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Combination form analysis and experimental study of
mechanical properties on steel sheet glass fiber
reinforced polymer composite bar
Chao WUa,b,c, Xiongjun HEa,b, Li HEd*, Jing ZHANGe, Jiang WANGf
aSchoolofTransportationandLogisticsEngineering,WuhanUniversityofTechnology,Wuhan430063,China
bHubeiProvinceHighwayEngineeringResearchCenter,Wuhan430063,China
cCERIS,InstitutoSuperiorTécnico,UniversidadedeLisboa,Lisboa1049-001,Portugal
dSchoolofCivilEngineering,GuizhouInstituteofTechnology,Guiyang550003,China
eChinaRailwayMajorBridgeReconnaissance&DesignInstituteCo.,Ltd,Wuhan430056,China
fWISDRIEngineering&ResearchIncorporationLimited,Wuhan430023,China
*Correspondingauthor.E-mail:lihe@git.edu.cn
©HigherEducationPress2021
ABSTRACT The concept of steel sheet glass fiber reinforced polymer (GFRP) composite bar (SSGCB) was put
forward. An optimization plan was proposed in the combined form of SSGCB. The composite principle, material
selection,andSSGCBpreparationtechnologyhavebeendescribedindetail.Three-dimensionalfiniteelementanalysis
was adopted to perform the combination form optimization of different steel core structures and different steel core
contentsbasedonthemechanicalproperties.Mechanicaltestssuchasuniaxialtensile,shear,andcompressivetestswere
carriedoutonSSGCB.Parametricanalysiswasconductedtoinvestigatetheinfluenceofsteelcontentonthemechanical
propertiesofSSGCB.TheresultsrevealedthattheelasticmodulusofSSGCBhadimprovementsandincreasedwiththe
riseof steelcontent.Shearstrengthwas alsoincreasedwith theadditionofsteelcontent. Furthermore,theyieldstateof
SSGCBwassimilartothesteelbar,both ofwhich indicatedamulti-stageyieldphenomenon.Thecompressivestrength
of SSGCB was lower than that of GFRP bars and increased with the increase of the steel core content. Stress-strain
curvesofSSGCBdemonstratedthatthenonlinear-stagecharacteristicsofSSGCB-8weremuchmoreobviousthanother
bars.
KEYWORDS steel sheet GFRP composite bar, combination form, numerical modeling, mechanical properties test,
strength
1Introduction
Fiber-reinforced polymer (FRP) bar is a hot research
topic that has drawn growing attention from scholars
worldwide.FRPiswildlyusedinthe industrybecauseof
its advantages of superior mechanical, thermal, and
chemical properties such as lightweight, high strength,
andhighcorrosionresistance.Owingtoitsanticorrosive
characteristics,the FRP bar is a promising substitutefor
conventionalsteelbars in reinforcing concrete structures
[1–9]. The recent investigation of glass fiber reinforced
polymer (GFRP) bars in manufacturing slabs [10],
columns[11],railwaysleepers [12], and durabilitystudy
[13] should be attached to vital significance. However,
FRPbarsdisplayedalinearelasticbehaviorupforfailure
withlowelasticmodulusandnoductility,whichadversely
affected the ductility of the concrete structures [7,14].
Bakisetal.[15]indicatedthatthemostusefulapplication
ofFRPwasasatension-onlymemberinstructures,rather
thantypicalreinforcementin concrete structures because
oftherelativelylowmodulusandhighstrengthcompared
to steel. Sonmez [16] reviewed the optimum design of
structuresaccordingtothetypeofthecompositestructure
optimized by the loading conditions, the objective
Articlehistory:ReceivedDec7,2020;AcceptedMay12,2021
Front.Struct.Civ.Eng.
https://doi.org/10.1007/s11709-021-0743-7
RESEARCHARTICLE
function, the structural analysis method, the design
variables, the constraints, the failure criteria, and the
searchalgorithm.
Despitethe significanceandconcernsabout steelsheet
GFRP composite bar (SSGCB), the current number of
studiesonthistopic isstillnotenough,namelyaboutthe
differentcombinationformsonthemechanicalproperties
of SSGCB. The most relevant issues of previous
investigationsinthisspecificresearchfieldareillustrated
next,togetherwiththeaspectsthatstillrequireadditional
research efforts and motivated the present investigation.
Therefore, it is vital to optimize the structural design to
improve the elastic modulus and ductility of FRP bars.
Consideringthedeficiencies of FRP bars, some scholars
proposedthe concept of hybrid fiber-reinforcedpolymer
(HFRP)toimproveFRPductility[17–21].CarbonandE-
glass fibers were used to develop a CFRP/GFRP
reinforcement,andthetensilestrengthofhybridrodswas
tested [22]. The results of the tests indicated that the
ultimate strain of hybrid bars with dispersed type
increased by 33% compared with that of FRP bars. The
ductilehybridFRP(DHFRP)wasstudiedandprovedthat
the elastic modulus of HFRP bars was much lower than
that of steel, and the deformation of concrete structures
withthecompositebarwashigherthanthatofsteel[23].
Based on HFRP bars, some scholars further propose the
concept of steel fiber reinforced polymer (SFRP).
According to the hybrid theory, the problem of low
modulus and ductility of FRP bars was solved by
combiningthe advantagesoffiberandsteel [24–29].Cui
et al. [30] summarized the development of ductile
composite reinforcement bars in concrete structures.
Behnam and Eamon [31] have analyzed alternative
ductilefiber-reinforcedpolymerreinforcingbarconcepts.
Some researchers concentrated on the innovative hybrid
reinforcement for flexural elements [32–39]. Zhao et al.
[40] investigated new types of the steel-FRP composite
bar with round steel bar inner core on the mechanical
propertiesandbondingperformancesinconcrete.
