Critical chloride content in reinforced concrete - Sintef

COIN Workshop on “critical chloride content” in reinforced concretePrefaceThis workshop has been carried out within COIN - Concrete Innovation Centre - one ofpresently 14 Centres for Research based Innovation (CRI), which is an initiative by theResearch Council of Norway. The main objective for the CRIs is to enhance the capability ofthe business sector to innovate by focusing on long-term research based on forging closealliances between research-intensive enterprises and prominent research groups.The vision of COIN is creation of more attractive concrete buildings and constructions.Attractiveness implies aesthetics, functionality, sustainability, energy efficiency, indoorclimate, industrialized construction, improved work environment, and cost efficiency duringthe whole service life. The primary goal is to fulfil this vision by bringing the development amajor leap forward by more fundamental understanding of the mechanisms in order todevelop advanced materials, efficient construction techniques and new design conceptscombined with more environmentally friendly material production.The corporate partners are leading multinational companies in the cement and buildingindustry and the aim of COIN is to increase their value creation and strengthen their researchactivities in Norway. Our over-all ambition is to establish COIN as the display window forconcrete innovation in Europe.About 25 researchers from SINTEF (host), the Norwegian University of Science andTechnology - NTNU (research partner) and industry partners, 15 - 20 PhD-students, 5 - 10MSc-students every year and a number of international guest researchers, work on presently5 projects:• Advanced cementing materials and admixtures• Improved construction techniques• Innovative construction concepts• Operational service life design• Energy efficiency and comfort of concrete structuresCOIN has presently a budget of NOK 200 mill over 8 years (from 2007), and is financed bythe Research Council of Norway (approx. 40 %), industrial partners (approx 45 %) and bySINTEF Building and Infrastructure and NTNU (in all approx 15 %).For more information, see www.coinweb.noTor Arne HammerCentre Manager3

COIN Workshop on “critical chloride content” in reinforced concrete6

COIN Workshop on “critical chloride content” in reinforced concreteIntroductionThe COIN workshop was held in June 5–6, 2008, at the Norwegian University of Scienceand Technology NTNU in Trondheim, Norway. It focused on critical chloride content(chloride threshold value) in reinforced concrete with emphasis on techniques fordetermining critical chloride contents and its relevance for practice.This report includes the abstracts and the foils presented at the workshop.The programmes for the two days and the list of participants are shown below.Workshop programme:Day 1, Thursday 5 June, 200808.00 – 08.50 Registration0850 – 09.00 Welcome by Tor Arne Hammer (Project Manager of COIN)09.00 – 10.00 Chris Page: Importance of the steel-concrete interface with regardto corrosion initiation in the presence of chloride.Discussion10.00 – 11.00 Josef Tritthart: Pore solution of concrete. The equilibrium ofbound and freeDiscussion11.00 – 12.00 Luca Bertolini: Depassivation of steel in case of pitting corrosion -detection techniques for laboratory studies.Discussion12.00 – 13.00 Lunch13.00 – 14.00 Maria Cruz Alonso: Chloride threshold values in the literature.Discussion14.00 – 15.00 Bernhard Elsener: Chloride sensors in concrete - accuracy and longterm stability.Discussion15.00 – 16.00 Ueli Angst: Laboratory experiments for detecting critical chloridecontent in reinforced concrete.Discussion16.00 – 17.00 Rob Polder: Relationship between electrical concrete resistivity andcritical chloride content.Discussion20.00 Dinner at Grenaderen restaurantDay 2, Friday 6 June,200809.00 – 10.00 Joost Gulikers: Critical considerations regarding laboratoryresearch on the critical chloride content in reinforced concrete.Discussion7

COIN Workshop on “critical chloride content” in reinforced concrete10.00 – 11.00 Gro Markeset: Critical chloride content and its influence on servicelife predictions.Discussion11.00 – 12.00 Tang Luping: A field study of critical chloride content in reinforcedconcrete with blended binderDiscussion12.00 – 13.00 Lunch in Realfagsbygget13.00 – 14.00 John Miller: Reference material for calibrating chloride analysis ofhardened concreteDiscussion14.00 – 15.00 Jens Frederiksen: On the need for more precise threshold values forchloride initiated corrosion in design, repair and maintenance ofreinforced concrete structures - a consultant's view.Discussion15.00 Closing the workshopThe workshop participants:Last name First name E mail Organization:Kjellsen Knut Ose knut.kjellsen@norcem.no Norcem ASGulikers Joost joost.gulikers@rws.nl Rijkswaterstaat BouwdienstHelland Steinar steinar.helland@skanska.no Skanska Norge ASMiller John B. millcon@online.no Millab Consult ASTritthart Josef tritthart@tugrat.at Graz, University of TechnologyFredriksen Jens Mejer jmf@alectia.com Alectia ASAlonso Maria Cruz mcalonso@ietcc.csi.esInstituto de Ciencias de laConstrucción Eduardo TorrojaBertolini Luca luca.bertolini@polimi.it Politecnico de MilanoPage Chris c.l.page@bham.ac.uk University of BirminghamPolder Rob rob.polder@tno.nl TNO BouwElsener Bernhard elsener@ethz.ch ETH HöngerbergNygaard Peter pvn@force.dk FORCE TechnologyManera Marco mmanera@mgtpatents.com MGT patentsRebolledo Nuria nuriare@ietcc.csi.esInstituto de Ciencias de laConstrucción Eduardo TorrojaSørensen Birgit biso@cowi.dk Cowi ASLuping Tang tang.luping@sp.se CBI BetonginstitutetBoubitsas Dimitrios dimitrios.boubitsas@sp.se CBI BetonginstitutetMarkeset Gro gro.markeset@sintef.no SINTEFVennesland Øystein oystein.vennesland@ntnu.no NTNUHammer Tor Arne tor.hammer@sintef.no SINTEFAngst Ueli ueli.angst@ntnu.no NTNULarsen Claus K. claus.larsen@vegvesen.no Staten vegvesenØstvik Jan Magnus jan-magnus.ostvik@vegvesen.no Statens vegvesen8

COIN Workshop on “critical chloride content” in reinforced concrete1 Initiation of chloride-induced corrosion“Initiation on Chloride-induced Corrosion of Steel inConcrete: Role of the Interfacial Zone”Chris L. PageUniversity of Birmingham9

COIN Workshop on “critical chloride content” in reinforced concrete10

COIN Workshop on “Critical chloride content in reinforced concrete”, Trondheim, 5-6 June 2008Initiation of Chloride-induced Corrosion of Steel in Concrete:Role of the Interfacial ZoneC L PageUniversity of BirminghamGiven that some of the basic phenomena associated with the passivity of ultrapureiron in aqueous solutions of NaOH and KOH are still incompletely understood (1,2),it would be surprising, to say the least, if the same were not true for aspects of theelectrochemical behaviour of reinforcing and prestressing steels in concrete. One of themany reasons for this is that pre-existing layers of mill-scale or rust, which may or maynot be present on the metal surface prior to its embedment in a concrete structure, areliable to affect the ease with which passivation and depassivation will occur, as has beennoted by many researchers (3-12). Such layers of mixed oxides and hydrated oxides ofiron, whose natures and thicknesses depend on features of the alloy composition, themanufacturing process and the vagaries of storage, are notoriously variable and difficultto characterise. Fortunately, however, the high pH values of the pore solutions formedduring cement hydration (13) are normally sufficient to induce passivation of embeddedsteel, as is evident from the noble potentials (> 200 mV SCE ) that are typically observed forreinforcing bars embedded in atmospherically exposed concrete structures which havenot become significantly contaminated by chlorides or carbonation.When studying the influence of variables, related to concrete composition,methods of manufacture, environmental exposure conditions etc, that affect the corrosionbehaviour of steel in concrete exposed to chloride ingress, it has usually been necessaryto start by cleaning the metal surface because particular samples of bars in their ‘asreceived’or ‘pre-rusted’ conditions are not representative or reproducible. For this reason,laboratory research has mainly been undertaken with steel bars, which have been cleanedby grit blasting or similar methods and then degreased and stored in dry air until use. Anexample of this kind of investigation, performed at Aston University (6,14), examinedrelationships between the corrosion responses of embedded steel bars and pore solutioncompositional features (determined at various depths corresponding to the bars) withinPortland cement concrete slabs of different types that were exposed for 6, 12 and 24months to external chloride solutions (5% NaCl). The main objective of the work was toelucidate the circumstances leading to depassivation of steel that had been allowed toform a stable passive layer over a long period in well-made laboratory specimens ofconcrete.The results demonstrated that, in concrete specimens, which were as free aspossible from macroscopic defects (voids, cracks, crevices etc) at the surfaces ofembedded bars, no significant corrosion of passive steel was induced until theconcentration of free chloride ions in the pore solution near the bars exceeded that of freehydroxyl ions by a factor of at least 3 times and, in some cases, as much as 20 times.These ratios of free [Cl - ]/[OH - ] were far higher than those that had previously been found(15,16) to cause depassivation of steel in bulk alkaline electrolytes of the same pH rangeas existed within the concrete at depths corresponding to the bars when depassivation was11

ecorded. This difference was believed to be due mainly to the buffering effect of a layerof cement hydration products deposited in intimate contact with the passive film on theembedded steel. It was concluded that a significant cause of the much lower tolerance tochloride exhibited by steel bars in reinforced concrete structures exposed to deicing salt(17) was the presence of macroscopic defects which disrupt the integrity of the layer ofhydration products deposited at the steel-concrete interface. Another contributory factorwas suggested to be the variable surface condition of the steel reinforcement prior to itsembedment in site-produced concrete (6).More recently, researchers at the University of South Florida (8) have comparedthe behaviour of steel bars, which were either sandblasted or not cleaned prior to beingimmersed in bulk aqueous solutions maintained at various constant pH values (12.6, 13.3and 13.6). After passivation had been induced, increased concentrations of chloride wereprogressively introduced into the solutions and it was shown that: (i) the sandblasted steelbars were more tolerant to chloride than the mill-scaled or pre-rusted bars, (ii) the criticalratios of free [Cl - ]/[OH - ], at which depassivation of the bars was observed, increased withincreasing solution pH value (being > 1 at pH 13.6). These resultsare relevant to reinforced concrete exposed under conditions where chloride ingress is notaccompanied by significant reduction in the pore solution pH. They do not, however,account for the very high ratios of free [Cl - ]/[OH - ] that were found around many of thepassive steel bars in the earlier work at Aston University. While the bulk internal pHvalues of the Aston specimens were originally in the expected range for Portland cementconcretes of w/c 0.5 (13.6 > pH > 13.3) the leaching of free hydroxyl ions thataccompanied chloride ingress was found to have reduced the pH to about 12.5 andresulted in free [Cl - ]/[OH - ] ratios > 10 at depths corresponding to several of the bars thatwere found to be maintained in a passive condition (6,14).That the high tolerance to chloride-induced pitting sometimes exhibited by steelin concrete exposed to the ingress of chloride ion might be due, in part, to the chemicalbuffering action of solid hydration products deposited in the interfacial zone was firstsuggested as long ago as 1975 (18) and discussed in further detail in subsequent papers ofsimilar vintage (19,20). In the original publication (18) it was noted that previous workershad found that voids at the interface were almost invariably the positions at whichcorrosion of steel reinforcement and prestressing tendons takes place (21) and thatmeasures which promote homogeneity of the interfacial zone, such as application of aslurry of neat Portland cement paste to the steel before embedding it in concrete, wereknown to be effective in reducing the risk of chloride-induced corrosion (22). At thattime, plain Portland cements were generally used for structural applications in salineenvironments and rather limited evidence was available concerning microstructuralfeatures of interfacial zones with embedded steel, the most relevant contribution cited in(18) being a paper by Moreau in which scanning electron microscopy (SEM) had beenused in secondary electron imaging mode to examine fracture surfaces of interfacesformed above and below horizontal reinforcing bars (23). Moreau’s work, which waspublished in French, showed that segregated portlandite (CH) crystals of varyingmorphology were a major component of the interfacial regions and demonstrated that,when compared with the relatively compact layers of hydration products deposited overthe upper surfaces of the bars, the undersides were characterised by more porous, open-12

textured zones with much larger single crystals of CH that had grown in the relativelythick solution-filled spaces formed by collection of bleed water beneath the bars.Several subsequent investigations involving the use of secondary electronimaging SEM have confirmed the presence of segregated CH-enriched interfacial zonesat clean mild steel or stainless steel fracture surfaces that had been in contact withPortland cement pastes, mortars and concretes (24-34) though the morphology and degreeof orientation of the CH have been found to vary, as has the distribution of C-S-H andother hydrate phases that have been identified. These different observations seem tosuggest that Portland cements of differing compositions, which give rise to significantdifferences in the early development of their pore solution chemistry (13), may formsomewhat distinct sequences of solid hydration products via the through-solution routesthat are generally believed to give rise to early-stage microstructural development atinterfaces. Until recently, only very limited studies of interfacial zones formed at precorrodedreinforcing steel appear to have been reported (35,36), mainly no doubt becauseof the difficulties involved in producing the required steel substrates with wellcharacterisedsurfaces that are adequately reproducible and representative.Although the examination of simply debonded surfaces by SEM is a usefulmethod for qualitative characterisation of steel/concrete interfaces, the approach is notcapable of providing quantitative compositional data for reasons that have been discussedin relation to the study of aggregate/concrete interfaces (37). Production of polishedsections cut perpendicular to the interfaces concerned is necessary for this purpose andthese sections can be analysed at moderately high resolution by means of back scatteredelectron imaging SEM. There are, however, problems of specimen preparation that mustbe avoided as the surfaces concerned have to go through a number of potentiallydamaging processes (viz. drying, evacuation and resin impregnation followed by severalstages of mechanical polishing) throughout which the possibility of introducing artefactsexists. Research undertaken at the University of Leeds, has attempted to overcome someof these experimental problems and a recent paper (38) has provided quantitative dataderived from a back scattered electron imaging SEM study of interfaces formed betweenconcrete made from a particular ordinary Portland cement and 9mm diameter ribbedreinforcing bars that were embedded vertically or horizontally. Prior to embedment, thesurface of the steel was either ‘as-received’ (with rust and millscale present) or, in somecases, cleaned by wire brushing. Among other things, the results reported appear to showthat, for the particular materials and conditions studied: (i) vertical steel bars hadinterfacial zones of higher CH content and higher porosity than the bulk cement matrix;(ii) for horizontal bars marked differences between the upper and lower interfaces werefound with higher porosity on the undersides, thus confirming the observations ofMoreau (23); (iii) cleaned steel bars had higher levels of CH at the interface thanuncleaned bars after long periods of embedment.Some remaining problems of specimen preparation for quantitative analysis byback scattered electron imaging SEM probably need further consideration if efforts are tobe made to resolve the detailed effects of cement compositional variables on interfacialzone microstructure. This may be of some interest in view of the increasing use nowbeing made of cements with high levels of supplementary cementitious materials (SCMs)that react with CH. If the aim is to develop new cements, which can offer enhancedprotection to embedded steel under conditions of exposure to chlorides, there may have to13

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COIN Workshop on “Critical chloride content in reinforced concrete”, Trondheim, 5-6 June 2008Pore solution of concrete: The equilibrium of bound and free chlorideJ. Tritthart1. IntroductionIt was in the second half of the fifties of the last century that it became apparent that chloridesmay trigger corrosion in the steel reinforcements of concrete. However, during subsequentinvestigations carried out in order to determine which chloride content is harmless and which isnot, greatly differing limit values were obtained. In Austria, a limit value of 0.4 % chloride relatedto the cement content of concrete was introduced as an assessment criterion. This amount ofchloride was considered to be harmless because based on test results it could be concludedthat cement is able to bind “firmly and permanently” up to 0.4% of chloride [1]. The author of thispaper was alerted to the question of chloride-induced corrosion of reinforced concrete in thesecond half of the seventies. Checks on road bridges in Austria in which the author was involved(such checks were performed more and more frequently in the aftermath of the collapse ofVienna’s Reichsbrücke in 1976) quite often showed areas of clearly enhanced chloride contentbut no signs of steel corrosion. This prompted the author to start investigations of his own.The threshold value of 0.4% and all the other threshold values of cement-based materials foundin literature were based on the total chloride content of the concrete related to the cement mass.As only the unbound chloride which remains dissolved in the pore solution can interact with theprotective layer on the steel surface, possibly destroy it and initiate corrosion, the investigationsfocused on the chloride concentration of the pore solution after binding and the factors ofinfluence. For this, pore solution was pressed out of hardened cement pastes which wereprepared i) with the addition of a chloride salt or ii) free of chloride but the chloride was addedsubsequently via diffusion. That way it should be found out whether there were differences inchloride binding between samples which contained chloride from the very beginning andsamples into which chloride penetrated after hardening.2. Chloride binding in cement when added during sample preparationFor sample preparation, the chloride salt was dissolved in the mixing water before mixing. Thefresh pastes, which had been mixed with a spoon, were filled into containers supported byframes that could be turned. After filling, the containers were tightly closed and tuned over nightto avoid sedimentation. Then the samples were wrapped in plastic bags and stored at 20°C untiltesting. Then, pore solution was partly squeezed out and analysed.Cl - -concentration in the poresolution (ppm)40000CEM I + NaClCEM II/B-S + NaCl30000C3A-free + NaClCEM I; CaCl220000CEM II/B-S; CaCl2C3A-free; CaCl21000000.0 0.4 0.6 0.8 1.0 1.5 2.0Total chloride content (% ww cement)19

Figure 1:Cl - -concentration of the pore solution versus total chloride contentFigure 1 shows the influence of total chloride content. It can be seen that the chlorideconcentration in the pore solution increased continuously with increasing total chloride contentand that there were differences depending on the type of cement as well as on the chloride saltadded. Most chloride was bound by the CEM II/B-S followed by the CEM I and least chloridewas bound by the C 3 A-free sulphate-resisting cement. The continuous increase of the Cl - -concentration in the liquid phase indicates that besides chemical binding adsorptive bindingtook place as well. Were only chemical binding to occur, the equilibrial Cl - -concentration of thepore water should have remained relatively constant as long as clinker phases such C 3 A, whichare able to bind the chloride chemically, were present and would increase rapidly if thesecomponents reacted. To verify the influence of C 3 A available for chloride binding, the sulphatecontent of cement was increased in various steps. The more C 3 A reacted with the sulphate andformed ettringite (“trisulfate”; 3CaO.Al 2 O 3 .3CaSO 4 .32H 2 O) the more gypsum was added and theless C 3 A remained available for chloride binding. As expected, the Cl - -concentration of the poresolution increased with increasing sulphate content of the cement but remained approx.constant from a C 3 A/SO 4 -2 -ratio of 3 upwards.Figure 1 shows furthermore that the Cl - -concentration of the pore solution was higher whenNaCl was added than when CaCl 2 was added. This is in agreement with findings described byother authors [2, 3]. However, it was surprising that the influence of the added chloride salt waseven higher than that of the type of cement. This can be explained by the changes in the OH - -concentrations of the pore solution.OH - -concentration in the poresolution120009000600030000CEM I; NaClC3A-free; NaClCEM II/B-S + CaCl2CEM II/B-S; NaClCEM I + CaCl2C3A-free + CaCl20.0 0.4 0.6 0.8 1.0 1.5 2.0Total chloride content (% mm cement)Figure 2: OH - -concentration of the pore solution versus total chloride contentAs can be seen from Figure 2, the hydroxide concentration increased with the use of NaCl butdecreased with CaCl 2 . The reason for the difference in OH - -concentration of the pore solutionbetween samples containing NaCl and CaCl 2 is that at the high pH-values of the pore solutioncalcium is insoluble (contrary to sodium) and precipitates as Ca(OH) 2 . Therefore the OH - -concentration of the pore solution is reduced the more the higher the amount of CaCl 2 [4, 5].Further studies with the use of other chloride compounds which reduce the OH - -concentration inthe same way as CaCl 2 does (MaCl 2 , HCl) showed that practically the same amount of chloridewas bound as was the case with CaCl 2 . When mixtures of chloride compounds were addedwhich affected the OH - -concentration of the pore solution differently (NaCl/CaCl 2 , etc.), the morechloride was bound (the less was the Cl - -concentration of the pore solution) the more the OH - -concentration decreased. The effect of the OH - -concentration of the liquid phase on chloridebinding was also found when the alkali content of the cement was reduced by washing outbefore sample preparation.20

