1
Strengthening Glue Laminated (Glulam)
Beams with FRP Composites
Babatunde O. Faleye, David B. Tann and Ray Delpak, Faculty of Advanced Technology

Abstract - This paper describes the strategy of improving the
strength and stiffness characteristics of timber and other wood
based glulam structures. This is achieved by bonding Fibre
Reinforced Polymer (FRP) composites to structural substrate
externally. The need for strengthening structures - timber
structures inclusive – is due to deterioration through extra
service use, age and the need for sustainable development
achieved by avoiding demolition and re-construction.
Bonded FRP Composite materials have been proven to be an
effective alternative to traditional steel plate strengthening
techniques for concrete structures from results of a variety of
researchers over the past two decades; this paper explores the
use of this material for same strengthening purpose in timber
structures.
The experimental programme consisted in testing glulam
flexural elements as follows: four different widths varying
between 50, 100, 150 and 200mm, both in natural form and
reinforced by CFRP. The tests were repeated thrice for each
category for quality assurance. This made a total of twentyfour control and CFRP externally bonded specimen. All
specimens were tested under four point bending at a loading
increment rate of 0.5kN, the lateral deflection were recorded
regularly. The data processing consisted of load, deflection and
moment curvature plots construction which were then used for
analysis and observations.
The results revealed that there was a marked increase in the
load bearing capacity of the timber flexural element, due to
FRP strengthening and there was also an increased rigidity in
the member characteristics.
Index
Terms—Carbon
Fibre
Reinforced
Polymer
Composites, Structural Strengthening and Reinforcement,
Timber
I. INTRODUCTION
T
imber is the most ancient and widely used traditional
construction material. Glued laminated (glulam) timber
material has been in existence since the 1800s [1]. It has
been used in numerous applications including arches for
airplane hangers, timber for floors, dome structures, timber
bridge decks etc. However, problems resulting from
Manuscript received October 9, 2007. (Paper submitted for review
February 21, 2008.) This work was supported in part by Network Rail and
Exchem Mining and Construction PLC.
B. O. Faleye of the Faculty of Advanced Technology, Civil Engineering
Department, University of Glamorgan, Pontypridd, Mid Glamorgan, CF37
1DL, United Kingdom (Phone: +44 (0)1443 48 2159; e-mail: bofaleye@
glam.ac.uk).
D. B. Tann is Head of the Civil Engineering Department, Faculty of
Advanced Technology, University of Glamorgan, Pontypridd, Mid
Glamorgan, CF37 1DL, United Kingdom (Phone: +44 (0)1443 48 2164; email: dbtann@ glam.ac.uk).
R. Delpak is professor with the Faculty of Advanced Technology, Civil
Engineering Department, University of Glamorgan, Pontypridd, Mid
Glamorgan, CF37 1DL, United Kingdom (Phone: +44 (0)1443 48 2152; email: rdelpak@ glam.ac.uk).
deterioration due to ageing of structures, among other
reasons such as population increase, shorten the expected
life of structures. This occurs by an increase in the design
load, usage time and traffic volume on structures, thereby
exceeding the design load and leading to failure of the
structure if not strengthened. Timber when compared to
other construction materials such as concrete, steel and
brick, has unique plastic characteristics and therefore can be
reinforced in the tension zone to efficiently utilize its
compressive characteristics [2].
In recent years scientific research had proven the use of
fibre reinforced polymer (FRP) composites as the most
promising technologies for repairing, strengthening or
retrofitting existing structures to resist higher loads and to
rectify damaged flexural structural elements. For sustainable
development it has been established that repair and retrofit
be considered before a decision is made to replace a
structure, since it is cost effective to rehabilitate and repair
structures in most cases rather than replacing them. It is also
less time consuming to repair than to replace, reducing
service interruption periods.
