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Long-term bond performance of GFRP bars in concrete under temperature

ranging from 20 C to 80 C

Radhouane Masmoudi a,*, Abdelmonem Masmoudi b, Mongi Ben Ouezdou b, Atef Daoud b

a Department of Civil Engineering, Faculty of Engineering, Sherbrooke of University, QC, Canada J1K 2R1b Civil Engineering Laboratory, National Engineering School of Tunis, Tunisia

a r t i c l e i n f o

Article history:

Received 28 March 2009

Received in revised form 8 December 2009

Accepted 17 December 2009

Available online 9 February 2010

Keywords:

Bond

Concrete

FRP bars

Pull-out testing

Bond–slip modelling

Temperature effect

a b s t r a c t

Eightypull-out specimens were used to study the effect of temperature ranging from 20 Cto80 C indry

environment on bond properties between Glass Fiber Reinforced Polymer (GFRP) bars and concrete. The

pullout-test specimens were subjected during 4 and 8 months to high temperatures up to 80 C and then

compared to untreated specimens (20 C). Experimental results showed no significant reduction on bond

strength for temperatures up to 60 C. However, a maximum of 14% reduction of the bond strength was

observed for 80 C temperature after 8 months of thermal loading. For treated specimens, the coefficient

b in the CMR model, which predicts the bond–stress–displacement behavior, seems to be dependant with

the temperature.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

Corrosion of steel in concrete has been identified as the prime

factor of deterioration and structural deficiency. Various remedies,

including replacing deteriorated concrete and using epoxy-coated

or galvanized steel, have been proven to be costly and inadequate

over the long run. Fiber-reinforced polymer (FRP) bars are a prom-

ising solution to this problem. Other attractive properties of FRP

materials include light weight, corrosion resistance, and high

strength. Glass FRP (GFRP) bars are gaining popularity as reinforce-

ment for concrete bridge deck slabs and other concrete structures

due to their low initial cost compared to carbon FRP bars [1–6].

However, the surface deformation and mechanical properties of

FRP reinforced bars are different from those of conventional steel

bars. Therefore, the design guidelines for steel reinforcing bars can-not be directly used for FRP reinforcing bars, Benmokrane et al. [7].

FRP materials are an isotropic and characterized by high tensile

strength only in the direction of the reinforcing fiber. The trans-

verse coefficients of thermal expansion (CTE) controlled by the re-

sin is up to three to six times the CTE of the concrete [8]. This

anisotropic behaviour should affect the shear strength action of

the FRP bar, as well as, the bond performance of FRP bars when

embedded in concrete and these effects need to be evaluated. High

temperatures, such as those occurring in extremely hot climates,

may decrease the mechanical and bond properties of FRP bars.

Many research studies have been carried out to evaluate the ef-

fect of high temperature on bond strength of FRP bars, Katz et al.

[9–11], Nanni et al [12]. However, very limited experimental data

is available on the bond effects due to high temperatures, when ap-

plied for a relatively long period of time. An experimental investi-

gation Galati et al. [13], was carried out on concrete specimens

reinforced with an FRP bar and subjected to thermal cycles. The

testing was completed using direct pull-out specimens. A 9.5 mm

GFRP bar with different embedment lengths was placed inside a

152 mm-cube of concrete. The treated specimens were placed into

an environmental chamber for 200 h at a temperature of 70 C and

at a humidity of 80%. The testing of the specimens was undertakenat room temperature. The influence of the thermal treatment is

more evident with the small values of the concrete cover. Such

behaviour was explained with the micro-cracking of the concrete

due to the thermal stresses induced during the thermal treatment.

In most of the specimens, the thermal treatment induced degrada-

tion in the bond performance of about 16%. A more pronounced ef-

fect was observed for the bond stress–slip curves in terms of slip

values due primarily to the degradation of the resin (Galati et al.

[13]).

