Concrete Cover

Composite Structures 73 (2006) 61–69

www.elsevier.com/locate/compstruct

Concrete cover requirements for FRP reinforced membersin hot climates

Hany Abdalla *

Department of Structural Engineering, Cairo University, Cairo, Egypt

Available online 14 March 2005

Abstract

Excessive corrosion problems exist in the Arabian Gulf countries. The exterior surfaces of reinforced concrete structures in these

countries are subjected to high temperatures and humidity. Fiber Reinforced Polymer (FRP) bars are currently contending conven-

tional steel in reinforced concrete members in highly corrosive environments or where non-magnetic fields are required. However,

the difference in thermal expansion between FRP bars and the surrounding concrete may cause significant splitting stresses within

the concrete around the bars during temperature increase. In this paper, the non-linear analysis of concrete members subjected to

high temperatures ranging from 20 �C to 100 �C is discussed. Several design parameters are varied such as FRP type, bar diameter,

concrete cover, and concrete strength. The results of the analytical study are substantiated by test results from ten FRP and steel

reinforced concrete beam specimens subjected to high temperatures and vertically applied loads up to failure. The experimental pro-

gram also included testing of 42 concrete cylinders reinforced with different types of FRP bars and subjected to high temperatures.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Concrete; Cracking; Cover; Fiber reinforced polymers; Finite elements; Temperature; Thermal stresses

1. Introduction

Temperature variations in concrete structures can

produce relatively high thermal stresses. Such stresses

develop when free expansion, contraction, or rotationdue to temperature is restrained. The restraint can be

external such as that provided by the supports in contin-

uous structures or internal when temperature distribu-

tion is non-linear across the section. In the latter case,

internal self-equilibrating stress develop in the longitudi-

nal direction due to an incompatibility of displacement

occurring within the section. Internal restraints can also

occur in FRP reinforced concrete members due to thedifference between the coefficient of thermal expansion

(CTE) of the FRP and that of the surrounding concrete.

0263-8223/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compstruct.2005.01.033

* Present address: Department of Civil Engineering, College of

Technological Studies, PAAET, Kuwait. Fax: +965 4843143.

E-mail address: [email protected]

FRP bars are generally made of longitudinal fibers

and a polymeric resin. The mechanical properties of

the FRP bars strongly depend on the type and volume

of fibers, while the polymeric resin plays an important

role in the transverse direction. Consequently, theCTE in the transverse direction is completely different

from that in the longitudinal direction. According to

the ACI-440 design guidelines [1], carbon fibre rein-

forced polymers, CFRP, for example, have a longitudi-

nal CTE, afl of almost zero, and a transverse CTE, aft ofabout 30 · 10�6/�C. A positive value of 0.68 · 10�6/�Cfor the longitudinal CTE of the CFRP was reported

by the manufacturer [2], and a negative value of�0.9 · 10�6/�C was reported by Daniel and Ishai [3].

For the Isorod type of glass fibre reinforced polymers,

GFRP, while the longitudinal CTE, afl, is similar to that

of hardened concrete, the transverse CTE, aft is over fivetimes higher [4]. The difference in thermal expansion in

the transverse direction may cause significant splitting

stresses in concrete around the bars under temperature

mailto:[email protected]

Nomenclature

a radius of FRP bar

Af area of FRP reinforcementAc cross-sectional area of concrete

b radius of concrete ring around the bar

c concrete cover

db bar diameter

Ec elastic modulus of concrete

Efl elastic modulus of the FRP reinforcement in

the longitudinal direction

Eft elastic modulus of the FRP reinforcement inthe transverse direction

Es elastic modulus of steel reinforcement

fc stress in concrete

ffu ultimate tensile strength of FRP reinforce-

ment

ft tangential stress

ftmax maximum tangential stress

fy yield stress of steel reinforcementnfl modular ratio in the longitudinal direction

nft modular ratio in the transverse direction

p radial pressurer radius measured from center of the bar

T change in temperature

ac coefficient of thermal expansion of concrete

afl longitudinal coefficient of thermal expansion

of FRP bars

aft transverse coefficient of thermal expansion of

FRP bars

al coefficient of thermal expansion in the longi-tudinal direction

as coefficient of thermal expansion of steel rein-

forcement

at coefficient of thermal expansion in the trans-

verse direction

b shape coefficient

lf FRP reinforcement ratio

mc Poisson�s ratio of concretemft in-plane Poisson�s ratio of FRP bar

62 H. Abdalla / Composite Structures 73 (2006) 61–69

increase [5], or separation of the bars from the concrete

under temperature decrease. This may affect the bond

between the FRP reinforcement and the surrounding

concrete [6] and cause cracking of the concrete cover

around the reinforcement [7].