The earliest recorded studies on wood beams reinforced
with fibre and FRP materials were in 1964 and 1965 by
Wangaard [3] and Biblis [4] respectively when they studied
the effect of bonding unidirectional fibre-glass/epoxy
reinforced plastic to the compression and tension faces of
wood cores of various species and Theakson [5] in 1965
studied the feasibility of strengthening both laminated and
solid wood beams with fibre-glass. In 1974, Krueger [6]
studied laminated timber reinforced in the tension zone with
a composite of high-strength bronze coated woven steel wire
and epoxy which was similarly worked on by Krueger and
Eddy [6] in that same year. Later in 1981, Spaun [7] studied
finger-jointed western hemlock cores reinforced with wood
veneers and fibre-glass roving.
By the 1990s, research into FRP strengthened beam
timber/wood had increased. Pelevis and Triantafillou (1992)
[8] researched into reinforcing fir wood with carbon FRP. In
the same year, prior to above, T. Triantafillou and N.
Deskovic [9] published a paper on prestressed FRP sheets as
external reinforcement for wood members. Plevris and
Triantafillou in 1995 [10] also discussed the creep behaviour
of FRP-reinforced wood. By the year 1997, Traiantafillou
[11] experimented on improving the shear capability of
wood reinforcing them with FRP Materials. Also in 1997,
Tingley, Chunxu Gai, and Giltner [12] investigated testing
methods to determine properties of FRP panels used for
reinforcing glulams.
By the 2000s, better enhanced FRP composites called
hybrid FRP and Ultra hybrid FRP composites were being
developed and more research work into their uses were
2
made. Fiorelli and Dias in 2002 [13] analysed the strength
and stiffness of timber beams reinforced with carbon fibre
and glass fibre composites. Lopez-Anido and Xu [14] that
same year published work they had done on the structural
characterization of hybrid FRP-glulam panels for bridge
decks. In 2003, Chen and Balaguru [2] studied the design of
(FRP) Strengthened timber beams. A method for flexural
reinforcement of old wood beams with CFRP materials was
published by Borri, Corradi and Grazini in 2004 [15].
Lopez-Anido, Michael, Sandford and Goodell in 2005 [16]
studied the repair of wood piles using prefabricated FRP
composite shells. In 2006 Dempsey and Scott [17] studied
the effects of mechanically fastening FRP strips to wood
members as a strengthening feature. That same year,
Johnsson, Blanksvard and Carolin [18] worked on some
glulam members strengthened by carbon fibre reinforcement
by first running pull-out tests on four pairs of glulam
members at different anchoring lengths; 100, 150, 200 and
250mm then performing four point beam tests on ten glulam
beam specimens using near surface mounted reinforcements
(NSMR) as the strengthening material. Three beams had 1
NSMR at their bottom, another three were reinforced with 2
NSMR, one beam was reinforced with a shortened NSMR
and finally, three were control beams. Results showed a 1027% increase in stiffness and an increase in mean load
capacity is in the range of 44–63%. The performance of
wood shear walls sheathed with FRP-reinforced OSB panels
was studied by Cassidy, Davids, Dagher, and Gardner [19],
also in 2006. This paper describes an experimental study of
variable width on externally bonded carbon FRP
strengthened glulam beam members.
Table 1: Mechanical Properties of the CFRP Used
Mechanical Properties
Ultimate tensile strength (MPa)
Elastic modulus (GPa)
Mean value
1900.0
222.5
Ultimate strain capacity (%)
0.9
3) Adhesive fastening material
This consisted of two parts, the primer and resin. These
materials consisting of the hardener and resin are mixed in
the ratio of 100:45. The primer required a 4-6hrs
reaction/drying time while the pot and curing times are 30
minutes and 24 hours respectively.
III. SPECIMEN PREPARATION
The plywood materials were cut to sizes consisting six beam
members for each width and cleaned with the aid of a sharp
sand paper to ensure a flat, clean and degreased surface. Saw
dust particles were removed with the aid of a hand vacuum
cleaner. Likewise, the CFRP reinforcing material was cut to
the same widths as the beams but at a length of 300mm and
vacuumed ready for application.
