Page 1
Flexural Behaviour of Concrete Beams ReinforcedWith GFRP Bars
L. Ascione, G. Mancusi and S. Spadea
Department of Civil Engineering, University of Salerno, Via Ponte don Melillo, 84084 – Fisciano (SA) Italy
ABSTRACT: The paper presents some of the results from a large experimental program undertaken
at the Department of Civil Engineering of Salerno University. The static behaviour under service
conditions of concrete beams reinforced with GFRP (Glass Fibre-Reinforced Polymer) as well as
CFRP (Carbon Fibre-Reinforced Polymer) bars and stirrups is examined. Within the whole
experimental program concerning forty beams, two different concrete strengths and two different
percentages of reinforcement are taken into consideration. The final aim is to investigate both
deflections at midspan and crack widths. Moreover, the ultimate behaviour up to failure is also
investigated. A part of this experimental program, concerning ten prototypes reinforced with GFRP
bars, is here presented and discussed. Finally, comparisons with analytical predictions given by CNR-
DT 203/2006 are also shown.
KEY WORDS: CNR-DT 203/2006, crack width, deflection, GFRP bars.
NOMENCLATURE
b cross section width
h cross section depth
d effective dept (distance from internal tensile
bars to the top of the section)
L beam span
La shear span
F single external force according to the four point
bending scheme
Deff effective diameter of FRP bar
DN nominal diameter of FRP bar
A0f amount of reinforcement (FRP) in compression
Af amount of reinforcement (FRP) in tension
Ec compressive Young modulus of concrete (eval-
uated according to [22])
Ef tensile Young modulus of FRP bars
fmax tensile strength of FRP bars
Rcm cubic compressive strength of concrete (mean
value)
Rck cubic compressive strength of concrete (fractile
5%)
fcfm tensile strength of concrete under flexure
(mean value evaluated according to [22])
Fcr cracking limit, referred to the external force F
Fu ultimate limit, referred to the external force F
Mcr bending moment at midspan corresponding to
cracking limit
Mu bending moment at midspan span correspond-
ing to ultimate limit
I1 moment of inertia of un-cracked cross section
I2 moment of inertia of the cracked cross section
f deflection of members
wk characteristic cracks width
srm average distance between cracks
Introduction
Over the last few years the use of FRP (Fibre-Rein-
forced Polymer) bars as internal reinforcement for
concrete structures has been sustained and particu-
larly encouraged in the international technical and
research community and specific technical instruc-
tions and guidelines have been published. Japan has
been one of the forerunners and drew up the first
design guidelines in 1996, subsequently translated
into English 1 year later [1]. Further guidelines were
published in Canada in 1996 [2], followed by those
published by ACI in 2000 and revised in 2006 [3].
More recently technical documents dealing with the
designing and execution of FRP concrete construc-
tions have been published in Europe [4] and in Italy
[5].
Despite the numerous advantages exhibited by
concrete structures reinforced with FRP bars,
there are still many issues demanding further
� 2010 Blackwell Publishing Ltd j Strain (2010) 46, 460–469460 doi: 10.1111/j.1475-1305.2009.00662.x
Page 2
investigation. The predictive models, the design for-
mulae developed over the years for structures rein-
forced with steel bars, cannot be directly applied to
FRP bars reinforced beams. It is worth considering the
different constitutive behaviour of the FRP bars
[6–10]. With respect to steel bars, FRP bars behave
linearly elastic up to failure, with quite different
stiffness and resistance values. Moreover, concrete
members reinforced with FRP bars very often exhibit
a lower post-cracking bending stiffness. Conse-
quently, the designing phase is substantially gov-
erned by limitations on serviceability (displacements
and crack width). These aspects are strictly connected
when it is considered that the kinematics of these
members depends on the inertia of the cross-section
that is influenced by the cracking phenomenon. On
the other side it is well known that the crack width
increases depending on the deformability of the
reinforcing internal bars.
The classical formulation in evaluating the
moment of inertia of concrete beams reinforced
with steel bars systematically results in excessive
errors in the case of FRP reinforced beams. This
produces an underestimation of the expected dis-
placements. In order to take into account the dif-
ferent behaviour in constitutive law as well as in
bonding to concrete of the composite bars, the
formulae have to be modified, introducing some
appropriate coefficients, which need to be experi-
mentally validated [11–21].
This work aims to give a contribution to the
design procedures of FRP reinforced beams by a
comparison between a large experimental database
and theoretical predictions, both at service and
ultimate conditions.
