Setting Bar-Bending Requirements for High-Strength · PDF fileSetting Bar-Bending Requirements for High-Strength Steel Bars ... Concrete Reinforcing Steel Institute ... Setting Bar-Bending

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SettingBar-BendingRequirementsforHigh-StrengthSteelBars

StephenD.ZhaoWassimM.GhannoumSponsoredby:TheCharlesPankowFoundationTheConcreteReinforcingSteelInstituteTheAmericanConcreteInstitute’sFoundation,ConcreteResearchCouncilJune30,2016

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CPFResearchGrantAgreement#01-15

Fundedby

CHARLESPANKOWFOUNDATIONP.O.Box820631

Vancouver,Washington98682Co-fundedby

TheConcreteReinforcingSteelInstitute’s,andTheAmericanConcreteInstitute’sFoundation

PrincipalInvestigator: Dr.WassimM.Ghannoum

GraduateResearchAssistant: StephenZhao

IndustrySupport:

IndustryChampions: MikeMota,VPEngineering,CRSI

AdvisoryPanel: SalemFaza,MMFX

DenisHunter,Gerdau

ErikNissen,NUCORSteelSeattle

JacobSeltzer,CMC

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Acknowledgements

This projectwasmade possible by funding from the Charles Pankow Foundation, the

ConcreteReinforcingSteelInstitute,andtheAmericanConcreteInstitute’sFoundation.

ThesteeldonationsofCMC,Gerdau,MMFX,andNUCORSteelInc.Seattlearegratefully

acknowledged.Thebendingwasperformedatnocosttothisprojectbythefabrication

facilityatHarrisRebarinNewBraunfels.ThetechnicalcontributionsofMikeMota,the

project’sindustrychampion,DavidMcDonald,CRSIPresidentandCEO,andtheadvisory

panelisgratefullyacknowledged.

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Abstract


The reinforcing steel industry is currently developing high-strength

reinforcingbarswithspecifiedyieldstrengthsof80and100ksiduetoincreased

demandforsuchgrades inconcreteconstruction.However,noneofthehigher

steelgradesareabletomatchthebenchmarkmechanicalpropertiesofgrade60

steel; with each high-strength variant diverging from benchmark behavior in

different ways. There is concern that the less ductile higher grade reinforcing

barsmayfractureatthebendsandmayrequirelargerbenddiameters.Limited

testsareavailablethatinvestigatetherelationbetweenbenddiameterandthe

ductility,orconverselythebrittleness,ofreinforcingbarsatbends.Nosuchtests

existforthenewlydevelopedhigh-strengthreinforcementhavingyieldstrengths

exceeding 80 ksi. Bend/re-bend (or re-straightening) tests were conducted on

grade60andhighergradereinforcingbarstoinvestigaterelationsbetweenbend

diametersandbendperformance.Thetestsweremonitoredusingdigitalimage

correlation technology from which never-before recorded comparative

measureswere obtained. Test results indicated significant differences in bend

performance betweenbars of varying grades, such thatwider bend diameters

maybenecessaryforcertainhighergradebars.

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TableofContents

ListofTables ........................................................................................................ viii

ListofFigures ......................................................................................................... ix

CHAPTER1:INTRODUCTION 1

1.1Motivation ......................................................................................................... 1

1.2ObjectivesandScope ........................................................................................ 1

CHAPTER2:BACKGROUND 3

2.1Metallurgy ......................................................................................................... 3

2.1.1QuenchingandTempering(QT) ............................................................ 3

2.1.2Micro-Alloying(MA) .............................................................................. 4

2.1.3PatentedMicrostructureManipulation(MMFX) ................................. 4

2.2StrainAging ....................................................................................................... 4

2.3BendTests ......................................................................................................... 6

CHAPTER3:EXPERIMENTALPROGRAM 9

3.1OverviewofProgram ........................................................................................ 9

3.1.1Bend/re-bendTests .............................................................................. 9

3.1.1.1SpecimenDetailsandPreparation ........................................... 9

3.1.1.2ControlledTestParameters .................................................... 11

3.1.1.3FixedParameters .................................................................... 14

3.1.1.4Instrumentation ...................................................................... 15

3.1.2StrainAgingTests ................................................................................ 18

3.1.3MonotonicTests ................................................................................. 18

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3.2SpecimenNomenclature ................................................................................. 18

CHAPTER4:TESTRESULTSANDGENERALOBSERVATIONS 20

4.1MonotonicTests ............................................................................................. 20

4.1.1SummaryofObservations .................................................................. 28

4.2StrainAgingTests ............................................................................................ 29

4.2.1StrainAgingTestData ......................................................................... 29

4.2.2StrainAgingPerformanceMeasures .................................................. 33

4.2.3StrainAgingResults ............................................................................ 34

4.2.3.1EffectsofStrainAgingDuration .............................................. 34

4.2.4.2EffectsofVanadiumonStrainAging ...................................... 37

4.3Bend/re-bendTests ........................................................................................ 39

4.3.1TypicalStressvs.StrainRelationsinBend/re-bendTests .................. 41

4.3.1TypicalStressvs.RemainingBendAngleRelationsinBend/re-bendTests ............................................................................................................. 43

4.3.2SummaryofResultsforBend/re-bendTests ...................................... 45

4.3.3EffectsofBarYieldStrengthonRe-bendPerformance ...................... 48

4.3.4EffectsofBarSizeonRe-bendPerformance ...................................... 53

4.3.6EffectsofBendDiameteronRe-bendPerformance .......................... 58

CHAPTER5:RECOMMENDATIONSFORMINIMUMBENDDIAMETERS 70

5.1PerformanceObjective ................................................................................... 71

5.2RelationbetweenInsideBendDiameterandThresholdStrainRatio(αf) ...... 72

5.3MinimumBendDiametersbasedonMinimumASTMElongations ............... 74

5.4MinimumBendDiametersbasedonCRSIMillDatabaseElongations ........... 80

5.4.1ProbabilitiesofFailureofBendsBasedonGrade60ProductionData82

5.4.2RequiredBendDiameterstoAchieveTargetProbabilitiesofFailureBasedonProductionData ............................................................................. 85

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CHAPTER6:SUMMARYANDCONCLUSIONS 88

References ............................................................................................................ 92

viii

ListofTables

Table1:Measuredβforvariousbarsizesvsβfrompinusedforbending...........................13

Table2:Re-bendtestloadingrates.......................................................................................17

Table3:Summaryofmeanmaterialpropertiescalculatedfrommonotonictensiontests.21

Table4:Ratioofεf/εun.........................................................................................................27

Table5:Summaryofstrainagingtestresults.......................................................................34

Table6:Bend/re-bendResultsfor#8and#11Bars..............................................................46

Table7:Bend/re-bendResultsfor#5Bars............................................................................47

Table8:TheoreticalStrainsatBends....................................................................................63

Table9:ASTMA615FractureElongationRequirements......................................................75

Table10:ASTMA706FractureElongationRequirements....................................................76

Table11:ASTMA1035FractureElongationRequirements..................................................76

Table 12: Comparison between minimum bend diameters to achieve the yield strength

performanceobjectiveandACI318-14minimumbenddiameters(assumingthesameACI318-

14benddiametersapplytohigh-strengthbars)..........................................................................77

Table13:Summaryof fractureelongationvalues fromCRSIMillDatabase forASTMA615

bars...............................................................................................................................................81

Table14:Summaryof fractureelongationvalues fromCRSIMillDatabase forASTMA706

bars...............................................................................................................................................81

Table 15: Summary of fracture elongation values fromCRSIMill Database for dual grade

ASTMA615/A706bars..................................................................................................................81

Table 16: Minimum required fracture strains for bars bent to current ACI 318-14 bend

diameters......................................................................................................................................83

Table 17: Probabilities of failing the performance objective for bars in production today

benttoACI318-14insidediameters.............................................................................................84

Table18:Targetbenddiameterratiosrequiredtoachieveagivenprobabilityoffailingthe

yieldperformanceobjective.........................................................................................................86

Table19:Recommendedtargetbarbenddiameterratios...................................................87

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ListofFigures

Figure1:Typical stress-straincurvesshowingtheeffectsofstrainaging (G.TVanRooyen,

1986)...............................................................................................................................................5

Figure2:Pictureofbend/re-bendtestcoupons(NISTGCR13-917-30).................................8

Figure3:RMSArnoldBenderusedtobendspecimens........................................................10

Figure4:Verifyingbendspecimentolerance........................................................................10

Figure 5: Specimen dimensions for #11, #8 and #5 barswith ACI 318-14minimum bend

diameter(db=barnominaldiameter)..........................................................................................11

Figure6:Exampleofbend/re-bendspecimenundertestingwithtargets...........................16

Figure7:Stress-straincurvesfrommonotonictestsofgrade60A706bars........................22



Figure10:Stress-straincurvesfrommonotonictestsofgrade80A706bars......................25

Figure11:Stress-straincurvesfrommonotonictestsofgrade100bars..............................26

Figure 12: Stress-strain curves for grades 60 and 80 A706 and grade 100MA bars from

Manufacturer1,notagedandstrainaged1month.....................................................................30

Figure 13: Stress-strain curves for grade 60 and 80 A706 bars fromManufacturer 4, not

agedandstrainaged1month......................................................................................................31

Figure14:Stress-straincurvecomparisonsbetweengrade80QTfromManufacturer4and

grade100MAfromManufacturer1,notagedandstrainaged3months...................................32

Figure15:Apparentyieldpointandlossofelongationafterstrainaging............................33

Figure16:Normalized∆!vsstrainagingduration...............................................................35Figure17:Normalized!fractureA.vsstrainagingduration........................................................36Figure18:∆!/!"vs%Vanadium..........................................................................................37

Figure19:Normalized!fractureAvs%Vanadium......................................................................38

Figure20:Photographofabarthatfracturedinthe90obendafterlimitedstraightening..40

Figure21:Typicalstressvsstrainrelationsfornon-straightenedbars.................................42

Figure 22: Typical stress vs strain relations for straightened bars exhibiting high ductility

duringre-bending.........................................................................................................................42

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Figure23:Typicalstressvs.remainingbendanglerelationsfornon-straightenedbars......44

Figure24:Typicalstressvs.remainingbendanglerelationsforstraightenedbars.............44

Figure25:Normalizedre-bendstressatfracturevsmeasuredbaryieldstrength...............49

Figure26:Normalizedre-bendstressatfracturevsmeasuredbaryieldstrength...............50

Figure27:Normalizedre-bendstrainatfracturevsmeasuredbaryieldstrength...............51

Figure28:Remainingbendangleatfracturevsmeasuredbaryieldstrength.....................52

Figure 29: Comparison of normalized stress at fracture vs measured yield strength for

variousbarsizes............................................................................................................................54

Figure 30: Comparison of normalized strain at fracture vs measured yield strength for

variousbarsizes............................................................................................................................55

Figure 31: Re-bend stress at fracture normalized by tensile strength vs measured yield

strengthfor#5bars.......................................................................................................................57

Figure32:NormalizedRe-bendFractureStressvsTargetBendDiameter...........................59

Figure33:NormalizedRe-bendFractureStrainvsTargetBendDiameter...........................60

Figure34:NormalizedRe-bendFractureAnglevsTargetBendDiameter............................61

Figure35:MaxTheoreticalStrainDerivation........................................................................62

Figure 36: Bar uniform strain vs measured yield strength overlaid with the estimated

maximumbendstrains(εma).........................................................................................................65

Figure37:Normalizedfracturestressvsnormalizedtheoreticalbendstrain......................67

Figure38:Normalizedfracturestress(fub/ft)vsnormalizedtheoreticalbendstrain............68

Figure39:Normalizedfracturestrainvsnormalizedtheoreticalbendstrain.......................69

Figure40:Normalizedfracturestressvsnormalizedtheoreticalbendstrain......................71

Figure 41: Relation between βTH and bar fracture strain (εf) for various threshold strain

ratios(αf).......................................................................................................................................73

Figure 42: βASTM,ADJ and bar fracture strain (εf) for various threshold strain ratios (αf) for

ASTMA615#5bars.......................................................................................................................78

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CHAPTER1:Introduction

1.1Motivation

Thereisanincreasingneedforhigherstrengthreinforcingsteelinseismicandnon-

seismic applications. A main driver for higher strengths is the need to reduce bar

congestion in seismic designs and reduce material quantities generally. Steel

manufacturersintheUnitedStatesarecurrentlydevelopingreinforcingbarswithyield

strengths reaching 120 ksi andwith varyingmechanical and chemical properties. The

newhigh-strengthbarsarebeingproducedusingvaryingmethods,themostcommonof

whicharequenchingand tempering,andmicro-alloying.However,noneof thehigher

steelgrades inproductionareable tomatch thebenchmarkmechanicalpropertiesof

grade60 steel;witheachhigh-strengthvariantdiverging frombenchmarkbehavior in

differentways.Thereisconcernthatthelessductilehighersteelgradesmayfractureat

the bends and may require larger bend diameters. Anecdotal evidence has been

reported of high-strength bars (HSRB) fracturing at the bends when dropped at a

constructionsite(particularlyincoldweather).