Twelve of the prepared beams and the cut CFRP material
were then primed and left to dry. After drying, the adhesive
matrix was mixed at the specified ratio and applied with the
aid of a roller onto the plywood and CFRP quickly minding
the pot life of the resin matrix. The CFRP was then placed
onto the tension face of the ply beam at equidistance from
the beam centre line and set firmly onto the substrate using a
suitable hand lay-on (washer) roller. The specimens were
then left to cure over the next 24 hours.
II. MATERIALS USED
IV. TEST PROCEDURE
1) Plywood
The strengthened glulam material consisted of twenty-four
plywood beam sections, prepared for experimental flexural
testing; samples were cut in lengths of 400mm with four
different widths varying between 50, 100, 150 and 200mm.
The thickness of the ply material used was 25mm.
Each beam was placed on the inverted test rig on a clear
span of 350mm. An external load was applied and split to
act from two points positioned at a distance of 200mm on
either side of the beam centre line. A picture of the beam test
arrangement is placed in Fig. 1. The mid-span displacement
was measured with the aid of a dial gauge positioned at the
beam centre line.
2) Carbon FRP
The upgrading material system used was Selfix Carbofibe
Wrap Type C, which is a carbon fibre uni-directional sheet,
of thickness 0.156mm made by Exchem Mining and
Construction PLC, and which is designed for the
strengthening of structural members against tensile, shear
and impact forces. The sheet is supplied with primer and
laminating resins and forms part of the Selfix Carbofibe
Externally Bonded Reinforcement System.
Fig. 1 – Beam test arrangement.
3
The beams were loaded to failure on the four point bending
test apparatus; the load increments were 0.5kN.
V. TEST RESULTS
Results from the experiment showed that the presence of
CFRP for each specimen greatly improved the load carrying
ability of the ply beams and reduced the mid-span
deflection. Table 2 shows results from the tests at failure.
Table 2: Average failure load and deflection results from experiments
Specimen
Description
50x25x400mm control
50x25x400mm CFRP
100x25x400mm control
100x25x400mm CFRP
150x25x400mm control
150x25x400mm CFRP
200x25x400mm control
200x25x400mm CFRP
Average
Failure
Load
3.00kN
3.75kN
6.00kN
8.50kN
8.00kN
12.00kN
12.00kN
16.00kN
Average
Deflection
at Failure
4.38mm
3.11mm
3.36mm
2.68mm
2.54mm
2.49mm
3.39mm
2.91mm
It is fair to say that with most strengthening methods there
tends to be an optimum limit whereby any increase in
reinforcement no longer gives the desired effect in terms of
increased performance. As a result, research engineers take
time to analyze test results to determine the optimum
reinforcement level in an effort to improve design. It can be
seen from Fig. 2 that with a strip of CFRP reinforcement the
optimum width was between 100-150mm; this is an
extremely important discovery and tells us that any further
increase in width no longer results in an efficient increase in
load bearing capacity. It is thought that further research into
this area is valid.
employed in one single operation, which involved lay-up,
application of vacuum and bonding of the CFRP layer.
All twenty-four plywood beams were subjected to fourpoint loading and the upgraded ply beam widths reached and
exceeded the plastic collapse load of like-width unstrengthened ply beams.
The widths of between 100mm and 150mm exhibited the
most effective increase in load bearing capacity when
strengthened with the one layer CFRP. Further investigations
are currently pursued to examine various beam widths,
strengthened with more than one layer of CFRP composites
hoping to further improve the beam properties.
ACKNOWLEDGEMENT
The author would like to thank Mr. P. Davies and Mr. P.
Marshman for their immense contributions towards the
experiment.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
Load per width - Deflection (Strengthened)
[8]
0.1
Load per width (kN)
50mm
100mm
150mm
200mm
[9]
[10]
0.05
[11]
[12]
0
0
1.75
Deflection (mm)
3.5
[13]
Fig. 2 – Load deflection graph for CFRP reinforced ply beams
[14]
VI. CONCLUSIONS
This paper has presented experimental results aimed at
establishing the effectiveness of CFRP composite systems
flexural strengthening of glulam beams. Four alternative
width measurements for the upgrading system have been
investigated. In all cases, the same adhesive scheme was
[15]
[16]
H. J. Dagher, T. E. Kimball, S. M. Shaler and B. Abdel-Magid
“Effect of FRP Reinforcement on Low Grade Eastern Hemlock
Glulams,” in National conference on wood transportation structures,
October 1996, pp. 207-213.