However,experimentalresearchonthesebarshasbeen
limited due to bond performance. The tensile stress
experimentalresultsofcomposite reinforcementrevealed
aslipdeformationphenomenonbetweenthesteelandthe
outer fiber when steel was yielding [41,42]. Therefore,
many investigations into steel-core-FRP composite bars
(SCFCB) and steel-wire-FRP composite bars (SWFCB)
were implemented [43]. Furthermore, it was difficult to
locate steel wire in the pultrusion process, and the
bondingeffectbetween steel wireandbasaltfiber would
be worse when using too much steel wire. Considering
steel content, construction technology, and mechanical
properties of composite bars, three kinds of steel forms,
including SSGCB-4, SSGCB-6, and SSGCB-8, were
applied to simulate the characteristics of composite bars
in this study. These results revealed that the steel and
FRP bond strength was the limitation of the SFRP. The
hybridbarscan bemanufacturedfromresin-impregnated
FRP to encase the steel core fully. And the interfacial
propertiesbetweenFRPandsteelbecomethemainfactor
that influences the yield and ductility performance of
SFRP [29]. Only tensile properties of GFRP were
researchedinthenumericalsimulation,andthefollowing
assumptionsweremade, including thesmoothsurfaceof
the steel, good adhesion between steel and GFRP, good
bonding performance between steel anchor pipe, and
GFRP, and no relative slip between anchor pipe and
GFRP. Several experiments have been conducted to
evaluate the mechanical properties of steel-FRP
composite bars. The SFCB pultrusion process produced
many composite bars, and the stress-strain curves of
SFCBrevealed aprominentdouble-linearpropertyunder
monotonicloading[44].
The main differences between this study and those
published in the literature are as follows. 1) The results
showthattheHFRPtendonscanyield,andtheductilityis
improved.The mainreasonis thattheelastic modulusof
most fiber materials is relatively low. The hybrid FRP
bars, after hybrid fiber composite, appear in the tensile
process yield stage, but its deformation is greater than
that of steel bars. High-performance fiber-reinforced
composites (such as carbon fiber) are needed, and the
economyofhybridFRPbarsispoor.2)Theeffectofthe
composite reinforcement with fiber braiding or spiral
wrapping outside the steel core is not ideal, resulting in
the waste of the mechanical properties of the fiber. The
steel wire and basalt fiber are combined with the
pultrusion forming process to get composite
reinforcement. The test results show that the composite
reinforcementhasahigh elasticmodulus,butthecontent
of steel wire should not be too large, because there is
unavoidableinterferencebetweenlongitudinalstressfiber
andsteelwireinthepultrusionformingprocess,thesteel
wire is difficult to locate, and the forming effect is not
good. For certain glass fiber reinforced composite bar
withsteelstrandassteelcore,theresearchshowsthatthe
bar has a high elastic modulus and yield characteristics.
However, after the outer fiber of the composite bar
breaks,thesteelstrandslipsduetothefailureofeffective
bonding, and the composite bar stops working after the
outer steel strand with greater stress breaks. Steel
continuous fiber-reinforced bar (SFCB) has a higher
elastic modulus and better ductility, but the surface
treatment of ribbed bars is more complex, which is not
conducive to industrial production. At the same time,
although the cladding of ribbed steel bar can achieve a
goodcombination withasteelbar andlongitudinalfiber,
italsoleadstothebendingoflongitudinalfiber,whichis
notconduciveto the exertion of fiber strength and leads
theadjacentlongitudinalfibertoprematurefailure.
To sum up, there are two technical bottlenecks in the
2 Front.Struct.Civ.Eng.
construction principle and mechanical properties of
GFRP composite bars. In this paper, the radial
arrangement of 304 steel sheet GFRP composite
reinforcement is proposed to increase the contact area
between the steel and GFRP reinforcement. By
optimizingthearrangementformofthesteelsheetinner
core and glass fiber and the improvement of
corresponding mechanical properties, the above two
breakthroughsarerealized.
Thekeyproblemof the SFRP composite bar washow
to improve the coordinated deformation between steel
and GFRP. The steel and FRP bond strength can be
improved by combing steel sheets with FRP. The idea
that a steel sheet was used to replace the steel core and
steel wire in SSGCB had two significant advantages.
First,thecontactareabetweenGFRP and steel could be
increasedto improvethebondingpropertytoachievethe
purpose of coordinated deformation of the composite
material; Secondly, steel sheet was much easier to be
located than steel wire during the production process.
Differentsteel-to-FRPcross-sectionalarearatioscouldbe
used to allow for different mechanical performance. To
investigatethe mechanicalpropertiesofSSGCB,a three-
dimensionalfiniteelementmodel (FEM)ofSSGCBwith
different steel content was established. And the
destructiveuniaxialtensileand shear test was conducted
on SSGCB to evaluate its mechanical properties. For
researchers and end-users, using SSGCB as the
reinforcingmaterialcanhelpachievepost-yieldstiffness
and good reparability of a structure. Based on this new
composite reinforcing bar, new damage-controllable
structurescanbedevelopedthatimplementperformance-
basedseismicdesigns more easily.Meanwhile,insevere
aggressive environments such as marine and coastal
concrete structures, SSGCB has a promising and cost-
effective advantage of durability over traditional steel
bars that avoids corrosive environmental properties. It
providesreferencesfortheresearch,andstructuraldesign
ofSSGCBreinforcedconcretestructures.
2Combination form analysis of SSGCB
Based on the formation and principle of steel-GFRP
composite bar (SGCB), the concept of SSGCB was put
forwardtoimproveco-deformationofsteelandGFRPby
increasingtheoutersurface areaofsteel.Inthelayoutof
thesteel sheet,ifonly thecontactareabetweensteeland
fiber was considered, the evaluation index of n (ratio of
perimeterand areaofsteelsheetsection)wasshownthat
theareabetweenfiberandsteelwaslargerwhenthesteel
sheetwasthinner. However,itwas difficult toproducea
thin steel sheet and locate it into composite bars due to
thelimitationsoffabricationtechnology, then the tensile
andshear propertiesofSSGCBwerereduced.Therefore,
thesizeand number ofsteelsheetsshould be adequately
controlledtoimprovethecontactareabyincreasingthen
value.On the overall layout, the surface ofSSGCB was
spiraled by GFRP fiber bundles and sandblasted to
enhance the adhesion between composite bars and
concrete. There are two kinds of interfaces in steel core
GFRPcompositebars,namelyresin-glassfiberandresin-
steel. When a silane coupling agent is used, the
contribution of the silane coupling agent to the bond
strengthofthetwointerfacesshouldbeconsidered.When
thebindercan bewellwettedon the reinforcedmaterial,
thesurfaceroughnessofthereinforcedmaterialshouldbe
increased, especially for the steel core with a relatively
small contact area ratio, the surface sanding treatment
should be carried out, and the interfacial transition zone
should be increased to increase the interfacial adhesion
and reduce the risk of debonding. To improve the
coordinated deformation between the steel core and
GFRP, the GFRP bars with radial steel sheets were
proposedbyincreasingthecontactareabetweenthesteel
core and resin. The overall layout model of SSGCB is
shown in Fig.1. The composition of the composite
reinforcement is as follows: 304 stainless steel sheet is
usedwithathicknessof1mmandawidthof6mm.The
fiberisanE-glassfiber,andthesurfacecouplingagentis
a silane coupling agent. The adopted resin is
Fig. 1OveralllayoutmodelofSSGCB:(a)crosssection;(b)longitudinal.