Another study of the effect of high temperature on the bond be-

tween GFRP reinforcing bars (rebars) and concrete was studied by

Katz and Berman [11]. Four types of rods (12.7 mm diameter),

0950-0618/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.conbuildmat.2009.12.040

* Corresponding author. Address: Faculté de génie Local C1-3002-4, Université de

Sherbrooke, 2500 Blvd. Université Sherbrooke, QC, Canada J1K 2R1. Tel.: +1 819 821

8000x62767; fax: +1 819 821 7974.

E-mail address: [email protected] (R. Masmoudi).

Construction and Building Materials 25 (2011) 486–493

Contents lists available at ScienceDirect

Construction and Building Materials

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o n b u i l d m a t

http://dx.doi.org/10.1016/j.conbuildmat.2009.12.040mailto:[email protected]://www.sciencedirect.com/science/journal/09500618http://www.elsevier.com/locate/conbuildmathttp://www.elsevier.com/locate/conbuildmathttp://www.sciencedirect.com/science/journal/09500618mailto:[email protected]://dx.doi.org/10.1016/j.conbuildmat.2009.12.040

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were embedded vertically (embedment length 5d), in a normal

concrete cylinder (150 mm diameter and 300 mm long). Rod CB

has molded deformations on the surface, similar to ordinary de-

formed steel rebars. The polymer at the surface is a urethane mod-

ified vinylester and the polymer at the core of the rod was recycled

polyethylene terephthalate. Rod CPH contains wraps of helical

braid of fibers on the surface and the polymer was vinylester

throughout the rod. Rod CPI contains wraps of a wide braid of fi-

bers on the surface. The polymer was vinylester throughout the

rod. Rod NG has tight wraps of a narrow braid of fibers which pro-

duced large deformations in the rod. The polymer was polyester.

Pull-out tests were conducted at high temperature from 20 to

250 C. A comparison between the behavior of the different rods

at 80 C, showed that the NG rod have an early reduction in the

bond strength (about 43%) which reflects its low glass transition

temperature (T g ) of the resin. For the CPH and CPI rods, the de-

crease of bond strength is relatively moderate, and they both have

approximately the same reduction (20%). For the CB rod, the reduc-

tion is the smallest (3%), indicating that the bond relies mainly on

the polymeric system. It is possible to conclude from the above

that improving one or some of the followings can modify the bond

at high temperature: (i) Use of a polymer with high T g

in order to

increase the temperature at which the reduction in bond begins.

(ii) Use of a polymer with a high extent of crosslinks to moderate

the gradient of bond loss. (iii) Improvement of the inorganic sys-

tem, which supports the bond at a high temperature where the

polymer practically ceases to contribute to the bond. We noted

that in the same study [11], and at 200 C, the bond strength exhib-

ited a severe reduction of 80–90% (for CB, CPH, CPI and NG rod).

We conclude from these works, that reduction in the bond

strength depends on the transverse coefficients of thermal expan-

sion. A limited experimental data is available on the GFRP bond ef-

fects due temperature ranging from 20 C to 80 C, when applied

for a relatively long period of time. The major focus of the present

paper is to evaluate the long term effect of temperature ranging

from 20 C to 80 C on bond properties of GFRP bars embedded

in normal concrete. Results from a total of 80 specimens 8 mm

and 16mm diameters GFRP bars, after more than 5000h

(240 days) of exposure under high temperatures up to 80 C are re-

ported. The thermal effects on the average bond strength are com-

pared to untreated specimens (20 C). Based on the available

experimental tests, the main parameters a of the Bertero–Popov–Elingehhausen (BPE) model and of the Cosenza, Manfredi Realfon-

zo (CMR) model (b, S r ) have been calibrated. The relationship be-

tween temperature and the parameters are established using the

CMR and BPE models.

