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Performance of outside filament-wound hybrid FRP-concrete beams

Anup Chakrabortty, Amar Khennane ⇑, Obada Kayali, Evgeny MorozovSchool of Engineering and Information Technology, UNSW@ADFA, Canberra, Australia

a r t i c l e i n f o

Article history:Received 30 July 2010Received in revised form 11 October 2010Accepted 8 January 2011Available online 15 January 2011

Keywords:A. Glass fibresD. Mechanical testingE. PultrusionE. Filament windingHybrid beam

a b s t r a c t

A novel configuration of a hybrid FRP-concrete beam is presented. The beam consists of a GFRP pultrudedprofile, a CFRP laminate, and a concrete block all wrapped up using filament winding. Three different con-crete blocks were used: high strength concrete, normal strength concrete and steel fibres reinforced highstrength concrete. The major feature of the design is that it does not mimic that of reinforced concrete asreported previously. The CFRP laminate is not designed to fail first to serve as a warning of imminent fail-ure, but rather to enhance the stiffness of the beam by compensating for the lack of stiffness of the GFRPprofile. The experimental results have shown that this approach is successful. The wrapping did not onlyeliminate the risk of premature failure as a result of the concrete block debonding from the pultrudedprofile, but it was also found to enhance the stiffness and load carrying ability of the beams. The beamswith a high strength concrete block showed increased stiffness and load carrying ability but failed in acatastrophic manner. On the other hand, the beams with normal strength and steel fibres reinforced highstrength concrete showed improved ductility. The amount of energy dissipative behaviour was found todepend on the thickness of the concrete block. When the latter is too thin, failure akin to shear punchingappears to take place.

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1. Introduction

Despite many advantages over conventional materials, compos-ites are making limited progress in the field of highway structures,where the only niche market for composites is in FRP deck con-struction over steel girders and externally bonded FRP repair. In-deed, there are very few all composite bridges. The reasons areto be found in their high initial cost compared to concrete, timber,and steel, but also in the low stiffness of glass fibre reinforced plas-tics (GFRP), which is favoured by the construction industry for itsaffordability as compared to aerospace grade composites such ascarbon fibre reinforced plastics (CFRP). While the problem of highinitial cost can be addressed using large volume automated pro-cesses such as pultrusion and filament winding, the lack of stiffnesscan only be addressed through innovative design.

Using GFRP typically results in a very thick girder cross section.An example is the Tech21 Bridge built in 1997 [1]: see Fig. 1. Theother alternative was to use hybrid girders [2–4] where a glass fi-bre reinforced plastic (GFRP) box beam is combined with a con-crete layer, cast onto the top flange, and a thin carbon fibrereinforced plastic (CFRP) laminate, bonded to the tension zone. Alimitation of such a design is that it mimics that of reinforced con-crete; the carbon laminate, with a lower failure strain, fails earlierthan the GFRP, thus serving the role of a sensor. The carbon lami-

nate is playing the role of steel yielding in a reinforced concretebeam. A consequence of this design is that the amount of carbonthat can be used in the cross section is limited. This in turn putsa limit on the stiffness of the beam. Consequently, because of theinherent lack of stiffness, when these beams are used in a bridge,they have to be put side by side as shown in Fig. 2.

In the following, an innovative hybrid beam design that not onlydoes not mimic that of reinforced concrete but also exploits effi-ciently the mechanical properties of the materials is presented.However, before describing the proposed design, it is worthwhileto review the existing designs for hybrid beams and identify theshortcomings.

2. Review of existing designs for hybrid beams

The concept of a hybrid beam is not new, it has already beentrailed by Deskovic et al. [2], Canning et al. [3], van Erp et al. [4],Zhao et al. [5], Hulat et al. [6] and Correia et al. [7]. Their respectivecross section designs are shown on Fig. 3. Deskovic et al. [2] Fig. 3a,Canning et al. [3] Fig. 3b, and van Erp et al. [4] Fig. 3d, use almostsimilar designs, where the beams consist of a custom made FRPbox, a concrete layer, and a CFRP laminate to serve as a warningof imminent collapse. This approach simply mimics that of rein-forced concrete in the sense that the CFRP failure is expected to re-place steel yielding. As a result, the CFRP laminate is usually verythin, and contributes very little to the stiffness of the beam. Havingrecognised the inherent lack of stiffness of GFRPs, Zhao et al. [5]

1359-8368/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.compositesb.2011.01.003

⇑ Corresponding author.E-mail address: a.khennane@adfa.edu.au (A. Khennane).