Over the past decade, many investigations have been

carried out on the behavior of concrete members rein-

forced or strengthened by FRP materials subjected tomechanically applied loads. Comparatively little atten-

tion has been directed toward the effect of high tem-

peratures on these members. Aiello [8] studied the

phenomenon of concrete splitting around the rebars

due to high temperatures. The theoretical model was

based on assuming linear behavior for the concrete sur-

rounding the FRP bars. The splitting phenomenon was

also reported experimentally by others [9,10].In order to examine the effects of high coefficient of

thermal expansion in the transversal direction, tests were

conducted on concrete cylinders reinforced with different

types of FRP bars and subjected to uniform increase in

temperature. A series of concrete beams (50 · 76 · 750

mm) reinforced by different types of FRP bars, were also

subjected to vertical loading up to failure after being sub-

jected to uniform increase in temperature.

2. Thermal stresses in reinforced concrete members

During the service life of a structure, thermal stres-

ses depend upon the non-linear temperature variation

within the structure. Geometric location and orienta-

tion of the structure, climatological conditions, cross-

section geometry, thermal properties of the material

and the exposed surfaces affect these stresses. If the

temperature varies in a non-linear fashion over the

cross-section, restraint stresses will develop even in

statically determinate structures. The values and distri-

bution of these stresses over a cross-section can be

found elsewhere [11]. Additional longitudinal andtransverse thermal stresses are developed in the con-

crete section due to the difference between the coeffi-

cient of thermal expansion of concrete and the

longitudinal and transverse coefficients of the reinforc-

ing FRP bars. Typical coefficients of thermal expan-

sion of concrete and different types of reinforcing

bars are given in Table 1 [1].

2.1. Thermal stresses in the longitudinal direction

Longitudinal self-equilibrating stresses develop in

reinforced concrete members in two cases; namely, when

the longitudinal coefficient of thermal expansion of the

reinforcement is different from that of the concrete,

and when the temperature distribution over the cross-

section is non-linear. In the first case, development ofstresses can be explained considering a concrete prism

symmetrically reinforced by FRP bars and subjected

to uniform rise of temperature, T. Assuming perfect

bond between the concrete and the FRP reinforcement,

and considering compatibility of strains and equilibrium

of forces, it can be shown that the stresses in the FRP

reinforcement, ff, is given by

Table 1

Typical coefficients of thermal expansion for concrete and reinforcing bars

Material Longitudinal CTE, al · 10�6/�C Transverse CTE, at · 10�6/�C

Concrete 7.2 to 10.8 7.2 to 10.8

Steel 11.7 11.7

Glass fiber reinforced polymer, GFRP 6 to 10 21 to 23

Carbon fiber reinforced polymer, CFRP �2 to 0 23 to 32

Aramid fiber reinforced polymer, AFRP �6 to �2 60 to 80

H. Abdalla / Composite Structures 73 (2006) 61–69 63

ff ¼ ðac � aflÞTEfl

ð1þ lfnflÞð1Þ

where lf = Af/Ac is the reinforcement ratio; Af is the area

of FRP reinforcement; Ac is the cross-sectional area of

the concrete; ac is the coefficient of thermal expansion

for the concrete; afl is the longitudinal coefficient of ther-

mal expansion for the FRP bars; nfl = Efl/Ec is the mod-

ular ratio; Efl is the modulus of elasticity of the FRP in

the longitudinal direction; and Ec is the modulus of elas-

ticity of the concrete.The stress in the concrete around the bars, fc, is given

by

fc ¼ �lfff ð2ÞIt has to be noted that from Eqs. (1) and (2), tensile

stress takes place in concrete for temperature increase

and when afl has a positive value greater than ac.In the longitudinal direction, self-equilibrating stres-

