Materials and Corrosion 2009, 60, No. 2 DOI: 10.1002/maco.200805019 93

Characterization of long-term corrosion ofrebars embedded in concretes sampled onFrench historical buildings aged from 50 to80 years

V. L’Hostis*, D. Neff, L. Bellot-Gurletand P. Dillmann

The aim of this paper is to understand what the chemicalinfluence of the concrete on the corrosion form present is, on rebarsembedded for tens or hundreds years. Therefore, metal/concreteinterface of samples coming from two French Historical Monumentshas been characterized: ‘Bourse du Travail’, Bordeaux, 80 years oldand ‘Maison du Bresil de la Cite Internationale de Paris’, 50 yearsold. The collected samples have been observed on transversesection in order to observe the whole corrosion system (metal/

� V. L’HostisCEA Saclay, DEN/DPC/SCCME/LECBA, Bat. 158, PC 25,91191 Gif sur Yvette Cedex (France)E-mail: valerie.lhostis@cea.fr

D. NeffCEA Saclay, DSM/IRAMIS/LPS, Bat 637, 91191 Gif sur YvetteCedex (France)

L. Bellot-GurletLADIR, UMR 7075, CNRS and Universite Pierre et MarieCurie-Paris 6 (France)

P. DillmannLRC CEA DSM 01-27, CNRS IRAMAT UMR 5060, IPSE etLaboratoire Pierre Sue, CEA Saclay, 91191 Gif-sur-Yvette Cedex(France)

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corrosion products/concrete). Morphology, elementary compo-sition and structure have been studied with complementaryanalytical approaches (optical microscope, scanning electronmicroscope coupled to energy dispersive spectroscopy andRaman microspectroscopy). Thus, a fine description of thecorrosion scales formed on the metallic rebars can be schema-tized. On the basis of these results, mechanisms hypotheses areproposed in the discussion.

1 Introduction

Corrosion of rebars in reinforced concrete structures is theprimary cause of structural deterioration of bridge decks, tallbuildings, tunnels and reinforced containers. Generally, thiscomposite material is able to withstand a wide range ofenvironments for a certain period of time. In order to evaluatethe service life, the corrosion mechanisms of steel inconcrete must be assessed and modelled. As concrete isporous and both moisture and oxygen can move through thepores and microcracks, the basic requirements for the onsetof active corrosion of mild or high strength ferriticreinforcing steels are present. Active corrosion does notoccur quickly in most cases because the pore fluid containshigh levels of calcium, sodium and potassium hydroxide,which maintain a pH between 12 and 13. At this range ofalkalinity, the steel remains passive [1], forming a passivecorrosion layer that is self maintaining and prevents activecorrosion. In the case of structures submitted to atmospheric

conditions, carbon dioxide is the component that penetratesthe concrete and is responsible for active corrosion.Usually, the associated deterioration process has the

following three stages [2]:

� I

bH

nitiation: during this long time the corrosion rate is verylow despite of the ingress of the carbonation front from theenvironment to the steel.

� D

epassivation: this step happens when the conditionsrequired for the onset of corrosion are fulfilled, thanks tothe carbonation conditions of concrete at the steel/concrete interface.

� P

ropagation: the reinforcement corrosion causes signifi-cant growth of expansive products. Then, internalmicrocracking and spalling of the concrete cover appear.They are due to the high tensile stresses generated by theexpansive volume of the corrosion products [3,4].

Corrosion mechanisms of steel rebars embedded inconcrete have been mostly studied in the case of laboratorystudies in controlled media [5,6]. In a more original way,Chitty et al. [7,8] collected iron bars embedded in mortar,plaster or in concrete after up to several hundred years. It hasbeen shown that in these cases, a corrosion layer of severalhundred micrometres thickness is formed surrounding theremaining metallic core. The corrosion scale can be dividedinto two distinct zones (Fig. 1). The first one is named denseproduct layer (DPL) and is mainly formed of ironoxyhydroxides (goethite, a-FeOOH) in contact with themetal and maghemite (g-Fe2O3)/magnetite (Fe3O4) stripsinside the goethite. The second one is named transformedmedium (TM) because it is composed of iron oxyhydroxidesand of characteristic minerals of the concrete. As this layercan extend to several millimetres around the metallic core, itcontains high quantities of iron coming from the metal. It has