Although the found correlations strongly suggest adsorptive binding of chloride and hydroxideapart from chemical binding, definite evidence can hardly be furnished due to overlappingequilibria and the complex overall situation. This can explain why the addition of silica-fume,which reduces the OH - -concentration of the liquid phase, was found to cause an increase inchloride binding in one study but a decrease in another study (different cements, different typesof silica fume, etc.) [5, 6].3. Subsequently added chlorideTo examine the binding of chloride added subsequently by diffusion, chloride-free cement slabsof 1 cm thickness and w/c-ratios of 0.5 and 0.7 were used [4, 5]. After 28 days of hardening, theslabs were immersed in chloride-containing solutions until no further chloride uptake occurred sothat the chloride concentration was practically the same in the pore solution and in the storagesolution. The slabs were immersed in storage solutions with different OH - -concentrations,namely in saturated Ca(OH) 2 (pH-value: 12.5), 0.1 m NaOH (pH-value: 13.0) and 0.5 m NaOH(pH-value: 13.7). Five different Cl - -concentrations were used. From time to time the Cl - -concentration of the storage solution was measured and used-up chloride replenished. That waythe Cl - -concentrations were kept more or less constant over the whole test period. Pore solutionwas pressed out after about 1 year in order to be sure that a state of equilibrium had beenreached and that the composition of the pore solution had become equal to that of the storagesolution. Only then the final determination of the total chloride content of the samples wascarried out. The results are shown in Figure 3.Total chloride content (% mm cement)1,81,51,20,90,60,301000 ppm Cl3000 ppm Cl5000 ppm Cl7000 ppm Cl10000 ppm Clw/c-0.50 w/c-0.70 w/c-0.50 w/c-0.70 w/c-0.50 w/c-0.70pH-12.5 pH-13.0 pH-13.7Figure 3:Chloride uptake of cement pastes immersed in chloride solutionsAs can be seen from Figure 3 the higher the chloride concentration of the storage solution wasthe more chloride was taken up by the hardened cement. The pH-value played a major role aswell; the higher the pH-value, the less chloride was taken up. The influence of the w/c-ratio onthe other hand was rather insignificant - samples with a w/c-ratio of 0.7 took up slightly morechloride than the ones with a w/c-ratio of 0.5. The calculation yielded that, as in the samplesprepared with the addition of chloride, the proportion of bound chloride remained practically thesame regardless of the w/c-ratio. Thus the influence of the w/c-ratio was almost exclusively dueto the different content of pore water in the samples.4. ConclusionsThe results showed that chloride is not bound in absolute quantities but rather in dependence onan equilibrium which is established between solids and the liquid phase. The state of equilibriumand thus the residual chloride concentration depends on the composition of the cement as wellas on the chemistry of the pore solution and in particular on its OH - -concentration.21

As regards the assessment of the corrosion risk, the results showed that the Cl - -concentration ofthe pore solution varied between samples made of the same cement and containing a totalchloride content of 1% between 2600 ppm and 23000 ppm. Other results showed that the Cl -/OH - -ratio of the pore solution - which is often mentioned to be a better indicator for theassessment of corrosion risk [7] - increased at a given total chloride content with decreasingw/c-ratio which, however, is inconsistent with practical experience. These results illustrate verywell that a certain total chloride content alone is not a suitable criterion for the assessment ofcorrosion risk.References1. W. Richartz: „Die Bindung von Chlorid bei der Zementerhärtung“ („Binding of chlorideduring hardening“), Zement-Kalk-Gips; 1969, H. 10, S. 447-456.2. P. Gunkel. „Die Zusammensetzung der flüssigen Phase erstarrender und erhärtenderZemente“ (Composition of the liquid phase of setting and hardenenig cements“), Beton-Information, V.1, pp 19-25, 1983.3. C. M. Hansson, Th. Fr∅lund and J. B. Markussen: “The Effect of Chloride Cation Type onthe Corrosion od Steel in Concretze by Chloride Ions”; Cement and Concrete Research, V15, pp 65-73, 1985.4. J. Tritthart: "Chloride Binding in Cement - II. The Influence of the HydroxideConcentration in the Pore Solution of Hardened Cement Paste on Chloride Binding",Cement and Concrete Research, V 19, pp 683-691, 1989.5. J. Tritthart.: "Choridinduzierte Betonstahlkorrosion"; Schriftenreihe Straßenforschung desBMfwA, Heft 346, 117 Seiten, 40 Abbildungen, 40 Tabellen, 1988.6. C. L. Page and ∅. Vennesland: “Pore soution composition and chloride binding capacity ofsiliac fume cement pastes“; Materieux et Constructions, V. 16, pp 19-25, 1983.7. D. A. Hausmann. „Steel corrosion in concrete“, Materials Protection, November 1967, pp.19-23.22

Graz, University of TechnologyPore solution of concreteThe equilibrium of bound and free chlorideJ. Tritthart Trondheim 05-06 June 2008Graz, University of TechnologyJ. Tritthart Trondheim 05-06 June 2008231

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Graz, University of Technology1. 3CaO.Al 2 O 3 (C 3 A) + 3CaSO 4 + 32H 2 O 3CaO.Al 2 O 3 . 3CaSO 4 .32H 2 O(Ettringite; “Trisulfate”)2. 3CaO.Al 2 O 3 + CaCl 2 + 10H 2 O 3CaO.Al 2 O 3 .CaCl 2 .10H 2 O (Friedel salt)3. 3CaO.Al 2 O 3 . 3CaSO 4 .32 H 2 O (Ettringite) + 2[3CaO.Al 2 O 3 ] 3[3CaO.Al 2 O 3. CaSO 4 .12 H 2 O] (Monosulfate)J. Tritthart Trondheim 05-06 June 2008Graz, University of TechnologyThreshold values (empirically;; total chlorid)Related to mass of concrete------approx. 1 lb Cl - /yd³ (= approx. 0,59 kgCl - /m³; = ca. 0,02%)Related to mass of cement2% CaCl 2 (~ 1,3 Cl - ) at a concereteporosity of max. 9,5 Vol-% and amin. Cover of ≥1,5 cm0,2% Cl -------authorTomek und Vavrin(1961)Clear & Hay (1973)Stratfull et al(1975)Stewart (1975)500 ppm Cl - Lukas (1980)0,028% Cl - at 330 kg PC/m³ and a watersoluble Cl - -content of 75%;0,051% Cl - at 390 kg PC/m³ and a watersoluble. Cl - -content of 50%------------2% CaCl 2 .2 H 2 O (=~1,3% Cl - ; atdense concrete; otherwise less)0,4% Cl -1,0% - 1,5% Cl - (at dense concrete;otherwise less)OECD-RoadResearch Group(1976)Working group„Monoliet“ (1976)Everett et al (1980)J. Tritthart Trondheim 05-06 June 2008286

Graz, University of Technologycritical Cl - -concentrationand Cl - /OH - - ratioalkaline solutionsconcrete/ mortarauthor

Graz, University of TechnologyCl - -concentration in thepore solution (ppm)12.00010.0008.0006.0004.0002.0000CEM I; NaClCEM II/B-S; NaClsulfate resistant; NaClCEM I; CaCl2CEM II/B-S; CaCl2sulfate resistant; CaCl20 25 50 75 100 125 150age of samles (days)J. Tritthart Trondheim 05-06 June 2008Graz, University of TechnologyCl - -concentration in the poresolution (ppm)40000300002000010000CEM I; NaClCEM II/B-S; NaClC3A-free; NaClCEM I; CaCl2CEM II/B-S; CaCl2C3A-free; CaCl200.0 0.4 0.6 0.8 1.0 1.5 2.0Total chloride content (% ww cement)J. Tritthart Trondheim 05-06 June 2008308

Graz, University of TechnologyCl - -concentration of the pore solution(ppm)1600012000800040000w/c: 0.60w/s: 0.600 ~0,5 ~1 ~3 ~4SO -2 4 /C 3 A-mole ratioJ. Tritthart Trondheim 05-06 June 2008Graz, University of TechnologyCl - -concentration in the poresolution (ppm)40000300002000010000CEM I; NaClCEM II/B-S; NaClC3A-free; NaClCEM I; CaCl2CEM II/B-S; CaCl2C3A-free; CaCl200.0 0.4 0.6 0.8 1.0 1.5 2.0Total chloride content (% ww cement)J. Tritthart Trondheim 05-06 June 2008319

Graz, University of TechnologyOH - -concentration in thepore solution (ppm)120009000600030000CEM I; NaClC3A-free; NaClCEM II/B-S; CaCl2CEM II/B-S; NaClCEM I; CaCl2C3A-free; CaCl20.0 0.4 0.6 0.8 1.0 1.5 2.0Total chloride content (% mm cement)J. Tritthart Trondheim 05-06 June 2008Graz, University of Technology• [Ca +2 ]. [OH - ]. [OH - ] = 5.47.10 -6• [Ca +2 ] = 5.47.10 -6 /[OH - ] 2 = 5.47.10 -6 / 10 -2 =• 5.47.10 -4 mole Ca +2 /l = ~ 22 ppmCa +2 -ions are not soluble at high pH-values andprecipitate as Ca(OH) 2J. Tritthart Trondheim 05-06 June 20083210

Graz, University of Technology14000Concentration in mg/L1200010000800060004000ClOH20000NaCl 0,25%HCl/0,75%NaCl0,5%HCl/0,5%NaClCaCl2 MgCl2 HCl 0,25%HCl/0,75%CaCl20,5%HCl/0,5%CaCl2J. Tritthart Trondheim 05-06 June 2008Graz, University of TechnologyOH - -concentration of the poresolution (ppm)16000120008000400000.4 0.5 0.6 0.7 0.8 0.9 1.0w/c-ratioNaClCaCl2J. Tritthart Trondheim 05-06 June 20083311

Graz, University of Technology25.000Cl - -concentration of the poresolution (ppm)20.00015.000NaCl10.000CaCl25.00000,4 0,5 0,6 0,7 0,8 0,9 1,0w/c-ratioJ. Tritthart Trondheim 05-06 June 20080,8Graz, University of Technology0,6NaClCl/OH-mole ratio0,40,2CaCl 23400,4 0,6 0,8 1,0w/c-ratioJ. Tritthart Trondheim 05-06 June 200812

Graz, University of TechnologyTotal Cl-content (mass-%)0,70,60,50,40,30,20,10w/c-0.50; 1000 ppm Clw/c-0.50; 3000 ppm Cl0 100 200 300 400 500Duration of storage (days)J. Tritthart Trondheim 05-06 June 2008Graz, University of TechnologyTotal chloride content (% mmcement)1,81,51,20,90,60,30w/c-0.50w/c-0.70w/c-0.50w/c-0.701000 ppm Cl3000 ppm Cl5000 ppm Cl7000 ppm Cl10000 ppm Clw/c-0.50w/c-0.70pH-12.5 pH-13.0 pH-13.7J. Tritthart Trondheim 05-06 June 20083513

Graz, University of TechnologyJ. Tritthart Trondheim 05-06 June 2008Graz, University of TechnologyJ. Tritthart Trondheim 05-06 June 20083614

Graz, University of TechnologyJ. Tritthart Trondheim 05-06 June 20083715

COIN Workshop on “critical chloride content” in reinforced concrete38

COIN Workshop on “critical chloride content” in reinforced concrete3 Depassivation of steel in case of pitting corrosionDepassivation of steel in case of pitting corrosion –Detection techniques for laboratory studiesLuca BertoliniPolitecnico di Milano39

COIN Workshop on “critical chloride content” in reinforced concrete40

COIN Workshop on “Critical chloride content in reinforced concrete”, Trondheim, 5-6 June 2008Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesLuca BertoliniPolitecnico di Milano – Dipartimento di Chimica, Materiali e Ingegneria Chimica “G.Natta”According to Tuutti’s model, methods for the design of durability of reinforced concrete structuresexposed to marine environments or the action of de-icing salts, divide the service life into aninitiation period, during which chloride ions penetrate the concrete cover and initiate pittingcorrosion, and a subsequent propagation period during which corrosion leads to a limit stateaffecting the serviceability or safety of the structure. In order to calculate the initiation period, thechloride threshold (Cl th ), i.e. the minimum amount of chloride ions that is required to breakdownthe passive film and initiate pitting corrosion, should be defined. If the penetration of chloride as afunction of depth (x) and time (t) is known Cl(x,t), the chloride threshold may be measured as thechloride content detected at the depth of the bars when corrosion initiates, i.e.:Cl th = Cl(x = c, t = t i )where c is the thickness of the concrete cover and t i is the initiation time.The definition of Cl th is quite complex for several reasons. First of all, pitting corrosion initiation isa stochastic phenomenon that is influenced by a great number of parameters. Cl th , in principle,should be defined through probability distributions that take into account the effect of thoseparameters. However, such an approach would require a huge amount of experimental data which isin practice impossible to collect. Statistical analysis are available in the literature only with regardto the effect of single influencing parameters. Furthermore, experimental details, such as the waychlorides are introduced in the concrete (e.g. added to the mixing water or penetrated by diffusionor migration) or the means of measuring the chloride content (e.g., total acid soluble chloridesexpressed by mass of cement or free chlorides from pore expression expressed as concentration insolution) may have a large influence on the resulting value of the chloride threshold. The techniqueused to detect the corrosion initiation of steel may also have a remarkable effect on the chloridethreshold.Since there are no standardized methods for the evaluation of the chloride threshold, as well as forthe detection of corrosion initiation, a large variety of techniques has been used by differentresearchers. Indeed, even the definition of the initiation of pitting corrosion is not simple. In fact, aninitial phase where breakdown of the passive film alternate to repassivation at incipient anodic sitesis followed by a permanent active corrosion stage, where sustained pit propagation takes place.From a practical point of view, it is often assumed that corrosion is initiated when a anodic currentdensity above a certain threshold (i.e. 1 mA/m 2 , corresponding to roughly 1 μm/year of averagecorrosion rate) is reached.The electrochemical behaviour of steel in chloride contaminated concrete can be described throughthe polarization curve depicted in Figure 1 (proposed by Pourbaix), that plots the anodic currentdensity (measuring the rate of steel dissolution) as a function of the steel potential, in concrete witha given content of chloride ions. This figure shows, at low potential values, the passive range wherethe corrosion rate is negligible. Pitting corrosion initiates when the potential of the steel is above apitting potential (E p ), which is a function of the chloride content, but also of several otherparameters (pH of the pore solution, temperature, microstructure and composition of the steelconcreteinterface, composition and surface finishing of the steel bar, etc.). This means that, if thecorrosion potential of the passive steel (E cor ) is known, Cl th can be defined as the amount of chloridethat brings about a value of E p equal to E cor .41

PotentialE pCEWEREE polPotentiostatI polSimulated concretepore solution + Cl -log (Current density)Figure 1 – Polarization curve of steel in chloridecontaminated concrete and definition of thepitting potential E p .Figure 2 – Set up of tests in solution.This presentation deals with the possible approaches used to detect depassivation of steel in relationto the definition of Cl th . Firstly, tests in alkaline solutions that simulate the concrete pore liquid willbe described. These are sometimes used to evaluate the resistance of steel to pitting corrosion, e.g.to compare different types of steel or the effect of preventative techniques (such as corrosioninhibitors). A steel specimen is immersed in the test solution, as depicted in Figure 2, and it isconnected to a potentiostat that can impose a polarization potential (E pol ) by means of a polarizationcurrent (I pol ). Potentiodynamic polarization tests can be used to plot the polarization curve of steelwith the aim of detecting E p . By repeating the tests in solutions with varying concentration ofchloride ions ([Cl - ]), the relationship between E p and [Cl - ] can be plotted and Cl th can beextrapolated in correspondence of a reference value of E cor . Potentiostatic polarization tests can beused, i.e. the potential of the steel is kept constant and, for instance, chloride are added step-wiseuntil corrosion initiation is detected by a sharp increase in I pol .Tests on steel bars embedded in concrete (or mortar) specimens are generally considered to be moreappropriate than solution tests for a quantitative evaluation of Cl th . Specimens are exposed tochloride penetration (by ponding, wet-dry cycles, exposure to natural marine environments,migration, etc.) and the chloride content is measured at the depth of the steel bars at the time whencorrosion initiation is detected. Alternatively, chlorides can be added to the concrete mix; althoughthis approach is generally considered less appropriate, since the steel is not allowed to form apassive film in chloride-free concrete, it can drastically reduce the time of testing, especially whenimpervious concrete is considered. In this case, the chloride threshold can be detected bycomparison of specimens with higher chloride contents where corrosion initiates and those withlower chloride contents where corrosion does not initiate. Corrosion initiation can be assessed byvisual inspection, i.e. when macroscopic consequences of corrosion take place (e.g. cracking ofconcrete, loss of steel mass or cross section). However, corrosion can be detected only in a laterstage and, if chlorides penetrate from the surface, Cl th may be overestimated. Monitoring ofelectrochemical parameters, usually the corrosion potential and the corrosion rate (measured bymeans of the linear polarization technique), are often used in order to be able to detect corrosioninitiation soon after it takes place (corrosion initiation of steel in concrete exposed to theatmosphere is normally accompanied by a sharp decrease in the corrosion potential and increase inthe corrosion rate). Alternatively, the steel may be polarized potentiostatically and corrosioninitiation may be detected by the increase of several orders of magnitude in the polarization current.Finally, depassivation may also be measured with macrocell tests (by potential difference or currentflowing between passive and active steel) .Examples of different methodology utilized for the detection of depassivation, based on the abovementioned techniques will be shown during the presentation and advantages and limitations of testin solution will be addressed.42

