ing. Based on the obtained results the BG annealing was
selected in order to achieve a higher tensile strength at
moderate elongation.
The average relative deviation between the predicted
and the experimental data for the tensile strength for
linear regression and genetic programming is 3.55 % and
3.08 %, respectively. The average relative deviation bet-
ween the predicted and the experimental data for elon-
gation for linear regression and genetic programming is
3.56 % and 2.65 %, respectively.
In the future the proposed concept will be used for
analyzing the properties of selected steel grades.
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M. KOVA^I^ et al.: INCREASING THE TENSILE STRENGTH AND ELONGATION OF 16MnCrS5 STEEL ...
888 Materiali in tehnologije / Materials and technology 51 (2017) 6, 883–888
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
889–895
DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY
RESIN
OBSTOJNOST FRP/LESNIH SKLOPOV, LEPLJENIH Z EPOKSI
SMOLO
Jan Vanìrek, Milan [mak, Ivo Kusák, Petr Misák
Brno University of Technology, Faculty of Civil Engineering, Veveøí 95, 602 00 Brno, Czech Republic
vanerek.j@fce.vutbr.cz
Prejem rokopisa – received: 2016-11-18; sprejem za objavo – accepted for publication: 2017-04-20
doi:10.17222/mit.2016.321
The paper describes the properties of FRP/wood bonds, made of different wood species commonly used in the timber industry
within Central Europe (oak, spruce, pine and larch). An FRP fabric (glass-fiber-reinforced polymer – GFRP,
carbon-fiber-reinforced polymer – CFRP) was applied as the reinforcement. The durability of all the reinforced FRP/wood
assemblies was verified with short-term exposure tests following the tensile-shear-strength and wood-failure criteria for the
shear area of a single-lap joint. To precisely define the effect of the mechanical interlocking mechanism of the bond, analyses of
porosity and surface roughness, and SEM were performed.
Keywords: durability, epoxy, FRP, wood adherend
^lanek opisuje lastnosti FRP/lesnih sklopov, narejenih iz razli~nih vrst lesa, ki se obi~ajno uporablja v lesni industriji v Srednji
Evropi (hrast, jelka, bor, macesen). FRP-materiali (polimeri, oja~ani s steklenimi vlakni – angl. GFRP, polimeri, oja~ani z
ogljikovimi vlakni, angl. CFRP) so bili uporabljeni za oja~anje. Obstojnost vseh FRP/lesnih sklopov so preverjali s
kratkotrajnimi stri`nimi preizkusi. Kriterij je bil povr{ina stri`nega preloma na enojni prekrivni ploskvi med FRP in lesom. Da
bi lahko natan~no definirali mehansko trdnost spoja, so izvedli analize poroznosti, povr{inske hrapavosti in SEM-analize.
Klju~ne besede: trajnost, epoksi, FRP, lepljivost lesa
1 INTRODUCTION
The main purpose of the FRP usage with timber in
the construction industry is generally to improve the
stiffness/strength of reinforced items without any
influence on their service life or any environmental
impact. From the perspective of the timber-rein-
forcement process, the optimum dimensional stability
during moisture changes in wood should be one of the
most important criteria for such joints.
Different studies were made for the FRP/wood bond
durability using different types of adhesives. The
optimum results of adhesion were achieved using
formaldehyde-based adhesives;1–3 different, contrary
results were found using epoxy resins.4,5 The results1 for
the epoxy resins showed an inability to reach the
requirement for a cohesive failure of 80 % (in wood)
after an exposure to cyclic hygrothermal conditions.
Moreover, positive results were found using priming
treatments (hydroxymetyl resorcinol – HMR;
resorcin-fenol – RF; hexamethylolmelamin metyl ether –
MME) before the gluing process.2 The durability of
FRP/wood joints using epoxy has not been completely
established. Therefore, the final aim of this experiment
was to establish the durability aspect of such joints. The
focus was on the observation of the wood adhering
parameters having a significant impact on the mecha-
nical interlocking process to understand the impact of
mechanical interlocking on the joint durability.
2 EXPERIMENTAL PART
2.1 Materials
Carbon, glass or aramid are common reinforcement-
fiber materials used for external polymer-composite
systems known as fiber-reinforced polymers (FRPs).
Reinforcing of wood with FRP lamellas/fabrics could be
applied without any restriction regarding the wood
species. To evaluate the influence of wood species, the
experiment used glass-fiber-reinforced polymer (GFRP)
and carbon-fiber-reinforced polymer (CFRP) fabrics
applied to different wood adherents. Oak (Quercus
robur), spruce (Picea abies), pine (Pinus sylvestris) and
larch (Larix decidua) with a uniform thickness of 25 mm
were chosen. The fabrics with carbon fibers (Tyfo
SCH-41, FYFE Co.) with a width of 1.0 mm and the
GFRP with glass fibre (Tyfo-SEH-51A, FYFE Co.) with
a width of 1.3 mm were used. As the adhesive, the epoxy
resin TYFO S (diglycidyl ether of bisphenol-A,
DGEBA) with an amine hardener was used. Pre-treat-
ment including penetration of the wood surface to ensure
stabilization of wood cells (200 g/m2) was applied.
