Chloride Ingress and Reinforcement Corrosion in Concrete under De-Icing Highway Environment A study after 10 years field exposure

Chloride Ingress and Reinforcement Corrosion in Concrete under De-Icing Highway Environment A study after years field exposure Tang Luping Peter Utgenannt SP Technical Research Institute of Sweden Building Technology and Mechanics SP Report 27:76

Chloride Ingress and Reinforcement Corrosion in Concrete under De-Icing Highway Environment A study after years field exposure Tang Luping Peter Utgenannt

4 Abstract Chloride Ingress and Reinforcement Corrosion in Concrete under De-Icing Highway Environment A study after years field exposure This report presents the results from a research project financed by the Swedish Road Administration. In this project about chloride and moisture profiles have been measured from various types of concrete specimens exposed to a de-icing highway environment for about years. A newly developed rapid non-destructive technique, RapiCor, for corrosion measurement was used to assess the conditions of steel embedded in concrete beams with different types of binder and water-binder ratios. Both the DuraCrete and ClinConc model were used to predict chloride ingress in concrete. The results show that chloride profiles measured after years exposure under the deicing highway environment are, in some cases, lower than those measured after the first ~2 years exposure. Application of new techniques for spreading de-icing salts and the coverage of specimens by the shovelled snow lumps might be possible reasons for the low chloride ingress. The DuraCrete model, when the current input data given in the guidelines are used, may significantly underestimate chloride ingress, while the ClinConc model in general gives better prediction results, but it contains a number of empiric parameters or factors which need to be further verified. Owing to the large scatter in chloride profiles, none of the present models can, so far, properly describe the chloride ingress under such a de-icing highway environment. Non-destructive corrosion measurement by RapiCor instrument is in general reasonably in fairly good agreement with chloride ingress. The corrosion rust observed from the destructive examination verified again that the non-destructive technique RapiCor is a useful tool for detection of ongoing corrosion of steel in concrete. Key words: Chloride, concrete, corrosion, durability, moisture, reinforcement SP Sveriges Tekniska Forskningsinstitut SP Technical Research Institute of Sweden SP Report 27:76 ISBN 978-9-85829-9-5 ISSN 284-572 Borås 27

5 Sammanfattning Kloridinträngning och armeringskorrosion i betong exponerad för tösaltad vägmiljö - Resultat efter års exponering vid Riksväg 4 Bakgrund Armeringskorrosion är en av de vanligaste orsakerna till skador på betongbroar. Normalt anses korrosion initieras av antingen kloridinträngning eller karbonatisering. För bägge dessa mekanismer är betongens motstånd mot klorider och dess fuktnivå av avgörande betydelse för om och när korrosion startar. För att kunna dimensionera betongkonstruktioner på ett ekonomiskt och säkert sätt samt göra livslängdsbedömningar på redan befintliga konstruktioner behövs modeller för hur klorider och fukt transporteras i betong. För att dessa modeller skall bli robusta och trovärdiga måste de baseras på resultat från forskning på betongkonstruktioner exponerade i fält. Ett sådant forskningsprojekt är det nationella projektet BMB Beständighet Marina Betongkonstruktioner som startade 99 med syfte att få fram beständighetsdata för betong exponerad i marint klimat. Detta projekt, som Vägverket aktivt deltog i, resulterade bland mycket annat i ett användarvänligt datorprogram (Clinconc) för beräkning av kloridinträngning i betong i marint klimat. Under 996 startade ytterligare ett nationellt projekt BTB Beständighet Tösaltade Betongkonstruktioner som har finansierats av Vägverket och Cementa för att studera hur det aggressiva vägklimatet vid Riksväg 4 påverkar betongen. Ett drygt 3-tal betongkvaliteter med olika cementtyper, restmaterial som slagg och flygaska, olika vattenbindemedelstal och olika lufthalter tillverkades och placerades ut på en fältprovplats utmed Riksväg 4 strax väster om Borås. Den första uppmätningen av kloridinträngning utfördes efter två vintersäsonger och den påföljande efter fem säsonger på tre utvalda betongkvaliteter. Efter tio års exponering stöttade Vägverket detta forskningsprojekt för att utföra en omfattande kartering av kloridinträngningen och fuktnivåerna hos samtliga betongkvaliteter. Dessa mätningar har kompletterats med uppmätning av pågående armeringskorrosion med en icke-förstörande mätteknik (RapiCor) samt genom att armeringsjärn frilagts för att kunna bedöma korrosionsomfattningen. Syftet med detta projekt är att ta fram underlag för livslängdsdimensionering av betongkonstruktioner utsatta för tösaltmiljö, förbättra kunskap om armeringskorrosion i samband med kloridinträngning och fuktvandring hos betong, kalibrera befintliga beräkningsmodeller för kloridinträngning, öka kunskapen om verkliga tröskelvärden för kloridinducerad armeringskorrosion, och kalibrera icke-förstörande metoder för mätning av pågående armeringskorrosion. Denna rapport redovisar resultaten från forskningsprojektet.

6 Betongprovkroppar och fältexponering Två typer av betongprovkroppar ett ren betongblock med storlek 4 3 3 mm och en armerad betongbalk med storlek 2 3 3 mm var gjutna på SP. Huvudvariationerna i betongsammansättningarna inkluderar vattenbindemedeltal (vbt =,3;,35;,4 och,5 samt en upp till,75); bindemedel (åtta typer med olika tilläggningar av kalksten, kiselstoft och slagg); och lufthalt (5 % luftpor och naturlig lufthalt). Betongprovkropparna placerades nära körbanan på en fältprovplats vid Riksväg 4 strax väster om Borås och exponerades för den aggressiva miljö som omger en tösaltad motorväg. Ett flertal av provkropparna har ingjuten armering med olika täckskikt och många av dessa är också tillverkade med sprickor. Riksväg 4 mellan Borås och Göteborg är en av Sveriges mest tösaltade vägar. Enligt Vägverkets informationer har en genomsnittlig mängd tösalt 2,2 kg per m² vägyta använts under 996-999 men den genomsnittliga mängden tösalt halverades efter 2 tack vara en ny metod för saltning med mindre mängder salt i saltlaken. Icke-förstörande mätteknik för bedömning av korrosionstillstånd Den nyutvecklade icke-förstörande mättekniken s k RapiCor har använts i detta projekt för att bedöma armeringsjärnets korrosionstillstånd. Tekniken är baserat på senare års forskningsresultat från ett par FoU projekt, bl a ett Vägverksprojekt. Liksom andra metoder för mätning av armeringskorrosion är RapiCor baserad på galvanostatisk polarisationsteknik. På armeringsytan finns det ett tunt ytskikt av järnoxid som i vanliga fall har mycket högt polarisationsmotstånd och skyddar armeringen från korrosion. När skyddsskiktet har brutits ner på grund av kloridangrepp eller karbonatisering, bildas ett rostigt område på armeringsytan. Den rostiga arean har då lågt polarisationsmotstånd och således fortsätter korrosionsprocessen. Genom att leda en känd galvanostatisk strömstyrka från betongytan till armeringsjärnet och samtidigt mäta potentialsignaler får man veta betong-armeringsjärnkretsens polarisationsmotstånd. Korrosionshastighet kan sedan beräknas ur polarisationsmotståndet. Inte bara korrosionshastigheten utan även armeringsjärnets halvcellpotential och betongens resistivitet kan uppskattas genom RapiCor tekniken. Halvcellpotentialen är en indikation på korrosionssannolikhet enligt ASTM C 876 och resistiviteten återspeglar betongens fuktighet. Ur de tre parametrarna korrosionshastighet som huvudparameter, halvcellpotential och resistivitet som kompletterande parametrar får man en säkrare bedömning av armeringsjärnets korrosionstillstånd med uttryck korrosionsgrad enligt nedanstående tabell.