ChaoWUetal.CombinationformandexperimentalstudyofmechanicalpropertiesonSSGCB 3
thermosettingepoxyresin.
2.1Compositeprinciples
Thethin steel sheets were used as the forcematerials in
SSGCB to replace the fiber partially. A perfect bond
between the steel sheet and GFRP was assumed by
neglecting the change in the bond stress with slip.
According to the composite law, the stress-strain
relationship of the steel-GFRP composite bar (SGCB)
was divided into three different stages: the elastic stage,
the plastic yielding stage, and the failure stage. The
period from the natural state to the steel yield state was
defined as the elastic stage of the composite bar, which
wasshowninEq.(1a).Inthisstage,thesteelcoreandthe
fiber were working together, and the modulus of the
composite bar was lower than that of steel and higher
thanGFRP.Equation(1b)expressedtheperiodfromsteel
yieldto theouterfiberbreakage,whichmeanttheplastic
yieldingstage.Theelastic modulus of the composite bar
decreased due to the yield of steel. The period of the
failurestage wasdefinedasshown inEq.(1c). Thefiber
waspulledout of theworkandall of theforcewasbore
bythesteeluntiltheincreasingstrainbrokeit.
σ=
ε(EsAs+EfAf)/A,0⩽ε⩽εsy,(1a)
(fsyAs+εEfAf)/A,εsy < ε ⩽εfu ,(1b)
fsyAs/A,εfu < ε ⩽εs,max,(1c)
ε
Es
As
εsy
εs,max
fsy
Ef
Af
εfu
A
A=As+Af
where =thestrainofSGCB; =theelasticmodulusof
steel; =thecross-sectionalareaofsteel; =theyield
strainofsteel; =thefracturestrainofsteel; =the
yieldstressofsteel; =theelasticmodulusoffiber; =
the cross-sectional area of fiber; = the fracture strain
offiber;and =thecross-sectionalareaofthecomposite
bar, .
2.2Manufacturingmethod
Glass fiber is an organic fiber to be used as a
reinforcement in advanced SSGCB due to its superior
performance at a relatively competitive cost. The main
justifications for applying the GFRP composite bar are
highstrength,lightweight, corrosion resistance,andanti-
fatigue. Meanwhile, good bonding properties and
plasticity of GFRP composite bars enabled the
preparation technology of SSGCB much more
manageable than others. Steel sheet is the primary
bearing component in SSGCB. Different shape features
ofsteelandsteelgradeand typecanbeusedtoallowfor
different mechanical performances of SSGCB. The
thermosetting resin is the matrix that bonds glass fiber
and steel sheets together to form SSGCB. Besides, the
resinhastwomaineffects,includingtransferringtheload
to each fiber and steel sheet; protecting SSGCB from
exposuretoenvironmentaleffects.Steelsheetsurfacehas
been usually smooth and coated with antirust grease,
which would inevitably affect the bonding strength
betweenresinandsteel.Therefore,thesteelsheetsurface
shouldberoughenoughtoenlargebondingstrength.The
pretreatment process to increase steel sheet surface
roughnessisillustratedinFig.2.
Glassfibers shouldbedistributed uniformlytotransfer
theloadtoeachfibereffectively.Glassfiberswereputon
the shelf and pull the materials out according to design
requirements and specifications, as shown in Fig.3(a).
Otherwise, the place where the spindle was in contact
with fiber should be smooth to avoid damage to glass
fiber.Glass fibers were introduced into the dippingtank
withthermosettingresinthroughtheguideroller,thenthe
redundant resin was cleaned through the carding board.
Finally, glass fibers were preformed by the pultrusion
mold. During the process, engineers should make sure
that the fibers and steel sheets in the dipping tank were
dispersedtoguaranteethestrengthofSSGCB.Figure3(b)
demonstrates the immersion process. It was critical for
materialsto infuse sufficiently with the resinmixture.If
glass fibers were not impregnated with the resin, the
strength of SSGCB would not attain the designed
strength.
The main function of preforming was to model the
resin-impregnated materials in the shape of the final
product. This process facilitated the close connection
between steel and fiber, and the excess resin was
removed. The preform model was used to transit the
material gradually from dispersion to centralization and
eventually reached the designed shape, and the SSGCB
moldisshowninFig.3(c).Afteryarnedandsandblasted,
the SSGCB was sent to the model for curing.
Thermosettingwasachemicalreactionprocesswithhigh
temperatureand highpressure,andthetemperature plays
adecisiverole intheprocess,as showninFig.3(d).The
curingratewasfasterwhenthetemperaturewashigh,but
thehigh temperaturewoulddamageSSGCB,however,if
the curing rate was too quick, it would lead to more
significant tension and influence the stress and
appearance of SSGCB. At the same time, too slow a
curing rate would lead to resin inhomogeneity and
affectedthequalityandpreparationefficiency.Therefore,
the temperature of the three heating zones was 160°C,
Fig. 2Thesteelpretreatmentprocess.
4 Front.Struct.Civ.Eng.
180°C,and170°C,respectively.
2.3FEmodeling
Due to the small cross-sectional size of the steel core
GFRP bar, the bonding interface between the steel core
andGFRPisthinner,andtheamountofslipissmall.The
slip between the steel core and GFRP interface is not
considered here, and the surface of the steel core is
assumed to be smooth and good bonding performance
with the GFRP bar. The integrated model is used for
analysis.The steelcoreandGFRPelementsarecommon
nodesonthejoint surface, and each material isregarded
as continuous and uniform material. Through numerical
analysis,theoveralltensileperformanceofthecomposite
barunder loadandtheeffect ofloadsharing bythesteel
core were studied. The yield strength, tensile strength,
elastic modulus, and Poisson’s ratio of glass fiber, 304
steel,andQ345steelarepresentedinTable1.