2. Experimental study

2.1. Test program

The main objective of the test program is to evaluate, under temperature rang-

ing from 20 C to 80 C (in dry environment) the performance of the bond strength

of FRP bars embedded in normal concrete. Specimens were submitted to three tem-

peratures of40, 60and80 C for4 and8 months in specially designed rooms, where

the temperature is controlled, as shown in Fig. 1. A total of 80 specimens were

tested. Table 1 presents the details of the experimental program.

2.2. GFRP bars

The Glass FRP bars ‘‘Combar” used in pull-out specimens were manufactured

by using fiber composites and were combined with synthetic resin to achieve im-

proved properties, such as higher strength and elevated modulus of elasticity [14].

The tensile properties of the bars used in this investigation are presented in

Table 2. These properties are based on the experimental tests conducted at the

laboratories of Schock Bauteile GmbH, Munich Technical University, and Syracuse

University [15]. Two nominal diameters were used in this study: 8 mm and16 mm for the GFRP bars.

2.3. Concrete design

Normal strength concrete was prepared in the laboratory. All the mixtures were

prepared in a 204 liters mixer, using a Portland cement type CEM I 42.5, and aggre-

gates with maximum size of 20 mm. Concrete mixture proportioning is presented

in Table 3. Standard concrete cylinders 160 320 mm were cast and cured at room

temperature (20 C). The pull-out specimens and the standard concrete cylinders

Fig. 1. Specimens submitted to accelerated ageing.

Table 2

Properties of the GFRP bars used in this study [14].

Type

of

bar

Nominal

diameter

(mm)

Tensile

modulus of

elasticity

(GPa)

Ultimate

tensile

strength

(MPa)

Coefficient of

thermal

expansion

(mm/mm/C)

Density

Glass 8 60 ± 1.9 738 ± 22 0.6 105

(axial)

2.2

Glass 16 2.2 105

(radial)

Table 1

Experimental program.

Bardiameter

(db, mm)

Embedmentlength

Temperature(C)

Samples numberfor each ageing

case

Timeexposure

(months)

8 5db 20 5

40 4

60 8

80

16 5db 20 5

40 4

60 8

80

Table 3

Concrete composition and characteristics.

Water (kg/m3) Cement I 42.5 (kg/m3) Sand (kg/m3) Aggregate 4/12 (kg/m3) Aggregate 12/20 (kg/m3) Compressive strength (MPa) Slump (mm)

204 300 857 296 691 30 ± 3 90 ± 2

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were cast in two layers and compacted using a vibrator. The compressive strength

and slump were 30 ± 3 MPa and 90 ± 2 mm, respectively after 28 days of curing at

20 C.

The pull-out specimens were stored in dry environment under temperature

ranging from 20 C to 80 C, until the date of testing.

2.4. Test procedure

Pullout bond testing were performed on specimens which consist on a 500 mmlong GFRP bar embedded vertically in 150 150 150 mm and 180 180

180 mm concrete cube, respectively for 8 and 16 mm bar diameters. This difference

in concrete cube dimensions is intendedto avoid theconcrete splitting. The embed-

ment lengthfor all specimensis 5db, where db is theGFRP-bar diameter. The desired

embedment length is obtained using PVC pipes which were placed around the bars

and sealed with silicon to avoid the contact of the concrete in this area. All speci-

mens were prepared following the specifications of ACI Guide Test Methods [16].

One additional specimen for GFRP rebars was instrumented with a thermocouple

placed at the surface of the bar before casting the concrete, for temperature moni-

toring during the timethat the specimens were subjectedto different temperatures.

After thethermal treatment(40, 60 and80 C) during 4 and8 months,pull-out tests

were performed. The pull-out specimen was installed on the machine testing

immediately after removing it out from the environmental chamber. The pull-out

test was performed about 3–5 min after the moment of removing it out from the

environmental chamber. During the test, which lasts approximately 5 min, it took

up to 10 min for the temperature at the FRPbar/Concrete Interface to decrease with

a couple of degree Celsius as shown in Fig. 2. So, we canconclude that at the time of the pull-out test, the temperature at the FRP bar/Concrete Interface is close to the

studied temperature levels (40, 60 and 80 C).

The pull-out tests were carried out using a calibrated LLoyed 50 KN testing ma-

chine with a displacement-rate control. The displacement-rate of loading was con-

stant during the tests (1.2 mm/min). Four LVDTs, with accuracy equal to 0.001 mm,

Fig. 2. Temperature distribution versus time.

Fig. 3. Setup of the pull-out test: (a) schematic and (b) photo.

Fig. 4. Load versus bar end slip behaviour.

Fig. 5. Failure mode of the rebar.

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were used for the GFRP bar to monitor the displacements. Three LVDTs were placed

at 120 segment orientation at the loadedend, and one LVDT at the freeend (Fig. 3).

2.5. Experimental results

2.5.1. Pullout load versus slip behavior

The obtainedexperimental results areplotted in theformof load versusend slip

curves. These curves contained mainly two phases as shown in Fig. 4. In theascend-

ing phase, the load increases rapidly with small slip until it reaches the maximum

load. In the descending phase, the load decreases gradually with significant slip in-

crease. The maximumbondstress forGFRPbars 8 mmdiameter was recorded at the

free end at a slip of 0.55, 0.53, 0.49 and 0.43 mm respectively for 20, 40, 60 and

80 C temperature. For 16 mm diameter, slips were 0.60, 0.58, 0.56 and 0.47 mm,

respectively for 20, 40, 60 and 80 C. It can be concluded that as the temperature

increases, the slip corresponding to the maximum pullout load decreases. For all

GFRP bars, the failure mode is shearing off the concrete corbels (Fig. 5).

2.5.2. Bond strength

The maximal bond stress sm was calculated using the following equation:

sm ¼ F m

pdbLebð1Þ

where F m, is the Peakrecordedload (N ) during the pull-out test, db is the nominal bar

diameter (mm), and Leb is the embedment length of GFRP bar (mm).

Table 4 presents the average bond strength values for each diameter and for

each temperature after 4 and 8 months of ageing. Fig. 6 shows the maximal bond

strengthin dryenvironment after 120 and 240 days of ageing. The 16 mm-diameter

bar developed lower bond strength than that of the 8 mm-diameter bar, (about

14 MPa for the 8 mm and 11 MPa for the 16 mm diameter).This diameter effect,

is dueto a differencein the contact surface (larger for16 mmdiameter) at theinter-

face bar/concrete. The bond strength decreases when the diameter increases (diam-

eter effect). This finding is in agreement with the results by Boyleand Karbhari [17],

Nanni and Faza [18], Tighiouart et al. [19].

After 120 days of ageing of the GFRP bar in dry environment, and at tempera-

ture up to 60 C, the average bond strengths do not show any significant reduction

(1.81% and3.36% respectively for the 8 mmand 16 mm). For the 80 C temperature,

the maximumreductions after 4 months of thermal loading were 9.39% and 13.71%,

respectively for the 8 mm and 16 mm GFRP bars, compared to the reference results

at 20 C.

After 240 days of ageing in dry environment at temperature up to 60 C, the

average bond strengths also did not show any significant reduction (1.96% and

3.54% respectively for the 8 mm and 16 mm). However, for the 80 C temperature,

the maximum reductions after 8 months of ageing in dry environment were 9.64%

and 14.14%, respectively for the 8 mm and 16 mm, compared to the reference re-

sults at 20 C. Fig. 7 presents the curve fittings of the thermal degradation of the

bond strengths for the two GFRP bars tested in this study. As shown in Fig. 7aand b, it is concluded that the third degree polynomial equations sm = f (T ) predictwith good accuracy the thermal degradation of the bond strength from 20 to

80 C. These equations are very useful to predict the bond strength for design

purpose.

In a similar study by Alvarez et al. [20], for the investigation of the thermal ef-

fect on bond properties with GFRP V-Rod bar, with a modulus of elasticity 44 GPa

and a CTE of 3.4 105 mm/mm/C, the average bond strength reduction is found

to be up to 27% and 32% for the specimens which are subjected to 60 and 80 C,

respectively. This comparison shows that the thermal effect on bond strength of

Table 4

Specimens and summary of test results.

Temperature

(C)

Average bonda (MPa)

4 months 8 months

8 mm

(GFRP)

16mm

(GFRP)

8 mm

(GFRP)

16mm

(GFRP)

20 14.37 ± 0.40 11.01 ± 0.25 14.32 ± 1.19 11.03 ± 0.92

40 14.27 ± 1.04 10.87 ± 0.36 14.22 ± 1.99 10.86 ± 0.21

60 14.11 ± 0.75 10.64 ± 0.15 14.04 ± 1.24 10.64 ± 0.44

80 13.02 ± 0 .22 9.50 ± 0 .27 12.94 ± 1 .49 9.47 ± 0 .93

a Based on five identical tests.

Fig. 6. Loss in bond strength in dry environment after 120 and 240 days of ageing. Fig. 7. Thermal degradation of the bond strength.


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GFRP Combar is less pronounced than that of GFRP V-Rod bar due to a lower CTE

and higher elasticity modulus (60 GPa).

The axial and radial GFRP coefficients of thermal expansion are respectively 0.6

and 2.2 105 mm/mm/C. For comparison, the coefficient of thermal expansion of

concrete is between 0.5 and 1.2 105 mm/mm/C, which may explain why there

is nosignificant thermal effect forthe specimenssubjected to temperatures from 20

to 60 C.

3. Analytical models of the bond–slip behaviour

In spite of a large number of formulations proposed in the past

for steel reinforcements and FRP bars, even though many experi-

mental programs have been conducted worldwide examining the

bond characteristics of FRP bars, very little work has been pub-

lished on analytical modelling. The available models for FRP rein-

forcement bond properties are reported hereafter.

3.1. Eligehausen, Popov and Bertero (BPE model)

Fig. 7 shows a schematic of the BPE model, the ascending

branch of the well-known bond–slip model proposed by Eligehau-

sen et al. [21], given by:

s ¼ s1s

s1

að2Þ

where s1 is the maximum bond strength, (MPa), s and s1 is the slipand maximum slip at maximum bond strength, (mm).Fig. 8. BPE model [17].

Fig. 9. Local bond–slip relationships GFRP 8 mm.


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Therefore s1 = sm and s1 = sm. In Eq. (2), a is a curve-fittingparameter that must be not larger than 1 to be physically meaning-

ful (a = 0.4 for steel bars).The value of parameter a, which the ascending branch depends

on, is evaluated by equating the area, As1, underneath the ascend-ing branch of the analytical bond–slip curve (see Fig. 8) given by

Eq. (3), to the area, Ass, underneath the ascending branch of eachactual curve:

As1 ¼

Z s1

0

sðsÞ ds ¼Z

s1

0

s1s

s1

a ds ¼

s1 s11 þ a

ð3Þ

In Eq. (3), s1 and s1 represent the bond strength and the correspond-ing slip, respectively. Therefore, a can be expressed as a function of

As1 given by:

a ¼ sm sm A

s1

1 ð4Þ

Fig. 10. Local bond–slip relationships GFRP 16 mm.

Table 5

Mean values for each temperature and diameter of GFRP bars.