7 Tabell: Kriterier för uppskattning av korrosionsgrad. Korrosionsgrad Korrosionshastighepotential [kω cm] Halvcell- Resistivitet Kriterier [µm/år] [mv (CSE) ] Försumbar < X L - - X L = µm/år X L X M E cr ρ cr X M = 3 ~ 5 µm/år* X L X M < E cr < ρ cr X H = µm/år 2 Lite X M X H E cr ρ cr E cr = -2 mv (CSE) X M X H < E cr < ρ cr ρ cr = ~ 2 kω cm 3 Måttlig > X H E cr ρ cr 4 Påtaglig > X H < E cr < ρ cr * 3 för medelvärde av mätarea och 5 för enskild mätning. beroende av betongkvalitet, ytbehandling, väder, etc. Korrosionsmätningar utfördes på fältplatsen under två väderförhållanden torrt och vått. Enligt fältmätningsresultaten togs fyra betongbalkar ut, d v s 22 BB(svenskt anläggningscement, vbt,5), 223 B (finskt snabbt cement, vbt,5), 236 AB och 236 BB (svenskt anläggningscement, vbt,75). De fördes till SP för förstörande mätningar inklusive okulär undersökning av korrosionstillståndet på de frigörliga armeringsjärnen, karbonatiseringsdjup och kloridhalt på täckskiktsnivåer (, 2 och 3 mm) för att verifiera de resultat som icke-förstörande tekniken har tagit fram. Resultaten visar god överenskommelse mellan den icke-förstörande mätningen och den förstörande undersökningen. Rostiga fläckar eller märken upptäcktes på armeringsjärnet där korrosionsgraden var 3-4 (måttlig respektive påtaglig korrosion pågår) enligt den icke-förstörande mätningen. Enligt den okulära undersökningen på de ospruckna betongbalkarna initierades korrosionen på armeringsjärnets undersida, där det ofta finns stora luftporer eller cementpasta med hög vattenhalt på grund av möjlig segregation under armeringsjärnet. På de spruckna betongbalkarna skedde korrosion på platser med bredda sprickor. Korrosionsmätningen under vått väder visar generellt högre korrosionsgrad än under torrt väder. Skillnaden i korrosionsgrad mätt mellan det torra och våta vädret är liten för den ospruckna betongen men relativt stor för den spruckna betongen. Det kan förstås att vatten är en av nödvändiga förutsättningar för korrosionsprocessen. Torrt väder kan lätt torka vattnet i sprickor och minska eller tillfälligt stoppa korrosionsprocessen. Karbonatiseringsdjupet är ca 5 mm på betongbalkar med vbt,75 (236 AB och BB) respektive 2 mm i betongbalkar med vbt,5 (22 BB och 223 B) efter års exponering under vägmiljön på fältplatsen. Placering nära marken där fuktigheten är relativt hög är en möjlig förklaring till det relativt låga karbonatiseringsdjupet. Det framgår från kloridmätningsresultaten att högre kloridinträngning upptäcktes på övre delen än undre delen av betongbalkens vertikala yta. Möjlig orsaken kan vara att de plogade snövallarna har blockerat vidareskvätt från bilar. Mätning av klorid- och fuktprofiler I denna undersökning har ett -tal klorid- och fuktprofiler mätts upp ur 34 st betongblock på betongsammansättningar med olika bindemedel och olika

8 vattenbindemedelstal. På varje betongblock mättes två kloridprofiler en från den övre horisontella exponeringsytan och en från den vertikala exponeringsytan och en fuktprofil från den vertikala exponeringsytan. Mätning av kloridprofiler utfördes av SP med hjälp från Chalmers Tekniska Högskola för profilfräsning. Fuktmätning utfördes av Lunds Tekniska Högskola. Resultaten visar att kloridinträngningen efter års exponering i några fall är mindre än vid uppmätningen efter två år. En orsak till detta kan vara att det nu används andra spridningstekniker som medför att mindre mängd salt sprids på vägarna idag jämfört med för år sedan. En annan orsak kan vara att kloriderna i ytan lakas ur under sommarhalvåret. Ytterligare en orsak kan vara att det finns lokala variationer, t ex snövallar och olika exponering mot skvätt från bilar vilket medför att kloridinträngningen kan variera något beroende på provkropparnas placering på provplatsen. Som förväntat har betongens vbt stor betydelse för kloridinträngningen. Betongen med lägre vbt visar i allmänhet mindre kloridinträngning. Normalt skulle pozzolantillsatsmaterial såsom kiselstoft och slagg bidra till bättre porstruktur och få till följd mindre kloridinträngning. Resultaten från detta projekt visar emellertid inte tydligt pozzolaneffekten. Sannolikt har den minskande saltspridningen bromsat vidare kloridinträngning men regnväder har lett till mer urlakning av klorider från betongen med enkelt cement än med pozzolantillsatsmaterial p g a den senares låga permeabilitet. Det framgår från uppmätta fuktprofiler att betongen med tillsatsmaterial visade lägre fuktprofiler än utan betongen med svenskt anläggningscement och finskt snabbt standardcement. Modellering av kloridinträngning under tösaltad vägmiljö Resultat från uppmätt kloridinträngning har jämförts med olika matematiska modeller för kloridinträngning såsom enkel ERFC modell, DuraCrete modell och ClinConc modell. ERFC modellen är baserad på enkel Ficks 2:a lag med ett komplement till fel-funktionen (ERFC). Det är beprövat att denna modell ofta leder till en för konservativ prediktion av kloridinträngning. Därför har ERFC modellen använts endast för beräkning av skenbar diffusionskoefficient och kloridhalt på betongytan. DuraCrete modellen togs fram i EUprojektet DuraCrete som avslutades 2 och är baserad på ERFC modellen med modifikationer av flera parametrar. ClinConc modellen har utvecklats på Chalmers i mitten av 99-talet och är baserad på de fysikaliska och kemiska processer som involveras i kloridtransportsteorier. Denna jämförelse visar att DuraCrete modellen i vissa fall kraftigt undervärderar kloridinträngningen. Jämförelse med ClinConc modellen ger generellt bättre överensstämmelse. Denna modell innehåller ett flertal empiriska parametrar som behöver verifieras. Beroende på den relativt stora spridningen i uppmätta kloridprofiler kan ingen existerande modell på ett tillräckligt säkert sätt beskriva kloridinträngningen i denna tösaltade vägmiljö. Slutsatser och rekommendationer Från jämförelsen mellan korrosionsmätningsresultaten och kloridinträngningsprofiler kan man dra följande slutsatser och rekommendationer: Liksom i tidigare undersökningar visade friläggning av korroderade armeringsjärn att den snabba icke-förstörande tekniken RapiCor stämmer bra med den förstörande mätningen. Resultaten från den icke-förstörande korrosionsmätningen uppvisar också bra överensstämmelse med uppmätt