ses may also occur due to non-linear temperature distri-

bution over the cross-section. In addition, continuity

thermal stresses develop in the longitudinal direction

of the member when end displacements due to tempera-

ture are externally restrained. The values of the self-

equilibrating stresses and continuity stresses are given

by Elbadry et al. [11]. This investigation is limited tothe stresses in the transverse direction affecting the con-

crete cover around the FRP bars.

Fig. 1. Cracking due to transverse thermal expansion of FRP: (a) axisymme

concrete cylinder reinforced with GFRP subjected to temperature increase.

2.2. Thermal stresses in the transverse direction

These stresses result from the high transverse thermal

expansion of FRP reinforcement with respect to that of

concrete, Table 1. Such stresses may lead to radial

cracking in concrete surrounding the FRP bars. This

may affect the bond between the concrete cover and

the reinforcement as well as the efficiency of that coverin protecting the reinforcement. The tangential stresses

that cause radial cracking can be estimated considering

a ring of concrete surrounding the FRP bar, as shown

in Fig. 1(a). At temperature increase, the large trans-

verse expansion of the FRP bar produces radial

pressure, p, on the concrete. This pressure can be calcu-

lated assuming plane stress conditions by [12]:

p ¼ ðaft � acÞTEft

nftðbþ mcÞ þ ð1� mftÞð3Þ

where mft is the in-plane Poisson�s ratio of the FRP bar;

mc is the Poisson�s ratio of concrete; and the coefficient bis a shape coefficient that depends on the FRP bar diam-

eter, db, and the concrete cover, c, and is given by

b ¼ b2 þ a2

b2 � a2ð4Þ

where b = c + (db/2) and a = (db/2) are the radii of the

concrete ring and the FRP bar, respectively.

tric model of FRP bar embedded in concrete; (b) plan and elevation of

0

1

2

3

4

5

0 5 10 15 20 25 30 35 40Radius r (mm)

Tens

ile s

tress

f t (M

Pa) T = 80 ºC

T = 50 ºC

T = 20 ºC

Fig. 2. Tangential tensile stress around a 16 mm diameter GFRP bar

in a 76 mm concrete cylinder (concrete cover = 30 mm).

2.5

3.0

3.5

4.0

4.5

5.0

5 10 15 20 25 30 35 40 45 50 55Concrete cover C (mm)

Tens

ile s

tress

f t (M

Pa)

db = 25 mm

db = 16 mm

db = 12 mm

= cover > 1.5 db

Fig. 3. Maximum tensile stress around GFRP for temperature

increase of 50 �C.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 6 12 18 24 30Distance from bar center r (mm)

Tens

ile s

tress

f t (M

Pa)

Cover = 1.5 db Cover = 2.0 db

CFRP

GFRP

Steel

Fig. 4. Tensile stress around 12 mm diameter bars of different

materials (temperature increase = 50 �C).


The tangential concrete stress, ft, induced in the

concrete at any radius, r, due to the radial pressure, p,

is given by

ft ¼a2ðb2 þ r2Þr2ðb2 � a2Þ

p ð5Þ

According to Eqs. (4) and (5), the maximum tangen-

tial stress occurs at the interface between the concrete

and the FRP bar, where

ftmax ¼ bp ð6ÞFig. 1(b) shows the cracking due to temperature in-

crease of 80 �C in a concrete cylinder reinforced with a

GFRP bar. Eq. (5) was used to estimate the tangential

tensile stresses in a 76 mm diameter concrete cylinder

reinforced with 16 mm diameter GFRP bar. The results

are shown in Fig. 2 for temperature increase of 20 �C, 50�C, and 80 �C. The stresses were estimated based on the

following properties for the GFRP bars: transverse CTEof 22 · 10�6/�C, transverse modulus of elasticity of 3300

