Enhancement in Elastic Modulus of GFRP Bars by Material Hybridization

doi:10.4236/eng.2013.511105

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Engineering, 2013, 5, 865-869

Published Online November 2013 (http://www.scirp.org/journal/eng)

http://dx.doi.org/10.4236/eng.2013.511105

Open Access ENG

Enhancement in Elastic Modulus of GFRP Bars by

Material Hybridization

Dong-Woo Seo*, Ki-Tae Park, Young-Jun You, Hyeong-Yeol Kim

Structural Engineering Research Division, Korea Institute of Construction Technology, Goyang, Republic of Korea

Email: *[email protected]

Received August 29, 2013; revised September 29, 2013; accepted October 10, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Fiber reinforced polymer (FRP) reinforcing bars for concrete structure has been extensively investigated for last two

decades and a number of FRP bars are commercially available. However, one of shortcomings of the existing FRP bars

is its low elastic modulus, if glass fibers are used (i.e., GFRP). The main objective of this study using the concept of

material hybridization is to develop a viable hybrid FRP bar for concrete structures, especially for marine and port con-

crete structures. The purposes of hybridization are to increase the elastic modulus of GFRP bar with acceptable tensile

strength. Two types of hybrid GFRP bar were considered in the development: GFRP crust with steel core and GFRP bar

with steel wires dispersed over the cross-section. Using E-glass fibers and unsaturated polyester resins, the hybrid

GFRP bar samples of 13 mm in diameter were pultruded and tested for tensile properties. The effect of hybridization on

tensile properties of GFRP bars was evaluated by comparing the results of tensile test with those of non-hybrid GFRP

bars. The results of this study indicated that the elastic modulus of the hybrid GFRP bar was increased by up to 270

percent by the material hybridization. The results of the test and the future recommendations are summarized in this

paper. To ensure long-term durability of the hybrid GFRP bars in waterfront structure applications, the individual and

combined effects of environmental conditions on hybrid GFRP rebar itself as well as on the interface between rebar and

concrete should be accessed.

Keywords: FRP; Glass Fibers; Tensile Test; Elastic Modulus; Pultrusion; Material Hybridization; Marine Structures

1. Introduction

Fiber Reinforced Polymer (FRP) is widely used as an

alternative material to resolve the corrosion problem of

the steel reinforcement and to increase the service life of

reinforced concrete (RC) structures. FRP rebar can pro-

vide high tensile strength as well as good resistance to

corrosion comparing to the steel reinforcement [1] for

RC structures, especially ones exposed to corrosive en-

vironments such as sea water. However, FRP has not

been actively applied as the reinforcement or structural

materials in civil engineering structures due to its low

elastic modulus and brittle fracture.

FRP is mainly composed of fibers and resin. Glass and

carbon are commonly used fiber materials. Carbon fiber

provides even higher tensile strength and more elastic

modulus than steel. These are advantageous features of

using carbon fiber in a structural point of view but not in

economics, since its price is almost ten times higher than

glass fiber. Use of glass fiber can be more beneficial ma-

terial in the initial cost. However, low modulus of elas-

ticity is a main disadvantage of using glass fiber, which

attains the elastic modulus less than a quarter of steel.

This leads to excessive deflection when FRP rebar was

used as the reinforcement for flexural members. With

this reason, the concept of “hybridization” was arisen for

the FRP rebar to overcome their shortcomings. The hy-

bridization of FRP has been investigated by many re-

searchers [2-5].

This paper discusses the recent development of FRP

hybrid bars using glass fiber and an experimentation of

their tensile properties. The purpose of this study is to

identify a feasible material hybridization of the glass

fiber reinforced polymer (GFRP) reinforcing bar to be

used for concrete structures. Two different materials,

mainly the combination of fibers and steel within the

cross-section of FRP bar, were considered for the hybrid

FRP bars. Two types of the hybrid GFRP bar were con-

sidered in the development: a) GFRP crust with steel

*Corresponding a uthor.

D.-W. SEO ET AL.

866

core; b) GFRP bar with steel wires dispersed over the

cross-section.

GFRP rebar with a circular cross-section was consid-

ered. Both vinylester and unsaturated polyester were

utilized as resin materials. For comparison purpose, the

existing GFRP bar developed and fabricated at Korea

Institute of Construction Technology (KICT, [6-8]) and

also two commercially available GFRP bars (Aslan and

V-rod [9,10]) were tested. The effect of material hy-

bridization on tensile properties of GFRP bars was

evaluated by comparing the results of tensile test with

those of the non-hybr id bars.

2. Development of Hybrid GFRP Bars

This study suggests two types of hybrid GFRP bars con-

sidering in the development: a) GFRP crust with steel

core; b) GFRP bar with steel wires dispersed over the

cross-section. Using E-glass fibers and unsaturated poly-

ester resins, the hybrid GFRP bar samples of 13 mm in

diameter were pultruded and tested for tensile p roperties.

Figure 1 shows the pultrusion process designed by KICT.

Vinylester (VE) and unsaturated polyester (PE) were

used as resins.

