Article

AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 13 (2020) 136–143

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Al-Qadisiyah Journal for Engineering Sciences

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* Corresponding author.

E-mail address: Haitham.althairy@qu.edu.iq (Haitham. Al-Thairy)

https://doi.org/10.30772/qjes.v13i2.635

Numerical Study on the Performance of GFRP RC Beams Exposed to

High Temperature

Nuha Hussien a, Haitham Al-Thairy a,*

a Civil Eng. Depart. / College of Eng./ University of Al-Qadisiyah, Iraq

A R T I C L E I N F O

Article history:

Received 03 April 2020

Received in revised form 22 May 2020

Accepted 15 June 2020

Keywords:

Numerical simulation

GFRP-RC beams

High temperature

ABAQUS

Heating rate

A B S T R A C T

This paper presents and validates a numerical model utilizing the nonlinear finite element software

ABAQUS/Standard to simulate the performance and failure of Glass Fibre Reinforced Polymers reinforced

concrete beams under high temperature. A numerical model was firstly developed by selecting the proper

geometrical and material modeling parameters with suitable analysis procedures available in

ABAQUS/Standard. The developed numerical model was verified by comparing numerical results with the

corresponding results of the experimental test extracted from the current study on Glass Fibre reinforced

concrete beams under different elevated temperatures ranges from (20 to 600ºC). Validation results have

indicated the accuracy of the suggested numerical model. The validated numerical model was implemented

to investigate the effect of important parameters on the performance and maximum load of Glass Fibre

reinforced concrete beams under different elevated temperatures that are not considered in the current

experimental tests. These parameters include the effect of exposure time or time-temperature history and

effect of temperature distribution around the beams

1. Introduction

In building design, fire endurance is one of the major safety requirements.

High-temperature effect on structures which are reinforced with FRP bars

considered one of the more important factors of the design, although this

factor has limited attention from the researchers with a limited number of

research studies. However, the deterioration in the mechanical properties of

FRP bars was investigated by several researches. Saafi [1] has established

a general equation for the degradation in the mechanical properties of FRP

material under different elevated temperatures. Saafi’s equations have been

used by a number of researches in the numerical simulation of FRP

bars[2][3] (see Figure 1).

Abbasi and Hogg [4] suggested two methods for predicting the effect of

elevated temperature on some properties of glass fibre reinforced concrete

beams. The two method was the suggested semi-empirical equation and FE

model based on experimental studies conducted by Sakashita et al. [5] and

Lin et al.[6] and Al-Baghdadi [7]. All beams were reinforced by FRP bars

at the tension zone and exposed to ISO-834 standard fire curve [8].A whole

concrete section with homogenous material was considered in finite

element model .While the thermal properties of concrete were taken as

proposed by Abbasi [9] assuming no concrete cracking during the fire test.

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https://doi.org/10.30772/qjes.v13i

NUHA HUSSIEN , HAITHAM AL-THAIRY /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 13 (2020) 136–143 137

In addition, GFRP RC beam with dimensions of (350mm width × 400mm

height) and overall length of 4250 mm and concrete cover of 70mm was

designed and simulated by using FE model. Comparison of time-

temperature curves between the FE model and semi-empirical equation has

shown that the FE model underestimates the time-temperature curve while

the semi-empirical equation shows good agreements.

a) Tensile strength

b) Elastic modulus

Figure 1 : Reduction factor of tensile strength and elastic modulus of

various type of FRP bars [1]

Rafi et al. [10] have carried out finite element simulations of RC beams

reinforced with CFRP bars under elevated temperature. The concrete was

assumed to be isotropic and thermal conductivity and coefficient of thermal

expansion was taken according to Eurocode 2[11] along with the

mechanical properties of concrete at elevated temperature . In addition, the

coefficient of thermal expansion of CFRP bars exposed to elevated

temperature was assumed constant according to ACI440.1R-15 [12] while

the mechanical properties of CFRP-bars under elevated temperature were

taken from the equations suggested by Saafi [1]. However, numerical

simulation results have shown large deviation from the experimental results.