Y. Chen and P. N. Balaguru, “Design of Fiber Reinforced Polymers
(FRP) Strengthened Timber Beams,” International Sampe
Symposium and Exhibition, vol. 48, pp. 2513-2524, 2003.
F. Wangaard, “Elastic deflection of wood-Fiberglass composite
beams,” Forest Product Journal, vol. 14, pp. 256-260. 1964.
E. J. Biblis, “Analysis of wood-fiberglass composite beams within
and beyond the elastic region,” Forest Product Journal, vol. 15, pp.
81-88, 1965.
F. H. Theakston, “A feasibility study for strengthening timber beams
with fibreglass,” Canadian Agricultural Engineering Journal, pp.
17-19, January 1965.
G. P. Krueger and F. M. Eddy, “Ultimate strength design of
reinforced timber: moment-rotation characteristics,” Wood Science,
vol. 6, pp. 330-344, 1974.
F. D. Spaun, “Reinforcement of wood with fibreglass,” Forest
Products Journal, vol. 13, pp. 26-33-, 1981.
N. Plevris and T. C. Triantafillou, “FRP-Reinforced Wood as
Structural Material,” Journal of Materials in Civil Engineering –
ASCE, vol. 4, pp. 300-317, 1992.
T. Triantafillou and N. Deskovic, “Prestressed FRP sheets as external
reinforcement of wood members,” Journal of Structural Engineering
– ASCE, vol. 118, pp. 1270–1284, May 1992.
N. Plevris and T. C. Triantafillou, “Creep Behavior of FRPReinforced Wood Members,” Journal of Structural Engineering ASCE, vol. 121, pp. 174-186, February 1995.
T. Triantafillou, “Shear Reinforcement of Wood Using FRP
Materials,” Journal of Materials in Civil Engineering – ASCE, vol.
9, pp. 65-69, May 1997.
D. A. Tingley, C. Gai and E. E. Giltner, “Testing Methods to
Determine Properties of Fiber Reinforced Plastic Panels Used for
Reinforcing Glulams,” Journal of Composites for Construction.ASCE, vol.. 1, pp. 160-167, November 1997.
J. Fiorelli and A. A. Dias, “Analysis of the Strength and Stiffness of
Timber Beams Reinforced with Carbon Fiber and Glass Fiber,”
Materials Research, vol.. 6, April-June 2003.
R. Lopez-Anido and H. Xu; “Structural Characterization of Hybrid
Fiber-Reinforced Polymer-Glulam Panels for Bridge Decks” Journal
of Composites for Construction.- ASCE, vol. 6, pp. 194-203, August
2002.
A. Borri, M. Corradi and A. Grazini, “A method for flexural
reinforcement of old wood beams with CFRP materials,” Composites
Part B: Engineering, vol. 36, pp. 143-153, March 2005.
R. Lopez-Anido, A. P. Michael, T. C. Sandford and B. Goodell,
“Repair of Wood Piles Using Prefabricated Fiber-Reinforced Polymer
Composite Shells” Journal of Performance of Constructed Facilities
vol. 19, pp. 78-87, February 2005.
4
[17] D. D. Dempsey and D. W. Scott, “Wood Members Strengthened with
Mechanically Fastened FRP Strips,” Journal of Composites for
Construction.- ASCE, vol. 10, pp. 392-398, September-October
2006.
[18] H. Johnsson, T. Blanksv¨ard and A. Carolin, “Glulam members
strengthened by carbon fibre reinforcement,” Materials and Structure
- ASCE, vol. 40, pp. 47-56, January 2006.
[19] E. D. Cassidy, W. G. Davids, H. J. Dagher and D. J. Gardner,
“Performance of Wood Shear Walls Sheathed with FRP-Reinforced
OSB Panels,” Journal of Structural Engineering, vol. 132, pp. 153163, January 2006.