Inthisreport,high-strengthreinforcingbars(HSRB)aredefinedassteelreinforcing

barswithyieldstrengthsof80ksiorhigher(i.e.,grade80orhigher).

1.2ObjectivesandScope

Efforts are underway to produce newASTM specifications for HSRB to give steel

mills a clear target to aim for in their production of HSRB. To complete the ASTM

specifications for HSRB, bar bending requirements need to be revisited given recent

evidence of HSRB fracturing at bends. Limited tests are available that investigate the

relation between bend diameter and the ductility, or conversely the brittleness, of

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reinforcing bars at bends. No such tests exist for the newly developed high-strength

reinforcementhavingyieldstrengthsexceeding80ksi.

Themain objectives of this study are to evaluate the performance under load of

bends satisfying ACI 318-14 (2014) in HSRB, and compare that performancewith the

benchmarkperformanceofgrade60barsbentinthesamewayandtosamediameters.

To achieve project objectives, bend and re-bend tests, such as those specified in

New-Zealand and United-Kingdom standards (AS/NZS 4761:2001; BS

4449:2005+A2:2009),wereconductedonHSRBandgrade60bars.Thetestsprovideda

measureof thereservestrengthandductilityofbarbends,whichcannotbeobtained

usingvisualinspectionofbendcracking,asdescribedinASTMstandardsforreinforcing

bars (ASTMA615, A706, A1035).HSRBwith varyingmanufacturing processes, grades,

diameters,andbenddiametersweretested.Therangeofparameterswasselectedto

represent the most typical bar properties and manufacturing processes currently in

production or development in theUnited States. All barswere fully strain-aged after

bending and prior to re-bending, to represent typical conditions of bar bends in

concretestructures.Strainagingtestswereconductedtoevaluatetheeffectsofstrain

agingonallbar types considered,and todetermine the requiredwait timeafterpre-

strainingorbendingbeforere-bendingcouldbeperformed.

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CHAPTER2:Background

2.1Metallurgy

ThreemainproductionmethodsarecurrentlyusedintheUnitedStatestoproduce

HSRB.Eachof thesemethodsgeneratesHSRBwithdifferingmechanical and chemical

properties. The three processes are quenching and tempering, micro-alloying, and

manipulation of the microstructure using alloying and heat treatment. Steel bars

produced through quenching and tempering typically exhibit relatively low tensile to

yieldstrength(T/Y)ratiosandrelativelyhighstrainsatfracture.Steelbarsproducedby

micro-alloying have a relatively high tensile to yield strength ratio and relatively high

strainsatfracture.HSRBproducedusingthethirdproductionmethodaretheonlyones

withASTMspecifications(ASTMA1035(2011)).Thesebarstypicallyhavelargetensileto

yieldstrengthratiosbutrelatively lowstrainsatfracture.Thedifferencesbetweenthe

threeproductionmethodsandthebarpropertiestheyproducearebrieflydiscussedin

thissection.

2.1.1QuenchingandTempering(QT)

The process of quenching and tempering (QT) consists of quenching the steel

immediately after rolling and then allowing the bar to be tempered by the heat

remaininginthecorewhilegraduallycooling.Asaresult,theQTprocessproducessteel

withmechanical properties that vary significantlybetween its inner core layer and its

outer skin layer,with the inner core having a lower yield strength andmoreductility

thantheouterlayer.QTtreatedbarsretaintheiryieldplateausincetheyhavenotbeen

strainhardenedand,sincetheoverallchemicalcompositionhasnotbeenaltered,they

canbeweldableiftheirchemistrysatisfiesASTMA706requirements.QTsteeltypically

exhibitsa lowT/Yratioontheorderof1.15forgrade100reinforcingbars.Slavinand

Ghannoum(2015)providesmoredetailsabouttheQTprocess.

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2.1.2Micro-Alloying(MA)

Micro-alloying is a process that involves introducing small amounts of alloys in

ordertoachievethedesiredpropertiesinsteelbars.Vanadiumisoneofthealloysmost

commonlyused to increase the strengthof reinforcingbars. It increases strengthand

fracture toughness primarily due to inhibition of grain growth during heat-treatment

and the precipitation of carbides and nitrides. The use of Vanadium can reduce the

amount of carbon needed to achieve higher strengths and is therefore useful for

achievingweldableHSRB.Micro-alloying can produce amarked yield point and a T/Y

ratio larger than that from quenched and tempered steels (on the order of 1.25 for

grade100reinforcingbars).SlavinandGhannoum(2015)providesmoredetailsabout

themicro-alloyingprocess.

2.1.3PatentedMicrostructureManipulation(MMFX)

ThepatentedMMFXprocess involvesmanipulating themicrostructureof steel to

obtainthedesiredmechanicalpropertiesandstrength.Theprocessgeneratesbarswith

stress-stainrelationsthatdonothaveawell-definedyieldpoint,exhibitarelativelyhigh

T/Yratio,buthaverelativelylowfractureelongations.TheMMFXsteelbarssatisfythe

ASTMA1035specifications.TheA1035specificationsmaintainthesamebenddiameter

for testing bar bends as in the ASTMA615 andA706 specifications used for the vast

majorityofbarscurrently inproductionintheUnited-States.ACI318-14(2014)allows

theuseofA1035grade100bars inconfinementapplicationsandrequiresthemtobe

bentatthesamediameterasothersteelgradesincludinggrade60.

2.2StrainAging

Strainagingisdefinedastheprocessbywhichsteelstrainedbeyonditselasticlimit

undergoes time dependent changes in itmechanical properties. Typically, reinforcing

bars strainedbeyond theirelastic limitwill,over time, seean increase in their tensile

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strengthandadecrease in theirductilityas illustrated inFigure1. Strainaging isalso

proven to affect the brittle transition temperature in steel (G.T. Van Rooyen, 1986).

Factorsaffectingstrainagingincludethesteelcomposition,temperature,andthetime

elapsedsince largestrainswere incurred.Strainaging ismostlyattributed tonitrogen

reallocation within the steel matrix (G.T. Van Rooyen, 1986). Higher temperatures

accelerate this process; hence strain aging occurs much faster in warmer regions.

Typically,mostoftheeffectsofstainaging insteelreinforcingbarswilloccurwithina

fewmonthsafterinelasticstrainsareincurred(G.T.VanRooyen,1986).

Figure1:Typicalstress-straincurvesshowingtheeffectsofstrainaging(G.TVanRooyen,1986)

Asreinforcingbarsarebent,theyexperiencelargeinelasticstrains.Barbendsare

therefore prone to strain aging embrittlement, which may cause them to fracture

prematurely and limit their ability to sustain inelastic deformations during structural

loading.

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ResearchdonebyG.TVanRooyen (1986) andRashid (1976) suggests thatmicro-

alloyedsteel includingtitaniumandvanadiumcan lowertheeffectsofstrainagingon

steelbars.Suchalloyshavepropertiesthatallowthemtobondwiththenitrogeninthe

composition to form nitrides. These reactions limit the amount of free nitrogen

throughoutthesteelthatisattributedtostrainagingeffects.

2.3BendTests

Three main categories of experimental tests are useful for investigating the

behaviorofbends in reinforcingbars,witheachcategoryof testsgeared toanswera

particularsetofquestions:

1-Visualinspectionsofbends(ASTMbendtests)

2-Bend/re-bendtests

3-Bendtestsinconcrete

1-ASTMreinforcingbarspecifications(suchasA615,A706,andA1035)specifythe

following bending requirement “The bend test specimen shall withstand being bent

aroundapinwithoutcrackingontheoutsideofthebendportion.”Therequiredbend

test therefore involves bending bars to 180o (or 90o for #14 and larger bars) at a

specifiedpinbenddiameter.Avisualinspectionisthenperformedtoidentifycrackingat

thebend. If no cracking is visually observed, a specimen is deemed topass thebend

test. The pin diameters specified in ASTM bend tests are tighter than those used in

construction,as specifiedbyACI318-14or theCRSIhandbook,and thereforeprovide

some degree of safety against observing cracking in bends during bar fabrication.

However, while this test is simple to perform, it does not provide ameasure of the

reservestrengthandductilityofbarbends,asaload-testcan.Itispossiblethatmicro-

crackingnotvisibletotheeyemaycompromisetheperformanceofbarsin-situ.

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2 - Bend and re-bend tests have been performed in New-Zealand (AS/NZS

4761:2001;HopkinsandPoole(2008)andtheUnited-Kingdom(BS4449(2005)).Inthese

tests, bar coupons are bent to the required angle and bend diameter (Figure 2), and

then straightened at either quasi-static or dynamic loading rates. For grade 60 bars,

workhardeningincreasessteelstrengthatthebendsandtypicallycausesthecoupons

to fracture away from the bends in a ductile manner. However, if bars have limited

ductilitysuchasHSRB,straindemandsatthebendsmaycausecracks,whichcanmake

bends weaker than the unbent portions of the bars and more susceptible to brittle

fracture. Ifabar fails inabrittlemanneratabend, it isconsideredtohavefailedthe

bend/re-bendtest.If,however,abarfailsinaductilemanner,thenitisdeemedtohave

passedthetest.Thistypeoftesthastheadvantageofputtingbar-bendsunderloadand

thereforeprovidesadirectmeasureof the strengthandductilityperformanceofbar-

bends.

HopkinsandPoole(2008)conductedbendandre-bendtestsonnewly introduced

grade500E(~72ksi)barsinNewZealandandAustralia.Thestudyaccountedforstrain

agingandexploredtheeffectsofcoldtemperatureontheperformanceofbends.The

study tested bars produced using micro-alloying (MA), as well as quenching and

tempering (QT). Test results confirmed that current bar bend diameters used inNew

Zealandwereadequateforthatgradeofsteel,regardlessofthemanufacturingprocess.

Amarkedworsening of the performanceof bar-bendswas observed at temperatures

below-10oCelsius.

It should be noted that the bend/re-bend tests apply larger demands on the bar

bendsthantheywouldnormallyseeinaconcretestructure.Forthisreason,itisbestto

comparethebend/re-bendperformanceofHSRBtothatofgrade60bars,whichhave

beenusedfordecadesandhaveshownadequateperformanceinconcretemembers.

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Figure2:Pictureofbend/re-bendtestcoupons(NISTGCR13-917-30)

3 - Bends in reinforcing bars can also be tested in concrete. In such tests, the

interactionbetweentheconcreteandbar-bendscanbeinvestigated.Simplifiedversions

ofthetestincludeembeddingahookedbarintoaconcreteblockandpullingonituntil

failure.Possiblefailuremodesthatcanbeexpectedinblocktestsinclude:barfracture

outside theblockwheredemandson thebar arehighest, bar failure inside theblock

closertooratthebend,orsplittingoftheconcreteblock.Suchtests,however,maynot

expose bends to the worst loading they could experience in a structure, as the

surroundingconcretecanrelievethebendsofsomeload.Incontrast,someoftheworst

loading on bar-bends can arise in confinement applications, where an expanding

concrete core partially straightens hoop bends while applying high tensile loads to

them.Anothercriticalapplicationforbar-bendsisindamagedregions,wherebondto

concrete and its beneficial effects on bends are reduced (e.g., joints under severe

seismicloading,orseverelycrackedregions).