Three-dimensional(3D)solidelements(Solid45)were
adopted to simulate GFRP and steel to simplify the
numericalcalculation. Asacrucial stepinthis modeling,
adetailed meshingcanensure arelativelyaccurate result
[45].Structural mesh generation technology was applied
inthismodeling.Because thesectionofthesteelsheetis
1mm×5mm, the mesh size was 1mmtohaveagood
analysisofthestructuralperformance.Thefiniteelement
modelofSSGCBisshowninFig.4.
2.4Analysisofdifferentsteelcorestructures
This section only considers the numerical calculation
results of three structural forms of composite
reinforcement. The material properties of the composite
reinforcementof eachstructuralformare thesame.Steel
wire,thespecificsectionformisshowninFig.5,andthe
structuralparametersareshowninTable2.
The composite bar has a stress concentration at the
anchoringjunction.Themainconsiderationinthissection
is the tensile force performance of the composite bar,
which is ignored here. For the stress of this part, select
the distal end 150 mm section for force analysis. The
axialstressdistributionsofthethreecompositebarsunder
variousworkingconditionsarenotmuchdifferent.Figure6
illustrates the SSGCB-6 axial tensile stress cloud
diagram. The stress on the steel core is about 258 MPa
when the tensile stress is 150 MPa. The stress is about
138MPawhenthetensilestressis600MPa,thestresson
the steel core is about 358 MPa, and the stress on the
GFRP is about 635 MPa. From the stress-strain
relationship of the two materials as mentioned above, it
can be seen that the elastic modulus of the steel core
decreasesrapidlyaftertensileyieldingandislower than
theelasticmodulusofGFRP,sothesteelcoreisstressed
duringtheelasticphaseduringthestretchingprocess.Itis
greater than the external GFRP material, and the stress
level of the steel core after yielding gradually decreases
and be lower than that of the GFRP material. It can be
seen that the high modulus steel core can increase the
Fig. 3Manufacturing method of SSGCB: (a) arrangement of glass fibers; (b) glass fiber immersion system; (c) mold; (d) pultrusion
process.
Table 1MaterialparametersofSSGCBmodel
materials density
(kg/m3)
yield
strength
(MPa)
tensile
strength
(MPa)
elastic
modulus
(GPa)
Poisson
ratio
glassfiber 2550 – 744 45 0.28
304steel 7800 257 540 193 0.30
Q345 7800 345 675 206 0.28
ChaoWUetal.CombinationformandexperimentalstudyofmechanicalpropertiesonSSGCB 5
elasticmodulus ofthecomposite ribintheearlystageof
the stretching process, and in the later stage, the overall
tensilemodulusofthecompositeribisreducedduetothe
rapiddecreaseofitsmodulus.
The steel core shape and layout of the three types of
composite bars are different, inevitably affecting the
stressdistribution oftheGFRP materialinthe composite
bar during tension, thereby affecting the force
transmissioneffectbetweenthe materials and the failure
form of the composite bar materials. In the composite
reinforcement, the force performance of the interface
betweenthesteelcoreandGFRPisnotonlyrelatedtothe
interfacebutalsoclosely related to theforceofthefiber
andresininthevicinityof thesteelcore.Iffiberfracture
or resin failure occurs in this area, the bonding
performanceoftheinterfaceisweakenedandeventually
fails.Figure7showsthestressclouddiagramsofGFRP
in composite bars under 150 and 600 MPa tensile stress
workingconditions.
Overall,theGFRPstressdistributionlevelsofthethree
typesofreinforcementsduring the stretching processare
equivalentandrelativelyuniform.Themaximumstressof
GFRPappearsinthevicinityofthecontactwiththesteel
core. The stress in this area of the three composite bars
under 150 MPa tensile stress is not much different, and
the difference is more evident under 600 MPa tensile
stress. For SSGCB-6, there is a specific stress
concentration phenomenon at the end of the steel sheet,
while the GFRP stress distribution near the side of the
steel sheet is relatively uniform, and there is no stress
concentration phenomenon. For SFCB-1 and SFCB-6
composite bars with circular cross-section steel cores,
larger stresses in the circumferential area contacts the
steel core. The maximum stress of GFRP in the
compositebarundertheactionofaveragetensilestressof
600 MPa of SSGCB-6, SFCB-1, SFCB-6 were 693.3,
672.9, 675.1 MPa, respectively. The maximum stress
valueof SSGCB-6isabout19MPalargerthan theother
two. The stress cloud diagram shows that in the same
kindofworkconditions, the GFRPstressintheside area
ofthe steelsheetintheSSGCB-6compositebar isabout
655 MPa, which is about 18 MPa lower than the other
two.Duetothesmallstressconcentrationareaattheend
of the steel sheet, the overall stress of the GFRP in the
contactareawiththethinsteelsheetislowerthanthatof
theothertwo,indicatingthattheGFRPontheouteredge
ofthesteelsheethasabetterstresscondition. Regardless
of interface slip, the shear stress cloud diagrams of the
interface between the steel core and GFRP in the three
composite reinforcements under a load of 600 MPa are
showninFig.8.
Fig. 4FEmodelsofSSGCB:(a)axisymmetricmodel;(b)sectionalmeshing.
Fig. 5Thecross-sectionalviewofthreetypesofsteelcoreGFRPcompositebars.
Table 2The steel core structural parameters of three different
compositebars
specimen diameter
(mm)
steelcore
content
steelcore
sectionform
steelcore
quantity
SSGCB-6 18 14.2%thinsteelsheet 6
SFCB-1 18 14.2%roundbar 1
SFCB-6 18 14.2%steelwire 6
6 Front.Struct.Civ.Eng.
Theload application methodinthis chapteristoapply
a load to the end of the composite bar anchorage. The
anchoragetransferstheloadtothecompositebarthrough
shearforce,sosignificantshearstressappearsasthesteel
core-GFRPinterface nearthetwoends oftheanchorage.
Among them, the maximum shear stress appears in the
transition section of the composite reinforcement
anchorage area, and the shear stress gradually reaches a
Fig. 6AxialstressclouddiagramofSSGCB-6underaxialload(MPa):(a)150MPa;(b)600MPa.