Yu and Kodur [3] presented finite element analysis to evaluate the fire

performance of concrete beam reinforced by FRP and steel bars. According

to Eurocode 2[11] , ASCE standard [13] and insulation layout suggested by

Bisby [14], heat transfer analysis and relevant high-temperature thermal

properties of concrete was applied to establish temperature distribution over

beam cross-section. Both specific heat and thermal conductivity of steel and

FRP bars were not considered in the analysis due to the neglected effect of

these properties on temperature distribution as suggested by Lie [15]. GFRP

bars were analyzed according to models suggested by previously studies

Saafi [1], Wang and Kodur [16], Bisby [14], FIB bulletin[17] and ACI

440.1R-15[12], which proposed a linear stress-strain curve up to failure and

a reduction of tensile strength about 25% of GFRP bars under a temperature

of 400 °C and 100% under a temperature of 1000 °C . Results of the

numerical model have shown good agreement with measured data by Rafi

and Nadjai [18] and Abbasi & Hogg [19]. The main conclusion extracted

from this Yu and Kodur [3] study is that the load capacity of FRP-RC beams

is less than the load capacity of Steel-RC beams under exposed to high

temperature. Parametric studies were conducted and indicate a notable

effect regarding the increasing of concrete cover thickness on the fire

capacity of FRP-reinforced concrete beams. Also, the existing of axial

restrain as well as provided FRP beams with fire endurances material reflect

efficiency in increasing fire capacity.

The above-mentioned research studies have reflected the ability of finite

element package ABAQUS in simulating the behavior and ultimate load of

FRP reinforced concrete beams under exposure to elevated temperature

with a very good accuracy .In addition, the stress-strain relationship of

normal weight concrete at high temperature proposed by Eurocode

2[11]and Eurocode 4[20]was successfully used to predict the degradation

of the concrete strength under high temperature. However, some of

previous numerical models have not considered the degradation of FRP

mechanical properties exposed to high temperature in there numerical

simulations [21]. Saafi’ equations was used in the numerical simulations of

current numerical study [1] (see Figure 1 ). Therefore, the main object of

the current study is to develop a more accurate numerical model to simulate

the performance of GFRP-RC beams exposed to high temperature using the

FE software ABAQUS/Standard.

2. Numerical model

2.1. Material modelling

Files must be in MS Word only and should be formatted for direct printing,

using the CRC MS Word provided. Figures and tables should be embedded

and not supplied separately. Concrete damage plasticity model (CDPM)

available within ABAQUS/Standard was utilized in current model to

simulate the performance and failure of the concrete material under ambient

and elevated temperature. Further, the concrete crushing and concrete

cracking are assumed as a basic failure criteria for concrete with

compression and tension deterioration of material response being taken into

consideration .Knowing that concrete was modelled as homogenous

material. To simulate the plastic behaviour of concrete, the CDPM in

Nomenclature

ACI American Concrete Institute

ASTM American Society for Testing and Materials
C3D8R Eight-node brick element with reduced integration

DC1D2 Two-node link heat transfer truss element

DC3D8 Eight-node liner heat transfer brick element
FEM Finite Element Modeling

FRP Fiber reinforced polymer

GFRP Glass fiber reinforced polymer

GF Fracture energy
RC Reinforced Concrete

T3D2 Two-node truss element

W1 Crack opinions at 0.15 of ultimate stress
WO Crack opinions at ultimate stress

138 NUHA HUSSIEN , HAITHAM AL-THAIRY /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 13 (2020) 136–143

ABAQUS uses several parameters some of which are obtained from

experimental tests. In the current model, the required plasticity parameters

for the concrete materials are taken as assumed in ABAQUS and shown in

Table 1.