Similarly, the FRP fabrics were saturated with epoxy
resin (400 g/m2) on both sides. After 30 min of such
Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895 889
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 67.017:674-419.3:678.686 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(6)889(2017)
pre-treatments, the FRP strips were applied to the wood
surface. After 24 h of the epoxy curing process, the final
layer of the epoxy matrix in the amount of 200 g/m2 was
also applied without any pressure. All the applications
were carried out at a laboratory ambient temperature of
20 °C. To ensure the post-curing process, the assemblies
were exposed to a constant temperature of 60 °C for
72 h. After the post-curing process, the test samples of
20 mm × 8 mm × 150 mm with 3-mm-thick notches in
the wood were adhered to the fabrics and the CFRP or
GFRP layers were made, defining the shear area.
2.2 Tensile-shear-strength testing
The FRP/wood specimens were subjected to accele-
rated tests, exposing them to all the climatic treatments
and different hygrothermal conditions (classes A2–A5,
specified in ^SN EN 302-1, Table 1). Tensile-shear tests
were performed on the test specimens, for each condition
class, and a total of 12 test samples were used. The tests
were performed on a Testometric M350-20CT machine
(Testometric Company Ltd., UK), digitally recording the
test progress at a crosshead speed of 1 mm/min. As an
additional evaluation of the bond durability, the wood
failures within the shear area (rounded to the nearest
value in multiples of 10 %) were visually assessed.
Table 1: Types of exposure treatments prior to the tensile-shear testing
Treatment Climatic treatment Specification
A1 Standard climate 7 d 20 °C/65 %
A2 Immersion in coldwater
7 d 20 °C/65 %
4 d in water (15±5 °C)
Tested in a wet state
A3 Immersion in coldwater
7 d 20 °C/65 %
4 d immersion in water
(15±5 °C), conditioning at
20 °C/65 %
Tested in a dry state
A4 Immersion inboiling water
7 d 20 °C/65 %
6 h in boiling water
2 h immersion in water
(15±5 °C),
Tested in a wet state
A5 Immersion inboiling water
7 d 20 °C/65 %
6 h in boiling water
2 h immersion in water
(15±5 °C)
7 d conditioning at
20 °C/65 %
Tested in a dry state
2.3 Microstructural analysis
To assess the effect of mechanical interlocking as
a possible aspect of a bond, the microstructures of all the
tested wood adherents were analyzed. The sizes of the
main structural elements of the transverse wood
microtome samples were assessed. Electron scanning
microscopy provided an accurate evaluation of a failed
bond based on the loss of adhesion.
2.4 Wood-surface-roughness analysis
The surfaces of all the tested wood adherents were
scanned using confocal laser microscopy in both the
optical and confocal mode. The automatic microscope
mode was used to determine the highest and lowest point
of the relief of a scanned area. The number of optical
sections depended only on the wood, assuming that the
other parameters such as magnification and the illumi-
nation mode (laser or optical light) remained unchanged.
Alternatively, the number of optical sections depended
on the illuminated thickness of the wood. Optical cuts
from 40 to 60 in the optical mode and from 70 to 90 in
the confocal mode were evaluated. The purpose of this
analysis was to see if the different surface roughness of
each wood species has an impact on the final durability
of a FRP/wood bond. The parameters of mean quadratic
heights (SRq, SPq) and mean arithmetic heights (SRa,
SPa) were assessed in five different appointed places for
each tested wood. Surface roughness parameters SRa,
SPa, SRq and SPq are based on one-dimensional
parameters Ra (the arithmetic mean of roughness), Pa
(the arithmetic mean of height of a curve profile), Rq
(the root mean square roughness), Pq (the root mean
square height of a curve profile). The calculation of these
one-dimensional parameters is specified in ^SN EN ISO
4287 and the conversion into surface parameters was
calculated based on the data acquired from two
perpendicular directions.
2.5 Porosity
The pore size and pore distribution within all the
tested wood specimens were evaluated with mercury
porosimetry using the PASCAL 140/240, Thermo
Finnigan equipment. Wood samples with a volume of
approx. 0.75 cm3 were used. To avoid an overestimation
of the portion of smaller pores, a gradual increase in the
rate of pressure during the process was chosen.
3 RESULTS
3.1 Tensile-shear-strength testing
The mean strength values obtained are shown in
Figures 1a to 1d; it is clear that the sets of dry-wet
cycling ageing tests show a reduction in the joint
strength. The most significant strength reduction in the
dry state for both FRP materials resulted from the
specimens subjected to both exposure treatments,
providing a significant hygrothermal-stress concentration
in the bond line.