& Co. KGaA, Weinheim

94 L’Hostis, Neff, Bellot-Gurlet and Dillmann Materials and Corrosion 2009, 60, No. 2

Fig. 1. Microphotograph and schematic representation of a transverse section of an iron rebar corroded in a hydraulic binder after Chittyet al. [7]

been demonstrated that, for this kind of corrosion system,corrosion rates are about few micrometres per year.From this general pattern, corrosion mechanisms of

archaeological analogues have been proposed. They are thebasis of a model [9] based on the behaviour (transport andreduction) of oxygen in the binder/DPL system. In order tovalidate this approach on contemporary concretes, sampleshave been taken on French historical buildings. This paperpresents the corrosion pattern that has been identified anddiscusses on the validity of previously proposed mechanisms.

2 Methodology

2.1 Corpus and sampling

Concrete samples containing rebar pieces have been coredon two monuments in France: the ‘Maison du Bresil’ of the‘Cite Universitaire’ of Paris (50 years old, referenced MdBin the following) and the ‘Bourse du travail’ in Bordeaux (80years old, referenced BdT).Phenolphthalein tests are realized on the samples in order

to determine the pH domain of the zone surrounding therebar. If colouration turns to purple, pH is above 9, if this isnot the case, pH is below this value. This on site macroscopictest allows to estimate if rebar should be in passive (i.e. pH ishigher than 9) or active state (i.e. pH is lower than 9).

2.2 Physico-chemical characterization

After the sampling step on site, reinforced concrete coresamples are mounted in epoxy resin and cut in order to workon cross-sections. They are grinded with SiC papers and thenpolished with a 3 mm diamond paste under ethanol.Complementary analytical techniques have been involved

to determine the characteristic of the corrosion layers at themetal/concrete interface. First, optical and scanning electronmicroscopes (SEM) are used. Then, elementary composi-tions are collected with energy dispersive spectroscopy(EDS) coupled to SEM (Jeol Stereoscan 120, acceleration

voltage of 15 kV). Detection is assumed by a Si(Li) detectorequipped with a beryllium window allowing to quantifymajor elements (including oxygen) with a relative error of2% and others (i.e. between 1 and 0.5 mass%) with 10%relative error. Lastly, Raman microspectroscopy has beencarried out locally in the corrosion scales in order todetermine and locate precisely, thanks to comparison withreference powders [10,11], the constitutive phases of thecorrosion system. Two set-ups have been used: a LabramInfinity with a Laser at 532 nm and a Labram HR with anexcitation at 514 nm (both Jobin Yvon Horiba). On bothapparatus the laser power is filtered under 100 mW in ordernot to transform under laser heat the corrosion phase duringmeasurements. The diameter of analysed area under the100� objective is of about 3 mm.

3 Results

3.1 Characterization of the concrete microstructureat the metal/concrete interface

Before the characterization of the steel/concrete interface,the concrete microstructure close to the rebar has beenassessed. Results are synthesized in Table 1. Some samplespresented air bubbles and/or macropores in contact with thesteel. Others presented a homogeneous microstructure,without any default.Concerning the phenolphthalein test, results showed that

some samples were carbonated at the metal/concrete inter-face, and some were not.These preliminary results show that the sampled corpus not

only allows to study carbonated and non-carbonated concretebut also the effect of interface defaults at the steel level.