Fig. 7vonMises stresscloud diagramunderaxial tensileload (MPa):(a) SSGCB-6150MPa;(b)SSGCB-6 600MPa; (c)SFCB-1 150
MPa;(d)SFCB-1600MPa;(e)SFCB-6150MPa;(f)SFCB-6600MPa.
ChaoWUetal.CombinationformandexperimentalstudyofmechanicalpropertiesonSSGCB 7
lower level in a smaller range. The shear stress
distributionlaw conforms tothelaw mentionedaboveof
interfacial shear force transfer in composite materials.
Duetothedifferenceinthethreetypesofstructureofthe
steelcoreGFRPcompositebars,theinterfaceshearstress
distribution is different, as shown in Fig.8. The
maximum shear stress at the interface of the SSGCB-6
composite bar is 98.0 MPa. At the stress concentration
area, the maximum shear stress at the interface of the
SFCB-6 composite bars is 74.8 MPa, and the maximum
interface shear stress in the SFCB-1 is 34.8 MPa. The
shearstress attheinterfaceof theSFCB-1bars isevenly
distributedalongthesteelcore circumferential direction.
For SSGCB-6 and SFCB-6, the outer edge of the
compositebarisclosertothesurfaceofthecompositebar
thanthesteelcore of the SFCB-1 bar, so the outer edge
shear stress is more significant that gradually decreases
from the outward axis direction, which conforms to the
shear stress distribution law in material mechanics. The
purpose of this paper is to find a steel core GFRP
composite bar to improve the mechanical properties of
GFRPsothatit can replace thestressedsteelbarand be
used in concrete. The reinforced concrete structure is
uniformly stressed, and there may not be such obvious
stress at the concentrated end. However, the above
analysisshowsthatwhilethethinsteelsheetincreasesthe
shearcapacity of the interface, it alsohas shortcomings;
that is, there will be relatively large shear stress on the
outeredge.Themechanicalproperties of composite bars
needtobestudiedinconjunctionwithrelatedtests.
AccordingtoASTMA304-20 [46],thetensilestrength
of304steelassteelcoreis540MPa,andthatofGFRP
reinforcement is 744 MPa. The convergence criteria of
different steel core cross-section bars are GFRP bars
ultimate tensile strength and steel sheet ultimate tensile
strength. It is considered that the ultimate bearing
capacity of composite bars is the corresponding bearing
capacity when the steel core and GFRP bars reach the
tensilestrength.ItcanbeseenfromFig.9.thattheGFRP
bar should reach an ultimate tensile strength before the
steelcore duringloading.Therefore,itcanbeconsidered
that the ultimate bearing capacity of the composite
reinforcement is the ultimate bearing capacity when the
GFRP Bars reach the yield limit. The ultimate bearing
capacity of SSGCB-6, SFCB-1, and SFCB-6 composite
bars is 168.53, 168.09, and 167.76 kN. The comparison
ofthethreecasesisshowninTable3andFig.9.
The ultimate bearing capacity of SSGCB-6, SFCB-1,
Fig. 8Shearstresscloud diagrams oftheinterface between the steelcoreand GFRP inthethree composite barsunder600 MPa stress
(MPa):(a)SSGCB-6;(b)SFCB-1;(c)SFCB-6.
8 Front.Struct.Civ.Eng.
and SFCB-6 composite bars is 168.53, 168.09, and
167.76kN.Thecomparisonofthethreecasesisshownin
Table3 and Fig.10. The ultimate bearing capacity of
SSGCB-6 is the largest, SFCB-6 is the smallest, and
SFCB−1isbetweenthem.Theultimatebearingcapacity
ofSSGCB-6 is0.77kNhigherthanthatofSFCB-6. The
main reason is that the steel sheet increases the contact
areabetweenthesteelandGFRPbarsrelativetothesteel
coreandimprovesthebondingcapacity.
2.5AnalysisofSSGCBwithdifferentsteelcorecontent
Researchersjust aimedatsteelcoreand steelwire,while
theoptimumsteel sheetcontentof SSGCB wouldnotbe
thesame asthem.The tensilepropertiesof SSGCBwere
studied to explore the influence of the steel content on
them. Considering the content of steel, construction
technology, and mechanical properties of SSGCB, three
kinds of steel structure forms including SSGCB-4,
SSGCB-6, and SSGCB-8, were applied in numerical
simulation to analyze its characteristics. In this section,
the modeling of SSGCB and the structure optimization
analysisofSSGCB based onmechanicalpropertieswere
carriedout.Tostudytheimprovementofelasticmodulus
and ductility of the SSGCB with different steel content
effectively, a simplified model of SSGCB with three
kindsofmaterialssuchasGFRP,innersteel,andthesteel
anchorpipewasadopted.ThreetypesofSSGCB,namely
SSCGC-4, SSGCB-6, and SSGCB-8 were chosen to
analyze their mechanical properties, of which the
structural parameters with different steel content are
tabulatedinTable4.
The materials of GFRP and steel were regarded as
continuoushomogeneousmaterialsto analyze the tensile
propertiesofSSGCB.GFRPwasassumedto behaveasa
linear-elastic material before the failure of stress and
Fig. 9TherelationshipbetweenvonMisesstressandtimehistoryofthreedifferentcompositebars:(a)SSGCB-6;(b)SFCB-1;(c)SFCB-6.
Table 3Theultimateloadingcapacityofdifferentcompositebarsunderuniaxialtensilestress
no. LS VMS(MPa) YP(MPa) P(MPa) BD(mm) STBD(mm) ULC(kN)
SSGCB-6-G 0.82857 744 744 563 18 28 168.53
SSGCB-6-S 0.82857 291.94 540
SFCB-1-G 0.82638 744 744 563 18 28 168.09
SFCB-1-S 0.82638 290.79 540
SFCB-6-G 0.82477 744 744 563 18 28 167.76
SFCB-6-S 0.82477 290.61 540
Notes:LS:loadstep;VMS:vonMisesstress;YP:yieldcriterion;P:pressure;BD:bardiameter;STBD:steeltubediameter+bardiameter;ULC:ultimate
loadingcapacity.
Fig. 10Ultimate loading capacity of different composite bars
underuniaxialtension.