MPa, and Poison�s ratio of 0.28. It can be seen from the

results that at the interface between the concrete and the

FRP bar for a temperature increase of 80 �C, the tensilestress reaches a value of 4.63 MPa which exceeds the

tensile strength of concrete. At the outer surface of the

cylinder (r = 38 mm) which represents a concrete cover

of 1.875db, the tensile stress is reduced to 0.4 MPa forthe same temperature increase. Fig. 2 shows also that

for a temperature increase of 80 �C, the tensile stress

at a distance equal to db from the bar surface (r = 24

mm), the tensile stress is reduced to 0.7 MPa which rep-

resents 15% of the maximum value in the concrete

around the bar. At a distance of 1.5db from the bar sur-

face (r = 32 mm), the tensile stress reaches a value of

0.47 MPa representing 10% of the maximum value.According to the ACI-440 design guidelines [1], for

static loading conditions, the concrete cover for FRP

reinforcement should not be less than the bar diameter

db, to avoid splitting bond failure. In case of using a con-

crete cover of db, a modification factor of 1.5 should be

used as a multiplier for the development length of the

FRP bar. This multiplier is taken as 1.0 in case of using

a concrete cover of 2db. The tensile stresses due to a tem-

perature increase of 50 �C were estimated for 12 mm, 16

mm, and 25 mm diameter GFRP bars for different con-

crete covers. The results are shown in Fig. 3, where it

can be seen that the maximum tensile stress is signifi-

cantly reduced when the concrete cover exceeds 1.5db.

Therefore, considering the results shown in Fig. 3, itcan be seen that a concrete cover ranging between

1.5db and 2db can satisfy both the loading and tempera-

ture requirements.

The tensile stress distributions in the concrete cover

for 12 mm diameter bars of different reinforcing materi-

als are shown in Fig. 4. It can be seen from the results

that increasing the concrete cover from 1.5db to 2db does

not significantly decrease the tensile stresses close to thebar perimeter. The reduction in the maximum tensile

stress at the bar surface (r = a) was 4% for the FRP bars,

and 2% for the steel bar. At the outside fibers of the con-

crete cover (r = b), the tensile stress decreases by 35%

when increasing the cover from 1.5db to 2db. Therefore,

increasing the cover to 2db may be helpful in case of

Fig. 5. Stresses in the GFRP and the surrounding concrete due

to temperature increase: (a) concrete cover = 1.25db; (b) concrete

cover = 1.875db.


using large bar diameters and high temperatures to re-

duce the tensile stress and hence avoid cracking at the

outside concrete fibers.

It has to be mentioned that the above estimation of the

tensile stresses is based on linear analysis, which can be

used to determine theses stresses before cracking. It canbe used also to determine the temperature that causes first

crack in the vicinity of the bar. After cracking, stress relief

takes place leading to different stress distributions and

stress values in the concrete cover. Therefore, the above

analysis can not be used to fully determine the behavior

of the concrete cover around the bars. A non-linear finite

element analysis is described below in an attempt to accu-

rately determine the tensile stresses around the FRP barsunder temperature increase.

According toEq. (5), for temperature increase of 50 �C,bar diameter of 16mm, and concrete cover of 30mm (cov-

er = 1.875db), the maximum tensile stress reaches 2.89

MPa. For concrete cover of 20 mm (cover = 1.25db), the

maximum tensile stress reaches 3.08 MPa. These cases

of concrete cover equal to 30mm and 20mm are analyzed

below using a non-linear finite element analysis for tem-perature increase up to 50 �C. These ratios of concrete

cover to bar diameter were chosen to evaluate the cases

of concrete cover less than or larger than 1.5db.

3. Non-linear analysis of thermal stresses

Thefinite element programABAQUS [13]was used forthe non-linear analysis of concrete reinforced with FRP

and subjected to temperature increase. In the program,

the temperature is applied gradually in small increments.