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opted for a CFRP shell filled with concrete as shown on Fig. 3c. Thecarbon shell has both hoop reinforcement (90� from longitudinalaxis) and ±10� longitudinal fibres, as well as helical ribs on the in-side to ensure full force transfer with the concrete. This design re-sulted in a stiff girder, but the load versus displacement curvesobtained under four-point bending were linear up to failure, anddid not exhibit any signs of imminent failure. Correia et al. [7]tested a hybrid beam which consisted of a pultruded section anda concrete layer. A preliminary test proved that the adhesion be-tween the pultruded profile and the concrete was insignificant.As a result, the shear connection in subsequent beams had to beprovided using mechanical means as shown in Fig. 3f. The beamsfailed without warning due to shear in the GFRP profile, which oc-curred in the top flange junction. In summary, the shortcomings ofthe previously described designs can be listed as follows:

� custom made GFRP boxes or carbon shell have a high initialcost;� being very thin to serve a warning for imminent failure, the

CFRP laminate contributes very little to the overall stiffness ofthe beam;

� when a cost effective pultruded profile is used, mechanicalmeans are needed to provide the shear connection with theconcrete.

3. Proposed design

The aim of the new proposed beam design, shown schemati-cally on Fig. 4, is to address the previously described shortcomings.The section is composed of a pultruded profile, an outer filamentwound GFRP laminate, a high strength concrete (HSC) block, anda CFRP laminate.

To minimize the cost of the beam, a pultruded GFRP box profile,available ‘‘off the shelf’’, is chosen. Thin walled box sections arevery efficient for beams, and they have a high resistance to lateraltorsional buckling.

To address the inherent lack of stiffness of hybrid beams, theCFRP laminate is not designed to fail first to give a warning ofimminent failure. Instead, its primary role is to provide the re-quired stiffness for the beam. As such, there is no upper limit onits thickness. Therefore, the stiffness of the beam can be tailored.The warning of imminent failure is hoped to be achieved throughthe crushing of the concrete.

Fig. 1. Cross section of the Tech21all composite bridge [1].

Fig. 2. Cross section of the first Australian composite bridge [4].

Fig. 3. Existing hybrid FRP-concrete beam designs. Fig. 4. Proposed design for a hybrid FRP-concrete beam.

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The addition of the filament wound outer laminate serves twopurposes. Its principal role is to provide some form of confinementto the system concrete block-pultruded profile as to ensure a com-posite action, thus eliminating the risk of debonding failure. Thelack of composite action between the concrete and the laminateswas reported as a serious shortcoming of these sections [3,7]. Fi-nally, with fibres oriented at ±45�, its second role is to improvethe shear strength of the pultruded profile, which has fibres pre-dominantly in the longitudinal direction. It is also worth mention-ing that the idea of wrapping is not new. It was first used by Fenget al. [8] to wrap GFRP pultruded profiles for use as bridge decking.It was found that the outside filament wound reinforcement en-hanced the load carrying ability of the profiles and shifted the fail-ure modes.

4. Testing program

4.1. Preliminary tests with high strength concrete (HSC) blocks

4.1.1. Description of the beamsAn initial series of beams with a high strength concrete (HSC)

block were tested in four-point bending. The beams were designedto carry a minimum load of 50 kN over a span 1.29 m and deflectless than L/600 as recommended in AS5100 [9] for reinforced con-crete beams. The cross section of the beams is schematically repre-sented in Fig. 5.

4.1.2. Section dimensionsIn reinforced concrete design, the span to depth ratio varies be-

tween 10 and 20. The lower limit of 10 is chosen so that a deeperbeam can be achieved and the problem of a lack of stiffness of athin walled GFRP box section can be addressed. Thus, for a spanof 1.29 m, the depth of the hybrid beam, H, is chosen as 200 mm.The height and width of the pultruded profile are imposed(152 � 102 � 6 mm). The height of the concrete block is chosenas 40 mm. The thickness of the wrapping is variable as differentlay-ups will be trailed. However, for initial dimensions, it was cho-sen to be equal to 2 mm.