9 kloridinträngning. Därför är denna snabba teknik ett användbart redskap och kan rekommenderas för bedömning av pågående armeringskorrosion. Resultaten från denna undersökning tyder på att kloridtröskelvärdet inte nödvändigtvis är,4 % av bindemedelsvikten vilket är ett vanligt antagande. I denna undersökning uppvisar inte armering i betong med pozzolantillsatsmaterial (slagg och kiselstoft) någon tendens att börja korrodera vid lägre kloridtröskelvärden än vad som antas gälla för betong med rena Portlandcement som bindemedel. Å andra sidan uppvisar den icke-förstörande tekniken en viss grad av korrosion i betongen med byggcement och finskt snabbt cement även när kloridhalt i närheten av armeringsjärnet är lägre än,4 % av bindevikt. Därför behöver kloridtröskelvärden för betong med och utan tillsatsmaterial undersökas vidare innan några säkra slutsatser skall kunna dras. Tillsats av 5 % slagg i finskt snabbt cement visar betydlig förbättring av betongens resistens mot kloridinträngning och armeringskorrosion medan tillsats av ~5 % slagg i finskt standardcement inte visar sådan förbättring. Å andra sidan måste frostbeständighet hos betong med tilläggning av hög volym slagg provas innan denna typ av betong används för tösaltade vägmiljöer. Ett positivt resultat är att tösalter tränger in i betongen med en reducerad hastighet efter bara ett par års exponering. För att få bekräftat att dessa resultat är representativa även för andra tösaltade vägmiljöer föreslås följande: Undersökning av några existerande motorvägsbroar för att mäta upp kloridinträngningen och på så sätt verifiera den reducerade inträngningshastigheten. Fortsätta verifiera och modifiera existerande modeller för kloridinträngning så att dessa stämmer med uppmätt kloridinträngning på verkliga betongkonstruktioner och så att modellerna kan användas vid livslängdsbedömning. Påbörja forskning kring kloridtröskelvärden där inte enbart kloridhalten beaktas utan även andra faktorer så som karbonatisering och lakning av kalcium som påverkar kloridbindningen och alkaliteten. Även inverkan av fuktinnehåll, luftblåsor i betongen och defekter på armeringsjärnets yta bör studeras.

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Contents Abstract 4 Sammanfattning 5 Introduction 3 2 Concrete Specimens and Exposure Conditions 4 2. Concrete specimens 4 2.2 Exposure conditions at the highway 4 field site 6 3 Corrosion Measurement 9 3. Technique for corrosion measurement 9 3.2 Technique for corrosion measurement 2 3.3 Field measurements 2 3.4 Laboratory measurements 22 3.4. Concrete beam 22 BB 23 3.4.2 Concrete beam 223 B 23 3.4.3 Concrete beams 236 AB and BB 26 3.4.4 Carbonation depths in concrete covers 29 3.4.5 Chloride ingress at the cover levels 3 4 Measurements of Chloride and Moisture Profiles 32 4. Sampling 32 4.2 Measurement of chloride profiles 32 4.3 Measurement of moisture profiles 33 4.4 Measured chloride profiles 33 4.4. Effect of binder types 33 4.4.2 Effect of water-binder ratios 38 4.5 Measured moisture profiles 4 4.5. Effect of binder types 4 4.5.2 Effect of water-binder ratios 4 5 Modelling of Chloride Ingress 43 5. Curve-fitting to the ERFC model 43 5.2 DuraCrete model 45 5.3 ClinConc model 46 5.3. Modelling of free chloride ingress 46 5.3.2 Calculation of total chloride content 49 5.3.3 Input parameters used in the modelling 49 5.4 Modelled results in comparison with the results from the field measurement of corrosion 5 5.5 Other chloride profiles in comparison with the results from the field measurement of corrosion 6 5.6 Discussions 67 5.6. DuraCrete model 67 5.6.2 ClinConc model 67 5.6.3 Uncertainty in chloride profiles 68 5.6.4 Chloride threshold value for corrosion 7 6 Concluding remarks and suggestions 7 7 References 73

2 Appendix Results of corrosion from the field measurement 75 Appendix 2 Chloride profiles from the Highway 4 field site 8 Appendix 3 Moisture data from the Highway 4 field site 49 Appendix 4 Curve-fitted results of apparent diffusion coefficient and surface chloride content 54