To compare the test results, variance tests (ANOVA)
were analyzed. The influence of the type of exposure
was tested using the one-way ANOVA. Based on this
analysis, the tensile strength is significantly influenced
by the type of exposure. In addition, the two-way
ANOVA was used when both the type of exposure and
the type of reinforcement were factors. The purpose of
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
890 Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895 891
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Cross-section of oak wood, apparent vessel elements and
libriform cells, magn. 100×
Figure 3: Cross-section of an early part of the annual ring of spruce
wood, magn. 200×
Figure 1: Mean tensile-shear strengths of the samples tested after the exposure to climatic treatment according to EN 302-1;  denotes the mean
value; the box plot – rectangle shows the interquartile range (IQR) with the median depicted within the box; for each exposure, the value of wood
failure is stated
Table 2: Mean values of the wood-element size proportion of the used wood species
Size (diameter) of wood elements (μm)
Lumina Cell-wall thickness
tracheids vessels libriform tracheids libriform
early wood late wood early wood late wood early wood late wood
Pine (Pinus sylvestris) 26.1 9.7 6.0 11.7
Larch (Larix decidua) 30.7 10.1 4.7 9.2
Spruce (Picea abies) 31.2 10.3 4.0 8.9
Oak (Quercus robur) 245.0 41.9 9.0 4.0
pre-treatments, the FRP strips were applied to the wood
surface. After 24 h of the epoxy curing process, the final
layer of the epoxy matrix in the amount of 200 g/m2 was
also applied without any pressure. All the applications
were carried out at a laboratory ambient temperature of
20 °C. To ensure the post-curing process, the assemblies
were exposed to a constant temperature of 60 °C for
72 h. After the post-curing process, the test samples of
20 mm × 8 mm × 150 mm with 3-mm-thick notches in
the wood were adhered to the fabrics and the CFRP or
GFRP layers were made, defining the shear area.
2.2 Tensile-shear-strength testing
The FRP/wood specimens were subjected to accele-
rated tests, exposing them to all the climatic treatments
and different hygrothermal conditions (classes A2–A5,
specified in ^SN EN 302-1, Table 1). Tensile-shear tests
were performed on the test specimens, for each condition
class, and a total of 12 test samples were used. The tests
were performed on a Testometric M350-20CT machine
(Testometric Company Ltd., UK), digitally recording the
test progress at a crosshead speed of 1 mm/min. As an
additional evaluation of the bond durability, the wood
failures within the shear area (rounded to the nearest
value in multiples of 10 %) were visually assessed.
Table 1: Types of exposure treatments prior to the tensile-shear testing
Treatment Climatic treatment Specification
A1 Standard climate 7 d 20 °C/65 %
A2 Immersion in coldwater
7 d 20 °C/65 %
4 d in water (15±5 °C)
Tested in a wet state
A3 Immersion in coldwater
7 d 20 °C/65 %
4 d immersion in water
(15±5 °C), conditioning at
20 °C/65 %
Tested in a dry state
A4 Immersion inboiling water
7 d 20 °C/65 %
6 h in boiling water
2 h immersion in water
(15±5 °C),
Tested in a wet state
A5 Immersion inboiling water
7 d 20 °C/65 %
6 h in boiling water
2 h immersion in water
(15±5 °C)
7 d conditioning at
20 °C/65 %
Tested in a dry state
2.3 Microstructural analysis
To assess the effect of mechanical interlocking as
a possible aspect of a bond, the microstructures of all the
tested wood adherents were analyzed. The sizes of the
main structural elements of the transverse wood
microtome samples were assessed. Electron scanning
microscopy provided an accurate evaluation of a failed
bond based on the loss of adhesion.
2.4 Wood-surface-roughness analysis
The surfaces of all the tested wood adherents were
scanned using confocal laser microscopy in both the
optical and confocal mode. The automatic microscope
mode was used to determine the highest and lowest point
of the relief of a scanned area. The number of optical
sections depended only on the wood, assuming that the
other parameters such as magnification and the illumi-
nation mode (laser or optical light) remained unchanged.
Alternatively, the number of optical sections depended
on the illuminated thickness of the wood. Optical cuts
from 40 to 60 in the optical mode and from 70 to 90 in
the confocal mode were evaluated. The purpose of this
analysis was to see if the different surface roughness of
each wood species has an impact on the final durability
of a FRP/wood bond. The parameters of mean quadratic
heights (SRq, SPq) and mean arithmetic heights (SRa,
SPa) were assessed in five different appointed places for
each tested wood. Surface roughness parameters SRa,
SPa, SRq and SPq are based on one-dimensional
parameters Ra (the arithmetic mean of roughness), Pa
(the arithmetic mean of height of a curve profile), Rq
(the root mean square roughness), Pq (the root mean
square height of a curve profile). The calculation of these
one-dimensional parameters is specified in ^SN EN ISO
4287 and the conversion into surface parameters was
calculated based on the data acquired from two
perpendicular directions.
2.5 Porosity
The pore size and pore distribution within all the
tested wood specimens were evaluated with mercury
porosimetry using the PASCAL 140/240, Thermo
Finnigan equipment. Wood samples with a volume of
approx. 0.75 cm3 were used. To avoid an overestimation
of the portion of smaller pores, a gradual increase in the
rate of pressure during the process was chosen.
3 RESULTS
3.1 Tensile-shear-strength testing
The mean strength values obtained are shown in
Figures 1a to 1d; it is clear that the sets of dry-wet
cycling ageing tests show a reduction in the joint
strength. The most significant strength reduction in the
dry state for both FRP materials resulted from the
specimens subjected to both exposure treatments,
providing a significant hygrothermal-stress concentration
in the bond line.