Nevertheless, testsofbarbends in concretemembers areessential for validating

theadequateperformanceofbarbendsinHSRB.However,suchtestsareexpensiveto

conductanddonoteasily lend themselves to the taskofdeterminingminimumbend

diameterswhileexploring thenumerousvariables thataffect theperformanceofbar-

bends.

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CHAPTER3:ExperimentalProgram

3.1OverviewofProgram

Theexperimentalprogramconsistedofthreetypesoftests.

1. Bend/re-bendTests2. StrainAgingTests

3. MonotonicTests

3.1.1Bend/re-bendTests

Objectives: Bend/re-bend testswere conducted to quantify residual strength and

elongation capacities under load in bar bends and compare high-strength reinforcing

barbendperformancewiththatofgrade60barbends.

3.1.1.1SpecimenDetailsandPreparation

Thebend/re-bendspecimensweresimilartothoseusedinNew-ZealandbyHopkins

andPoole(2008)anddescribed inBS4449:2005+A2:2009(2005).Barspecimenswere

constructedbybendingstraightcouponsintoa“V”shapehavingtwo45degreebends

andone90degreebend(Figure2).Couponswerebentbyalocalfabricatoraccording

to typical bending practices using an RMS Arnold Bender (Figure 3). Bars were bent

abouttheirweakaxis,withthelongitudinalribsfacingverticallyinthebender.Bending

wasconductedataroomtemperatureofabout20°C.Barswerethenlefttostrainage

prior to re-bending them in tension until fracture in a uniaxial testing machine.

Tolerancetemplateswereusedtoensurethatspecimenswerebentaccurately(Figure

4).The internalbenddiametersforthefirstbatchofbarspecimenswerethesmallest

specified in ACI 318-14. The dimensions of bend/re-bend coupons with those bend

diameters are presented in Figure 5. In subsequent bending, some #5 bar bend

diameterswereincreasedto5dband6db(withdb=barnominaldiameter).

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Figure3:RMSArnoldBenderusedtobendspecimens

Figure4:Verifyingbendspecimentolerance

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Figure5:Specimendimensionsfor#11,#8and#5barswithACI318-14minimumbenddiameter(db=bar

nominaldiameter)

3.1.1.2ControlledTestParameters

a- SteelGradeandSpecifications:A706 (highductility)andA615 (lowerductility)

grade 60 barswere tested to provide benchmark performance.Grade 60 bars

were obtained from three mills to obtain a representative sample of current

production.HSRBproducedusingthemainthreeproductionmethodsintheU.S.

were also tested: MA (Micro-Alloying), QT (Quenching and Tempering), and

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MMFX(ASTMA1035).ThemainfocusinHSRBwasongrade100steel(havinga

yieldstrengthequaltoorhigherthan100ksi)butlimitedtestswereconducted

ongrade80barsaswell.Inall,steelbarsfromfourmanufacturersweretested:

a. Manufacturer1(M1):MicroAlloyedSteel(MA)

b. Manufacturer2(M2):PatentedMicrostructureMMFX(ASTMA1035)

c. Manufacturer3(M3):CombinationofQuenchandTemperingandMicro

Alloying(QT)

d. Manufacturer4(M4):CombinationofQuenchandTemperingandMicro

Alloying(QT)

b- BarSize:Threebarsizesweretestedinthisstudycoveringacommonrangeof

sizesusedinconstruction.Thebarssizesusedwere#5,#8,and#11.

c- Bend Diameters: Bar specimens were first bent to the current ACI 318-14

minimumbenddiameters.For#11bars,theinternalbenddiameterwasabout8

db. For #8 bars, the internal bend diameter was about 6db. For #5 bars, the

minimum bend diameter for transverse reinforcement was initially targeted,

which is4db.However,afterobservingpoorerperformance in#5HSRBat that

benddiameter,#5barswerebentandtestedwith5dband6dbbenddiameters.

The latterbenddiameterof6dbcorrespondstotheACI318-14minimumbend

diametersfor#5longitudinalorotherbarsintension.It isnoteworthythatthe

selectedbarsizes (#5,#8,and#11)correspondto the largestsizewithagiven

benddiameterbeforethenextbiggersizerequiresalargerbenddiameterinACI

318-14.

Due to bending pin availability and spring-back in the bending process, final

internal bend diameters were close to but not exactly equal to their target

values.Table1 lists thetargetACI318-14,aswellasthemeanachieved inside

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benddiameterratiosforthe90degreebendsinthespecimens(βACI=ACI318-14

benddiameter/dbandβActual=meanmeasuredinsidebenddiameter/db).The

table also lists the pin diameters usedduring bending and its associatedbend

ratio(βPin=pindiameter/db).

Table1:Measuredβforvariousbarsizesvsβfrompinusedforbending

TargetβACI(db)

MeanMeasuredβActual(db)

PinDiameter

(in.)

βPin(db)

Spring-backDifference=(βActual-βPin)/

βActual

ErrorfromTarget=(βACI-βActual)/βACI

#11(8db)4Samples

8 7.82 10.0 7.30 7.1% 2.3%

#8(6db)4Samples

6 5.64 5.0 5.00 12.7% 6.1%

#5(6db)2Samples

6 5.39 3.0 4.80 12.3% 10.1%

#5(5db)3Samples

5 4.80 2.5 4.00 20.0% 4.0%

#5(4db)3Samples

4 3.59 2.0 3.20 12.3% 10.2%

As can be seen in Table 1, themean actual bend diameterswere less than 10%

lower than the target bend diameters. Bending pins were only offered in ½”

increments, hence thepindiameterswere chosen such that the final benddiameters

wereasclosetotargetdiametersaspossiblebutnotlarger.Theactualbenddiameters

werehigherthanthepindiametersby7to20%duetospring-backafterbending.

14

3.1.1.3FixedParameters

a- BendAngle:Aprimarybendangleof90°was tested in this study.Otherbend

angleswerenotincludedinthescopeofthisproject.

b- LoadingRatewasappliedquasi-staticallytore-bendtestsinthisproject.

c- #ofSpecimens:Atleast3barspecimenspertypeweretested.

d- Strain Aging: Bar types sensitive to strain aging (as determined in the strain

agingtests)werebentandallowedtostrainagepriortobeingsubjectedtothe

re-bend tests. Thiswasdone toaccount for thepossibledeleteriouseffectsof

aging on bar ductility, and to reproduce actual in-situ conditions of bends in

concretestructures.

e- Temperature:Thefocusofthisprojectwasonassessingthebendperformance

ofHSRBatrelativelywarmambienttemperatures(20to25°C).Thisevaluationis

a necessary first step prior to bending and testing bars in cold climate at or

below their brittle transition temperatures. Cold temperature bending and

testingshouldbeevaluatedinfuturework.

f- Bending Equipment: An RMSArnold Benderwith the capability to bend at 16

RPMand14RPM(RevolutionsPerMinute)wasusedforallspecimens.

g- BendingRate:Allspecimenswerebentat16RPM.

15

3.1.1.4Instrumentation

Theload-cellintheuniversaltestingmachineprovidedreadingsoftheappliedload.

Strains and deformations of the bars were obtained during testing using a high-

resolution opticalmeasurement system reported by Sokoli et al (2014). Targetswere

appliedalongthefourstraightportionsofbentbarspecimens(Figure6).Thelocations

of thetargetsweretrackedusingtheopticalsystemwhile thebarswerebeingtested

(Figure6).

16

Figure6:Exampleofbend/re-bendspecimenundertestingwithtargets

17

3.1.1.5TestProtocol

Barwere gripped at each end using hydraulic grips thatwere 6 inches long. The

loading during re-bend tests consisted of three loading rates. The loading protocol

startedwithalowloadingratetopickupinitialloadinthespecimens.Thiswasdonein

order toobtain sufficientdata in casesofbar fracture at low forces.Once specimens

carriedalargerforce(~10%ofyield)theloadingratewasincreaseduntilthebarswere

almoststraight.Thentheratewasagainloweredtoobservefractureinmoredetail.All

threebarsizeswereloadedwiththesameratespresentedinTable2.

Table2:Re-bendtestloadingrates

18

3.1.2StrainAgingTests

Objectives: Toquantify strainagingeffectsonall thebars tested in thebend/re-

bendtestsandidentifythedurationafterwhichmoststrainagingeffectsleveloff.

Strain-agingtestswereconductedonstraight#5barcouponsforallsteeltypes.All

bars,exceptthosesatisfyingASTMA1035,werestrainedintensiontoapredetermined

strain value of 0.04 in a universal testmachine and then unloaded. A1035 barswere

strainedintensiononlytoastrainof0.02duetotheirrelativelylowuniformelongation

values.Uniformelongationisdefinedasthebarelongationatpeakstress.Tensiontests

were then performed on the pre-strained bars immediately after pre-straining, one

month after pre-straining, and three months after pre-straining. Strain aging was

allowed tooccurat room temperature (~20°C). Thebarswere strained in the strain-agingtestsatthestrainrateusedinthemonotonictensiontests.

3.1.3MonotonicTests

Monotonictensiontestswereperformedonat least threestraightspecimensper

bartypetoidentifythemechanicalpropertiesofthesteelbars.Thetestsfollowedthe

procedures specified in ASTMA370 (ASTM StandardA370-15),with strainsmeasured

overagaugelengthof8inches.

3.2SpecimenNomenclature

Formonotonic and re-bend tests, the following nomenclature is used to identify

specimens:

19

Manufacturer#_Grade_Bar Type or Specification_Diameter (in eighth of an

inch)_Testuniqueidentifier

e.g.:(M1_Gr60_A706_5_01orM1_Gr100_MA_5_01)

Forstrainagingteststhefollowingnomenclatureisused:

Manufacturer#_Grade_Bar Type or Specification_Diameter (in eighth of an

inch)_MonthsAged_Testuniqueidentifier

e.g.:(M1_Gr60_A706_5_3Mo_01)

20

CHAPTER4:TestResultsandGeneralObservations

4.1MonotonicTests

Monotonictestswereconductedonthreeormorecouponsforeachbartypeused

in the bend/re-bend testmatrix. Themeanmechanical properties and typical stress-

strainrelationsofeachbar typearesummarized inTable3andFigure7toFigure11.

Uniform elongation measures were obtained by following the ASTM E8 procedures

(ASTMStandardE8/E8M-15a).