Table 1 summarizes four cross-section types consid-

ered in this study, categorized by steel volume fraction

from 0% to approximately 48%. Type A (e.g., KICT,

Aslan, and V-Rod) was selected as a reference case that

was considered as a non-hybrid GFRP bar. For the de-

signing purpose, the tensile strength of the hybrid GFRP

bar was assumed to be 800 MPa.

For type B, diameter equal to 4 mm steel bar was in-

serted with volume fraction 9.5% in the cross-section. A

Figure 1. Fabricating method for GFRP hybrid bars [6]: (a)

Detailed view of braiding method; (b) Pultrusion process

while braiding.

Table 1. Type of FRP hybr id bar samples.

Type A B C D

Steel volume fraction by

cross-section area (%) None 9.5 30.8 47.9

Cross-Section

total of 13 numbers of 2 mm steel wire were inserted

with volume fraction 30.8% fo r type C. In case of type D,

9 mm steel rebar was inserted into GFRP to be an outer

diameter equal to 13 mm, steel volume fraction was

47.9%.

The hybrid bars were fabricated with a circular cross-

section of diameter equal to approximately 13 mm. E-

glass fiber (SE1200-2200TEX, Owens Corning Korea

[11]), steel wire (KS D3510 C-type, Korea) and steel

rebar (nominal strength with 400 MPa) were used in this

study. Vinylester and unsaturated polyester are known as

effective resins for the pultrusion process of fabrication

because they offer economical advantage, low viscosity,

and rapid hardening. The material properties of the fiber

and resins are provided in Table 2.

3. Experiments

3.1. Tensile Test

The tensile tests on the specimens were carried out in

accordance with ASTM D 3916 [12]. The total length of

the specimens was 2000 mm and the gauge length was

1070 mm. An UTM w ith a cap acity o f 100 0 k N wa s used.

Strain gauges were attached at the center and the quarter

of the specimens within th e gauge length. The specimens

were fixed both at the top and the bottom with steel grip

adapters shown in Figure 2. Mortar was filled into the

Table 2. Material properties of fiber and resins [8].

Material Tensile strength

(MPa) Elastic modulus

(GPa) Elongation

(%)

E-glass fiber 2410 79.0 3.04

Vinylester res in9 3.7 7.00

Unsaturated

polyester resin62 3.1

Steel wire 16.5 200

Steel rebar 400 200

Figure 2. Test setup and fracture types: (a) Tensile test

setup; (b) Hybrid GFRP fracture; (c) Aslan fracture; (d) V-

Rod fracture.

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D.-W. SEO ET AL. 867

grip adapters and cured for two weeks to ob tain the com-

pressive strength approximately 60 MPa. Figure 2 shows

the tensile test with the loading rate equal to 5 mm/min

[13].

Brittle fractures of GFRP bars, including Case A,

Asaln, and V-rod were seen in Figure 3 one of short-

comings of FRP was a brittle fracture and this issue was

improved by material hybridization proved in this study.

Table 3 summarizes the list of specimens that tested in

this study. 7 cases of tested specimens were selected for

tensile test associated with 4 types explained in Table 1.

A total of 21 samples consisting 3 specimens for each

case was tested.

Cases A through C were correspond ing to the types A,

B, and C in Table 1. For cases D-1 and D-2, type D in

Table 1 was subdivided into two types depending on a

steel type; Case D-1 with circular shape of rebar and D-2

with the deformed rebar. Cases A through D-2 were de-

veloped and fabricated by KICT [6]. Two commercially

available GFRP bars (i.e., Aslan and V-Rod, [9,10]) were

also considered and their tensile strength was compared

to other hybrid GFRP bars developed at KICT.

3.2. Results and Discussion

The tensile strength of the specimen can be calculated by

dividing the measured maximum load by the cross-sec-

tional area of the GFRP bar (Ahybrid). The elastic modulus

of the GFRP bar (Ehybrid) can be given by the following

expression as recommended in [13].



hybrid

12 hybrid





. (1)

In Equation (1) P1 and P2 are the applied loads corre-

sponding to 50% and 25% of the ultimate load respec-

tively, and ɛ1 and ɛ2 are the c o rresponding strains.

Table 4 and Figures 3 and 4 summarize the result of

tensile tests. Figure 3 shows a linear increment of elastic

The linear stress-strain relationship of the specimens was

found for Case A, Aslan and V-rod, in which no material

hybridization was considered.

In these cases the brittle fracture was occurred shown

Figures 1(c) and (d). Small change of the curvature was

Table 3. List of specimens for tensile test.

Case Description

A D13 with GFRP only

D-1

D-2

Aslan

V-Rod

D13 with GFRP and D4 steel wire inserted

D13 with GFRP and D2 × 13EA (steel wire) inserted

D13 with GFRP and D9 rebar inserted

D13 with GFRP and D9 deformed rebar inserted

D13 Aslan 100 [9]

D13 V-Rod GFRP [10]

Table 4. Results of tensile tests at the location L/2.