To compare the test results, variance tests (ANOVA)
were analyzed. The influence of the type of exposure
was tested using the one-way ANOVA. Based on this
analysis, the tensile strength is significantly influenced
by the type of exposure. In addition, the two-way
ANOVA was used when both the type of exposure and
the type of reinforcement were factors. The purpose of
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
890 Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895 891
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Cross-section of oak wood, apparent vessel elements and
libriform cells, magn. 100×
Figure 3: Cross-section of an early part of the annual ring of spruce
wood, magn. 200×
Figure 1: Mean tensile-shear strengths of the samples tested after the exposure to climatic treatment according to EN 302-1;  denotes the mean
value; the box plot – rectangle shows the interquartile range (IQR) with the median depicted within the box; for each exposure, the value of wood
failure is stated
Table 2: Mean values of the wood-element size proportion of the used wood species
Size (diameter) of wood elements (μm)
Lumina Cell-wall thickness
tracheids vessels libriform tracheids libriform
early wood late wood early wood late wood early wood late wood
Pine (Pinus sylvestris) 26.1 9.7 6.0 11.7
Larch (Larix decidua) 30.7 10.1 4.7 9.2
Spruce (Picea abies) 31.2 10.3 4.0 8.9
Oak (Quercus robur) 245.0 41.9 9.0 4.0
this was to establish the individual influences of these
factors and the interaction between them. The results of
the two-way ANOVA show that each factor is statisti-
cally significant for the tensile strength, but the interac-
tion between the factors is not significant. All the
statistical tests were carried out at a significance level of
0.05.
3.2 Microstructural analysis
Results of the size-distribution evaluation did not
show any significant difference between the coniferous
species; the biggest size of the cell-wall thickness was
found for the pine species, concurring with the smallest
size of the pores (lumina) of tracheid elements. Details
of all the observed elements are shown in Table 2. The
photomicrographs of the hardwood and conifer species
with the size measurements are shown in Figures 2 and
3.
3.3 Wood-surface-roughness analysis
A comparison of the results of the profile parameters
(SPa, SPq) indicates deviations from the interspaced
centerline of the test surface; the highest deviations were
detected on the oak surface, arising from the existence of
open vessel elements (Figure 5). The smallest deviation
was found on the pine; the drop of the profile-parameter
value for the pine was lower by almost 50 % compared
to the other coniferous woods.
The achieved values of the roughness parameters
(SRa, SRq), where the longwave part is eliminated,
showed the highest values of the roughness for the oak
sample; the highest decrease of nearly 40 % was ob-
served for the pine sample, compared to the other
softwood species. All the achieved values are shown in
Table 3.
3.4 Porosity
Among all the other parameters, porosity has the
most significant influence on the impregnability of
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
892 Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Surface relief of the pine sample used for calculating square
roughness parameters (magn. 120×; area of 2560 μm × 1920 μm)
Figure 4: Surface relief of the pine sample used for calculating square
roughness parameters (magn. 120×; area of 2560 μm × 1920 μm)
Table 3: Values of selected roughness parameters of used wood
species obtained with confocal microscopy
Wood
species
Mean values and S. D. of roughness parameters
SPa SRa SRq SPq
Pine 9.14 1.20 5.82 0.18 8.00 0.41 12.31 1.29
Oak 42.89 9.20 9.66 1.04 14.77 1.73 53.68 10.35
Larch 19.95 2.20 8.05 0.95 11.22 1.52 24.84 3.18
Spruce 17.41 3.03 7.47 0.63 11.20 1.26 22.95 3.51
Figure 6: Cumulative pore volume as a function of the pore diameter
of softwood and hardwood species
Table 4: Achieved results of mercury porosimetry of tested wood
samples
Wood
species
Total
porosity
Specific
density
Bulk
density
Cumulative
pore
volume
% kg m–3 kg m–3 mm3 g–1
Oak 47.6 1410.0 738.6 644.7
Larch 54.0 1518.4 699.2 771.6
Pine 60.3 1381.1 549.0 1097.3
Spruce 62.8 1423.6 528.9 1188.1
wood. Consequently, the bulk density and specific
density for each tested sample were established from the
other parameters. The resulting values are shown in
Table 4, confirming the presumption of a decreasing
total porosity despite the increasing bulk density. The
cumulative pore-volume curves as a function of the pore
diameter of oak (hardwood) and pine (softwood) are
shown in Figure 6.
4 DISCUSSION
4.1 Tensile-shear-strength testing
A stronger effect of the thermal exposure ensuring
the hygrothermal exposure for condition classes A4 and
A5 compared to the exposure without a thermal effect
(A2, A3) was not proven (Table 5).
Regarding the type of fabrics, there was no indication
of any dependency on the durability aspect. The
requirements of the EN standard for strength limits
(related to the beech/beech bond) for all the exposures
were not exceeded by any of the tested samples. The
most significant result of durability was achieved with
the pine as an adherent when no losses in the strength
occurred under all the severe exposures. Furthermore,
the values of the cohesive failure expressed with a higher
wood failure (WF) for the pine/FRP bonds supported this
durability aspect. The values of the wood failure showed
that the minimum limit of 80 % was not reached for the
dry test samples of larch as a softwood; also, the WF
percentage dropped below the 80 % criterion for the
FRP/larch samples tested in a wet condition. Comparing
the effects of the strength and wood-failure parameters, it
can be clearly indicated that the more decisive criterion
for the durability aspect shows exceptional values for the
type of failure as shown for the larch/FRP bonds.