4.1.3. CFRP laminate thicknessTotal maximum mid-span deflection of a simply supported

beam under four-point bending can be expressed as:

dmax ¼ dbend þ dshear ð1Þ

where dbend represents the deflection due to bending and expressedas:

dbend ¼Pað3L2 � 4a2Þ

24EIð2Þ

and dshear obtained as:

dshear ¼Pa

RGA

� �fs ð3Þ

where a represents the shear span and fs represents the shape fac-tor. The shape factor, fs, is an important variable in these types of

cross sections [6]. The detailed calculations of the shape factor aregiven in Appendix B, while the material properties are given inAppendix A.

Expressing the stiffness and the neutral axis of the beam respec-tively as:

EI ¼ Ec bcc2þ hþ t þ tg � YN

� �2þ bc3

12

!

þ Ep btw hþ t þ tg �tw

2� YN

� �2

þ bt3w

12

!

þ 2EP dtwd2þ tw þ t þ tg � YN

� �2

þ twd3

12

!

þ EP btw YN � tw

2� t � tg

� �2

þ bt3w

12

!

þ Eg b2tg H � tg

2� YN

� �2

þb2t3

g

12

!

þ 2Eg ðH � 2tgÞtgH � 2tg

2þ tg � YN

� �2

þ tgðH � 2tgÞ3

12

!

þ Eg b2tg YN � tg

2

2� �þ b2tg

12

3 !

þ EL b1t YN � t2� tg

� �2

þ b1t3

12

!ð4Þ

and

YN ¼ 1D� Ecbc

c2þ hþ t þ tg

� �þ Epbtw hþ t þ tg �

tw

2

� ��

þ 2EPtwdd2þ tw þ t þ tg

� �þ EPbtw

tw

2þ t þ tg

� �

þ ELb1tgt2þ tg

� �þ Egb2tg H � tg

2

� �

þ 2EgðH � 2tgÞtgH � 2tg

2þ tg

� �þ

Egb2t2g

2

!ð5Þ

with

D ¼ ECbC þ 2Epbtw þ 2EPtwdþ ELb1t þ 2Egb2tg þ 2EgtgðH � 2tgÞ

where Ec, EP, EL and Eg are the elastic modulus of concrete, pultrudedprofile, CFRP and GFRP laminate respectively, and solving theequation:

dmax ¼ dbend þ dshear ¼L

600ð6Þ

for t using Maple yields the thickness of the CFRP laminate, which isobtained as 1.14 mm. A CFRP laminate of 1.2 mm thickness is cho-sen as this is a standard thickness.

4.1.4. Load carrying capacity4.1.4.1. Lateral stability. Generally pultruded box sections have highresistance to lateral torsional buckling. However, to satisfy the de-sign requirements and to prevent lateral instability, height towidth ratio is often checked by the following relation [2]:

C þ hb

< k ð7Þ

According to Deskovic et al. [2], for thin-walled composite hybridsections the value of k is around 3. For this beam cross sectionthe value of k is obtained as 1.88, which is well within the acceptedvalue.Fig. 5. Hybrid beam cross section.

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4.1.4.2. Web buckling load. Another possible mode of failure of thisbeam is through buckling of the pultruded box section webs. Thistype of failure mechanism has been reported in [10], where thebeam undergoes large global flexural deformation and local celldeformations, and the failure occurs along the corners of the boxprofile. To make sure that the beam’s webs are stiff enough to takethe design load and would not buckle, the web buckling load wastheoretically estimated using the formulas given in [2,11]. It wasfound to be equal to 1303 kN, which is well above the assumed de-sign load. The detailed calculations are given in Appendix C.

4.1.4.3. Flexural capacity. The ideal scenario is for the beam to failthrough concrete crushing. Using strain compatibility, as shownon Fig. 6, the moment capacity of the beam at which the concreteshould crush is calculated and compared to the external appliedmoment caused by the 50 kN load. Based on a crushing strain ofconcrete, equal to ec = 0.0026 measured on a cylinder, the ultimatemoment capacity of the section was found to be equal to40.9 kN m, which is four times greater than the design momentof 10.75 kN m. The detailed calculations are given in Appendix D.

4.1.5. Test resultsTo investigate the effect of filament wound wrapping on the

performance, three beams were wrapped with different lay-ups.The first beam, HSC_B1, had no CFRP reinforcement and waswrapped up with a ±45�/90� lay-up. The second and third beamslabelled HSC_B2 and HSC_B3, were reinforced with a 1.2 mm thickCFRP laminate, and wrapped up with ±45� and ±45�/90� lay-upsrespectively. All the beams were tested in four-point bending, asschematically shown on Fig. 7, using a Shimadzu UDH-100 univer-sal testing machine with a capacity of 100 tonnes. All the beamswere tested under displacement control at a rate of 1 mm/min.

