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Corrosion of reinforcing steel in concrete

Corrosion of reinforcing steel in concrete is the main cause of premature aging of reinforced concrete structures. This natural pathology is no exception and represents an omnipresent threat to the sustainability of essential structures such as buildings, bridges, tunnels, nuclear power plants, water towers, or churches.


The impact of corrosion on our concrete structures


According to a recent report by the American Corrosion Association (NACE), the direct costs of corrosion of reinforcing steel in concrete represent 3.5% of the GDP of industrialized countries [1]. To these direct costs, are added indirect costs that are difficult to quantify such as operating losses related to the immobilization of the structure, insurance in the event of a disaster on the structure, ...

 

Morandi Bridge's collapse due to the corrosion of the reinforcing steel in concrete
Figure 1 : Morandi Bridge after its collapse, partly due to the corrosion of the reinforcing steel in concrete

The collapse of the Morandi Bridge in Genoa in August 2018 left a profound impression on people’s minds, thus triggering a collective awareness of the worrying aging of concrete infrastructures [2]. An Italian study has highlighted the predominant role of corrosion of reinforcement in the collapse of the Genoese structure [3]. This tragedy reminded project managers that concrete infrastructures have a limited lifespan and that it is their responsibility to monitor and maintain them. 


Besides the economic weight for the global society, the corrosion of infrastructures is truly problematic in terms of safety of people, degradation of the quality of life (unavailability of facilities, intensification of road traffic...) and significantly increases the carbon footprint (consumption of resources and energy for repair or reconstruction). The proactive management of steel corrosion in concrete thus becomes imperative to ensure the sustainability of infrastructures and the safety of all. 


Passivation of steel: a fleeting shield against corrosion


Concrete initially offers steel a favorable environment for its preservation. At the time of pouring, the strong alkaline character of the concrete (pH 12 to 13) leads to the formation of a protective oxide layer on the surface of the metal, giving the reinforcement a good chemical resistance [4]. This passivation layer considerably slows down the rate of corrosion, bringing it to a negligible level regarding the usual service lives of civil works.  Steel is then described as passive or passivated in healthy concrete.

 

La passivation constitue l'une des clés du succès du béton armé dans l'histoire de la construction. Sa facilité de fabrication, de mise en œuvre, et son coût limité expliquent son utilisation intensive depuis près d’un siècle.


Passivation is one of the keys to the success of reinforced concrete in the history of construction. Its ease of manufacture, implementation, and limited cost explain its intensive use for nearly a century.


However, concrete is a porous material that allows aggressive environmental agents such as chlorides (cf. part 4) or atmospheric CO2 (cf. part 3) to seep. These agents inevitably lead to the local destruction of the protective film of the steel, thus initiating a state of active corrosion of the rebar.


Corrosion of reinforcing steel by carbonation of concrete


Carbonation of the concrete cover is one of the two causes of reinforcement steel corrosion. This risk of corrosion is taken into account by exposure class XC of the NF EN 206/CN standard. This perfectly natural phenomenon results from the natural reaction between atmospheric carbon dioxide (CO2) and the calcium phases of the cementitious matrix [5].


CO2, present in the air at 400 ppm, penetrates through the open porosity of concrete. This gas dissolves in the interstitial solution forming carbonic acid which gradually neutralizes its alkalinity. This decrease in pH causes the dissolution of portlandite Ca(OH)2, then the decalcification of C-S-H. This phenomenon leads to the formation of calcium carbonate in the matrix, which is intrinsically rather favorable for the durability of concrete because of the progressive clogging of its porosity.


However, the drop in the pH of the pore solution from 12-13 to values below 9 results in the dissolution of the oxides composing the protective passive layer. The steel in the carbonated concrete is then no longer passivated and a galvanic cell is created with the rest of the passive rebars in the healthy concrete. The impact of concrete carbonation on steel depassivation can be illustrated by referring to the Pourbaix diagram, which defines the areas of iron stability in water (Figure 2).



Pourbaix Diagram and corrosion of steel by concrete carbonation
Figure 2 : Figure showing how concrete carbonation impacts steel depassivation, reffering to Pourbaix diagram of iron evolution in water.

There are then two parts within the structure: the healthy part in-depth, and the carbonated part in contact with the ambient air. The easiest technique to use to measure the carbonation depth of concrete is to project a colored indicator on a fresh concrete fracture. Phenolphthalein is the most widely used indicator by the profession because it has a turning pH around 9. We distinguish the carbonated zone (pH < 9) which remains colorless from the non, or low, carbonated zone (pH> 9) colored purple. The depth of carbonation is then compared to the minimal concrete cover of the steel to highlight the involvement of the pathology in the corrosion process.