4.2 Porosity
The microscopy analysis of the main structural ele-
ments showed similar sizes of tracheids for all the soft-
wood species; the highest porosity was established for
spruce, the lowest for larch. But differences occurred
between the results for the pore distribution as a function
of the pore diameter. The differences occurred between
the distributions of pores for the softwood species where
the biggest drop in the distribution occurred for the pore
diameters of 10 μm, contrary to the shape of the distri-
bution curves for the oak as the ring-porous wood, which
showed a consistent increase in the pore volume for pore
diameters from 12 nm to 100 μm. The distribution of
pore diameters for softwood is affected by the amount of
tracheids elements, which form the main structure of
softwoods. The lowest value of the porosity was estab-
lished for the oak-wood sample; however, this had no
effect on the final durability of the FRP/wood bond.
Similar strength decreases were found when comparing
the FRP/oak samples with the samples of the other wood
species. The values of the pore distribution correlate with
similar results,6 performed for the European wood
species.
4.3 Bond-line failure
After the accelerated tests, the FRP/wood bond lines
were assessed at the macroscopic and microscopic levels.
At the macroscopic level, the adhesive failure occurred
mostly in the late part of the annual ring, despite the
cohesive failure in the early part of the annual ring for
the soft-wood species. For oak as a hardwood, the most
cohesive failures were detected at the shear areas of the
specimens tested in a dry state.
To precisely specify the mechanism of failure, elec-
tron microscopy (SEM) was performed to observe the
microscopic structure of the wood at the FRP/wood
interphase region. For the softwood species, the adhesion
failure did not show the existence of structural tracheid
elements in the early parts of the annual rings. Never-
theless, the existence of ray elements filled with resin
was observed (Figure 7); a deeper penetration of the
adhesive was observed due to their perpendicular orien-
tation to the application area, which was wetted by the
adhesive. This clearly indicates that the existence and
quantity of rays partake in the tensile-shear strength of
the softwood species.
For the hardwood species, a good adhesive penetra-
tion into the lumina of all the conductive elements
(vessel, libriform) was observed. This phenomenon was
proven by a higher wood-failure percentage for the
specimens tested in a dry state after a hygrothermal
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895 893
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Table 5: Resulting values of decreasing strength after exposing the test samples to different exposure criteria
Decreasing-strength and wood-failure values (%)
Exposure A1 A2 (wet) A3 (dry) A4 (wet) A5 (dry)
Standard requirements for beech 10 MPa -40 -20 -40 -20
Oak/CFRP (WF) 9.3 (98) -34.3 (45) -6.0 (73) -21.1 (93) -6.4 (93)
Oak/GFRP (WF) 8.6 (100) -20.8 (59.2) -0.5 (98.3) -18.0 (70.8) -4.6 (100.0)
Pine/CFRP (WF) 8.3 (100) -29.7 (98) 7.5 (100) -29.7 (70) 13.0 (100)
Pine/GFRP(WF) 7.9 (100) -29.3 (95.0) 1.9 (98.3) -29.5 (70.8) 6.1 (91.7)
Larch/CFRP (WF) 8.3 (97.0) -22.0 (22.5) 28.0 (60.0) -22.3 (38.0) 2.9 (86.0)
Larch/GFRP (WF) 7.9 (92.5) -24.7 (0) -1.4 (46.7) -28.6 (56.3) 12.1 (78.3)
Spruce/CFRP (WF) 8.1 (100) -36.8 (100) -6.5 (84) -22.3 (74.0) -1.1 (85.0)
Spruce/GFRP (WF) 7.7 (80.0) -37.0 (88.3) -15.2 (100.0) -28.6 (43.3) -8.2 (81.7)
this was to establish the individual influences of these
factors and the interaction between them. The results of
the two-way ANOVA show that each factor is statisti-
cally significant for the tensile strength, but the interac-
tion between the factors is not significant. All the
statistical tests were carried out at a significance level of
0.05.
3.2 Microstructural analysis
Results of the size-distribution evaluation did not
show any significant difference between the coniferous
species; the biggest size of the cell-wall thickness was
found for the pine species, concurring with the smallest
size of the pores (lumina) of tracheid elements. Details
of all the observed elements are shown in Table 2. The
photomicrographs of the hardwood and conifer species
with the size measurements are shown in Figures 2 and
3.
3.3 Wood-surface-roughness analysis
A comparison of the results of the profile parameters
(SPa, SPq) indicates deviations from the interspaced
centerline of the test surface; the highest deviations were
detected on the oak surface, arising from the existence of
open vessel elements (Figure 5). The smallest deviation
was found on the pine; the drop of the profile-parameter
value for the pine was lower by almost 50 % compared
to the other coniferous woods.
The achieved values of the roughness parameters
(SRa, SRq), where the longwave part is eliminated,
showed the highest values of the roughness for the oak
sample; the highest decrease of nearly 40 % was ob-
served for the pine sample, compared to the other
softwood species. All the achieved values are shown in
Table 3.