Fig. 8 shows the load–displacement curves for the beams. For thesake of comparison, an extra curve for a beam with a hand lay-upwrapping [12] is added. It can be clearly seen that filament woundwrapping increases not only the stiffness but also the load carryingability of the beams. The beam without any CFRP reinforcement dis-plays enough stiffness when compared to the beam having a handlay-up wrapping, which also had CFRP reinforcement.

The beam without CFRP reinforcement failed through localbearing capacity at the support as shown on Fig. 9. Even thoughthis constitutes a premature failure, the load displacement curvefor this beam still shows that dissipation of energy can be obtainedthrough tearing and splitting of the pultruded profile as previouslyreported by Bank et al. [13] when testing pultruded profile for useas safety crash barriers.

To avoid this sort of premature failure, the remaining beamsHSC_B2 and HSC_B3 were reinforced at the support as shown onFig. 10 with a block made of Delrin, a plastic material that is easily

Fig. 6. Strain compatibility.

Fig. 7. Testing arrangement for beams HSC_B1, HSC_B2, and HSC_B3.

Fig. 8. Mid-span load displacement curves for the HSC beam series.

Fig. 9. Failure of beam HSC_B1.

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machined and having similar stiffness properties as a pultrudedprofile. This inhibited the failure at the support and allowed thebeams to fail through concrete breaking in the zero shear area.However, the load displacement behaviour of the beams did notshow enough ductility. The concrete did not crush, but rather failedin a sudden fashion accompanied by a loud sound. Fig. 11 showsthe failure of the beam, and Fig. 12 the failure of the concrete block.The failure is akin to a punching shear failure. Once the concreteblock fails, it punches and tears through the pultruded profile caus-ing its walls to buckle outside. In resume, the brittleness of thehigh strength concrete did not allow for energy dissipativebehaviour.

Fig. 13. shows the recorded strains in the top and bottom fibresat mid-span for beam HSC_B2.

At peak load, the top fibre recorded a compressive strain of0.0033. Although this strain is recorded on the wrapping, it doessuggest that the strain in the concrete top fibre is within the vicin-ity of this value. This proves that the wrapping does confine theconcrete since the ultimate compressive strain of the concrete re-corded in a standard compressive test was only 0.0026. However,the confinement hence introduced is not enough as to provide duc-tility to the concrete.

4.2. Enhancing the pseudo-ductility of the beams

4.2.1. Description of the beamsIt is very clear from the preceding results that the high strength

concrete block fails in a sudden catastrophic manner. From a civilengineering perspective, this is problematic since the much soughtafter ductile failure is not achieved. In the following two solutionsare proposed to improve the post failure behaviour of the beam.The first solution consists in using normal strength concrete in-stead of high strength concrete. The second solution consists inadding short steel fibres to the high strength concrete mix. Indeed,steel fibres are commonly used to improve fracture toughness andpost peak behaviour of high strength concrete [14].

It was also shown previously that it is the failure of the concreteblock that governs the peak and post peak behaviours of the beam.It was decided therefore to trial three different concrete blockthicknesses: 60, 75 and 90 mm, and observe the effects on the fail-ure mode of the beams.

In total six (06) beams were tested: three with normal strengthconcrete (NSC) blocks, labelled NSC_B1, NSC_B2, and NSC_B3, andthe other three with steel fibres reinforced high strength concreteblocks (SFHSC), labelled SFHSC_B1, SFHSC_B2, and SFHSC_B3.Somewhat larger pultruded profiles (200 � 100 � 10 mm) wereused for these tests. These offered the flexibility to increase thespan length of the beams up to 2 m with a loading span of1.71 m. All the beams were wrapped up with a ±45�/90� lay-upand reinforced with a CFRP laminate1.4 mm thick. Fig. 14 showsa picture of the beams. They were tested in four-point bendingas shown on Fig. 15.

4.2.2. Test resultsFig. 16 shows the experimental load displacement curves. The

beams with steel fibres reinforced high strength concrete blocksdisplay somewhat linear curves as compared to the curves of thebeams with normal strength concrete blocks which could beapproximated with a second order parabola. It is also very clear

Fig. 10. Support reinforcement with a plastic block.

Fig. 11. Failure beam HSC_B3.

Fig. 12. Failure of the high strength concrete block.

Fig. 13. Strains in top and bottom fibres for bean HSC_B2.

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from the plots that, when all the dimensions are equal, the beamswith a steel fibre reinforced concrete block display better stiffnessand load carrying ability compared to those having only a normalconcrete block.

However, the most interesting feature of these tests is that ex-cept for SFHSC_B1 none of the beams failed in a catastrophic man-ner. All of them showed some imminent signs of failure; firstthrough the debonding of the wrapping then followed by thecrushing of the concrete block as shown respectively on Figs. 17and 18. A ductile plateau is clearly visible on Fig. 16 for the beamwith a 90 mm steel fibres reinforced concrete block labelledSFHSC_B3.

All the curves show a somewhat increased ductility as the thick-ness of the concrete block increases. This suggests that as the con-crete thickness increases, the akin to shear punching failureobserved previously is alleviated. This remains an experimentalobservation, and we have yet to understand the true mechanismsof failure of the concrete block in order to determine a minimumvalue for the concrete thickness to ensure failure through crushing.

Using the approximate analysis based on strain compatibilityand a rectangular stress block, as shown on Fig. 6 and detailed in

the Appendix, the theoretical failure loads are determined for allthe beams and compared to the experimental results as shownon Table 1. For the beams with a normal strength concrete block,the predictions appear to be consistent with the observed experi-mental behaviour. The capacity of the beam with the smallest con-crete block thickness, 60 mm, was over predicted by about 15%.This can be explained owing to the fact that the shear punchingfailure prevailed over concrete crushing causing the beam to failprematurely. Indeed, as can be seen on Fig. 16a, it is the beam thatdisplays less ductility. For the beams NSC_B2 and NSC_B3, theagreement with the experimental values is very satisfactory andwithin the safer side. The same thing cannot be said about the pre-dictions for the beams with steel fibre high strength concreteblocks; they are inconsistent with the experimental observations.

Fig. 14. NSC and SFHSC series of beams with different concrete blocks’ thicknesses.

Fig. 15. Testing arrangement for NSC and SFHSC series of beams.

Fig. 16. Mid-span load displacement curves for the NSC and SFHSC beam series.

Fig. 17. Debonding of the wrapping prior to failure.

Fig. 18. Concrete crushing in beam SFHSC_B3.

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For instance consider the beam SFHSC_B1 having a concrete blockthickness of 60 mm. According to the experimental results asshown on Fig. 16b, it is the beam that displays less pseudo-ductility and minimal concrete crushing, yet the models underesti-mate the failure load. On the other hand, the models overestinatethe failure loads of the beams SFHSC_B2 and SFHSC_B3 whichdisplay enough pseudo-ductility and concrete crushing. Eventhough the number of tests is not statistically significant, onemay presume that the models, based on strain compatibility anda rectangular concrete stress block, initially developed on the basisof normal concrete, are not suitable for steel fibres reinforced highstrength concrete.

5. Conclusions

The results obtained from the experimental data and analysesshow that it is possible to design and manufacture a hybridcomposite-concrete beam with adequate stiffness and still achievethe most sought after ‘‘pseudo-ductile’’ behaviour necessary forcivil engineering structures.

The stiffness of the beam can be tailored through the use of anovel design approach that does not mimic that of reinforced con-crete. In this approach the CFRP laminate is not used as to serve asa warning of imminent failure but rather to enhance the stiffness ofthe beam by compensating for the lack of stiffness of the pultrudedprofile.

The lack of composite action between the concrete block andthe GFRP box that hindered the use of cheap pultruded profilesin the past has been solved by externally wrapping the beam withfilament winding. The wrapping did not only eliminate the risk ofpremature failure as a result of the concrete block debonding fromthe pultruded profile, but it was also found to enhance the stiffnessand load carrying ability of the beams.

The use of high strength concrete has many advantages such asincreased stiffness and load carrying ability. On its own, however,it was found to be brittle and results in catastrophic failure of thebeam. To improve therefore the pseudo-ductility of the beam, nor-mal strength concrete and steel fibres reinforced high strengthconcrete were trailed. Both materials were found to result in im-proved ductility provided the concrete block is thick enough toavert a premature failure akin to shear punching failure.

