Carbonation of concrete bridge.docx

Carbonation of concrete bridge.docx

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Carbonationofconcretebridge

Carbonationofconcretebridge

structuresinthreeSouthAfricanlocalities

aDepartmentofCivilEngineering,UniversityofCapeTown,Rondebosch,7701,SouthAfrica

bDepartmentofCivilEngineering,UniversityofCanterbury,NewZealand

cHHOConsultingEngineers,CapeTown,SouthAfrica

Received15February2007; 

revised1June2007; 

accepted7June2007. 

Availableonline17July2007.

Abstract

Ratesofcarbonationforreinforced

concretebridges

wereinvestigatedforthreelocalitiesinSouthAfrica:

CapePeninsula,DurbanandJohannesburg.Carbonationdatafromapproximately90in-service

bridges

agedbetween11and76yearswereinterpretedintermsofinfluenceofmaterialsandenvironmentoncarbonationrates.Thedataweregroupedwithrespectto

concrete

strengthgradeandexposureconditionpriortostatisticalanalysis.

Bridges

inJohannesburghadthehighestratesofcarbonationowingtotherelativelydryenvironment.Durban

bridges

hadlowercarbonationratesthanJohannesburg

bridges,

buthigherthan

bridges

intheCapePeninsula,ascribedtodifferencesinambienttemperatureandthenatureofprecipitation.Overall,averagecarbonationratesforGrade30

concretes

overa30-yearperiodvariedfromapproximately0.3 mm/annuminCapePeninsulastructurestoapproximately0.7 mm/annumforJohannesburgstructures.Exposurecondition,characterizedbydegreeofshelter,hadlittleinfluenceoncarbonationrateinDurbanandJohannesburg

bridges,

ascribedtotheaveragerelativehumidityanddurationofprecipitationattheselocalities.Carbonationratesforolder

bridges

werelowerthanfornewerstructures,attributedpossiblytochangesincementcharacteristicswithtimerelatedtotheneedforfasttrackconstructioninmodernstructures.

Keywords:

Carbonation;Carbonationrate;Exposureconditions;

Concretebridges

;Relativehumidity

ArticleOutline

1.Introduction

2.Climaticconditions

3.Analysisofinsitucarbonationdata

3.1.Groupingthedata

3.1.1.Exposureconditions

3.1.2.Concretestrengthgrade

3.2.Statisticalanalysis

3.3.Detectionofgrossoutliers

4.Rateofcarbonation,andpredictionmodels

4.1.Workedexample:

CapePeninsula,exposedgrade40concreteelements

4.2.DurbanandJohannesburglocalities

4.3.Comparisonofcarbonationratesbetweenlocalities

5.Carbonationratesin“OLD”and“NEW”structures

6.Carbonationpredictionfordesign

7.Closure

Acknowledgements

Appendix.Statisticalanalysisexample（ExposedGrade40elements,CapePeninsula）

References

1.Introduction

Carbonationofconcreteisacommonchemicalreactioninwhichatmosphericcarbondioxidereactsmainlywithcalciumhydroxidefromcementhydrationtoformcalciumcarbonateinthepresenceofwater.Thisreactionoccursrapidlyinhighlypermeableconcrete（e.g.concreteofhighwater/cementratiowithinadequatecuringand/orinsufficientcompaction）,exposedintherelativehumidityrangebetween50%and75%（theoptimumRHrangeforcarbonation）（ACI[1],Richardson[2]）.

CarbonationofconcretereducesthepHoftheporesolutionasitconvertscalciumhydroxidetocalciumcarbonate.Oncethisacidificationprocessadvancestothesteelreinforcement,depassivationofreinforcementoccurs.Thereafter,corrosionofreinforcementmayensueinthepresenceofbothmoistureandoxygen.Eventually,productsofcorrosionwillbeformed,whichmaycausecrackingandspallingofthecoverconcrete.Inevitably,structuralandaestheticaspectsofthestructurearetherebycompromised,leadingtohighrepairandrehabilitationcostsforthesedamagedstructures.

Thispaperreportsastudyoncarbonationofreinforcedconcretebridgesagedbetween11and76years,inthreeSouthAfricanlocalities:

theenvironsofthecoastalcitiesofCapeTown（theCapePeninsula）（30bridges）andDurban（32bridges）,andtheinlandcityofJohannesburg（30bridges）.Thestudyaroseoutofconcernoverincreasingincidencesofcarbonation-inducedcorrosionofbridgestructuresandthustheneedtoquantifyinsitucarbonationrates.Thepaperprovidesinformationonthemajorclimaticfactorsintheselocalities,givesanalysesofinsitucarbonationdataandcarbonationrates,andcommentsonthelikelyeffectsofchangesincementcharacteristicswithtime.Theanalysesallowedderivationofcarbonationpredictionmodelstoassistindecision-makingformaintenanceofreinforcedconcretestructurestomanagecarbonation-inducedcorrosionriskandassociatedcostsofrepair.Usingtheinformation,coverdepthsfordesignoffuturestructurescanbeassessed,providedimplicationsofanychangesincementcharacteristicsareaccountedfor.