3.4 Porosity
Among all the other parameters, porosity has the
most significant influence on the impregnability of
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
892 Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Surface relief of the pine sample used for calculating square
roughness parameters (magn. 120×; area of 2560 μm × 1920 μm)
Figure 4: Surface relief of the pine sample used for calculating square
roughness parameters (magn. 120×; area of 2560 μm × 1920 μm)
Table 3: Values of selected roughness parameters of used wood
species obtained with confocal microscopy
Wood
species
Mean values and S. D. of roughness parameters
SPa SRa SRq SPq
Pine 9.14 1.20 5.82 0.18 8.00 0.41 12.31 1.29
Oak 42.89 9.20 9.66 1.04 14.77 1.73 53.68 10.35
Larch 19.95 2.20 8.05 0.95 11.22 1.52 24.84 3.18
Spruce 17.41 3.03 7.47 0.63 11.20 1.26 22.95 3.51
Figure 6: Cumulative pore volume as a function of the pore diameter
of softwood and hardwood species
Table 4: Achieved results of mercury porosimetry of tested wood
samples
Wood
species
Total
porosity
Specific
density
Bulk
density
Cumulative
pore
volume
% kg m–3 kg m–3 mm3 g–1
Oak 47.6 1410.0 738.6 644.7
Larch 54.0 1518.4 699.2 771.6
Pine 60.3 1381.1 549.0 1097.3
Spruce 62.8 1423.6 528.9 1188.1
wood. Consequently, the bulk density and specific
density for each tested sample were established from the
other parameters. The resulting values are shown in
Table 4, confirming the presumption of a decreasing
total porosity despite the increasing bulk density. The
cumulative pore-volume curves as a function of the pore
diameter of oak (hardwood) and pine (softwood) are
shown in Figure 6.
4 DISCUSSION
4.1 Tensile-shear-strength testing
A stronger effect of the thermal exposure ensuring
the hygrothermal exposure for condition classes A4 and
A5 compared to the exposure without a thermal effect
(A2, A3) was not proven (Table 5).
Regarding the type of fabrics, there was no indication
of any dependency on the durability aspect. The
requirements of the EN standard for strength limits
(related to the beech/beech bond) for all the exposures
were not exceeded by any of the tested samples. The
most significant result of durability was achieved with
the pine as an adherent when no losses in the strength
occurred under all the severe exposures. Furthermore,
the values of the cohesive failure expressed with a higher
wood failure (WF) for the pine/FRP bonds supported this
durability aspect. The values of the wood failure showed
that the minimum limit of 80 % was not reached for the
dry test samples of larch as a softwood; also, the WF
percentage dropped below the 80 % criterion for the
FRP/larch samples tested in a wet condition. Comparing
the effects of the strength and wood-failure parameters, it
can be clearly indicated that the more decisive criterion
for the durability aspect shows exceptional values for the
type of failure as shown for the larch/FRP bonds.
4.2 Porosity
The microscopy analysis of the main structural ele-
ments showed similar sizes of tracheids for all the soft-
wood species; the highest porosity was established for
spruce, the lowest for larch. But differences occurred
between the results for the pore distribution as a function
of the pore diameter. The differences occurred between
the distributions of pores for the softwood species where
the biggest drop in the distribution occurred for the pore
diameters of 10 μm, contrary to the shape of the distri-
bution curves for the oak as the ring-porous wood, which
showed a consistent increase in the pore volume for pore
diameters from 12 nm to 100 μm. The distribution of
pore diameters for softwood is affected by the amount of
tracheids elements, which form the main structure of
softwoods. The lowest value of the porosity was estab-
lished for the oak-wood sample; however, this had no
effect on the final durability of the FRP/wood bond.
Similar strength decreases were found when comparing
the FRP/oak samples with the samples of the other wood
species. The values of the pore distribution correlate with
similar results,6 performed for the European wood
species.
4.3 Bond-line failure
After the accelerated tests, the FRP/wood bond lines
were assessed at the macroscopic and microscopic levels.
At the macroscopic level, the adhesive failure occurred
mostly in the late part of the annual ring, despite the
cohesive failure in the early part of the annual ring for
the soft-wood species. For oak as a hardwood, the most
cohesive failures were detected at the shear areas of the
specimens tested in a dry state.
To precisely specify the mechanism of failure, elec-
tron microscopy (SEM) was performed to observe the
microscopic structure of the wood at the FRP/wood
interphase region. For the softwood species, the adhesion
failure did not show the existence of structural tracheid
elements in the early parts of the annual rings. Never-
theless, the existence of ray elements filled with resin
was observed (Figure 7); a deeper penetration of the
adhesive was observed due to their perpendicular orien-
tation to the application area, which was wetted by the
adhesive. This clearly indicates that the existence and
quantity of rays partake in the tensile-shear strength of
the softwood species.