3 Introduction Reinforcement corrosion is one of the most common reasons for the damage of concrete bridges. It is generally believed that reinforcement corrosion is initiated by either chloride ingress in concrete or carbonation of concrete. Moisture is a prerequisite for both mechanisms of chloride transport and carbonation. In order to design concrete structures based on the expected service life and performance, models are needed to predict chloride ingress and moisture conditions in concrete. In the middle of 99 s, the first mechanismbased model ClinConc has been developed at Chalmers University of Technology [Tang & Nilsson, 994; Tang 996]. This model has been verified with the field data after exposure under a marine environment over years Tang, 23b], under the financial support by the Swedish Road Administration. In the beginning of 2 s, also under the financial support by the Swedish Road Administration, a rapid technique for corrosion measurement was developed at SP Technical Research Institute of Sweden [Tang, 22]. This technique has been verified on the laboratory specimens and the reinforced concrete slabs exposed under a marine environment for over 3 years [ Tang et al, 25]. Since 996 a large number of reinforced concrete specimens with different qualities have been exposed at the field station by Highway 4. Measurements of chloride and moisture profiles have been made after and 2 years exposure. In 26 the specimens were exposed for years. In order to investigate these specimens and obtain the first-hand information about the long term behaviour of concrete with regard to chloride ingress and reinforcement corrosion under the de-icing highway environment, the Swedish Road Administration financed this project. It was a good opportunity to use these unique specimens for measurements of chloride profiles and moisture profiles, and also for the verification of prediction models and non-destructive techniques. Therefore, the primary objectives of this project include to form the basis for service life design of concrete structures exposed to de-icing environment; to improve the knowledge of reinforcement corrosion related to chloride ingress and moisture condition in concrete; to verify the existing models for prediction of chloride ingress; and to verify the non-destructive technique for measurement of on-going corrosion. This report presents the results from the above mentioned research project (Contract No. AL 9 B 25:686), financed by Vägverket Swedish Road Administration.

4 2 Concrete Specimens and Exposure Conditions 2. Concrete specimens The relevant mixture proportions of concrete are summarised in Table 2. and the detailed information about the raw materials and hardened properties of each mixture proportion was published elsewhere [Utgenannt, 998]. The main variations include water-binder ratio (.3,.35,.4 and.5, one up to.75), binder type (eight types of binder with different additions of limestone, silica fume and blast-furnace slag), and air content (5% entrained air and non-aea). Two types of concrete specimens, one plain concrete block of 4 3 3 mm and another reinforced concrete beam of 2 3 3 mm, were cast at SP Technical Research Institute of Sweden (previously Swedish National Testing and Research Institute). The plain concrete blocks were designed for sampling of chloride penetration profiles and the reinforced concrete beams were designed for testing corrosion resistance under uncracked and pre-cracked conditions. A typical structure of the reinforced concrete beam is shown in Figure 2. and the detailed information about the reinforcement placement in each beam was published elsewhere [Nordström et al, 998]. The specimens were cured in the laboratory for 35 to 7 days before placed at the field site. Reinforcement bars B Stainless steel 6 3 5 B B B Pre-cracks Exposure zone Epoxy coating Notes: ) In most of the beams there was no rebar at the level with cover 6. 2) Each steel bar was soldered with a copper wire whose another end was connected to the plinth in the electronic box outside the specimen. This wire supplies the connection to the rebar for electrochemical measurement. Figure 2. Schematic of reinforcement beams exposed to the Highway 4 field site.

5 Table 2. Mixture proportions of concrete exposed at the Highway 4 field site. Mix No. Binder type Binder kg/m 3 Waterbinder ratio ) Fine aggreg. -8 mm kg/m 3 Coarse aggreg. 8-6 mm kg/m 3 Sp 2) % of binder AEA 3) % of binder Air % of vol. 28d compr. strength 4) MPa 2 42.4 886.4 85.6.97.28 4.8 65.4 22 38.5 89.2 82.8.2 4.5 5.8 23 5.3 833.5 978.5 3.25.3.6 24 % Anl 5) 45.35 88.3 953.7 2.27.9 9.3 25 38.5 938.6 866.4.3 54.8 236 26.75 7.4 79.6.2 4.5 3 26 42.4 86 86.2.4 4.7 78. 27 95% Anl 38.5 865.5 83.5.22 4.4 58.2 28 +5% SF 6) 5.3 86 985..7.2 9.7 29 45.35 846.5 954.5.5. 3.5 2 38.5 95.8 87.2.9 62.6 2 42.4 863 863.2.2 4.9 48.8 22 % 38.5 885 87.8 4.5 42. 23 FinStd 7) 54.3 767.7 938.3 3.6 2.5 66.9 24 39.5 95.2 844.8.2 47.4 25 42.4 88.3 845.7.7.27 4.9 6.7 26 % 39.5 874. 86.9. 4.4 46 27 SliteStd 8) 52.3 799.9 939. 3.8 2.3 85.7 28 4.5 892.3 823.7.4 59.9 29 56% 42.4 858 858.5.27 4.5 55 22 FinRpd 9) 37.5 89.8 823.2. 4.7 4.5 22 +44% SL ) 54.3 76.9 93.2 3.8..6 82 222 42.4 863 863 2.27 4.7 57.6 223 % 38.5 885 87.3 4.9 43.4 224 FinRpd 54.3 767.7 938.3 4.5 2.4 66.5 225 42.4 85.5 85.5.42.8 4.8 8.4 226 9% Anl 5.3 796.5 973.5 2..5 26.9 227 +% SF 45.35 844. 95.9.7.9 2.7 228 42.4 88.3 845.7.7.3 4.8 68.9 229 39.5 874. 86.9.4 4.7 5.8 23 P Kalk C ) 53.3 78.2 954.8 3.6 2.2 98.5 23 47.35 85 922 2.62.8 86.3 232 4.5 93.8 834.2.2 6.9 ) Mass ratio of water to binder without consideration of the efficiency factor for silica fume or blast-furnace slag. 2) Sp Super-plasticizer, Cementa Melcrete 3) AEA Air entraining agent, Cementa L7 4) According to SS 3 72 5) Anl Anläggningscement (Swedish SRPC, CEM I) 6) SF Silica fume (Elkem. Norway) 7) FinStd Finnish standard Portland cement with -5% blast-furnace slag (CEM III/C) 8) SliteStd Swedish standard Portland cement with 5-8% limestone filler, made in Slite (CEM II/A-LL) 9) FinRpd Finnish rapid Portland cement ) SL Finnish blast-furnace slag ) P Kalk C Swedish Portland cement with -5% limestone filler (CEM II/A-LL)

6 2.2 Exposure conditions at the highway 4 field site The field exposure site at highway 4 was established in the autumn of 996. It consists of a 2 meter long and a couple of metres wide gravel area along the highway, with specimens mounted in steel frames at road level, as shown in Figure 2.2. A guard rail was installed to separate the exposure site from the traffic. It was placed in such a way as to ensure the traffic safety and to have the specimens fully exposed to the splash water from the traffic. The climate around the specimens is moist, and the specimens are exposed during the winter to low temperatures and de-icing salts, producing a climate corresponding to exposure class XD 3/XF 4 in EN 26- (2). Concrete blocks & beams Guard rail.76 m.45 m Gravel layer Figure 2.2 Field exposure site at Highway 4. Specimens placed in steel frames behind a guard rail (right).