Experimental Program
The experimental tests presented here concern ten
concrete beams reinforced with GFRP bars and
stirrups. They have been grouped into two sets,
each relating to a different strength of concrete.
Each set is composed of five identical specimens
cast in place with the same concrete mixture and
tested according to a four point bending scheme.
The cross-section of the specimens is rectangular:
b = 150 mm · h = 200 mm; the total length is
equal to 2300 mm; the span, L, is equal to
2000 mm; the ratio between shear span and effec-
tive depth, La/d, is equal to 4.12. Details of the
specimens are reported in Figure 1.
An additional thirty specimens were fabricated and
will be tested in subsequent phases. They differ from
the previous ones with regard to the amount of ten-
sile reinforcing area and/or the type of FRP internal
Figure 1: Test specimen: geometry, reinforcement detail and test scheme
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L. Ascione, G. Mancusi and S. Spadea : Flexural Behaviour of GFRP RC Members
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reinforcement (GFRP/CFRP). Two different casting
phases have been performed: in the first, concerning
sets I, II, V, VI, a C20/25 concrete was used, in the
second, relative to sets III, IV, VII, VIII, a C28/35
concrete was used. Figure 2 shows the reinforcing
cage of the specimen in the formwork.
For each phase, 15 cubic concrete samples
(150 mm · 150 mm · 150 mm) were tested under
compression. Tensile tests on reinforcing bars (Fig-
ure 3) were carried out too, in order to determine
their mechanical characteristics.
The mechanical properties taken into consideration
within the whole experimental program are given in
Table 1, where the symbol ‘*’denotes the specimens
already tested and discussed in this paper. FRP
mechanical properties are based on experimental test
carried out at Milan Polytechnic University, within the
activity of the same research project [23]. With regards
to tensile strength of CFRP bars, the nominal value
furnished by the producer was reported.
Instrumentation, Test Setup and Loading
Each beam was instrumented by electric strain gauges
(SGs) as well as Linear Variable Differential Trans-
ducers (LVDTs), as shown in Figure 4. Five strain
gauges were bonded to the concrete faces at midspan
section (SG1, SG2, SG3, SG4, SG5). Furthermore,
three displacement transducers (V1, V2, V3) were
installed: one at midspan section and two other ones
at distance La=2 from each support. Another group of
four LVDTs (H1, H2, H3 and H4) were applied to the
concrete front face.
The above described equipment is introduced with
the following purposes:
• Five strain gauges (SG1-SG5), placed in the mid-
span section, are utilised to evaluate the flexural
curvature before cracking. The flexural curvature
is also calculated by data acquired from LVDTs
H1 and H3.Figure 2: Specimens before casting
Figure 3: Reinforcing bars: GFRP (Deff = 10 mm – DN = 10 mm) and CFRP (Deff = 12 mm – DN = 10 mm)
Table 1: Mechanical properties of specimens
Set
Concrete Rebars
Class
Ec
[Mpa]
Rcm
[Mpa]
Rck
[Mpa]
fcfm
[Mpa] GFRPCFRP
Ef
[Mpa]
fmax
[Mpa]
A0f[mm2]
Af
[mm2]
I* C20/25 27086 24.1 22.4 2.53 Glass 46 000 970 2/7 2/10
II C20/25 27086 24.1 22.4 2.53 Glass 46 000 970 2/7 5/10
III* C28/35 29490 32.0 30.1 3.08 Glass 46 000 970 2/7 2/10
IV C28/35 29490 32.0 30.1 3.08 Glass 46 000 970 2/7 5/10
V C20/25 27086 24.1 22.4 2.53 Carbon 115 000 2000 2/7 2/10
VI C20/25 27086 24.1 22.4 2.53 Carbon 115 000 2000 2/7 5/10
VII C28/35 29490 32.0 30.1 3.08 Carbon 115 000 2000 2/7 2/10
VIII C28/35 29490 32.0 30.1 3.08 Carbon 115 000 2000 2/7 5/10
*Specimens already tested and discussed.
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Flexural Behaviour of GFRP RC Members : L. Ascione, G. Mancusi and S. Spadea
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• Once cracks have occurred, the flexural curvature
is evaluated only by means of LVDTs H1 and H3.
• Moreover, vertical LVDTs V1, V2 and V3 are uti-
lised to evaluate the deflections of the beam over
its whole length.
• Finally three horizontal LVDTs (H2, H3, H4) are
utilised to evaluate the crack width.