Table 4StructuralparametersofSSGCBwithdifferentsteelcontent
specimenID. diameter(mm) sheetsize(mm) sheetnumber steelcontent
SSGCB-4 18 1×5 4 9.4%
SSGCB-6 18 1×5 6 14.2%
SSGCB-8 18 1×5 8 18.9%
ChaoWUetal.CombinationformandexperimentalstudyofmechanicalpropertiesonSSGCB 9
strain in longitudinal tension. Steel was anisotropic
material,sothemultilinearisotropic hardening plasticity
model was used to simulate. The inner steel of SSGCB
was 304 thin steel sheets in accordance with the
experiment protocol. The yield strength was 257 MPa,
andthestress-straincurvedid not have an obvious yield
platform. The tensile characteristics of SSGCB with
different steel content were researched in the finite
element models. The constant loads (150 and 600 MPa)
wereappliedattheendoftheanchorpipetoavoidstress
concentration and the corresponding boundary condition
was followed as the experimental test. Figure11
illuestrates the von Mises stress of composite bars
(includingSSGCB-4,SSGCB-6,andSSGCB-8).
Theresultsindicatedthatthestressofsteelwasgreater
thanfiberanddecreasedslightlywiththeincreaseofsteel
contentwhenSSGCBunderthetensilestressof150MPa.
The maximal stresses of the SSGCB were, respectively,
263.3, 260.6, and 257.6 MPa, which was because the
modulus increased with the increase of steel content.
Therefore, the stress was decreased with the increase of
steelcontentunderthesameload.Underthestressof600
MPa,thestressofGFRPwasgreaterthansteelduetothe
yield of steel. Also, there was a significant increase in
stress with the increase of steel content, and they were
634.3, 653.3, and 672.2 MPa, respectively. Because the
elastic modulus of SSGCB decreased faster when steel
was yielded, and the stress of GFRP will increase.
However,because thecapacityofGFRPwaslimited,the
strengthofSSGCB would belowerwiththe higher steel
content.TofactuallyanalyzethedeformationofSSGCB,
thepartsofwhich were kept awayfromthesteelanchor
pipewere chosenasthe effectivepartof thespecimenin
thepaper.ThedisplacementcurvesofSSGCBareshown
Fig. 11vonMises stressofSSGCB underaxial load:(a)SSGCB-4 under150 MPa;(b)SSGCB-4 under600 MPa;(c)SSGCB-6 under
150MPa;(d)SSGCB-6under600MPa;(e)SSGCB-8under150MPa;(f)SSGCB-8under600MPa.
10 Front.Struct.Civ.Eng.
in Fig.12. The stress-displacement curves of SSGCB
were presented with a convex linear in both the elastic
stageandtheplasticstage.Fromthestress-displacement
curve, it can be observed that the elastic modulus of
SSGCB was larger than that of GFRP under the elastic
stage.TheelasticmodulusandductilityofSSGCBwould
increase with the increase of steel content. Then the
elasticmodulus ofSSGCBwasgradually decreasedwith
theincreaseofthestress.Andwhenthesteelyielded,the
modulus of the composite would be decreased and
showed the ductility characteristics. Also, the nonlinear-
stagecharacteristicsofSSGCB-8weremoreobviousthan
thatofothercompositebars.
3Experimental optimization analysis of
SSGCB of combination form
The slip phenomenon between steel and fiber appeared
because the steel and glass fiber have different
deformationratioswhenthesteel yields. Therefore, four
groupsofSSGCBspecimensconsistedofGFRPandsteel
sheet were tested experimentally to evaluate the tensile,
shear, and compressive strengths of SSGCB with
differentsteelcontents,whichthespecimenidentification
isSSGCB-0,SSGCB-4,SSGCB-6,andSSGCB-8.
3.1Materialsdesign
304 stainless steel was selected as the test material, and
thethickness ofthesteelsheetwas1mm, thewidthwas
5 mm; glass fiber type was E-glass fiber, and silane
couplingagentwasselectedasasurface coupling agent;
thebondingagentwasepoxyresin.Thisbarwasmadeup
ofunidirectionalrovingof E-glass and epoxyvinylester
resinasmanufacturedthroughthepultrusionprocess.The
appearancewasmilkywhite,andthesurfaceofthespiral
sandblasting, each screw length was 14 mm, and the
height was 0.325 mm. SSGCB specimens are shown in
Fig.13.
Considering the size and layout of the steel sheet, the
test was carried out for four kinds of steel content
compositebars,includingSSGCB-0,SSGCB-4,SSGCB-
6, and SSGCB-8. The parameters of SSGCB specimens
areshowninTable5.
3.2Experimentmethodology
Figure14(a)depicts thatthespecimensfortensile testing
were prepared by anchoring two ends of the SSGCB in
steel plugs filled with epoxy resin. The interspace
betweenSSGCBandthesteelanchorpipewasfilledwith
epoxy resin. To ensure the anchorage performance
betweenSSGCB andsteelanchorpipe, therewerescrew
threadsontheanchorpipeinterior.Thelengthofthesteel
pipewas300mm,thediameterwas 35mm,andthewall
thickness was 6 mm. The epoxy resin was used as
adhesive to fix the composite. The free length between
Fig. 12Stress-displacementcurveofSSGCB.
Fig. 13SSGCBspecimens:(a)longitudinal;(b)horizontal.
Table 5MechanicaltestparametersSSGCBspecimens
specimenID D(mm) SC(%)SSN TTS STS CTS
SSGCB-0 18 0 0 4 4 5
SSGCB-4 18 9.4 4 4 4 5
SSGCB-6 18 14.2 6 4 4 5
SSGCB-8 18 18.9 8 4 4 5
Notes:D:diameter;SC:steelcontent;SSN:steelsheetnumber;TTS:tensiletestspecimens;STS:sheartestspecimens;CTS:compressiontestspecimens.
ChaoWUetal.CombinationformandexperimentalstudyofmechanicalpropertiesonSSGCB 11
steel plugs was about 300 mm to ensure the anchor
bonding strength is higher than the tensile stress,
accordingtotheguidelinesasspecifiedinACI440.3R-04
[47].Auniversaltestingmachine(SHT4106-G)wasused
inthetensile test, and an extensometer of 50 mm gauge
length was mounted with clips at the center of the test
specimen, as can be seen from Fig.14(b). The
displacement control mode was adopted with a loading
rate of 2 mm/min, and an automatic data acquisition
system was used to collect the test data with a data
acquisition frequency of 1 Hz. The extensometer model
(YYU-10/50) was used in the electronic extensometer
measurementtest.
