Construction and Building Materials 41 (2013) 563–569

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Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

Investigation of flexural behaviors of hybrid beams formed with GFRPbox section and concrete

Ferhat Aydın ⇑, Mehmet SarıbıyıkSakarya University, Technology Faculty, Civil Engineering Department, 54187 Sakarya, Turkey

h i g h l i g h t s

" We developed five different hybrid GFRP-Concrete beams groups." We applied physical and mechanical experiments on the beams." Flexural strength and fracture toughness of developed hybrid beams increased about 3 and 10 times." It becomes possible to produce elements with smaller sections with the same strength.

a r t i c l e i n f o

Article history:Received 25 August 2012Received in revised form 24 December 2012Accepted 28 December 2012Available online 30 January 2013

Keywords:GFRP box profileConcreteHybrid beamFlexural strength

0950-0618/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.12.060

⇑ Corresponding author.E-mail addresses: ferhata@sakarya.edu.tr, ferhata5

a b s t r a c t

This study investigated the use of concrete, which is known to have high compressive strength, as ahybrid beam with GFRP box sections which have a high tensile stress. Hybrid construction elements wereformed by placing concrete in plastic form inside GFRP box profiles and experimental tests were per-formed on the materials. Firstly, the properties of the GFRP profiles were determined and the flexuralbehaviors of the hybrid beams produced with different profiles were analyzed. The developments ofthe hybrid material to its components, which are concrete and GFRP profiles, were examined. To increasethe adherence of concrete to GFRP profile, sand particles were pasted in the interior surface of the profileusing epoxy and the hybrid beams were improved by increasing the profile felt. Study results showedthat in addition to many advantages due to its formation, the hybrid design showed superior physicaland mechanical properties. It was found that the flexural strength and fracture toughness values ofhybrid beams significantly increased when compared to reference values.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Like in all technical fields, the needs and demands of humans inthe field of material technologies increase each day in parallel tothe problems experienced in materials. Researchers investigatenew material types and applications and try to produce new de-signs to decrease these problems and to satisfy these demands.In recent years, many researchers have concentrated on compositematerials and hybrid designs, which can be considered as a deriv-ative of these materials. Composite materials have required prop-erties and are preferred in a wide variety of fields including theconstruction sector. Fiber Reinforced Plastic (FRP) composites areone of these composite types (Fig. 1). In addition to their high resis-tance and good performance towards environmental factors, thesematerials are preferred since they have all the properties desiredby the researchers and they can be produced in different combina-

ll rights reserved.

8@hotmail.com (F. Aydın).

tions [1]. In addition to their superior mechanical resistance, thesenew generation composite materials draw the attention ofresearchers due to the properties such as their lightweight struc-ture, corrosion resistance and high resistance to chemicals, electricinsulation, low density and high resistance/density ratio [2–4].

FRP composites are generally used in curtain wall systems, pe-destrian and vehicle bridges, soil improvements, pipes, repair andreinforcement works in the construction industry [1]. Theconstruction sector constitutes a significant part of the FRP com-posite market, followed by the automotive sector. However, sincethese materials are not yet well recognized by users and designers,they are not considered as a replacement for other materials. It isestimated that FRP composites can be a good solution in a signifi-cant part of available applications [5]. Recently, the use of compos-ite materials has rapidly increased and it is gradually developing inmany technical fields including the construction sector. In thisdevelopment process, the construction industry is constantlyworking to develop new construction technologies design andobtain more economical solutions [6]. These new generation com-posites, which are generally preferred in secondary constructions

Fig. 1. FRP composite profiles.

564 F. Aydın, M. Sarıbıyık / Construction and Building Materials 41 (2013) 563–569

which are not considered as bearing elements in the constructionsector, are today also used as bearing elements, as main construc-tion elements. Particularly after the increase of the serial produc-tion of FRP composites, they began to be used more effectively inbuildings for different purposes. The use of fiber reinforced com-posites, which are lightweight and have a high resistance, in rein-forcement, repair and improvement works has increased [7].Reinforcement and improvement works involving the wrappingof FRP laminates on the bottom surfaces of beams and FRP fabricson all surfaces of columns are the most widely known applicationsof these types of composites with concrete [8–12].