Politecnico di MilanoDipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”COIN Workshop onCritical chloride content in reinforced concreteTrondheim, 5-6 June 2008L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesDepassivation of steel in caseof pitting corrosionDetection techniques for laboratory studiesLuca Bertoliniluca.bertolini@polimi.itChloride induced corrosionCorrosionCorrosion limitL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiest it pTime431

Chloride induced corrosionChlorideCorrosionCorrosion limitL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesH 2 OCl -Cl(x,t 1)Coverthickness (c)ChloridethresholdDepth (x)RebarConcretet 1t it pTimeChloride induced corrosionL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesH 2 OCl -ChlorideCl(x,t 2)Coverthickness (c)ChloridethresholdDepth (x)RebarConcreteCorrosiont 2Corrosion limitt it pTime442

Chloride induced corrosionChlorideL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesH 2 OCl -Cl(x,t i)CorrosionCorrosion limitCoverthickness (c)ChloridethresholdDepth (x)RebarConcretet it it pCl th = Cl(x=c, t=t i )t i = time whencorrosion "initiates"TimeChloride induced corrosion1) Depassivation/repassivation(incipient anodic site)2) Permanent active corrosionPassive layerCl -L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesI cor > 0.1 μA/cm 2 (≈1 μm/year)(C. Andrade, COST 521 Final Report, 2002)H 2 OCl -RebarConcreteCl th = Cl(x=c, t=t i )t i = time whencorrosion "initiates"453

Chloride induced corrosion1) Depassivation/repassivation(incipient anodic site)CorrosionCorrosion limitL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesH 2 OCl -2) Permanent active corrosionI cor > 0.1 μA/cm 2 (≈1 μm/year)(C. Andrade, COST 521 Final Report, 2002)RebarConcretet it pt 4TimeElectrochemical behaviour of steel in concretePotentialL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesPittingcorrosionImperfectpassivityPerfectpassivityE pconcreteH 2 O, O 2 OH -pH > 12,5PassiveH 2 O H + Cl -layere -Steellog (Current density)Active zone (pit) - pH < 5M. Pourbaix, Lectures on Electrochemical Corrosion, 1973464

Potential(mVSCE)Potential(mVSCE)Electrochemical behaviour of steel in concretePotentialE pCl - 1E corL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesI corlog (Current density)Electrochemical behaviour of steel in concretePotentialE pCl - 2 >Cl- 1E corI cor47L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studieslog (Current density)5

Potential(mVSCE)Electrochemical behaviour of steel in concretePotentialE p= E corCl - thL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesI corlog (Current density)Electrochemical behaviour of steel in concrete400L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesPotential (mV SCE)2000-200-400-600-800-1000-1200CorrosionImperfect passivityPerfect passivityABCpitting can initiateand propagatepitting does not initiatebut can propagatepitting does not initiatenor propagate0 0.5 1 1.5 2Chlorides (% cement mass)E pE rP. Pedeferri, L'Edilizia, 1992 / Constr. Build. Mat., 10, 1996486

L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesChloride induced corrosionH 2 OCl -ChlorideCoverthickness (c)ChloridethresholdDepth (x)RebarConcrete1) potential (moisturecontent)2) type of binder (pH of poresolution, binding, ...)3) voids at the steelconcreteinterface4) temperature5) steel compositionand surface finishing6) ...Electrochemical tests in solutionsCEWEREE polPotentiostatL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesI polSimulated concretepore solution + Cl -497

Potential(mVSCE)Potential(mVSCE)Potentiodynamic polarization testsPotentialE polPotentiodynamicpolarization curveL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesE pE polEE polcorE polE polE polE polI polI polI polE corI polI polI polI corlog (Current density)I corE plog(I pol)Potentiodynamic polarization testsL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesE (mV vs SCE)6004002000-200-4000.2Ca(OH) 2 satpH = 12.6carbon steelAISI 304AISI 3160.5-6000.1 0.21112 3 52 3 4Chloride concentration (% by mass)2358810L. Bertolini et al., Brit. Cor. Journ., 31, 3, 1996508

Potential(mVSCE)Potential(mVSCE)Potential(mVSCE)Potentiodynamic polarization testsL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesPotential (mV vs SCE)0-200-400-600Ca(OH) 2 saturated solution (pH ≈12.6)T = 20°C-8000.01 0.1 1 10 NChloride concentrationE pE rR. Cigna, 0. Fumei, L’industria italiana del cemento, 9, 1981Potentiostatic polarization testsPotentialE polPotentiostaticpolarization curveE pE pL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesE corE polI corrE corI polpolE pol51log (Current density)I corrlog(I pol)9

Potential(mVSCE)Potential(mVSCE)Potential(mVSCE)Potential(mVSCE)Potentiostatic polarization testsPotentialCl -E pE polI polPotentiostatic polarization tests(Constant potential / added chloride)I polL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesE corI corrlog (Current density)[Cl - ]Electrochemical behaviour of steel in concreteL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesPotentialE polE corI corrI pollog (Current density)I polPotentiostatic polarization tests(Constant potential / added chloride)[Cl - ] th[Cl - ]5210

Potentiostatic polarization testsL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesCl - (% by mass)1086420no pit pitpH 7,5pH 9pH 12,6pH 13,9carbonsteelAISI410AISI304Imposed potential: +200 mV vs SCEChloride added at intervals of 48 hAISI304LAISI316AISI316L23Cr4Ni254SMOL. Bertolini et al., Brit. Cor. Journ., 31, 3, 1996Limitations of solution tests- composition of the testing solution- steel/solution interface vs steel/concrete interfaceL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studies- chloride content (% mass in solution vs % mass cement)- time of polarization / scan rate- ...5311

Tests in concrete (or mortar)CEWEPotentiostatREE polL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesSteel barConcreteVariables:- concrete composition and properties- steel composition and properties- exposure (temperature, humidity, etc.)Chlorides:- penetrated (ponding, wet/dry cycles,migration, ...)- added to the mixing waterDetection techniques:- Visual inspection / Mass loss- Free corrosion conditions (E cor, I cor)- Polarization tests- Macrocell tests (ΔE, I)Visual inspection / mass loss80Bridge decks70L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesProbability (%)60504030201000 0.5 1 1.5Chloride (% by mass of cement)P. R. Vassie, Proc. of Inst. Of Civ. Eng., 19845412

Visual inspection / mass lossMarine exposure(tidal zone)4 yearsL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesM. Thomas, Cement and Concrete Research, 26, 1996Visual inspection / mass loss5 years ponding with 3.5% NaClCathodiccurrentdensity(mA/m 2 )Corrodedarea (%)Maximumdepth ofattack(mm)E (mV) vsTi/MMO*Cl - vscementcontent(%)L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesControl0.40.81.74.22.50.70.7001.510.800-400 / -500-400 / -500-500 / -600-600 / -700-700 / -9001.51.72.53.5L. Bertolini et al., COST 521 Final Workshop, 200225513

Visual inspection / mass lossStainless steel barsFerritic (405)13%CrMixed-in chloridew/c =0.6; 0.75L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesFerritic (430)17%CrAustentic (302)18%Cr, 9%NiAustenitic (315)17%Cr, 10%Ni, 1.4%MoAustenitic (316)17%Cr, 12%Ni, 2%Mo0 0.32 0.96 1.9 3.2Chloride content (% cement mass)Outside exposure(10 years)Observation+ mass lossK. W. Treadaway et al., Proc. of Inst. Of Civ. Eng., 1989Visual inspection / mass lossGalvanized steel barsMixed-in chloride120L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesZinc depletion (μm)1008060402000 0.5 1 1.5 2 2.5 3 3.5Chloride (% by mass of cement)2.5 years exposureU. Nuernberger, W. Buel, Materials and Corrosion, 42, 19955614

Monitoring corrosion potential and corrosion rateL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesCorrosion potentialCorrosion ratePotentialE corE corEE corPassivelog (Current density)I corActivelog (I)Potential (mV vs CSE)-100-200-300-400-5000 1 2 3 4 5Corrosion potentialCathodiccurrent densityPotential ( E )ΔEAnodiccurrent density ( I)I cor[mA/m 2 ] = B [mV] / R p[Ω⋅m 2 ]ΔICorrosioninitiationTime (year)L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesV corr (μm/anno) /ρ (Ω . m)I cor (mA/m 2 )Monitoring corrosion potential and corrosion rate1000100101CEM II/A-Lw/c = 0.61-100-200-3005,5-40054,50,1-500 40 100 200 300 400 5003,5Time Tempo (day) (giorni)32001000Cl Cloruri - (% mass (% vs cemento) cem)E corr (mV) vs SCE2,521,510,50Penetratedchlorides(ponding)"Depassivation":E cor< -350 mV vs SCEI cor> 2.5 mA/m 2Chloride profile0 10 20 30 40 50 60Profondità Depth (mm)F. Lollini, PhD Thesis, Politecnico di Milano, 20085715

Monitoring corrosion potential and corrosion rateL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesChloride Cloruri (% mass vs cemento) cement)2.521.510.50OPC 15% LI30% LI 30%FA30% PZ 70% BF0 50 100 150 200 250 300 350 400 450Tempo Time (giorni) (day)F. Lollini, PhD Thesis, Politecnico di Milano, 2008Monitoring corrosion potential and corrosion rate10%SF concrete -Mixed-in chlorideL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesI cor (mA/m 2 )654321020°C 35°CAR.rSB.rAR.sSB.s50°C0.6-1.2%0 0.5 1 1.5 2 2.5Chloride (% by mass of cement)E cor (mV SCE)0-100-200-300-400-500-6000.6-1.2%0 0.5 1 1.5 2 2.5Chloride (% by mass)M. Manera et al., Corrosion Science, 50, 20085816

Potentiostatic polarization testsPotential steps (constant Cl - )L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesPotential (mV vs SCE)700316L60022-05500400304L3002001000-100Stainless steel bars - 5%Cl --2000.01 0.1 1 10 100 1000 10000Current density (mA/m 2)L. Bertolini et al., 15th ICC, Granada, 2002Potentiostatic polarization testsPotential steps (constant Cl - )L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesPotential (mV vs SCE)700600500PickledAs received400300200Welding oxide1000-100304 stainless steel bars - 5% Cl --2000.01 0.1 1 10 100 1000 10000Current density (mA/m 2)L. Bertolini et al., 15th ICC, Granada, 20025917

Potentiostatic polarization testsConstant potential (penetrating chloride)L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesJ. M. Frederiksen, Rilem Workshop, 2000M. C. Alonso et al., Rilem Workshop, 2000Potentiostatic polarization testsConstant potential (penetrating chloride)L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesMortarw/c = 0.57 day curing6 mm ribbed barcover = 5 mmM. C. Alonso et al., Electrochimica Acta, 47, 20026018

Potentiostatic polarization testsL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesMortarw/c = 0.57 day curing6 mm ribbed barD. Izquierdo et al., Electrochimica Acta, 49, 2004Macrocell testsCl -L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studies12PotentialE cor,1ΔEPotential1 2E cor,2ΔElog Ilog I6119

Macrocell tests1Cl -I macroL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studies2PotentialE cor,1Potential1 2I macroE cor,2I macroIRlog Ilog IMacrocell testsL.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesM. Raupach, P. Schiessl, Const. Build. Mat., 11, 19976220

Concluding remarksDifferent techniques have been utilized for the detection of "corrosioninitiation" or "depassivation" of steel during laboratory tests, dependingon the type of test adopted for the study of the chloride threshold.Solution tests → Polarization tests (E pitvs Cl - or Cl thvs E pol)L.Bertolini, Depassivation of steel in case of pitting corrosionDetection techniques for laboratory studiesTests in concrete → Mass loss / Visual observation→ E cor, I corr→ Potentiostatic polarization tests→ Macrocell testsDifferent techniques focus on specific aspects of the "depassivation"process.The methodology as well as the criterion defined for detectingcorrosion initiation/depassivation may have a significant influence onthe evaluation of the chloride threshold.6321

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COIN Workshop on “critical chloride content” in reinforced concrete4 Chloride theshold values in the litterature“Chloride threshold values in the literature”Maria Cruz AlonsoInstitute of Construction Science Eduardo Torroja65

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COIN Workshop on “Critical chloride content in reinforced concrete”, Trondheim, 5-6 June 2008CHLORIDE THRESHOLD VALUES IN THE LITERATUREM Cruz AlonsoInstitute of Construction Science Eduardo Torroja (IETcc-CSIC)Madrid (Spain)(mcalonso@ietcc.csic.es)Penetration of chloride ions from de-icing salts or marine environments are common causes fordegradation of concrete structures and responsible for most of the damages and investment in repairworks. By these reasons efforts are being devoted in standards and construction codes in order togive rules and guidelines for designing durable structures. Also efforts are being derived to thedeveloping of models, deterministic or probabilistic, to more accurately approaches for thecalculation of the service life of concrete structures, although most of them are based on Tuutti´sModel.Service life models consider that reinforced concrete exposed to chloride polluted environmentsinitiate corrosion when a certain amount of chlorides arrives to the rebar surface. The so-calledchloride threshold level is considered an essential parameter for assessing the probability ofreinforcement corrosion, and becomes one of the key parameters needed for service life prediction,being of interest to have tests methods and expressions to introduce in the models. The specificconditions that allow the chlorides to reach the threshold for corrosion initiation are still notestablished, which prevents the use of a representative value in the calculation of service life ofconcrete structures.In spite of the numerous studies performed to establish the critical chloride level for the onset ofcorrosion, a wide range of chloride threshold values have been suggested even using apparentlysimilar testing conditions and materials, which makes not feasible to define a single value. Thus, areliable quantification of the critical chloride content is considered a prerequisite for service lifecalculation. The reason has been attributed to the number of variables that affect the chloridethreshold, among them are: the type and content of cement, the exposure conditions, the distancefrom the sea in marine environments, the time of exposure, the environmental temperature, theoxygen availability at the rebar surface, the type of steel, the electrical potential of the rebar surface,presence of cracks etc. Other reasons exist for a non unique chloride threshold value, derived fromthe methodology employed to detect depassivation and the parameter to express the threshold. Fromthis analysis it can be concluded that, at present, a significant lack of critical analyses on the chloridethreshold is observed resulting in a large uncertainty in the values used for reinforcement corrosiononset. It is therefore a great practical importance to reduce this uncertainty based on a commonapproach supported by appropriate experimental investigations.One general aspect still not fully agreed, that also contribute for no founding a unique chloridethreshold value, is the form to express the threshold, in percentage of total or free chloride by weightof binder in concrete, or as [Cl - ]/[OH - ] ratio. Although [Cl - ]/[OH - ] seems to be the most accurateexpression, as include not only the critical chloride but also informs on the ability of the concrete topassivate the reinforcement and inhibit corrosion onset. However due to the difficulty in measuring[OH - ] in concrete, the free and total chloride contents have been more widely used, in particularconstruction codes refer to total chloride. Besides the free %Cl and the [Cl - ]/[OH - ] expressions showwidest scatter of threshold values in literature.Most of the authors for prediction of critical chloride concentration for depassivation ofreinforcements have used stochastic models, nevertheless values defined in standards have usually67

een adopted; however in many cases these values are very conservative. To make a more senseapproach, statistical analysis has been used. Hausman in the sixties was the first that gave anstatistical threshold distribution to predict rebar depassivation, using a [Cl - ]/[OH - ] ratio and giving amean value of 0.6 but with a wide variation coefficient of 30%. Since then other authors have triedto make statistical approaches, proposing uniform distributions of chloride for critical concentrationsgiving mean values of 0.25 % of total chloride by weight of cement with a variation coefficient of 19%, or 0.5 % with variation coefficient of 36 %, also fittings for a log normal distribution have beenmade giving values, with 95% provability for reinforcement depassivation, from 0.496% to 0.569%for free chlorides and from 0.623% to 0.771% for total chlorides, with a minimum value for 5%depassivation risk of 0.280% for free chlorides and 0.316% for total chlorides. Finally in a recentfitting made by present author using all chloride threshold values reported in the literature haveallowed to define histograms as those of figures 1 and 2 for free and total chlorides.6050%Frequency4030201000,1 0,5 0,9 1,3 1,7 2,1 2,5 2,9%Total Cl-Figure 1. Histogram of the total chloride threshold from the values published.6050%Frequency4030201000,2 0,6 1 1,4 1,8 2,2 2,6 3%Free Cl-Figure 1. Histogram of the free chloride threshold from the values published.68

COIN Workshop on “critical chloride content” in reinforced concrete5 Chloride sensors in concrete“Chloride sensors in concrete - Accuracy, long-termstability and application”Bernhard ElsenerETH Zürich and University Cagliari69

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COIN Workshopon Critical Chloride Content in Reinforced ConcreteTrondheim 5./6. June 2008Chloride Sensors in Concrete -Accuracy, Long-term Stability, ApplicationBernhard ElsenerETH Zürich and University Cagliarielsener@ethz.ch71COIN Workshop on Critical ChlorideContent in Reinforced ConcreteTrondheim 5./6. June 2008

BackgroundCondition assessmentVisual InspectionHalf-cell potential mappingConcrete resistivityCorrosion rateMonitoringPotential of the reinforcementConcrete resistancePolarization resistanceChloride sensorspH sensorsReuss-bridge, 30 - 40 yearsDirect information on “critical chloride content” only with sensorsChloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 1/20Critical chloride contentDetermine pitting potential is only half of the truth....< 10 µm >In practical situation - what parameters determine if the potential is above orbelow the pitting potential ?Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 2/2072

Critical chloride contentCondition for depassivationE corr > E pitPitting potential E pitChloride content (free Cl - )Reinforcement material, ...Corrosion potential E corrpH of pore solutionOxygen availability, ...Pitting potential [mV SCE]4003002001000-100-200stainless steellower pH0.01 0.1 1 10Cl concentration [Mol/l]Open circuit potential [mV SCE]-80-120-160-200-240BreitHausmannthis workAndradelong timep a s s i v a t i o n12 12.5 13 13.5 14 14.5pH of solutionChloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 3/20Chloride sensorCombined chloride and resistivity sensor element.120402 5 46311 silver wire, 2 AgCl coating, 3 teflon tube (isolation),4 stainless steel tube (1.4301), 5 epoxy sealing, 6 insulation.Total length ca. 12 cmB. Elsener et al., Proc. RILEM Int. Workshop, ed. L.O. Nilsson, RILEM (1997) p. 17 - 26B. Elsener et al., Materials and Corrosion 54 (2003) 440 - 446Chloride sensors in concrete - accuracy, term stabilty, application B. Elsener 4/2073

TheoryChainAg / AgCl // a Cl- / KCl // Hg 2 Cl 2 / AgNernst LawE (AgCl/Cl - )= E o + RT/nF * ln ([Cl - ] / [AgCl])E (AgCl/Cl - ) = E o + 0.059 * log ([Cl - ])Calibration curvesFurther questions- Activity coefficients- Stability of reference electrode- Stability of chloride sensor- Junction potentialsChloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 5/20Calibration curvesSynthetic pore solution (pH 13.5)0.15Ca(OH) 2solution sat. (pH 12.5)0.15Potential [V SCE]0.10.050solution B-0.05-2.5 -2 -1.5 -1 -0.5 0 0.5log Cl - [ Mol / l!]y = - 6.5 ± 1.4 - 54.4 ± 1.3 * log xPotential [V SCE]0.10.050sol. Ca(OH) 2sat.-0.05-2.5 -2 -1.5 -1 -0.5 0 0.5log Cl - [ Mol / l!]y = - 4.3 ± 1.2 - 59.4 ± 1.1 * log xSimilar values of the slope were reported for other solutionsB. Elsener et al., Proc. RILEM Int. Workshop, ed. L.O. Nilsson, RILEM (1997) p. 17 - 26L. Zimmermann, PhD Thesis No. 13870 ETHZ (2000)Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 6/2074

