20320130405006

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. 42-54
© IAEME: www.iaeme.com/ijciet.asp
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IJCIET
©IAEME

DUCTILITY OF TIMBER BEAMS STRENGTHENED USING CFRP 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 ductility 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 FRPreinforced wood section. The strength of timber beams was significantly improved upon
strengthening with maximum percentage increase being 114.28% and 140% for deodar and Kail
respectively.The ductility was dramatically improved where the highest ductility index was 6.81 for
Deodar beams and 4.33 for Kail beams. From this study, it was found that 0.59% is the optimum
value of CFRP area to achieve maximum ductility index. All beams in this study did not fail due to
peel off or de-bonding. Load–deflection curves provided an insight on the performance of the CFRP
strengthened beams.
Keywords: Retrofitting, Fiber Reinforced Polymers, Flexural Strengthening, Timber Failures,
Ductility, Ductility Index.
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.

42

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
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.
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.[4]
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.[8]
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%.[5]
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
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.[16]
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
43

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 includeCedrus Deodara commonly known as
Deodarand 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
wareCedrus Deodaraand 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
The properties of materials used are summarized in Table 1 whereas beam designations are
shown in Table 2.

44

Table 1: Material Properties
Property
Deodar
Kail
Ultimate tensile strength,
35
20
N/mm2
0.40
0.38
Ultimate tensile strain, %
Tensile modulus of
10
8
elasticity, KN/mm2

CFRP
2000
1.65
175

Table 2: Beam Designation
Designation
Deodar

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

FPD-30-1
FPD-40-1
FPD-50-1
FPD-70-1

Bf = 30mm
ACFRP = 0.36%

Bf = 40mm
ACFRP = 0.47%

Thickness of
CFRP Strip,
mm
-

30
40
50
70

Kail

Control Beam- D

Width of
CFRP Strip,
mm
-

1
1
1
1

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 appli on the bottom
applied
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.
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
and
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.

45


(a) Front View

(b) Side View
Fig 3: Testing Arrangement

3. DUCTILITY
The ductility of a beam can be defined as its ability to sustain inelastic deformation without
loss in its load carrying capacity prior to failure. The deformations can be deflections, curvatures, or
strains. A ductile system displays sufficient warning before catastrophic failure. Based on this
definition, ductility can be expressed in terms of deformation or energy absorption. In the case of
uctility
steel reinforced beams, where there is clear plastic deformation of steel at yield, ductility can be
calculated as the ratio of ultimate deformation to deformation at yield.
yi
However, for beam strengthened with FRP, the determination of yield point is a difficult task.
So, ductility is studied in terms of energy parameters.
Typical load-deflection behavior of a flexural member (Fig4(a)) is shown in the Fig4(b). The
deflection behavior
Fig4
beam either exhibits brittle or ductile behavior. The use of material with brittle failure should be
ither
avoided. In the extreme event of a structure loaded to failure, it should be able to undergo large
deflections at near its maximum load-carrying capacity i.e. ductile behavior. This will give a warning
load
ctile
before failure.

Fig 4: Load deflection curve of flexural member
:
46

Different researcher has expressed the ductility in different quantitative basis. For instance,
Spadeaet al. (1998)[23], Harris et al. (1998)[12], Stehn and Johansson (2002)[24] have evaluated the
ductility in terms of structural characteristics such as mid span deflection, curvature, and area under
the load-deflection diagram at two different stages, namely, the yielding of the tension material, and
at ultimate failure. The ductility indices for beams are defined on the basis of;
a) Deflection
b) Curvature
c) Area under the load-deflection curve (Spadeaet al., 1998).[23]
These are briefly explained below;
1) Deflection ductility
It is defined as the ratio of ultimate deflection to yield deflection at the mid span of beams. It
is expressed as;

ߤ௱ ൌ

∆ೠ

……1

∆೤

2) Curvature ductility
It is defined as the ratio of curvature or slope at ultimate load to curvature at the yield load
measured at the mid span of beams. It is expressed as;

ߤః ൌ

ఃೠ

……2

ః೤

Where ∆u = deflection at ultimate load
∆y = Deflection at yield load
Φu = curvature at ultimate load
Φy = Curvature at yield load
3) Energy Ductility
It is defined as the ratio of total energy determined as the area under load deflection curve up
to failure load, to elastic energy determined as the area under load deflection curve up to 75%
of failure load (Fig 5). It is expressed as;

ߤா ൌ

ௐ೟೚೟

……3

ௐబ.ళఱ,ುೠ

Where; Wtot = Total energy, computed as the area under the load deflection curve up to the failure
load (Fig 5(b)).
W0.75, Pu = Area under the load-deflection diagram up to 75% of the ultimate load (elastic
energy)
With FRP reinforced beams, there is no exact yield point; consequently, these definitions are not
suitable to be applied (Grace et al., 1998)[10]. Therefore, Naaman and Jeong (1995)[17], stated by
(Harris et al., 1998)[12] suggested other definition for the ductility index µEbased on energy
considerations that is applicable to FRP reinforced beam, i.e.