The section analyses using concrete block parameters from dif-ferent sources were all shown to predict the flexural capacity of the

beams with a normal strength concrete block that failed throughcrushing. Most importantly, the results were consistent withexperimental observations. However, this was not the case forthe beams with a steel fibres reinforced high strength concreteblock. Most of the models over estimated the failure load.

Finally, the results from these tests have highlighted the need tocarry out advanced analysis techniques to understand the truemechanisms of failure of the concrete block in order to determinea minimum value for the concrete thickness to ensure failurehappens through crushing. Current work is also focussed ondetermining analytically the shear punching capacity of the con-crete block. The results from the tests and the analytical modelswould serve to design the beam to achieve its full capacity withoutfailing prematurely.

Appendix A. Material properties

See Tables 2–8.

Appendix B. Determination of the shape factor

The hybrid beam is broken down into various eight components– (1) concrete, (2) GFRP box top flange, (3) GFRP box webs, (4)GFRP box bottom flange, (5) GFRP wrap (top flange), (6) GFRP wrap(webs), (7) GFRP wrap (bottom flange) and (8) CFRP laminate. Thefirst moment of area, Q and area, A are calculated for each of theeight components. The shear factor is obtained as:

fs ¼A

I2

ZQ 2

c

b2 dAþZ Q 2

p1

b2 dAþZ Q 2

p2

t2w

dAþZ Q 2

p3

b2 dAþZ

Q 2L

b2 dA

"

þZ Q2

g1

b22

dAþZ Q2

g2

t2g

dAþZ Q 2

g3

b22

dA

#ðB-1Þ

with the first moments of areas of the different parts obtained as:

Table 1Predicted and experimental failure loads.

Beam notations Concrete thickness (mm) Predicted failure load (kN) Experimental failure load (kN)

NZ std. [15] Ozbakkaloglu et al.[16] Mertol et al. [17]

NSC-B1 60 246.57 244.78 246.57 214.40NSC-B2 75 276.74 274.19 276.74 279.80NSC-B3 90 306.35 302.96 306.35 332.10SFHSC-B1 60 338.88 334.87 343.08 361.60SFHSC-B2 75 381.76 376.12 381.71 371.00SFHSC-B3 90 421.77 414.38 421.77 416.00

Table 2Concrete mix designs.

Mix type Mix contents Mix ratios

Normal strength concrete(NSC)

Portland cement, 12 mm coarse aggregate, fine aggregate and water 2:3.94:4.53:1

High strength concrete(HSC)

Portland cement, silica fumes, water, coarse aggregate, sand, and super plasticisers 1:0.1:0.33:1.3:0.66:0.02

High strength concretewith steel fibres (SFHSC)

Portland cement, silica fume, water, super/hyper plasticizer, viscosity modifier, 12 mmcoarse aggregate, 5–1.18 mm fine aggregate, steel fibres and fly ash

1:0.15:0.35:0.014:0.004:1.24:0.84:0.10:0.38

Table 3Mechanical properties of concrete.

Concrete type Elastic modulus(MPa)

Compressivestrength (MPa)

Ultimatecompressive strain

NSC 23,140 38.83 0.0032HSC 36,640 84.80 0.0026SFHSC 33,330 88.20 0.0034

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Qc ¼ bC y1� C2� tg

� �for the concrete block;

QP1 ¼ btw y1� C � tw

2� tg

� �for the pultruded box top flange;

QP2 ¼ dtwðYN � d2� tw � t � tgÞ � 2 for the pultruded box webs;

QP3 ¼ btw YN � tw

2� t � tg

� �for the pultruded box bottom flange;

QL ¼ btw YN � t2

� �for the CFRP laminate;

Qg1 ¼ b2tg y1� tg

2

� �for the GFRP wrapping ðtopÞ;

Q g2 ¼ 2� ðH � 2tgÞ

tg YN � tg �H � tg

2

� �for the GFRP wrapping ðwebsÞ;

Q g3 ¼ b2tg YN � tg

2

� �for the wrapping ðbottomÞ;

and; y1 ¼ H � YN

Appendix C. Determination of the web buckling load

To ensure that the webs of the hybrid beam are stiff enough totake the load and not buckle, the theoretical web shear buckling

stress is calculated by Eqs. (C-6) and (C-7) which are given byHolmes and Just [11].