Long term stabilityCyclic concentration change (5 sensors)0.1Long term stability (solution)15Potential [V SCE]0.1 M NaCl0.0502.0 M NaCl-0.050 10 20 30 40 50 60 70time [ hours]80Potential [mV SCE]105Sensor 1Sensor 2solution BSensor 30.5 M NaClSensor 4Sensor 500 20 40 60 80 100time [days]Standard deviation < 2 mVStandard deviation < 2 mVVery low drift (no T correction)B. Elsener et al., Proc. RILEM Int. Workshop, ed. L.O. Nilsson, RILEM (1997) p. 17 - 26L. Zimmermann, PhD Thesis No. 13870 ETHZ (2000)Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 7/20Experiments in mortarSmall cylindrical mortar samples (ø 4 cm, height 4 cm, w/c 0.5, 500 d)Cl- concentration sensor (Mol/l)10.80.60.40.200 100 200 300 400 500Time of immersion (days)1 Mol/l0.5 Mol/l PS0.5 Mol/l0.3 Mol/l0.1 Mol/l0 Mol/lChloride sensor in mortar (Mol/l)10.80.60.40.2pore water expressionDifferencechloride sensor00 0.2 0.4 0.6 0.8 1Chloride concentration solution (Mol/l)Follow chloride uptake into samplesGood agreement between sensorsand pore water expression, but...L. Zimmermann, PhD Thesis No. 13870 ETHZ (2000) - Final report COST 509Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 8/2075

Field - monitoringInstrumented coresWork well in the laboratory. When mountedon structures, additional problem:!E + I * RMacrocell currentPotential difference betwenn sensor andreference electrode is affected by macrocellcurrent -> wrong chloride contentY. Schiegg, PhD Thesis No. 14583 ETHZ (2002)Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 9/20Field - monitoringInfluence of macrocell current!E + I * RMacrocell currentPotential difference betwenn sensor and reference electrode is affected by macrocellcurrent -> wrong chloride content. Application limited.Y. Schiegg, PhD Thesis No. 14583 ETHZ (2002)Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 10/2076

Pitting initiationCan be detected by drop in potential...is a stochastic processrebars potential E [V SCE]depassivation0.000.5-0.050.4-0.10Cl -E-0.150.3-0.20-0.250.2-0.300.1-0.35-0.4000 100 200 300 400 500time [hours]Cl - [mol/l]Probability of initiation [%]100806040200% at 5 C% at 14 C% at 22 C0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Chloride concentration [M/l]Time of depassivation can be detectedFree chloride content measuredCritical chloride content is distributed,mean value and standard deviation.L. Zimmermann, PhD Thesis No. 13870 ETHZ (2000), B. Elsener et al., EUROCORR ‘98Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 11/20Pitting initiationComparison mortar / solutionProbabilita of initiation %100806040200Solution 22 Cmortar w/c 0.50 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Free chloride [Mol/l]Pore solution, OH - conc. 0.64 Mol/lMortar w/c 0.5, ca. 0.5 Mol/lPit initiation (depassivation) inmortar is more distributedMore variables to control:- age of sample- cover depth (oxygen availability)- corrosion potentialIn agreement with practicalexperience - critical chloridecontent is not a constant.L. Zimmermann, PhD Thesis No. 13870 ETHZ (2000), B. Elsener et al., EUROCORR ‘98Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 12/2077

Surface finishInfluence of surface quality on E pitPitting potential [mV SCE]6004002000-200synthetic pore solutioncoarsefine-4000 1 2 3 4 5Cl concentration [Mol/l]Rough surface shows lower pittingpotentialPittingPit propagation - other controlling factorsL. Zimmermann, PhD Thesis No. 13870 ETHZ (2000), B. Elsener et al., EUROCORR ‘98Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 13/20Cl - /OH - ratioProbability of initiation [%]1008060402000 0.5 1 1.5 2 2.5[Cl - ] / [OH - ] ratioAccording to Hausmann (1967)Pore solution, pH 13.2 - 13.5Stochastic feature of pit initiationand distribution remains validSeveral works of different authorscan be “normalized”Most important is not the value of the Cl - /OH - ratio but its probabilisticdistribution. This has to be taken into account for modeling andservice life predictionChloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 14/2078

Corrosion potentialOpen circuit potential [mV SCE]-80-120-160-200-240BreitHausmannthis workAndradelong timep a s s i v a t i o n12 12.5 13 13.5 14 14.5pH of solutionFor passive steel in concrete E corris a function of pH and timePercentage Fe(II) and Fe(III) [%]10080604020Fe(III)Fe(II)0-0,45 -0,4 -0,35 -0,3 -0,25 -0,2 -0,15 -0,1Potential (V SCE)Increasing potential leads to a passivefilm rich in Fe(III) - more protective! The passive film on the reinforcement shows “ageing” phenomena! Protective properties improve with time of exposure to alkaline environmentA. Rossi, B. Elsener, EUROCORR 2001Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 15/20Passive film composition10080 Fe(III)60this work4020Fe(II)0-0,9 -0,8 -0,7 -0,6 -0,5 -0,4 -0,3 -0,2 -0,1Electrochemical potential of steel inalkaline solutions is related to (controlledby ?) surface compositionFe3O4a-FeOOHFe2O3Presence of different oxide phaseson steel in 0.1 M NaOH solutioninitial24 hPotentialA. Rossi, B. Elsener, EUROCORR 2001Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 16/2079

Chloride ingressInstrumented mortar blocks with chloridesensors at different depthsPotential [V SCE]0.20.150.10.050-0.0502127MPK 23242 mm10 20 30 40time [hours]61 mmDrop in potential indicates increase inchloride concentration50depth mmComparison of water uptake (resistance)and chloride ingress1008 06 04 02 0water00 2 4 6 8 1 0 1 2time 0.5 [h 0.5 ]free chlorideWater ingress follows " t law. Chlorideingress slowed down by bindingB. Elsener et al., Proc. RILEM Int. Workshop, ed. L.O. Nilsson, RILEM (1997) p. 17 - 26Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 17/20Control of ECEElectrochemical chloride removal controlled by sensorsFree chloride concentration (mole/l)Part I Part II Part IIIFree Cl- content measuredwith chloridesensors “in-situ”Mortar, w/c 0.5Chloride ingress bycapillary suctionCurrent 1 A/m 2 steelTime (h)! free chloride concentration decreases rapidly within few days!After current switch off an increase in free Cl is foundU. Angst, B. Elsener, Corrosion Science (2008)Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 19/2080

Free and bound chlorideProposed types of chloride binding in chlorideInfluence ofcement typemeasured withchloride sensorOH - competitionfor binding siteSet free under the effectof the electric current! Equilibrium between chemically bound and free chloride! but also between adsorbed and free chloride - effect of currentU. Angst, B. Elsener, Corrosion Science (2008)Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 20/20Summary• The chloride sensitive AgCl element measures the free chloridecontent in solutions and in porous materials like mortar or concrete• The measurement requires a stable reference electrode• The measured potential (especially with surface RE) may be affectedby liquid junction potentials, membrane potentials etc. This willdecrease the accuracy of the sensor• The combined chloride / resistance sensor has numerous applicationsin research in laboratory as has been shown in this presentation• Field applications are difficult due to macro-cell currentsChloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 21/2081

Critical chloride contentThe critical chloride content is an important design-parameter for service life• It is not a constant but distributed due to the probabilistic nature ofpitting corrosion• Defined as free chloride content that leads to depassivation of the steell• Influenced by a number of parametersFor service life prediction• Take into account total chloridemight be dissolved !• Take into account pH, O 2 changesI would not rely on the assumption of lowcorrosion rate...Chloride sensors in concrete - accuracy, long-term stabilty, application B. Elsener 22/20Thank you for your attention -and your discussions82

COIN Workshop on “critical chloride content” in reinforced concrete6 Laboratory experiments for detecting chritical chloride content“Laboratory experiments for detecting critical chloridecontent in reinforced concrete”Ueli AngstNTNU, Norwegian University of Science and Technology83

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Abstract Ueli AngstLaboratory experiments for detecting critical chloride content inreinforced concreteUELI ANGSTNTNU Norwegian University of Science and Technology, Department of Structural EngineeringRichard Birkelandsvei 1A, N-7491 Trondheim, Norwayueli.angst@ntnu.noIntroductionThe knowledge of chloride threshold values for initiation of reinforcement corrosion in concrete isimportant for service life predictions and service life design. A lot of research has been devoted to findingsuch values. The reported results for critical chloride contents scatter over a wide range, e.g. from 0.02 to3.08% when expressed in the form of total chloride by weight of binder [1]. This is mainly related to thevariety of possible measurement techniques, both for field studies and laboratory setups. At present, nostandardised or accepted testing method for critical chloride content exists.A literature review over nearly 40 references reporting chloride threshold values has concluded thatmany studies were not practice-related [1]. Main pitfalls in laboratory work on critical chloride contenthave been identified to be the procedure to introduce chlorides and the quality of the steel-concreteinterface. The latter is affected by both the rebar characteristics and the properties of the matrix. In manyworks, smooth rebars have been used instead of ribbed bars and the rebars have been prepared bysandblasting, polishing, cleaning, etc. Experiments have been conducted in alkaline solutions, or incement paste, mortar or concrete. It is evident that the properties of the steel-concrete interface in the caseof, for instance, smooth and polished rebars embedded in cement paste differ from reality. This differenceis considered to be important since the steel-concrete interface has been identified to be one of the majorinfluencing factors with regard to critical chloride content [2]. Also, compaction is usually better forlaboratory concrete in comparison with real concrete. With regard to the chloride introduction, severaltechniques are available to accelerate chloride penetration in order to avoid time-consuming experimentsas in the case of pure diffusion. However, the situation is different from reality, e.g. in the case of mixedinchlorides where the steel might not be able to initially passivate.The literature evaluation [1] also revealed that most critical chloride contents have been measured interms of total chloride content by weight of binder; free chloride contents have primarily been reported inthe case of experiments dealing with solutions or porous cement paste/mortar/concrete. This is mainly dueto the limitations of the pore solution expression technique, which is usually the one used for obtainingsamples for analysis of free chloride concentrations in concrete pore solution. Up to now, there is a lack ofinformation on the critical chloride content in dense concrete (low w/c ratio, alternative cement types) onthe basis of free chloride.Current projectThe current project aims at measuring the critical chloride content based on both free and total chloridecontents in laboratory concrete samples with a dense matrix. Emphasis is put on realistic and practicerelated conditions with regard to the factors mentioned in the previous section.For the measurement of the free chloride content, embedded “chloride sensors” are used. These sensorsconsist of Ag/AgCl electrodes and have been used earlier by other researchers [3].COIN Workshop on Critical Chloride Content in Reinforced Concrete Page 1 / 35–6 June 2008, Trondheim, Norway85

Abstract Ueli AngstChloride sensors – direct potentiometry in concreteThe use of ion selective electrodes (ISE) in direct potentiometry is well established and has long beenused in many fields such as analytical chemistry to determine the ionic activity of a certain species inaqueous solutions by a potential measurement. However, the situation in a cement based material such asconcrete is more complicated. With regard to this, preliminary investigations have been undertaken by thepresent author. Some of the considerations and results are presented here; a more detailed description willfollow in the full paper.Sensitivity and error sourcesPotentiometric measurements are very sensitive to variations in potential. The sensitivity directlyfollows from the slope of measured calibration curves or from the theoretical slope of 59.2 mV/decade. Ifonly small deviations in potential are considered (in the range of only a few mV), it can be calculated thatan error in potential of 1 mV results in a relative error in chloride concentration of ca. 4 percent [4].Several phenomena have been identified to disturb accurate measurements of sensor potentials whenmeasuring in cement based materials [5]. When an external reference electrode is used by establishingcontact to the concrete surface with a wetted sponge the situation can be schematically depicted as in Fig.1. Several components contribute to the measured potential, namely the liquid junction potential at theinterface between inner solution of the reference electrode and the wetting agent in the sponge E RE-S , theliquid junction potential at the boundary between the wetting agent in the sponge and the concrete poresolution E S-C , as well as membrane potentials E C across the concrete, and eventually iR drops. Both liquidjunction and membrane potentials are all so-called diffusion potentials and arise due to concentrationgradients in combination with differences in mobility between the diffusing ions. The significance ofdiffusion potentials for electrochemical measurements in concrete has been recently discussed by theauthor in ref. [5].Fig. 1. Disturbing phenomena when measuring potentials in concreteConsequences for experimental setupsIn order to measure reliable free chloride contents in concrete with potentiometric sensors one has to bevery careful with regard to the setup. Liquid junction potentials at the interface between an externalreference electrode and the sample surface can be minimised (but never completely avoided) if thereference electrode is contacted to the concrete with an appropriate solution [5]. With regard to membranepotentials the position of the reference electrode appears to be decisive: To minimise contributions frommembrane potentials arising from internal concentration gradients the reference electrode should ideallybe placed at the same depth as the chloride sensors (e.g. by use of embedded reference electrodes).COIN Workshop on Critical Chloride Content in Reinforced Concrete Page 2 / 35–6 June 2008, Trondheim, Norway86

Abstract Ueli AngstConclusionsThe following sums up the current project plans and the results obtained so far:−−The current project aims at using a practice-related testing setup for determining the critical chloridecontent in dense concrete, based on both free chloride and total chloride contents.Ag/AgCl electrodes can be used as ion selective electrodes embedded in concrete to measure thefree chloride content non-destructively. However, the technique is highly sensitive to errors inpotential measurements arising from phenomena such as diffusion potentials. This is not a problemof the sensor itself, but of potential measurements through concrete as such.References1. U. Angst and Ø. Vennesland. Critical chloride content in concrete - state of the art. In 2nd Int.Conf. on Concrete Repair, Rehabilitation and Retrofitting. Cape Town, South Africa. 2008,(accepted for publication).2. R. Cigna, C. Andrade, U. Nürnberger, R. Polder, R. Weydert and E. Seitz, ed. COST 521: Finalreport “Corrosion of steel in reinforced concrete structures”. Luxembourg, 2002.3. B. Elsener, L. Zimmermann and H. Böhni. Non destructive determination of the free chloridecontent in cement based materials. Materials and Corrosion 54 (2003), 440-446.4. J. Koryta. Theory and applications of ion-selective electrodes. Analytica Chimica Acta 61 (1972),329-411.5. U. Angst, Ø. Vennesland and R. Myrdal. Diffusion potentials as source of error inelectrochemical measurements in concrete. Materials and Structures (2008), (in press.DOI:10.1617/s11527-008-9387-5).COIN Workshop on Critical Chloride Content in Reinforced Concrete Page 3 / 35–6 June 2008, Trondheim, Norway87

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COIN Workshop on Critical Chloride Content in Reinforced Concrete5–6 June 2008, TrondheimLaboratory experimentsfor detecting critical chloride contentin reinforced concreteUeli AngstPhD Candidate at NTNUNorwegian University of Science and TechnologyOverview – my projectPart I• Brief background and motivation for my project• My plans and aims• Expected resultsPart II• Results so far891

Critical chloride content C crit – State of the ArtLiterature evaluation:Large scatter in reported results0.02…3.04% total chloride by weight of cementDifferentdefinitions usedDifferent measurementtechniques usedA lot of influencingparameters on criticalchloride contentsuch as:• pH of the pore solution• Steel-concrete interface• Potential of the steel• Moisture content•etcCritical chloride content C crit – State of the Art (2)A lot of studies have already been undertaken on this topic……on steel bars in solution or steel in cement paste / mortar / concrete…under laboratory conditions or outdoor exposure…with mixed-in chloride or chloride ingress by capillary suction and/ordiffusion or migration…with smooth or ribbed steel bars…with polished or sandblasted or cleaned or pre-rusted or as-received rebars…by measuring total or free chloride contents or Cl – /OH – ratiosMain outcome:• Extremely large scatter in the results• Major influencing factors identified (see slide above)902

Critical chloride content C crit – State of the Art (2)A lot of studies have already been undertaken on this topic……on steel bars in solution or steel in cement paste / mortar / concrete…under laboratory conditions or outdoor exposure…with mixed-in chloride or chloride ingress by capillary suction and/ordiffusion or migration…with smooth or ribbed steel bars…with polished or sandblasted or cleaned or pre-rusted or as-received rebars…by measuring total or free chloride contents or Cl – /OH – ratios Many studies were not practice-related In the majority of the studies the total chloride content was measured;Free chloride thresholds were only reported in connection with solutionsor porous mortar / concrete.State of the Art – Summary• A lot of studies have been undertaken using a lot of different techniques anddefinitions. The reported results scatter in a wide range.• Most researchers reported total chloride contents; free chloride contents orCl – /OH – ratios were reported in connection with solution experiments orporous mortar/concrete. There is a lack of results in the range of low porosityconcrete and free chloride contents.• Many studies were not practice-related (but indicated the major influencingfactors).• For the critical chloride content, no standardised or accepted testing methodexists at present.• Confusing results with regard to the role of bound and free chlorides. What isthe role of bound chlorides in the corrosion initiation process?913

Plans and aims for my project• Measure both the free and total chloride content…Role of bound and free chloride?Form to express chloride threshold favoured?• …in dense concrete (low w/c ratio)• Non-destructive “free chloride sensors” (Ag/AgCl ion selective electrodes)• Practice-related testing methodIon selective electrodes – chloride sensors• Ag/AgCl electrodes for potentiometric chloride determination:Nernst’s law:AgClAgCl coatedsilver wireStainlesssteel tube• Calibration in various media calibration curves924

Experimental setup and procedure (1)• Cast concrete cube with an embedded carbon steel barSteel barEquipped with• Chloride sensors• Embedded referenceelectrode(s)• Stainless steel bar(counter electrode for LPR)How to get the chlorides into the concrete samples?TechniqueMixed-inPure diffusionCapillary suctionand diffusionMigrationPro- Fast- Homogeneously distributed- Total chloride content wellknown- Affinity to practice- Affinity to practice- Rather fastContra- Initial formation of passive filmquestionable- Homogeneously distributed- Chloride binding different- Porosity affected- Very time consuming- Sample water saturated(limited oxygen availability)- Time-consuming- Initial drying for capillary suction mightchange the porosity- Current field- Migration of hydroxide ions(affecting pH in concrete)- Sample water saturated(limited oxygen availability)935

Experimental setup and procedure (2)• After curing, cutting and/or grinding to reduce the concrete coverExperimental setup and procedure (3)Chloride penetration• Capillary suction & diffusion• (Migration)no chloridechlorideconcretesample• Follow free chloride concentration with sensors946