Studies reveal that research which uses carbon or glass fiber FRPfabric or laminates with different fiber contents will further devel-op in the future and concentration on hybrid systems, which com-bine profile FRP elements with bearing properties andconventional construction materials such as concrete or steel, willincrease [13]. Like in various study units, the most recent researchand development studies have concentrated on hybrid systemswhere conventional construction materials particularly such asconcrete and composite materials are used in combination [6]. Re-cently much of the research has focused on hybrid FRP columnsformed by concrete-filled or hollow FRP pipes [14–17]. The ten-dency in scientific studies clearly shows that in the near future,

Hybrid Use of Concrete with GFRP box profiles

Formwork Advantages

Cross-Section

Advantages

Preventing of local

fractures in the profile

Insulation

and Impermeability

Advantages

Concrete Cure

Advantage

Increases in strength

Increases in stiffness

Fig. 2. Advantages of hybrid use of GFRP box profiles with concrete.

the use of FRP composites in new buildings will mainly concentrateon the use of hybrid structure [18]. Many studies have shown thatthe use of FRP composites with conventional materials like con-crete was one of the solutions to eliminate certain deficienciesand disadvantages in construction elements fully made of FRP [19].

The first studies on hybrid designs, where FRP profiles and con-crete were used in combination, began in 1981 [20]. In the firststudies, positive results were obtained by using concrete to in-crease rigidity and compressive strength inside FRP profile. Theidea of using FRP-Concrete hybrid system as a flexural elementwas first introduced in 1990 [21]; it was reported that the FRP pro-file used in the formed system offered advantages in formwork,lightweight structure and resistance and could yield more than50% lightweight structure when compared to conventional platesystems. In addition, some researchers [22] investigated hybridfloor covering systems. They formed permanent formwork byusing concrete in a T-section GFRP profile and increased materialresistance. In previous studies on hybrid systems which wereformed by filling concrete inside the FRP profile; the behavior ofa hybrid system under uniaxial load [23,24] flexural behavior in1997 were again analyzed by the same researchers in 1999 [24–26]. Various studies were conducted on the long term creep andshrinkage effects of hybrid beams in 2002 [27], on behaviors underrepetitive loads in 2005 [28,29], on the effects on impact loads in2007 [30], on shear behavior and material fatigue in 2008 [31–34] and on frost-thaw effects in 2008 [30,35].

2. Aim of the study

This study was carried out in parallel to the increasing attentionon fiber reinforced composites. The study analyzed hybrid use ofhighly preferred Glass Fiber Reinforced Plastic (GFRP) compositebox sections among FRP composites and concrete, which is theprinciple construction material and has long been used in con-struction. GFRP profiles, which have many positive properties,were combined with concrete which is the most widely preferredconstruction material. New hybrid beams, which are produced bybenefitting from positive aspects of both components, were ana-lyzed; improvements were conducted and solutions were offeredfor identified problems.

In this study, the aim was for two materials to contribute toeach other by making use of the superior properties of concreteand GFRP profiles. The aim was to obtain the advantages presentedin Fig. 2.

Formwork advantages: forming small parts to prepare the nec-essary formwork to give the concrete the desired shape requires along time and extra cost. Since GFRP box sections serve as a form-work, there is no need for a secondary element to shape the con-crete, which fills inside the box section (Fig. 3). This isconsiderably time saving and cost efficient. This property, whichis termed as permanent formwork in literature, is considered tobe one of the principle advantages of hybrid materials [36–42].

Cross-section advantages: with the combination of these twomaterials, GFRP profiles with high tensile stress are expected tomeet tensile stresses, and concrete, whose most important mechan-ical property is compressive strength, is expected to meet compres-sive stresses. Thus, thanks to two-component hybrid material, it willbe possible to produce elements which have the same strength asbox profile and plain concrete with smaller cross-sections.

Preventing local damage in GFRP profile: these materials, whichhave many superior properties, are exposed to regional and localfractures in profile form particularly under flexural effect [43,44].In GFRP profiles formed by filling concrete, since the concrete willbe in a hardened form, local fractures will be reduced oreliminated.

Fig. 3. Placing concrete into the GFRP box section.

Table 1Ratios fiber of GFRP profiles.

Total Longitudinal fiber Felt fiber Matrix

The ratios fiber of standard GFRP profileWeight (%) 100 39.10 19.18 41.71Volume (%) 100 26.76 13.12 60.12

The ratios fiber additional felt GFRP profilesWeight (%) 100 31.91 19.96 48.12Volume (%) 100 21.83 13.65 64.52

Table 2GFRP tensile test results.