For the hardwood species, a good adhesive penetra-
tion into the lumina of all the conductive elements
(vessel, libriform) was observed. This phenomenon was
proven by a higher wood-failure percentage for the
specimens tested in a dry state after a hygrothermal
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895 893
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Table 5: Resulting values of decreasing strength after exposing the test samples to different exposure criteria
Decreasing-strength and wood-failure values (%)
Exposure A1 A2 (wet) A3 (dry) A4 (wet) A5 (dry)
Standard requirements for beech 10 MPa -40 -20 -40 -20
Oak/CFRP (WF) 9.3 (98) -34.3 (45) -6.0 (73) -21.1 (93) -6.4 (93)
Oak/GFRP (WF) 8.6 (100) -20.8 (59.2) -0.5 (98.3) -18.0 (70.8) -4.6 (100.0)
Pine/CFRP (WF) 8.3 (100) -29.7 (98) 7.5 (100) -29.7 (70) 13.0 (100)
Pine/GFRP(WF) 7.9 (100) -29.3 (95.0) 1.9 (98.3) -29.5 (70.8) 6.1 (91.7)
Larch/CFRP (WF) 8.3 (97.0) -22.0 (22.5) 28.0 (60.0) -22.3 (38.0) 2.9 (86.0)
Larch/GFRP (WF) 7.9 (92.5) -24.7 (0) -1.4 (46.7) -28.6 (56.3) 12.1 (78.3)
Spruce/CFRP (WF) 8.1 (100) -36.8 (100) -6.5 (84) -22.3 (74.0) -1.1 (85.0)
Spruce/GFRP (WF) 7.7 (80.0) -37.0 (88.3) -15.2 (100.0) -28.6 (43.3) -8.2 (81.7)
exposure. Despite this, the proper penetration of the
adhesive into the lumina did not show a positive effect
on the durability for the A2 and A4 condition classes.
5 CONCLUSIONS
The durability of FRP/wood bonds with different
wood adherents was established using accelerated
durability tests. According to the EN standard (declared
for the beech/beech bond), all the tested samples of
GFRP and CFRP fabrics met the standard criterion
regardless of the wood species as the adherent. The wood
species did not affect the durability of the FRP/wood
bonds. The achieved strength for all the tested specimens
in the conditions with a thermal exposure (A4, A5) was
not significantly different from the one achieved in the
conditions without a thermal exposure (A2, A3).
Therefore, the inability of epoxy to withstand the volume
changes of a wood adherent plays a more dominant role
than the temperature influence on a possible resin degra-
dation. The wood failure percentage proved to be a better
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
894 Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 10: SEM micrograph of adhesive failure for FRP/oak,
existence of cohesive failure of vessels, predominant adhesive failure
of the FRP/oak bond line, magn. 200×
Figure 8: SEM micrograph, typical existence of rays (R) and
tracheids (Tr) filled with resin, FRP/pine, magn. 500×
Figure 9: SEM micrograph of adhesive failure for FRP/oak, existence
of the adhesive in the lumina of vessels (V) and libriform (L), magn.
200×
Figure 7: SEM micrograph of the border between early and late larch
wood, different types of failure, magn. 300×
means to assess the durability of bonded specimens
compared to strength-loss aspects.
The principle of a good adhesion of the FRP/wood
bonds via a mechanical interlocking process was con-
sidered for the experimental work. The size of the pores
is dominant for the permeability of the adhesive into the
cell lumina. Therefore, the surface roughness, porosity
and size distribution of pores for all the tested wood
samples were analyzed. The dependency between the
porosity and the FRP/wood bond durability was not
proven. The spruce wood having the highest porosity did
not exhibit the lowest drop in the strength, or even the
biggest value of the cohesive failure. Furthermore, the
FRP/wood durability of all the tested softwoods showed
no dependency on the pore-size distribution. According
to the laser confocal microscopy analysis, it was found
that good durability of the FRP/pine wood bonds
exhibits a good relationship with the parameters of the
surface roughness. A plain surface and elimination of
any debris on the surface allow proper adhesive wetting.
All the results showed that the mechanical inter-
locking of the FRP/wood bond does not have the domi-
nant role anticipated; on the contrary, the chemical bonds
of the adhesive contribute to the higher durability of the
epoxy/wood bonds. This is because an interpenetrating
polymer (adhesive) network occurs in the cell walls and
in the mesopores between the cellulose fibers in the cell
walls. A proper application technique, ensuring a proper
diffusion of the epoxy adhesive between the cellulose
microfibrils in the cell walls seems to be necessary for
more durable FRP/wood bonds.
Acknowledgment
This paper was prepared with the financial help of
project TA04010425 “A complex system of special
repair materials using secondary raw materials for
industries” supported by The Technology Agency of The
Czech Republic; and the internal project of specific
research No. FAST-S-16-3772 “Research of an adhesive
modification increasing the durability of FRP/wood
bonds at moisture exposure“ supported by Brno
University of Technology, Faculty of Civil Engineering.
6 REFERENCES
1 B. S. Trimble, Durability and mode-I fracture of fiber-reinforced
plastic (FRP)/wood interface bond, Doctoral dissertation, West
Virginia University, 1999, 192
2 H. V. S. Ganga Rao, Sawn and laminated wood beams wrapped with
fiber reinforced plastic composites, Wood Design Focus, 8 (1997) 3,
13–18
3 D. J. Gardner, J. F. Davalos, U. M. Munipalle, Adhesive Bonding of
Pultruded Fiber-Reinforced Plastic to Wood, Forest Products Journal,
44 (1994) 5, 62–66
4 G. M. Raftery, A. M. Harte, P. D. Rodd, Bond quality at the FRP-
wood interface using wood-laminating adhesives, International
Journal of Adhesion & Adhesives, 29 (2009) 2, 101–110,
doi:10.1016/j.ijadhadh.2008.01.006
5 J. P. Alexander, S. M. Shaler, D. J. Gardner, Evaluating wood/FRP
bond durability through chemical kinetics, Proc. of Wood Adhesive,
Madison WI, 2000, 289–299
6 M. Plötze, P. Niemz, Porosity and pore size distribution of different
wood types as determined by mercury intrusion porosimetry,
European Journal of Wood and Wood Products, 69 (2011) 4,
649–657, doi:10.1007/s00107-010-0504-0
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895 895
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
exposure. Despite this, the proper penetration of the
adhesive into the lumina did not show a positive effect
on the durability for the A2 and A4 condition classes.