7 Highway 4 leads from Gothenburg to the east, through Borås and towards Jönköping. Over the year, the daily average number of vehicles passing the field exposure site is around 2, of which 25 are heavy vehicles (data from measurements carried out by the Swedish Road Administration in 2). For safety reasons, de-icing salts are used during the winter in many parts of Sweden to keep road surfaces free from snow and ice. The de-icing agent used is sodium chloride, which is spread either in the form of a solution (about 24% NaCl) as a preventive measure, or as crystals when spread on snow. In this region, de-icing salts are normally used between October and April. Table 2.2 shows an estimate of the total amount of salt spread on the highway per square metre and year. The figures in Table 2.2 are based on the data from the Swedish Road Administration. The table also shows the number of occasions de-icing salts were spread on the highway each winter season. As can be seen in Table 2.2, the amount of salt spread on the road was markedly reduced around year 2. This was due to the introduction of the new method of applying salt, as a solution, which uses a smaller amount of salt. Table 2.2 Estimated total annual amount of salt spread on Highway 4 per square metre and the number of occasions de-icing salts were spread on the road each winter season. Winter 96-97 97-98 98-99 99- - -2 Amount of salt (kg/m 2 ).9 2.4 2.3 2...2 Number of occasions 26 57 5 4 7 48 Winter 2-3 3-4 4-5 5-6 6-7 Amount of salt (kg/m2).*.3*.3*.2*.* Number of occasions 28 56 5 4 23 * Estimated from the number of occasions. The annual precipitation data between 996 and 22 obtained from the climate station about km from the exposure site, run by the Swedish Meteorological and Hydrological Institute (SMHI), are shown in Table 2.3. It can be seen that the average annual precipitation is about 7 mm. Table 2.3 Annual precipitation data from SMHI. Year 996 997 998 999 2 2 22 Precipitation, mm 796 95 256 859 323 94 27 The monthly air temperature registered at the climate station near the field exposure site is shown in Figure 2.3. From these data the annual average temperature of 7 ºC can be estimated. If the freezing period is excluded, the average temperature will be about ºC. It should be noticed that the actual chloride concentration in the highway environment is unknown, although some data of salt spread is available as shown in Table 2.2. Wirje &

8 Offrell (996) investigated chloride ingress into mortar specimens placed at different locations along the road. The results showed that the ingress of chlorides decreases with increasing height above the road level. Tang & Utgenannt (2) present results from collecting splash water at different locations around the exposure site. The results confirm the findings by Wirje & Offrell (996). 2 Monthly air temperature, C 5 5-5 - Oct. Nov. Dec. Jan. Feb. Mar. Apr. May Jun. July Aug. Sep. Winter 95/96 Winter 96/97 Winter 97/98 Figure 2.3 Temperature data near the field exposure site at Highway 4.

9 3 Corrosion Measurement 3. Technique for corrosion measurement In this project, a handheld instrument RapiCor was used for measurement of corrosion of rebars in concrete, see Figure 3.. The detailed descriptions of this measurement technique have been published elsewhere [Tang 22; Tang & Fu 26]. The instrument measures half-cell corrosion potential, and then generates two galvanostatic pulses for corrosion rate and resistivity measurements. The measurement is quick and only needs a few seconds to obtain three parameters: corrosion potential, corrosion rate of steel and resistivity of concrete. These three parameters contribute a more accuracy estimation of the corrosion status of steel. Figure 3. Handheld instrument RapiCor. The measurement principle of the new rapid technique is illustrated in Figure 3.2. In order to facilitate the modelling of electrical current distributions using 2-D numerical model, the rectangular shape of counter and guard electrodes were used in this new technique. A wet sponge is place on concrete surface in order to improve the contact between concrete and the electrodes unit. Similar to the typical galvanostatic pulse measurement, the instrument firstly measures the corrosion potential E corr by the reference electrode placed at the centre of the electrodes unit and afterwards imposes galvanostatic currents I CE and I GE through the counter electrodes CE and the guard electrodes GE to the steel bar embedded in concrete. Immediately after having imposed the currents I CE and I GE, the data acquisition system starts to record the signal responses of potential E a (t) at a time interval of less than.2 seconds. The recorded potential-time curve is directly displayed on the screen of the instrument and can be used for calculation of various parameters such as ohmic resistance R Ω, polarization resistance R p, etc. For an endless long steel bar embedded in concrete, the imposed total current I tot = I CE + I GE will disperse along the steel bar to a certain distance, depending on the conductivity of

2 concrete, σ c, the thickness of concrete cover, l c, and the conductivity of the surface film of the steel, σ f. Therefore, it does not necessarily mean that the current I CE is equal to the polarization current I p through the specified polarization length L p. In order to calculate the proper polarization current I p through the specified polarization length L p the numerical modelling must be used. In this new technique, a 2-D FEM (2-Dimensional Finite Element Method) was employed to model the current distributions under the galvanostatic measurement conditions. With the help of modelling the effective polarization current I p flowing through the specified polarization length L p can be estimated from the ratio of polarization potential to Ohmic drop, both of which can be obtained from the measurement. Therefore, the true Ohmic and polarization resistances, R Ω and R p, can be calculated using Ohm s law and, consequently, the resistivity of concrete and the corrosion rate can be obtained. Results display, data storage/output Data treatment Pulse generator Data acquisition I GE I CE E RE Ep = IpRp E = IpR I p Current Potential signals t GE CE CE GE R Randles model Concrete L p Wet sponge Surface layer Cover l c Cdl Rp Steel bar Figure 3.2 Measurement principle of RapiCor. 3.2 Technique for corrosion measurement Thanks its rapidity, this new technique can be used for mapping a large area of the structure in a short time. For the purpose of structural assessment the corrosion conditions can be classified into four levels: negligible, low, moderate and high corrosion [Rodríguez & Andrade, 22]. Since the instrument measures not only corrosion rate, but also the half-cell potential and ohmic resistivity of concrete, it would reduce the uncertainty if all these three parameters are utilised in the assessment. From the previous investigations it has been shown that, among these three parameters, the corrosion rate correlates best to the actual chloride content in concrete near reinforcement steel [Tang & Malmberg, 25; Tang & Utgenannt, 27]. It has, therefore, been suggested to take corrosion rate as main parameter, and take half-cell potential and resistivity as complementary parameters in the assessment of corrosion level. The criteria of each parameter, especially resistivity, may be dependent on the type of concrete structures, the surface treatment and the weather when the measurement is carried out. An example of criteria is shown in Table 3..