Sample Tensile strength (N/mm2)

Modulus of elasticity (E) (N/mm2)

R2

1 620.45 28981.3 0.992 505.78 29751.5 0.993 521.77 29645.9 0.994 576.71 30676.3 0.995 547.83 27668.1 0.996 567.22 30827.3 0.997 574.25 31209.2 0.998 542.29 26846.2 0.999 609.42 31092.9 0.9910 543.38 26636.4 0.99Average 560.91 29333.5 0.99

y = 28981x + 27.25

R2 = 0.99

0

100

200

300

400

500

600

700

0 0.005 0.01 0.015 0.02 0.025

Stre

ss (

N/m

m2 )

Strain (εε)

Fig. 4. GFRP tensile chart.

Fig. 5. Flexural beams.

F. Aydın, M. Sarıbıyık / Construction and Building Materials 41 (2013) 563–569 565

Curing advantage in concrete: GFRP prevents the concrete inplastic thickness which is placed in box section from losing itswater and moisture and makes this vital procedure highly advan-tageous [45]. In standards [46,47], concrete is required to be curedfor 28 days in 100% relative humidity. However, in this procedurethe concrete will not lose its water and the hydration process willbe achieved without any problems.

Strength and rigidity increases: hybrid material is formed byplacing concrete inside a GFRP box section. With the collectivebehavior of these two components, local fractures in the profileare expected to decrease or be eliminated and similarly, increasesof varying ratios are expected in material resistance since the GFRPprofile meets tensile stresses.

Impermeability and insulation advantages: the epoxy materialsthat are one of the components of GFRPSs and which wrap the fi-bers are impermeable due to their plastic based structure. Thus,the GFRP profile which wraps the concrete protects the concreteby preventing permeation of exterior water or moisture and pre-vents the minerals inside the water from damaging the concrete.

3. Experimental studies

Although many properties of concrete which has a long history are well-knownby researchers, the fact is that GFRPs are new generation materials and have limitedproduction in Turkey. Therefore, firstly some physical and mechanical properties ofprofiles were determined. Thus, experimental studies are presented in two sectionswhich are determining properties of GFRP profiles and beam tests.

3.1. Determining properties of GFRP profiles

The unit weight, specific weight, fiber ratios and tensile properties of the GFRPbox sections used in beam tests were determined using related test methods.

Firstly, 10 samples cut from box sections were used to determine the unitweight and specific weight values. The unit weight values and specific weight val-ues of GFRP materials were found to be 1.738 g/cm3 and 1.822 respectively.

To determine the fiber ratios, the resin burn-off method was applied to the sam-ples obtained from the profiles to determine the fiber and matrix ratios affecting themechanical behaviors of GFRP box sections [48]. Samples were prepared by cutting2 cm wide parts from the GFRP box sections. The samples were kept in an oven for2 h at a temperature of 600 �C and the felt and longitudinal fiber weights weredetermined. The lateral fiber ratios of the profiles were increased by wrapping extra

felt fiber around the exterior surface of the profile. The fiber ratio test results ofstandard GFRP profiles and the ones to which extra fiber was added are presentedin Table 1.

Evaluation was made by calculating the current felt amount of the profiles withadditional felt. It was found that the felt amount increased by 50% in profiles withadditional felt according to the felt weight in the unit section. The effects of lateralfiber amount used in the profiles in varying ratios on the flexural resistance of thehybrid samples were determined and analyzed in detail.

Tensile tests were performed to determine the tensile properties and modulusof elasticity of GFRP materials. Samples were prepared and tested in line with re-lated standards for GFRP [49–51] and test conditions. The necessary data were re-corded, and the modulus of elasticity, tensile and unit deformations of GFRPmaterial were calculated in Table 2.

GFRP material was found to have a modulus of elasticity (E) of 29,333 N/mm2

and a tensile strength of 561 N/mm2. Stress-unit deformation graphs were drawnfor each sample. The sample obtained from the tests is presented in Fig. 4.

Fig. 6. Sandy samples.

Fig. 7. Extra felt sample.

Fig. 8. Four-point flexural test system.

Table 3Flexural test results.

Maxload (N)

Flexural strength(N/mm2)

Fracture toughness(N mm)

GFRP box profiles 11,991 16.19 80,981Hybrid 16,537 22.32 196,244Sandy hybrid 18,055 24.37 389,801Additional felt hybrid 18,534 25.02 439,906Additional felt and

sandy hybrid35,482 47.90 815,142

0

5000

10000

15000

20000

0 5 10 15 20L

oad

(N)

Deflection (mm)

GFRP box profile

Hybrid

Fig. 9. Load–deflection curves for box profile and hybrid.

0

5000

10000

15000

20000

0 10 20 30

Loa

d (N

)

Deflection (mm)

Hybrid

Sandy Hybrid

Fig. 10. Load–deflection curves for hybrid and sandy hybrid.

566 F. Aydın, M. Sarıbıyık / Construction and Building Materials 41 (2013) 563–569

3.2. Beam tests

In beam tests, the results of the flexural test performed on GFRP box profileswith cross-section dimensions of 4–100–100 mm, length of 1500 mm and effectivespan of 1350 mm and different hybrid beams were presented (Fig. 5). A total of fivedifferent beam types were produced. Of these beams, three were hybrid beams. Thefelt amount was increased in some samples and the internal surfaces were sandblasted in some samples. Hybrid beams were obtained by filling concrete inside

the profile starting from GRFP box profiles. Hybrid beams with increased adherenceby plastering 2 mm diameter sand particles on the interior surfaces of profiles(Fig. 6) and hybrid beams with increased lateral felt amount by wrapping extra felt(Fig. 7) were produced. As the final sample type, hybrid beams were produced fromsand-blasted profiles in the interior and wrapped with extra felt were produced.Four-point flexural tests were performed on these hybrid beams (Fig. 8).

Load–deflection graphs were drawn for the five beams in each test group; theflexural load, flexural strength and fracture toughness values were calculated andcompared. The mean fracture loads, flexural strength and fracture toughness ob-tained from flexural tests are presented in Table 3.

Analysis of the beam tests showed that the GFRP box profiles had a mean flex-ural load of 11,991 N, flexural strength of 16.19 N/mm2 and fracture toughness of80,981 N mm. After the improvements, hybrid samples with additional felt andsand had a mean flexural load of 35,482 N, flexural strength of 47.90 N/mm2 andfracture toughness of 815,142 N mm. Load–deflection graphs drawn for hybridbeams which were formed by filling the GFRP box profiles with concrete and sam-ple graphs representing the GFRP box profiles and hybrid beams to analyze the ef-fect of hybrid formation on flexural behavior are compared in Fig. 9. It is understoodfrom the graphs that there were significant increases in the flexural strength,deflection amount and rigidity of hybrid beams.

Hybrid sandy samples were prepared to prevent formation of interface betweenGFRP box sections and concrete and to increase adherence by sand-blasting theinterior surfaces of the profiles. Load–deflection graphs of the hybrid and hybrid

0

5000

10000

15000

20000

0 10 20 30 40 50

Loa

d (N

)

Deflection (mm)

Additional Felt Hybrid

Sandy Hybrid

Fig. 11. Load–deflection curves for sandy hybrid and additional felt hybrid.

0

10000

20000

30000

40000

0 10 20 30 40 50

Loa

d (N

)

Deflection (mm)

Additional Felt+Sandy HybridAdditional Felt Hybrid

Fig. 12. Load–deflection curves for additional felt hybrid and additional felt + sandyhybrid.

Fig. 13. Deformation of GFRP profiles.

0

5000

10000

15000

20000

25000

30000

35000

40000

0 10 20 30 40 50

Loa

d (N

)

Deflection (mm)

Hybrid (Sandy+Additional Felt)

Additional Felt Hybrid

Sandy Hybrid

Hybrid

Box Profile

Fig. 14. Comparison of all flexural charts.

1

23 4

5

0

10

20

30

40

50

60

Fle

xura

l Str

engt

h(N

/mm

2 )

1- GFRP box profiles

2- Hybrid

4- Sandy Hybrid

3- Additional Felt Hybrid

5- Hybrid (Additional Felt + Sandy)

Fig. 15. Comparison of flexural strength.

1

2

34

5

0

100000

200000

300000

400000

500000

600000

700000

800000

900000

Fra

ctur

e T

ough

ness

(Nm

m)

1- GFRP box profiles

2- Hybrid

4- Sandy Hybrid

3- Additional Felt Hybrid

5- Hybrid (Additional Felt + Sandy)

Fig. 16. Comparison of fracture toughness.

F. Aydın, M. Sarıbıyık / Construction and Building Materials 41 (2013) 563–569 567

beams with sand were compared to analyze the effect of adherence of concrete andprofile on the flexural behavior of the material (Fig. 10). It was found that the flex-ural strength, deflection amount and thus, the fracture toughness of hybrid beamswith sand increased.

A flexural graph comparing hybrid samples with additional felt produced fromthe profiles with 50% increase of the felt amount in the unit section and hybrid sam-ples with sand is presented in Fig. 11. Analysis of the graph shows that rigidity in-creased when compared to hybrid samples with additional felt. On the other hand,in the hybrid samples with additional felt, additional felt fibers increased the deflec-tion amount and thus, fracture toughness without losing load value.

It was found that in hybrid felts, an increase in felt amount or sand-blasting ofinterior surfaces increased flexural performance and thus, both samples both withsand and additional felt were prepared. Load–deflection graphs of hybrid sampleswith additional felt and hybrid samples with additional felt + sand beams werecompared (Fig. 12).

Analysis of the flexural test graphs of hybrid beams with additional felt andsand shows that all samples showed a linear behavior up to a 35,000 N load valueand sudden fractures were observed. Analysis of the comparison graph shows thatthe flexural strength of hybrid beams with additional felt + sand increased by

approximately 90% when compared to hybrid samples with additional felt, andrigidity increased by sand-blasting the interior surfaces of the profile and increasingthe adherence with the concrete. As a result, it was found that improvement ofshear behavior of GFRP profiles by increasing the felt amount and simultaneouslyincreasing adherence with concrete significantly contributed to material behavior.In addition, the fracture situation of hybrid beam is presented in Fig. 13.

After the tests were performed on the beams, the graphs representing eachsample group were shown and compared on a single graph (Fig. 14). It is under-stood from the graph that after each improvement work performed on the material,flexural strength increased particularly in the box profiles, and hybrid beams withadditional felt + sand showed a great performance.

Comparisons of the flexural strengths of the specified sample species are pre-sented in Fig. 15. Thus, it was found that the flexural strength of hybrid sampleswith additional felt + sand increased approximately 2.8 times when compared tothe hollow profiles of the same size and approximately 2 times when comparedto standard hybrid samples.

568 F. Aydın, M. Sarıbıyık / Construction and Building Materials 41 (2013) 563–569

The fracture toughness values of the different sample types were calculated andcompared in Fig. 16. Similarly, it was found that the fracture toughness values ofhybrid samples with additional felt + sand increased approximately 10 times whencompared to the GFRP profiles and approximately 4 times when compared to thestandard hybrid samples.

4. Conclusions

The results of improvement works on hybrid design and hybridbeams are summarized below:

� In addition to their numerous superior properties, GFRP profilesare more advantageous than other construction materials dueto their lightweight structure, corrosion resistance and hightensile stress. Thus, the use of these materials as an alternativematerial in the construction industry can solve many construc-tion material problems.� A hybrid construction system which involves the combination

of traditional construction materials and new generation com-posites improves the inadequate aspects of the existing con-struction materials and new generation materials becomemore feasible. It was found that hybrid construction materialsproduced with the combination of concrete-GFRP box sectionhad superior properties than other components.� Since GFRP box sections serve as formwork in a hybrid system,

there is no need for a second formwork element to shape theconcrete in plastic form. Thus, the system significantly savestime and formwork costs.� As both materials function collectively in a hybrid construction

element, it shows a higher strength than both hollow GFRP pro-file and plain concrete. Thus, it becomes possible to produceelements with smaller sections with the same strength. Localfractures in GFRP profiles under flexural load decrease in ahybrid system which is formed with concrete filling and thus,flexural strength increases.� In a hybrid system, the GFRP profile will protect the concrete

from the minerals in water by preventing exterior water andmoisture. Since GFRPs are good insulators, the problems causedby energy consumption due to heat transfers and problemsrelated to thermal stresses will decrease. In addition, it will pre-vent the concrete in plastic form placed inside GFRP box sectionfrom losing its water and moisture and will significantly con-tribute to the curing procedure.� It was found that in hybrid samples with additional felt, flexural

strength increased approximately 3 times when compared tohollow GFRP profiles of the same size and approximately2 times when compared to hybrid samples. It was found thatthe fracture toughness of hybrid samples with additional feltincreased approximately 10 times when compared to hybridsamples and approximately 4 times when compared to stan-dard hybrid samples.� It is believed that performing the procedures of developing pro-

file properties by wrapping felt fiber to the exterior surface ofGFRP box sections in a laboratory with special set ups will fur-ther improve and increase ratios in material strength. On theother hand, the fact that as a result of all the tests performedon the available profiles, GFRP box sections were not deformedfrom longitudinal fibers, but, that the latitudinal fibers rupturedindicates that decreasing longitudinal fiber ratios and increas-ing latitudinal fiber ratios will provide positive results in termsof economy and strength.� Hybrid systems which are formed by a combination of GFRP box

sections and concrete have the potential to address durabilityand corrosion problems as a separate column or beam appear-ing in costal buildings or the ones which are exposed to sea

water. In addition, GFRP-Concrete hybrid construction elementscan be used in buildings such as chemical production facilities,bridge beams and docks.

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