ߤா ൌ

ଵ ௐ೟೚೟
ଶ

ቀ ௐ ൅ 1ቁ

……4

೐೗

Where

Wel = Elastic energy (Fig 5(a))
47


Fig 5: Ductility Index a) Elastic energy b) Total Energy

4. RESULTS AND DISCUSSIONS
SIONS
The beams were tested and the load deflection curves were plotted as shown in Fig 6 and Fig
7. All the beams exhibited elastic behavior followed by nonlinear and plastic behavior afterwards.
.
The results from laboratory testing for the strengthened beams are compared with control beam (un
beams
(unstrengthened) in order to study the behavior of strengthened timber beams in terms of ductility. Table
3 and 4 summarizes typical properties of load deflection curves for Deodar and Kail beams
respectively.

70
60

Load, KN

50
Control Beam Deodar

40

FPD-30-1

30

FPD-40-1
20

FPD-50-1

10

FPD-70-1

0
0

10

20

30

40

Midspan Deflection, mm

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

48

60
50

Load, KN

40
Control Beam Kail
30

FPK-30-1
FPK-40-1

20

FPK-50-1
FPK-70-1

10
0
0

5

10

15

20

25

30

35

Midspan Deflection, mm

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

Table 3: Properties of Load Deflection Curves
Control
Beam - D
Failure Load, Pu,
KN
Maximum Midspan
Deflection, δu, mm
Proportionality
Limit, KN
Midspan Deflection
at Proportional
Limit, mm

FPD – 30-1

FPD – 40-1

FPD – 50-1

FPD – 70-1

29.05

37.35

41.5

62.25

43.16

23

22.1

17.7

16.5

36.76

16.6

14.94

20.75

18.26

12.45

10.5

8.25

5.84

5.82

15.9

Table 4: Properties of Load Deflection Curves
Control
Beam - K
Failure Load, Pu,
KN
Maximum
Midspan
Deflection, δu, mm
Proportionality
Limit, KN
Midspan
Deflection at
Proportional
Limit, mm

FPK – 30-1

FPK – 40-1

FPK – 50-1

FPK – 70-1

20.75

29.05

41.5

49.8

33.2

15

14.76

15.4

14.18

28.8

16.6

16.6

22.41

25.73

12.45

12.5

5.86

7.43

6.7

13.35

49

As an illustration, beam FPD-70-1 is used for discussion of ductility. The Load-deflection
curve for the beam is shown in Fig 8. From the curve, the maximum elastic load, ultimate load, and
the corresponding deflections were determined.
50

y = -1E-05x5 + 0.000x4 - 0.023x3 + 0.224x2 + 0.189x + 0.044
R² = 0.997

45
40

Load, KN

35
30
25
FPD-70-1

20

Poly. (FPD-70-1)

15
10
5
0
0

10

20

30

40

50

Mid Span deflection, mm

Fig 8: Load Deflection Curve of FPD – 70 – 1
In this study none of the CFRP plate has yielded because the yield strain for CFRP is higher
than the yield strain of the timber. Hence the compressive zone of the timber will reach its yield
point before CFRP. From the curve, the elastic deflection, and the ultimate deflection were ∆e = 15.9
mm, and ∆u = 36.76 mm, respectively. The total failure occurred when the deflection at mid-span
was 36.76 mm which is considered high. This value provides good performance in the ductility point
of view where the people will have ample time to escape from the building before collapse.
Using the procedures mentioned earlier, the ductility indices were calculated based on energy
methods and the summary of the results is shown in Table 5 and Table 6.
For energy method, the equation for the curve is required to calculate the energy under the
curve. Thus, a polynomial regression analysis was carried using Microsoft Excel to determine the
equation. The order of the polynomial was decided by best fit curve of observed plot. The value for
R2 was determined and used as an indicator for the accuracy of the equation for best curve fitting.
For each curve, the energy on the elastic zone and the total energy up to failure were computed.
For the beam under consideration, elastic energy, Weis equivalent to the area under the curve
between ∆ = 0 and ∆e= 15.9 mm which is given by the following integration as;

ܹ ൌ න
௘
ܹ ൌ ቤන
௘

ଵହ.ଽ

଴

ଵହ.ଽ

଴

‫ݔ݀ ݕ‬

ሺെ1‫ ܧ‬െ 05‫ ݔ‬ହ ൅ 0.0009‫ ݔ‬ସ െ 0.0232‫ ݔ‬ଷ ൅ 0.2244‫ ݔ‬ଶ ൅ 0.1896‫ ݔ‬൅ 0.0443ሻ݀‫ ݔ‬ቤ

We = 110.64KNmm = 110.64Nm or 110.64 Joules

50

Similarly, the total energy, Wtotis equivalent to the area under the curve between ∆ = 0 and
∆u= 36.76 mm which is given by the following integration as;

ܹ௧௢௧

ൌ ቤන

ܹ௧௢௧ ൌ න

଴

ଷ଺.଻଺

଴

ଷ଺.଻଺

‫ݔ݀ ݕ‬

ሺെ1‫ ܧ‬െ 05‫ ݔ‬ହ ൅ 0.0009‫ ݔ‬ସ െ 0.0232‫ ݔ‬ଷ ൅ 0.2244‫ ݔ‬ଶ ൅ 0.1896‫ ݔ‬൅ 0.0443ሻ݀‫ ݔ‬ቤ