The length of the shear specimen was 300 mm to
ensure the anchor bonding strength according to the
specification specified in JG/T 406−2013 [48]. The
universal testing machine (SHT4106-G) was adopted in
the shear test, and the shearing mechanism is shown in
Fig.15(a).AndtheforcearrangementofSSGCBcanbe
seen from Fig.15(b). The test loading rate was 40
MPa/min, and an automatic data acquisition system was
applied to collect the test data with the data acquisition
frequencyof1Hz.
ThestandardGB/T 1448−2005 [49] stipulates that the
testpiecewithadiameterof5–16mmhasacompression
test height of 2.5 d. The “Metal Compression Test
Method” stipulates that the length of metal material
specimens is generally 2.5–3 times the diameter.
Combining the specifications and the above tests, the
heightofthespecimenusedinthecompression testis 50
mm. During the production process of the test piece,
ensure that the material section is flat and intact, and
ensure that the test is vertically compressed during the
test.
3.3Resultsanddiscussion
The failure modes of SSGCB in tensile, shear, and
compression tests are depicted in Fig.16. There was no
specimen split off the anchor due to the sufficient bond
strength between the specimen and anchor pipes during
thetest.Thefailureprocessshowedanobviousphaseand
a stable post-yielded stiffness after the yield of the steel
sheet.Thetensile failure position ofSSGCBwasmainly
concentratedon the middle of the specimensand a little
closetothe endofthe anchor. Themainfailure modeof
SSGCBwasthatthefiberwasbrokenawayfromthesteel
sheet,afterwhichwasyielded.There would be a micro-
crack when the load reached about 40% of the ultimate
load,andpartoffiberwouldbe fractured when the load
reached up to about 70% of the ultimate load, and then
the breakage of fiber would increase sharply, which
finallyledtothedamageofthespecimens.Nosteelsheet
was pulled out or slipped from the SSGCB, and the
failuremodesofglassfiberinSSGCBwerenotthesame
asGFRP. Inthesheartest,the soundsofsteelbrittleand
fiberdamage couldbeheard,andthefrequencyof sound
would be quickened with the increase of load. Then the
whole GFRP bar and SSGCB were cut off, and the
Fig. 14UniaxialtensiletestofSSGCB:(a)SSGCBanchorage;
(b)uniaxialtensiletestsetup.
Fig. 15SSGCBsheartest:(a)sheartestsetup;(b)shearforcediagram.
12 Front.Struct.Civ.Eng.
sectionsof GFRP and steel sheetwere trim. There were
no apparent cracks and compression deformation of the
section during the shear test. Figure16 shows the shear
test failure of the SSGCB. During the compression
process, there will be multiple brittle noises. After the
loadreachesthepeak, itwilldrop,andthentherewill be
asmallerrisingstage.Finally,theloadwilldropatalow
level, and the specimen will be damaged. The main
failuremode of GFRP tendons is split failure. The main
failuremodes ofSSGCBcompositebarspecimensareas
follows.1) End failure. The endof the composite bar is
fracturedintoatruncatedconeshape,andpartofthesteel
core end is unstable. 2) Split failure. The main split
failureinthe composite barisconcentratedat the end of
thesteelsheet,mainlyduetotheinconsistentdeformation
ofthesteelcorewiththeGFRPdeformationandsplitting
failureduringthecompressionprocess.
ThetensiletestdataofSSGCBareillustratedinTable6.
The elastic modulus and ultimate tensile strength of the
SSGCB are shown in Fig.17. The elastic modulus of
SSGCBimprovedandincreasedwiththeincreaseofsteel
content.Theelasticmodulus ofSSGCB-8 was67.4GPa,
which was 49.8% bigger than that of GFRP. However,
thetensile strength declined because ofthe low strength
ofsteel,andtheelasticmoduluswoulddecreasequickly
afterthesteelwasyielded.Therefore,theultimatetensile
strength of SSGCB was lower than that of GFRP and
decreasedwiththeincreaseofsteelcontent.Theultimate
tensilestrengthof SSGCB-8 was 663.4MPa,whichwas
17.0%lowerthanthatofGFRP.
Figure18 demonstrates the stress-strain curves of
SSGCB. In the nonlinear stages, characteristics of steel-
sheet-GFRP composite bars’ stress-strain curves became
more obvious with the increase of steel content. The
results suggested that the deformation effect of SSGCB
was better, and the mechanical properties were more
stable. Besides, the test results were in good agreement
with the numerical simulation results, which indicated
that the adhesion between the steel sheet and fiber was
prettygood.TheGFRPstress-straincurvewaslinear,and
there was no yield phenomenon. However, the stress-
straincurvesoftheSSGCBwerenonlinear.Thesecurves
were presented a ‘convex’ trend, and the trend was
increasedwiththeincreaseofsteelcontent.Theuptrend
of curves could be slow, which showed a yield
phenomenonthatwasmoreobviouswith the increase of
steel content. And the SSGCB-8 had a distinct plastic
stageundertheuniaxialtensileload.Theelasticmodulus
of steel and SSGCB declined sharply when the steel
enteredthe plasticstage.Afterthefractureofglassfiber,
the load was transferred to the steel, and then the steel
wasdestroyedrapidly.
Asthe sheartestresultsoftheSSGCBaretabulated in
Table6.TheaveragecompressivestrengthoftheSSGCB
isshown inFig.18.Therewasno brittlefailurefromthe
failure mode of the specimen, and no steel sheet was
pulled out of SSGCB. This phenomenon indicated that
the bonding property between the steel sheet and fiber
Fig. 16FailuremodesofSSGCB:(a)tensilefailure;(b)shearfailure;(c)compressionfailure(splitfailureofGFRPbar);(d)compression
failure(SSGCBbarendfailure);(e)compressionfailure(splitfailureofSSGCBbar).