The size of each increment depends on the convergence of

the iteration process in the previous increments. The con-

crete under compression is modeled by an elastic-plastic

theory, using a simple form for the yield surface expressed

in terms of the equivalent pressure stress and the Misesequivalent deviatoric stress [13]. Cracking is assumed to

occur when the stress reaches the failure surface repre-

sented by a simpleCoulomb line in terms of the equivalent

pressure, and the Mises equivalent deviatoric stress. The

model is a smeared crack model, in the sense that it does

not track individual micro cracks. Instead, constitutive

calculations are performed independently at each integra-

tion point of the finite element model, and the presence ofcracks enters into these calculations by the way in which

the cracks affect the stresses and material stiffness associ-

ated with the integration point.

Fig. 5(a) shows the non-linear finite element results

for the compression stresses in the GFRP bar and the

tensile stresses in the surrounding concrete due temper-

ature increase up to 50 �C. The stresses are shown for a

16 mm diameter GFRP bar with 20 mm concrete cover.This cover of 1.25db (cover > db) is accepted by the ACI-

440 design guidelines [1] as discussed before. It can be

seen from the results that the maximum tensile stress

around the bar from the finite element method was

11% less than that estimated according to Eq. (5). In

Fig. 5(b), where the concrete cover was increased to

1.875db, the maximum tensile stress in the concrete

around the bar was decreased by 10%. It can be seenalso from the results that the maximum tensile stress

around the bar from the finite element method was

14% less than that estimated according to Eq. (5). There-

fore, Eq. (5) can be used conservatively to determine the

maximum tensile stresses due to temperature increase as

long as the tensile stresses are less than the concrete

cracking strength. Once the concrete cracks, stress relief

takes place leading to less tensile stresses around the bar.Fig. 5 shows also that the tensile stress around the bar

decreases at the outer fibres of the concrete beam away

from the bar. It reaches zero or turns to compression at

the corners of the beam cross-section. Due to tempera-

ture increase, the principal stresses in the plane of the

bar cross-section were compressive, with a value of

3.85 MPa and 3.91 MPa for concrete cover of 1.25dband 1.875db, respectively.

4. Experimental program

In the experimental program, load tests were carried

out on 10 reinforced concrete beams subjected to tem-

perature increase. Splitting tests were also conducted

Table 3

Details of the tested beams

Beam Temperature Reinforcement Bar diameter (mm)

BP1 20 Plain concrete –

BP2 80 Plain concrete –

BI3 20 GFRP, Isorod 12.7

BI4 80 GFRP, Isorod 12.7

BC5 20 CFRP, CFCCa 5

BC6 80 CFRP, CFCC 5

BS7 20 Steel 10

BS8 80 Steel 10

BS9 80 Steel 16

BL10 80 CFRP, Leadline 10

a CFCC = Carbon Fiber Composite Cables.


on 42 concrete cylinders reinforced with different types

of FRP reinforcing bars after being subjected to temper-

ature increase.

4.1. Material properties

Five types of reinforcing bars were used in the exper-

imental study, namely, GFRP (Isorod), GFRP (C-bar),

CFRP (Leadline), CFRP (CFCC), and steel. The GFRP

(Isorod) bars are manufactured by pultrusion of E-glass

continuous fibers and thermosetting polyester resin. To

enhance the bond characteristics, the surface is wrapped

by helical glass fiber strands and covered by a mixture of

a known grain size of sand and polyester resin [4]. TheGFRP (C-Bar) rod is manufactured by the hybrid pul-

trusion process [14]. C-Bar rods are produced using four

different fiber types, namely, E-Glass, Carbon, Aramid,

and a hybrid of Carbon and E-Glass, designated as

Type 1, Type 2, Type 3, and Type 4, respectively. Type

1 reinforcing bars are manufactured in two grades,

Grade A and Grade B, according to the surface defor-

mations and characteristics. Type 1-Grade B was usedin this study. The CFRP (Leadline) rods are pultruded

using linearly oriented coal tar pitch-based continuous

fiber epoxy resin [2]. The CFRP (CFCC) is composed

of a prepreg in which polyacrylonitrile based carbon fi-

bers are impregnated with epoxy resin. A seven-wire

cable of 5 mm diameter with effective cross-sectional

area of 10.1 mm2, and guaranteed breaking load of

17.7 kN was used in this study. The CFCC configurationas a cable allows excellent flexibility and adhesion to

concrete, as well as ease of preparation, [15,16].