Experimental setup and procedure (4)Monitor freechloride contentcontinuouslynoDepassivation?yesLPR &potentialmeasurementCastingCuringCutting /GrindingChlorideingressExposeto airCoredrillingStop at a certainconcentration beforecorrosion initiationUntil internalRH at steel 70–80%(as in practice)TotalchlorideExpected results• Values for critical chloride content in…• …dense concrete…• …based on both the free and total chloride content• Improved non-destructive measurement technique for the free chloridecontent in concrete (embedded chloride sensors)• Suggestion for a practice-related testing methodProbably not for finding C critvalues for use in servicelife calculations; more as a testing method tocompare different systems (rankings)957

Part IIResults so far – Chloride sensorsCalibration of Ag/AgCl electrodes in solution (1)Setup:Ion selective electrodes in various solutions with known chlorideconcentrationPotential measured vs. sat. Calomel electrode (SCE)Corrections:1Activity vs. concentrationActivity coefficients2 Liquid junction potential at boundaryCorrect liquid junctionbetween SCE and solutionpotential error968

Calibration of Ag/AgCl electrodes in solution (2)Calibration curves. Example for synthetic pore solution:Theoretical (T = 25°C)E = –59.2 loga –18.9 (mV)E = –57.9 loga –19.4 (mV)R 2 = 0.9991Sensitivity & accuracy of Ag/AgCl electrodesIn direct potentiometry:1 mV measurement error ≈ 4% error in chloride activityVery accurate potential measurement requiredDisturbing phenomena for potential measurements in concrete:Solution in RE differentfrom solution in the spongedifferent from pore solutionLiquid junction potentialsMembrane potentialsiR dropSensor potential of interest979

Diffusion potentialsTwo solutions of different composition in contact » Liquid junction «Diffusion takes placeDiffusing ions have different mobilitiesDifferent diffusion rateCl –Cl –OH –K +Na + OH –Charge separationElectric field E dPotential difference acrossthe boundaryE daccelerates the slow ionsand holds back the fast ions» Diffusion potential «Steady stateDiffusion potentials – Henderson equationTwo solutions of different composition in contact » Liquid junction «α β » Diffusion potential «Henderson equation:Estimate diffusionpotentialsAssumptions:– linear concentration gradients– mobility independent of concentration– concentration equivalent to activity9810

Diffusion potentials – DefinitionsTwo solutions of different composition in contactαβPore walls / double layeraffects mobilityD Cl–>> D Na+Contact?CementpasteNon-selective(diaphragm,e.g. porous frit)Permselective(permeable for some,but impermeable forother substances)» Liquid junctionpotentials «» Membranepotentials «How diffusion potentials affect measurements (1)Mortar sample withembedded Cl – sensorAdmixed chloride:2% per cem wtOne drop of tap waterOne drop of 0.3 M NaOH(another drop of0.3 M NaOH)thin clothOne drop ofexpressed poresolution9911

How diffusion potentials affect measurements (2)In direct potentiometry:1 mV measurement error ≈ 4% error in chloride activityLiquid junctionpotentialsNegligible?ca. 16 mVWhat does this mean for my setup?Chloride ingress from surface chloride profileIn case of migration OH – concentration profileWhere to place the reference electrode?OH – 100Chloride1Cl – concentrationOH – concentration23TimedependentTimedependent12

How large could this effect be?pH profileAssume OH – concentration gradient (linear) use Henderson equationpH αpH βIn bulk solutionD OH–>> D Cl–≈ D Na+In cement pasteD OH–>> D Cl–>> D Na+No exact valuesavailableHow large could this effect be?Chloride profilepH α = pH βAssume chloride concentration gradient (linear) use Henderson equation0 MNaClx MNaClExpectedC critIn bulk solutionD OH–>> D Cl–≈ D Na+In cement pasteD OH–>> D Cl–>> D Na+No exact valuesavailable10113

What does this mean for my setup?Impossible to calculate membrane potentials along concentration profiles inconcrete accurately; pH profiles are presumably more serious than chloride profiles.These phenomena are a serious error source for the measurement.Position of the reference electrode is very important.OH –Chloride1Cl – concentrationOH – concentration23TimedependentTimedependentComparison with pore solution expressionThree parallel mortar samples (2% total chloride by cem wt)Measured potential of chloride sensors:8 – 12 mV vs ext. SCECalibration curveand corrections(liquid junction, activity)Concentration = 0.5 mol/lBut: pore solution expression 0.75 mol/lPossible reasons:• Membrane potentials?• Activity affected by the pore walls?thin cloth• Pressure increases free Cl – ? More experiments to answer this.10214

Conclusions for the use of chloride sensors• Direct potentiometry is very sensitive.• Chloride sensors (Ag/AgCl electrodes) can be used to measure the freechloride content in concrete non-destructively.• But, one has to…• Be careful with what solution is used to contact the concrete surface• Be careful with the position of the reference electrode (RE)• Avoid pH gradients and chloride concentration gradients between theRE and the chloride sensorThank you for your attention.10315

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COIN Workshop on “critical chloride content” in reinforced concrete7 Critical chloride content and concrete resistivity“Critical chloride content for reinforced concrete and itsrelationship to concrete resitivity”Rob B. PolderTNO105

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Critical chloride content for reinforced concrete and its relationship to concrete resistivityDr. Rob B. Polder, TNO Built Environment and Geosciences, Delft, The NetherlandsCOIN Workshop on Critical Chloride Content in Reinforced Concrete, NTNU, June 5-6, 2008Introduction: Problematic issuesService life design (including re-design or assessment) of reinforced and prestressed concretestructures requires a number of input variables. One of the variables that is difficult toquantify is the critical chloride content or threshold value. This is due to issues of:- concept and definition- experimental parameters- measuring method- influences in the field.In engineering terms, the underlying concept is that no or low (insignificant) corrosion ofembedded reinforcing (or prestressing) steel occurs at chloride contents below the criticalcontent; significant corrosion occurs above the critical value. Under external chloride load,the content at the bar surface increases over time and at some point, a transition takes placefrom absence to presence of corrosion, or, in electrochemical terms, from passive to active.Definitions for the critical value usually refer to:- the transition from passive to active (onset of corrosion)- an unacceptable amount of damage (usually of concrete cracking and spalling).The "active-passive" definition is problematic from the point of view of measuring methodsand criteria. Some effect of reaching the critical level at a reinforcing bar should bemeasurable; and next, the effect should be significant. For instance, a drop of steel potentialmay not be due to corrosion initiation but to reduced oxygen access. In any case, thisdefinition provides the best option to proceed; see below.The "unacceptable" definition is problematic because it cannot be generalised; it is based onmanagement policies and lies outside the realm of materials science. A rational approach ofacceptable damage-due-to-corrosion-due-to-chloride could be based on reliability (safety) orrisk (economy) considerations; thus, individual owners may use different levels ofacceptability.The introduction of reliability or more generally probabilistic considerations is very useful,though. Initiation and pitting corrosion are stochastic processes, e.g. as demonstrated byelectrochemical noise measurements. Moreover, various influences on corrosion initiation andcritical content may have considerable spatial variation, e.g. the density of the cement paste orthe presence of aggregate particles and air voids near the steel surface. Others vary in time,either periodically or continuously, such as temperature, moisture content and pH changes.Field studies have shown that the probability of corrosion increases with the chloride contentat the depth of the steel (Vassie, Everett).Sometimes a third "definition" is used: the acceptable amount of chloride in the fresh concreteas given by Standards. This is not a proper definition, however, as it includes an unknown107

safety margin and as the effect of mixed-in chloride on passivation and corrosion is differentfrom that of penetrated chloride.Practical solutionsAndrade and a group within COST 521 have proposed that depassivation is a process thattakes a certain amount of time, at the end of which corrosion has been firmly established.Thus, the critical content can be defined as the value above which a sustained corrosion rateof more than 1 mA/m 2 (c. 1 µm/year) occurs. Advantages of this definition are that thecorrosion rate is directly related to the cross section loss and that it is measurable, at least inprinciple. For experimental determination, a test setup has to be found that must addressseveral problematic issues:- exposure (moisture content, temperature, oxygen access)- compositional and interfacial factors (cement type and content, w/c, aggregate, airvoids, steel surface properties)- electrochemical effects (free or imposed potential, cathodic area, measuring method,e.g. polarisation resistance, macro cell current)- high precision of measuring methods- and finally, statistical treatment of the results.Breit has succeeded in solving most issues, providing the basis for a statistical expression ofthe critical content. From his experiments on mainly ordinary Portland cement materials, thecritical level has a mean of about 0.5% by mass of cement with a standard deviation of about0.2%. The type of distribution is slightly problematic, though: it could be normal, lognormalor beta. This is due to the relatively low number of results at the low end, where theprobability is low but significant. Nowadays, service life design aims at a probability ofcorrosion (initiation) of 10% or less. So, a practical solution could be accepting Breit's meanand standard deviation; in the author's opinion, a lognormal distribution provides the best fitto the low end and should be chosen.Analysis of another data setA large number of data was obtained from specimens with penetrated chloride and embeddedelectrodes for corrosion potential, polarisation resistance and concrete resistivitymeasurements (Polder). These data are analysed in view of the discussion given above.Further research neededUsable data on the critical chloride content including statistical parameters appears to beavailable, in particular for ordinary Portland cement concrete, although the actual distributionat the low but critical end is still open for debate. Remaining questions regard the influence ofblending agents (slag, fly ash, silica fume) and the effect of lower binder contents which aredesirable from the point of view of raw materials and energy conservation. A simple testmethod that can produce good statistical information needs to be designed and tested. Finally,confirmation from field data is needed.ReferencesAndrade, C., et al., section 2.4 in: COST 521, 2003, Corrosion of steel in reinforced concretestructures, Final Report, Eds. R. Cigna, C. Andrade, U. Nürnberger, R. Polder, R. Weydert, E.108

Seitz, European Commission, Directorate-General for Research, EUR 20599, ISBN 92-894-4827-X, 238 pp.Breit W., 2001, Critical corrosion inducing chloride content – State of the art and newinvestigation results, Betontechnische Berichte 1998 - 2000, Verein Deutscher Zementwerkee.V., Verlag Bau+Technik, Düsseldorf, 145-167Everett, L.H., Treadaway, K.W.J., 1980, Deterioration due to corrosion in reinforced concrete –BRE Information IP12/80, Building Research EstablishmentPolder, R.B., Peelen, W.H.A., Bertolini, L., Guerrieri, M., 2002, Corrosion rate of rebars fromlinear polarization resistance and destructive analysis in blended cement concrete afterchloride loading, 15 th International Corrosion Congress, Granada, September 22 – 27Vassie, P.R., 1984, Reinforcement corrosion and the durability of concrete bridges, Proceedingsof the Institution of Civil Engineers, Part 1, Vol. 76, paper 8798, 713109

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Critical chloride content forreinforced concrete and itsrelationship to concreteresistivityRob PolderContents• Ccrit general remarks• Experiments• Discussion• Conclusions2Ccrit and resistivityNTNU/COIN June 20081111

Ccrit general remarks• Service life design modelling; need input variables• Ccrit problematic• Concept, definition• Experimental parameters• Measuring method• Field influences• Concept• Cl < Ccrit negligible corrosion• Cl > Ccrit significant corrosion• Definition• Transition: passive -> active• Unacceptable amount of damage• Allowed by Standards (fresh concrete)3Ccrit and resistivityNTNU/COIN June 2008(un)acceptable amount of damage• Over which time?• Management policy (owner dependent), not materialsscience issue!• Should be based on reliability or risk of damage…• Risk = Cost * probabilityX• Probability useful concept• Corrosion (initiation), load, concrete stochastic• Prob(corrosion) increases with Cl (BRE, Vassie, 1980s),e.g. 0.4 -> 1% (binder)• Ccrit = statistical distribution!• Most interesting: Low Prob!4Ccrit and resistivityNTNU/COIN June 20081122

Allowed by Standards in fresh concrete• Safety margin unknown• 0.4% reinforced, 0.2% prestressed• used to be 0.3 and 0.1%!• Economically negotiated values• Effect mixed in Cl =/= penetrated ClX5Ccrit and resistivityNTNU/COIN June 2008Transition: passive -> active• COST 521 (ao Andrade, Frederiksen, Glass, Page)• Initiation = process (takes time)+• Ccrit: sustained CR > 1 mA/m 2• Measurable.. but• Accuracy moderate > 2 mA/m 2• Exposure (T, H 2 O, O 2 )• Concrete/mortar (cement, w/c, air voids, steel surface)• Potential, cathodic area..• Statistical treatment6Ccrit and resistivityNTNU/COIN June 20081133

Example: Breit 2001• E 0.5 V(H), migrate Cl, Monitor I(t),• I=0 passive• I>0 active, stop experiment• Split specimen, take out steel• Reassemble, drill dust in hole (1 mm), test Cl• Mortar, 11 comp., 5 bindersC W RIProbability1.00.9pN0.8plogN0.7exp Breit0.6Lineair (exp Breit)0.50.40.30.20.10.00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Chloride (% cement)Mean c. 0.5%Cl7Ccrit and resistivityNTNU/COIN June 20080.400.350.300.25pNplogNexp BreitLineair (exp Breit)Probability0.200.150.100.050.000.0 0.1 0.2 0.3 0.4Chloride (% cement)• Type of Distribution!10% value depends on type/fit• Low end! 5..10% probability of corrosion• Mean 0.5%; std 0.2%, logNormal8Ccrit and resistivityNTNU/COIN June 20081144

Experiments TNO• Chloride and corrosion rate (COST 521 project NL-2)• Accelerated penetration, blended cements• Monitor CR(LPR), Esteel, Resistivity; test Cl(x,t)• 1 week fog room; 3 weeks 20C 80%RH• Salt loading/drying: week 5 - 30, 1st chloride profile (n=6/mix)• Three climates: week 31 – 130• Fog room (n=2/mix)• Outside unsheltered (n=2)• 20C 80%RH (n=2)• week 130 2nd chloride profile (n=3/mix*)9Ccrit and resistivityNTNU/COIN June 2008Compositions D max 8 mmX = test destr. 130 weekscementwater-to-cement ratio0.40 0.45 0.55CEM I 32.5 R Portland cement x x xCEM II/B-V 32.5 R Portland fly ashcement, 27% fly ashCEM III/B 42.5N LH HS Blast furnaceslag cement, 75% slagCEM V/A 42.5N Composite cement,25% fly ash & 25% slagcement content (kg/m 3 ) 336 318 283xxxx> Considerable amount of air voids (~practice….)10Ccrit and resistivityNTNU/COIN June 20081155

previous salt/drying loadVR2nd Clprofile1st Clprofilepotentiostat103050Ti* reference300 mm11Ccrit and resistivityNTNU/COIN June 200812Ccrit and resistivityNTNU/COIN June 20081166

2nd Cl profiles at 130 weekschloride(%cement)2.52.01.51.00.5CEM I0.40 20C80%0.40 outside0.40 fog room0.45 20C80%0.45 outside0.45 fog room0.55 20C80%0.55 outside0.55 fog roombar 10 mmbar 30 mm0.00 5 10 15 20 25 30 35 40 45 50depth (mm)13Ccrit and resistivityNTNU/COIN June 2008Results• 130 weeks: Cl(1), CR(3), Res(1); for x = 10, 30 mm• ! Fog room, outside, 20C 80%RH; CEM I, II/B-V, III/B, V/A; w/c• Cl(10/30) 0.1 .. 3%• CR(10/30) 0 .. 30 m/y• All data fit problematic..• CR = 1.9 Cl + 3.0 (R 2 =0.11)• Separate CEM, exposureCR(microm/y)3025201510CEM I..Vmaxmeanmin500.0 0.5 1.0 1.5 2.0 2.5 3.0Chloride (% cement)14Ccrit and resistivityNTNU/COIN June 20081177

CEM I, 20C 80%RH, outside (6 spec., 36 bars)• CR = 6.62 Cl – 0.04 (R 2 =0.83)• Cl = 0% CR = 0 m/y• CR = 1 m/y Cl = 0.16%• CR = 2 m/y Cl = 0.31%• CR = 5 m/y Cl = 0.76%54• needs further statistical treatment!y = 6.62x - 0.04R 2 = 0.83315Ccrit and resistivityCR(microm/y)2mean5&7min5&71max5&7Lineair (mean5&7)00.0 0.2 0.4 0.6 0.8 1.0Chloride (% cement)NTNU/COIN June 2008CEM II/B-V; III/B; V-A; 20C 80%RH, outsideCR(microm/y)2018161412108642CEM III/B-VIII/BV/ALinear (CEM I)Linear (II/B-V)Linear (III/B)Linear (V/A)y = 6.62x - 0.04R 2 = 0.83y = 1.61x + 1.44R 2 = 0.52y = 2.29x + 0.80R 2 = 0.77y = 1.84x + 0.20R 2 = 0.8700.0 0.5 1.0 1.5 2.0 2.5 3.0Chloride (% cement)CEM ICEMII..V16Ccrit and resistivityNTNU/COIN June 20081188

Overview CEM I .. V; Cl(CR) (% binder)CR (m/y)125CEM I0.160.310.76OKCEM II/B-V fly ash0.430.982.6Transition of control?CEM III/B slag0.090.521.8CEM V/Afly ash & slag-0.352.2Bad fit..CEM (II,) III, V: n = low; further study17Ccrit and resistivityNTNU/COIN June 2008Steel potential:if negative; some low, some high CR;if positive: low CRCorrosion rate = f(Cl, wetness; cement)CR (m/year, mean) in 3 climates, wk 52-130w/c0.40 0.45 0.55cementCEM I 1 - 5 2 - 10 5 – 15CEM II/B-V 1 - 5 1 – 5CEM III/B 1 – 6CEM V/A 1 - 518Ccrit and resistivityNTNU/COIN June 20081199

Concrete resistivity2500200050 20/80 50 outside50 fog room 50 fog room50 20/80 50 outsideCEM II/B-V 0.551500resistivity(ohmm)1000500CEM I 0.5500 20 40 60 80 100 120 140time (week)19Ccrit and resistivityNTNU/COIN June 2008Resistivity control?16141220C80%RH, outsideCEM III/B-VIII/BCR(microm/y)1086V/A3450/res *Macht (3450/res *)Cl> Ccrit20Ccrit and resistivityNTNU/COIN June 200812010

Resistivity results "bulk“ 30-130 weekw/cCEM0.400.450.55w/cCEM0.400.450.55III/B-VIII/BV/A1901601200110100016001300III/B-VIII/BV/A5004502200380170020002700Fog roomOutside,20C80%RH21Ccrit and resistivityNTNU/COIN June 2008Ccrit & resistivity, "if any"• Nature of Ccrit?• Not thermodynamic• Kinetic; breaks down passive film• Pit: Cl, pH• Concentration & transport of Cl• Cl transport ~ resistivity• But: also OH transport ~…22Ccrit and resistivityNTNU/COIN June 200812111