ChaoWUetal.CombinationformandexperimentalstudyofmechanicalpropertiesonSSGCB 13
was good. And the shear strength increased with the
increase of steel content, mainly owing to the shear
capacity of steel is better than that of GFRP. The shear
capacity of SSGCB improved when steel was added to
SSGCB, and the shear strength was 22.2% greater than
that of GFRP. The measured data shows that the
compressive strength of the steel core added to the
composite bar has decreased slightly, with a maximum
decrease of 11.5%, but the compressive strength will
increase as the steel core content increases. Combining
the failure morphology, due to the uncoordinated lateral
deformation of the steel core and fiber during the
compressionprocess,theresin’sbondingperformanceis
not enough to provide the uncoordinated lateral force,
which leads to the separation of the materials under the
actionoflargecompressivestress.Thefiberandthesteel
Table 6SSGCBmechanicaltestresults
test parameters no. specimenidentification
SSGCB-0 SSGCB-4 SSGCB-6 SSGCB-8
tension elasticmodulus(GPa) ①43.7 57.5 61.7 67.8
②45.3 54.7 60.5 66
③44.2 52.9 66.2 65.7
④46.1 53.3 62.3 69.2
tensilestrength(MPa) ①793.1 731.3 674.9 660.8
②815.4 739.5 686.1 671.3
③782.3 688.7 696.4 680.6
④806.5 706.5 671.9 640.9
shear load(kN) ①88.3 97.2 106.0 107.5
②92.5 95.4 101.6 110.8
③86.2 96.3 102.3 102.5
④88.4 98.6 97.6 113.7
shearstrength(MPa) ①173.5 191.1 208.3 211.3
②181.8 187.6 199.8 217.8
③169.5 189.3 201.1 201.5
④173.7 193.9 191.9 223.5
compression ultimateload ①77.6 75.3 75.9 79.6
②79.1 73.4 71.8 81.9
③78.3 69.2 79.4 78.8
④72.8 68.3 79.7 83.9
⑤79.2 73.8 81.4 71.6
compressivestrength(MPa) ①386.1 296.1 298.4 313.0
②311.0 288.6 282.3 322.0
③307.9 272.1 312.2 309.8
④286.2 268.5 313.2 329.9
⑤311.4 290.2 320.0 281.5
Fig. 17Mechanical properties and steel content’s relationship of SSGCB: (a) average shear strength; (b) elastic modulus; (c) ultimate
tensilestrength.
14 Front.Struct.Civ.Eng.
sheetare destabilized anddamaged.It istheexistenceof
this uncoordinated deformation that causes the overall
compressive strength of the composite bar to decrease,
but at the same time, the steel core itself has better
compressive performance, which will contribute to the
compressive strength of the SSGCB composite bar. The
compressive strength will increase with the increase of
thesteelcorecontent.
Tosumup,accordingtothecombinationformanalysis
from mechanical evaluations and experimental test, the
optimumcombinationformwasSSGCB-8,whichhadthe
best comprehensive performance of elastic modulus,
displacement, stress-strain, tensile strength, and shear
strength.
4Conclusions
Thecombinationofsteelsheetand GFRP for improving
the tensile modulus of the composite bars has proven to
be a viable technology for applications in concrete
structures.Steelsheetwasusedtoreplacesteelcore/wire,
whichwouldincreasethecontactareabetweenGFRPand
steeltoimprovethebondingpropertysoastoachievethe
purpose of coordinated deformation for composite
materials. Three-dimensional finite element models and
mechanical tests were conducted on SSGCB to evaluate
its mechanical properties, and the following conclusions
weredrawnbasedontheresultspresentedanddiscussed.
1)InacombinationformanalysisofSSGCB,basedon
the interface characteristics of GFRP and the bond
mechanismofthecomposites,theconceptofSSGCBwas
putforwardtoimprove the tensile modulusandductility
ofthecompositebar. The composite principle, materials
selection, and preparation technology of SSGCB were
describedindetail.
2) The selection and comparison of SSGCB based on
mechanicalpropertyanalysiswerecarriedout.Theelastic
modulusofSSGCBwasgreaterthanthatof GFRPat the
elasticstage,whichgraduallydecreasedwiththeincrease
of stress. After the yield of steel, the elastic modulus of
the SSGCB decreased and presented ductility
characteristics.
3) The elastic modulus and ductility of SSGCB
increased with the increase of steel content. When the
steel was yielded, the modulus of the composite
decreased and illustrated the ductility characteristics.
Furthermore, the nonlinear stage characteristics of
SSGCB-8weremuchmoreevidentthanthatofothers.At
the elastic stage, the main force component of SSGCB
was the steel sheet. With the enlargement of load, the
steel sheet was yielded, and fiber became the main
forcingcomponent.
4) According to the selection and comparison of
SSGCB based on the mechanical properties experiment,
the tensile strength decreased because of the low steel
strength,and theelasticmodulus woulddecreasequickly
aftertheyieldofsteel.Theultimatetensilestrengthofthe
SSGCBwaslowerthanthatofGFRPanddecreasedwith
the increase of steel content. The shear capacity of
Fig. 18Stress-straincurveofSSGCB:(a)SSGCB-0;(b)SSGCB-4;(c)SSGCB-6;(d)SSGCB-8;(e)average.
ChaoWUetal.CombinationformandexperimentalstudyofmechanicalpropertiesonSSGCB 15
SSGCB would be improved when steel was added into
the composite bar, which indicated that the bonding
performancebetweenthesteelsheetandfiberwasgood.
5) The tensile failure process illustrated an obvious
phase and a stable post-yielded stiffness occurred after
thesteelsheetwas yielded. The primary failure modeof
SSGCB was that fiber was broken away from the steel
sheet when steel yielded, and the outer glass fiber was
scattered.
6) In the compression process, due to the inconsistent
lateral deformation of the steel core and fibers, the
compressive strength of SSGCB is lower than that of
GFRP bars, but it will increase with the increase of the
steelcorecontent.
AcknowledgementsThe authors may wish to express their sincere
appreciation for the financial support provided by the National Key
Research and Development Program of China (No. 2017YFC0806008),
Scienceand TechnologyProject ofDepartment ofTransportation ofHubei
Province (No. 2018-422-1-2), National Natural Science Foundation of
China(GrantNo.51178361), MajorProjectofTechnologicalInnovationof
Hubei Province (No. 2018AAA031), China Scholarship Council (No.
201906950026), and the Fundamental Research Funds for the Central
Universities(No.2019-YB-015)forthiswork.
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