The tests were performed at Structural Engineering
Laboratory of the University of Salerno. The experi-
mental setup includes (Figure 5):
• reaction frame for vertical loads,
• servo-controlled hydraulic actuator,
• 350 kN load cell.
The load was gradually increased up to failure. All
sensors (strain gauges, LVDTs, load cell) were
recorded by an automatic data acquisition system
(Vishay System 5000; Vishay Electronic GmbH, Selb,
Germany) connected to a computer.
Results and Discussion
The experimental results regarding ten beam speci-
mens are presented to highlight the peculiar features
of concrete members reinforced with GFRP bars and
stirrups. Comparison between experimental evidence
and theoretical predictions are also shown. With
regards to the latter, predictions have been estab-
lished using force equilibrium equations and strain
compatibility, by assuming the following hypothe-
ses:
• plane section preservation;
• perfect reinforcement-concrete bonding;
• no shear deformations.
Moreover, in order to compare the experimental
values, partial factors and environmental conversion
factors (see the design formulae given in [5]) have
been assumed unitary.
With regard to the theoretical value of cracking
load level, the following assumptions have been
introduced:
• concrete behaviour is linear elastic, with a ratio
between tensile to compressive Young modulus
equal to 0.50;
• concrete tensile strength is equal to fcfm, evaluated
according to the Italian code [22] (fcfm = 1.2 Æ fctm
where fctm ¼ 0:3 � f 2=3ck – unit strength N/mm2).
In Table 2, both experimental and theoretical values
of the cracking load level are summarised. Experi-
mental values are easily determined analyzing the
Load-Displacement curves and detecting the point
where the loss in term of secant stiffness is more than
10% of the initial tangent stiffness.
As expected, Table 2 confirms the low reliability of
the analytical prediction of the initial cracking load
level Fcr.
Figure 5: Testing setup
Figure 4: Instrumentation
Table 2: Initial cracking load (unit load: kN)
Set Test 1 Test 2 Test 3 Test 4 Test 5 Mean value Fcr
I 4.74 3.52 4.55 3.81 3.56 4.04 4.40
III 3.35 2.71 4.88 3.09 4.00 3.61 5.34
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L. Ascione, G. Mancusi and S. Spadea : Flexural Behaviour of GFRP RC Members
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In addition, the ultimate load has been evaluated
as follows:
• the concrete contribution has been accounted by
means of the equivalent rectangular stress-block;
• tensile concrete contribution has been neglected;
• compressive FRP bars have not been considered, as
suggested in [5];
In Table 3 the experimental values of failure loads are
presented. Comparisons with theoretical values for
both flexure and shear failure are also shown.
The average experimental ultimate load is, in both
cases, very close to the theoretically predicted one.
However, though the specimens were pre-designed
for a flexural collapse (rupture due to concrete
crashing), measured values of compressive deforma-
tion in concrete at the collapse are normally far from
ones expected. Moreover the actual failure mode has,
in many cases, similarities with a brittle one
(Figure 6). A great discordance between CNR-DT 203
and ACI 440 shear failure load formulae is found, due
to the strain limitation for FRP stirrups introduced in
the latter. It is worth noting that the latter prediction
seems to work better.
In Figure 7(A)–(E) an analysis of cracking phe-
nomenon up to failure of specimens is reported with
reference to Set I.
In Figure 8(A) and (B) bending moment versus
experimental flexural curvature response is shown for
beams (both set I and set III are referred to).
As it is easy to observe, the pre-cracking behav-
iour is clearly separated from the post-cracking
response: the moment-curvature relationship in
post-cracking state is quite linear, no relevant
tension stiffening contributions appeared in these
specimens.
In view of these considerations, a conservative
theoretical moment curvature relation can be repre-
sented through a tri-linear curve depending only on
three parameters, to be calculated according with the
aforementioned assumptions:
• cracking moment, Mcr;
• flexural stiffness of un-cracked cross section, EI1;
• flexural stiffness of totally cracked cross section,
EI2;
The idealised curves shown in Figure 9(A) and (B) still
represent bending moment versus experimental cur-
vature. Each curve, which refers to a single test, has
been obtained by the corresponding one shown in
Figure 8(A) and (B) by means of two linear regres-
sions, without any interdependency, respectively
based on the experimental data concerning the pre-
cracking behaviour and the post-cracked response. In
the same figures the theoretical relationships
between bending moment and curvature are shown.