Table 1 : Plasticity parameter used in current

numerical model

Parameters Assumed value Parameters

Dilation angle Ψ 40 Dilation angle Ψ

Potential eccentricity ϵ 0.1 Potential

eccentricity ϵ

fb0/fc0 1.16 fb0/fc0

The mathematical model of compressive stress-strain relationship of

concrete that suggests by Eurocode 4 was adopted in current numerical

model for ambient and elevated temperature. Figures 2 shows graphical

representations of the compressive behaviour of concrete at ambient and

elevated temperatures according to Eurocode 4 [20]. To account for the

reduction in the mechanical properties of concrete due to elevated

temperature, compressive strength reduction factors obtained from the

experimental tests conducted in current study was used as shown in Figure

Figure 2: Stress - strain relationships of concrete at high temperature

used in the numerical model of current study

For concrete under ambient and elevated temperature, tension stiffening

was simulated in the numerical model through using fracture energy (Gf)

(see Figure 3) .The value of Gf depending on the maximum aggregate size

and the required concrete strength as presented in Table 3. Further,

reduction factors of concrete tensile strengths results from the experimental

tests was used in current model (see Table 2).

Table 2 : Results of tensile strength (ft) concrete cylinders under

different elevated temperatures

Temp. ,˚C 𝐟𝐭𝐞𝐱𝐩.. , MPa 𝐊𝐭 Redaction factor

20˚C 2.4 /

300˚C 1.9 20%

400˚C 1.6 33.4%

500˚C 1.4 41%

600˚C 0.9 62.5%

Figure 3: Bilinear softening tension of normal weight concrete used in

current model (Hillerborg, 1985)

Table 3 : Values of Gf for different aggregate sizes and concrete

strengths

𝑫𝒎𝒂𝒙.,
𝒎𝒎

Gf (N/m)

C12 C20 C30 C40 C50 C60 C70 C80

8 40 50 65 70 85 95 105 115

16 50 60 75 90 105 115 125 135

32 60 80 95 115 130 145 160 175

The crack initiation and propagation of concrete crack at ambient and

elevated temperature was simulate using the tension damage parameters (dt)

which determined from equation 1 suggested by ABAQIUS (Hibbitt, 2008)

as follows:

𝐝𝐭 = 𝟏 −
𝝈𝒕

ƒ𝒄𝒕𝒎
(1)

On the other hand, the mathematical model of the stress-strain relationship

of the reinforcing steel bars proposed by Eurocode 4 [20] was used in

modelling the behaviour of the steel bars at ambient and elevated

temperature. The modulus of elasticity and yielding stress of steel bars at

ambient temperature were taken as 414 MPa and 2×105MPa respectively

according to experimental tests. The reduction factors of reinforcing steel

bars properties (yield strength, proportional limit and young modulus) at

elevated temperature were taken from Eurocode 4 .

2.1 Material modeling of GFRP bars at ambient and elevated

temperature

The mechanical properties of GFRP reinforcement bars provided by the

manufactured company was used in the numerical model at ambient

temperature. However, the reduction factors of the ultimate tensile strength

and of elastic modulus of GFRP bars proposed by Saafi [1] was used in the

numerical model to simulate the degradation of the mechanical

characteristics of the GFRP bars at elevated temperature .Also ,the

reduction factors of the specific heat ,density and thermal conductivity of

GFRP bars proposed by Kodur et al. [3] was used in the numerical model

as indicated in appendix A. Knowing that bar element was used to model

the reinforcement with a 120 linear line element of type T3D2.

NUHA HUSSIEN , HAITHAM AL-THAIRY /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 13 (2020) 136–143 139

2.2 Geometrical modelling parameters

Three-dimensional eight-nodes solid with reduced integration hourglass

control and reduced integration(C3D8R) available in ABAQUS/standard

was used in current study to model the GFRP-RC beams, bearing plates as

well as supporting plates during strucural analysis (see Figure 4-a). While,

the eight-node solid linear with a temperature degree of freedom( DC3D8)

was used in modeling the GFRP-RC beams, bearing plates and supporting

plates in the heat transfer analysis. Furtheramore three-dimensional two

nodes linear displacement truss element (T3D2) and a two nodes heat

transfer link element (DC1D2) with a temperature degree of freedom were

used in modeling of reinforcing GFRP and steel bars during structural and

thermnal analyses respectively .Table 4 and Figure 4 show elements used

in structural analysis and thermal analysis of current study.