21

Table3:Summaryofmeanmaterialpropertiescalculatedfrommonotonictensiontests

BarSize Manf. GradeYield

Strengthf y(ksi)

TensileStrengthf u(ksi)

T/YRatioUniformElongationεun(%)

FractureElongation

εf(%)

A70660 64.3 93.2 1.45 12.5% 21.9%A70680 81.7 111.2 1.36 10.3% 18.2%MA100 110.4 139.6 1.26 8.8% 12.7%A61560 63.2 104.0 1.64 11.2% 17.9%A61580 80.5 121.1 1.50 9.1% 14.4%

2 A1035100 125.0 162.1 1.30 4.9% 11.7%A70660 77.4 102.8 1.33 9.1% 14.5%A70680 83.1 109.7 1.32 9.1% 13.8%A61560 63.6 90.7 1.43 12.1% 17.1%A70660 67.5 95.8 1.42 11.5% 16.0%A70660 60.5 90.0 1.49 11.5% 18.9%A70680 84.4 114.1 1.35 9.8% 16.4%MA100 99.0 125.2 1.27 8.9% 13.0%A61560 63.7 101.3 1.59 10.7% 16.2%A61580 84.4 123.5 1.46 9.2% 14.6%

2 A1035100 131.4 164.3 1.25 5.2% 10.8%A70660 80.9 101.7 1.26 9.0% 14.9%A70680 81.6 104.0 1.27 8.9% 14.6%QT100 98.7 126.0 1.28 7.2% 9.9%A61560 68.1 95.8 1.41 12.0% 18.3%A70660 66.7 90.9 1.36 12.3% 18.5%A70660 65.7 93.9 1.43 10.5% 14.7%A70680 86.5 115.2 1.33 9.5% 13.8%MA100 113.0 135.1 1.20 8.2% 11.9%A61560 63.0 97.9 1.55 11.2% 16.5%A61580 81.8 112.9 1.38 9.9% 13.9%

2 A1035100 125.6 163.6 1.30 5.4% 9.5%A70660 81.6 99.7 1.22 8.8% 12.7%A70680 83.3 102.7 1.23 8.8% 12.6%QT100 90.0 129.7 1.44 6.9% 8.4%A61560 80.2 102.9 1.28 10.3% 15.0%A70660 66.0 90.8 1.38 12.1% 18.4%QT100 106.1 125.6 1.18 7.6% 10.7%

#5

1

3

4

#11

1

3

4

#8

1

3

4

22

A706Grade60

Figure7:Stress-straincurvesfrommonotonictestsofgrade60A706bars

#5 #8 #11

Manufacturer1

Manufacturer3

Manufacturer4

23

A615Grade60


#5 #8 #11Manufacturer1

Manufacturer4

24

A615Grade80


#5 #8 #11

Manufacturer1

25

A706Grade80


#5 #8 #11

Manufacturer1

Manufacturer3

26

Grade100

Figure11:Stress-straincurvesfrommonotonictestsofgrade100bars

#5 #8 #11

Manufacturer1

Manufacturer2

Manufacturer3

Manufacturer4

27

Table4summarizestheratiosofεf/εunforvariousbarsgradesandsizes.

Table4:Ratioofεf/εun

#5 #8 #11

60 1.46 1.60 1.5780 1.45 1.66 1.6460 1.47 1.52 1.5180 1.40 1.59 1.57

A1035 100 1.77 2.09 2.37

A706

A615

εf/εun

28

4.1.1SummaryofObservations

Theyieldstrengthsofgrade60A706barstestedinthisstudyrangedfrom63-77ksi.The

fracturestrainsofthosebarswereintherangeof15%to22%.Theyieldstrengthsofgrade60

A615bastestedinthisstudywerearound63to64ksiwiththeexceptionofManufacturer4’s

#5bars thatcame inatyield strengthof80.2ksi.The fracturestrainsofgrade60A615bars

wereintherangeof15%to19%.

Grade80A706barshadyieldstrengthsrangingfrom81to87ksi.Thefracturestrainsof

those barswere in the range of 13% to 19%. Grade 80 A615 barswere only obtained from

Manufacturer 1 and had yield strengths ranging from 80 to 84 ksi for all sizes and fracture

elongationsrangingfrom14%to15%.

Grade 100 bar from Manufacturers 1 and 4 exhibited a yield plateau and had yield

strengthsintherangeof105to110ksi,withfracturestrainsrangingfrom11to13%.Barsfrom

Manufacturers2and3showednoyieldplateau.Yieldstrengthsforthosebarswereobtained

using a 0.2% strain offset method (ASTM A1035, 318-14). Yield strength values for

Manufacturer 2 grade 100 bars ranged from 125 to 131 ksi while those forManufacturer 3

rangedfrom90to99ksi.Manufacturer2grade100barsexhibitedfracturestrainsfrom10to

12% whereas Manufacturer 3 grade 100 bars exhibited fracture strains from 9 to 10%.

Manufacturer 2 grade 100 bars differ from other bars as they exhibited a relatively shallow

descendingrelationbetweenstressandstrainspasttheiruniformelongation.

29

4.2StrainAgingTests

4.2.1StrainAgingTestData

Strainagingtestswereconductedon#5barsofalltypesusedinbend/re-bendtests.Figure

12toFigure14presenttypicalstress-strainrelationsobtainedfromthestrainagingtests.The

arrows in the figures are used to show the difference from non-strain aged to strain aged

results.Asexpectedbasedonpast research (Rashid,1976), grade80and100barsproduced

usingmicro-alloyingwithVanadium(Manufacturer1)exhibited limitedstrainagingcompared

withgrade60barsfromthesamemanufacturer(Figure12).BarsfromManufacturer4,which

contained relatively small amounts of micro-alloys, exhibited more pronounced strain aging

effects as indicatedbyapparent gains in tensile strengthanddecreasedductility (Figure13).

Figure 14 highlights how different manufacturing processes result in different strain aging

results.

30

Figure12:Stress-straincurvesforgrades60and80A706andgrade100MAbarsfromManufacturer1,notagedandstrainaged1month

31

Figure13:Stress-straincurvesforgrade60and80A706barsfromManufacturer4,notagedandstrainaged1month

32

Figure14:Stress-straincurvecomparisonsbetweengrade80QTfromManufacturer4andgrade100MAfromManufacturer1,notagedandstrainaged3months

33

4.2.2StrainAgingPerformanceMeasures

Two parameters were used to quantify the effects of strain aging, (∆!/!!) and(!fractureA/ !fracture).∆! isthedifferencebetweentheapparentyieldpointuponreloadingafterspecimenagingandthestressatunloadingasseenintheFigure15.∆!wasnormalizedwith

respect to the measured yield strength of the bars, !!, for comparison between different

grades. The strain at bar fracture of aged bars normalized by the strain at fracture prior to

aging (!fractureA /!fracture)wasalsoused toassess the severityof strainaging.The strainsweremeasuredoveraneightinchgaugelength.

Figure15:Apparentyieldpointandlossofelongationafterstrainaging

∆!

!fractureA !fracture

34

4.2.3StrainAgingResults

Table5summarizesthestrainagingresultsaveragedoverthecouponstestedperbartype.

Atleastthreecouponsweretestedperbartype.TheconcentrationofVanadiumwasfoundto

behighly correlatedwith strainagingeffects,namely the increase inapparent yield strength

andthereductioninfactureelongation.Table5alsosummarizestheVanadiumconcentrations

foreachbartype.

Table5:Summaryofstrainagingtestresults

4.2.3.1EffectsofStrainAgingDuration

Figure 16 and Figure 17 illustrate the variation of (∆!/!!) and (!fractureA / !fracture) withstrainagingduration.Dashed lines representgrade60bars, shaded lines representgrade80

bars and solid lines represent grade 100bars. As can be seen in the figures, themajority of

strainagingeffectsoccurwithinthefirstmonthwithlimitedchangesobservedthereafterforall

bartypes.Basedonthesefindings,bend/re-bendtestswereconductedonbarsstartingatleast

onemonthaftertheinitialbendingwasconducted.

35

Figure16:Normalized∆!vsstrainagingduration

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0

0123

36

Figure17:Normalized!fractureA.vsstrainagingduration

37

4.2.4.2EffectsofVanadiumonStrainAging

In Figure 18 and Figure 19, clear relationships can be seen between Vanadium

concentrationsandtheeffectsofstrainaging.Theapparentyieldstrengthofstrain-agedbars

decreaseswith increasingconcentrationsofVanadium,uptoaconcentrationofabout0.08%

(Figure18).Beyondthatconcentration,(∆!/!!)appearsto leveloffat0.03regardlessoftheconcentration of Vanadium. Changes in fracture elongation also appear to vary with the

concentrationofVanadiumwithstrain-agedbarsbecomingmoreductilewithhigherVanadium

concentrations. An increase in ductility post-aging was observed at relatively high

concentrationsofVanadium(inexcessof0.35%).Onlytwodatapointsareavailablehowever

withthosehighconcentrations.

Basedontheseobservations, the followingrelationsweredevelopedbetweenVanadium

concentrationsinpercentages(%V)and(∆!/!!)or(!fractureA/ !fracture).

∆! !! = −1.26 ∗ %! + 0.13 0.00≤%V≤0.08 (Equation4.2.1)

∆! !! = 0.03 0.08<%V (Equation4.2.2)

Figure18:∆!/!!vs%Vanadium

38

!!"#$%&"'( !!"#$%&"' = 0.9 ∗ %! + 0.9 0.00≤%V≤0.26 (Equation4.2.3)

!!"#$%&"'( !!"#$%&"' = 1.12 0.26<%V (Equation4.2.4)

Figure19:Normalized!fractureAvs%Vanadium

0.8

0.9

1

1.1

1.2

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

! fractureA/! fr

acture

%Vanadium

1Month

3Month

39

4.3Bend/re-bendTests

A total of 60 bend/re-bend testswere performed. The following performancemeasures

wereusedtocomparetheperformancebetweenthevariousbartypesandgrades:

a) Fracturelocation(in90obend,45obendorinstraightregions)

b) Remainingbendangleatfractureduringre-bending(θb)

c) Axialstraininbaratfracturenormalizedbyitsuniformelongationstrain(!b/!un)d) Axialstressinbaratfracturenormalizedbyitsyieldstrength(fub/fy)

e) Axialstressinbaratfracturenormalizedbyitstensilestrength(fub/ft)

Figure 20 shows a fracture in the 90 degree bend. The remaining bend angle during re-

bending was measured using the targets bracketing the 90 degree bend. Two slopes were

calculatedfromthetwostraightregionsadjacenttothe90degreebend.Usingtherelationship

between the two slopes, the remaining bend angle was calculated, which starts around 90

degrees and goes to almost zero when a bar is fully straightened. Axial strain in bars was

obtainedbyaveragingthestrainsbetweenthetargetsfurthestawayfromeachotherinthetop

andbottom straight regions as indicatedby thearrows in Figure20.Axial stress inbarswas

obtainedbydividingtheloadreadingofthetestmachinebythenominalareaofthebars.

40

Figure20:Photographofabarthatfracturedinthe90obendafterlimitedstraightening

41

4.3.1TypicalStressvs.StrainRelationsinBend/re-bendTests

Theaxialstress-strainrelationsforthreere-bendtestsinwhichfractureoccurredpriorto

fullbendstraighteningareshowninFigure21.Theaxialstress-strainrelationsforbarsinwhich

fracture occurred after straightening and after significant inelastic straining occurred in the

straightregionsareshowninFigure22.AscanbeseeninFigures21and22,theinitialportion

ofthestressstrainrelationsmeasuredarenotlinear.Thisisattributedtothebendingmoments

thatdevelopduringre-bendinginthetopandbottomstraightportionsofthespecimenswhere

thestrainsweremeasured(Figure20).Thesemomentsdieoutoncethebarsstraigthen.

42

Figure21:Typicalstressvsstrainrelationsfornon-straightenedbars

Figure22:Typicalstressvsstrainrelationsforstraightenedbarsexhibitinghighductilityduringre-bending

0

0.2

0.4

0.6

0.8

1

1.2

0 0.002 0.004 0.006 0.008 0.01 0.012

f ub/f "

!b

M1_60_A615_5_01

M1_60_A615_5_02

M1_60_A615_5_03

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.02 0.04 0.06 0.08 0.1

f ub/f "

!b

M1_60_A706_5_01

M1_60_A706_5_02

M1_60_A706_5_03

43

4.3.1TypicalStressvs.RemainingBendAngleRelationsinBend/re-bendTests

Typicalbarstressversus remainingbendangle relationsareplotted inFigure23 forbars

that fractured prior to full straightening. Typical bar stress versus remaining bend angle

relationsareplottedinFigure24forbarsthatfracturedafterfullstraightening.Inthefigures,

90degreesdenotestheinitialbendangleand0degreesdenotesthatabar-bendhasbeenfully

straightened.TestshighlightedinFigure23andFigure24arethosehighlightedinFigure21and

Figure22,respectively.