2.Climaticconditions

Climaticconditionsgoverntheexposurerelativehumidity（RH）,temperature,andprecipitation（rainfall）.Theclimatesforthethreelocalitiesvaryfromcool,wetwintersandhot,drysummersfortheCape,tomildorhotconditionswithhighRHinDurban,tocold,drywintersandhotsummerswithmainlyshortprecipitationperiodsinJohannesburg.CarbonationofconcreteisadiffusionprocessandissensitivetotheinternalRHoftheconcrete.AlthoughinternalRHrelatestotheexternalatmosphericRH,itdoesnotvaryasrapidlyasexternalRH.Thus,theaverageexternalRHfor“wet”and“dry”seasonsforthelocalitieswasused,relatedtotheperiodwithhighandlowrainfall,respectively.Table1showsaveragelocality-specificRHandtemperaturevaluesforthe“wet”and“dry”seasons.

Table1.

AverageRH（basedonmeanhourlyvalues）andtemperatures（basedonaveragedailyvalues）forhighandlowrainfallseasonsforthethreelocalities（S.A.WeatherService[3]）

Rainfall

Locality

CapePeninsula

Durban

Johannesburg

Month

Ave.MonthlyRH（%）

Ave.RH（%）

Month

Ave.MonthlyRH（%）

Ave.RH（%）

Month

Ave.MonthlyRH（%）

Ave.RH（%）

High（“wet”）

April

76

78[14]

Oct.

78

79[23]

Nov.

66

68[19]

May

80

Nov.

79

Dec.

68

June

80

Dec.

79

Jan.

69

July

80

Jan.

80

Feb.

70

Aug.

78

Feb.

80

March

68

Sept.

76

March

79

April

64

–

–

April

78

−

−

Low（“dry”）

Oct.

72

71[19]

May

75

73[18]

May

56

51[13]

Nov.

70

June

71

June

52

Dec.

70

July

71

July

49

Jan.

70

Aug.

73

Aug.

46

Feb.

70

Sept.

77

Sept.

46

March

73

–

–

Oct.

57

Full-sizetable

Thevaluesinsquareparenthesesaretheaveragetemperature（°C）forthe“wet”and“dry”seasons.

ViewWithinArticle

Table1suggeststhattheCapePeninsulaandDurbanwouldhaverelativelylowratesofcarbonationduringthewetseasonwiththeirhighaverageseasonalRH,asthesemi-saturatedconcreteporeswouldhinderdiffusionofcarbondioxide.CarbonationwouldbefasterforDurbanthanforCapeTownduringthisseasonduetothedifferenceintheaveragetemperatures.Bycontrast,theJohannesburgareawouldhaveahighrateofcarbonationinbothhighandlowrainfallseasons,theaverageseasonalrelativehumiditiesbeingintherangefortheoptimumRHforcarbonation.Therefore,structuresinJohannesburgshouldhaveahigherrateofcarbonationthansimilarstructuresintheCapePeninsulaandDurban.

Importantly,thenatureofprecipitationinthethreelocalitiesisnotthesame.RelativelyshortrainfalldurationsgenerallyoccurinDurbanandJohannesburg,whilstrainfallperiodsarelongerintheCape,givingdeeperpenetrationofmoistureintotheconcrete.

Table1alsogivesanindicationoftheriskofreinforcementcorrosionafterdepassivation.Richardson[2]statesthattheoptimumconcreteRHforcorrosionisabove80%.Therefore,structuresintheCapePeninsulaandDurbanwilltendtoexperiencecorrosioninthehighrainfallseason,withcarbonationoccurringpredominantlyinthelowrainfallseason.Particularattentionshouldbepaidtothedesignandmonitoringofreinforcedconcretestructuresinthesetwolocalitiessincecarbonation-inducedcorrosionislikelytobeacauseofdeterioration.Incontrast,structuresintheJohannesburgareawillingeneralnotexperiencerapidcorrosionastheaverageseasonalRHiswellbelowthecriticalcorrosionRHofabout80%.Therefore,carbonationofconcretemayberapidinthislocalitybutseriouscorrosionisnotinevitable.However,ifmoisturefromexternalsources（e.g.leakage,ponding）penetratesintothestructuresafterdepassivation,corrosioncouldtakeplace.Also,structureswithverylowcovertosteelmayexperiencecorrosion-induceddamage,sincemoisturecouldpenetratethelimitedcoverlayer.

3.Analysisofinsitucarbonationdata

DatawereobtainedfromRonné[4]（CapePeninsula）,andfromforensicstudiesconductedforpublicroadsagenciesbyMoore[5]（Durbanandadjacentareas;JohannesburgMotorwayandadjacentNationalRoute3（N3）Bridges）.ThedataofBallimandLampacher[6]（Johannesburg/N3Bridges）werealsoused.Ingeneral,averagecarbonationdepthswereobtainedfrommultiplemeasurementsusingphenolphthaleinsolutionsprayedoncoresorwedgescutfromelementsofthestructures.

Fig.1a–cshowinsitucarbonationdataasafunctionofageofthestructure.Thedataexhibitaremarkablywidescatter,notonlybecauseconcreteitselfisavariablematerial,butalsoduetovariableconstructionfactorsandclimaticconditions.Therefore,thedataweregroupedintermsofexposureconditionsandgradeofconcreteinordertominimisethescatterandgainabetterunderstandingofcarbonationrate.

Full-sizeimage（46K）

Fig.1. Overviewofcarbonationdata（a）CapePeninsula（b）Durbanand（c）Johannesburglocalities.

ViewWi