Table 4: Element used in current numerical model

Structural analysis Heat transfer analysis

Part
Type of

element
Part

Type of

element

RC beams C3D8R RC beams DC3D8

bearing plates, and

supporting plates
C3D8R

bearing plates, and

supporting plates
DC3D8

GFRP and Steel

bars
T3D2

GFRP and Steel

bars
DC1D2

a ) C3D8R and DC3D8 elements b) T3D2 and DC1D2 elements

Figure 4 : Types of elements used in the current numerical model

2.3 Assembly of the numerical model

The assembly of all parts of GFRP-RC beams used by the present numerical

model is shown in Figure 5. In order to preclude intensity of the stress at

the supports and loading points, steel plates with dimensions of

(150×80×10) mm and (150×100×10) mm (length ×width× depth) have

been used as supporting and bearing plates respectively. To simulate the

same boundary condition used in the experimental tests, displacement

/rotation type of the boundary condition option available in ABAQUS was

used. Further, the embedded region interaction option was used to simulate

the interaction between reinforcement bars and concrete beams. Surface to

surface tie interaction was used in simulating the relationship between

GFRP-RC beams and bearing and supporting plates using the constraint

option available in ABAQUS. Figure 5 show the assembled parts of GFRP-

RC beams.

a) GFRP-RC beam system

b) Reinforcement system

Figure 5: Schematic representation for the assembled parts of GFRP-

RC beams

2.4 Analysis procedure

The sequential thermal-structural analysis procedure in

ABAQUS/Standards was used in the current study to analyse the GFRP-

RC beams exposed to elevated temperature followed by a static load. This

analysis procedure includes two stages as follows:

2.4.1 Heat transfer analysis

In heat transfer analysis, the temperature was applied on a selected set of

concrete external surface as a boundary condition in the heat transfer

analysis step using the corresponding time –temperature history obtained

from experimental tests (see Figure 6).The “Amplitude” option available

in ABAQUS/standard was used to specify time–temperature histories from

experimental tests.

2.4.2 Structural analysis

After completing the heat transfer analysis, structural analysis is initiated to

obtain the structural performance of GFRP- RC beams after exposure to

elevated temperature. A displacement controlled structural analysis was

conducted by applying a lateral displacement as a boundary condition in

the center of GFRP-RC beams using boundary condition option in

ABAQUS

Bearing plate

Supporting plate

RC beams

GFRP bars

Stirrup

140 NUHA HUSSIEN , HAITHAM AL-THAIRY /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 13 (2020) 136–143

a ) T=350˚C

b) T=500˚C

c ) T=600˚C

Figure 6: Time-temperature histories of experimental tested GFRP-

RC beams under different temperatures

3. Validation of the numerical model

To ensure the reliability of the developed numerical model, experimental

tests were carried out in this study on RC beams subjected to elevated

temperature followed by concentrated static load at the beam’s mid-

span .The numerical simulation results obtained from the suggested model

will be compared with the experimental tests results. The section below

describes the experimental tests used in the validation of current numerical

model.

3.1. Experimental test

Four GFRP-RC beams with cross-sectional dimensions of 250mm ×160

mm (width × height) and a total length of 1250 mm were designed

according to ACI440.1R-15[12](see figure 7). All GFRP-RC beam

specimens are made from normal weight concrete with an average

compressive strength of 46 MPa .One beam specimens was tested at

ambient temperature and the other three beams were firstly exposed to

elevated temperatures of 350°C, 500°C and 600°C using electrical furnace

then subjected to a monolithically increased load up to failure.

The internal dimensions of the furnace are 130×300×290 mm (length

×depth ×height) with maximum temperature of 900 °C was used to heat up

the RC beam specimens .The average heating rate of the furnace is about

10°C /min and decrease gradually to the end of the heating process. An

external thermocouple was attached to the specimens through a small hole

in one long side of the furnace. Two thermocouples were used to measure

and record the temperatures-time history at the concrete and at the GFRP

bars. In addition, Figure 8 show set up of electric furnace.