As it can be seen, the actual flexural stiffness is
always lower than that predicted by theoretical for-
mulae when the reinforcement is made of GFRP bars,
though the totally cracked cross section was consid-
ered. It is to be remarked, however, that the experi-
mental curvatures evaluated by means of LVDT signals
may be affected by the short LVDT gauge-length
compared to the average distance between cracks.
Deflections
The Document CNR-DT 203/2006 [5] suggests dif-
ferent procedures for evaluating deflection in FRP
reinforced members.
The classical integration of moment curvature
diagram may be computed with non-linear analysis
by taking into account both cracking and tension
stiffening concrete.
Figure 6: A typical rupture pattern
Table 3: Failure load (unit load: kN)
Set Test 1 Test 2 Test 3 Test 4 Test 5 Mean value Fflexure* Fshear* Fshear**
I 21.15 20.06 20.98 19.98 20.10 20.45 19.81 64.74 25.04
III 21.64 23.34 26.01 23.76 22.79 23.51 23.64 68.44 25.58
*, CNR-DT 203/2006; **, ACI 440.
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Flexural Behaviour of GFRP RC Members : L. Ascione, G. Mancusi and S. Spadea
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(A)
(B)
(C)
(D)
(E)
80
24
20 20
6
7
7 7 7
14
30 30
30 30 20
35 39 38
9 15 13
37 35
30
35 30
25 35 30
15 15
15 15
15 25 11 11 11
7 7 7 7
20
20
35 35 35
26 30
30
35 35 35
35
30 23
20
20 20
20
26
8
8 8
8 8
8
8
8 14
11
11
11
11 11 11 11 11
11
11
11
11
11
11 10
10
10 10
10
10 20 20
7
7
8
8 8 8
915
6 12
16 29
8
8 8
19 19
19
19 19 19
35
12
36 25
25
30 8 25
25 25 25
10
10
10 10
25 25
25 9 9 16
16
16 20
20
38 35 35 25
25
25 25
6
6
6
6 8 6
15
15
15
15 15
17 17
17
17
17
19
19
19
9
9
9
9
9 9
9 9 9
9
32
32
7
7 7
7
7
31 19 24
24 24
9 15
15 15
15 15
20
20
6 6 9 9 9 9
20 20 9 9
39 10
11 20
20 14 12 13 37
19 15 10 37
37 19 13
A B C
A B C
A B C
A B C
A B C
A B C
A B C
A B C
A B C
A B C
Front
Front
Front
Front
Front
Back
Back
Back
Back
Back
15 12 14 10 37
24
20 20
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
Figure 7: Cracking patterns (A) specimen I.1, (B) specimen I.2, (C) specimen I.3, (D) specimen I.4, (E) specimen I.5
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L. Ascione, G. Mancusi and S. Spadea : Flexural Behaviour of GFRP RC Members
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A simplified formulation is derived from the one
proposed in Eurocode 2 for traditional reinforced
concrete members:
f ¼ f1b1b2
Mcr
Mmax
� �m
þ f2 1� b1b2
Mcr
Mmax
� �m� �(1)
where
• terms f1 and f2 are, respectively, the theoretical
deflection of un-cracked and totally-cracked sec-
tion;
• parameters b1 and b2 account for bond properties
of FRP bars and load duration, respectively;
• exponent m is a coefficient to be set equal to 2.00
[5].
Moreover appendix E of [5] reports a procedure to be
used by the manufacturer of the bars to determine
FRP bar-concrete bond in order to accurately evaluate
deflections. It substantially consists of calibrating the
exponent m on the basis of the comparison between
analytical and experimental results – evaluated over
the range between 20% and 60% of ultimate load –
setting coefficient b1 and b2 unity and using an
appropriate statistical analysis.
In this work the calibration of exponent m has been
carried out using data experimentally obtained and
adopting two possible statistic procedures. A minimi-
zation of standard error, e1, and mean error, e2, was
executed separately on data related to Set I and Set III,
as well as on the entire population of data.
e1 ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1
n
Xn
i¼1
fðexpÞi � f
ðthÞi
� �2
fðthÞi
� �2
vuuuut ; (2)
e2 ¼1
n
Xn
i¼1
fðexpÞi � f
ðthÞi
fðthÞi
: (3)
The statistical results obtained lead to the finding
that for the peculiar bond property of GFRP bars
used, the coefficient best fitting experimental data is
equal to 1.50.