The average shear buckling load is determined as follows:

s�bw ¼4Kwb

ffiffiffiffiffiffiffiffiffiffiffiDLD2

T4q

twd2 ; for h > 1 ðC-1Þ

s�bw ¼4Kwb

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiDT Hwbp

twd2 ; for h > 1 ðC-2Þ

where h ¼ffiffiffiffiffiffiffiffiffiffiffiDLDTp

HwbðC-3Þ

Hwb ¼12ðmLDT þ mT DLÞ þ

Gwt2w

6ð1� mLmTÞðC- 4Þ

DL ¼EwLt2

w

12ð1� mLmTÞand DT ¼

EwT t2w

12ð1� mLmTÞðC-5Þ

and the value of Kwb depends on h, which is provided by Holmes andJust [11] in Table 9.

Shear buckling occurs when the average shear stress equals theshear buckling stress. The average shear stress is given as:

Vu ¼ 2twds�bw ðC-6Þ

The theoretical web buckling load is obtained after calculating theaverage shear stress. The web buckling load is given as:

Pwb ¼VuL

aðC-7Þ

Appendix D. Section analysis

Using strain compatibility, as shown on Fig. 6, the moment car-rying capacity of the beam is calculated from the ultimate crushingstrain of the concrete block. The stresses in the concrete block arecalculated based on a rectangular stress block approach. Thedimensions of the rectangular stress blocks are given as follows:

Ozbakkaloglu and Saatcioglu [16]:for f 0C 6 30 MPa

a ¼ 0:85c ¼ 0:85

for f 0C P 30 MPa

a ¼ 0:85� 0:0014 f 0c � 30� �

P 0:72c ¼ 0:85� 0:0020 f 0C � 30

� �P 0:67

New Zealand standard for high strength concrete (NZS-3101 1995).For concrete compressive strength f 0C > 50 MPa

� �,

a ¼ 0:85� 0:004 f 0c� �

limit a P 0:75

c ¼ 0:85� 0:008 f 0c� �

limit 0:65 6 c 6 0:85

For normal strength concrete f 0C > 50MPa� �

, it is the same as theAustralian Standard AS 3600

a¼ 0:85c¼ 0:85�0:007 f 0C �28

� �¼ 0:78 ðfor 38 MPaÞ limit 0:656 c6 0:85

Mertol et al. [17]:

Table 4Pultruded profiles used in beams HSC_B1, HSC_B2.

Pultruded profilecross section

Longitudinalmodulus, EL

(MPa)

Transversemodulus, ET

(MPa)

Shearmodulus GLT

(MPa)

152 � 102 � 6.35 22,400 11,600 2890

Table 5Pultruded profiles used in beams NSC_B1, NSC_B2, NSC_B3, and SFHSC_B1, SFHSC_B2,SFHSC_B3.

Pultruded profilecross section

Longitudinalmodulus, EL

(MPa)

Transversemodulus, ET

(MPa)

Shearmodulus GLT

(MPa)

200 � 100 � 10(new profile)

28,870 3510 2980

Table 6Mechanical properties of CFRP laminate used in beams HSC_B1, HSC_B2.

Laminate crosssection

Longitudinal modulus, EL

(MPa)Ultimate tensile strength(MPa)

100 � 1.2 140,000 2400

Table 7Mechanical properties of CFRP laminate used in beams NSC_B1, NSC_B2, NSC_B3, andSFHSC_B1, SFHSC_B2, SFHSC_B3.

Laminate crosssection

Longitudinal modulus, EL.(MPa)

Ultimate tensile strength(MPa)

90 � 1.4 165,000 3100

Table 8Mechanical properties of the GFRP laminate wrapping.

Laminatetype

Longitudinal modulus,EL (MPa)

Transverse modulus,ET (MPa)

Shear modulus(MPa)

±45� 17,500 17,500 6940±45�/90� 8540 11,490 4630

Table 9Values of h versus Kwb.

h 0 0.2 0.5 1.0 2.0 3.0 5.0 10.0 20.0 40.0

Kwb 18.6 18.9 19.9 22.2 18.8 17.6 16.6 15.9 15.5 15.3

914 A. Chakrabortty et al. / Composites: Part B 42 (2011) 907–915

Author's personal copy

a ¼0:85 for f 0c 6 69 MPa0:85� 0:0029 f 0c � 69

� �P 0:75; for f 0c > 69 MPa

(

c ¼0:85 for f 0c 6 69 MPa0:85� 0:007252 f 0c � 28

� �P 0:65; for f 0c > 69 MPa

(

Considering the equilibrium of the resultant forces on Fig. 6, thecorresponding moment is obtained as:

Mint ¼ �Fp1hþ t þ tg � YN

3þ C � y

� �� Fg1

tg

2þ y

� �

þ Fp223ðYN � t � tgÞ þ ðhþ t þ tg � YNÞ þ ðC � yÞ

� �

þ FLt2þ C þ h� y

� �þ Fg2

tg

2þ C þ t þ hþ y

� �

where y ¼ cC2 .