2 Table 3. Example of criteria for classification of corrosion level Corrosion Corrosion Half-cell Resistivity Criteria level rate potential [kω cm] [µm/yr] [mv (CSE) ] Negligible < X L - - X L = µm/yr X L X M E cr ρ cr X M = 3 ~ 5 µm/yr* 2 Low X L X M < E cr < ρ cr X H = µm/yr X M X H E cr ρ cr E cr = -2 mv (CSE) 3 Moderate X M X H < E cr < ρ cr ρ cr = ~ 2 kω cm > X H E cr ρ cr 4 High > X H < E cr < ρ cr * 3 for average measurement and 5 for single measurement. depending on the type of concrete, surface treatment, weather, etc. According to Table 3., the parameter corrosion rate is used for primary classification. If the two complementary parameters are not lower than the criteria for possible corrosion, the corrosion condition will be classified as one level lower. For example, if the corrosion rate is > µm/yr corresponding to high, but the half-cell potential is higher than -2 mv CSE or the resistivity is larger than 2 kω cm, the corrosion level will be classified as moderate. In this way the assessment would be safer than that based on only one parameter, considering the complication of reinforcement corrosion in the real structures. 3.3 Field measurements The field measurements were carried out under two weather conditions, one in June after a few days of sunny weather (dry condition) and another in August and October after a few days of rainy weather. The measured results in the order of the beam placement from the east to the west at the field exposure site are listed in Appendix. The average values from the same type of concrete are summarised in Table 3.2. It should be kept in mind that the actual corrosion of steel in concrete is dependent on many factors, such as chloride content, moisture content, cover thickness, crack wideness, etc.. The average values give us a rough comparison only.

22 Table 3.2 Average values of corrosion from each type of concrete Binder P Kalk C % SliteStd %FinStd 95%Anl + 5%SF %Anl 56%FinRp d +44%SL %Fin Rpd 9%Anl +%SF Measured on 6-6-9 & 3 (dry weather) Measured on 6-8-23-24 & -27 (wet weather) Half-cell potential Corrosion rate Resistivity Corr Half-cell potential Corrosion rate Resistivity Corr w/b Mix mv[cse] µm/yr kω.cm index mv[cse] µm/yr kω.cm index.3 23-23 2. 3. -65 4.5 9.3.35 24-39.2 95. -38.7 86..4 2-9 3.8 87. -7 4. 69.3.5 22-38 4. 78.3-23 6.6 78 2..5 25-6.2 63 2. -25.6 66 2.5.75 236-27.5 6 3.3-342 2. 44 3..3 28-73.6 277. 37 2. 235..35 29-34.7 8. -6.3 7..4 26-95.5 29. -75 2.2 2..5 27-46 2.6 67. 2.5 89..5 2-56 4.8 69.5-92 6. 73 2..3 23-4 2. 87.3-46 2.7 85.5.4 2-23.6 27. -58 3.2 52.2.5 22-26 3.7 67.8-285 4.3 76 2..5 24-95 6.5 73 2.3-285 8.9 79 2.8.3 27-75.6 86. -4 2.3 97.3.4 25-5.7 86. -44 2.2 84..5 26-24 3. 55. -39 7.8 62 2..5 28-55 2.8 43. -87 3.3 54.5.3 22-66.7 234. -7. 28..4 29-46.9 228. -8.9 94..5 22-34.3 283. -27.8 85..3 224-55.9 9. -5 3.4 87..4 222-85 2. 77. -89 9. 58 2.5.5 223-9 5. 52 2. -249 4.3 52 3.8.3 226 3.9 37. - 3.5 85.3.35 227 75.2 39. -64 2. 76.3.4 225 28. 95. -4 2.4 52..3 23-4.8 4. 6 2.6 88..35 23-8.9 29. -33.9 59..4 228-225.9 35.3-38 4.9 38.5.5 229-44.4 9. -49 2.4 7..5 232-254.7 98.5-229 5. 5.5 From Table 3.2 it can be seen that more corrosion was detected from the measurement under the wet weather than under the dry weather. This should be understandable because under the wet weather the concrete contains more electrolyte in the pores which is prone to corrosion process. For concrete mixes 236 (w/b.75) and 24 (w/b.5), significant or moderate corrosion could be detected both under dry and wet weather. Generally, if any corrosion could be detected under the dry weather, the corrosion would be confirmed under the wet weather. Some comparisons between chloride ingress and corrosion will be presented later in Chapter 5 Modelling of chloride ingress. 3.4 Laboratory measurements The purpose of laboratory measurements was to verify the field measurement using the non-destructive technique RapiCor. Since the copper wires connecting each steel bars in the adjacent concrete beams are connected to the same joint box, both the results from the