44

Figure23:Typicalstressvs.remainingbendanglerelationsfornon-straightenedbars

Figure24:Typicalstressvs.remainingbendanglerelationsforstraightenedbars

0

0.2

0.4

0.6

0.8

1

1.2

020406080100

f ub/f "

Remainingbendangleduringre-bending,θb(degrees)

M1_60_A615_5_01

M1_60_A615_5_02

M1_60_A615_5_03

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

020406080100

f ub/f "

Remainingbendangleduringre-bending,θb(degrees)

M1_60_A706_5_01

M1_60_A706_5_02

M1_60_A706_5_03

45

4.3.2SummaryofResultsforBend/re-bendTests

Table 6 and Table 7 summarize values of the bend/re-bend test performancemeasures,

averagedoveratleastthreetestsperbartype.Table6containsresultsfrom#8and#11bars,

andTable7containsresultsfrom#5bars.

Thebend/re-bendtestssubjectedbarbendstoharsherstressandstrainhistoriesthanthey

typically encounter in concrete structures. As such, defining acceptance criteria values that

delineatedeficientin-situbendperformanceisnotstraightforward.However,selectingvalues

ofperformancemeasuresabovewhichbendperformancecanbedeemedadequateinconcrete

canbedoneconservatively.Inthisstudy,barswithvaluesofthenormalizedstressatfracture

(fub/fy) exceeding 1.0 are deemed to have adequate bend performance, as these bars have

reached their design strength prior to fracture. Likewise, barswith values of the normalized

fracture strain (!sb / !un)exceeding 0.2 are deemed to have adequate bend performance, as

thesebarstypicallystraightenfully,reachstressesinexcessofyield,andstraintoatleast20%

oftheiruniformelongationpriortofracture.

46

Table6:Bend/re-bendResultsfor#8and#11Bars

StraightRegions

45 90

A70660 M1_60_A706_11 64.3 93.2 12.5% 1.05 1.47 0 1/3 1/3 1/3 3/3A70680 M1_80_A706_11 81.7 111.2 10.3% 0.96 1.38 0 3/3 3/3MA100 M1_100_MA_11 110.4 139.6 8.8% 0.52 0.83 28 2/3 1/3 2/3A61560 M1_60_A615_11 63.2 104.0 11.2% 1.05 1.64 0 3/3 3/3A61580 M1_80_A615_11 80.5 121.1 9.1% 0.77 1.46 1 2/3 1/3 3/3

M2 A1035100 M2_100_A1035_11 125.0 162.1 4.9% 0.47 1.23 1 3/3 3/3M3 A70680 M3_80_A706_11 83.1 109.7 9.1% 0.86 1.32 0 3/3 3/3

A61560 M4_60_A615_11 63.6 90.7 12.1% 1.11 1.42 0 3/3 3/3A70660 M4_60_A706_11 67.5 95.8 11.5% 1.36 1.42 0 3/3 3/3

A70660 M1_60_A706_8 60.5 90.0 11.5% 1.01 1.50 0 3/3 3/3A70680 M1_80_A706_8 84.4 114.1 9.8% 0.92 1.35 0 3/3 3/3A706100 M1_100_MA_8 99.0 125.2 8.9% 1.11 1.28 0 3/3 3/3A61560 M1_60_A615_8 63.7 101.3 10.7% 1.12 1.61 0 3/3 3/3A61580 M1_80_A615_8 84.4 123.5 9.2% 0.73 1.45 0 3/3 3/3

M2 A1035100 M2_100_A1035_8 131.4 164.3 5.2% 0.43 1.17 1 3/3 3/3A70680 M3_80_A706_8 81.6 104.0 8.9% 0.99 1.28 0 3/3 3/3QT100 M3_100_QT_8 98.7 126.0 7.2% 0.38 1.16 1 1/3 2/3 3/3A61560 M4_60_A615_8 68.1 95.8 12.0% 1.06 1.41 0 2/3 1/3 3/3A70660 M4_60_A706_8 66.7 90.9 12.3% 1.18 1.36 0 1/3 2/3 3/3

YieldStressf y(ksi) θb<5degrees

TensileStressf t(ksi)

UniformStrainεun(%)

NormalizedStrainatFracture(εb/εun)

NormalizedStressatFracture(f ub/f y)

AngleatFracture

θb(degrees)

FractureLocationBarSize(Bend

Diameter)Manf. Grade

SpecimenName

M4

#11(8db)

#8(6db)

M3

M1

M1

M4

47

Table7:Bend/re-bendResultsfor#5Bars

48

4.3.3EffectsofBarYieldStrengthonRe-bendPerformance

AscanbeseeninFigure25to28theperformanceofbarbendsgenerallydecreasedasthe

measuredyieldstrengthofthebarsincreased.Thistrendholdsacrossallbarsizes.

A negative correlation can be observed between (fub/fy) during re-bending and the

measured bar yield strength (Figure 25). For #8 bar specimens with a target inside bend

diameter of 6db (or #8 (6db)) and for #11 (8db) specimens, test data exhibited relatively low

variability about the observed trends highlighted by the linear regression lines in Figure 25.

However, for #5 (4db) specimens, test data exhibited relatively large variability about the

observedtrend.The#8(6db)and#11(8db)specimensconsistentlyfailedatstresseswellabove

theyieldstrengthandclosertothetensilestrengthsofthebarsatallbarstrengthlevels;with

theexceptionoftheM1_Gr100_MA_11specimen.Todecouplethetrendof lowertensile-to-

yield-strength ratios with increasing yield strength from bend performance, the stresses at

fracturenormalizedbythetensilestrengthofthebars(fub/ft)areplottedversusthemeasured

yieldstrengthofthebarsinFigure26.Ascanbeseeninthefigure,thenegativecorrelationcan

beobservedfor#5barsasyieldstrengthincreases.#8and#11barsseemtoreachmorethan

95%offtwiththeexceptionofM3_Gr100_QT_8andM1_Gr100_MA_11.

Anegativecorrelationbetweentheyieldstrengthandthenormalizedstrainatfracturecan

also be observed in Figure 27. Most #8 bars (6db) and #11 (8db) bars, regardless of yield

strength, straightenedalmost fullyduring re-bending (Figure28). For#5 (4db)bars,however,

there is a clear increase in the remaining bend angle at fracture as the bar yield strength

increased.

49

Figure25:Normalizedre-bendstressatfracturevsmeasuredbaryieldstrength

50

Figure26:Normalizedre-bendstressatfracturevsmeasuredbaryieldstrength

51

Figure27:Normalizedre-bendstrainatfracturevsmeasuredbaryieldstrength

52

Figure28:Remainingbendangleatfracturevsmeasuredbaryieldstrength

53

4.3.4EffectsofBarSizeonRe-bendPerformance

Therelationshipbetween(fub/fy)andbaryieldstrengthiscomparedfordifferentbarsizes

in Figure 29. As can be seen in the figure, #11 (8db) and #8 (6db) bars reached significantly

largerstressesat fracturethan#5(4db)bars. In fact,exceptforonebartype,all#11and#8

barsreachedstressesinexcessoftheiryieldstrengthduringre-bending.However,asthebend

diameterof#5barswasincreasedfrom4db,thebendperformanceof#5barsimprovedand

becamecomparabletothatofthe largerbarswhenthetarget insidebenddimeterofthe#5

barswas6db(sameasthatof#8bars).Testdatathereforesuggesta limited influenceofbar

sizeonthebarstressat fractureduringre-bending,butasignificant influenceofbend inside

diameter on stress at fracture. Similar trends can be observed between bar size and the

normalizedstrainatfractureFigure30.

54

Figure29:Comparisonofnormalizedstressatfracturevsmeasuredyieldstrengthforvariousbarsizes

55

Figure30:Comparisonofnormalizedstrainatfracturevsmeasuredyieldstrengthforvariousbarsizes

56

Giventherelativelylargescatterintheperformanceof#5barscomparedwiththatofthe

largerbars,thepossibleeffectsofthemanufacturingprocessesontheperformanceof#5bars

areexploredinFigure31.The#5barsfromManufacturer1bentat4db,withtheexceptionof

M1_Gr60_A706bars,fracturedatstressesbelowthelinearregressiontrendlineforall#5(4db)

bars.However,thefracturesstressesofbarsfromManufacturer1bentat5dbweredistributed

aboveandbelowthetrendline,butexhibitedhighvariability.BarsproducedbyManufacturer

3showedthehighvariabilityaswell,withsomebarsfailingatsignificantlyhigherstressesthan

thetrendlinesandothersatsignificantlylowerstresses.BarsfromManufacturers2and4were

consistentlyabovethetrendlines.

57

4db

5db

6db

Figure31:Re-bendstressatfracturenormalizedbytensilestrengthvsmeasuredyieldstrengthfor#5bars

58

4.3.6EffectsofBendDiameteronRe-bendPerformance

The #11 (8db) and #8 (6db) bar specimens performed reasonably well across all grades.

However,the#5(4db)barsdidnot.Forthisreason,additional#5barswerebentatinsidebend

diametersof5dband6db.

Theeffectsofincreasingbenddiameterson(fub/fy)canbeseeninFigure32.Inthefigure,

lineswith short dashesdenotebarswith yield strengths around60 ksi (grade60), lineswith

longerdashesdenotebarswithyieldstrengthsaround80ksi(grade80),andsolidlinesdenote

barswithyieldstrengthsaroundorexceeding100ksi(grade100).Ascanbeseenin

Figure 32, the stress at fracture of grade 60 bars is not affected significantly when

increasingbenddiametersfrom4dbto5db;withtheexceptionofM1_60_A615specimens.This

is because these bars are failing at stresses close to their tensile strength at both bend

diameters(Figure31).Ontheotherhand,thehighergrades80and100,fracturedatstresses

significantlybelowtheirtensilestrengthatabenddiameterof4db.Thismaybethereasonwhy

bars of these grades experienced significantly higher stresses at fracture with larger bend

diameters.

Whilemostgrade60barsfracturedatstressesaboveyieldwithbenddiametersof4db,the

majorityofgrade80and100barsdidnot.Evenwithabenddiameterof5db,themajorityof

grade 80 and 100 bars still did not reach their yield strengths prior to fracture during re-

bending.Itisonlywhenthebenddiameterisincreasedto6dbthatthemajorityofgrade80and

100barsreachedtheiryieldstrengthsatfracture.

Consideringthestrainperformancemeasure(!b/!un),allgradessawincreasesinstrainsatfracturewith increasing bend diameters,with only a few reaching their uniform elongations

duringre-bending(Figure33). Interestingly,eventhoughgrade60barsdidnotseesignificant

increasesintheirstressesatfracture,theydidseemarkedincreasesintheirstrainatfracture

duringre-bendingwithincreasingbenddiameters.

Considering the remaining bend angle at fracture (Figure 34), all grade 60 and 80

specimens bent at 5db essentially straightened during re-bending andmost did so with 4db

59

bends.Mostgrade100bars,ontheotherhand,fullystraightenedonlywhentheywerebentat

6dbandmostdidnotstraightenwhenbenttotighterdiameters.

Figure32:NormalizedRe-bendFractureStressvsTargetBendDiameter

TargetInsideBendDiameter(db)

60

Figure33:NormalizedRe-bendFractureStrainvsTargetBendDiameter


61

Figure34:NormalizedRe-bendFractureAnglevsTargetBendDiameter

Insummaryforallperformancemeasuresconsidered,increasingthebenddiameterfrom

4db to 6db was found to generally have a positive effect. Overall, #5 bars of all grades saw

increasedductilityatthebendswithincreasingbenddiameters.Grade80and100#5barssaw

increases instressatfractureduringre-bendingwith increasingbenddiameters.Grade60#5

bars,howeverdidnotastheyreachedstressesclosetotheirtensilestrengthevenwithabend

diameterof4db.


62

4.3.7TheoreticalStraininBentSpecimens

TheimpactofthelowerductilityofHSRBonbendperformanceisinvestigatedby

comparingthestrainsinducedbybendingwiththestraincapacityofbars,namelytheiruniform

elongation(!un).Uniformelongatingisusedhereasitislessdependentonbarsizeand

measurementgaugelengththanfractureelongation.Sincebendingstrainsaredifficultto

measure,themaximumtheoreticaltensionstrainattheoutersurfaceofthebarswas

estimatedasfollows,assumingzeroelongationattheneutralaxis(!!!!"#!$%&'().

Figure35:MaxTheoreticalStrainDerivation

!!!!"#!$%&'( = !!!!"

(Equation4.3.1)

!! = 2 ∗ ! ∗ ! ∗ !! + !! ∗ !!"# (Equation4.3.2)

!!" = 2 ∗ ! ∗ ! ∗ !! + !!!! ∗ !

!"# (Equation4.3.3)

!!!!"#!$%&'( = !∗!!!!!!∗!!!!!!!− 1 = !!!

!!!!− 1 (Equation4.3.4)

As canbe seen inEquation4.3.4, the theoretical strain is independentofbar sizeand is

onlyafunctionofβ,theratioofbendinsidediametertobardiameter.

! d!

63

UsingEquation4.3.4,themaximumtheoreticalstrain,εmt,experiencedattheedgeofthe

specimensusing the targetedACI318-14minimumbenddiameterswas calculated. Similarly,

themaximumtheoreticalbendstaincorrespondingtotheactualpinsizesusedduringbending,

εma,wascalculatedusingEquation4.3.4.Thestrainsbasedonpindiameterratherthanthose

based on the final bend diameter after spring back (Table 1) were considered as theywere

deemedtobemore representativeof themaximumstrainsexperiencedduringbending.The

calculatedstrainvaluesarepresentedinTable8.

Table8:TheoreticalStrainsatBends

BarSpecimens

#5(4db) #5(5db) #5(6db) #8(6db) #11(8db)

PinDiametersUsed(in.) 2.0 2.5 3.0 5.0 10.0

βACI=TargetBendDiameter/db 4 5 6 6 8

βPin=PinDiameter/db 3.20 4.00 4.80 5.00 7.27

TargetMaximumTheoreticalStrain,εmt

0.110 0.091 0.077 0.077 0.059

MaxTheoreticalStrainBasedonPinSize,εma

0.135 0.111 0.094 0.091 0.064

Figure 36 compares the maximum theoretical strains associated with the bending pin

diameters(εma)withtheuniformstrainvaluesofthebarstested.Apointbelowthetheoretical

strain linesuggeststhatthebarspecimenwasbentpast itsuniformstrain.Ascanbeseen in

the figure, all but one of the #11 bars were bent to a maximum estimated strain that is

considerablylowerthantheuniformstrainsofthebars.Thisisowingtothelargeratioofbend

tobardiameterusedfor#11bars(βPin).Most#8barswerealsostrainedsignificantlylessthan

uniform strains (Figure 36). All #5 bars, including grade 60 bars, were strained past their

uniform strain with βPin = 3.2 (for a 4db target bend diameter). In addition, most #5 bars,

especiallyhighergradebars,werestrainedpast theiruniformstrainwithβPin=4.0 (fora5db

targetbenddiameter). It isonlywhen#5barsarebendtoaβPin=4.8 (fora6db targetbend

diameter), that grade 60 and 80 bars experience an estimatedbend strain at or below their

64

uniformstrains.Grade100#5bars,however,stillappearedtohavebeenstrainedhigherthan

theiruniformstrainvaluesatβPin=4.8.

65

Figure36:Baruniformstrainvsmeasuredyieldstrengthoverlaidwiththeestimatedmaximumbendstrains(εma)

66

These observed trends between strains experienced and bar uniform strains may help

explain the poorer performance of #5 bars compared with the larger bars, and the poorer

performance of higher grade bars that typically have lower uniform elongations than their

lower grade counterparts. To explore these relations further, (εma/εun) values were plotted

versus(fub/fy)forallbarsinFigure37,versus(fub/ft)inFigure38andversus(εb/εun)inFigure39.

These figures indicateaclearnegativecorrelationbetweenthe ratioof theoreticalmaximum

strainsincurredduringbendingandbaruniformstrains(εma/εun)forallperformancemeasures

considered.Thefiguresthereforecorroboratethehypothesisthatbendsin#5andhighergrade

bars showed poorer performance because they were strained higher with respect to their

uniformelongations.

Relations can be drawn between performance measures and the demand parameter

(εma/εun)asseeninFigure37toFigure39.Givenatargetperformancemeasure,thesefigures

allow the selection of the maximum permissible bending strain to uniform strain (εma/εun)

allowable in bending. For a performance objective defined as fub/fy ≥ 1.0 during re-bending,

Figure 37 indicates that εma/εun should not exceed about 1.2 during bending. With the

exception of an outlying #11 data point and one #5 bar data point, we can see that all bar

specimenswithεma/εun≤1.2wereabletodeveloptheiryieldstrengthduringre-bending(Figure

37).Moreover,withtheexceptionofa limitednumberofspecimens, thosewithanεma/εun≤

1.2 were also able to achieve stresses during re-bending that exceed 80% of their tensile

strength(Figure38),andstrainsthatexceed50%oftheiruniformstraincapacities(Figure39).

Accordingtoanεma/εun≤1.2criteria,thebendpindiametersusedfor#11(8db),#8(6db)

and#5(6db)bars(Table8)resultinadequatebendperformanceforallbarsandgradestested

inthisstudy(Figure36).Atighterpindiameteroffourtimesthebardiameter,βPin=4.0,used

for #5 (5db) specimens, can be permitted for grade 60 and 80 bars but not grade 100 bars.

FinallyaβPin=3.2valueshouldonlybeusedforgrade60bars.

Otherlimitsonεma/εuncanalsobeselectedbasedonotherperformanceobjectivesusingFigure37toFigure39.

67

Figure37:Normalizedfracturestressvsnormalizedtheoreticalbendstrain

68

Figure38:Normalizedfracturestress(fub/ft)vsnormalizedtheoreticalbendstrain

69

Figure39:Normalizedfracturestrainvsnormalizedtheoreticalbendstrain

70

CHAPTER5:RecommendationsforMinimumBendDiameters

In this chapter, recommendations forminimum inside bend diameters for reinforcing

barsaregivenforvariousbarsizesandASTMsteelgrades.Therecommendationsaregivenasa

function of the minimum required fracture elongation of the bars, as given in the ASTM

standards A615, A706, and A1035. Minimum bend diameter recommendations are also

provided as a function of the historic elongation data obtained from the CRSIMill Database

(CRSI).Theminimumbenddiametersprovidedareintendedtodeliverbendsthatarecapable,

at aminimum, of sustaining their yield strength prior to fracture in a re-bend test. Fracture

elongation,!f,isusedhereasitisthestraingiveninASTMstandardsforreinforcingbarsandin

the CRSI Mill Database; even though this measure is dependent on the ratio between the

standard8”measurementgaugelengthandbardiameter.

71

5.1PerformanceObjective

AsdemonstratedinChapter4,theratiooftheoreticalmaximumtensilestrainatabend

basedonpindiameters(εma)tobaruniformelongation(εun)governsbendperformanceunder

load.Asimilarrelationcanbeobservedwithrespecttothefractureelongationofbars(εf)

(Figure40).AscanbeseeninFigure40,allbarsbenttopindiametersthatdidnotgeneratea

theoreticalpeakstrainexceeding0.8εf(εma≤0.8εf)wereabletoreachtheyieldperformance

objective;exceptforoneoutlying#11bartest.Forathresholdstrainratioαf=εma/εfbetween

0.8and1.0,8bartypesexceedtheyieldstressperformanceobjectivewhile5donot(Figure

40).Forαf=εma/εf>1.0,allbutonebartypesfailtheperformanceobjective.Barbendsthat

donotexceedthethresholdvalueofεma=0.8εfarethereforedeemedtosatisfytheminimum

performanceobjectiveandshouldhaveadequateperformanceunderloading.

Figure40:Normalizedfracturestressvsnormalizedtheoreticalbendstrain

72

5.2RelationbetweenInsideBendDiameterandThresholdStrainRatio(αf)

RecallEquation4.3.4relatingthetheoreticalpeaktensilestrain inabend(!!!!"#!$%&'()tothebenddiameterratio(! = !"#$ !"#$%&' !!).

!!!!"#!$%&'( =! + 1! + 1/2− 1

Substituting !!!!"#!$%&'( with !!!!, the threshold peak strain, we get the theoreticalminimumbenddiameterratiotobe:

!!" =!!!

!!!!!

!!!! (Equation5.1.1)

Figure41illustratesthegeometricrelationshipbetweenβTHandabar’sfracturestrain

(εf) for various threshold strain ratios (αf) based on Equation 5.1.1. This graph indicates the

minimumbendpindiametertousetogenerateapeakbendstrainwithinaprescribedfraction

(αf)ofthebarfractureelongation.Forexample,forabarwithafractureelongationεf=0.1,a

pindiameterβTH=5.75shouldbeusedtoachieveatheoreticalpeakstrainof80%ofεf(orαf=

0.8).

73

Figure41:RelationbetweenβTHandbarfracturestrain(εf)forvariousthresholdstrainratios(αf)

0123456789

10

0 0.05 0.1 0.15 0.2

β TH

εf

α=0.6

α=0.8

α=1.0

f

f

f

74

5.3MinimumBendDiametersbasedonMinimumASTMElongations

Theminimum bend pin diameter (βASTM) based on ASTMminimum elongation values

thatachievesthedesiredyieldstrengthperformanceobjectivecanbeobtainedbysettingthe

bar fractureelongationεf,ASTM=minimumASTMspecified fractureelongation,andαf=0.8 in

Equation5.1.1.Thisestimateofthepindiameterisconservativebecauseallbarswithanαf=

0.8 are expected to achieve the performance objective while all bars satisfying the ASTM

specificationhavefractureelongationsexceedingεf,ASTM(Table9,

75

Table 10,andTable11).Alessconservativeestimateonpindiametercanbeobtained

byusingεf,ASTMbutαf=1.0.Table12presentsthebendpindiameters(βASTM)basedonASTM

elongations forvarioussteel specifications,grades,andαfvalues.Aspring-back resulting ina

15%increaseinbenddiameterfromthepindiametertothefinalat-restinsidebenddiameter

canbeconservativelyassumedbasedonmeasuredspring-backvalues in thisstudy (Table1).

Thisassumptionresults in theadjustedvaluesβASTM,ADI =βASTM*1.15 thatcanbecompared

with current ACI 318-14 minimum inside bend diameters (βACI). Figure 42 illustrates this

comparison forA615,#5barsofvarioussteelgrades. InTable12,boldedratiosofβASTM,ADJ /

βACIindicatethatACIbenddiametersachievethedesiredperformanceobjective.

Table9:ASTMA615FractureElongationRequirements

ASTMA615FractureElongationRequirementsεf,ASTM=Min.elongationin8in.%

BarDesignationNo. Grade40 Grade60 Grade80 Grade100

3 11 9 7 7

4,5 12 9 7 7

6 12 9 7 7

7,8 … 8 7 7

9,10,11 … 7 6 6

14,18,20 … 7 6 6

76



BarDesignationNo. Grade60 Grade80

3,4,5,6 14 12

7,8,9,10,11 12 12

14,18 10 10



BarDesignationNo. Grade100 Grade120

3,4,5,6,7,8,9,10,11 7 7

14,8 6 6

77

Table12:Comparisonbetweenminimumbenddiameterstoachievetheyieldstrength

performanceobjectiveandACI318-14minimumbenddiameters(assumingthesameACI

318-14benddiametersapplytohigh-strengthbars)

βASTM,ADJ βASTM,ADJ/βACI βASTM,ADJ βASTM,ADJ/βACI40 4.0 5.4 1.35 4.3 1.0660 4.0 7.4 1.84 5.9 1.4780 4.0 9.7 2.42 7.6 1.90100 4.0 9.7 2.42 7.6 1.9040 6.0 5.4 0.90 4.3 0.7160 6.0 7.4 1.23 5.9 0.9880 6.0 9.7 1.61 7.6 1.27100 6.0 9.7 1.61 7.6 1.2760 6.0 8.4 1.40 6.7 1.1180 6.0 9.7 1.61 7.6 1.27100 6.0 9.7 1.61 7.6 1.2760 8.0 9.7 1.21 7.6 0.9580 8.0 11.4 1.42 9.0 1.12100 8.0 11.4 1.42 9.0 1.12

ASTMA615 Grade βACIαf=0.8 αf=1.0

#5

transverse

#5

longitu

dinal

#8

#11

βASTM,ADJ βASTM,ADJ/βACI βASTM,ADJ βASTM,ADJ/βACI60 4.0 4.6 1.15 3.6 0.8980 4.0 5.4 1.35 4.3 1.0660 6.0 4.6 0.77 3.6 0.5980 6.0 5.4 0.90 4.3 0.7160 6.0 5.4 0.90 4.3 0.7180 6.0 5.4 0.90 4.3 0.7160 8.0 5.4 0.68 4.3 0.5380 8.0 5.4 0.68 4.3 0.53

αf=1.0

#5

tran.

#5

long.

#8

#11

ASTMA706 Grade βACIαf=0.8

βASTM,ADJ βASTM,ADJ/βACI βASTM,ADJ βASTM,ADJ/βACI100 4.0 9.7 2.42 7.6 1.90120 4.0 9.7 2.42 7.6 1.90100 6.0 9.7 1.61 7.6 1.27120 6.0 9.7 1.61 7.6 1.27100 6.0 9.7 1.61 7.6 1.27120 6.0 9.7 1.61 7.6 1.27100 8.0 9.7 1.21 7.6 0.95120 8.0 9.7 1.21 7.6 0.95

#5

tran.

#5

long.

#8

#11

ASTMA1035 Grade βACIαf=0.8 αf=1.0

78

Figure42:βASTM,ADJandbarfracturestrain(εf)forvariousthresholdstrainratios(αf)forASTMA615#5bars

Ascanbe seen inTable12,allASTMA615bar types,exceptone,bent toACI318-14

diametersaredeemednot toachievetheyieldstrengthperformanceobjectivewithαf=0.8,

withonlyafewmoreachievingitwithanαfof1.0.Ontheotherhand,Table12indicatesthat

all longitudinal bars satisfying the ASTM A706 specifications can achieve the desired

performance with an αf of 0.8. Transverse bars satisfying ASTM A706 specifications having

tighterACI318benddiametersthanlongitudinalbarsdonotmeetthisobjectivebutonlybya

smallmargin.SimilarlytoA615bars,allA1035barsbenttoACI318-14diametersaredeemed

nottoachievetheyieldstrengthperformanceobjectivewithαf=0.8,withonlythe#11bars

achievingitwithanαfof1.0.

TheseresultsthereforeindicatethatmostA615andA1035barbendssatisfyingACI318-14

shouldexhibitpoorperformancewhileA706barsbendsshouldexhibitadequateperformance.

012345678910

0 0.05 0.1 0.15 0.2

β ASTM,ADJ

εf

ASTMA615#5

Grade40Grade60Grade80&100

α=0.8α=1.0

βACIforlong.reinf.

βACIfortrans.reinf.

79

However,thatisnotthecaseinconcretestructureswhereA615grade60barsandA1035bars

have been shown to have adequate performance. The disconnectmay be attributed to the

conservative nature of the acceptance criteria selected. Of particular concern is the overly

conservative use of the minimum specified ASTM values for fracture strains (εf,ASTM) in the

evaluation.Thefollowingsectioninvestigatesthispointfurther.

80

5.4MinimumBendDiametersbasedonCRSIMillDatabaseElongations

Because the ASTM values for εf are minimum values and are typically exceeded by a

significantmargininproduction(especiallyforthelowergradebars),itisusefultocomparethe

βTHobtainedbyusingthestatisticalvaluesoffractureelongations(εf)andthecurrentACI318-

14benddiameterratios(βACI).

Thiscomparisonisfirstmadeusingstatisticalelongationsforgrade60barsobtainedfrom

the Concrete Reinforcing Institute’s (CRSI) Mill Database (CRSI, 2015), presented in Section

5.4.1.Theprobabilitiesoffailingtheyieldstressperformanceobjectiveforgrade60barsbent

to ACI 318-14 diameters were evaluated using statistical elongation data, and used to

demonstratethattheyieldstressobjectiveisinlinewithactualbendperformanceinconcrete

structures(Section5.4.2).

InSection5.4.3,thetargetbenddiameterratiosrequiredtoachieveagivenprobabilityof

failingtheyieldperformanceobjective(βPf)areevaluatedforallbartypesandgrades,forwhich

sufficient elongation data was available in the CRSI database. Particularly, the fracture

elongationvalues forASTMA615grade60,80and100andA706grade60and80barswere

used.Benddiameterrecommendationsaregivenforthosebartypesandgradesbasedonthis

analysis.

5.4.1CRSIMillDatabase

TheCRSIMillDatabase (2015) summarizes thevariousmonotonic-testpropertiesofbars

produced in the United-States in 2015. The mean (μ) and standard deviation values (σ) for

fractureelongationsobtainedfromthedatabasearesummarizedforgrade60barsinTable13

toTable15forthevariousbarsizesusedinthisstudy.

81

Table13:SummaryoffractureelongationvaluesfromCRSIMillDatabaseforASTMA615

bars

Table14:SummaryoffractureelongationvaluesfromCRSIMillDatabaseforASTMA706

bars

Table15:SummaryoffractureelongationvaluesfromCRSIMillDatabasefordualgrade

ASTMA615/A706bars

Themean(μ)andstandarddeviationvalues(σ)forfractureelongationsobtainedfrom

thedatabasearesummarizedforgrade80and100barsinTable13toTable15forthevarious

barsizesusedinthisstudy.

μ σ μ-2×σ

#5 0.132 0.018 0.096

#8 0.137 0.021 0.095

#11 0.131 0.026 0.079

A615GR60εf

μ σ μ-2×σ

#5 0.16 0.017 0.126

#8 0.162 0.026 0.110

#11 0.149 0.027 0.095

A706GR60εf

μ σ μ-2×σ

#5 0.154 0.017 0.120

#8 0.165 0.017 0.131

#11 0.172 0.021 0.130

DUALA615/A706εf

82

5.4.1ProbabilitiesofFailureofBendsBasedonGrade60ProductionDataInthissection,thepercentagesofgrade60barsinproductionthatwouldfailtheyield

strengthperformanceobjectiveifbenttoACI318-14minimumbenddiametersareevaluated

basedonbarpropertiessummarizedinTable13-Table15.

To achieve this, the following probabilities of failing the yield strength performance

objectiveareassumedbasedontestdataillustratedinFigure40:

Pf1=P[fub/fy≤1.0|αf≤0.8]=0.0 Equation5.4.1

Pf2=P[fub/fy≤1.0|0.8<αf≤1.0]=0.5 Equation5.4.2

Pf3=P[fub/fy≤1.0|1.0<αf]=1.0 Equation5.4.3

Then the probability of failure of bends for a given bar type, size, and target bend

diameterratio(βACI)canbeestimatedas:

PfTotal=P[εf<εf,1.0]*Pf3+P[εf,1.0<εf≤εf,0.8]*Pf2+P[εf≥εf,0.8]*Pf1 Equation5.4.4

With:

P[εf<εf,1.0],P[εf,1.0<εf≤εf,0.8],andP[εf≥εf,0.8]beingtheprobabilitiesthatbarsinabar

typehaveameasuredfractureelongationwithintherangesspecifiedbytheminimumrequired

fracturestrainsforbarsbenttocurrentACI318-14benddiametersandforagivenαf.Inthese

calculations,thefractureelongation(εf)isassumedtofollowanormaldistributionwithmean

(μ)andstandarddeviationvalues(σ)giveninTables13to15.

!!,!.! =!!"#/!.!"!!!!"#/!.!"!!/!

!!!.! Equation5.4.5

83

!!,!.! =!!"#/!.!"!!!!"#/!.!"!!/!

!!!.! Equation5.4.6

Table16summarizestheminimumrequiredfracturestrainsforbarsbenttocurrentACI318-14

benddiametersbasedonEqs.5.4.5and5.4.6.

Table16:MinimumrequiredfracturestrainsforbarsbenttocurrentACI318-14bend

diameters

Table17summarizestheprobabilitiesoffailingtheperformanceobjectiveofbendsfor

ASTMA615andA706grade60barsproducedin2015intheU.S.Ascanbeseeninthetable,all

barsbenttoachieveafinalbenddiameterratioof6.0ormore,regardlessofsize,haveverylow

probabilitiesoffailureandshouldexhibitadequateperformanceunderload.Atatargetbend

diameterratioof5.0,grade60A706anddualgradebarsstillhaverelativelylowprobabilitiesof

failure. However, at a target ratio of 4.0, Table 17 indicates that A615 bars have a 64%

probability of not reaching the performance objective, while those probabilities remain

relativelyhighbutdropto23%and31%forA706anddualgradebars,respectively.

Given that this failure analysis,whichutilizes actual productiondata, determines that

the vast majority of bars bent to ACI 318-14 specifications achieve the yield strength

performanceobjective,which in linewithobservedbendperformance in concrete structure,

this performance objective is deemed to be a reasonable measure of performance for bar

bendsinconcretestructures.

βACI εf,0.8 εf,1.04.0 0.157 0.126

5.0 0.129 0.103

6.0 0.109 0.087

8.0 0.084 0.067

84

Based on these findings and results in Table 17, it may therefore be warranted to

increase theminimumbenddiameter requirement inACI318-14 form4db to5db for#5and

smallergrade60transversebars.

Table17:Probabilitiesoffailingtheperformanceobjectiveforbarsinproductiontoday

benttoACI318-14insidediameters

BarSize TargetBendDiameterRatio Pf,Totalβtarget=4db 64.1%

βtarget=5db 24.3%

βtarget=6db 5.5%

#8 βtarget=6db 5.1%


#5

A615Grade60


βtarget=5db 1.7%

βtarget=6db 0.1%



A706Grade60

#5


βtarget=5db 3.6%

βtarget=6db 0.2%



DUALA615/A706Grade60

#5

85

5.4.2RequiredBendDiameterstoAchieveTargetProbabilitiesofFailureBasedonProductionData

The target bend diameter ratios required to achieve a given probability of failing the

yield performance objective (βPf) are presented in Table 18 for all bar types and grades, for

whichsufficientelongationdatawasavailableintheCRSIdatabase.Theratioswerecalculated

fortheprobabilitiesoffailureof2%,5%,10%,and20%.

AscanbeseeninTable17abenddiameterratioof4.0,whichisspecifiedinACI318-14

for#5andsmaller transversebars, shouldnotbeused foranyof thegradesandtypesof#5

barsconsidered,evenifprobabilitiesoffailureof20%areaccepted.However,grade60ASTM

A706anddualgrade#5barsdocomeclosetobeingabletobebenttoaratioof4.0.

Table19presentsrecommendedtargetbarbendinsidediametersthroughbendratiosthat

achieveprobabilitiesoffailurewithin5%formostbartypesandgrades.Incaseswhereresults

support reducing the bend diameters prescribed in ACI 318-14, the recommended bend

diameterswerenotreducedasthatmayhaveunintendedconsequencesontheperformance

of concrete around thebends.Departures fromcurrentlyprescribedbenddiameter ratios in

ACI318-14arehighlightedinthetable.Notably,A615andA706#5andsmallertransversebars

arerecommendedtobebenttoatleastabenddiameterratioof5.0forgrade60and80and

6.0forgrade100.Grade100A615longitudinalbarsarerecommendedtobebenttoaratioof

9.0for#9to#11barsand8.0for#6to#8bars.Itisnoteworthythelatterrecommendationfor

#6to#8grade100barsismadewithoutthesupportofelongationdata.

86

Table18:Targetbenddiameterratiosrequiredtoachieveagivenprobabilityoffailing

theyieldperformanceobjective

μ σ

5 0.132 0.018 6.6 6.1 5.6 5.28 0.137 0.021 6.6 6.0 5.5 5.011 0.131 0.026 8.0 7.0 6.2 5.55 0.123 0.016 7.0 6.5 6.0 5.58 0.132 0.013 6.0 5.7 5.4 5.011 0.123 0.02 7.7 6.9 6.3 5.75 N/A N/A N/A N/A N/A N/A8 N/A N/A N/A N/A N/A N/A11 0.096 0.017 10.4 9.3 8.4 7.5

5 0.16 0.017 4.9 4.6 4.4 4.18 0.162 0.026 5.6 5.1 4.6 4.211 0.149 0.027 6.6 5.8 5.3 4.75 0.129 0.011 6.0 5.7 5.4 5.18 0.134 0.014 6.0 5.6 5.3 5.011 0.139 0.018 6.1 5.6 5.3 4.8

5 0.154 0.017 5.2 4.9 4.6 4.38 0.165 0.017 4.7 4.4 4.2 3.911 0.172 0.021 4.7 4.4 4.1 3.85 N/A N/A N/A N/A N/A N/A8 N/A N/A N/A N/A N/A N/A11 N/A N/A N/A N/A N/A N/A

βPf=2% βPf=5% βPf=10%

DUALASTM

A615/A7

06 Grade

60

Grade

80

βPf=20%Grade

60

Grade

80

ASTMA615

ASTMA706

BarSizeεf

Grade

80

Grade

100

Grade

60

87

Table19:Recommendedtargetbarbenddiameterratios

BarSize βACI βRecom

ASTMA61

5

Grade

60

3to5Transverse 4.0 5.0*

3to5Longitudinal 6.0 6.0

6to8 6.0 6.0

11 8.0 8.0Grade

80

3to5Transverse 4.0 5.0*


6to8 6.0 6.0

11 8.0 8.0

Grade

100

3to5Transverse

NotSpecified 6.0

3to5Longitudinal

NotSpecified 6.0

6to8Not

Specified 8.0

9to11Not

Specified 9.0

ASTMA70

6 Grade

60

3to5Transverse 4.0 5.0


6to8 6.0 6.0

9to11 8.0 8.0

Grade

80

3to5Transverse 4.0 5.0


6to8 6.0 6.0

9to11 8.0 8.0

*thesebendratiosresultinhigherthan5%probabilitiesoffailure,whichisdeemedacceptablesincetheA615barsareusedinnon-seismicapplications

88

Chapter6:SummaryandConclusions

Summary

Bend/re-bendtestswereconductedonreinforcingbarswithyieldstrengthsrangingfrom

60ksitoapproximately120ksi.Strainagingtestswerealsoconductedonthebarstoensure

that the bar bends were re-bent after most of the strain aging embrittlement effects had

occurred. The bend/re-bend test variables were: bar grade, bar manufacturing process, bar

diameter (db),andbend insidediameter.High-strengthreinforcingbars (grade80andabove)

producedusingthemostprevalentmethods intheU.S.wereobtainedfromfourofthemain

manufacturersintheU.S.Barsizeswere#5,#8,and#11andthebarswerebenttomeetthe

minimumspecifiedACI318-14benddiameters foreachof the sizes. #5barswerebentwith

inside bend diameters of 4 bar diameters (transverse steel requirement) to 6 bar diameters

(longitudinal steel requirements). Thebar specimenswerebent into aV-shapeandpulled in

tensionuntilfracture.Allbarswerebentandtestedatatemperatureofabout20°C.Anoptical

measurementsystemwasusedtorecordbarsstrainsandchangesinthebendangleduringre-

bending.Performancemeasuresusedtoquantifytheperformanceofbendsincluded:

a) Theremainingbendangleatfractureduringre-bending(θb)

b) Theaxialstrainatfracturenormalizedbythebaruniformelongationstrain(!b/!un)c) Theaxialstressatfracturenormalizedbythebaryieldstrength(fub/fy)

d) Theaxialstressinbaratfracturenormalizedbythebartensilestrength(fub/ft)

89

Conclusions

Thechemistryofthebarswasfoundtoaffecttheextentofstrainagingsignificantly.The

highertheconcentrationofVanadiuminthesteel,whichwasusedbysomemanufacturersto

increase strength, the lower the embrittlement due to strain aging was found. Overall,

however, strain aging embrittlement never resulted in more than a 20% reduction in bar

fracturestrains.

Overall,forallbarsizesandtypes,asbarstrength(orgrade)increased,bendperformance

decreased as demonstrated by lower stresses, strains, and changes in the bend angle at

fractureduringre-bending.Moreover,forallbartypes,asthebendinsidediameterincreased,

bendperformancewasseentoimprove.

Overall, bends in #8 and#11barswere found toperformadequately at the currentACI

318-14minimumbenddiametersforallgrades.Most#8and#11barbendsstrengthenedfully,

priortofracturingatstressesaboveyieldandatrelativelylargeinelasticstrains.

Bends in #5 bars showed significantly varied performance. Grade 60 #5 bars, bent to

achieve a target inside diameter of 4db, were able to reach stresses close to yield prior to

fractureduringre-bending.Bendsingrade80and100bars,however,onlyreachedfractionsof

their yield strength during re-bending when bent to achieve a 4db inside diameter. The

performanceofbendsinhighergrade#5barsreachedlargerstressesandstrainsasthebend

diameterwasincreased,withgrade80barsreachingstressesclosetotheiryieldwith5dbbends

andgrade100barsreachingyieldstrengthswithbenddiametersof6db.

A relationship was derived between the bend inside diameter and the theoretical peak

tensile strain at the bend. This relationship indicates that the ratio of bend diameter to bar

diameter(β)governsthelevelofstrainappliedduringbending;i.e.,giventhesamebendratio,

thetheoreticalpeakstrainsincurredatbarbendswillbethesameregardlessofbarsize.The

relation derived assumes zero net strain at the bar centerline and therefore only applies to

bendingprocesseswherebarsarefreetomovelongitudinallyduringbending.Therelationdoes

not apply to simultaneous double bending applications that can introduce additional tensile

strains due to longitudinal restraint during bending. Strain parameters were obtained by

90

normalizingthetheoreticalpeakstrainsincurredduringbendingbythebaruniformorfracture

straincapacities(εma/εunorεma/εf).Theparameterswerefoundtobenegativelycorrelatedwith

all performance measures. As bending strains increased with respect strain capacities, bent

specimens were found to sustain lower stress and strain at fracture during re-bending. To

achieve specimens that reached at least their yield stress at fracture, it was found that the

εma/εun should not exceed 1.2, or εma/εf should not exceed 0.8, which corresponds

approximatelytotargetinsidebenddiametersofnolessthan4dbforthegrade60,5dbforthe

grade80,and6dbforthegrade100barstestedinthisstudy.

To generalize recommendations for bar bend diameters, relations between ASTM

minimumfractureelongationvaluesaswellasactualproductionelongationvaluesobtainfrom

theCRSIMillDatabasewerestudied.Inthisstudy,barbendsthatareabletosustaintheiryield

strengthduringre-bendingweredeemedtohaveacceptableperformance.

When setting bar fracture elongations equal to theminimum ASTM fracture elongation

limits, most bar bends of grade 60 and 80 ASTM A706 were found to have acceptable

performance.On the other hand, the vastmajority of grade 60 A615 andA1035 bar bends,

regardless of grade, were found to have unacceptable performance, which is contrary to

observed bend performance in concretemembers. Thiswas attributed to the fact that bars

generallyexceedtheminimumASTMspecifiedelongationbyasubstantialmargin.

Astatisticalanalysiswasconductedtodeterminetheprobabilityoffailure,ornotachieving

the performance objective, for bar bends using the statistical distribution of fracture

elongationsobtainedfromtheCRSIMillDatabasefor2015.Theanalysisshowedthatgrade60

bars in production today should have acceptable performance based on their measured

fractureelongationdowntoabenddiameterof5db.Thisfindingcorroboratedthattheselected

performance objective is a reasonable measure of performance for bar bends in concrete

structures.Atabenddiameterof4db,however,alargeportionofgrade60barsdidnotachieve

acceptable performance. Increasing theminimumbenddiameter for 4 db to 5db for # 5 and

smallerbarsisthereforerecommendedforgrade60bars.

91

The statistical analysiswas extended using elongation values for grade 80 and 100 bars

obtained from theCRSIMillDatabase.Recommendations forbarbenddiameters for various

A615andA706gradesandbarsizeswereprovidedtoachieveaprobabilityoffailingtheyield

stressperformanceobjectivearound5%,formostcases.Departuresfromcurrentlyprescribed

benddiameterrationinACI318-14werehighlighted.Notably,A615andA706#5andsmaller

transverse bars were recommended to be bent to a bend diameter ratio of at least 5.0 for

grade60and80and6.0forgrade100.Grade100A615longitudinalbarsarerecommendedto

bebenttoaratioofatleast9.0for#9to#11barsand8.0for#6to#8bars.

Since the performance evaluations of bends based on ASTM minimum elongations

estimate poor bend performance for a majority of bar types while those based on actual

elongationvaluesdidnot, results imply that theACI318-14benddiameter specificationsare

relyingontheinherentadditionalstraincapacityofbarsbeyondtheASTMminimumvaluesto

achieveacceptablebendperformance.Thesameistruefortherecommendedbenddiameters.

Currently, ACI 318-14 provides minimum bar-bend diameters as a function of bar size

mainlyandbarapplication(transverseor longitudinalbars).However,asproductionmethods

evolveorhigherandlessductilegradesofbarsareintroduced,itmaybecomenecessarytoset

minimumbend diameters as a function of uniformor fracture elongations and possibly as a

function of loading (i.e., seismic or non-seismic), such that bends achieve performance

objectivesthatmaybedifferent.

Therecommendationsforminimumbarbenddiametersprovided inthisstudyarebased

oncurrentproductionelongationstatistics.Theserecommendationswillneedtobeadjustedif

actual bar elongation values change significantly over time.Moreover, since all bending and

testing was conducted at relatively warm ambient temperatures, cold weather bend

performancemayresult indifferentbendperformanceattherecommendedbenddiameters.

Lowtemperaturebendperformanceshouldbeassessedforcoldweatherregions.

92

References

[1]AS/NZS4671:2001(2001)."SteelReinforcingMaterials."StandardsAustralia/StandardsNewZealand,Wellington,NewZealand.

[2]ASTMStandardA615/A615M-13(2013)."StandardSpecificationforDeformedandPlainCarbon-SteelBarsforConcreteReinforcement."ASTMInternational,WestConshohocken,PA.

[3]ASTMStandardA706/A706M-13(2013)."StandardSpecificationforLow-AlloySteelDeformedandPlainBarsforConcreteReinforcement."ASTMInternational,WestConshohocken,PA.

[4]ASTMStandardA1035/A1035M-11(2011)."StandardSpecificationforDeformedandPlain,Low-carbon,Chromium,SteelBarsforConcreteReinforcement."ASTMInternational,WestConshohocken,PA.

[5]ASTMStandardE8/E8M-15a(2015)."StandardTestMethodsforTensionTestingofMetallicMaterials."ASTMInternational,WestConshohocken,PA.

[6]CRSIMaterialPropertiesandBarProducersCommitteeandMillDatabaseSub-Committee(2015)CRSIMillDatabase.Schaumburg,IL. [7]BS4449:2005+A2:2009(2005)."Steelforthereinforcementofconcrete.Weldablereinforcingsteel.Bar,coilanddecoiledproduct.Specification",BSI.ISBN:9780580645853

[8]G.T.VanRooyen(1986)."Theembrittlementofhardened,temperedlow-alloysteelbystrainaging."JournaloftheSouthAfricanInstituteofMininingMetallurgy,vol.86,no.2.(1986):67-72

[9]Hopkins,D.,andPoole,R.(2008)."Grade500EReinforcingSteel:TestsonMicro-alloyandQuenchedandTemperedsamplesavailableinNewZealand."DepartmentofBuildingandHousing,Wellington,NewZealand,11.

[10]RashidM.S.(1976)."StrainAgingKinteticsofVanadiumorTitaniumStrengthenedHigh-StrengthLow-AlloySteels."MetallurgicalTransactionsA,vol.7A,(1976):497-503.

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