The measured average cylinder compressive strength

of the concrete used for the beam specimens ranged

from 30 MPa to 35 MPa at the time of testing, with a

maximum aggregate size of 13 mm. The reinforcing steel

was of Grade 400 (fy = 435 MPa). Table 2 shows the

physical properties of the different types of reinforce-ments used in the experimental program.

4.2. Test procedure

The experimental program includes testing of ten

beam specimens. Two beams were tested without rein-

forcement as control beams, and the remaining eight

Table 2

Properties of reinforcements used in the experimental study

Reinforcement Specific

gravity

ffu or fy(MPa)

Efl or Es

(GPa)

al · 10�6/�C

GFRP, Isorod 2.0 692 42 9.0

GFRP, C-bar 2.1 746 42 9.0

CFRP, Leadline 1.6 1970 147 0.68

CFRP, CFCC 1.5 1780 137 0.6

Steel 7.8 435 200 10.6

specimens were reinforced with different types of rein-

forcement. Table 3 shows the type of reinforcement

and the maximum temperature for each of the tested

beams. All beams had the same dimensions of 50 · 76

mm cross-section and 750 mm length. The pre-heated

beams were tested under two point loads up to failure.

Fig. 6 shows the loading test of beam BI4 after being

heated to temperature increase of 80 �C.The experimental program also included conducting

splitting tests on concrete cylinders reinforced with dif-

ferent FRP materials. A total of 42 concrete cylinders,

of 76 mm diameter and 152 mm height, were tested.

Three cylinders were tested for each bar diameter. The

reinforcing bars used in the tests were: GFRP, Isorod,

of diameters 9.5 mm, 15.9 mm, 19.7 mm and 25.4 mm;

CFRP, Leadline, of diameters 8 mm and 10 mm; CFRP,CFCC cables, of diameters 5 mm and 15 mm; and steel

of diameters 11 mm and 20 mm. For each diameter of

the reinforcing bars, one cylinder was subjected to split-

ting tension test after being subjected to a uniform rise

in temperature of 100 �C and one cylinder was tested un-

der room temperature. Tests were also conducted on cyl-

inders reinforced with GFRP of diameters 12.7 mm and

19.7 mm after being subjected to a uniform rise in tem-perature of 50 �C. Six plain concrete cylinders were also

Fig. 6. Load testing of beam BI4.

Fig. 7. Heating box used for the beam and cylinder specimens.

Plai

n Co

ncre

teIs

orod

, d=9

.5 m

mIs

orod

, d=1

5.9

mm

Isor

od, d

=19.

7 m

mIs

orod

, d=2

5.4

mm

Lead

line,

d=8

mm

Lead

line,

d=1

0 m

mCF

CC, d

=5 m

mCF

CC, d

=15

mm

Stee

l, d=

6 m

mSt

eel,

d=11

mm

Stee

l, d=

20 m

m

0

1

2

3

4

Split

ting

Stre

ngth

(MPa

)

T=20 ºC T=100 ºC

Cylinder

Fig. 8. Splitting strength of concrete cylinders reinforced with GFRP,

CFRP and steel.

T=20 ºC T=50 ºC

Isorod, d=19.7 mm

Isorod, d=12.7 mm

Cylinder

Plain Concrete

Split

ting

Stre

ngth

(MPa

)

4

3

2

1

0

C-Bar, d=12 mm

C-Bar, d=15 mm

Fig. 9. Splitting strength of concrete cylinders reinforced with GFRP.


tested, for comparison purposes, after being subjectedto: temperature of 100 �C, temperature of 50 �C, androom temperature.

The temperature increase in the beams and cylinders

tests was produced using the heating box shown in Fig.

7. The insulated wooden heating box contained 10

mounted infrared 250 W heating bulbs used to provide

a uniform heat flux. The temperature inside the concrete

beams or cylinder specimens was monitored using suffi-cient number of thermocouples for each specimen to

reach the required temperature.