Figure 7: Dimension and rienforement details of RC-beams used in

current study

(a)Electric furnace during thermal test (b) Thermally tested RC-

beam

Figure 8 : Set up of the electric furnace

All GFRP-RC beams were tested using the universal testing machine

obtained at the structural laboratory of College of Engineering/ The

University of Al-Qadisiyah. Deflection at mid span corresponding to each

load increment during the structural tests was monitored and recorded using

a linear variable displacement transducer (LVDT) with a maximum

capacity of 50 mm.

3.2. Validation of the numerical model against thermal tests

The results obtained from heat transfer analysis were presented in terms of

temperature - time- histories at the surface of main reinforcement GFRP

bars and compared with temperature - time- histories of corresponding

experimental thermal tests as shown in Figure 9 .The comparisons show

reasonable agreement between the numerical simulation results and the

experimental tests results especially at maximum temperature transferred

from concrete to the GFRP bars. This agreement indicates the accuracy of

the suggested model in capturing the temperature distribution inside the

concrete cross-section. In addition, Figure 9 clearly shows that as the

distance from concrete surface increased the corresponding temperature is

considerably decreases.

NUHA HUSSIEN , HAITHAM AL-THAIRY /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 13 (2020) 136–143 141

a) Under 350 ºC

b) Under 500 ºC

c) Under 600 ºC

Figure 9 : Temperature- time histories at the surface GFRP bars of

the tested beam

3.3. Validation the numerical model against the structural tests

The results of numerical model in terms of ultimate load-lateral

displacement relationships of GFRP-RC beams were compared with the

corresponding experimental load-displacement relationships .Figures (10-

a-d) illustrate the comparison for GFRP RC beams. Good agreement can

be seen between the two sets of results. This agreement confirms that the

proposed numerical model have the ability to estimate and predict the

response and maximum load of GFRP- RC beam under different elevated

temperatures up to failure. Numerical simulation results reveal that the

maximum load of GFRP-RC beams was found to be decreased by 1%, 8%,

and 9% compared to the maximum load of control beam under exposure to

a temperature of 350, 500, and 600 ºC respectively (see Figure 10 and Table

5).Furthermore, Table 5 shows comparison of failure load between

numerical model and experimental tests Comparison indicated very

reasonable agreement can be seen between the two results.

a) NC

b) N350

c) N500

d) N600

Figure 10 : Numerical versus experimental load-displacement

relationship of GFRP-RC beams exposed to different elevated

temperatures.

142 NUHA HUSSIEN , HAITHAM AL-THAIRY /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 13 (2020) 136–143

Table 5 : Comparison of failure load between numerical model

(Abaquse) results and conducted experimental results of GFRP-RC

beams under exposure to high temperature.

Beam

designation

Ultimate load(kN)-

Experimental

Ultimate load(kN)

Abaquse

% Difference between

exp. and num. results

NC 174 170 2%

N350 167 170 1.7%

N500 147 155 5%

N600 141 154 8.4

4. Parametric study

The validated numerical model was utilized to carry out a parametric

investigation to demonstrate the effect of two important parameters on the

performance GFRP reinforced concrete beams exposed to an elevated

temperature which have not addressed in the current experimental study

these parameters as follows:

(a) Effect of temperature - time history.
(b) Effect of temperature distribution around the cross-section of the

GFRP-RC beams.

4.1. Effect of temperature distribution around GFRP-RC beams

A temperature value of 600 ºC was considered in the current parametric

study with three cases of temperature distribution: one side temperature

exposure at the top layer of the section, one side temperature exposure at

the bottom layer of the section and three sides temperature exposure in

addition to the four sides exposure which is previously analyzed as shown

in Figure 11. Load -displacement relationships of the beam N600 under

above selected cases of thermal exposure were shown in Figure 12. Top

section, bottom section and three sides’ temperature exposure results

revealed increasing in maximum load by 11.5%, 13% and 13% respectively

compared to maximum load of the beam N600 obtained from experimental

test. The reason for this increase may be attributed to the resistance of

unheated and undamaged layer of beam. The un-heated and un-damaged

layer of the beam can resist more loading capacity than all heated beam

layers due to un-deterioration of mechanical characteristic of unheated

concrete and reinforcement bars.

