Steel ﬁ ber reinforced concrete panels subjected to impact projectiles with different caliber sizes and muzzle energies

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Steel ﬁ ber reinforced concrete panels subjected to impact projectiles with different caliber sizes and muzzle energies

Sittisak Jamnam

^a

, Buchit Maho

^a

, Apisit Techaphatthanakon

^a

, Yoshimi Sonoda

^b

, Doo-Yeol Yoo

^c

, Piti Sukontasukkul

^a,

*

aConstructionandBuildingMaterialsResearchCenter,DepartmentofCivilEngineering,KingMongkut’sUniversityofTechnologyNorth Bangkok,Thailand

bDepartmentofCivilandStructuralEngineering,KyushuUniversity,Fukuoka,Japan

cDepartmentofArchitecturalEngineering,HanyangUniversity,SouthKorea

ARTICLE INFO Articlehistory:

Received7February2020

Receivedinrevisedform7April2020 Accepted9April2020

Keywords:

Fiberreinforcedconcrete Impactprojectile Calibersize Muzzleenergy Impactresistance

ABSTRACT

Inspecialsituationsliketerroristattacks,concretestructuresareoccasionallysubjectedto impactloadssuchasfirearms.Sinceconcreteisbrittle,itoftenshattersintoseveralpieces under impact loadings. In order to alleviate this brittleness, fibers are generally incorporatedintoconcrete.Inthisstudy,steelfiberreinforcedconcretepanelssubjected toprojectileimpactloadswithdifferentgeometrieswasinvestigated.Theimpactorsinthe formofbulletscameinthreedifferentcalibersizes9,11,and7.62mm,providingmuzzle energiesof468,1972,and3259J,respectively.Hookedendtypesteelfiberswereusedat 3volfractionsof1–3%.Thespecimenswerecastinsquarepanels withdimensionsof 400400mmandvaryingthicknessfrom10to100mm.Eachpanelwassubjectedtoa singleimpactatthecenter.Dataintheformofvelocity(priortoandafterimpactevent), failuremodes,andspallingdiameterswerecollected.Resultsshowedthatfourtypical failure modes were commonly found in panels: perforation, scabbing, spalling, and cracking.Forpiercingtypebullets,thethicknessplayedanimportantroleontheimpact resistanceofthepanels.However,forlargeandblunttipbullets,boththicknessandfiber volumefractionmustbeconsideredtogethertoprovidesufficientimpactresistance.

1.Introduction

Afirearmreferstoaportablehandheldguncapableofdrivingabulletinaprojectilepathtoatargetbyhighlyexpansive pressuregasresultingfromthechemicalexothermiccombustionofpropellantwithinacartridge[1].Widevarietiesof firearmscanbeclassifiedbybarrellength(handgun,rifle),barreltype,loadingmechanism(automaticormanual),firing mechanism,borediameter,orcalibersize,etc.Themainfunctionofafirearmistoignitethegunpowder,thuscreating pressuretopushoutabullet(aprojectile)downthebarrelandoutofthemuzzle.Bulletsizecanbeclassifiedbydiameteror caliber.Afterignition,abulletturnsintoakineticprojectilewithdifferentmuzzleenergiesandvelocitiesrangingfrom subsonictosupersoniclevels(120–1200m/s)[2].

Underan attackbyﬁrearm,concretestructures mayexperiencedifferenttypesoffailuremodes suchas,complete fracture,perforation(fullpenetration),penetration,scabbing,spalling,orcracking.Amongthem,theperforationmodecan

*Correspondingauthor.

E-mailaddresses:[email protected],[email protected],[email protected](P.Sukontasukkul).

https://doi.org/10.1016/j.cscm.2020.e00360

4.0/).

ContentslistsavailableatScienceDirect

Case Studies in Construction Materials

j o u r n a lh o m e p ag e :w w w . e l s e v i e r . c o m / l o c a t e / c s c m

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causethemostseriousdamageandmustbeconsideredcarefully.Foranybulletproofelement,a minimumprotection requirementisthatitmustalsobeabletopreventperforationordamagefromperforation.