5. Experimental results

The cylinder splitting tests were carried out to study

the effect of the difference between the transversal coef-ficient of thermal expansion of the FRP reinforcement

and that of the surrounding concrete. Fig. 8 shows the

splitting strength of the tested concrete cylinders. The re-

sults are shown for cylinders tested after being subjected

to uniform rise of temperature up to 100 �C and for cyl-

inders tested at room temperature, 20 �C. It can be seen

that increasing the temperature to 100 �C resulted in

decreasing the tensile strength of all the cylinders. Theconcrete cylinders reinforced with GFRP exhibited the

highest reduction in the splitting strength. The cylinder

of 76 mm diameter reinforced with 25.4 mm diameter

GFRP Isorod was fully cracked due to temperature

only. The crack shown in Fig. 1(b) for that cylinder

was observed at a temperature increase of 70 �C.

Fig. 9 shows the results of the splitting tests con-

ducted on cylinders reinforced with GFRP after being

subjected to a temperature rise of 50 �C. The cylinder

reinforced with a 19.7 mm diameter of Isorod bar exhib-

ited a reduction in splitting strength of 25%. The results

of the cylinder tests clearly show the effect of radial

cracking of concrete surrounding the FRP bars due to

the difference in transverse thermal expansion betweenFRP and concrete. It has to be noted that the reduction

in splitting strength of the cylinders reinforced with steel

is attributed to the small difference in thermal expansion

between concrete and steel which is usually ignored for

normal temperatures. The cylinders reinforced with

CFRP have exhibited reduction in strength similar to

that observed in cylinders reinforced with steel. This

indicates that the difference between the transverse ther-mal expansion of concrete and CFRP would not cause a

problem in the normal weather temperature increase

particularly for bars of small diameters. This is not the

case for GFRP bars.

Fig. 11. Cracking of concrete beams reinforced with different materials


Fig. 10 shows the deflection results for pre-heated

concrete beams having different types of reinforcement.

The deflection of plain concrete beams is also shown for

comparison purposes. The results show that the uniform

increase of temperature up to 80 �C generally leads to an

increase in deflection for all types of reinforcement. Thismay be attributed to the reduction in modulus of elastic-

ity of concrete and reinforcement accompanying the

temperature increase. It can be seen from Fig. 10 that

the largest deflection increase due to temperature took

place in the beam reinforced with GFRP. This may be

attributed to the development of radial cracks around

the GFRP bars due to the difference between the

transverse thermal expansion of concrete and GFRP.

012345

0 1 2 3 4 5 6 7 8 9 10Deflection (mm)

Load

(kN

)

Reinforced with IsorodT=20 oC, = 1.8 %


Plain ConcreteT=20 oC


5

6

Load

(kN

)

Reinforced with CFCCT=20 oC, = 0.52 %






Reinforced with SteelT=80 oC, = 0.75 %






6789

Reinforced with IsorodT=20 oC, ρ = 1.8 %








0

1

2

3

4

0 1 2 3 4 5 6Deflection (mm)













0

1

2

3

4

5

6

0 1 2 3 4 5 6Deflection (mm)

Load

(kN

)







Reinforced with SteelT=80 oC, = 0.75 %Reinforced with SteelT=80 oC, = 0.75 %

Reinforced with SteelT=20 oC, = 0.75 %Reinforced with SteelT=20 oC, = 0.75 %

ρ

ρ

ρ

ρ

ρ

Fig. 10. Deflection of concrete beams reinforced with different

materials.

at high temperatures.

These cracks contribute in bond reduction between the

bars and the surrounding concrete leading to higher

deflections. Fig. 11 shows the cracking pattern for the

concrete beams tested in this study. The beams were

reinforced with different materials and tested at high

temperatures. The results show that the thermal behav-

ior of the beams reinforced with CFRP was similar tothat of the beams reinforced with steel.

6. Summary and conclusions

Results from tests as well as from non-linear finite

element analysis were utilized to investigate the effect

of concrete cover on the behavior of members reinforcedwith FRP bars in hot climates. Based on the results of

this investigation, the following conclusions can be

made:

1. The high transverse coefficient of thermal expansion

of the GFRP bars creates bursting tensile stresses

in the concrete surrounding the bars at high

temperatures.2. Concrete cylinders reinforced with GFRP exhibited

wide cracks around the bars at high temperatures.