a) Top layer exposure b) Bottom layer exposure

c) Three side exposure d) all side exposure

Figure 11 : Temperature distribution around GFRP-

RC beam

Figure 12 : Load -displacement relationships of N600 beam under

different temperature distribution around beams

4.2. Effect of temperature distribution along the GFRP-RC beams

The investigated beam length was as follows: half-length exposed and

three-quarter length exposed in addition to the full length exposure which

is previously analysed of N600 beam as shown in Figure 13.The load-

displacement curves are shown in Figure 14. The results of half exposed

length shows increasing in ultimate load capacity by 7% compared to

ultimate load capacity of full length exposure While, the results of three-

quarter length exposed shows very little decreased in ultimate load capacity

by about 1% compared to ultimate load capacity of full length exposure

case of N600 beam obtained in experimental study (see Figure 14). This

difference in the ultimate load capacity is due to the influence of high-

temperature exposure on the mechanical characteristic of the exposed

length of GFRP-RC beam.

a) Half-length exposure b) Three-quarter length exposure

c ) Total length exposure

Figure 13 : Temperature distribution along GFRP-RC beam

Figure 14 : Load -displacement relationships of N600 beam under

different temperature distribution along the beams.

NUHA HUSSIEN , HAITHAM AL-THAIRY /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 13 (2020) 136–143 143

5. Conclusions

The main conclusions and findings extracted from the numerical

simulations carried out in this study were summarized as follows:

1. The finite element software ABAQUS/Standard can reasonably

predict the performance and ultimate load of GFRP-RC beams under

different elevated temperatures if suitable geometrical, thermal

dependent material and mechanical properties are selected and

implemented correctly.

2. The distribution of the temperature around the GFRP-RC beam has a

considerable effect on ultimate load of GFRP-RC beam. At the

temperature of 600°C, when the temperature is distributed above one

side (top section) , one side (bottom section) and three sides ,ultimate

load capacity increased by 11.5%, 13% and 13% respectively

3. The distribution of the temperature over the length of GFRP

reinforced concrete beam has reflected a slight effect on the maximum

capacity of GFRP-RC beams. At temperature value of 600°C, the

numerical results of the half temperature exposed length shows

increasing in ultimate load by 7% compare with maximum load of

full-length temperature exposure. While, the results of three-quarter

length exposed shows very little decreased in ultimate load capacity

by 1 %.

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user’s manual. Vol. I-III, Version 6.8. Pawtucket, 2008.

Appendix A

Table A.1: Reduction factor of mechanical properties of GFRP bars

T
e
n

si
le

S
tr

e
n
g

th
[1

]

𝑘𝑓=
𝑓𝑓𝑢,𝑇

𝑓𝑓𝑢,20°C
, (T in °C)

𝑘𝑓=1-0.0025 T for 0≤ 𝑇 ≤ 400

𝑘𝑓=0 T for 400 ≤ 𝑇

M
o
d
u

lu
s

o
f

E
la

st
ic

it
y

[1
]

𝑘𝐸 =
𝐸𝑇

𝐸20°C
, (T in °C)

𝑘𝑓𝐸=1 for 0≤ 𝑇 ≤ 100

𝑘𝑓𝐸=1.175-0.00175 T for 110≤ 𝑇 ≤ 300

𝑘𝑓𝐸= 1.625-0.00325 T for 300≤ 𝑇 ≤ 500

𝑘𝐸 =0 for 500 ≤ 𝑇

S
p

e
c
if

ic
H

e
a
t

[3
]

C=(1.25+
0.95

325
Ƭ) × 103

For 20℃ ≤ Ƭ ≥ 325℃

T
h

e
rm

a
l

C
o
n
d

u
c
ti

v
it

y
[

3
]

𝐾𝑓,Ƭ = 1.4 +
−1.1

500
Ƭ

For 20℃ ≤ Ƭ ≥ 500℃

D
e
n

si
ty

[
3

]

ρ= ρ(20℃)= 𝑅𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑑𝑒𝑛𝑠𝑖𝑡𝑦

For 20℃ ≤ Ƭ ≥ 510℃