Corrosion of reinforcing steel by chlorides


Corrosion of steels in concrete can also be initiated by chlorides because of their high oxidizing power [6]. Chlorides in reinforced concrete may come from an exogenous source resulting from the marine environment and de-icing salts. Chlorides can also be found during the construction due to the use of activator adjuvants, the use of unwashed marine aggregates, or the storage of rebars exposed to sea spray. However, pollution by exogenous chlorides, present in the environment, is the most frequent and is associated with exposure classes XS and XD of the NF EN 206/CN standard (Figure 3).


Examples of exogenous chlorides, causes of the corrosion of reinforcing steel in concrete
Figure 3 : Spreading of de-icing salt on a highway bridge (exposure class XD standard NF EN 206/CN) – Concrete dock partially immersed in seawater (exposure class XS standard NF EN 206/CN)

Chlorides in the cementitious matrix of concrete come in two main forms:

  • Free chlorides, present in ionic form and dissolved in the interstitial solution; these ions are likely to initiate corrosion locally when their concentration exceeds a critical threshold (Ccrit) [7].

  • Bound chlorides, forming chemical precipitates such as Friedel salts, or physically binding to the hydrates of the cementitious matrix. The physical fixation being reversible, the chlorides initially bound can return to solution and contribute to the initiation of corrosion.


The depth of penetration of chlorides (free and total) into the cover concrete is crucial for the diagnosis of corrosion. The determination of chloride content is based on laboratory chemical analysis of concrete cores or powders taken directly onsite.


Comparing this depth of penetration with the minimal embedding of the reinforcement helps identify whether these ions are responsible for corrosion. In case the chlorides have not yet reached the rebars, the profile knowledge in the concrete cover allows the anticipation of the future risk of corrosion and the implementation of potential preventive measures, whether physical (e.g., protective coating) or electrochemical (e.g., dechlorination). On the other hand, if the chlorides have penetrated deeply, initiating the corrosion of the steel, the implementation of a cathodic protection becomes essential to ensure the durability of the structure. The chloride profile also indicates the nature of the latter: a flat profile indicates endogenous pollution, while a decreasing profile suggests exogenous pollution.


The galvanic corrosion cell


Despite the differences between the two pathologies in the mechanisms of degradation of the passive layer, the consequences remain the same: the initiation of an active corrosion zone. The dissolution of the passive layer leads to the decrease of the electrochemical potential of the active steel. Because of this difference in the electrochemical behavior, a galvanic cell is formed between the active zone and the rest of the passive reinforcement [8].


In this galvanic coupling, the dissolution of the metal corresponds to the anodic reaction which provides electrons, consumed by the cathodic reaction. In the basic interstitial solution of concrete, this cathodic reaction corresponds to the reduction of dissolved dioxygen. This results in a corrosion current exchange between the active corrosion zone and the rest of the passive reinforcement as shown in Figure 4.


Corrosion galvanic cell of reinforcing steel in concrete
Figure 4 : Illustration of the principle of the corrosion galvanic cell mechanism between active steel and still healthy reinforcement

Structural damages on structures due to corrosion of steel in concrete
Figure 5 : Examples of structural damages induced by corrosion (Sicile, Italie)

This electrochemical process initially leads to the formation of expansive iron oxides at the interface between steel and concrete. The growth of these corrosion products quickly causes the bursting of the embedding concrete, causing at this stage only aesthetic problems for the structure (cracking, rust drips). In the absence of proper treatment, corrosion gradually leads to mechanical damages to the structure, resulting from the reduction of reinforcement steel sections, deterioration of steel-concrete bond, and loss of ductility of the steel [9].


Conclusion


Corrosion is the main cause of premature aging of concrete structures. This natural pathology can be initiated by the penetration of aggressive agents through the porosity of concrete, such as atmospheric CO2 or chlorides present in sea spray in coastal areas as well as in de-icing salts used in cold regions of the globe.


The initiation of an active corrosion zone leads to the formation of a galvanic corrosion cell with the rest of the reinforcement, still healthy. Over time, this pathology inevitably leads to a mechanical weakening of the concrete structure, which thus loses its ability to support the loads (own weight, road traffic, etc.) for which it was initially sized, inevitably resulting in the brutal ruin of the structure.


Fortunately, there are solutions such as cathodic protection to halt the spread of corrosion and the resulting structural damage. A diagnosis of corrosion on site and in laboratory is essential to identify the origin and extent of the pathology in order to recommend the most appropriate treatment.


Bluespine vous offre un accompagnement personnalisé et certifié, allant du diagnostic de votre ouvrage à la conception de la solution de traitement, grâce à notre méthodologie innovante basée sur 60 ans d'expérience cumulée dans ce domaine.


Bluespine offers a customized and certified support, from the diagnosis of your structure to the design of the treatment solution, thanks to our innovative methodology based on 60 years of cumulative experience in this field.



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