23 field measurements and the possibility for removal of concrete beams without destroying the connections of the adjacent beams have to be considered in the selection of concrete beams for the laboratory investigation. Finally, four concrete beams were selected, that is, 22 BB(Swedish structural cement, w/b.5), 223 B (Finnish rapid cement, w/b.5), 236 AB (Swedish structural cement, w/b.75) and 236 BB (ditto). These four concrete beams were transport to the laboratory at SP. further non-destructive measurements were carried out at a distance of cm from the left side of each rebar. Afterwards the rebars were released from each concrete beam by sawing and splitting. Since the ribbed steel was used in the concrete beam, it is difficult to carry out any gravimetric measurement for a quantitative evaluation. The corrosion condition on each rebar was, therefore, visually examined only. 3.4. Concrete beam 22 BB From the field measurement concrete beam 22 BB showed a low corrosion under the dry weather and a moderate corrosion under the wet weather. The results from the laboratory measurements and visual examinations are shown in Figure 3.3. It can be seen from Figure 3.3 that there is significant corrosion rust on Rebar at the distance about ~5 cm from the left side, which is in a good agreement with the non-destructive measurements. From the non-destructive measurement, rebar 2 showed a moderate corrosion at the distance 4 cm from the left side. A corrosion mark can also be seen at the distance around 44~46 cm, implying a good agreement between non-destructive measurement and destructive observation. It is surprising that the corrosion, no matter by the non-destructive measurements or visual observations, was more severe in rebar with 3 mm cover than in rebar 2 with 5 mm cover. One possible explanation could be that the lower part of concrete beam was covered by the snow lump under the winter, which hindered the splashing water in direct contact with the concrete, resulting in less chloride ingress. This will be discussed later in 3.3.5. 3.4.2 Concrete beam 223 B From the field measurement concrete beam 223 B showed a low corrosion under the dry weather and a high corrosion under the wet weather. This is a pre-cracked beam with three major vertical cracks at 2~22 cm (wideness.~.2 mm), 3~33 cm (wideness.2~.3 mm), and 37~39 cm (wideness.~.2 mm), respectively, from the left side. The results from the laboratory measurement and visual examination are shown in Figure 3.4. Corrosion marks can be seen at corresponding cracked positions, especially on rebar 2 at position 38 cm. The results from the visual observation are in a fairly good agreement with the results from the non-destructive measurement. The corrosion at cracked positions could be more sensitive to the weather changes. This could be the reason why the results measured on the beam 223 B under the dry and the wet weather vary from low to high corrosion. This may also explain the large variation in the results measured on the other cracked beams under the dry and the wet weather, as shown in Appendix.

24 Concrete 22, SRPC w/b.5 Corrosion rate, µm/yr High Moderate Low Negligible Rebar, Cover 3 mm Rebar, After wet sawing. Rebar 2, Cover 5 mm 2 3 4 5 6 Distance from left side, cm Figure 3.3 Corrosion of steel in concrete beam 22 BB, with Swedish SRPC (CEM I), w/b.5, no pre-cracking.

25 Corrosion rate, µm/yr. Crack. mm High Concrete 223, Finnish Rapid C, w/b.4 Crack.2.3 mm Crack. 2 mm Moderate Low Negligible Rebar, Cover 3 mm Rebar, After wet sawing Rebar 2, Cover5 mm 2 3 4 5 6 Distance from left side, cm Figure 3.4 Corrosion of steel in concrete beam 223 B, with Finnish rapid cement, w/b.5, pre-cracked.

26 3.4.3 Concrete beams 236 AB and BB From the field measurement concrete beams 236 generally showed a high corrosion under both the dry and the wet weathers. There was no signal response on beam 236 BB in the field measurement (see Appendix ). It was believed that the wire connection was broken due to severe corrosion. In the laboratory, each rebar in beam 236 BB was directly connected for the non-destructive measurement using RapiCor. Concrete beam 236 AB is a pre-cracked beam with four major vertical cracks at 6~ cm (wideness about. mm), ~5 cm (wideness.~.3 mm), 8~23 cm (wideness.2~.5 mm), and 3~32 cm (wideness.~.5 mm), respectively, from the left side. The results from the laboratory measurements and visual examinations are shown in Figures 3.5 and 3.6. It can be seen from Figure 3.5 that corrosion in beam 236 AB is more localised than in beam 236 BB (see Figure 3.6). This more localised corrosion in beam 236 AB is surely attributed to the pre-cracks which initiated corrosion before the non-cracked parts of concrete were contaminated by chlorides ingress. The results from the visual observation are, again, in a fairly good agreement with the results from the non-destructive measurement. Concrete beam 236 BB was not subjected to pre-cracking. The corrosion of steel in this beam should be induced by either carbonation or chloride ingress through the bulk concrete. The carbonation depth in this beam is, however, about 5 mm only, as shown in Table 3.3 and Figure 3.7 in 3.3.4. The corrosion must, therefore, be induced by chloride ingress. Figure 3.6 shows that severe corrosion occurred on rebar, while relatively less severe corrosion on rebar 2. This is in good agreement with the non-destructive measurements. Again, the corrosion was more severe in rebar with 3 mm cover than in rebar 2 with 5 mm cover, as will be discussed later in 3.3.5.

27 Concrete 236, SRPC w/b.75 Corrosion rate, µm/yr High Moderate Low Negligible Rebar, Cover 3 mm Rebar, After wet sawing. Rebar 2, Cover 5 mm 2 3 4 5 6 Distance from left side, cm Figure 3.5 Corrosion of steel in concrete beam 236 AB, with Swedish SRPC (CEM I), w/b.75, pre-cracked.

28 Crack due to splitting Crack due to splitting Concrete 236, SRPC w/b.75 Crack.2 mm Crack..2 mm Corrosion rate, µm/yr High Moderate Low Negligible Rebar, Cover 3 mm, after wet sawing. Rebar 2, Cover 5 mm, after wet sawing 2 3 4 5 6 Distance from left side, cm Figure 3.6 Corrosion of steel in concrete beam 236 BB, with Swedish SRPC (CEM I), w/b.75, no pre-cracking.

29 3.4.4 Carbonation depths in concrete covers After removal of rebars from the concrete beams, carbonation depths were measured on the split surfaces using colourimetric method with phenolphthalein solution. The results are summarised in Table 3.3. Some of the photos were shown in Figures 3.7 and 3.8. It can be seen from the measured results that carbonation depths in the concrete cover are in general very low. Even for the concrete with the highest water-binder ratio (mix 236, w/b.75), the carbonation depth was about 5 mm after over years exposure under the highway environment. The placement of concrete near the ground where the average moisture level is relatively high could be one of the reasons to the low carbonation depth. As expected, the cracked zone can easily be carbonated and also give the paths for chloride ingress, initiating corrosion at an early age. Figure 3.8 shows the carbonation front in the concrete cover of 5 mm for rebar 2 in beam 223 B at about 38 mm distance from the left side, where there is a pre-crack with wideness of.~.2 mm. The corresponding corrosion can be seen in Figure 3.4. Table 3.3 Carbonation depths [mm] in concrete covers Concrete beam 22 BB 223 B 236 AB 236 BB Rebar, cover 3 mm ~2 ~ 2~4 3~6 Rebar 2, cover 5 mm ~2 ~* 3~5 4~6 * 5 mm around a cracked zone, see Figure 3.8. Figure 3.7 Carbonation depth in concrete cover of beam 236 BB.