5 CONCLUSIONS
The durability of FRP/wood bonds with different
wood adherents was established using accelerated
durability tests. According to the EN standard (declared
for the beech/beech bond), all the tested samples of
GFRP and CFRP fabrics met the standard criterion
regardless of the wood species as the adherent. The wood
species did not affect the durability of the FRP/wood
bonds. The achieved strength for all the tested specimens
in the conditions with a thermal exposure (A4, A5) was
not significantly different from the one achieved in the
conditions without a thermal exposure (A2, A3).
Therefore, the inability of epoxy to withstand the volume
changes of a wood adherent plays a more dominant role
than the temperature influence on a possible resin degra-
dation. The wood failure percentage proved to be a better
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
894 Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 10: SEM micrograph of adhesive failure for FRP/oak,
existence of cohesive failure of vessels, predominant adhesive failure
of the FRP/oak bond line, magn. 200×
Figure 8: SEM micrograph, typical existence of rays (R) and
tracheids (Tr) filled with resin, FRP/pine, magn. 500×
Figure 9: SEM micrograph of adhesive failure for FRP/oak, existence
of the adhesive in the lumina of vessels (V) and libriform (L), magn.
200×
Figure 7: SEM micrograph of the border between early and late larch
wood, different types of failure, magn. 300×
means to assess the durability of bonded specimens
compared to strength-loss aspects.
The principle of a good adhesion of the FRP/wood
bonds via a mechanical interlocking process was con-
sidered for the experimental work. The size of the pores
is dominant for the permeability of the adhesive into the
cell lumina. Therefore, the surface roughness, porosity
and size distribution of pores for all the tested wood
samples were analyzed. The dependency between the
porosity and the FRP/wood bond durability was not
proven. The spruce wood having the highest porosity did
not exhibit the lowest drop in the strength, or even the
biggest value of the cohesive failure. Furthermore, the
FRP/wood durability of all the tested softwoods showed
no dependency on the pore-size distribution. According
to the laser confocal microscopy analysis, it was found
that good durability of the FRP/pine wood bonds
exhibits a good relationship with the parameters of the
surface roughness. A plain surface and elimination of
any debris on the surface allow proper adhesive wetting.
All the results showed that the mechanical inter-
locking of the FRP/wood bond does not have the domi-
nant role anticipated; on the contrary, the chemical bonds
of the adhesive contribute to the higher durability of the
epoxy/wood bonds. This is because an interpenetrating
polymer (adhesive) network occurs in the cell walls and
in the mesopores between the cellulose fibers in the cell
walls. A proper application technique, ensuring a proper
diffusion of the epoxy adhesive between the cellulose
microfibrils in the cell walls seems to be necessary for
more durable FRP/wood bonds.
Acknowledgment
This paper was prepared with the financial help of
project TA04010425 “A complex system of special
repair materials using secondary raw materials for
industries” supported by The Technology Agency of The
Czech Republic; and the internal project of specific
research No. FAST-S-16-3772 “Research of an adhesive
modification increasing the durability of FRP/wood
bonds at moisture exposure“ supported by Brno
University of Technology, Faculty of Civil Engineering.
6 REFERENCES
1 B. S. Trimble, Durability and mode-I fracture of fiber-reinforced
plastic (FRP)/wood interface bond, Doctoral dissertation, West
Virginia University, 1999, 192
2 H. V. S. Ganga Rao, Sawn and laminated wood beams wrapped with
fiber reinforced plastic composites, Wood Design Focus, 8 (1997) 3,
13–18
3 D. J. Gardner, J. F. Davalos, U. M. Munipalle, Adhesive Bonding of
Pultruded Fiber-Reinforced Plastic to Wood, Forest Products Journal,
44 (1994) 5, 62–66
4 G. M. Raftery, A. M. Harte, P. D. Rodd, Bond quality at the FRP-
wood interface using wood-laminating adhesives, International
Journal of Adhesion & Adhesives, 29 (2009) 2, 101–110,
doi:10.1016/j.ijadhadh.2008.01.006
5 J. P. Alexander, S. M. Shaler, D. J. Gardner, Evaluating wood/FRP
bond durability through chemical kinetics, Proc. of Wood Adhesive,
Madison WI, 2000, 289–299
6 M. Plötze, P. Niemz, Porosity and pore size distribution of different
wood types as determined by mercury intrusion porosimetry,
European Journal of Wood and Wood Products, 69 (2011) 4,
649–657, doi:10.1007/s00107-010-0504-0
J. VANÌREK et al.: DURABILITY OF FRP/WOOD BONDS GLUED WITH EPOXY RESIN
Materiali in tehnologije / Materials and technology 51 (2017) 6, 889–895 895
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS