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Citation: Thongchom, C.; Bui, L.V.H.;
Poonpan, N.; Phudtisarigorn, N.;
Nguyen, P.T.; Keawsawasvong, S.;
Mousa, S. Experimental and
Numerical Investigation of Steel- and
GFRP-Reinforced Concrete Beams
Subject to Fire Exposure. Buildings
2023,13, 609. https://doi.org/
10.3390/buildings13030609
Academic Editor: Francisco
López Almansa
Received: 25 December 2022
Revised: 9 February 2023
Accepted: 22 February 2023
Published: 25 February 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
buildings
Article
Experimental and Numerical Investigation of Steel- and
GFRP-Reinforced Concrete Beams Subject to Fire Exposure
Chanachai Thongchom 1, Linh Van Hong Bui 2, Natthanuch Poonpan 1, Natcha Phudtisarigorn 1,
Phuoc Trong Nguyen 3,* , Suraparb Keawsawasvong 1and Saeed Mousa 4
1Department of Civil Engineering, Faculty of Engineering, Thammasat School of Engineering,
Thammasat University, Pathumthani 12120, Thailand
2
Advanced Retrofit Technology International Center, Advanced Research Laboratories, Tokyo City University,
1-28-1 Tamazutsumi, Tokyo 158-8557, Japan
3Faculty of Civil Engineering, Ho Chi Minh City Open University, 97 Vo Van Tan, District 3,
Ho Chi Minh City 700000, Vietnam
4Faculty of Engineering, Jazan University, Jazan 706, Saudi Arabia
*Correspondence: phuoc.nguyen@ou.edu.vn
Abstract:
This study investigates the behavior of three concrete beams reinforced with steel and GFRP
bars under fire exposure. The fire tests of three beams were conducted including one control steel-
reinforced concrete (RC) beam and two GFRP-RC beams. The beams were exposed to fire according
to the standard fire curve ISO 834 for 3 h. The investigation parameters included the reinforcement
types (i.e., steel and GFRP bars) and diameter of GFRP bars. Based on the experimental results,
during fire exposure, the deflection rate of the steel-RC beam was lower than the ones reinforced with
GFRP bars. The critical temperatures measured at steel and GFRP bars in the steel-RC and GFRP-RC
beams were 593
◦
C and 300–330
◦
C, respectively along with the fire durations of 83 and 33–36.4 min,
respectively. The different GFRP bar sizes did not affect the fire resistance process. The steel-RC beam
had greater fire resistance than the GFRP-RC beams. All test specimens had a fire resistance time
lower than two hours. In addition, the 2D simplified finite element method (FEM) using commercial
software ANSYS was performed to predict the thermal response of the beam section. Compared with
experimental results, the FE model can reasonably predict the thermal responses of the beam sections.
Keywords: glass fiber-reinforced polymer; fire resistance; concrete beam; finite element; fire
1. Introduction
Reinforced concrete (RC) structures are still common in the construction industry in
both developing and developed countries. Many benefits of the RC structures, such as
high and durable strength, shape variety, and economical value, can be clearly identified.
However, when the RC structures are in use for a certain period of time, they can suffer
deterioration due to various reasons, such as inefficient design, construction control, aging,
and environmental impact. Much research has reported that steel-RC structures are deemed
to be affected by corrosion. The aforementioned factors affect the properties of concrete
following the decrease of the performance of RC structures. Permeability of air and moisture
from the environment, which can cause rust in the reinforcing steel, leads to volume change
and loss of bonding between steel and concrete. Engineers and researchers have paid
much attention to the development of new reinforcement systems that could overcome the
drawbacks of conventional steel bars.
In the past few decades, fiber-reinforced polymer (FRP) composites, which are popular
in the aerospace field, have been considered for application in the construction [
1
–
5
].
FRP materials consist of a polymer matrix and reinforcement fibers. The fibers used as
reinforcement in FRP composites can be made of various materials such as carbon, glass,
aramid, or basalt, and they provide the material with its enhanced mechanical properties.
Buildings 2023,13, 609. https://doi.org/10.3390/buildings13030609 https://www.mdpi.com/journal/buildings
Buildings 2023,13, 609 2 of 19
FRP materials have been increasingly used in various industries, such as aerospace,
construction, and transportation, due to their unique combination of mechanical and
physical properties. FRP composites offer several advantages over traditional reinforcement
materials. The outstanding features of FRP make them lightweight and easy to install. The
anticorrosion and chemical resistance of the FRP composites require little maintenance and
are highly durable and four times lighter than traditional reinforcement materials [
6
–
8
].
The FRP composites can be used for all components of the structures, including slabs,
columns, and beams [
8
–
12
]. Conversely, as summarized in the studies [
11
–
13
], FRP behaves
linearly until rupture; thereby, the elements with FRP reinforcement may experience a
brittle and sudden failure. Recently, several studies proposed the hybrid use of steel and
FRP for reinforcing the concrete members and proposed extra strengthening of FRP to
existing steel-RC structures. FRP reinforcement has a higher tensile strength compared to
steel rebars. A number of research works indicated that the beams with FRP reinforcement
had a higher load-carrying capacity than the those with conventional steel reinforcing
bars [5,14–17].
Aside from the above-mentioned impacts, fire is one of the most unexpected threats
to building structures. When a fire accident occurs, it will cause a lot of damage to both
the life and property of the building occupants [
18
,
19
]. Regardless of whether the fire
occurred intentionally or unintentionally, when the fire has occurred, it will spread to
other areas rapidly. When the fire is exposed to the FRP-reinforced concrete structures,
it affects the mechanical properties of the concrete and reinforcements. When exposed
to high temperatures, the concrete can undergo physical and chemical changes that can
weaken its structure and reduce its strength. At low temperatures, the concrete may only
experience surface cracking and spalling, which can lead to the loss of the surface layer. As
the temperature increases, the concrete can undergo thermal expansion and contraction,
which can cause the concrete to crack and delaminate. At even higher temperatures, the
hydration process of the concrete can be reversed, leading to the release of water and the
formation of steam. This may cause the concrete to expand and spall, leading to further
loss of material. The properties of concrete, such as its compressive and tensile strength,
are reduced as a result of fire exposure. In general, concrete will spall after exposure to
temperatures between 200
◦
C and 325
◦
C. The explosive spalling of concrete appears to
coincide with high pore pressure buildup and a high thermal gradient [
20
]. When the
concrete is exposed to fire at 300
◦
C, the strength reduction will be in the range of 15–
40% [
21
]. When FRP is exposed to fire, FRP reinforcements can undergo significant changes
in their mechanical properties, such as tensile strength and modulus of elasticity. As the
temperature increases, the FRP rebar can soften and lose its strength. Additionally, the fire
can cause thermal degradation of the polymer matrix, leading to the release of toxic gases
and the formation of cracks and voids. This can further reduce the strength and durability
of the FRP rebar, making it more susceptible to corrosion and other forms of degradation.
In particular, when the FRP is exposed to temperatures at the glass transition level (
Tg
),
the resin matrix of FRP is affected to induce a small crack and to soften the FRP surface.
At higher glass transition temperature levels, the softness of the FRP occurs more quickly.
When the FRP is exposed to the critical temperature (
Tcr
), the tensile strength of the FRP is
reduced by 50% [
19
]. This can be a significant concern in high-temperature fire scenarios,
as the fire can quickly spread and cause extensive damage to the building or structure.
Therefore, it is extremely important to focus on the fire resistance of a building struc-
ture as its structural integrity might be the last line of defense, as stated in the works
of [19,22–27].
Fire resistance is a critical factor in the design and construction of buildings
and structures. The fire resistance time of FRP-reinforced concrete beams refers to the
amount of time that a concrete structure can withstand high temperatures without collaps-
ing or losing its structural integrity. The fire resistance time of a concrete structure is crucial
in ensuring the safety of the building and its occupants. The fire resistance time of GFRP-
reinforced concrete beams is influenced by a number of factors including the properties
of the concrete, the thickness of the concrete covering, the heating rate, the cooling rate,
Buildings 2023,13, 609 3 of 19
and the properties of the FRP reinforcement. The properties of the concrete, such as its
compressive strength, permeability, and water-to-cement ratio, play a significant role in
determining its fire resistance time. The thickness of the concrete covering is also important,
as thicker concrete provides more insulation and protection against high temperatures.
The heating and cooling rates of the concrete can also have a significant impact on its fire
resistance time.
The progress of the application of FRP for construction requires an understanding of
the behavior of the FRP-reinforced structures under extreme actions and agents. Various
types of FRP have been applied and studied, in which the common FRP types are carbon
FRP (CFRP), glass FRP (GFRP), and aramid FRP (AFRP). The benefit of GFRP compared
to other FRPs is that the GFRP has low elastic modulus but high rupturing strain. This
behavior may provide better ductility of the GFRP-RC members in comparison to the
CFRP/AFRP-RC elements. Therefore, the studies of the behavior of GFRP-RC beams under
fire conditions are necessary to gain insights into the safety of building occupants. The
present study experimentally and numerically investigates the responses of two concrete
beams reinforced with GFRP bars (GFRP-RC beams) and one concrete beam reinforced
with steel reinforcement bars (steel-RC beam) subjected to fire exposure. The thermal
responses of the steel-RC beams and GFRP-RC beams exposed to a standard fire curve
for three hours are investigated. The fire resistance time based on the critical temperature
for both steel-RC beams and GFRP-RC beams are examined. The temperature and time
dependencies among beams are assessed. Additionally, the effects of different GFRP bar
sizes on the thermal behavior of RC beams are evaluated. Conversely, the temperature
distribution along the beam section obtained from the fire tests is compared with that
simulated by a simplified two-dimensional finite element method (2D FEM) using the
numerical software ANSYS 15.0.
2. Materials and Methods
2.1. Tested Beam Specimens
Three full-scale beams were tested in this study. The purposes of the tests were
to compare the behavior of RC beams reinforced with GFRP bars under fire exposure
considering the different amount of GFRP reinforcement. An overview of this study is
shown in Figure 1. The beams were 3850 mm long, 150 mm wide, and 300 mm high. The
length of the beams from support to support were 3750 mm. The concrete cover thickness
was 25 mm, while the stirrups in all beams were made from RB9 and spaced at 100 mm.
The details of the beams are shown in Table 1. The beam RC12 was the control beam, which
includes two tensile steel bars and two compressive steel bars. Meanwhile, the beams BF12
and BF20 had two GFRP12 bars and two GFRP20 bars for reinforcing the bending zone,
respectively. These beams had two GFRP12 for compressive reinforcement. The beam
configurations are demonstrated in Figure 2.
Table 1. Details of test beams.
Beam No. Tension Reinforcement Compressive Reinforcement
RC12 2DB12 2DB12
BF12 2GFRP12 2GFRP12
BF20 2GFRP20 2GFRP12
Buildings 2023,13, 609 4 of 19
Buildings 2023, 13, x FOR PEER REVIEW 4 of 20
Figure 1. Flowchart of the research program.
300
3750
3850
1250 12 50
2DB12
RB9@100
150
2DB12
A
A
2DB12
2DB12
50
50
50
50
1250
SECTION A
RB9@100
(a)
3750
3850
1250 12 50
RB9@100
150
300
2GFR P12
2GFR P12
RB9@100
A
A
2GFR P12
2GFR P12
50
50
1250
50
50
SECTION A
(b)
Figure 1. Flowchart of the research program.
Buildings 2023, 13, x FOR PEER REVIEW 4 of 20
Figure 1. Flowchart of the research program.
300
3750
3850
1250 1250
2DB12
RB9@100
150
2DB12
A
A2DB12
2DB12
50
50
50
50
1250
SECTION A
RB9@100
(a)
3750
3850
1250 1250
RB9@100
150
300
2GFRP12
2GFRP12
RB9@100
A
A2GFRP12
2GFRP12
50
50
1250
50
50
SECTION A
(b)
Buildings 2023, 13, x FOR PEER REVIEW 5 of 20
3750
3850
1250 12 50
RB9@100
150
300
2GFR P20
2GFR P12
RB9@100
A
A
2GFR P12
2GFR P20
50
50
1250
50
50
SECT ION A
(c)
Figure 2. Beam details. (a) Beam RC12; (b) Beam BF12; (c) Beam BF20.
Table 1. Details of test beams.
Beam No. Tension Reinforcement Compressive Reinforcement
RC12 2DB12 2DB12
BF12 2GFRP12 2GFRP12
BF20 2GFRP20 2GFRP12
2.2. Material Properties
Ready-mixed concrete was used in this experimental program. The average compres-
sive strength of the concrete from three standard cylinder specimens (ASTM C39/C39M
[28]) was 28 MPa with standard deviation of 0.43. The three samples of steel reinforcement
were tested under tensile loading according to ASTM A370 [29]. The steel reinforcements
DB12 (Standard deformed bars 40, SD40) had a yield strength of 466 MPa, an ultimate
strength of 540 MPa, and an elastic modulus of 210 GPa. The steel stirrups RB9 (Standard
round bars, SR24) had a yield strength of 270 MPa, an ultimate strength of 410 MPa, and
an elastic modulus of 206 GPa. Five GFRP bars were also tested under tensile loading
according to ASTM D7205/D7205M [30]. The GFRP bars with 12 mm diameter had a ten-
sile strength of 851 MPa and an elastic modulus of 45 GPa. In addition, the GFRP bars
with 20 mm diameter had a tensile strength of 935 MPa and an elastic modulus of 45 GPa.
The mechanical properties of reinforcements are summarized in Table 2. The standard
composition of steel reinforcement typically consists of 98.6% to 99.2% iron (Fe), 0.15% to
0.30% carbon (C), 0.60% to 1.20% manganese (Mn), and 0.15% to 0.35% silicon (Si), with a
maximum of 0.05% sulfur (S) and phosphorus (P). Additionally, trace amounts of chro-
mium (Cr), nickel (Ni), and molybdenum (Mo) are included, typically in amounts less
than 0.10%. Table 3 shows the chemical composition of both RB9 and DB20 steel reinforce-
ments provided in the TIS guidelines [31–32].
Table 2. Mechanical properties of materials.
Materials Yield Strength
(MPa)
Ultimate Strength
(MPa)
Elastic Modulus
(GPa)
DB12 466 540 210
RB9 270 410 206
GFRP12 - 851 45
GFRP20 - 935 45
Figure 2. Beam details. (a) Beam RC12; (b) Beam BF12; (c) Beam BF20.
Buildings 2023,13, 609 5 of 19
2.2. Material Properties
Ready-mixed concrete was used in this experimental program. The average compres-
sive strength of the concrete from three standard cylinder specimens (ASTM C39/C39M [
28
])
was 28 MPa with standard deviation of 0.43. The three samples of steel reinforcement were
tested under tensile loading according to ASTM A370 [
29
]. The steel reinforcements DB12
(Standard deformed bars 40, SD40) had a yield strength of 466 MPa, an ultimate strength
of 540 MPa, and an elastic modulus of 210 GPa. The steel stirrups RB9 (Standard round
bars, SR24) had a yield strength of 270 MPa, an ultimate strength of 410 MPa, and an elastic
modulus of 206 GPa. Five GFRP bars were also tested under tensile loading according to
ASTM D7205/D7205M [
30
]. The GFRP bars with 12 mm diameter had a tensile strength
of 851 MPa and an elastic modulus of 45 GPa. In addition, the GFRP bars with 20 mm
diameter had a tensile strength of 935 MPa and an elastic modulus of 45 GPa. The mechani-
cal properties of reinforcements are summarized in Table 2. The standard composition of
steel reinforcement typically consists of 98.6% to 99.2% iron (Fe), 0.15% to 0.30% carbon (C),
0.60% to 1.20% manganese (Mn), and 0.15% to 0.35% silicon (Si), with a maximum of 0.05%
sulfur (S) and phosphorus (P). Additionally, trace amounts of chromium (Cr), nickel (Ni),
and molybdenum (Mo) are included, typically in amounts less than 0.10%. Table 3shows
the chemical composition of both RB9 and DB20 steel reinforcements provided in the TIS
guidelines [31,32].
Table 2. Mechanical properties of materials.
Materials Yield Strength
(MPa)
Ultimate Strength
(MPa)
Elastic Modulus
(GPa)
DB12 466 540 210
RB9 270 410 206
GFRP12 - 851 45
GFRP20 - 935 45
Table 3. Chemical composition of steel (TIS, 2016).
Steel Type
Chemical Composition % (Max)
Carbon Manganese Phosphorus Sulphur Carbon+
Manganese/6
RB9 0.28 - 0.060 0.060 -
DB20 - 1.85 0.060 0.060 0.500
During fire test, a linear variable differential transformer (LVDT) was installed in the
center of the beams of all test samples to measure the deflection of beams during a fire. The
temperature gauges (thermocouple) were glued on positions A, B, and C of each beam to
measure the temperature inside the beam cross-section. The details of installation of LVDTs
and thermocouples are shown in Figure 3.
In this test, all sample beams are exposed to fire. The dimensions of the furnace are
3500 mm width, 4500 mm length, and 1600 mm depth. The details of the front, top, and
side cross-sections are shown in Figure 4a, Figure 4b, and Figure 4c, respectively, and the
photographs of the furnace front, top, and sides are shown in Figure 5a, Figure 5b, and
Figure 5c, respectively.
For the fire resistance test, the RC12, BF12, and BF20 specimen beams were exposed to
fire in accordance with ISO 834 [
33
] simultaneously for all three samples until the beams
failed under critical temperature. This means when the temperatures at the location of the
reinforcing bar and the GFRP bar reached 593
◦
C [
34
,
35
] and 300–330
◦
C [
27
,
36
], respectively.
The installation of the sample beam in the furnace and the thermocouple signal installation
are shown in Figure 6.
Buildings 2023,13, 609 6 of 19
Buildings 2023, 13, x FOR PEER REVIEW 6 of 20
Table 3. Chemical composition of steel (TIS, 2016).
Steel Type
Chemical Composition % (Max)
Carbon Manganese Phosphorus Sulphur Carbon+
Manganese/6
RB9 0.28 - 0.060 0.060 -
DB20 - 1.85 0.060 0.060 0.500
During fire test, a linear variable differential transformer (LVDT) was installed in the
center of the beams of all test samples to measure the deflection of beams during a fire.
The temperature gauges (thermocouple) were glued on positions A, B, and C of each beam
to measure the temperature inside the beam cross-section. The details of installation of
LVDTs and thermocouples are shown in Figure 3.
3750
3850
1250 1250
RB9@100
A1 ,A2
B1 , B2
B3
B9 , B10
C1,C2
625
625
C
C
A
A
B
B
50
50
50
50
LV DT
B4 ,B5 ,B6 ,B 7,B 8
B1 1
150
300
SECTIO N A
A1 A2
150
300
SECTION B
B1 B3
B9
B4
B11
B2
B5
B6
B7
B8
B10
150
300
SECTION C
C1 C2
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🔥
🔥
🔥
🔥🔥🔥
🔥
🔥
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Figure 3. LVDT and thermocouple installation location of the sample beam.
In this test, all sample beams are exposed to fire. The dimensions of the furnace are
3500 mm width, 4500 mm length, and 1600 mm depth. The details of the front, top, and
side cross-sections are shown in Figure 4a, Figure 4b, and Figure 4c, respectively, and the
photographs of the furnace front, top, and sides are shown in Figure 5a, Figure 5b, and
Figure 5c, respectively.
Figure 3. LVDT and thermocouple installation location of the sample beam.
Buildings 2023, 13, x FOR PEER REVIEW 7 of 20
(a) (b)
(c)
Figure 4. Fire furnace (unit: millimeter); (a) front view; (b) top view; (c) side view.
(a) (b)
(c)
Figure 4. Fire furnace (unit: millimeter); (a) front view; (b) top view; (c) side view.
Buildings 2023,13, 609 7 of 19
Buildings 2023, 13, x FOR PEER REVIEW 7 of 20
(a) (b)
(c)
Figure 4. Fire furnace (unit: millimeter); (a) front view; (b) top view; (c) side view.
(a) (b)
(c)
Figure 5. Fire test furnace; (a) front view; (b) top view; (c) side view.
Buildings 2023, 13, x FOR PEER REVIEW 8 of 20
Figure 5. Fire test furnace; (a) front view; (b) top view; (c) side view.
For the fire resistance test, the RC12, BF12, and BF20 specimen beams were exposed
to fire in accordance with ISO 834 [33] simultaneously for all three samples until the beams
failed under critical temperature. This means when the temperatures at the location of the
reinforcing bar and the GFRP bar reached 593 °C [34,35] and 300–330 °C [27,36], respec-
tively. The installation of the sample beam in the furnace and the thermocouple signal
installation are shown in Figure 6.
Figure 6. Fire test setup.
3. Results and Analyses
3.1. Distribution of Temperature along the Beam Section
In Figures 7–9, the temperature distribution along the beam sections in the beams
RC12, BF12, and BF20, measured by the thermocouples, shows that the temperature ex-
posed to the beam components increased rapidly with a nonlinear relationship to time. It
can be seen that the temperature measured at the beam bottom was higher than that at
the beam center and top. This is due to the fact that the fire was set to start exposing the
beam from the bottom. Generally, the temperature at the beam center (positions B4, B5,
B6, B7, and B8) was higher than that at the concrete bottom and top of the beam. However,
before 1.5 h, the temperature was found to be slightly higher than the beam top at position
B11 because the top of the beam was covered with ceramic fiber to prevent fire exposure.
Figure 6. Fire test setup.
Buildings 2023,13, 609 8 of 19
3. Results and Analyses
3.1. Distribution of Temperature along the Beam Section
In Figures 7–9, the temperature distribution along the beam sections in the beams RC12,
BF12, and BF20, measured by the thermocouples, shows that the temperature exposed to
the beam components increased rapidly with a nonlinear relationship to time. It can be
seen that the temperature measured at the beam bottom was higher than that at the beam
center and top. This is due to the fact that the fire was set to start exposing the beam from
the bottom. Generally, the temperature at the beam center (positions B4, B5, B6, B7, and B8)
was higher than that at the concrete bottom and top of the beam. However, before 1.5 h, the
temperature was found to be slightly higher than the beam top at position B11 because the
top of the beam was covered with ceramic fiber to prevent fire exposure. After 1.5 h, the
temperature at B11 was higher than the temperature at the center of the beam. A possible
reason is that the increase in temperature, and the increase in deflection, led to the top of
the beam becoming heavily exposed to fire. The aforementioned observations imply that
the material properties of tension reinforcement (steel or GFRP) did not affect the trend
of the temperature distribution under fire in the long-span RC beams. Effects of various
parameters on the performance of beams under fire are shown in the following sections.
Buildings 2023, 13, x FOR PEER REVIEW 9 of 20
After 1.5 h, the temperature at B11 was higher than the temperature at the center of the
beam. A possible reason is that the increase in temperature, and the increase in deflection,
led to the top of the beam becoming heavily exposed to fire. The aforementioned obser-
vations imply that the material properties of tension reinforcement (steel or GFRP) did
not affect the trend of the temperature distribution under fire in the long-span RC beams.
Effects of various parameters on the performance of beams under fire are shown in the
following sections.
(a) (b)
Figure 7. Temperature versus time relationship in the beam RC12. (a) Reinforcement; (b) concrete.
(a) (b)
Figure 8. Temperature versus time relationship in the beam RF12. (a) Reinforcement; (b) concrete.
(a) (b)
Figure 9. Temperature versus time relationship in the beam RF20. (a) Reinforcement; (b) Concrete.
Figure 7. Temperature versus time relationship in the beam RC12. (a) Reinforcement; (b) concrete.
Buildings 2023, 13, x FOR PEER REVIEW 9 of 20
After 1.5 h, the temperature at B11 was higher than the temperature at the center of the
beam. A possible reason is that the increase in temperature, and the increase in deflection,
led to the top of the beam becoming heavily exposed to fire. The aforementioned obser-
vations imply that the material properties of tension reinforcement (steel or GFRP) did
not affect the trend of the temperature distribution under fire in the long-span RC beams.
Effects of various parameters on the performance of beams under fire are shown in the
following sections.
(a) (b)
Figure 7. Temperature versus time relationship in the beam RC12. (a) Reinforcement; (b) concrete.
(a) (b)
Figure 8. Temperature versus time relationship in the beam RF12. (a) Reinforcement; (b) concrete.
(a) (b)
Figure 9. Temperature versus time relationship in the beam RF20. (a) Reinforcement; (b) Concrete.
Figure 8. Temperature versus time relationship in the beam RF12. (a) Reinforcement; (b) concrete.
Buildings 2023,13, 609 9 of 19
Buildings 2023, 13, x FOR PEER REVIEW 9 of 20
After 1.5 h, the temperature at B11 was higher than the temperature at the center of the
beam. A possible reason is that the increase in temperature, and the increase in deflection,
led to the top of the beam becoming heavily exposed to fire. The aforementioned obser-
vations imply that the material properties of tension reinforcement (steel or GFRP) did
not affect the trend of the temperature distribution under fire in the long-span RC beams.
Effects of various parameters on the performance of beams under fire are shown in the
following sections.
(a) (b)
Figure 7. Temperature versus time relationship in the beam RC12. (a) Reinforcement; (b) concrete.
(a) (b)
Figure 8. Temperature versus time relationship in the beam RF12. (a) Reinforcement; (b) concrete.
(a) (b)
Figure 9. Temperature versus time relationship in the beam RF20. (a) Reinforcement; (b) Concrete.
Figure 9. Temperature versus time relationship in the beam RF20. (a) Reinforcement; (b) Concrete.
3.2. Effects of GFRP and Steel
Figure 10a,b presents the comparison in the temperature versus time responses at the
tension reinforcement layers between RC12 (beam reinforced with steel tensile bars) and
BF12 and BF20 (beams reinforced with GFRP tensile bars). Obviously, Figure 10a implies
that with the same bar diameter, the beam with steel longitudinal reinforcement (RC12)
provided a higher maximum temperature than the beam with GFRP tensile bars (BF12).
Indeed, the maximum temperatures measured in steel and GFRP tension reinforcement in
those two beams were over 1000 and over 800
◦
C, respectively. At the same temperature
level, the beam with GFRP bars exhibits an earlier time than the beam with steel bars.
This is attributable to the fact that the thermal properties of steel in terms of the thermal
conductivity, specific heat, and coefficient of thermal expansion have better fire resistance
compared to those of GFRP. In Figure 10b, the temperature-time performance of the beam
with bigger GFRP bars is greater than that of the beam with smaller GFRP bars. a possible
reason is that the larger GFRP bar size provided the greater thermal capacity to increase fire
resistance. To obtain 1000
◦
C at the tension reinforcement, the beam reinforced by GFRP
bars with 20 mm diameter furnished the same temporal behavior as the beam reinforced
with steel bars with 12 mm diameter. As seen in Figure 10a,b, under the increase in
temperature, all specimens failed at approximately 2 h or less.
3.3. Fire Resistance
Table 4presents the times when the thermocouples obtained critical temperatures.
Note that the critical temperatures for steel and GFRP bars were obtained from the com-
bined thermal and tensile tests. As a result, it was found that the thermocouples at positions
A2, B1, B2, and C1 in the steel-RC beam (RC12) reached critical temperature
(Tcr = 593 ◦C)
at 80, 90, 91, and 71 min, respectively. Meanwhile, the thermocouples in the GFRP-RC
beams reached critical temperature at around 31-39 min. This could be due to the early
deterioration of the GFRP bars under fire exposure. The steel-RC beam performed with
better fire resistance than the GFRP-RC beams, while the GFRP-RC beam with larger GFRP
tension bars provided a longer fire resistance time. According to fire resistance based
on critical temperature [
34
], all beams failed under fire at three hours or less. The main
reason is that the long span of the beams could increase deformation, which induces more
microcracks. This condition would accelerate the exposure of fire to the reinforcement,
leading the premature failure of the beams.
Buildings 2023,13, 609 10 of 19
Buildings 2023, 13, x FOR PEER REVIEW 10 of 20
3.2. Effects of GFRP and Steel
Figure 10a,b presents the comparison in the temperature versus time responses at the
tension reinforcement layers between RC12 (beam reinforced with steel tensile bars) and
BF12 and BF20 (beams reinforced with GFRP tensile bars). Obviously, Figure 10a implies
that with the same bar diameter, the beam with steel longitudinal reinforcement (RC12)
provided a higher maximum temperature than the beam with GFRP tensile bars (BF12).
Indeed, the maximum temperatures measured in steel and GFRP tension reinforcement
in those two beams were over 1000 and over 800 °C, respectively. At the same temperature
level, the beam with GFRP bars exhibits an earlier time than the beam with steel bars. This
is attributable to the fact that the thermal properties of steel in terms of the thermal con-
ductivity, specific heat, and coefficient of thermal expansion have better fire resistance
compared to those of GFRP. In Figure 10b, the temperature‒time performance of the beam
with bigger GFRP bars is greater than that of the beam with smaller GFRP bars. a possible
reason is that the larger GFRP bar size provided the greater thermal capacity to increase
fire resistance. To obtain 1000 °C at the tension reinforcement, the beam reinforced by
GFRP bars with 20 mm diameter furnished the same temporal behavior as the beam rein-
forced with steel bars with 12 mm diameter. As seen in Figure 10a,b, under the increase
in temperature, all specimens failed at approximately 2 h or less.
(a)
(b)
Figure 10. Comparisons in the temperature: (a) RC12 and BF12; (b) BF12 and BF20.
Figure 10. Comparisons in the temperature: (a) RC12 and BF12; (b) BF12 and BF20.
Table 4. Critical temperature of reinforcing steel and GFRP rods of sample beams.
TC Position
RC12
(minutes)
(Tcr = 593 ◦C)
BF12
(minutes)
(Tcr = 300–330 ◦C)
BF20
(minutes)
(Tcr = 300–330 ◦C)
A1 - 33–36 29–33
A2 80 33–36 33–37
B1 90 31–34 32–35
B2 91 34–38 -
C1 71 - 30–34
C2 - 34–38 35–39
3.4. Failure of Beam under Fire
Figure 11 shows the failure of all beams under fire process. It can be seen that all
beams collapsed and fell down to the kiln floor. At failure completion, heavy damage
with large cracks and spalling occurred. The primary causes are (1) the long span of the
beams provides large deformation that speeds up the failure process due to the quick
aggressiveness of fire affecting the reinforcement and (2) under fire, the concrete is spalled
and the melting of steel reinforcements might decrease the bond strength between steel
Buildings 2023,13, 609 11 of 19
and concrete, leading to cracks in concrete. To prevent the premature failure of concrete
structures exposed to fire, the recommended methods from previous works are (1) us-
ing lightweight aggregates [
37
] and (2) using fire-resistant coatings such as intumescent
coatings, etc. [38].
Buildings 2023, 13, x FOR PEER REVIEW 12 of 20
Figure 11. Failure of beams subject to fire.
3.5. 2D Thermal Analysis
The finite element model for temperature analysis was conducted using a 2D finite
element analysis using the ANSYS program. The concrete element was modeled using
PLANE55 [39]. It is a planar element or is an axially symmetrical annular element with
2D thermal conductivity. The element has four nodes with temperature-independent de-
grees at which each element node is applicable to thermal analysis of 2D, steady state, or
transient [39]. Figure 12 represents the element of PLANE55.
Figure 12. Characteristics of the PLANE55 element.
Figure 13 shows a simplified 2D beam section for heat transfer analysis. Three sides
of the beam were directly exposed to fire and one was unexposed. Based on the assump-
tion that the reinforcements did not significantly influence the temperature distribution
in the beam section, they were not included in the 2D FE model [40,41]. In this study, the
carbonate aggregate concrete was assumed. The thermal properties were needed in the
K
J
I
L
ts
X (or radial)
Y (or axial)
12
3
4
Figure 11. Failure of beams subject to fire.
3.5. 2D Thermal Analysis
The finite element model for temperature analysis was conducted using a 2D finite
element analysis using the ANSYS program. The concrete element was modeled using
PLANE55 [
39
]. It is a planar element or is an axially symmetrical annular element with
2D thermal conductivity. The element has four nodes with temperature-independent
degrees at which each element node is applicable to thermal analysis of 2D, steady state, or
transient [39]. Figure 12 represents the element of PLANE55.
Buildings 2023, 13, x FOR PEER REVIEW 12 of 20
Figure 11. Failure of beams subject to fire.
3.5. 2D Thermal Analysis
The finite element model for temperature analysis was conducted using a 2D finite
element analysis using the ANSYS program. The concrete element was modeled using
PLANE55 [39]. It is a planar element or is an axially symmetrical annular element with
2D thermal conductivity. The element has four nodes with temperature-independent de-
grees at which each element node is applicable to thermal analysis of 2D, steady state, or
transient [39]. Figure 12 represents the element of PLANE55.
Figure 12. Characteristics of the PLANE55 element.
Figure 13 shows a simplified 2D beam section for heat transfer analysis. Three sides
of the beam were directly exposed to fire and one was unexposed. Based on the assump-
tion that the reinforcements did not significantly influence the temperature distribution
in the beam section, they were not included in the 2D FE model [40,41]. In this study, the
carbonate aggregate concrete was assumed. The thermal properties were needed in the
K
J
I
L
ts
X (or radial)
Y (or axial)
12
3
4
Figure 12. Characteristics of the PLANE55 element.
Figure 13 shows a simplified 2D beam section for heat transfer analysis. Three sides of
the beam were directly exposed to fire and one was unexposed. Based on the assumption
Buildings 2023,13, 609 12 of 19
that the reinforcements did not significantly influence the temperature distribution in
the beam section, they were not included in the 2D FE model [
40
,
41
]. In this study, the
carbonate aggregate concrete was assumed. The thermal properties were needed in the
thermal analysis. In this study, the thermal properties of concrete were proposed by
Eurocode2 [41] and included thermal conductivity, specific heat, and density.
Buildings 2023, 13, x FOR PEER REVIEW 13 of 20
thermal analysis. In this study, the thermal properties of concrete were proposed by Eu-
rocode2 [41] and included thermal conductivity, specific heat, and density.
Figure 13. Concrete beam, 2D (units in millimeters).
The thermal conductivity (
k
) properties of normal strength concrete are proposed
to be between the upper limit and the lower limit in the temperature range 20–1200 °C
since concrete is a composite material, as shown in Equation (1):
2
2
2.0 0.2451 0.0107 for upper limit
100 100
1.36 0.136 0.0057 for lower limit
100 100
cc
cc
TT
k
TT
−+
=
−+
(1)
The specific heat of concrete (
c
c
) normally varies with the moisture content in the
concrete and the temperature. For concrete in a dry state (moisture 0%), the temperature
range of 20–1200 °C is proposed as shown in Equation (2):
()
()
= 900 for 20 C 100 C
= 900+ 100 for 100 C < 200 C
200
= 1000+ for 200 C < 400 C
2
= 1100 for 400 C 1200 C
cc
cc c
c
cc
cc
cT
cT T
T
cT
cT
°≤ ≤ °
−°≤°
−°≤°
°< ≤ °
(2)
The density of concrete (
c
p
) varies with temperature, with a decreasing value due
to internal water loss of the concrete as shown in Equation (3). In this work, the concrete
density of 2400 kg/m
3
is used.
Figure 13. Concrete beam, 2D (units in millimeters).
The thermal conductivity (
k
) properties of normal strength concrete are proposed to
be between the upper limit and the lower limit in the temperature range 20–1200
◦
C since
concrete is a composite material, as shown in Equation (1):
k=
2.0 −0.2451Tc
100 +0.0107Tc
100 2for upper limit
1.36 −0.136Tc
100 +0.0057Tc
100 2for lower limit
(1)
The specific heat of concrete (
cc
) normally varies with the moisture content in the
concrete and the temperature. For concrete in a dry state (moisture 0%), the temperature
range of 20–1200 ◦C is proposed as shown in Equation (2):
cc=900 for 20 ◦C≤Tc≤100 ◦C
cc=900+(Tc−100)for 100 ◦C<Tc≤200 ◦C
cc=1000+(Tc−200)
2for 200 ◦C<Tc≤400 ◦C
cc=1100 for 400 ◦C<Tc≤1200 ◦C
(2)
Buildings 2023,13, 609 13 of 19
The density of concrete (
pc
) varies with temperature, with a decreasing value due to
internal water loss of the concrete as shown in Equation (3). In this work, the concrete
density of 2400 kg/m3is used.
pc=pc,RT for 20 ◦C≤Tc≤115 ◦C
pc=pc,RT1−0.02 Tc−115
85 for 115 ◦C<Tc≤200◦C
pc=pc,RT0.98 −0.03 Tc−200
200 for 200 ◦C<Tc≤400 ◦C
pc=pc,RT0.98 −0.03 Tc−200
200 for 400 ◦C<Tc≤1200 ◦C
(3)
An average furnace temperature was applied as convection on lines (section sides)
with convection film coefficient values of 25 W/m
2·
K for exposed surface and 9 W/m
2·
K
for unexposed surface [41–44].
3.6. Results and Discussion
The comparison of the temperature versus time relationship between the tests and 2D
FEM simulations is shown in Figure 14a,b, Figure 15a,b, and Figure 16a,b. Generally, the
results indicate that the 2D FE analysis can predict the temperature development of the
components in the beams along the time axis. However, the discrepancy between the test
curves and numerical curves remains, due to the assumption of the 2D model for temporal
transfer. In addition, the FEM prediction has a lower temperature distribution inside the
cross-section than the temperature distribution obtained from the tests. This is because the
prediction did not consider the effect of the cracks during the temperature heating.
Buildings 2023, 13, x FOR PEER REVIEW 14 of 20
,
,
,
,
= for 20 C 115 C
115
= 1 0.02 for 115 C < 200 C
85
200
= 0.98 0.03 for 200 C < 400 C
200
= 0.98 0.
ccRT c
c
ccRT c
c
ccRT c
ccRT
pp T
T
pp T
T
pp T
pp
°≤ ≤ °
−
−°≤°
−
−°≤°
−200
03 for 400 C 1200 C
200
c
c
TT
−
°< ≤ °
(3)
An average furnace temperature was applied as convection on lines (section sides)
with convection film coefficient values of 25 W/m
2
·K for exposed surface and 9 W/m
2
·K
for unexposed surface [41–44].
3.6. Results and Discussion
The comparison of the temperature versus time relationship between the tests and
2D FEM simulations is shown in Figures 14a,b, 15a,b, and 16a,b. Generally, the results
indicate that the 2D FE analysis can predict the temperature development of the compo-
nents in the beams along the time axis. However, the discrepancy between the test curves
and numerical curves remains, due to the assumption of the 2D model for temporal trans-
fer. In addition, the FEM prediction has a lower temperature distribution inside the cross-
section than the temperature distribution obtained from the tests. This is because the pre-
diction did not consider the effect of the cracks during the temperature heating.
(a)
Figure 14. Cont.
Buildings 2023,13, 609 14 of 19
Buildings 2023, 13, x FOR PEER REVIEW 15 of 20
(b)
Figure 14. Comparison between prediction and testing of RC12 beams. (a) Reinforcements; (b) con-
crete.
(a)
Figure 14.
Comparison between prediction and testing of RC12 beams. (
a
) Reinforcements;
(b) concrete.
Buildings 2023, 13, x FOR PEER REVIEW 15 of 20
(b)
Figure 14. Comparison between prediction and testing of RC12 beams. (a) Reinforcements; (b) con-
crete.
(a)
Figure 15. Cont.
Buildings 2023,13, 609 15 of 19
Buildings 2023, 13, x FOR PEER REVIEW 16 of 20
(b)
Figure 15. Comparison between prediction and test of the BF12 beam. (a) Reinforcements; (b) con-
crete.
(a)
Figure 15.
Comparison between prediction and test of the BF12 beam. (
a
) Reinforcements;
(b) concrete.
Buildings 2023, 13, x FOR PEER REVIEW 16 of 20
(b)
Figure 15. Comparison between prediction and test of the BF12 beam. (a) Reinforcements; (b) con-
crete.
(a)
Figure 16. Cont.
Buildings 2023,13, 609 16 of 19
Buildings 2023, 13, x FOR PEER REVIEW 17 of 20
(b)
Figure 16. Comparison between prediction and test of the BF20 beam. (a) Reinforcements; (b) con-
crete.