Hansson & Sørensen, 1990• E 0.0 V SCE, migrate Cl until I a >> 0.1 mA/m 2• t I , Cl@steel, resistance ()• various binders, w/c, curing• +fly ash: high resistivity, same Ccrit (t ilonger..)• w/c effect; Ccrit ~ resistivity !• Curing effect; Ccrit not ~,,0.20• -> ?????0.180.16• origin of resistivity..0.14Ccrit/XRF(%mortar)0.120.100.080.060.040.02Danish OPCcuringw/c0.000 50 100 150 200 250 300 350 400 450resistance (ohm)23Ccrit and resistivityNTNU/COIN June 2008Initiation modelCRm/yCl, pHinitiationResistivity!passiveactiveCl24Ccrit and resistivityNTNU/COIN June 200812212

Conclusions• Service life design: requires Ccrit• Need distribution• Target probability 1 .. 5 .. 10%: low end!!• Need clear relationship• Concrete composition• Exposure• Execution (compaction, curing)• Methods: Est X; CR useful• Resistivity decreases• w/c 0.40 > 0.45 > 0.55 influence on Ccrit!!• slag>composite>fly ash>Portland cement (?)• 20C80%RH > outside > fog room (?)• Anyone for further study of old results?25Ccrit and resistivityNTNU/COIN June 200812313

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COIN Workshop on “critical chloride content” in reinforced concrete8 Critical considerations regarding laboratory research“Critical considerations regarding laboratory research onthe critical chloride content in reinforced concrete”Joost GulikersRijkswaterstaat Bouwdiens125

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COIN Workshop on “Critical chloride content in reinforced concrete”, Trondheim, 5-6 June 2008CRITICAL CONSIDERATIONS REGARDING LABORATORY RESEARCH ONTHE CRITICAL CHLORIDE CONTENT IN REINFORCED CONCRETEJoost GulikersRijkswaterstaat BouwdienstP.O. Box 200003502 LA Utrecht, The Netherlandsjoost.gulikers@rws.nlIntroductionThere is a general acceptance that the so-called critical chloride content, C crit , is one of the keyparameters to estimate service life for new as well as existing reinforced concrete structuresexposed to chloride-laden environments. Consequently a strong practical need emerges tohave a better understanding of the mechanisms involved in chloride-induced corrosion as wellas to obtain more reliable values for C crit . This renewed attention for the critical chloridecontent has resulted in a number of initiatives to perform laboratory investigations on ascientific level. However, the experience with previous laboratory research activities indicatesthat the test set-up does not reflect realistic conditions as commonly observed in practice. Inview of that the practical significance of the outcome of such a laboratory research can bequestioned.Laboratory practice versus site conditionsTime constraintsOne of the major boundary conditions in laboratory research is that results should be availablewithin a timeframe of less than 3 years. Consequently, a number of concessions is made as toachieve this. Most often chlorides are introduced in the fresh concrete mix in contrast torealcrete where chloride ions have to diffuse though to the pore system of the concrete coverfor a period of several decades before they reach the level of the reinforcing steel. This has 4significant implications:‣ for corrosion initiation the so-called free chloride content is of importance. The use ofmixed-in chorides will result in a higher amount of chlorides to be chemically boundby the cement hydration products. Thus a lower free chloride content will beencountered in the pore solution resulting in a higher critical chloride content.‣ generally the chlorides are introduced in the mixing water and this results in a uniformchloride distribution through the concrete material. However, in realcrete a nonuniformchloride distribution will prevail and this provides a more favourablecondition for inducing potential differences along the steel surface which is one of themajor reasons for corrosion-initiation.‣ the use of dissolved chloride in the mixing water accelerates the hydration processresulting in a coarser pore structure as compared to realcrete without admixedchlorides.‣ most often chlorides are dissolved in such an amount as to allow for initiation of steelcorrosion from the very beginning, i.e. even before the concrete has hardened. Thereinforcement steel embedded in realcrete is given sufficient time to passivate and will127

emain in this passive condition during many decades. In contrast to labcrete apassivating layer has to be attacked in real structures.Geometrical constraintsFor practical reasons it is common in laboratory research to employ small size specimenshaving a relatively shallow cover and containing a single rebar. Chloride-induced corrosion isstrongly localized and is thus accompanied by the formation of macro-corrosion cells. Theshort length of the embedded rebar may hinder the depassivation process as only a relativelysmall cathodic surface area is available to support anodic dissolution. For corrosion to occurpotential differences should be promoted to occur along the steel surface. However, inlaboratory research the concrete cover will be of uniform thickness and the steel surfacethoroughly cleaned, sandblasted and degreased. These conditions are not likely to give rise todifferential electrochemical behaviour. In addition, mortars rather than concrete is used inresearch. The presence of large aggregate particles in the concrete cover will result in a spatialvariation of chloride, water and oxygen transport. Thus, some parts of the steel surface maybe effectively shielded against penetration of deleterious substances, whereas in mortars thetransport properties will be more homogeneous.Structural constraintsIn real structures inadequate compaction and curing may have resulted in spatial differencesin concrete quality, accompanied by surface cracking, gravel pockets and bleeding water. Inaddition, after loading, the steel reinforcement is under tension giving rise to flexural cracksextending from the exposed concrete surface to the embedded rebars. For higher stress levelsthe bond between concrete and steel may be severely compromised. As initiation of corrosionis strongly dependent on the quality of the steel-concrete interface, the critical chloridecontent in the flexural zone of a structure could be completely different from that in thecompressive zone. Thus it would also be likely that in prestressed concrete structures in theabsence of major cracking, the critical chloride content could be significantly higher.Concluding remarksMany factors may impact the eventual level of the critical chloride content in real structures.In this respect laboratory research may not be adequate to assess the real level of C crit in viewof the time, geometrical and structural constraints outlined. In effect, additional factors thatare not discussed here, may play an important role. In order to achieve results from laboratoryresearch that are of practical relevance for real structures, the test set-up, specimen geometry,and exposure conditions should reflect outdoor conditions. In this respect long-term researchusing exposure sites in the vicinity of real structures could be of great help.128

COIN Workshop on “critical chloride content” in reinforced concrete9 Critical chloride content and service life predictions“Critical chloride content and its influence on service lifepredictions”Gro MarkesetSINTEF129

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COIN Workshop on Critical Chloride Content in Reinforced Concrete, 5-6 June 2008CRITICAL CHLORIDE CONTENT AND ITS INFLUENCE ON SERVICE LIFEPREDICTIONSGro Markeset, Dr. ing.Project Manager, COIN - Concrete Innovation CentreSINTEF, NorwayINTRODUCTIONIn the design of new concrete structures exposed to chloride induced corrosion the service lifepredictions are commonly based on the time to onset of corrosion. When the chloride contentreaches a critical level at the depth of the reinforcement, the reinforcing steel becomes depassivated,and corrosion starts.In service life modelling the critical chloride content (or the chloride threshold level) is requiredas input parameter. However, there is a lack of reliable data for this parameter, especially fromfield exposure of real structuresThe wide scatter in observed chloride threshold values may, in addition to material and environmentalfactors, also be attributed to different measuring techniques and definitions of the thresholditself. For instance, some relate depassivation of the reinforcement to a certain shift in the corrosionpotential, other use visual inspection of the reinforcement after chiselling, and finally, otherrelate depassivation with a certain level of the corrosion current.MEASURING CRITICAL CHLORIDE CONTENT IN REAL STRUCTUREInstalling corrosion sensors in existing structures exposed to chloride environments is one practicalmeans of determining the chloride threshold value in an actual structure, see Figure 1. The basicmeasuring principle is to place steel electrodes into different depths related to the concrete surfaceand to measure the onset of corrosion of these electrodes one by one. The measuringelectrodes are made of steel with a similar composition as reinforcing steel to ensure that they willstart to corrode at the same time as a rebar at the same depth.Figure 1: Basic principle to determine the time-to-corrosion (to the left) /1/. Mounting of corrosionsensors in existing quay (to the right).The small titanium pin as cathode placed just above the instrument131

COIN Workshop on Critical Chloride Content in Reinforced Concrete, 5-6 June 2008The corrosion monitoring system shows then directly the depth of the critical chloride content.The actual values of the critical chloride concentrations can be determined by comparing the chlorideprofile of the cores (drilled out for the placement of the sensors) with the level of corrosioncurrents for each sensor ring measured shortly after installation.Such measurements have been performed on a 37 years old quay in the northern part of Norway/2/, based on 14 corrosion sensors installed above the tidal zone. The quay was made of a coarsegrainedPortland cement. Based on petrographical analyses of the concrete the water/cement-ratiowas found to vary between 0.40 and 0.50.Due to the uncertainties in the measurements and variations among the corrosion sensors the criticalchloride content is considered in terms of probability or risk of corrosion. The critical chloridecontents obtained for the Norwegian quay are compared with Browne’s classification of corrosionrisk, see Table 1. A lognormal distribution is found to give the best fit to the measurements, givinga mean value for critical chloride content of 0.77 % weight of cement and a coefficient ofvariation equal to 32%, See Figure 2. It should be kept in mind that the corrosion sensors measurethe time to depassivation (or at a very low level of corrosion current) and may therefore result inlower values than if the critical chloride content was based on visual inspection of the reinforcementafter chiselling and identification of rust and corrosion. This might be the background for thehigh values suggested by Browne. In fib “Model Code for Service Life Design” /3/ a betadistributionwith mean value of 0.6 by % weight of cement is recommended, which is somewhatlower than found for the investigated quay, see Figure 2.Table 1: Critical chloride content (% weight of cement)Probability/risk ofcorrosionCritical Chloride ContentAccording to Browne /4/ Norwegian quay /5/ * )Negligible 1.3* assuming 350 kg cement/m 3 and density 2350 kg/m 3100 %80 %Probability of corrosion60 %40 %Model20 %MeasurementsFib Model Code0 %- 0,50 1,00 1,50 2,00 2,50 3,00Critical chloride content [% w eight of cement]Figure 2: Critical chloride content: field data from a Norwegian quay and statistical model for thesedata /5/ compared with statistical model according to fib Model Code for Service Life Design /3/132

COIN Workshop on Critical Chloride Content in Reinforced Concrete, 5-6 June 2008SERVICE LIFE PREDICTIONSIn probabilistic service life predictions of concrete structures exposed to marine environment, thecritical chloride content is modelled as a stochastic variable characterized by a mean value, astandard deviation and a type of probability density function. The limit state function is defined asthe difference between the critical chloride concentration and the calculated chloride concentrationat the reinforcement:( x t)g( X ) = Ccr − C ,where X is a vector of the statistical parameters as diffusion coefficient, surface chloride concentration,concrete cover etc.Probabilistic calculations of required concrete cover for design service lives of 50 and 100 years,respectively, are performed. The chloride ingress is calculated based on Ficks’ 2. law of diffusion,including the time dependency of the diffusion coefficient. The results are presented in Figure 3.As seen, the lognormal distribution gives approximately the same results as using a characteristicvalue of C cr = 0.67 % by weight of cement. To illustrate the sensitivity of C cr a conservative valueof 0.34 is also included. For an acceptance limit (probability of corrosion initiation) of 10 % thecover thickness needed to obtain 50 years service life becomes 73 mm and 92 mm for C cr =0.67and 0.34, respectively. For a 100 year design service life the corresponding cover thickness becomes90 mm and 114 mm respectively.The sensitivity of the critical chloride content, C cr , is clearly illustrated by the following observations:If a characteristic value C cr of 0.67 is used, a concrete cover of about 90 mm results in a designservice life of 100 years. However, if C cr is reduced to 0.34 the service life is reduced to 50years!Figure 3: Effect of C cr on required concrete cover for design service lives of 50 and 100 years, respectively(C cr given in % weight of cement)REFERENCES1. Schiessl, P. and Raupach, M. (1992):“Monitoring system for the corrosion risk of steel inconcrete structures”, Concrete International V. 14, No. 7, July 1992, pp 52-552. Markeset, G.(2000):“Service Life Predictions of Marine Concrete Structures – Consequencesof Uncertainties in Model parameters”; Proceedings, Nordic Mini Seminar: "Prediction Modelsfor Chloride Ingress and Corrosion Initiation in Concrete Structures", Chalmers Institute ofTechnology, Gothenburg, Sweden, Publikation P-01:63. fib (2006):“Model Code for Service Life Design”, Bulletin 344. Browne, R.D.(1982):“Design prediction of the life of reinforced concrete in marine and otherchloride environments”, Durability of Building Materials, Vol. 3, Elsevier Publishing5. Larssen, R.M.(2001):”Parametervariasjon i levetidsmodeller for marine betongkonstruksjoner”(in Norwegian), Dr.Ing. A.Aas-Jakobsen AS, Oslo, Norway.133

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Critical chloride content and itsinfluence of service life predictionsGro Markeset, Dr. ing.Project Manager, COINDeterioration and service lifeThreshold valueTimeDeteriorationNew structuresTuutti’s two phase modelInitiation phasePropagation phaseCOCO22-Cl-ClOO22H H 2O2O1351

Chloride induced corrosion• Initiation phase:• modelling of chloride ingress• Propagation phase:• modelling of concrete/steel deteriorationOnset ofcorrosionTimeCrackingDeteriorationDelaminationCritical limitBond failureCollapseInitiation phasePropagation phaseDefinition of critical chloride content (C cr )• C cr can be defined in two different ways:1. Chloride content required for depassivation of thereinforcement surface2. Chloride content associated with some acceptabledeterioration• From “fib Model Code for Service Life Design, 2006”:• “The total chloride content which leads to the depassivation ofthe reinforcement surface and initiation of iron dissolution,irrespective of whether it leads to visible corrosion damage onthe concrete surface”.1362

Experience from real structures• Observed C cr in real structures are often based on visualinspection of the reinforcement after chiselling, incombination with determined chloride content• The evaluation of rebar corrosion are classified incorrosion levels:0.40Bridge investigationsChloride content (mean values) & corrosion level A-ECl - in % of concrete mass0.300.200.10(approximately 300 inspections)Ref: NPRAGimsøystraumenOther coastal bridgesCABDE0.00No corrosionStartdepassivationCorrosion Heavy corrosion Heavy corrosion& pittingExample from Gimsøystraumen bridge:B: C crit= 0.07 % Cl - of concrete mass, variation coeff. 98%C: C crit= 0.12 % Cl - of concrete mass, variation coeff. 78%Chloride content for onset of corrosion – is it B or C or?1373

Investigation of C cr in existing structureusing corrosion sensors• Procedure1. Installation of corrosion sensors in existingstructure to determine the depth of the criticalchloride content2. Measurement of chloride profile from the coresdrilled out where sensors have been installed3. Critical chloride content obtained by comparingthe chloride profiles with the sensor reading atthe depth of depassivationCorrosion sensors type “Expansion-Ring-AnodeSystem” for non-submerged conditions Ref: S+R Sensortec• Consists of six measuring rings in different distances from theconcrete surface, in 1 cm -steps from 1 to 6 cm.• Ingress of chlorides and/or carbonation from the concrete surfaceinto the concrete and the subsequent corrosion risk of thereinforcement can be measured• As long as the critical chloride content have not reached the surfaceof the outer anode, all electrical currents are low.• Limit values for activity (exceeding depassivation) of the singleanodes under usual conditions:• Voltage between anode and cathode of about 400 mV• Electrical current of about 10 µA1384

Principle for the determination of C crDepth below surface, mm0,000,00 20,00 40,00 60,00 80,00 100,00 120,00 140,0,100,20C cr0,300,400,500,60Measuring of C cr in a Norwegian quay• 14 corrosion sensors installed in a 37 years old concretequay in the Northern part of Norway-• Concrete properties• coarse-grained Portland cement• water/cement-ratio varying between 0.40 and 0.50.• Chloride profiles determined on 50 mm cores extracted fromthe drilled holes for sensor installation1395

Installation of the corrosion sensorsConclusions from the corrosionsensor measurements• The cored hole in which the sensor shall be placed should beextremely precise in alignment and in free diameter. A tolerance indiameter of +/- 0.5mm was specified.• the robustness and the general installation tolerances of thesystem should be increased• Recordings from apparently undamaged sensors have shownconsiderable variations in individual readings seemingly withoutcorrelation between current flow, voltage and electric resistivity.• data interpretation needs additional user-oriented developmentand guidance1406

Determination of C crDue to the uncertainties and variations in themeasurements the critical chloride content is considered interms of probability or risk of corrosion.Table: Results compared to Browne’s corrosion risk classificationProbability/riskof corrosionNegligiblePossibleProbableCertainNorwegian Quay1.3Browne2.0Statistical modelling of C cr100 %80 %Probability of corrosion60 %40 %Ccr: LN (0.77, 32%)Model20 %MeasurementsFib Model Code0 %- 0,50 1,00 1,50 2,00 2,50 3,00Critical chloride content [% weight of cement]Example from Gimsøystraumen bridge in Norway:B: Mean C crit= 0.46 % Cl - by weight of binder, variation coeff. 98%C: Mean C crit= 0.76 % Cl - by weight of binder, variation coeff. 78%1417

C( x,t)Service life predictionsModelling of chloride ingress by Fick’s 2. law of diffusion⎛ ⎡= C ⎜s1−erf⎜⎢⎝ ⎢⎣2x ⎤⎞⎥⎟D(t)⋅t⎥⎟⎦⎠D(t)= Dconstanttime dependantwhere t: Exposure periodx: Distance from exposed surfaceC s: Chloride surface concentrationD(t): Chloride diffusion coefficient at time tD:oChloride diffusion coefficient determined at t=t 0t 0: Reference periodα : Age factorCorrosion initiation:C( c,t ) ≥ Ccrc: Concrete coverC cr: Critical chloride content (threshold value)0⎛ tD(t)= D00 ⎜⎝ tα⎞⎟⎠diffusiondiffusionFick’s 2. law of diffusionDeterministic vs. probabilistic• Deterministic analyses:• Parameters given as unique values, e.g. the meanvalue.• Each set of parameters give a service life for a givenaccept limit of the chloride concentration• Probabilistic analyses:• Variation of parameters described by mean values andstandard deviations and distributions.• Each set of parameters give a service life and aprobability for exceeding the given accept limit of thechloride concentrationg(X)= Ccr−( )C x,t1428

Surface chloride concentration obtainedon Norwegian quay slabsSurface chloride concentration(% of concrete mass)1,201,000,800,600,400,200,000,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 2,2Height above highest high water level (m)Service life calculations – An Exampleinput parameters and distributions:• Chloride concentration on concrete surface, C sC s= LN(2.67, 60 %) (% of cement mass)• Cover thickness, xx = LN(x, 10) [mm]• Chloride diffusion coefficient, D 0D 0= LN(7.0 10 -12 , 30%) [m 2 /s]• Ageing factor, αα = N(0.4, 15 %)• Critical chloride concentration, C crC cr: LN (0.77, 32%)Characteristic values: C cr= 0.34 and 0.67 (% of cement mass)• Acceptance criterion for corrosion initiation:10 % probability of corrosion (SLS)1439

Reliability based service life predictions- Example: design life 50 yearsEffect of Ccr (% of weight of cement)Mean value20mm difference inconcrete cover10% probability ofcorrosionReliability based service life predictions- Example: design life 100 yearsEffect of Ccr (% of weight of cement)50 years SL for C cr0.3410% probability ofcorrosion14410

Concluding remarks• The calculated service life are very sensitivity to the criticalchloride content as illustrated;• for the same cover thickness, the service life is 50 years for acharacteristic value of Ccr = 0.34% and 100 years for Ccr=0.67%(by weigth of cement).• The definition of Ccr is decisive in order to know how muchof the “propagation phase” that are included in the limitstate named “onset of corrosion”.C cr is a parameter needing much more attention thanrecognized in the service life predictions!14511

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COIN Workshop on “critical chloride content” in reinforced concrete10 A field study of chritical chloride content“A field study of critical chloride content in reinforcedconcrete with blended binder”Luping TangCBI and Chalmers University of Technology147

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A Contribution to COIN Workshop, 5-6 June 2008, Trondheim 1A Field Study of Critical Chloride Content in Reinforced Concrete with Blended BinderTANG, Luping 1,2 and UTGENANNT, Peter 11CBI Swedish Cement and Concrete Research Institute, Borås, Sweden2 Chalmers University of Technology, Gothenburg, SwedenExtended AbstractIn the beginning of 1990’s over 40 reinforced concrete slabs with different types of binder andwater-binder ratios were exposed in a marine environment at Swedish west coast. After over13 years’ exposure, the conditions of steel bars in the concrete slabs were investigated usingboth non-destructive and destructive methods. This paper presents the results from theinvestigation.The field site is situated in the Träslövsläge harbour in the south western part of Sweden,80 km south of Gothenburg. At the field site the average annual temperature in the air is about11 °C and the average chloride concentration in the seawater is about 14 g/l. The testspecimens are slabs (height 100 cm, width 70 cm, thickness 10-20 cm). Three steel bars, onesmooth stainless and two ribbed carbon steel, were embedded in each slab with coversbetween 10-30 mm in most cases and 35-55 mm in some cases. Half of the slab was exposedunder the seawater and another half to the air.A newly developed rapid technique, RapiCor [1,2], was used for non-destructivemeasurement of corrosion. Based on the results from the non-destructive measurement, theactual corrosion of steel bars in five concrete slabs was visually examined and the chlorideprofiles in the penetrating direction as well as at the cover level were measured. Theinformation about these five slabs is listed in Table 1.Table 1: Concrete slabs taken from Träslövsläge for the laboratory investigationBinder typeBinderkg/m 3WaterbinderratioAircontent%Compr.strengthMPa, 28dRebardiam.mmCovermmBar 1/2100%Anl 450 0.35 6.0 70 20 20/1590%Anl+10%SF 420 0.40 6.6 65 12 15/15100%Anl 420 0.40 2.1 79 12 10/1578.5%DK+17%FA+ 4.5%SF 420 0.40 6.1 69 12 15/1080%Anl+20%FA 616 0.30 3.0 98 12 15/10These five slabs were taken from the field site to the laboratory, where the non-destructivecorrosion measurements were carried out again in a small grid in order to identify the activecorrosion positions. Afterwards a groove along each rebar was cut from the back of the slab,so as to facilitate the release of rebar for visual examination. After release of rebar, concretesamples were taken at the depth where the rebar was embedded and at the positions where thecorrosion was measured by the RapiCor technique. The acid soluble chloride and calciumcontent in each sample were determined by titration.Based on the visual examination of the steel bars from the five selected slabs, it has beenverified that the non-destructive RapiCor technique gives satisfactory estimation of corrosion.Therefore, this technique was used for non-destructive measurement on the rest of slabsexposed in the Träslövsläge field site.149

A Contribution to COIN Workshop, 5-6 June 2008, Trondheim 2Relationships between corrosion rate and chloride content are shown in Figure 1. From theresults it can be seen that, the criterion for actual high corrosion (10 μm/year) are alwaysrelated to chloride content about 1% by mass of binder, despite of the types of binder. It isunderstandable that high chloride content may not necessarily mean high corrosion rate, buthigh corrosion rate should be caused by higher chloride content with regard to chlorideinducedcorrosion.33Cl, mass% of binder21PCBlandedCl, mass% of binder21Little corrosion markPCBlanded00.1 1 10 100Corrosion Rate [μm/yr]01 10 100Corrosion Rate [μm/yr]aFigure 1: Relationships between corrosion parameters and chloride contentexposed in the field, a – all the data, and b – data with active corrosionmeasured by the RapiCor and observed by visual examination.Since the corrosion measurement is for the instantaneous status, the actual initiation ofcorrosion is unknown from this study. Therefore, the chloride level 1% by mass of binder maynot be the same as the conventionally defined threshold value, but can be taken as the criticallevel for significant on-going corrosion that is visible by destructive visual examination.In the literature, it is believed that critical chloride content in the concrete with blendedbinder should be lower than that in the OPC concrete due to lower alkalinity in the poresolution with blended binder [3,4]. The results from this field study do not show thistendency. Since the pore structures in the concrete with blended binder are normally finer thanthose in the OPC concrete, these finer pore structures may prevent the reinforcement fromdepassivation. Therefore, the factor of pore structures should also be taken into account whendefining “critical chloride content” for initiation of reinforcement corrosion in concrete.References[1] Tang L., “Mapping Corrosion of Steel in Reinforced Concrete Structures”, SP RAPPORT2002:32, SP Sveriges Provnings- och Forskningsinstitut, Borås, 2002.[2] Tang L. and Fu Y., “A rapid technique using handheld instrument for mapping corrosionof steel in reinforced concrete”, Restoration of Buildings and Monuments, 12(5-6) (2006)387-400.[3] Glass, G.K. & Buenfeld, N.R., “Chloride threshold levels for corrosion induceddeterioration of steel in concrete”, in “Chloride Penetration into Concrete”, ed. by L.-O.Nilsson et al, pp 429-440, 1995.[4] Frederiksen, J.M., “Chloride threshold values for service life design”, in “Testing andModelling the Chloride Ingress into Concrete”, ed. C. Andrade and J. Kropp, pp. 397-414,2000.b150

COIN Workshop on “critical chloride content” in reinforced concrete11 Reference material for calibrating chloride analysis“Reference material for calibrating chloride analysis ofhardened concrete”John B. MillerMillab Consult A.S.151

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COIN Workshop on “Critical chloride content in reinforced concrete”, Trondheim, 5-6 June 20081. IntroductionReference Material for Calibrating Chloride Analysis of Hardened ConcreteJohn B. Miller, (Millab Consult A.S., Oslo, Norway)When analysing, it is important to calibrate against relevant materials containing known amounts of thesubstance to be determined, otherwise it is difficult to know whether a particular analytical method isreturning correct results. In simple cases, standardised solutions containing precisely weighed-in amounts ofpure substances can be used for, for example, titration calibration. In more difficult cases, such as that ofdetermining chloride content in concrete, the task of producing a reference material for calibration is muchmore complex and onerous. The creation of concrete reference material containing precisely known amountof chloride, though simple in principle, is in fact a demanding and expensive operation.Reference materials with two different water contents and 6 different chloride contents, have been produced[1,2], and have been used to conduct two, previously published, independent Round-Robin tests, the resultsof which demonstrated very clearly the large discrepancies that can occur between methods and laboratories,when analysing identical material. In these tests, results deviating by more than 100% from the true valueswere returned [3]. Other parties have found similar results [5].2. Preliminary ConsiderationsThe analysis of hardened concrete for total chloride content entails three main operations, assuming thatrepresentative concrete samples have been obtained in the first place. These operations are:- the reduction of the material to a fineness that allows easy dissolution- the dissolution of the material in acid- the analysis properIn the laboratory, hardened concrete can be reduced using crushers, grinders and mills to a fineness that isconvenient for dissolution in acids, thus freeing the chloride to solution. In the field, the majority of samplesare taken by the collection of the tailings produced by 16 to 22 mm hammer drilling, which produces aparticulate material that is much coarser than laboratory reduced material, and where complete aciddissolution needs greater care. With both types of material, it is essential that the chloride extraction iscomplete, and thus the grain size distribution of the calibration material must resemble that of samplescommonly taken by hammer drilling.Many analytical methods exist, ranging from common volumetric or potentiometric titration, to methodsusing proprietary equipment such as Quantab, RCD, and RCT. It is the author’s contention that the actualmethod used is of little or no importance providing correct results are returned for known material.3. Production of precisely known reference material.In order to produce materials that are dependable, the following factors need to be controlled and/or known:• Chloride content of raw materials (water, sand, gravel, cement and any additives used)• Content of the raw materials (sand, gravel, cement)• Cleanliness of the equipment and machinery used for mixing• Exact weights of weighed in materials and amount of mixing water• Addition of precisely known amounts of chloride to produce a range of chloride contents• Proper hardening of the concrete bodies produced• Drying of the bodies to constant weight• Crushing and grinding of the concrete to the correct grain-size distribution• Homogenisation of the reduced material• Hermetically packing the reduced materials in suitable amounts (10 g per package)153

• Withdrawal of sufficient numbers of randomly selected sample packs for chloride analysis• Statistical treatment of the results in order to estimate standard deviation and deviation from themean of the chloride contents of the sample packs.• Production of enough material to supply the world community for upwards of 30 yearsSince samples are commonly taken in the field by hammer drilling, the first step was to collect sufficientmaterial from various types of hammer drill to allow grain size distribution curves to be made. These curveswere then used to examine crushing and grinding methods that could produce material having a distributionclose to the average of the drilled materials. This is important, especially when using commercial methodssuch as the Rapid Chloride Test (RCT) or Rapid Chloride Determination (RCD), since the extractiontechnique must be able to completely disband cemented particles. To find such a method entails sendingblocks of concrete to various laboratories to identify one that could produce such material. We found onesuch laboratory – Taylor Engineering Laboratory, London, UK.3.1 Preparation of mixing equipment and constituent materials.Thus ensured that it was possible to produce material with the required grain size, the next step was toproduce suitable batches of concrete. To do this, all materials were first homogenised in excess of requiredquantities, and samples removed by quartering for chloride analysis using classic potentiometric titration ofacid extracts. The materials, including the cement, were then dried to constant weight at 105 o C, andthereafter kept in airtight containers until use.To avoid contamination, all mixing equipment was either carefully cleaned of all previous residues, or newequipment was used depending on the cost of the needed items. Water used for final rinsing, and for mixingthe concrete batches, was distilled water.3.2 PrecautionsDuring crushing and grinding, any dust produced had to be collected and returned to the material since it wasnot known whether the dust contains more or less chloride than the main material. This required someadaptation of the equipment used.To protect the material from weight changes that could result from moisture exposure or carbonation, it wasnecessary to pack the material in hermetically sealed containers. We used the same type of sachet as used forpackaging shampoos, liquid soaps, wet tissues and the like, such as are found in hotel rooms or used byairlines. These are made from plastic coated aluminium foil, and are sealed by welding. The packing machineused had equipment for collecting dust released at the hopper and doser, and this was later analysed forchloride enrichment, since it was feared this could disturb the chloride content of the material being packed.However, no such enrichment was found.3.3 Statistical sampling and determination of standard deviations from true chloride contentDuring packing, the packages issuing from the machine were sampled statistically in accordance with arandom number generator. The contents of these sachets were then analysed and the results used to calculatethe standard deviation of the various chloride contents of each of the 12 batches of concrete [4]. The resultscompiled in the table show that the produced material is entirely suitable for the calibration of analyticalmethods of chloride analysis in hardened.4. Nordic and Dutch Round Robin TestsThe reference material was used in a previously reported Round Robin (3) test to evaluate three chlorideanalysis techniques commonly used in Norway for accuracy, precision and reproducibility. This programmewas funded by NORTEST, a Nordic body within the field of technical testing and measurements, and fiveNordic laboratories participated. The chloride analysis techniques selected were the Quantab Test, ChlorideSelective Electrode Potentiometry and Volhard Titration. The reported results were summarised and discussed foreach of the chloride analysis techniques separately, and are presented in tables showing the mean values of thefour parallels, the standard deviations of the means and deviations in % of the nominal values. Deviations werevery large for all 4 methods.154

Conflicting results of chloride analysis have also been experienced by the Civil Engineering Division of theMinistry of Transport, Public Works and Water Management in Holland. The Civil Engineering Division,therefore, had concrete reference material, with both Portland cement and blast furnace slag cement, produced inorder to examine the reliability of chloride analysis [5]. A total of 17 Dutch laboratories received nine sachets ofreference concrete samples and were asked to perform chloride analysis in duplicate, according to their standardprocedure, and to report the results as chloride content by weight of cement. None of the laboratories knew thatthe results were to be used in a Round Robin test. Ten of the laboratories used Volhard Titration, one used MohrTitration, while Chloride Selective Electrode Potentiometry, Potentiometric Titration and Spectrophotometrywere used by two laboratories. The results of this test also showed unacceptably large variations, thus confirmingthe necessity of calibration against known material.Reference materials, their chloride contents & standard deviationsTarget Cl content w/c True Cl content in % w/w of Standard Added asConcrete Cement deviation**0.0000 0.58 0.006 0.051 1.5 x 10 -3 NaCl0.0250 0.58 0.028 0.236 1.6 x 10 -3 NaCl0.0500 0.58 0.056 0.472 1.9 x 10 -3 NaCl0.1000 0.58 0.097 0.818 4.5 x 10 -3 NaCl0.2000 0.58 0.192 1.619 4.4 x 10 -3 NaCl0.5000 0.58 0.468 3.947 8.3 x 10 -3 NaCl0.1000 0.58 0.097 0.818 2.3 x 10 -3 CaCl 20.0000 0.45 0.004 0.034 7.2 x 10 -4 NaCl0.0250 0.45 0.028 0.236 1.6 x 10 -3 NaCl0.0500 0.45 0.050 0.422 1.0 x 10 -4 NaCl0.1000 0.45 0.094 0.793 1.7 x 10 -3 NaCl0.2000 0.45 0.187 1.577 2.4 x 10 -3 NaCl0.5000 0.45 0.451 3.803 3.9 x 10 -3 NaCl0.1000 0.45 0.095 0.801 1.8 x 10 -3 CaCl 2** Of chloride content in sachetReferences1. Gran H.C and Farstad, T: "Production of reference concrete with known chloride contents" Project no. 02877,Norwegian Building Research Institute, Oslo, Norway, 1990.2. NCT documentation package on the production of standardised concrete dusts. (Certificate of Tests on:Concrete Crushing Analysis, Chloride Contents, Moisture Content of Ground Concrete Dust Sample andAnalysis of Sample Packets for the Effect on Heating); Norwegian Concrete Technologies, Oslo, Norway,1992.3. Nustad, Guri E. “Experience on accuracy of chloride and sodium analysis of hardened concrete”Proc. Eurocorr ’97. European Federation of Corrosion, Event NO 208, Trondheim, Norway, September1997.4. Gran, H. C. "Measurement of chlorides in concrete - an evaluation of three different analysis techniques" NBIProject Report 110 -1992, Oslo, Norway 1992.5. Gulikers, J. “Reliability of chloride analysis in determining the corrosivity of concrete”, Workshop US-Europe on Bridge Engineering: Evaluation, management and repair, Barcelona, Spain, 15-17 July 1996.155

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COIN Workshop on “critical chloride content” in reinforced concrete12 On the need for more precise thresholds values“On the need for more precise threshold values for chlorideinitiated corrosion in design, repair and maintenance ofreinforced concrete structures – a consultant’s view”Jens Mejer FredriksenALECTIA A/S157

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On the need for more precise threshold valuesfor chloride initiated corrosionThe thresholdto the NTNUCorrosionlaboratory!in designrepairmaintenanceof reinforced concrete structures- a consultant's viewJens Mejer Frederiksenjmf@alectia.com, ALECTIA A/S, Denmark”Service life prediction”is in many cases clo sely related to”Prediction of time to corrosion initiation”MosquéeThe GreatHassan II,Belt LinkCasablancaConstructed (Constructed:1987-1993) 1993)1987-19981998No requirements!RequirementRepaired (!!):100 years2005-20082008Mosquée Hassan II, Casablanca, MoroccoNordNew PortTangerMed. IConstructed2004-20072007Requirement100 yearsxx159

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Confinement at its last stage – demolition started”Service life prediction”Preparation for casting while demolition proceedsWhat do we really want?A widely accepted and strictly logicalmodel/system to take us throughdurability design for concretestructures in saline environments”Service life design etc.”Including guidelines for:‣ Design‣ Specifications‣ Tender documents‣ Contract‣ Concrete manufacturing‣ Construction and concreting‣ Commissioning…and may be even:‣ Operation‣ Maintenance‣ Repair…we do however not have sucha model or system today!What we have is a part of a frameworkIt is neither strictly logical nor widelyacceptedIt does not give us clear information onhow to make specificationsIt is based on both short time experienceand on expert opinions!161

…but we might be able to reachour goal in a (4-8 years) timeService life definitionsWhen we have a widelyaccepted ingress model we willbe focusing on the soft spotsWe already know – and we haveknown it for many years – thatwe need to understand thecorrosion initiation processIf we do not understand it wecan not make sensible servicepredictions and hence we willnot reach our goal.Structural integrityInitiation phaseAcceptable level of deteriorationService lifePropagation phaseTimeService life– or technical service lifeAlmost anything can bemaintained to have a service lifeof 100 years or moreIt is almost just a matter ofmaintenance and costsSometimes maintenance ishowever so costly and annoyingthat we rather demolish andreconstructThis is why we are asked by theowners to predict service life!”Service life prediction”- many interconected disciplinesActualenvironmentObservationsExperiencesInputparametersConcreteObservationsChlorideingress model(mathematics)Quality testingAcceptancecriteriaThresholdvalues forinitiationDefinition ofservice lifeProbabilismSafety/reliabilityChloride ingressChloride penetrates throughthe concrete surface andbuild up a profile in theconcrete cover with acharacteristic shape The chloride profile at anytime can be fitted by theerror function solution toFick’s 2 nd law the describedby the parameters C s and D:C(x,t) CsChlorideconcentration ( Cs Ci)erf 2C s(surfaceconcentration)D(ingress rate(curvature))Depthx t D Chloride profileChloride concentration,%mass binder121086420SUB 25 yHigh thresholdLow thresholdIntermediate0 10 20 30 40 50 60 70Depth below exposed surface, mmThe chlorideprofile is usedto evaluatethe risk forreinforcementcorrosionThe thresholdvalue is animportantparameter inthis evaluationThe value stillhas to bechosen byestimates162