GFRP 8 mm GFRP 16 mm

20 C 40 C 60 C 80 C 20 C 40 C 60 C 80 C

4 months CMR model b 0.458 0.463 0.476 0.496 0.416 0.425 0.456 0.512

S r (mm) 0.134 0.149 0.145 0.137 0.155 0.172 0.149 0.105

BPE model a 0.087 0.088 0.09 0.095 0.085 0.089 0.087 0.084

8 months CMR model b 0.458 0.463 0.477 0.498 0.417 0.425 0.458 0.515

S r (mm) 0.148 0.146 0.147 0.136 0.166 0.161 0.147 0.112

BPE model a 0.093 0.089 0.092 0.094 0.088 0.090 0.086 0.089


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3.2. CMR model

Cosenza et al. [22], proposed a law to model the first branch of

the s–s curve

sðsÞ ¼ sm 1 e ssr

bð5Þ

where sm is the maximum bond strength, (MPa), S r and b are

parameters based on curve-fitting of the experimental data. Param-eters S r and b were calibrated for each diameter of bar and temper-

ature by the least-square method.

The local bond–slip laws of the considered bars after thermal

treatment have been determined via the BPE and the CMR models.

The ascending branch is the most important branch because this

branch gives the bond strength–slip of the bar below the ultimate

load. A comparison of the ascending branch obtained from the ana-

lytical curves with the modified BPE and CMR models, and the

experimental results submitted to different temperatures are pre-

sented in Figs. 9 and 10 respectively, for the diameter 8 and 16 mm

after 8 months of ageing. Table 5 presents the mean values of a andb parameters for each temperature and each diameter of bar stud-

ied and calibrated to the experimental phase after 4 and 8 months

of ageing in dry environment. The CMR model appears to be themost reliable for all specimens; the ascending branch of the

bond–slip law is well interpreted by the CMR model valid for

06 s 6 sm.

The average values obtained for the coefficient a, from the firstbranch of the BPE model for reference (20 C) GFRP-Combars spec-

imens is 0.089. It is noted that the average value found by Cosenza

et al. [22] for sand-coated bars is 0.067. No significant effect was

detected after 8 months of ageing on the calibrated coefficient afor specimens submitted to temperature ranging from 20 C to

80 C.

For the CMR model and after 8 months of ageing, the coefficient

b calibrated to the experimental data for specimens after condi-

tioning depends on the temperature T . The coefficient b , from the

first branch of the CMR model increases as temperature increases,

as shown in Fig. 11. The third degree polynomial equations b = f (T )

for each diameter predicts this dependence on temperature as pre-

sented by Eqs. (6) and (7). These equations are fit for temperature

ranging from 20 C to 80 C with this particular kind of rebar and

diameters used in this investigation

Diameter 8 mm :

bðT Þ ¼ 0:0002T 3 þ 0:005T 2 0:0088T þ 0:462 ð6Þ

Diameter 16 mm :

bðT Þ ¼ 0:0005T 3 þ 0:008T 2 0:0185T þ 0:426 ð7Þ

4. Conclusions

The following conclusions are deduced from the experimental

and analytical results:

– For temperature up to 60 C applied for periods of 4 and

8 months, the average bond strengths did not show any signifi-

cant reduction.

– For the 80 C temperature, the maximum reductions after

8 months of ageing in dry environment were 10% and 14%,

respectively for the 8 mm and 16 mm GFRP bars, compared to

the reference results at 20 C.

– No significant damages were observed on the interface GFRP

rebars/concrete after 240 days of thermal loading in dry

environment.

– No significant effects were detected on the coefficient a of theBPE modified model submitted to temperature ranging from

20 C to 80 C.

– The bond strength decreases when the diameter increases

(diameter effect).

– The thermal effect on bond strength of GFRP Combar is less pro-

nounced than that of GFRP V-Rod bar due to a lower CTE and

higher elasticity modulus.

– To predict the bond–stress slip behavior, the CMR model pro-

vides better accuracy with the experimental results, than the

BPE model.

– The coefficient b, from the first branch of the CMR model

increases as temperature increases. This finding is fit for temper-

ature ranging from 20 C to 80 C with this particular kind of rebar and diameters used in this investigation.

Acknowledgments

The authors would like to thank the manufacturer of the GFRP

Combar (Schöck, Baden-Baden, Germany) for providing the GFRP

bars. The opinion and analysis presented in this paper are those of

the authors.

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