In Figures 14a, 15a, and 16a, both experimental and numerical results indicate that
the temperature duration at the bottom reinforcement was larger than that at the top re-
inforcement due to the heat transfer scheme. As can be seen in Figures 14, 15, and 16,
similar to the experimental observation, the FE analysis demonstrates that the concrete
soffit had higher temperature distribution than the steel reinforcement. Furthermore, the
2D FEM simulation is suitable to assess the temperature distributed in concrete rather
than to predict the temperature distribution in the reinforcement. Conversely, the temper-
ature measurements at different longitudinal sections of the beams were different because
of the crack effect. This phenomenon could not be predicted by the 2D FEM. Therefore, in
future works, the 3D FEM simulation is recommended to reflect the actual behavior of the
test beams.
4. Conclusions
This study provides a valuable contribution to the state of the art by presenting new
findings related to GFRP-RC beams exposed to fire. To explore the new findings, the con-
crete beams reinforced with steel or GFRP bars subjected to fire exposure were experi-
mentally and numerically investigated against the standard ISO 834 fire curve. The ther-
mal behavior of the test beams was examined, while the 2D FEM was used to predict the
temperature distributions on the beam sections. The main new findings obtained from the
present study can be summarized, as follows:
1. The temperature in the bending parts of the steel-RC beam was lower than that of the
GFRP-RC beam. The average fire resistance rates of the steel-RC beam and the GFRP-
RC beam were 83 min and 33–36.4 min, respectively. The critical temperatures meas-
ured at the steel rebar and at the GFRP rods were 593 °C and 300–330 °C, respectively.
This means that the steel-RC beam had greater fire resistance than the GFRP-RC
beam, and all beams failed due to the fire exposure less than the resistance time of 2
h.
2. The fire resistance of the beam reinforced with GFRP bars of 20 mm diameter (BF20)
was better than that of the beam reinforced with GFRP bars of 12 mm diameter
(BF12). The fire durations of the beams BF12 and BF20 were similar with the range of
Figure 16.
Comparison between prediction and test of the BF20 beam. (
a
) Reinforcements;
(b) concrete.
In Figures 14a–16a, both experimental and numerical results indicate that the tempera-
ture duration at the bottom reinforcement was larger than that at the top reinforcement due
to the heat transfer scheme. As can be seen in Figures 14–16, similar to the experimental
observation, the FE analysis demonstrates that the concrete soffit had higher temperature
distribution than the steel reinforcement. Furthermore, the 2D FEM simulation is suitable
to assess the temperature distributed in concrete rather than to predict the temperature
distribution in the reinforcement. Conversely, the temperature measurements at different
longitudinal sections of the beams were different because of the crack effect. This phe-
nomenon could not be predicted by the 2D FEM. Therefore, in future works, the 3D FEM
simulation is recommended to reflect the actual behavior of the test beams.
4. Conclusions
This study provides a valuable contribution to the state of the art by presenting
new findings related to GFRP-RC beams exposed to fire. To explore the new findings,
the concrete beams reinforced with steel or GFRP bars subjected to fire exposure were
experimentally and numerically investigated against the standard ISO 834 fire curve. The
thermal behavior of the test beams was examined, while the 2D FEM was used to predict
the temperature distributions on the beam sections. The main new findings obtained from
the present study can be summarized, as follows:
1.
The temperature in the bending parts of the steel-RC beam was lower than that of
the GFRP-RC beam. The average fire resistance rates of the steel-RC beam and the
GFRP-RC beam were 83 min and 33–36.4 min, respectively. The critical temperatures
measured at the steel rebar and at the GFRP rods were 593
◦
C and 300–330
◦
C,
respectively. This means that the steel-RC beam had greater fire resistance than the
GFRP-RC beam, and all beams failed due to the fire exposure less than the resistance
time of 2 h.
2.
The fire resistance of the beam reinforced with GFRP bars of 20 mm diameter (BF20)
was better than that of the beam reinforced with GFRP bars of 12 mm diameter
(BF12). The fire durations of the beams BF12 and BF20 were similar with the range of
31.8–46.4 min.
It was found that the increase of the GFRP bar diameter for reinforcing
the beams slightly enhanced fire resistance.
Buildings 2023,13, 609 17 of 19
3.
The deflection of the GFRP-RC beams was larger than that of the steel-RC beam
due to the small elastic modulus of GFRP bars. The FEM simulation is an effective
package for modeling the beams reinforced with GFRP and steel bars under the fire
condition. The numerical prediction had a lower temperature distribution inside the
cross-sections of the beams than that of the experimental measurements.
Author Contributions:
Conceptualization, C.T., N.P. (Natthanuch Poonpan) and S.K.; methodology,
C.T., L.V.H.B., and N.P. (Natcha Phudtisarigorn); validation, N.P. (Natthanuch Poonpan) and N.P.
(Natcha Phudtisarigorn); formal analysis, C.T., N.P. (Natthanuch Poonpan). and N.P. (Natcha Phudti-
sarigorn); investigation, C.T. and L.V.H.B.; resources, P.T.N. and S.M.; data curation, P.T.N. and S.M.;
writing—original draft preparation, N.P. (Natthanuch Poonpan). and N.P. (Natcha Phudtisarigorn);
writing—review and editing, C.T. and L.V.H.B.; visualization, C.T. and L.V.H.B.; supervision, P.T.N.
and S.K.; project administration, C.T. and L.V.H.B.; funding acquisition, C.T., L.V.H.B., P.T.N., S.K.
and S.M. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments:
This study was supported by Thammasat University Research Fund, Contract
No. TUFT 51/2564. This research was also supported by Thammasat University Research Unit in
Structural and Foundation Engineering, Thammasat University and by the Thailand Science Research
and Innovation Fundamental Fund fiscal year 2023.
Conflicts of Interest: The authors declare no conflict of interest.
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