In concrete beams reinforced with GFRP, the cracks

in the vicinity of the bars resulted in weakening of

bond between the concrete and the GFRP and, con-

sequently a reduction in the tension stiffening of the

concrete and an increase in deflection.

3. The thermal behavior of concrete beams reinforcedwith CFRP was similar to that of beams reinforced

with steel. This behavior was better than that of

beams reinforced with GFRP.

4. The tensile stresses around the bars due to tempera-

ture increase are significantly reduced when the con-

crete cover exceeds 1.5db. A concrete cover ranging

between 1.5db and 2db can satisfy both the loading

and temperature requirements for beams in hotclimates.


5. For temperatures higher than 50 �C, it is not recom-

mended to use GFRP bars of diameters larger than

12 mm in order to avoid the high bursting stresses

in the vicinity of the bars which cause a reduction

of bond.

References

[1] ACI Committee 440. Guide for the design and construction of

concrete reinforced with FRP bars. American Concrete Institute,

Detroit, May 2001.

[2] Mitsubishi Chemical Corporation. Leadline carbon fiber rein-

forced plastic rod. Technical Data, Japan, 1995.

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rials. Oxford, UK: Oxford University Press; 1994.

[4] Challal O, Benmokrane B. Physical and mechanical perfor-

mance of an innovative glass fiber reinforced plastic rod for

concrete and grouted anchorages. Canad J Civil Eng 1993;

20(2):254–68.

[5] Gentry TR, Husain M. Thermal compatibility of concrete and

composite reinforcement. J Compos Construct ASCE 1999;

3(2):82–6. May.

[6] Katz A, Berman N, Bank LC. Effect of high temperature on bond

strength of FRP rebars. J Compos Construct ASCE 1999;

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[7] Gentry TR, Bank LC. Application of FRP reinforcement

in structural precast concrete. In: Proc 3rd Material

Engineering Conference, ASCE, Reston, VA, USA, 1994.

p. 575–82.

[8] Aiello MA. Concrete cover failure in FRP reinforced beams under

thermal loading. J Compos Construct ASCE 1999;3(1):46–52.

[9] De Sitter WR, Tolman F. Uni-directional fibre pretensioned

concrete elements. In: Second International Symposium on

Non-Metallic (FRP) Reinforcement for Concrete Structures

(FRPRCS-2), RILEM Proceedings 29, Ghent, Belgium, 1995. p.

49–56.

[10] Matthys S, De Schutter G, Taerwe L. Influence of transverse

thermal expansion of FRP reinforcement on the critical concrete

cover. In: Proc the 2nd International Conference on Advanced

Composite Materials in Bridges and Structures, ACMBS-II,

Montreal, Canada, 1996. p. 201–8.

[11] Elbadry MM, Abdalla HA, Ghali A. Effects of temperature on the

behaviour of fiber reinforced polymer reinforced concrete mem-

bers: experimental studies. Canad J Civil Eng 2000;

27(5):993–1004.

[12] Rahman AH, Kingsley CY, Taylor DA. Thermal stress in FRP-

reinforced concrete. In: Proc the Annual Conference of the

Canadian Society for Civil Engineering, CSCE, Montreal,

Canada, 1995. p. 605–14.

[13] Hibbitt HD, Karlsson BI, Sorensen EP. ABAQUS Finite Element

Program version 5.5. Providence, RI: Hibbitt, Karlsson, and

Sorensen, Inc.; 1995.

[14] Faza SS, GangaRao HVS. Theoretical and experimental correla-

tion of behavior of concrete beams reinforced with fibre reinforced

plastic rebars. In: Fibre-Reinforced-Plastic Reinforcement for

Concrete Structures, SP-138, American Concrete Institute,

Detroit, 1993. p. 599–614.

[15] Tokyo Rope Mfg., Ltd., Technical Data on CFCC, Japan,

October 1993.

[16] Nanni A. Fiber-reinforced-plastic (FRP) reinforcement for con-

crete structures: Properties and applications. Amsterdam, The

Netherlands: Elsevier Science Publishers; 1993.

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