Elastic Modulus (E) Tensile Strength (P)

Case GPa N (E) MPa N (P)

A 49.6 1.00 754.4 1.00

D-1

D-2

Aslan

V-Rod

53.7

98.3

129.2

133.2

52.5

46.2

1.08

1.98

2.60

2.69

1.06

0.93

762.1

688.2

715.4

601.8

574.6

0.94

0.85

0.88

0.74

0.71

found for cases B and C after steel wire was likely

yielded earlier than GFRP. The bilinear type of fracture

behavior was detected for cases D-1 and D-2. In these

cases, failure mechanism is clearly dominated by steel

rebar in the initial stage and GFRP holds the applying

loads after approximately 350 MPa.

Most of the specimens failed in the gauge length, but

some of them presented ruptures at the grip adapters. The

averaged value of the three specimens for each case re-

sults, measured at the location L/2, was presented in Ta -

ble 4. A negligible difference of strains between the two

locations, L/2 an d L/4, was found. In Table 4, values for

both elastic modulus (E) and maximum tensile strength

(P) were normalized to case A for comparison purpose.

Case A was considered as a non-hybrid GFRP bar de-

veloped at KICT.

Enhancement in elastic modulus was investigated by

material hybridization up to 269%. However, regarding

the tensile strength, a small reduction was found for all

cases. This reduction may occur due to damage, the size

of specimen, the gripping method, or slip between two

materials (i.e., GFRP and steel). More detailed study for

this issue is planned by the authors.

Cases D-1 and D-2 shows the h ighest hybrid effect for

the GFRP bar in terms of elastic modulus with steel frac-

tion of 47.9%.

The commercial GFRP bars, Aslan and V-Rod pro-

vided maximum tensile strength approximately 30%

lower than “KICT GFRP bar” while elastic modulus was

a similar value supposed to be around 50 GPa.

4. Conclusions

In this study material hybridization of GFRP bar was

considered to overcome its low elastic modulus to be

used as reinforcement for concrete structures built in the

corrosive environment. The existing GFRP bar devel-

oped at Korea Institute of Construction Technology

(KICT [6]) was hybridized by adding steel as a high

strength material. Various combinations of mixing ratio

etween GFRP and steel were investigated.

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D.-W. SEO ET AL.

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868

00.002 0.004 0.0060.0080.010.012 0.0140.016

200

400

600

800

1000

Case A

S t ress ( MP a)

Strain

at L/2

at L/4

00.005 0.01 0.015 0.02

200

400

600

800

1000

Case B

Stress (MPa)

Strain

at L/2

at L/4

(a) (b)

00.002 0.004 0.006 0.0080.010.012 0.014

200

400

600

800

1000 Case C

Stress (MPa)

Strai

at L/2

at L/4

00.005 0.01 0.0150.02 0.025

200

400

600

800

1000

Case D-1

S tress (MPa)

Strain

at L/2

at L/4

00.005 0.01 0.015 0.02

100

200

300

400

500

600

Case D-2

S t res s ( MPa)

Strai

at L/2

at L/4

00.005 0.01 0. 015

100

200

300

400

500

600

700

800

Aslan

S tress (MP a)

Strai

at L/2

at L/4

00.005 0.01 0.015

100

200

300

400

500

600

700

V-R od

Stress (MPa)

Strain

at L/2

at L/4

(e) (f) (g)

Figure 3. Stress vs. strain curves at the location L/2 and L/4 for hybrid GFRP specimens: (a) Case A; (b) Case B; (c) Case C;

(d) Case D-1; (e) Case D-2; (f) Aslan; (g) V-Rod.

010 20 30 40 50

100

120

140

Elastic Modulus, G Pa

St eel Volume Frac t ion , %

Case D-2

Case C

Case B

Case A

As a result of tensile test, the elastic modulus of the

hybrid rods was increased by 8% to 269% with material

hybridization in comparison with the non-hybrid GFRP

bar. However, a small reduction of tensile strength was

found. This reduction may occur due to damage, mis-

placement of fibers during the fabrication, the size of

specimen, the gripping method, or slip between two ma-

terials (i.e., GFRP and steel). More detailed study for this

issue is planned by the authors.

One of shor tcomings of FRP was a brittle fracture and

this issue was improved to “pseudo-ductile” behavior by

material hybridization.

The hybrid effect was the largest where steel was

added the most in the section. Furthermore, the incre-

ment of elastic modulus was proportional to the quantity

Figure 4. Relationship between steel volume fraction in the

cross-section and elastic modulus.

D.-W. SEO ET AL. 869

of steel added in the section.

Further investigation should be conducted to study the

effect of the stress redistribution mechanism on the

“pseudo-ductile” behavior regarding to the quantity as

well as the dispersion of steel. Economic feasibility of

the hybrid FRP bars should also be investigated.

5. Acknowledgements

This research (2013 Basic Research: Development of

Hybrid FRP Bars for Concrete Waterfront Structures)

was supported by Korea Institute of Construction Tech-

nology and funded by the Ministry of Science, ICT, and

Future Plannin g of Korea n G overnment .

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