3.2 Heterogeneous patterns around rebars

Optical observations on the different samples highlightthat the corrosion pattern is very heterogeneous for a given

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Materials and Corrosion 2009, 60, No. 2 Characterization of long-term corrosion of rebars embedded in concretes 95

Table 1. Sample references and metal/concrete characteristics

Sample Origin Concrete microstructure at metal/concrete interface Phenolphtalein turn red

1-1 BdT1 Air bubble Yes1-2 Macropores Yes1-3 Macropores Yes2-1 MdB2 Air bubble No2-2 Air bubble No3-1 MdB3 Crackþ air bubble Partially3-2 Crack Partially3-3 Crack Partially4-1 BdT4 Air bubble Yes4-2 Air bubble No6-1 As received bars Generalized rust —

Fig. 3. Microphotograph of a typical type 1 ‘initial’ zone encoun-tered on sample reference 1-3

cross-section. For instance, Fig. 2 shows that the corrosionlayer can vary from 10–30 to 200 mm on the same sample.Because of this heterogeneity, it has been decided to

consider separately the different zones of a same sample andmore especially ‘active’ and ‘passive’ zones.Not only the layer thickness but also phase characteriza-

tion of each zone on every samples, has allowed to determinetwo typical corrosion patterns.

3.3 Type 1 corrosion pattern: ‘Initial’corrosion layers

The ‘type 1’ layout, illustrated in Fig. 3, presents thincorrosion layer (between 10 and 50 mm). This layout wasalso found on the ‘as received bars’ that were stored during 3years in the laboratory (sample reference 6-1).Corrosion scales are constituted mainly of Fe3O4 sur-

rounding the metallic core (Fig. 4), wustite (FeO) (Fig. 5) isalso present as a dense layer in contact with the metal.Locally, on the external part of the corrosion layer, hematite(a-Fe2O3) is detected (Fig. 6). The succession of these threephases from the metallic core to the external zone is in goodagreement with a high temperature corrosion layout (over570 8C) [12]. In fact FeO cannot form under this temperature.Its presence could be due to the hot working of the rebars

Fig. 2. Microphotograph of rebar from Maison du Bresil (sample3-1) and separation into representative zones

Fig. 4. Representative Raman spectrum of Fe3O4 obtained onsample 1-3

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96 L’Hostis, Neff, Bellot-Gurlet and Dillmann Materials and Corrosion 2009, 60, No. 2

Fig. 5. Representative Raman spectrum of FeO obtained onsample 3-2

Fig. 7. Scheme of the initial layout

Fig. 8. Microphotograph of a typical ‘thick corrosion layer’ zone(sample reference 3-1)

during their manufacturing. They have then been put in theconcrete without removing this calamine. Very locally, onfew micrometres, goethite can be present at the metal/corrosion products interface. A schematic representation ofthis layout is given in Fig. 7.

3.4 Type 2 corrosion pattern: Thick corrosion layers

On the second type of corrosion form, layer thicknessesare significantly higher than for the type 1 and about 100–300mm (Fig. 8).In this layout, traces of the ‘initial’ corrosion type (type 1)

often remain in the form of a FeO/Fe3O4 mix embedded intwo thick layers of goethite (Fig. 9, Fig. 11a). In some cases,lepidocrocite (g-FeOOH) can be evidenced in the externalpart of the CPD (Fig. 10). A variation of this layout can beobserved on several samples on which the Fe3O4/FeO mix islocated at the external part of the CPD (Fig. 11b).

Fig. 6. Representative Raman spectrum of a-Fe2O3 obtained inthe external part of the corrosion layer of sample 1-3

Fig. 9. Representative Raman spectrum of goethite obtained onsample 3-1

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Materials and Corrosion 2009, 60, No. 2 Characterization of long-term corrosion of rebars embedded in concretes 97

Fig. 10. Representative Raman spectrum of lepidocrocite obtainedon sample 3-1

Fig. 12. Representative Raman spectrum of less crystallized(d-FeOOH/Ferrihydrite like) obtained on sample 3-1

In both cases, the presence of less crystallized phases offeroxyhite (d-FeOOH) or ferrihydrite type (5Fe2O3, 9H2O,whose chemical formula is until now not well established)can be noticed (Fig. 12). It can be questioned whether thesephases are the product of a more recent evolution of thecorrosion layers as it will be discussed in the next part.Lastly, in some cases neither Fe3O4 nor FeO is detected in

the thick corrosion layer.

4 Discussion

The different observed corrosion layouts can be explainedas follows.First, a thin corrosion layer is formed on the rebars during

hot working of the metal. The rebars are then embedded inthe concrete with this initial oxidation layer that is contitutedof FeO, Fe3O4 and a-Fe2O3. At this stage, it can be questio-ned whether this specific layer could passivate the metalduring the first period of embedding.After a certain period, corrosion products characteristic of

aerated confined medium and mainly composed of goethiteare observed surrounding the initial layer. This fact showsthat the corrosion front progresses inside the material (i.e.from the hot temperature layer to the metal). Nevertheless, in

Fig. 11. Schematic representation of the thick corrosion layer layout.in the external part of the CPD

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some case, the presence of goethite on the outer part of thecorrosion layer could be representative of dissolution/reprecipitation phenomena or other mechanisms that haveto be explained in the future. An important point is thatthe initial phases (i.e. a-Fe2O3, Fe3O4 and FeO) reactduring the aerated corrosion processes and can completelydisappear from the corrosion layout.Moreover, no links could be established between the pre-

sence of a type of layout and the progression of the carbona-tion front inside concrete. As a matter of fact, on somesamples, the phenophtalein test indicates that carbonationfront has reached the rebar/concrete interface. Nevertheless,at the same location, the ‘initial’ corrosion layout type can beobserved but also, in other places, the ‘thick layers’ profile(Fig. 2).However, thick corrosion layers have been correlated to

two kinds of defaults in the concrete. First one are cracks thatcan be connected with external atmosphere through theentire wall. It seems that the most active corrosion phe-nomena (i.e. thicker layers) occur at the intersection betweencracks and rebars. The second type of default is linked to thepresence of vacuoles near the rebar, so that an air reservoir islocally available just after the embedding operation. It seemsthat this could be responsible for a more active corrosionprocess on the metal linked to the presence of important

(a) Fe3O4/FeO mix embedded in oxyhydroxides; (b) Fe3O4/FeO mix

98 L’Hostis, Neff, Bellot-Gurlet and Dillmann Materials and Corrosion 2009, 60, No. 2

quantities of available oxygen in the system. This fact hadalready been observed by other authors [13]. Indeed, fromthese observations it is still not evident to determine if thecorrosion products conduct to the formation of macrocracksor if the cracks lead to an activation of the corrosionprocesses. In all likelihood, both phenomena could occur inthe system.

5 Conclusions

Corrosion patterns formed on ancient rebars in concrete inuse for periods up to 80 years have been characterized byoptical and electron microscopy and Raman microspectro-scopy. It has been evidenced that before being embedded inconcrete, the iron rebars are recovered by a thin oxidationlayer of several ten micrometres. This layer is constituted ofFeO, Fe3O4 and locally on the external part of a-Fe2O3. Aftera long period of degradation in concrete, it has been observedon some part of the collected rebars that thicker corrosionlayers of 100–300 mm have developed. The corrosion phasesformed are mainly iron oxyhydroxides (goethite) in which, insome cases, a Fe3O4/FeO mix is present as a presumed traceof the initial layer. Moreover, these thick corrosion layersare correlated with the presence of cracks or vacuoles insidethe concrete, at the interface with the corrosion. But thequestion remains whether the corrosion products growth isresponsible for the crack formation or, in contrast, if theformation of the cracks leads to change locally theenvironmental conditions of the concrete so that thickcorrosion layers are formed.

Acknowledgements: Authors would like to thank LaurentVincent from CEA for his contribution. Elisabeth Marie-Victoire and Emmanuel Cailleux (LRMH) are also thanked

for their precious help for the sampling of specimens. Thisstudy is financially supported by Electricite de France andANDRA. Results are also included in a PNRC project of theFrench Culture Ministry. Its participants are kindly thankedfor their participation.

6 References

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[2] K. Tuutti, Corrosion of Steel in Concrete, Swedish Cement

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[4] I. Petre-Lazar, Ph.D. Thesis, Universite de Laval, Quebec,

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(Received: February 8, 2008) W5019(Accepted: February 25, 2008)

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