Additionally, the rational procedure proposed by
Razaqpur et al. [15], consisting on the integration of a
tri-linear moment-curvature relation without taking
into consideration tension stiffening effects, is taken
in consideration. In case of a four point bending
scheme the expression, given in [15], results:
f ¼ f2 �FL3
3EcI21� I2
I1
� �L�
L
� �3" #
(4)
(A)
(B)
Figure 8: Bending moment versus experimental flexural cur-
vature (A) (set I), (B) (set III)
(A)
(B)
Figure 9: Idealized bending moment versus experimental
flexural curvature (A) (set I), (B) (set III)
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Flexural Behaviour of GFRP RC Members : L. Ascione, G. Mancusi and S. Spadea
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where L* = 2Mcr/F represents the extension over
which the beam is un-cracked.
In Figure 10(A) and (B) the experimental mid-span
displacements versus the total applied load, 2F, are
shown with reference to specimens of set I and set III.
Moreover, three analytical predictions, evaluated only
in the post-cracked phase, are depicted:
• curve a corresponds to the predicting formula
given in Equation (1), where b1 = 0.50, b2 = 1.00,
m = 2.00;
• curve b corresponds to the same formula, where
b1 = 1.00, b2 = 1.00, m = 1.50;
• curve c corresponds to Equation (4).
All the analytical models are in good agreement with
the experimental data with conservative predictions.
The model b, based on the assumption of unitary
values of b1, b2 and the calibration of exponent m,
gives the curve of best fit with the experimental data.
Crack Width and Distance Between Cracks
According toCNR-DT203/2006 [5], bothcharacteristic
crack width, wk, and average distance between cracks,
srm, have been evaluated over the range between 20%
and 60% of the ultimate load. Figure 11(A) and (B)
illustrates a comparison between experimental values
of crack width and the theoretical values wk.
In Table 4 both experimental and theoretical values
of theaveragedistancebetweencracksaresummarised.
Experimental data seem to assess the reliability of
the predicting formula proposed in [5] for crack width,
though a comparison with characteristic values is
done in Figures 11. On the other hand the theoretical
prediction of average distance between cracks is defi-
nitely high if compared with the experimental values.
The result is that cracking phenomenon experi-
mentally observed is, generally, more relevant than
the one predicted.
Conclusions
A large experimental program on the mechanical
behaviour of concrete members with FRP internal
reinforcement is still under development at the
(A)
(B)
Figure 10: Total applied load (2F) versus mid-span deflection,
experimental data and analytical predictions (A) (Set I), (B) (Set
III)
(A)
(B)
Figure 11: Bending moment versus crack width, experimental
data and analytical predictions (A) (Set I), (B) (Set III)
Table 4: Average distance between cracks (unit length: mm)
Set Test 1 Test 2 Test 3 Test 4 Test 5 Mean value Srm
I 143 118 105 108 110 117 193
III 112 110 100 110 133 112 193
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L. Ascione, G. Mancusi and S. Spadea : Flexural Behaviour of GFRP RC Members
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Department of Civil Engineering of Salerno Univer-
sity. A part of this research, which has been recently
completed, was presented and discussed. The results
obtained by the authors show the main features of
the behaviour of such members.
• The predicting formula proposed by CNR-DT 203
for evaluating shear failure load seems to largely
overestimate the actual strength of FRP rein-
forced members. This point requires further
investigation in order to improve the reliability
of the design approach.
• The moment-curvature relation of FRP reinforced
members is basically linear in both the pre-
cracked and post-cracked phases, no relevant
tension stiffening contributions appear.
• Experimental results substantially assess the reli-
ability of the predicting formulae proposed in [5]
for both deflections and crack width.
• The procedure of calibration of the exponent m
seems to be consistent and could be enlarged to
other experimental data, to be obtained in sub-
sequent tests.
• Neglecting effects of tension stiffening in calcu-
lating the deflection of flexural members leads to
results in good accordance with experimental
data.
• Cracking phenomenon experimentally observed is
more relevant than the one predictable according
with [5].
Subsequent experimental tests will be useful for
analyzing these topics in members with different
characteristics. More investigation is also required in
order to better predict the actual failure mode. With
this aim, the experimental response of concrete
beams reinforced with steel bars and/or stirrups is
going to be investigated by the authors in order to
compare their mechanical behaviour with that one
here investigated, both at service and ultimate con-
ditions.
ACKNOWLEDGEMENTS
This research is a part of a main national research project
[23] financially supported by Italian Ministry of Research
whose contribution is gratefully acknowledged.
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