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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 5, September – October (2013), © IAEME

TECHNOLOGY (IJCIET)

ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 4, Issue 5, September – October, pp. 61-77
© IAEME: www.iaeme.com/ijciet.asp
Journal Impact Factor (2013): 5.3277 (Calculated by GISI)
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IJCIET
©IAEME

FLEXURAL STRENGTHENING OF TIMBER BEAMS USING CARBON
FIBER REINFORCED POLYMER PLATES
Javaid Ahmad1, Dr. Javed Ahmad Bhat2
1

2

Graduate Student, National Institute of Technology, Srinagar
Associate Professor, National Institute of Technology, Srinagar

ABSTRACT
The aim of current study is to investigate the effect of Carbon Fiber Reinforced Polymer
(CFRP) composites on flexural behavior of timber beams. Ten beams with cross section of 70mm x
120mm were tested, where two served as control beams (without CFRP strengthening). Two species
of timber were used in this study based on their availability in this region viz. Cedrus Deodara
(Deodar) and Pinus Wallichiana (Kail).An experimental investigation was conducted on the behavior
of FRP-reinforced wood section. The strength of timber beams was significantly improved upon
strengthening with maximum percentage increase being 114.28%for Deodar beams and 140% for
Kail beams. This study, in turn, led to an empirical procedure based on modification factors, suitable
for application in the design of FRP reinforcement of existing timber beams under different
configuration of strengthening materials applied.
Keywords: Retrofitting, Fiber Reinforced Polymers, Flexural Strengthening, Timber Failures,
Bending Strength, Stiffness.
1. INTRODUCTION
Rehabilitation of deteriorated civil engineering infrastructure such as buildings, bridge decks,
beams, girders, marine structures, roads etc. has been a major issue in last decade. The deterioration
of these structures might be due to aging, poor maintenance, corrosion due to unfavorable
environmental conditions, poor initial design or construction and accidental situations like
earthquakes. The need to upgrade the deteriorated civil engineering infrastructure is necessitated due
to the ever increasing demand e.g. unprecedented loads on buildings which have not been considered
in design and likewise.
New technology options in rehabilitation are being developed from polymers, metals,
ceramics and composites of these materials and some of these high performance materials are
61

already being utilized in construction. While the concept of composites has been in existence for
several millennia, the incorporation of fiber reinforced polymer (FRP) is less than a century old.
These composites combine the strength of fibers with the stability of polymer resins.
Market penetration of timber and fiber composites within the construction industry will
ultimately be determined by final cost. CFRP is more expensive. However, material cost alone is not
the prime consideration for many applications using these materials. It is important to consider the
total built cost rather than just comparing the material component costs with traditional materials but
the costs of design, manufacture, fabrication and erection need to be considered as well. Building
owners, when looking at new construction, have not given a great deal of consideration to long term
rehabilitation methods, costs or effects on projected service. Although research has been done to
strengthen timber using FRP, but the comprehensive analysis and design are not established in detail.
This is one of the reasons why the application of FRP to timber is very limited.
From investigations that have been conducted in the last decades to some of the bridges in
Canada, United States, Czech Republic, Germany, Great Britain, Japan, and Switzerland showed that
the soffits of the bridge have been strengthened using various carbon FRP materials (Horyna et al.,
2001).[13]
In the plate bonding or externally bonded technique, the FRP is situated on the external
tensile surfaces of the concrete beam to improve the flexural strength or on the vertical surfaces of
the beam to increase its shear capacity (Allbones, 1999)[3]. Arduini and Nanni (1997) have
investigated plates in the form of flexible carbon FRP sheets, to be externally bonded to the concrete
surfaces. The strengthening technology consisting of externally bonded CFRP sheets to concrete is
easy to perform and significant improvement was found for ultimate load capacity and to a lesser
extent in flexural stiffness.[5]
Plevris and Triantafillou (1992) provided an analytical study of the short-term flexural
behavior of timber beams and beam-columns reinforced on the tension face only with epoxy-bonded
unidirectional CFRP sheets. This work demonstrated that even a small amount of fibers, as low as
1% of the cross-sectional area, of thin carbon FRP bonded to timber beams could result in a strength
increase on the order of 60%.[18]
There was another research done by Fiorelli and Antonio (2002) to evaluate the structural
behavior of timber beams strengthened with FRP. The research was focused on the experimental and
theoretical analysis of timber beams of the species Pinus Caribea Hondurensis which were reinforced
with GFRP and CFRP fabrics. The results of this research showed that the flexural stiffness (EI)
determined experimentally was greater than the theoretical values. These values are in favor of
structural safety. It shows that the increase of stiffness varied from 15% to 29% for beams
strengthened with glass and carbon fabric. The use of FRP provides better results in load capacity
and in the vertical displacement of the beam.[9]
Gentile et al. (2002) have investigated creosote-treated sawn Douglas Fir timber beams
strengthened with GFRP bars. The beams were obtained from a dismantled bridge that was in service
for over 30 years and were tested to obtain initial stiffness before strengthening. The percentage
reinforcement ratios were between 0.27 and 0.82%. The results have shown that the failure mode has
changed from brittle tension to compression failure. The flexural strength increased by 18 to 46%.[10]
Buell and Saadatmanesh (2005) have conducted research on creosote-treated solid-sawn
Douglas Fir strengthened with bidirectional CFRP fabric. The results show that applying carbon
fabric to the timber beams provides significant increase in the bending and shear capacity, and
nominal increase in the stiffness of the beams. The ultimate bending strength was increased between
40 to 53% and the horizontal shear strength was increased between 36 to 68%.[6]
Micelli et al. (2005) have investigated on flexural reinforcement of glulam timber beams with
CFRP rods. Flexural behavior of CFRP-reinforced beams was compared with unreinforced beams
that were used as control specimens. Experimental results showed a significant influence of the
62

CFRP rods because the reinforced beams demonstrated an increase in ultimate capacity and stiffness.
An increase in ultimate moment of 26% and 82% was recorded with respect to unreinforced beams
for 0.51% and 1.03% cross sectional reinforcement.[17]
The present research focuses on application of pre-fab CFRP strips for strengthening timber
beams. The strips or plates are attached to beams by means of specified adhesive. The flexural tests
are carried out on timber beams strengthened with CFRP in varying proportion with an object of
studying the improvement in load carrying capacity, modulus of rupture and flexural rigidity of these
beams. However, the scope of this study was limited to seasoned dry timber only which infers the
applicability of research findings to beams used in dry condition or in the interior of structure.
The timber species used in this research include Cedrus Deodara commonly known as
Deodar and Pinus Wallichiana commonly known as Kail. These species are being widely used in
upper northern region (primarily Jammu and Kashmir) of India for structural purposes due to its
prime availability in this region. Besides, this these timber species arehaving good mechanical
properties as compared to other timber species found in northern region of India. Having been widely
used in structures constructed decades ago, it has become necessary torehabilitate the timber
structures which have suffered damage. Leaving the application of conventional rehabilitation
techniques aside, we are left with application of fiber composites as best alternatives for
rehabilitation of these structures. So, research has been conducted at National Institute of
Technology, Srinagar (NIT Srinagar) to study the feasibility of utilizing CFRP for rehabilitation or
design of new structures using Deodar & Kail.
2. METHODOLOGY
This section describes features of beam specimens, beam designation, loading equipment,
instrumentation and testing schemes. In the present study, ten timber beams were prepared for testing
among which eight beams were strengthened using CFRP Plates of different widths. The beams
without strengthening served as control beams and used as reference level for checking improvement
in properties of strengthened timber beams. As already mentioned timber species utilized for beams
ware Cedrus Deodara and Pinus Wallichiana. The ultimate tensile strength and modulus of elasticity
of specimens of Deodar were observed to be 35 MPa and 10 GPa respectively. Whereas for Kail,
ultimate tensile strength and modulus of elasticity were observed to be 20 MPa and 8 GPa
respectively. The typical geometry and testing arrangement is shown in Fig1.

480mm(Deodar)
360mm(Kail)

P

P

480mm(Deodar)
360mm(Kail)

70mm
120mm

1524mm(Deodar)
1473mm(Kail)

CFRP Strip

Fig 1: Flexural Test – Loading Arrangement

63

The properties of materials used are summarized in Table 1 whereas beam designation are
designations
shown in Table 2.
Table 1: Material Properties
Property

Deodar

Ultimate tensile strength,
N/mm2

35

Ultimate tensile strain, %

Kail

0.40

2000

20
0.38

10

Tensile modulus of
elasticity, KN/mm2

CFRP

1.65
175

8

Table 2: Beam Designation
Designation

Width of CFRP
Strip, mm

Thickness of
CFRP Strip, mm

Deodar

Kail

Control Beam- D

Control Beam- K
Beam

-

-

FPD-30-1

FPK-30-1

30

1

FPD-40-1

FPK-40-1

40

1

FPD-50-1

FPK-50-1

50

1

FPD-70-1

FPK-70-1

70

1

Bf = 30mm
ACFRP = 0.36%

Bf = 40mm
ACFRP = 0.47%

Bf = 50mm
ACFRP = 0.59%

Bf = 70mm
ACFRP = 0.83%

Fig 2: Graphical representation of Beam cross
:
cross-sections
Reinforcement was used in the form of CFRP strips of varying widths applied on the bottom
face of the beams using adhesive. The CFRP sheets were supplied by GSP Superb Technology
(India), a New Delhi based supplier who imports it from Korea. The mechanical properties of CFRP
are shown in Table 1. Retrofitted beam specimens were strengthened with a single strip of CFRP of
thickness 1mm and different widths in all specimens varying from 30mm to 70mm.
64

The tests were carried on beam models in a loading frame of capacity 500KN. Prior to testing
dial gauges were set up at mid span and two alternate points. The loading was applied by means of
hydraulic jack till failure and deflections were noted every 4KN interval load. The failure pattern was
studied for each beam discussion regarding which is given afterwards. Fig 3 shows schematics of
flexural testing.

(a) Front View

(b) Side View
Fig 3: Testing Arrangement

3. RESULTS AND DISCUSSIONS
The results from laboratory testing for the strengthened beams are compared with control
beam (un-strengthened) in order to study the behavior of strengthened timber beams in terms of load
carrying capacity, bending strength, stiffness and ductility. In addition, the modes of failure for all
beams have been studied and discussed.
3.1 Unstrengthened Beams
Two unstrengthened timber beams (Deodar& Kail) were tested under two point loading. This
is to determine the initial bending stiffness whilst in the elastic zone. The Deodar Control beam
started to behave non-linearly when the load was 16.6 kN and the corresponding deflection was 10.5
mm. The failure load was 29.05KN for Deodar Control Beam. Any non-linearity in the behavior of
timber beam was due to plasticity in the compression zone and the level of plasticity will determines
whether the beam fails in compression or otherwise. Similarly, Kail Control beam started to behave
non-linearly when the load was 16.6 kN and the corresponding deflection was 12.5 mm. The failure
load was 20.75KN for Kail Control Beam.
Fig 4 shows load versus deflection at mid-span of control beam for every 4 kN load interval.
It is clear that all beams exhibited linear elastic as expected. The gradients of the lines were
determined from which the stiffness of the beams can be calculated. This was done so that the
uniformity of initial stiffness could be judged. Table 3 contains other properties of control beams.
Moreover, Fig 5 shows failure pattern of control beam.

65


35
30

Load, KN

25
20
Control Beam Deodar

15

Control Beam Kail
10
5
0
0

5

10

15

20

25

30

Midspan Deflection, mm

Fig 4: Load Deflection Curve for Control Beams
Table 3: Control beam test results

Failure Load, Pu, KN
Maximum Midspan
Deflection, δu, mm
Flexural Rigidity, EI,
KNm2
Modulus of Rupture,
N/mm2

Deodar Control
Beam
29.05

Kail Control Beam

23

15

76.40

62.19

41.5

22.23

(a) Deodar Control Beam

20.75

(b) Kail Control Beam
Fig 5: Failure Pattern
66

3.2 Strengthened Beams
The beams were tested under two point loading until failure. For comparison purposes, the
results of load-deflection curves for all beams are plotted. Main discussion from curves focuses on
load carrying capacity, bending strength, stiffness and ductility of the strengthened beams relative to
the un-strengthened beams (control beams). In addition deflection, crack patterns and failure
behavior are also discussed.
In general, all control beams and strengthened beams behaved linearly elastic initially and as
the load increased, the flexural cracks increased in number, width, and depth, and the beams tend to
behave non-linearly until failure. There are some slight drops observed in some of the curves as a
result of internal cracking of the beam, which is hardly recognized through the observation with eyes
and ears. All figures clearly indicate that all the strengthened beams have higher ultimate load
carrying capacity than control beam. When there is significant failure, the corresponding load will be
taken as ultimate load carrying capacity. Besides that, the strengthening beams with FRP also
enhance the flexural stiffness. In other words, the strengthened beams experienced lower deflection
than the un-strengthened control beam at the same load level. This low deflection phenomenon is
desirable in the aspect of serviceability limit state in design to ensure comfortability of timber
structures. In timber design, generally the deflection will govern the design.
Beams were strengthened using CFRP plates with of thicknesses 1mm and four different
widths i.e. 30, 40, 50 and 70 mm. The span of Deodar beams was 1.524m and that of Kail beam was
1.473m. In this study, the same control beams were used for comparison. The beams were tested and
the graphs of load versus mid-span deflection were plotted in Fig 6 and Fig 7.
70

60

Load, KN

50

40

Control Beam Deodar
FPD-30-1

30

FPD-40-1
FPD-50-1

20

FPD-70-1

10

0
0

10

20

30

40


Fig 6: Load Deflection curves for Deodar beams strengthened using CFRP

67

60

50

Load, KN

40
Control Beam Kail
30

FPK-30-1
FPK-40-1
FPK-50-1

20

FPK-70-1
10

0
0

5

10

15

20

25

30

35


Fig 7: Load Deflection curves for Kail beams strengthened using CFRP

Beam FPD-30-1 starts to behave non-linearly when the load was 14.94KN and corresponding
deflection was 8.25mm. This point was the limit for proportionality and the beam shows non-linear
behavior beyond this point where from the stiffness decreased gradually. It should be noted that
although the beam seems to have impending crushing failure, but it was not. This phenomenon
usually happens in timber beams subjected to point load. No matter how the point load is applied,
high bearing stress will definitely occurs at the point of application. This cannot be totally eliminated
during testing although wider bearing can be provided for that particular zone but it will not give
good simulation to the point load. When the load was increased to 30.71 kN, partial crushing
occurred at top layer of compression zone. The load was then increased and the beam still resisted
extra load till it reached 33.2 kN whilst the corresponding deflection was 19.55 mm, tensile cracks
propagated towards neutral axis, however no slip appeared. Suddenly at 37.35 kN, a big sound was
heard and the load decreased abruptly. A big tensile crack occurred with many small cracks were
distributed over longer area and the CFRP plate was delaminated due to timber crack. Shear cracks
were also observed which give contribution to the total failure. In fact, this beam failed in tension
although crushing failure occurred too but it was initiated by high bearing stress. There were few
crushes observed and were distributed to a small area of compression zone. The CFRP plate was not
ruptured indicating that the yield or ultimate stress was not reached.
For beam FPD-40-1 the proportionality limit was 20.75 kN which was slightly higher
compared to beam FPD-30-1. This beam has shown good performance at the beginning of the testing
where the slope of the curve was greater than the slope for beam FPD - 301. First sound of crack at
tension zone was heard when the load was 24.9 kN. At this stage, any additional load was taken by
the strengthening material. When the load was increased to 41.5 kN, the compression zone failed in
crushing caused large deflection to the beam and total tensile crack took place. It is very common to
timber beams that when the beam fails in tension, the compression fiber fails too and vice versa due
68

to large deflection. Number of crushes was similar to the previous beams but the cracks were shorter
and distributed over small area. Thus, by increasing the CFRP area, it helps the beam to reduce the
serious crack but the strengthened area was not sufficient to prevent the beam from tension failure.
However, the CFRP plate did not fail because the ultimate strain was not reached. In this case the
main function of CFRP plate is to share the stress at the tension fiber and reduced the deflection and
hence delays the tensile crack and plastic formation. From this test the beam failed in tension. It is
very interesting that there was no peel off at both end of the plate and also no de-bonding occurred
between CFRP plate and the bonding agent and between bonding agent and timber substrate.
When the beam was strengthened with more area of CFRP plate such as beam FPD-50-1, the
load-carrying capacity was increased as well as the stiffness which will be discussed latter. This
beam starts to behave non-linearly when the load was 18.26KN and corresponding deflection was
5.82mm. Beyond this load the beam behaved non-linearly. Very little cracks were observed at 47.31
KN and the cracks were localized. The cracks occurred at the beam corner at midspan where the
maximum bending stress occurred. Corners or edges are weak zone for timber beams. Hence, the
cracks were considered not serious because the main timber fibers were not cracked. Many crushes
were observed clearly on both front and back side of the beam when the beam reached its ultimate
load at 62.25 kN. At failure, there was no peel off at both end of the plate and also no debonding
occurred between CFRP plate and the bonding agent and between bonding agent and timber
substrate. It seems that the maximum tensile and compressive capacity of timber was fully utilized.
The tensile strength of CFRP plate was too high relative to the tensile strength of timber. Hence the
CFRP plate was not fully utilized.
It is expected that the beam has a maximum limit for compressive capacity regardless how
the strengthening is done. To confirm this hypothesis, a beam called FPD-70-1 was strengthened
using wider and thicker section where the cross section dimension was 70 mm by 1 mm. During the
testing the beam looks strong and stiff. The proportionality limit was 12.45 kN. The first crush was
started at 20.75 kN and as the load increased, the crushes propagated beyond the neutral axis. The
beam started to crack when the load reached at 41.5 kN, however the cracks were prevented to
propagate to wider area by the help of CFRP plate. The tensile cracks were small and only the
bottom layer at midspan was affected. The total failure occurred after almost 80% of the beam depth
was crushed at the ultimate load of 43.16 kN.It very obvious that the compression zone has reached
it ultimate capacity. Any CFRP area of more than 0.59% of beam cross section will not give
enhancement to the ultimate capacity unless the compression zone is strengthened. The movement of
crushing line up to 80% of the beam depth showed that the strengthening material was very strong
which prevent the tension zone from tensile failure. Thus, when the beam was strengthened using
more area or wider plate, the ultimate load as well as the slope of load-deflection curve was
increased and the failure mode was changed from tension to compression. Small cracks were
common to all timber beams as the grains were not perfectly parallel to the beam length. At failure,
there was no peel off at both end of the plate and also no debonding occurred between CFRP plate
and the bonding agent and between bonding agent and timber substrate. This was exactly similar to
other strengthened beams. Bear in mind that this beam has moderate defect as a knot. Knot is a weak
zone to timber and will affect the strength, but the CFRP plate is able to reduce the effect of defect.
This is one of the advantages of strengthening the timber beam using CFRP plate.
Similar behavior was observed in case of kail beams. The ultimate load in case of FPK-30-1,
FPK-40-1, FPK-50-1 and FPK-70-1 was 29.05KN, 41.5KN, 49.8KN and 33.2KN respectively.
Moreover the maximum mid span deflection for FPK-30-1, FPK-40-1, FPK-50-1 and FPK-70-1 are
14.76mm, 15.4mm, 14.18mm and 28.8mm respectively. The balanced reinforcement for kail beams
was observed at 0.59% beyond which the beams were observed to be over-reinforced. The maximum
increase in load capacity relative to control beam was 140% at CFRP area of 0.59%. After the tensile
reinforcement was increased beyond 0.59%, load capacity decreased as was observed that load
69

capacity at reinforcement of 0.83% was 33.2KN showing just 60% improvement as compared to
140% improvement in case of reinforcement area of 0.59%.
All beams in this study did not fail due to peel off at plate ends. This showed that the external
anchorage systems are not required. It is expected that when the bigger section is strengthened where
the bending capacity is higher, the CFRP plates might fail due to peel off because the shear stress at
plate ends become very high. At this stage the external anchorage systems should be carefully
designed to take this stress. The research on this part was studied by Spadeaet al. (1998) and they
found that the use of external anchorage would increase the beam capacity.
The application of CFRP plate gives advantage where large area of the bottom layer of the
tension zone can be protected. The larger the area protected, the lesser the weak zone and hence the
lesser the crack. Failure mode of a beam is shown in Fig 8 and Fig 9.

Fig 8: Failure pattern of beam FPD-30-1

Fig 9: Failure pattern of beam FPK-30-1
3.2.1 Effects on Load Carrying Capacity
The summary of load carrying capacity of all tested beams is shown in Table 4. The
strengthened beams have greater load carrying capacity when compared to control beam. Generally,
the ultimate and service load carrying capacity was increased as the percentage of CFRP increased.
The ultimate load was increased significantly when the percentage of CFRP plate was 0.36%. This
percentage was considered small when compared to beam cross section but the strength achievement
was very encouraging where the increase in ultimate load was 28.57% for Deodar beam and 40% for
Kail beam. The highest increment for ultimate load among these beams was 114.28% for Deodar
beams and 140% for Kail beams, when the beam were strengthened using CFRP area of 0.59%.

70

Table 4: Effect of CFRP area on Load Carrying Capacity of Deodar Beams
Ultimate
Midspan
Increase in
Area of
Load,
Deflection,
Ultimate
Beam
CFRP
Pu
δu
Load
(%)
(KN)
(mm)
(%)
Control Beam-D
0
29.05
23
FPD-30-1
0.36
37.35
22.10
28.57
FPD-40-1
0.47
41.5
17.7
42.86
FPD-50-1
0.59
62.25
16.5
114.28
FPD-70-1
0.83
43.16
36.76
48.57
Table 5: Effect of CFRP area on Load Carrying Capacity of Kail Beams
Ultimate
Midspan
Increase in
Area of
Deflection,
Ultimate
Load,
Beam
CFRP
Pu
δu
Load
(%)
(KN)
(mm)
(%)
Control Beam- K
0
20.75
15
FPK-30-1
0.36
29.05
14.76
40
FPK-40-1
0.47
41.5
15.4
100
FPK-50-1
0.59
49.8
14.18
140
FPK-70-1
0.83
33.2
28.8
60
To get better and clearer understanding regarding this relationship, the percentage increase in
ultimate load were plotted against percentage area of CFRP as shown in Fig 8. The relationship was
not linear. When the beam was strengthened using more than 0.59% CFRP, it would not enhance the
load carrying capacity as the compression zone has achieved maximum capacity. Thus, It can be
concluded that the beam experiences over-reinforced if the percentage of CFRP is greater than
0.59%. This is clearly shown by the curve in Fig 10.
70
60

Load, KN

50
40
Effect of CFRP Area (Deodar)

30

Effect Of CFRP Area (Kail)
20
10
0
0

0.2

0.4

0.6

0.8

1

Area of CFRP, %

Fig 10: Effect of CFRP area on ultimate load
71

It seems that the strengthened beams were over reinforced especially for beam FPD
FPD-70-1 in the
ultimate limit state point of view but the provided area is beneficial to the service limit state design
because it increases the service load carrying capacity up to 48.57%. In summary, the load carrying
capacity of the strengthened beams increased between 28.57% to 114.28% for Deodar and 40% to
140% for Kail, when the CFRP area is between 0.36% to 0.59%.
3.2.2 Effects on Bending Strength (Modulus of Rupture)
The bending strength or modulus of rupture (MOR) has been defined earlier.For beams
(
)
strengthened using CFRP plate, the plate was transformed into equivalent timber area. First, the
modular ratio, n needs to be calculated. The average elasticity modulus of CFRP plate as obtained
upon experimentation was 175 kN/mm2. Hence, the modular ratio is;

Where, ETd= Modulus of Elasticity of Deodar = 10GPa
Beam FPD-30-1 is taken for typical calculation of Modulus Of Rupture. This beam was
1
.
strengthened using CFRP plate with 30 mm wide and 1mm thick as shown in Fig 11 Applying the
n
11.
following relationship, the equivalent timber area is;
AT = ndACFRP = 17.5 x 30 x 1 = 525 mm2
Now, if the thickness of the plate is maintained, the new plate width, bTof the timber cross
section at bottom layer becomes;
bT = AT/t = 525 mm

Fig 11: Section of Timber Beam
11

The same procedures were applied for other beams. The summary of location of neutral axis
(NA), moment of inertia and Modulus of Rupture is shown in Table 6 and Table 7.

72

Table 6: Bending strength of Deodar beams strengthened using CFRP
Area of
CFRP

Beam

(%)

Location of
NA from
bottom,

Moment of
Inertia
(mm4)

(mm)

Modulus of
Rupture
(N/mm2)

Modification
Factor

Control Beam-D

0

60

10080000

41.5

1

FPD-30-1

0.36

57.44

11888637.88

43.31

1.04

FPD-40-1

0.47

56.35

12445143.08

45.09

1.08

FPD-50-1

0.59

55.29

12980648.44

63.64

1.53

FPD-70-1

0.83

53.3

13993242.08

34.14

0.82

Table 7: Bending strength of Kail beams strengthened using CFRP

Beam

Area of
CFRP
(%)

Location of
NA from
bottom,

Moment of
Inertia
(mm4)

(mm)

Modulus of
Rupture
(N/mm2)

Modification
Factor

Control Beam-K

0

60

10080000

22.23

1

FPK-30-1

0.36

56.63

12300945.75

24.08

1.08

FPK-40-1

0.47

55.31

12971638.59

31.85

1.43

FPK-50-1

0.59

54.05

13611518.56

35.59

1.60

FPK-70-1

0.83

51.69

14806961.38

20.86

0.94

It is observed that neutral axis moves down below the beam mid-depth as the area of CFRP
plates was increased. For Deodar beams, the maximum movement was 6.7 mm when the area of
CFRP is 0.83% where for Kail beams, the maximum movement was 8.31 mm when the area of
CFRP is 0.83%..This movement causes lower stress at tension zone and higher stress in compression
zone. This movement has good agreement with the strain reading during testing. Moment of inertia
was also increased as the area of CFRP was increased. The maximum increment was 38.82% for
deodar and 46.89% for Kail beams. The MOR values are plotted versus CFRP area as shown in Fig
12.The results suggest that by using CFRP plates increases the MOR level and when the area of
CFRP plates is about 0.59%, the MOR achieved its maximum value and then decreases as shown in
graph. It was found that the MOR for beam FPD-70-1 was slightly lower compared to beam FPD-501. The main reason is that the ultimate load for beam FPD-70-1 was lesser than beam FPD-50-1 but
the section modulus is higher which yields to lower MOR value.

73

70
60

MOR, N/mm2

50
40
MOR Kail

30

MOR Deodar
20
10
0
0

0.2

0.4

0.6

0.8

1

CFRP Area, %

Fig 12: MOR Versus CFRP Area
3.2.3 Effects on Flexural Rigidity (EI)
The flexural rigidity of beams strengthened using CFRP plate is given in Table 8 and Table 9.
The flexural rigidity of beams under the loading arrangement shown in Fig 1 is calculated using
equation as;
௉௔
∆max=
ሺ3‫ܮ‬ଶ െ 4ܽଶ )
ଶସாூ

Where, L = 1.524m & a = 0.48m (for Deodar)
L = 1.473m & a = 0.36m(for Kail)
∆max= Maximum midspan deflection
Generally, the flexural rigidity was increased as the percentage of CFRP plate increased. The
effect of plate width can be confirmed by comparing beams FPD-30-1, FPD-40-1, FPD-50-1 and
FPD-70-1. Wider plate is more effective to prevent the initial crack at bottom layer because most of
the tensile cracks started at the unstiffened portion. The flexural rigidity of the beams could be
increased between 33.66% to 198.5% for Deodar beams and 42.21% to 153.67% for Kail beams,
when the timber beams were strengthened using CFRP plate of area between 0.36 – 0.83%.
Table 8: Effect of CFRP area on stiffness of Deodar beams
Percentage of
Flexural
Increase in EI,
Modification
Beam
CFRP Area,
Rigidity,
%
Factor
2
%
EI, KNm
(EI)
Control
0
76.40
1
Beam-D
FPD-30-1
0.36
102.12
33.66
1.34
FPD-40-1
0.47
141.75
85.53
1.85
FPD-50-1
0.59
228.07
198.5
2.98
FPD-70-1
0.83
85.49
11.89
1.11
74

Table 9: Effect of CFRP area on stiffness of Kail beams
Percentage of
Flexural
Increase in EI,
Modification
CFRP Area,
Rigidity,
%
Factor
(EI)
%
EI, KNm2

Beam
Control
Beam-D
FPK-30-1
FPK-40-1
FPK-50-1
FPK-70-1

0

76.40

-

1

0.36
0.47
0.59
0.83

102.12
141.75
228.07
85.49

42.21
94.59
153.67
-16.91

1.34
1.85
2.98
1.11

Fig 13 shows the relationship between the flexural rigidity and the area of CFRP plates. It is
observed that there must be a limiting value for the flexural rigidity after certain amount of CFRP
area and hence the relationship would yields to optimum value for CFRP area.
250

200

EI, KNm2

150
EI Vs CFRP Area (Deodar)

100

EI Vs CFRP Area(Kail)
50

0
0

0.2

0.4

0.6

0.8

1

CFRP Area

Fig 13: Flexural rigidity Versus CFRP Area
4. CONCLUSIONS
I.

II.
III.
IV.
V.

All the strengthened beams exhibited linear elastic behavior in the first stage followed by
non-linear in a short period and showed almost linear plastic behavior in the last stage before
the beams failed.
In general, the load carrying capacity of the strengthened beams was increased.
The balanced reinforced occurred when the CFRP was about 0.59%. Thus, the beam
experiences over-reinforced if the percentage of CFRP is greater than 0.59%.
The maximum increase in load carrying capacity was 114.28% for Deodar beams and 140%
for Kail beams when CFRP area was 0.59%.
Moment of inertia was increased as the area of CFRP was increased. The maximum
increment was 38.82% for Deodar beams and 46.89% for Kail beams.
75

VI.
VII.
VIII.

IX.

X.

XI.

The strengthening using wider plate was very efficient to enhance the stiffness. Wider plate
was effective to prevent the initial crack at bottom layer.
The stiffness of the beams was increased as the area of CFRP plate increases but up to CFRP
area of 0.59% beyond which it decreased owing to over-reinforced section.
It is concluded that 0.59% was the optimum value of CFRP area for maximum ductility
index. This finding was synchronized with the results for strength where the optimum value
for CFRP area that can provide maximum strength was also 0.59%.
All beams in this study did not fail due to peel off and also no de-bonding occurred between
CFRP plate and the bonding agent and between bonding agent and timber substrate because
the bonding length for all beams (1.5 m) was sufficient.
When FRPs were bonded to the timber, randomization of defects (e.g., timber knots and
drying cracks) reduces the variability of properties and consequently increases the tensile
strength. Other effect was that the "bridging effect" exerted by the FRP composite skin on
the bonded timber also increases the tensile strength.
Although steel bars or plates are still used for strengthening works, but if high strength,
modulus and ductility of FRP are used, the FRP could be the right choice provided the
overall price is considered.

5. REFERENCES
[1]

A Borri, Dr M Corradi, Andrea Grazini (2003), FRP Reinforcement of Wood Elements under
Bending Loads.
[2] Alann André and Robert Kliger, (2009), Strengthening Of Timber Beams Using Frp, With
Emphasis On Compression Strength: A State Of The Art Review. The second international
conference of International Institute for FRP in construction for Asia-Pacific Region.
[3] Allbones, C. (1999). The Use of Pultruded Composites in the Civil Engineering and
Construction Industry.Proceedings of Composites and Plastics in Construction. 16-18 Nov.
Watford, UK, pp (5) 1-3.
[4] A Yusof and A. L. Saleh, (2010), Flexural Strengthening Of Timber Beams Using Glass
Fibre Reinforced Polymer Electronic Journal of Structural Engineering
[5] Arduini, M. and Nanni, A. (1997).Behaviour of Precracked RC beams Strengthened with
Carbon FRP Sheets. Journal of Composites for Construction. Vol. 1, pp 39-80.
[6] Buell, T. W. and Saadatmanesh, H. (2005). Strengthening Timber Bridge Beams Using
Carbon Fiber. Journal of Structural Engineering.
[7] Chaallal, O, Nollet, M. J. and Perraton, D. (1998). Shear Strengthening of RC Beams by
Externally Bonded Side CFRP Strips. Journal of Composites forConstruction. Vol. 2,
pp 69-114.
[8] Dagher, H. J. and Altimore, F. M. (2005).Use of Glass-Fiber-Reinforced Polymer Tendons
for Stress-Laminating Timber Bridge Decks.Journal of BridgeEngineering.Vol 10, pp 21-27.
[9] Fiorelli, J. and Dias, A. A. (2002). Evaluation of the Structural Behaviour of Wood Beams
Reinforced With FRP. The 7th World Conference on TimberEngineering, WCTE Broughton,
J. G. and Hutchinson, A. R. (2001).Effect of timber moisture content on bonded-in
rods.Journal of Constr. and Building Materials.
[10] Gentile, C., Svecova, D. and Rizkalla, S. H. (2002). Timber Beams Strengthened with GFRP
Bars: Development and Applications. Journal of Composites forConstruction.
[11] Grace, N. F., Soliman, A. K., Abdel-Sayed, G.And Sale, K. R. (1998).Behaviour and
Ductility of Simple and Continuous FRP Reinforced Beams. Journal ofComposites for
Construction. Vol. 2, pp 149-203.

76

[12] Halliwell, S. M. and Moss, R. (1999).Polymer Composites in Construction the Way
Ahead.Proceedings of Composites and Plastics in Construction. 16-18 Nov. Watford, UK,
pp (30) 1-5.
[13 ]Horyna, T. Foschi, R. O. and Ventura, C. E. (2001).Response of Timber Bridge to Traffic
Loading.Journal of Bridge Engineering.Vol 6, pp 69-77.
[14] Humphreys M.F, Francey K.L (2005), An investigation into the rehabilitation of timber structures
with fiber composite materials.
[15] Keble, J. (1999). Alternative Structural Strengthening With Advanced Composites.Proceedings of
Composites and Plastics in Construction. 16-18 Nov. Watford, UK, pp (18) 1-8.
[16] Kirn, Y. and Davalns, J. F. (1997). Delamination buckling of FRP layer in laminated wood
beams. Journal of Composite Structures.Vol 37, pp 311-320.
[17] Micelli, F., Scialpi, V. and La-Tegola, A. (2005).Flexural Reinforcement of Glulam Timber
Beams and Joints with Carbon Fiber-Reinforced Polymer Rods. Journal of Composites for
Construction.
[18] Plevris, N. and Triantafillou, T. C. (1992).FRP-Reinforced Wood as Structural Material. Journal
of Materials in Civil Engineering
[19] Plevris, N. and Triantafillou, T. C. (1995). Creep Behavior of FRP-Reinforced Wood Members.
Journal of Structural Engineering
[20] Radford, D. W., Goethem, D. V., Gutkowski, R. M. and Peterson, M. L. (2002). Composite repair
of timber structures.Journal of Constr. and BuildingMaterials.Vol 16, pp 417-425.
[21] Saadatmanesh, H. and Malek, A. M. (1998). Design Guidelines for Flexural Strengthening of RC
Beams With FRP Plates. Journal of Composites forConstruction. Vol. 2, pp 149-203.
[22] Serrano et.al (2007), Technical Report, Design of safe timber structures.
[23] Tingley, D. A., Gai, C. and Giltner, E. E. (1997). Testing Methods to Determine Properties of
Fiber Reinforced Plastic Panels Used for Reinforcing Glulams. Journal of Composites for
Construction.
[24] Triantafillou, T. C. (1997). Shear Reinforcement of Wood using FRP Materials. Journal of
Materials in Civil Engineering.Vol 9, pp 65-69.
[25] ACI 440 (2002), Guide for the design of externally bonded FRP systems for strengthening
concrete structures.
[26] FIB Bulletin 14 (2001), Externally bonded FRP reinforcement in RC structures.
[27] Italian National Research Council (2004).CNR-DT 200/2004. Guide for the Design and
Construction of Externally Bonded FRP Systems for Strengthening Existing Structures. Rome,
Italy.
[28] Dr. Salim T. Yousif, “New Model of CFRP-Confined Circular Concrete Columns: Ann
Approach”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4,
Issue 3, 2013, pp. 98 - 110, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.
[29] A.S Jeyabharathy, Dr.S.Robert Ravi and Dr.G.Prince Arulraj, “Finite Element Modeling of
Reinforced Concrete Beam Column Joints Retrofitted with GFRP Wrapping”, International
Journal of Civil Engineering & Technology (IJCIET), Volume 2, Issue 1, 2011, pp. 35 - 39,
ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.
[30] Javaid Ahmad, “Seismic Evaluation & Retrofit Assessment of Jlnm Hospital, Rainawari
Srinagar”, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 4,
2013, pp. 278 - 283, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.
[31] Shaikh Zahoor Khalid and S.B. Shinde, “Seismic Response of FRP Strengthened RC Frame”,
International Journal of Civil Engineering & Technology (IJCIET), Volume 3, Issue 2, 2012,
pp. 305 - 321, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.
[32] Wani Ahmad and Javed Ahmad Bhat, “Pre-Tensioned Precast Elements as a Replacement to
Wooden Bracings in the Armature Cross Wall System: An Abstract Attempt to Revive the
Forgotten Heritage”, International Journal of Civil Engineering & Technology (IJCIET), Volume
3, Issue 2, 2012, pp. 305 - 321, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.
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