Chloride ingressIngress of the position ofthe critical chlorideconcentration, x cr can beestimated by:C s , C cr and D are parametersin the penetrat ionparameter K:K 2D erfx cr K1t Cs C Cs CcriThe parametersThe D valueWe cancontrol thevalue of D bychoice ofconcretecompositionTheenvironmentinfluences thevalue of DThe C s valueTheenvironmentcontrols thevalue of C sWe caninfluence the C svalue by choiceof concretecompositionThe C cr valueWe can controlthe C cr valueby choice ofconcretecompositionTheenvironmentinfluences thevalue of C crI.e.: We can control both D and C cr !Measured chloride profiles, TräslövslägeChloride concentration,% mass binderD a65432100.67 y; Da=38.8 mm²/y ; Csa= 1.4 %2.07 y; Da=8.1 mm²/y ; Csa= 1.3 %5.23 y; Da= 5.2 mm²/y ; Csa= 1.6 %10.23 y; Da=6.7 mm²/y ; Csa= 2.3 %ID #12-35 AT M(w /b =0.33; 10% FA; 5% SF)0 20 40 60Depth below exposed surface, mmis decreasing andC sa is increasing!Chloride concentration,% mass binder65432100.67 y; Da=56.1 mm²/y ; Csa= 1.9 %2.07 y; Da=19.9 mm²/y ; Csa= 3.7 %5.23 y; Da=22.8 mm²/y ; Csa= 4.9 %10.23 y; Da=13.6 mm²/y ; Csa= 5.1 %ID #12-35 SUB(w/ b=0.33; 10% FA; 5% SF)0 20 40 60Depth below exposed surface, mmWe do not know why, butanyhow we can model theresult of both!Fitted parameters from chloride profilesD a is decreasingAchieved diffiusion coefficientmm²/year10001001010ATM (40 profiles)SUB (46 profiles)Exponential (ATM)Exponential (SUB)0 2 4 6 8 10 12Exposure time, yearsFitted parameters from chloride profilesC sa is increasingAchieved surface concentration%mas binder876543210ATM (40 profiles)SUB (46 profiles)Log. (ATM)Log. (SUB)0 1 10 100Exposure time, yearsThe Mejlbro-Poulsen Modelis the complete solution to Fick’s2 nd law – it has four parametersBoundarycondition (C s )increasing in timeTimeC(x,t) C ( C ( t tisex) C ) (Cs Ci S (t tex) DpS S /( t D )pexaexiaexpDiffusivity (D a )( t t tex t p0.5xexdecreasingin timeTime) D ( t taD ( t Da )aexex)) t tex 163

The Mejlbro-Poulsen Modelhas four parametersD aexS ppThey can be foundby conversionformulas when thefollowing fourparameters areknown:‣ D 1‣ D 100‣ C 1p Conversion formulas1 log2log10C10100100tlog10 1texex1t exC1DD1001SD aexp D D1 D DC 1 1 D1100100 t ex 1 texp‣ C 100But how do we find‣ D 1‣ D 100‣ C 1‣ C 100…we got the ideas from laboratoryexperiments…Laboratory tests, NT BUILD 443Plain concrete CEMI, 0.3≤w/c≤0.7, 0.7,Paste volume 27 %Dpex, mm²/year140012001000800600400200DpexCspC sp = 11×w/cR 2 = 0.9D pex =50 ,00 0×exp(-10/(w /c) )987654321Csp , % mass bin der00.3 0.4 0.5 0.6 0.7w /c ratio by mass0Therefore the basic formulas aresuggested to have the form:C 1 = (11/A) eqv (w/c) b k C1, env [% mass binder] 10 D 1 B 6250 exp,eqv c D eqv(w / c)property XC kkDenvw 1 FA,property XwFAk[mm²/year]MS , property XMSThe basic extrapolation formulasare suggested to be:C 100 = C 1 k C100, env [% mass binder] = (U eqv (w/c) D + V) k , envD 100 D 1 1100164

A 16 parameter systemOptimisation of parameters5ParametersDiffusioncoefficientsCoefficientBAUVk D1, envk D1, envk , envk , envatmsubatmsubSurfaceconcentrationEfficiencyfactors,transportEfficiencyfactors,bindingCoefficientk C1, envk C1, envk C100, envk C100, envk FAk MSk FAk MSatmsubatmsubPoints estimated by the model,% mass bind er432R ²ATM = 0 .65x=y1ATM (332 points)Atmosphere00 1 2 3 4 5Measured points up to 10.3 years of exposure,% mass b inderOptimisation of parameters5Optimisation of parameters5Points estimated by the model,% mass bind er4321x=yR ²SUB = 0.81Points estimated by the model,% mass bind er4321R ²SUB = 0.81R ²ATM = 0 .65x=yATM (332 points)SUB (465 points)SUB (465 points)SubmergedSubmergedAtmosphere00 1 2 3 4 5Measured points up to 10.3 years of exposure,% mass b inder00 1 2 3 4 5Measured points up to 10.3 years of exposure,% mass b inderDerived design parametersParametersDiffusioncoefficientsCoefficientBAUVValue730.10.9SurfaceconcentrationCoefficientk C1, envk C1, envk C100, envk C100, envatmsubatmsubValuek D1, env atm 0.6Efficiencyk FA6k D1, env sub 0.7 transport k MS1factors,Efficiencyk , env atm 0.7factors,k FA– 0.1k , env sub 0.6binding k MS– 0.60.61.983.5Predicted chloride profilesChloride concentratio n,%mass binderEffect of w/c ratioParameters:(D aex ,, S p , p)= (2035, 0.66, 0.25, 1.30)121086420ATM 5 yATM 10 yATM 25 yATM 50 yATM 100 y0 10 20 30 40 50 60 70Depth belo w exposed surface, mmCh loride concentrat ion,%mass binder1210Binder composition:w/b = 0.45;CEM I 100 %;fly ash 0 %;silica fume 0 %Parameters:(D aex ,, S p , p)= (1745, 0.57, 1.35, 0.62)86420SUB 5 ySUB 10 ySUB 25 ySUB 50 ySUB 100 y0 10 20 30 40 50 60 70Depth below exposed surface, mm165

Predicted chloride profilesPredicted chloride profilesEffect of w/c ratioBinder composition:w/b = 0.40;CEM I 100 %;fly ash 0 %;silica fume 0 %Effect of w/c ratioBinder composition:w/b = 0.35;CEM I 100 %;fly ash 0 %;silica fume 0 %Parameters:(D aex ,, S p , p)= (1512, 0.66, 0.22, 1.29)Chloride concentration,%m as s binder121086420ATM 5 yATM 10 yATM 25 yATM 50 yATM 100 y0 10 20 30 40 50 60 70Depth below exposed surface, mmParameters:(D aex ,, S p , p)= (1298, 0.56, 1.20, 0.61)Chloride concentration,%mass binder121086420SUB 5 ySUB 10 ySUB 25 ySUB 50 ySUB 100 y0 10 20 30 40 50 60 70Depth below exposed surface, mmParameters:(D aex ,, S p , p)= (1058, 0.66, 0.19, 1.28)Chloride c oncent ration,%mass binder121086420ATM 5 yATM 10 yATM 25 yATM 50 yATM 100 y0 10 20 30 40 50 60 70Depth below exposed surface, mmChloride co ncent ration,Parameters:(D aex ,, S p , p)= (910, 0.56, 1.05, 0.61)%mass binder121086420SUB 5 ySUB 10 ySUB 25 ySUB 50 ySUB 100 y0 10 20 30 40 50 60 70Depth below exposed surface, mmPredicted chloride profilesPredicted chloride profilesEffect of FA & SFBinder composition:w/b = 0.40;CEM I 95 %;fly ash 0 %;silica fume 5 %Effect of FA & SFBinder composition:w/b = 0.40;CEM I 80 %;fly ash 15 %;silica fume 5 %Parameters:(D aex ,, S p , p)= (823, 0.65, 0.24, 1.27)Chl oride conc ent ration ,%mass binder121086420ATM 5 yATM 10 yATM 25 yATM 50 yATM 100 y0 10 20 30 40 50 60 70Depth below exposed surface, mmParameters:(D aex ,, S p , p)= (1298, 0.56, 1.30, 0.61)Chloride concentration,%mass binder121086420SUB 5 ySUB 10 ySUB 25 ySUB 50 ySUB 100 y0 10 20 30 40 50 60 70Depth below exposed surface, mmParameters:(D aex ,, S p , p)= (182, 0.64, 0.29, 1.24)Chloride concentration,%mass binder121086420ATM 5 yATM 10 yATM 25 yATM 50 yATM 100 y0 10 20 30 40 50 60 70Depth below exposed surface, mmParameters:(D aex ,, S p , p)= (250, 0.55, 1.58, 0.60)Ch loride concentrat ion,%mass binder121086420SUB 5 ySUB 10 ySUB 25 ySUB 50 ySUB 100 y0 10 20 30 40 50 60 70Depth below exposed surface, mmPredicted chloride profilesEffect of FA & SFParameters:(D aex ,, S p , p)= (182, 0.64, 0.28, 1.24)Chloride conc ent ration,%mass binder121086420ATM 5 yATM 10 yATM 25 yATM 50 yATM 100 y0 10 20 30 40 50 60 70Depth below exposed surface, mmBinder composition:w/b = 0.40;CEM I 85 %;fly ash 15 %;silica fume 0 %Parameters:(D aex ,, S p , p)= (157, 0.55, 1.53, 0.60)Chl oride concent ration,%mass binder121086420SUB 5 ySUB 10 ySUB 25 ySUB 50 ySUB 100 y0 10 20 30 40 50 60 70Depth below exposed surface, mmChloride ingress modelling is onthe threshold to succeed!We can produce data thatcorresponds with ourexperience and intuitionWe need to verify the models onreal structuresWe are still lacking good data…butwhat about chloridethreshold values?166

HETEK-1– AEC, CTH, CEMENTAOur scope was to make acomplete frameworkWe did not promise that itwould not need verification –but it would be complete…and it is!Paul Sandberg delivered thethreshold values for design!Evaluation of published chloridethreshold levelsAdvantages / disadvantagesField testing of modern low w/b ratio laboratory cast concretespecimen, with small covers (normally in the range 10-20 mm) -missing the effects of cover and of variable compaction inpractise.Laboratory testing of modern low w/b ratio concrete, sometimeswith the environmental impact simulated by potentiostaticallycontrolled steel potentials - missing the effects of cover and ofvariable compaction, and usually also the effect of avarying microclimate and of leaching of alkali hydroxide.Field studies of existing good quality old structures, with covers> 30 mm but w ith higher w/b ratios (typically > 0.5) - missingthe effect of a low w/b ratio and the effect of new binders.Laboratory or field testing of concrete with cast-in chloride,thereby allowing for the use of low w/b ratio concrete and a thickcover - missing the effect of steel passivation in chloridefree concrete.Measured ranges of chloride threshold levels(black steel) in macro crack free concrete invarious exposure regimes, Glass & Buenfeld [1995].Total chloride%Wt. cement0.17 - 1.40.2 - 1.50.250.25 - 0.50.3 - 0.70.40.4 - 1.60.5 - 20.50.5 - 1.40.61.6 - 2.51.8 - 2.2FreeChlorideMole/l0.14-1.8[Cl][OH]3 – 202.5 - 60.26 - 0.80.30.61 – 40ExposureTypeFieldFieldFieldLabo ratoryFieldOutdo orsLabo ratoryLabo ratoryOutdo orsLabo ratoryLabo ratoryLabo ratoryFieldLabo ratoryLabo ratoryLabo ratoryLabo ratoryLabo ratoryReferenceSt ratful et al. [1975]Vass ie [1984]West & Hime [1985]Elsene r & Böhni [1986]Henrik sen [1993]Bamforth & Chapma n-Andrews [1994]Hansso n & Sø ren sen [199 0]Sch iess l & Raupach [1990]Thomas et al. [ 1990]Tuutti [1993]Locke & Siman [1980]Lambe rt et al. [1991]Lukas [1 985]Pettersson [1993]Goni & Andrade [1990]Diamond [1986]Hausmann [1967]Yonezawa et al. [1988]Suggested design values for chloride thresholdlevels (black steel) in various Nordic exposurezones, [Paul Sandberg 1997, HETEK report 83]• Chloride threshold levels vary extensively in fieldexposed concrete, as a consequence of the varyingmicroclimate at the steel surface.• As a consequence the chloride threshold level dependson the cover thickness and on the physical bondingbetween concrete and reinforcement.• The chloride threshold levels are only valid for “ma crocrack free” concrete w ith a maximum crack w idth of 0.1mm and a minimum cover of 25 mm.• The data are not valid for calculations of the initiationtime in cracked concrete with crack widths > 0.1 mm.Suggested design values for chloride threshold levels(black steel) in various Nordic exposure zonesType ofbinder(PO = cement+ puzzolanas)w/b 0.50100 % CEM I5 % SF10 % SF20 % FAw/b 0.40100 % CEM I5 % SF10 % SF20 % FAw/b 0.30100% CEM I5% SF10% SF20 % FASubmergedzoneCcr %Cl of PO1.51.00.60.72.01.51.01.22.21.61.21.4Mari ne/de-icingatmosphere +Marine splash zoneCcr %Cl of PO0.60.40.20.30.80.50.30.41.00.60.40.5De-icing saltsplash zoneCcr %Cl of PO0.40.30.20.20.60.40.20.30.80.50.30.4From the table created by Sandberg anapproximating formula was made:C k exp 1.5 eqv wcr cr, envcThe constant k cr, env for the road environment.Environment:Constant:k cr, envWet RoadenvironmentSplash (WRS)1Dry RoadenvironmentSplash (DRS)0.8Distant RoadAtmosphere (DRA)The constant k cr, env for the marine environment.Environment:Constant:k cr, envSubmerged marineenvironment (SUB)0.3crMarine environmentSplash (SPL)[% mass binder]0.80.8Marine Atmosph.(ATM)0.8167

C k exp 1.5eqv wcr cr, envceqv(w / c)The activity factors for corrosion initiationto be used when calculating the eqv(w/c) crratio:Activityfactorkproperty XC kSilica fume-4.7crFA,property X[% mass binder]Fly ash-1.4wFA kMS , property XMSEstimation of the threshold valueRoad environment Marine environmentFormula, % mass binder1,0%0,8%0,6%0,4%Ccr(WRS)0,2%Ccr(DRS/DRA)y=x0,0%0,0% 0,2% 0,4% 0,6% 0,8% 1,0%Values from Table, % mass binderFormula , % mass binder2,5%Ccr(SUB)Ccr(SPL/ATM)y=x2,0%1,5%1,0%0,5%0,0%0,0% 0,5% 1,0% 1,5% 2,0% 2,5%Values from Table, % mass binderThreshold value, % Cl- of binder2,42,22,01,81,61,41,21,00,80,60,40,20,0Cenvcr kcr, exp1.5 eqvw Ccr kcr,exp1.5 eqvwenv Marine submerged0,30 0,35 0,40 0,45 0,50 0,55w/b ratioccrSUB ce 100; fa 0; ms 0SUB ce 95; fa 0; ms 5SUB ce 85; fa 15; ms 0SUB ce 80; fa 15; ms 5Threshold value, % Cl- of binder2,42,22,01,81,61,41,21,00,80,60,40,2Marine atmosphere0,00,30 0,35 0,40 0,45 0,50 0,55w/b ratioccrATM ce 100; fa 0; ms 0ATM ce 95; fa 0; ms 5ATM ce 80; fa 15; ms 5ATM ce 85; fa 15; ms 0Depth of penetration, mmChloride ingress (time-dependentdiffusivity and boundary condition)60504030Effect of w/b = 0.40 ± 0.0520 years50 years20w/b rati o 0.3510w/b rati o 0.400w/b rati o 0.450 20 40 60 80 100Time of exposure, yearsA change of w/bwith ± 0.05 (andhence a change ofdiffusivity) changesthe estim ated tim eto corrosion from 50years to 20 years ormore than 100>100 yearsyears!Nowadays we arefocused on how tocontrol w/b andthereby D. Wemust pay the sameattention to C cr !Chloride ingress (time-dependentdiffusivity and boundary condition)Depth of penetration, mm605040302010025 yearsEffect of C cr ± 50%50 yearsHigh thresholdIntermediateLow threshold0 20 40 60 80 100Time of exposure, yearsThe value of C crplays animportant rolefor the servicelife estimate -95 yearsand it greatlyaffects thedecision of whenand what to doA change of C crwith ±50 %changes the timeto corrosion from50 year to 25years or 95 years168

The two ways of making the estimation - by atable or by a formula correlate satisfactory -remember they are both estimates!…and what more do we need?Are we satisfied now?We have been focussed onpenetration methods – now it is time for initiation methodsD and C cr deserve the same attention!… but can we do it better?Why we should make itbetter?…because we must:Avoid disputes and/or frustrationMeet the demands for greener production…therefore be able to handle new typesof cement and other materialsMeet the demands for more economicalservice and maintenance… therefore face the fact that use ofservice life models will be required morefrequentlyOur models are more sophisticated – wehave included a time dependence…Why laboratory tests?We know that a large number ofparameters influences the threshold valueSome have been identified in the labWe know there is much more parameters inthe real worldWhy laboratory tests?We can try to limit the number ofvariables in the labWhen we can reproduce results we cancarry onWith time we will learn more and moreabout the decisive parametersIt is a long way to go…But we started a long time agoThreshold values - the recent development- In Denmark and Sweden – up to 2003:1985-1991 – Birgit Sørensen (and Carolyn M. Hansson)in lab tests.1991-1995 Karin Pettersson and Paul Sandberg - in laband in Träslövsläge.1993-1999 Hans Arup, Paul Sandberg and Henrik E.Sørensen – multi-probe in lab and in Träslövsläge.1999-2001 NORDTEST project AEC-CBI an attempt tomake a standard test method was made.2002-2003 Peter V. Nygaard, master thesis.…It is all published and in English169

The work by Peter V. Nygaardgave a breakthrough, by:Accelerating chloride ingress by first drying the testspecimens in a controlled climate……after which the specimens were exposed to a salinesolution allowing capillary suction of the saline solution...…and thereby avoiding the densification due to formation ofa dense invisible (to the naked eye) layer of calciteDesigning the c orrosion c ell to solve the earlier enc ounteredproblems with unwanted corrosion attacks at vital parts ofthe corrosion cell not intended as anode.Partial drying prior to partialcapillary suctionTest specimen layout by Peter V. Nygaard2DchlorideprofileUnevenchlorideingressCasting direction170

Potentiostatic test set-upThe set-up isused to determinethe onset ofcorrosionThe current to thecounter electrodein order tomaintain thepotential ismonitoredA sudden rise incurrent gives thecorrosion onsetDetection of corrosion onsetCorrosion initiated171

Definition of Cl - -thresholdCross section in specimen10 10CoverSteel electrodeThe threshold concentration ismeasured here (!!) at corrosioninitiatio n – because it is what wemodel – for the time being!Test principleGeneral requirementsShall give repeatablethreshold valuesShall relate to servicelife modellingShall be rational - i.e.economical andsensibleShall also considerexisting structuresProvisional measuresProven specimen des ignand proceduresMeasurements in bulkconcreteProven equipment andsmart conduction of testHas to be furtherinvestigatedWe need to know the threshold better!The first step must be to do it in thelaboratoryOtherwise we will walk like blind people!Peter V. Nygaard showed ushow to measure it!Come on!What are wewaiting for?172

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