbon footprint.
The widespread ageing of concrete structures
In developed countries, the age profile of reinforced concrete structures reflects a peak of construction in the 60s. With a theoretical service life of about 50 years for a building and 70 years for a bridge, this longevity can be significantly reduced in case of corrosion. Some works may show the first signs of corrosion after only a few years.
Following the Morandi Bridge disaster, an information mission undertaken by the French Senate highlighted the worrying state of the bridges of the French national road network (NRN) .
In the continuity of this parliamentary mission, the National Bridge Program, led by CEREMA, has made it possible to refine the photography of the state of French heritage. A bridge identified out of 3 has significant structural flaws that will require repair actions in the upcoming years.
This observation becomes even more alarming for large prestressed structures, with major flaws detected in more than 7 out of 10 cases. The Saint Cloud viaduct is a perfect example. The Parisian structure had the same marks of corrosion as its Genoese counterpart. The Ile-de-France road management opted for cathodic protection in order to prevent corrosion and extend its service life by 50 years.
Le cas particulier des ponts reflète le mauvais état global des infrastructures en France, qu'il s'agisse de bâtiments, de centrales nucléaires, de tunnels, etc. Aucune structure n'est épargnée. Ce constat peut être généralisé à l'ensemble des pays industrialisés, faisant de la durabilité des structures en béton l'un des défis majeurs du XXIe siècle.
The specific case of bridges reflects the poor overall state of infrastructures in France, whether it is buildings, nuclear power plants, tunnels, etc. No structure is spared. This can be generalized to all industrialized countries, making the durability of concrete structures one of the major challenges of the 21st century.
Conceptual diagram of the service life of a concrete structure exposed to corrosion
La corrosion des aciers dans le béton est une pathologie évolutive. Selon les travaux de Tutti, la durée de vie d’un ouvrage en béton armé ou précontraint affecté par la corrosion peut être subdivisée en plusieurs phases, de sa construction à sa ruine.
Corrosion of steels in concrete is an evolutionary pathology. Depending on Tutti’s work, the service life of a reinforced or prestressed concrete structure affected by corrosion can be subdivided into several phases, from construction to ruin.
Figure 2 illustrates the temporal evolution of the degree of corrosion of a concrete structure during four main phases: incubation, initiation, propagation and degradation.
Incubation
The incubation stage of corrosion represents the period during which aggressive agents (carbon dioxide and chlorides) penetrate through the porosity of the concrete and progress towards reinforcement. During this phase, the steel remains protected by the oxide layer constituting the passivation layer.
The duration of the incubation phase depends on various parameters including:
The concrete cover of steels, which acts as a protective physical barrier. The EN 206 standard specifies the minimum thickness to be respected according to the exposure conditions of the structure. Improper positioning of the reinforcement cage during the execution phase is the main cause of early initiation of corrosion.
The physicochemical characteristics of concrete. High porosity will make the transport of aggressive agents easier.
The conditions of the environment. For instance, a concrete dam in contact with brackish water will have a longer incubation time than the same structure in contact with seawater, which is more concentrated in chlorides.
If the aggressive agents have not yet reached the reinforcement, knowledge of the thickness of the contaminated concrete makes it possible to assess the risk of future corrosion. As a result, physical prevention (protective coating) or electrochemical treatment of concrete (de-chlorination and re-alkalization) can be applied to delay the initiation process and ensure the durability of the structure.
Initiation
When the aggressive agents reach the less covered steels in sufficient quantity, the passivation layer is locally destroyed. Steel, initially passive, becomes active, generating a galvanic corrosion pile in the concrete structure.
The initiation criterion depends on the corrosion pathology:
For carbonation, the depassivation of steel occurs when the pH of the pore solution drops below 9.
For chlorides, corrosion initiation occurs when the chloride concentration in the steel exceeds a critical threshold. This threshold varies from one structure to another and depends in particular on the concrete formulation and the quality of the steel-concrete interface.
Propagation
After its initiation, corrosion spreads gradually. The active steel dissolves, generating current for the rest of the healthy reinforcement where the initiation criterion has not yet been reached.
Corrosion kinetics depend on several factors, including the resistivity of concrete. A lower resistivity accelerates the dissolution of steel. Similarly, oxygen availability plays a crucial role, being an essential component of the cathodic corrosion reaction. A small amount of oxygen causes latent corrosion, for example, in underwater structures.
During this phase, the active steel zone expands, and the front of aggressive agents (chlorides and carbonation) keeps spreading, initiating other zones.
To stop the propagation of corrosion and avoid structural damages, cathodic protection stands as the most effective treatment. Its early implementation avoids very expensive repair work.
Degradation
The spread of corrosion leads inexorably to the structural degradation of the concrete structure.
Initially, the expansive iron oxides formed during the corrosion process exert pressure on the cementitious matrix, causing cracking and loss of grip of the steel. These aesthetic alterations due to the degradation of the cover concrete most often appear well before the structural failures. Although early, this phenomenon alerts the structure manager on the imminent risk of collapse.
Corrosion of the steel in the active zones also leads to the reduction of the reinforcement sections, thus reducing the load bearing capacity of the structure which can no longer withstand the expected loads.
Furthermore, the corrosion of steel significantly changes its mechanical behavior. Unlike healthy steel, which has a certain ductility, corrosion reduces the plastic bearing of steel, making it less compliant and more fragile. This decrease in ductility exposes a corroded structure in a seismic zone to a risk of sudden collapse, endangering the safety of people, with risks of physical injury or even death.
Lorsqu'un stade avancé de corrosion de l’acier est atteint, une approche curative est essentielle pour stopper sa propagation et remédier aux désordres structurels induits. Cependant, la restauration du béton dégradé est une étape préalable incontournable à la mise en place d'un traitement électrochimique. Il convient même de prévoir un renforcement pour les éléments de structure particulièrement dégradés.
When an advanced stage of steel corrosion is reached, a curative approach is essential to stop its spread and remedy the induced structural disorders. However, the restoration of degraded concrete is an essential requirement for the implementation of an electrochemical treatment. Strengthening should even be provided for particularly degraded structural elements.
Ensuring the durability of reinforced or prestressed concrete structures
Controlling the corrosion of steel in concrete is essential to ensure the durability of our built heritage. In response of this progressive pathology, a diagnosis of corrosion, based on measurements on site and in the laboratory, becomes imperative to identify its stage of advancement and recommend the most effective treatment. The diagnosis also provides the necessary data for optimal design of the treatment.
For new constructions, the prediction of the incubation phase duration allows preventive maintenance at a lower cost. In addition, many projects include, from construction, a cathodic protection system to extend the lifespan of the structure.
Whatever the type of project, construction or rehabilitation, solutions exist to fight corrosion of steels in concrete. Bluespine offers customized and certified support to assess the residual lifespan of the structure, recommend and then design treatment solutions suitable regarding the stage of corrosion advancement.
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