Engineering, 2013, 5, 865-869
Published Online November 2013 (
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: *
Received August 29, 2013; revised September 29, 2013; accepted October 10, 2013
Copyright © 2013 Dong-Woo Seo et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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.
core; b) GFRP bar with steel wires dispersed over the
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
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
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
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
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].
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
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
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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00.002 0.004 0.0060.0080.010.012 0.0140.016
Case A
S t ress ( MP a)
at L/2
at L/4
00.005 0.01 0.015 0.02
Case B
Stress (MPa)
at L/2
at L/4
(a) (b)
00.002 0.004 0.006 0.0080.010.012 0.014
1000 Case C
Stress (MPa)
at L/2
at L/4
00.005 0.01 0.0150.02 0.025
Case D-1
S tress (MPa)
at L/2
at L/4
(c) (d)
00.005 0.01 0.015 0.02
Case D-2
S t res s ( MPa)
at L/2
at L/4
00.005 0.01 0. 015
S tress (MP a)
at L/2
at L/4
00.005 0.01 0.015
V-R od
Stress (MPa)
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
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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