3 Figure 3.8 Carbonation front in concrete cover of beam 223 B. 3.4.5 Chloride ingress at the cover levels It has been observed from concrete beams 22 BB and 236 BB (both without precracking) that corrosion of rebar with 3 mm cover is more severe than that with 5 mm cover, see Figures 3.9, in contrast to the conventional knowledge of steel protection in concrete. One possible explanation could be that the lower part of concrete beam was covered by the snow lump under the winter, which hindered the splashing water in direct contact with the concrete, resulting in less chloride ingress. To verify this, small cores of diameter 5 mm were taken from these two beams at the heights where rebars were embedded. Chloride contents at the depths of, 2 and 3 mm were determined using the same methods as will be described in 4.2. The results are shown in Figure 3.. It can be seen that the chloride ingress in the upper part of the beam where rebar was embedded with cover 3 mm is more than in the lower part of the beam where rebar was embedded with cover 5 mm, especially in the concrete beam 236 BB. Figure 3.9 shows that, for non-cracked concrete beams 22 BB and 236 BB, corrosion were initiated from the undersides of rebars, where often exist voids or high water content paste due to segregation under the rebars. For the pre-cracked concrete beams, the corrosion should be initiated at the positions with wider cracks.

3 Figure 3.9 Corrosion in rebars. upper photo: upsides of rebars; lower photo: undersides of rebars. Cl [% by wt of binder].2.8.6.4 22 BB, upper cover 3 mm 22 BB, lower cover 5 mm 236 BB, upper cover 3 mm 236 BB, lower cover 5 mm.2 2 3 4 5 Depth [mm] Figure 3. Chloride ingress in upper and lower parts of concrete beams.

32 4 Measurements of Chloride and Moisture Profiles 4. Sampling Totally 34 concrete blocks of size 4 3 3 mm were taken to the laboratory at SP for measurements of chloride and moisture profiles. Three cores of diameter mm, two from the vertical exposure surface and one from the upper horizontal exposure surface, were taken at the positions as shown in Figure 4.. When the cores became surface dry after coring, they were individually sealed in double thick plastic bags to prevent from further evaporation of moisture. One of the cores from the vertical surface was sent to Lund Institute of Technology for moisture measurement and the rest were divided between Chalmers University of Technology (the st set) and SP (the rest sets) for chloride profiling. Φ, L: -2 Marking: X-S2 Φ, L: -2 Marking: X-T Direction to highway 2 2 2 Φ, L: 5 Drill first! Marking: X-S X = Concrete block number, e.g 2 DK3 (see right side) Drilling arranged in three sets in order to facilitate the moisture measurements at Lund Institute of Technology st set 2 DK3 22 CK2 22 CK6 23 GK2 23 EK 24 BK2 25 K2 26 DK3 27 BK2 28 AK 28 CK 29 BK2 2nd set 2 K2 2 BK2 2 BK6 22 K2 23 BK2 24 K2 27 BK2 28 K2 29 K2 22 K2 22 BK2 222 K2 3rd set 223 K2 224 BK2 225 K2 226 BK2 227 BK2 228 BK3 23 BK2 23 BK2 232 K2 236 CK2 Figure 4. Illustration of sampling positions at concrete block. 4.2 Measurement of chloride profiles The same techniques as used in the previous investigations (e.g. Tang, 23b) were used in this project for measurement of chloride profiles. Powder samples were taken from each core by means of dry-grinding on a lathe with a diamond tool, successively from the exposed surface to a certain depth. The depth of each sample was measured from the lathe with an accuracy of.5 mm. After the grinding, the powder samples were immediately dried at 5 C and then stored in a desiccator for later chloride and calcium analysis. The acid soluble chloride content in each sample was determined principally in accordance with AASHTO T26 using potentiometric titration on an automatic titrator Metrohm Titranor 76 with chloride selective electrode and Ag/AgCl reference electrode. A sample size of about gram was used to facilitate the parallel calcium analysis. The soluble calcium content in each powder sample was determined parallel to the determination of chloride content, using the technique reported by Tang (23a).

33 4.3 Measurement of moisture profiles The measurement of moisture profiles was carried out at the department of Building Materials, Lund Institute of Technology. After arrival of the cores individually sealed in double thick plastic bags, they were stored in the laboratory at the room temperature not longer than a few days prior to sampling. A slice of about 2 mm thick was split from each concrete core at depths of about 2~4, 4~6, 6~8 and 8~ mm starting from the exposure surface, with the help of a compression jack. A large piece of sample of about ~3 g and a number of small pieces of sample were immediately taken, using hammer and chisel, from the central portion of the freshly split slice. The large piece was immediately weighed and then placed in a box for measurement of degree of capillary saturation, while the small pieces were stored in a glass test tube for measurement of RH (Relative Humidity). The technique for measurement of RH has been well described by Nilsson (98) and for degree of capillary saturation by Hedenblad and Nilsson (985). After the above sampling, another slice was successively split and samples were taken. The above sampling process was repeated until all the samples were taken from each core. 4.4 Measured chloride profiles The results of chloride and calcium profiles in each core are given in Appendix 2. 4.4. Effect of binder types The chloride profiles from concrete with different binders are summarised in Figures 4.2 to 4.9. It can be seen from these figures that, for concrete with low water-binder ratios (w/b <.4), the pozzolanic additions reveal reduction of chloride penetration, while for concrete with w/b.4~.5, the pozzolanic additions reveal unclear effect and, in some cases, the chloride ingress in the concrete with pozzolanic additions is even deeper than in the Portland cement concrete (see figures 4.7 to 4.9). Addition of blast-furnace slag tends to increase chloride binding, while addition of limestone as in the cement P Kalk C seems decrease chloride binding. Normally, the addition of pozzolanic materials, such as silica fume, flyash and blastfurnace slag, in concrete will improve the pore structures through the secondary hydration, resulting in less permeable concrete. As a consequence, the concrete with pozzolanic additions should have better resistance to chloride ingress. The chloride profiles in figures 4.7 to 4.9 do not show the positive effect of pozzolanic additions on chloride ingress. One possible reason is probably due to the application of new techniques for salt spreading which results in less splashing of de-icing salts to the sides of highway. With the decreased sources of chlorides, the previously penetrated chlorides in concrete may be washed out during the non-freezing period. More permeable the concrete is, more chlorides may be washed out. However, more data are needed to confirm this explanation. Another possible reason could be the large scatter in chloride ingress in such an environment. The chloride profiles taken from the same type of concrete reveal significant differences, e.g. the replicate profiles in figures 4.7 and 4.9. The chloride profiles taken from earlier exposure periods also showed very large scatters [Lindvall, 22], as will be discussed later in 5.6.