The forces in the different part of the section as obtained asfollows:

for the concrete block:

Fc ¼ af 0ccCb

for the pultruded profile above the neutral axis,

FP1 ¼EPecbtwðhþ t þ tg � YN � twÞ

Y1þ EPecbt2

w

2Y1

þ EPectwðhþ t þ tg � YN � twÞ2

Y1

for the pultruded profile below the neutral axis,

FP2 ¼EPectwðYN þ tw � t � tgÞ2

Y1þ EPecbt2

w

2Y1

þ EPecbtwðYN � t � tg � twÞY1

for the CFRP laminate,

FL ¼ELecb1tðYN � t � tgÞ

Y1þ ELecb1t2

2Y1

for the wrapping above the neutral axis,

Fg1 ¼Egecb2t2

g

2Y1þ Egecb2tg

and for wrapping below the neutral axis,

Fg2 ¼Egecb2t2

g

2Y1þ Egecb2tgðYN � tgÞ

Y1

References

[1] Farhey N. Long-term performance monitoring of the Tech 21 all-compositebridge. J Compos Construct ASCE 2005;9(3):255–62.

[2] Deskovic N, Triantafillou TC, Meier U. Innovative design of FRP combined withconcrete: short term behavior. J Struct Eng ASCE 1995;121(7):1069–78.

[3] Canning L, Hollaway L, Thorne AM. Manufacture, testing and numericalanalysis of an innovative polymer composite/concrete structural unit. In:Proceedings – institution of civil engineers, structures and buildings , vol. 134;1999. p. 231–41.

[4] van Erp GM, Heldt TJ, Cattell CL, Marsh R. A new approach to fibre compositebridge structures’. In: 17th Australasian conference on the mechanics ofstructures and materials, ACMSM17, Australia; 2002. p. 37–45.

[5] Zhao L, Burgueno R, La Rovere H, Seible F., Karbhari VM. Preliminaryevaluation of the hybrid tube bridge system. Final test report submitted toCalifornia DoT 2000. <http://www.dot.ca.gov/hq/esc/earthquake_engineering/Research/modgirde.pdf>.

[6] Hulatt J, Hollaway L, Thorne A. Short term testing of hybrid T beam made ofnew prepeg material. J Compos Construct ASCE 2003;7(2):135–44.

[7] Correia JR, Branco FA, Ferreira JG. Flexural behaviour of GFRP-concrete hybridbeams with interconnection slip. Compos Struct 2007;77(1):66–78.

[8] Feng P, Ye L, Li T, Ma Q. Outside filament-wound reinforcement: A novelconfiguration for FRP bridge decks. In: The 9th international symposium onstruct engrg for young experts/Fuzhou & Xiamen, China; 2006.

[9] AS 5100. Bridge design code. Australian Standards; 2004.[10] Charoenphan S, Bank LC, Plesha ME. Progressive tearing failure in pultruded

composite material tubes. Compos Struct 2004;63(1):45–52.[11] Holmes M, Just DJ. GRP in structural engineering. UK: Applied Science

Publishers; 1983.[12] Khennane A. Manufacture and testing of a hybrid beam using a pultruded

profile and high strength concrete. Aust J Struct Eng 2009;10(2):145–55.[13] Bank LC, Gentry TR. Development of a pultruded composite material as a

highway guardrail. Compos Part A 2001;32(9):1329–38.[14] Kayali O. Effect of high volume fly ash on mechanical properties of fiber

reinforced concrete. Mater Struct 2004;37:318–27.[15] NZS-3101. The design of concrete structures. Standards New Zealand,

Wellington, New Zealand; 1995.[16] Ozbakkaloglu T, Saatcioglu M. Rectangular stress block for high-strength

concrete. ACI Struct J 2004;101(4):475–83.[17] Mertol HC, Rizikalla S, Zia P, Mirmiran A. Characteristics of compressive stress

distribution in high-strength concrete. ACI Struct J 2008;105(5):626–33.

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