Wtot = 1224.34 KNmm = 1224.34 Nm or 1224.34 Joules
The ductility indices for all beams are shown in Table 5 & 6.
Table 5: Ductility Index for Strengthened Deodar beams
Beam

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

CFRP Area,
%

0
0.36
0.47
0.59
0.83

Energy
Elastic
We
Nm or J

Ultimate
Wtot
Nm or J

96.30
71.09
58.48
43.44
110.64

392.28
453.95
482.51
548.35
1224.34

Ductility index
Based on Energy
ܹ௧௢௧
ߤா ൌ 0.5 ൬
൅ 1൰
ܹ
௘
2.53
3.69
4.62
6.81
6.03

Table 6: Ductility Index for Strengthened Kail beams
Beam

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

CFRP Area,
%

0
0.36
0.47
0.59
0.83

Energy
Elastic
We
Nm or J

Ultimate
Wtot
Nm or J

136.27
46.12
88.27
77.75
73.00

238.08
199.76
421.32
595.91
353.83

Ductility index
Based on Energy
ܹ௧௢௧
ߤா ൌ 0.5 ൬
൅ 1൰
ܹ
௘
1.37
2.66
2.88
4.33
2.92

The polynomial regression equations for the other beams are shown in Table 7. There was
significant increase in ductility when the timber beams are strengthened using CFRP plates. Even
after ultimate failure, the beams still held together. In other words, there was no catastrophic failure
when the beams were externally plated. This shows that the CFRP plates provide effective
strengthening material to the timber beams. By taking control beam as a reference, the highest
ductility index for Deodar based on energy method was 6.81 where the percentage increase was
169.17% relative to control beam.
From these results, there is a relationship between the CFRP area and the ductility index. The
relationship is shown in Fig 8. The curve shows that ductility index increased nonlinearly as the area
of CFRP plates increased. When the area of CFRP is about 0.59%, we get maximum value for the
ductility index and any increases in CFRP area beyond this value will not improve the ductility
performance. Although ductile material is important in design, consideration should not be given to
too ductile element which will lead to a decrease in the load-carrying capacity and an increase in
total deflections of the structural system. Both these effects are regarded as negative for practical
design (Stehn and Johansson, 2002). [22]
51

It seems possible to make ductile timber beams by adequately strengthening the tension zone.
There is possibility to design the reinforced timber beams up to plastic limit as steel design does. The
plastic design approach promises an advantage in timber beam design which are strengthened in the
tension zone.
8
7

Ductility Index

6
5
4
Ductility Index Deodar
3

Ductility Index Kail

2
1
0
0

0.2

0.4

0.6

0.8

1

Area of CFRP, %

Fig 8: Ductility index Versus CFRP Area
Table 7: Polynomial Regression Equations of load deflection Curves of Beams
Control BeamD
FPD-30-1
FPD-40-1
FPD-50-1
FPD-70-1

Control BeamK
FPK-30-1
FPK-40-1
FPK-50-1
FPK-70-1

Deodar Beams
y = -0.0007x3 - 0.0096x2 + 1.7841x + 0.3608;
[R² = 0.9959]
y = -0.0006x4 + 0.0286x3 - 0.4143x2 + 3.5929x - 0.2622;
[R² = 0.9976]
y = -0.0002x5 + 0.0101x4 - 0.1689x3 + 1.0258x2 + 1.5664x + 0.0659;
[R² = 0.9997]
y = -0.0002x5 + 0.0093x4 - 0.1352x3 + 0.905x2 + 0.7197x - 0.0965;
[R² = 0.9987]
y = -1E-05x5 + 0.0009x4 - 0.0232x3 + 0.2244x2 + 0.1896x + 0.0443;
[R² = 0.9974]
Kail Beams
y = -0.0001x5 + 0.0052x4 - 0.061x3 + 0.2315x2 + 1.3275x + 0.0283;
[R² = 0.999]
y = -0.0003x5 + 0.0112x4 - 0.1703x3 + 0.9944x2 + 0.9387x + 0.0094;
[R² = 0.9996]
y = -0.0001x5 + 0.0049x4 - 0.0617x3 + 0.2673x2 + 2.8106x + 0.2385;
[R² = 0.9994]
y = -0.0001x6 + 0.0054x5 - 0.093x4 + 0.693x3 - 1.994x2 + 4.788x - 0.0825;
[R² = 0.9992]
y = 5E-06x5 - 0.0002x4 + 0.0004x3 + 0.0517x2 + 0.4575x + 0.0224;
[R² = 0.9928]
52

5. CONCLUSION
I.
II.

III.

Ductility index obtained from energy method was observed to vary in the range 2.53 – 6.81
for Deodar beams and 1.37 – 4.33 for Kail beams.
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.

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[1]
[2]

[3]

[4]
[5]
[6]

[7]

[8]

[9]
[10]

[11]

[12]
[13]
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