Concreteisabrittlematerial,andtoimproveitsbrittlenesssmallﬁbersmaybemixedanddistributeduniformlyinit.The effectof ﬁber bridgingacrossthe crackscanhelp increase ductilityand prevent catastrophicfailure. Fiberreinforced concrete(FRC)isalsoknowntoprovideexcellentimpactresistance[2–6].

Foracertaintype ofimpactforce,suchasprojectileimpact,somestudieshavebeencarriedouttoinvestigatethe resistanceofFRC toprojectileimpactforce.Zhangetal.[7] testedprojectileimpactresistanceofplainand steelfiber reinforcedhighstrengthconcreteusinga12.6mmogivenoseprojectileatvelocitiesrangingfrom620700m/s.Theyfound thatincorporationofsteelfibersintheconcretecouldreducecraterdiameterandcrackpropagation.Luoetal.[8]usedan armor piercingimpact head withdiameter of 37mm to teston highstrength reinforced concrete (RHSC) and high- performancesteelfiberreinforcedconcrete(HPSFRC).Theimpactvelocityofabout365378m/swasusedintheirtest.Their resultsshowedsmashfailureintheRHSCtargetswhiletheHPSFRCtargetsremainintactwithseveralradialcracksonthe frontfaces.Almusallametal.[9]usedabi-conicimpactheadwithdiameterof40mmtravellingatasubsoniclevelvelocityof 300m/s.Thetargetsincludedplainandhybrid(steelandpolypropylene)fiberreinforcedconcreteslabs.Theyconcludedthat theexistenceoffiberinconcreteledtosmallercratervolumesandreductioninspallingandscabbingdamage.

Sukontasukkuletal.[10,11]testeddoublelayerconcretepanelsmadeofrubberizedandsteelﬁberreinforcedconcrete subjectedtoimpactprojectilesof9mmand11mmbulletswithmuzzlevelocityof362and270m/s,respectively.Thetest resultsshowedthattheinsertionofasoftmaterial(rubberizedconcrete)layerhelpedabsorbpartoftheimpactenergyand allowedlessimpactforcetoreachtheFRClayer.Mahoetal.[12]testedmultilayerSFRCpanelswitharubbersheetinsertion subjectedto9mmFMJbulletwithmuzzlevelocityof398m/s.Theyconcludedthat,inordertopreventperforation,the impactenergyabsorptionofthepanelmustbeequalorhigherthantheimpactenergyofthebullet.Theincorporationof rubbersheetinsertionhelpedintrappingbulletsandpreventedricocheting.

Basedontheaboveliteraturereviews,mostballistictestsoncementitiousmaterialswerecarriedoutusingprojectiles withvelocityrangingfrom270to700m/s.Thereisstillalackofinformationinprojectilewithvelocityhigherthan700m/s.

Thisstudywasintendedtoﬁllthegapofsuchinformationbyusingbulletswithvelocityuptoabout842m/s(muzzleenergy of3259J).

Inthisstudy,theimpactresistanceoffiberreinforcedconcretepanelssubjectedtoimpactprojectileswithdifferent muzzleenergieswasinvestigated.Threetypesofbulletswereused:9,11(.44magnum),and7.62mm.The9mmcaliber providedmuzzleenergyof468Jandvelocityof342m/swhichisinthesamerangetothesoundvelocityof340m/s.The11 and7.62mmcalibersprovidedmuzzleenergiesof1972and3259Jandvelocitiesof499and842m/s,respectively,whichare consideredinthesupersonicrange(higherthansoundvelocity).Hookedendsteelfiberswereusedat1–3%volumefraction withthicknessofconcretepanelsvariedfrom10to100mm.Resultsintermsoffailuremodes,velocitypriortoandafter impact,andimpactenergyabsorptionwerecollected.Therelationshipbetweenmuzzleenergiesandfailuremode,panel thickness,andfibercontentwasdiscussed.

2.Experimentalprocedure 2.1.Materials

MaterialsusedinthisstudyconsistedofPortlandcementtypeI(ASTMC150),riversandwithFMof2.75,tapwater,and superplasticizerTypeF.Singlehookedendsteelﬁberswithtensilestrengthofabout1000MPawasusedat1–3%volume fractions(Table1).Threetypesofbulletswithdifferentdiameterandmuzzleenergieswereused(Table2).

2.2.Mixproportionandspecimenpreparation

Mixproportionoffiberreinforcedconcretewassetatwater/cementratioof0.30,cement/fineaggregateratioof0.50,and fibervolumefractionof1–3%(Table3).Thisyieldedthecompressivestrengthofabout77.1,89.5and89.7MPafor1,2and3%

SFRC,respectively.Topreparethespecimens,cementandsandweremixedtogetherfor3min,thenwaterwasadded,and themixingcontinuedforanother3min.Steelﬁberswereaddedintofreshmortarandthemixingcontinuedfor3more minutestoensureuniformdistributionofﬁbers.

Thespecimensintheformofsquarepanelswithdimensionsof400400mmandvaryingthicknessesfrom100to 600mmwereprepared.Afterthemixinghadﬁnished,thefreshconcretewaspouredintosteelmolds,compactedwitha steelrodfor25timesandvibratedonashakingtable.Aftercompletion,theexcessconcretewasremovedfromthetop surfaceandthespecimenswerecoveredwithplasticsheetsfor24h.Thespecimensweredemoldedonthenextdayand curedinwaterfor28days.

Table1

PropertiesofSteelFiber.

Materials SpeciﬁcGravity Shape Length(mm) Section(mm) AspectRatio(l/d) TensileStrength(N/mm²)

Steel 7.8 HookedEnd 35 dia.-0.55 64 1000

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2.3.Projectileimpacttest

General test setup began with placing and locking a specimen on a 400400mm steel support. Two ballistic chronographscapableofmeasuringvelocityfrom1.5–3047m/swereplacedatthefront andbackofthetargetwitha distanceof2500and300mm,respectively.ThreetypesofbulletswereusedasmentionedinTable2.Twostrikingdistances wereadopted:5mfor9mmand11mmcalibertestand15mfor7.62mmcalibertest(Fig.1).Thetestwascarriedoutin accordancetoBallisticResistanceofBodyArmorNIJStandard-0101.06.

Toanalyzetheresults,thespecimenwasshotfromthedistancementionedabove.Thefailuremodesandbulletvelocity priortoimpactwereobtained.Inthecasewheretheperforationfailuremodeoccurred,thebullet’sresidualvelocity(after impact)wasalsomeasured.Bothvelocitiespriortoandafterimpactwerethenusedincalculatingkineticenergiesusing Eq.1.Assumingnoenergylosstootherformsandconstantbulletmassduringtheimpactevent,theenergylossbythebullet canbecalculatedusingEq.2.Alsobyfurtherassumingthatthekineticenergylossbythebulletisequaltotheenergy absorbedbythepanel,theenergyabsorptionbythepanelcanbecalculatedbyEq.3.

InitialKineticEnergy,Ei¼^mv2²ⁱ

ResidualKineticEnergy, Er¼^mv2²^r (1)

EnergyLossbyabullet,

D

^E¼EiEr¼^m^ð^vⁱ^v2 ^r^Þ² (2)

EnergyAbsorptionbyPanel, EAb¼

D

^E ⁽³⁾

Table2

Propertiesofbullet.

No. Caliber BulletWeight BulletDescription MuzzleVelocity MuzzleEnergy Classiﬁcation

1 9mm 8.0g FMJ 342m/s 468N.m LowImpactEnergy

2 11mm

(.44Magnum)

15.6g SJHP 499m/s 1942N.m MediumImpactEnergy

3 7.62mm 9.6g FMJ 824m/s 3259N.m HighImpactEnergy

Table3 CastingSchedule.

Name Fibrecontent

(%)

Impacttest Thickness (mm.)

9mm .44

Magnum

7.62 Riﬂe

1S10 1 10 3 3 –

1S20 20 3 3 –

1S30 30 3 3 –

1S40 40 3 3 –

1S50 50 3 3 –

1S60 60 – 3 –

2S10 2 10 3 3 3

2S20 20 3 3 3

2S30 30 3 3 3

2S40 40 3 3 3

2S50 50 3 3 3

2S60 60 – 3 3

2S70 70 – – 3

2S80 80 – – 3

2S100 100 – – 3

3S10 3 10 3 3 3

3S20 20 3 3 3

3S30 30 3 3 3

3S40 40 3 3 3

3S50 50 3 3 3

3S60 60 – 3 3

3S70 70 – – 3

3S80 80 – – 3

3S100 100 – – 3

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where

D

Ê⁼ênergy^loss^by^the^bullet^duringîmpact^(Joule)

EAb=energyabsorbedbythepanel(Joule) m=bulletmass(kg)

V_i=initialbulletvelocity(priortostriking)(m/s) Vr=residualbulletvelocity(afterstriking)(m/s) 2.4.Castingschedule

Foreachtest,atleastthreespecimensweretested.ThetotalnumberofspecimensaresummarizedinTable3.

3.Resultsanddiscussion 3.1.Failurepatterns

Typicalfailurepatternsofapanelsubjectedtoasinglestraightshotcanbeclassiﬁedinto4modeswhichwerealso deﬁnedpreviouslybySukontasukkuletal.[13](Fig.2):

1)Perforation+Backspalling(P/Sp)–Bulletfullypenetratedthroughthepanelthicknesscreatingacircularpuncturehole (orcrateratthefrontsurfaceandbacksurfaces)andspallingareaatthebacksurfacewithﬂyingdebris(Fig.2a).Thiswas theonlymodethatthebulletpenetratedthroughthethicknessandallowedthemeasurementofbulletvelocityafteran impactevent.AsimilarmodeoffailurewasalsoreportedbyAtapekandKaragoz[14]whenimpacttestswascarriedout ontemperedbainiticsteelusing7.62mmarmorpiercingprojectile.Althoughthetestswerecarriedoutwithasteel material,theperforationfailurewithsmallfrontcraterwasalsoobservedinspecimens.

2)Frontscabbing+Backspalling(Sc/Sp)–Bulletpartiallypenetratedthefrontsurfacecreatingscabbingzoneandlarge spallingareaatthebacksurfacewithflyingdebris(Fig.2b).Thismodewasoftenfoundinpanelshitbyabulletwith mediumtohighmuzzleenergy(11mmor7.62mm)becausetheenergymustbehighenoughtonotonlycreatespalling piecesbutalsodebondingthemfromfibers(becomingflyingdebris).Typically,thebackspallingdiameterwasalways largerthanthefrontscabbingdiameter.Thisisbecausethecompressionstresswavewaslargeronthebackthanonthe frontsurface[15].

3)Frontscabbing+Backcracking(Sc/Cr)–Bulletpartiallypenetratedthefrontsurfacecreatingscabbingzoneandcracksat thebacksurfacewithoutflyingdebris(Fig.2c).Thismodeoftenoccurredinpanelswithjustsufficientthicknesstoresist bulletenergy.Thebullet’senergylevelwassufficientincausingcracksbutnotenoughtoovercomethebondstrength betweenfibersandspallingpieces.

Fig.1.ImpactTestSetup.

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Fig.2.TypicalFailurePatternsofaSingleStraightShot(a)Perforation,(b)Frontscabbing+Backspalling,(c)Frontscabbing+Backcrackingand(d)Front scabbingonly.

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4)Frontscabbing+Nobackdamage(Sc/ND)–Bulletpartiallypenetratedthefrontsurfacecreatingscabbingzonebutno damageatthebacksurface(Fig.2d).Thismodeusuallyoccurredinpanelswithamuchlargerthicknessthatwascapable ofovercomingbulletenergyentirely.

3.1.1.9mmcaliber

The9mmbulletisthelightestamongthethreebullettypesintermsofweightandmuzzleenergy(8gand468J).The perforationwithbackspallingfailuremode(P/Sp)wasobservedinthepanelswiththicknessesof10and20mmregardlessof theirﬁbervolumefractions(1S,2Sand3S).ThefailurepatternschangedtoSc/Crmodewhenthepanelthicknessincreased to30mm.Atthethicknessequaledtoorlargerthan40mm,thedamageatthebacksurfacedisappearedandtheSc/NDmode wasobserved.DetailsfailuremodeandbackspallingdiameteraresummarizedinTable4andFig.3.

3.1.2.11mmcaliber(.44magnum)

The.44magnumcaliberistheheaviestbullet(about15.6g)amongthethreecalibers.Althoughithasarelativelylow velocityofabout499m/s(ascomparedto842m/sof7.62mmbullet),becauseoftheweight,itisabletoprovidequitelarge muzzleenergy ofabout1942J.Undertheimpactbythisbullet type,allFRCpanels withthicknessfrom10to30mm experiencedP/Spfailuremode.Atthethicknessof40mm,thefailuremodeshiftedtoSc/Spmodefor2S40and3S40while theperforationmodestilloccurredin1S40.Withtheincreasingthicknessto50mm,thefailuremodeofallpanelschanged Table4

DetailSummaryonPanelType,BulletTypeandFailuremode.

Name 9mm. 11mm. 7.62mm

1S10 Perforation Perforation –

2S10 Perforation Perforation Perforation

1S20 Perforation Perforation –

1S30 Scabbing+Cracking Perforation –

2S30 Scabbing+Cracking Perforation Perforation

3S30 Scabbing+Cracking Perforation Perforation

1S40 Scabbing+Nodamage Perforation –

2S40 Scabbing+Nodamage Scabbing+Spalling Perforation

1S50 Scabbing+Nodamage Scabbing+Cracking –

2S50 Scabbing+Nodamage Scabbing+Cracking Perforation

3S50 Scabbing+Nodamage Scabbing+Nodamage Perforation

1S60 – Scabbing+Nodamage –

2S60 – Scabbing+Nodamage Scabbing+Spalling

3S60 – Scabbing+Nodamage Scabbing+Spalling

2S70 – – Scabbing+Spalling

3S70 – – Scabbing+Spalling

2S80 – – Scabbing+Cracking

3S80 – – Scabbing+Nodamage

Fig.3.FailureMode,BackSpallingDiametervs.FRCPanelThickness(9mm.Caliber).

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toSc/Crwithcracklinesatthebacksurface.Thedamageatthebacksurfacedisappeared(Sc/NDmode)whenthethickness wasequaltoorlargerthan60mm.DetailsaresummarizedinTable4andFig.4.

3.1.3.7.62caliber

Althoughthe7.62caliberisthesmallestintermsofsize,duetoitshighvelocity(824m/s),itprovidesthehighestimpact energyof3259J.Withlargemuzzleenergytogetherwiththesharptips,the7.62mmbulletsareconsideredarmorpiercing bullets.TheresultsshowedthattheywerecapableofpenetratingthroughalltypesofSFRCpanelswiththicknessranging from10to50mmeasily(perforationfailuremode).Withincreasingthicknessto60mm,thefailuremodechangedtoSc/Sp modewithﬂyingdebris.Atthethicknessof80mm,thebackspallingreducedtocrackingandthefailuremodeshiftedtoSc/

Crmode.Beyond80–100mm,thedamageatthebacksurfacedisappearedcompletely(Sc/NDmode).Moredetailsare summarizedonTable4andFig.5.

3.2.Impactenergyabsorption

InordertodeterminetherelationshipbetweentheenergyabsorptionandthicknessofFRCpanels,thedataonvelocities priorandafterimpactofthosefailedunderperforationfailuremodewereused.Althoughthebulletwasdeformedduringan impactevent,itsmasswasassumedconstant.UsingEq.1–3insection2.3,energyabsorptionbyapanelcanbecalculatedand theresultsareshowninFig.2–4.Regardlessofbullettypeandgeometry,theimpactenergyabsorptionwasfoundtoincrease withtheincreasingthickness[16].

Fig.4. FailureMode,BackSpallingDiametervs.FRCPanelThickness(.44magnumCaliber).

Fig.5.FailureMode,BackSpallingDiametervs.FRCPanelThickness(7.62mmCaliber).

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3.2.1.9mmcaliber

TherelationshipbetweentheenergyabsorptionandthicknessofFRCpanelssubjectedto9mmbulletsareshowninFig.6 andexpressedbyEq.4–6.

Energyabsorptionof1Spanel, E^1S₉ ¼22:56:t (4)

Usinganaverageinitialkinetic(muzzle)energyofa9mm.bulletof468JandEq.4to6,theminimumthicknessesfor1S, 2S,and3Spanelsrequiredtomeetthebulletenergywaspredictedequalto20.7,19.5,and18.8mm,respectively.The predictedthicknessesweresomewhatinlinewiththeexperimentalresultsinwhichtheendofperforationfailuremode occurredatthethicknessaround20mmforallFRCpanels.

Although,thethicknesswasfoundtodecreasewiththeincreasingfibercontent,thedifferenceinthicknesswasrelatively small(about0.95mmor5%pervolumefractionoffiber).Thesmalldifferenceimpliedthattheresistanceofthepanel increasedverylittlewiththeincreasingfibercontentanditwasthethicknessthatplayedanimportantroleonimpact resistanceofpanelsunder9mmcaliber.Thereasonwasperhapsduetothenatureofitstip(pointy)andtherelativelysmall calibersizeascomparedtothespecimensize;thedamagebecameverylocalized.

3.2.1.1.11mm.Caliber(.44magnum). TherelationshipbetweentheenergyabsorptionandthicknessofFRCpanelssubjected to.44magnumcalibersareshowninFig.7andexpressedbyEq.7–9.

Energyabsorptionof1Spanel, E^1S₄₄¼49:75:t (7)

UsingEq.7to9,thethicknessesof1S,2S,and3Spanelscorrespondingtothebulletmuzzleenergyof1942Jwere predictedat39.0,32.9and29.2mm,respectively.Similarly,thethicknesswasfounddecreasewithincreasingﬁbervolume fraction.Thepredictedthicknessesofallpanelswerewellagreedwiththetestresults.Theperforation(P/Sp)wasobserved in1Spaneluptothethicknessofabout40mm,thepredictedthicknessof39wasabout1mmunderestimatedbyabout 1mmor2.5%.For2Sand3Spanels,theperforationmodestoppedatthethicknesslargerthan30mmwhileEq.8and9 yieldedthethicknessof32.9and29.2mmfor2Sand3S,respectively.

Thedifferenceinthicknessbetweenpanelswithdifferentfibervolumefractionswasfoundinaverageof4.9mm/volume fractionoffiber(or16.7%per volumefractionoffiber). Unlike9mmcalibers, the11mmcaliberyieldedmuch larger differencesinthickness.Itissuspectedthatthetipgeometryandimpactbehaviorofthe.44magnumplaysanimportantrole onthis.The.44magnumisknownasaheavybulletwithflattenedpointheadthatcanproducealotofrecoilandhasalarge

Fig.6.EnergyAbsorptionvs.Thickness(9mm).

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muzzleenergyThisallowsthebullettoinsertmorepressureatthetargetandsurroundingarea.Unlikethepiercingtype bulletswithsmalldiameters,whereimpactzoneislocalized,theflattenedpointbulletcreatesalargerimpactzonewhich,in turn,weakensthesurroundingarea.Thisshowedthatwhensubjectedtoimpactloadfrombulletswithlargercalibers,the failurebecomemore globalized.The considerationonthicknessalone maynotbesufficienttoprovideresistanceand preventperforationfromoccurring,boththicknessandfibervolumefractionmustbeconsideredtogetherforoptimum design.

3.2.1.2.7.62mmcaliber. TherelationshipbetweentheenergyabsorptionandthicknessofFRCpanelssubjectedto7.62 magnumcalibersareshowninFig.8andexpressedbyEq.10–11.

Energyabsorptionof2Spanel, E^2S₇₆₂¼58:01:t (10)

Energyabsorptionof3Spanel, E^3S₇₆₂¼62:47:t (11)

For7.62mmcaliber,theminimumthicknessesrequiredtomeetitsmuzzleenergyof2Sand3Spanelswerepredictedat 56.2and52.2mm,respectively.Basedontheexperimentalresults,theminimumthicknesswhichallowedtheperforationto occurwasat50mm.Atthethicknessof60mm,thefailuremodechangedtoSc/Sp.Thepredictedthicknessesfellinbetween 50–60mmwhichwereconsideredwellagreedwiththetestresults.Thedifferenceinthicknessbetween2Sand3Spanels wasfoundaround4mmor7.6%pervolumefractionoffiber,whichwasconsideredrelativelylowascomparedtothe11mm (or.44Magnum).Similartothe9mmbullet,the7.62mmisapiercingtypebulletwithpointytip,ittendstopenetrate throughthetargetratherthandamagingthesurroundingarea.Thissimilarlyimpliesthatthepanelthicknessmostlikely playedamoreimportantrolethanthefibercontentinthecaseof7.62mmbullet.However,whendesigningapanelforthis kindofarmorpiercingbullettype,boththicknessandfibervolumefractionmustbeconsideredtogethertoobtainthe

Fig.8.EnergyAbsorptionvs.Thickness(7.62mm).

Fig.7. EnergyAbsorptionvs.Thickness(11mm).

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desiredfailuretype.Ifthepurposeistopreventperforation,thenperhapsthethicknesscanbeconsideredalone.However,if thepurposeistopreventperforationanddamageonthebacksurface,thenboththicknessandﬁbervolumefractionshould beconsideredtogether.

4.Conclusion

Steelﬁberreinforced concretepanels behaveddifferently underimpactfromprojectileswithdifferentgeometries, muzzlevelocities,andenergies.Thefailuremodeswerefoundtodependmainlyonthepanelthicknessandﬁbercontent.

Underthesametypeofprojectile,theshiftingoffailuremodesfromperforationtoscabbingorspallingornodamagewasthe resultofincreasingpanelthicknessandﬁbercontent.

Forpanelswithathinnerthickness,theperforationfailuremodewasmainlyobserved.Asthethicknessincreased,the failuremodeshiftedtopenetrationwithbackspalling,andthentopenetrationwithoutdamageinthebacksurface.

ConsideringSFRCpanelswiththesameﬁbercontent,thethicknessinwhichthefailuremodebegantoshiftfromone modetoanotherwasfoundtodependstronglyonthebullettypeandmuzzleenergy.Thethicknessinwhichtheperforation failuremodedidnotoccurwasfoundtobethickerwhensubjectedtobulletswithlargermuzzleenergy.

Regardlessofthebullettype,theimpactenergyabsorptionincreasedwiththeincreasingthicknessandﬁbercontent.

Underthesamethicknessandﬁbercontent,theconstantawasobservedtodependonthebullettype.Underimpactby piercingtypeprojectiles(9and7.62mm),thepanelthicknessseemedtoplayanimportantroleontheimpactresistantof FRCpanels.Thiswasbecausethefailuremodeswereverylocalized.Underimpactbybluntandlargerdiameterprojectiles (11mmor.44magnum),ontheotherhand,themainconsiderationshouldbegivenontheﬁbercontentratherthan thicknessbecausethefailuremodesbecamemoreglobalized.

DeclarationofCompetingInterest

Regardingthesubmissionof a manuscriptonthetitle“SteelFiberReinforcedConcretePanelssubjectedtoImpact ProjectileswithDifferentCaliberSizesandMuzzleEnergies”foryourconsiderationtobepublishedontheCaseStudiesin ConstructionMaterials,theauthorsdeclarethattheyhavenoconﬂictofinterest.

Acknowledgement

ThisresearchisfundedbyKingMongkut’sUniversityofTechnologyNorthBangkok(contractno.KMUTNB-61-NEW-005) andThailandResearchFundunderResearchandResearchersforIndustries(contractno.MSD62I0063andPHD59I0023).

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