Composites in Construction 2005 – Third International Conference
Lyon, France, July 11 – 13, 2005
DURABILITY OF GLASS FIBRE REINFORCED POLYESTER (GFRP) PULTRUDED PROFILES
USED IN CIVIL ENGINEERING APPLICATIONS
J.R. Correiaa, S. Cabral-Fonsecab, F.A. Brancoa, J.G. Ferreiraa, M.I. Eusébiob and M.P. Rodriguesb
a
Civil Engineering Department, Instituto Superior Técnico (IST), Technical University of Lisbon
Av. Rovisco Pais, Lisboa, 1049-001 Lisboa, Portugal
b
Laboratório Nacional de Engenharia Civil (LNEC)
Av. do Brasil 101, 1700-066 Lisboa, Portugal.
jcorreia@civil.ist.utl.pt sbravo@lnec.pt fbranco@civil.ist.utl.pt joaof@civil.ist.utl.pt
isabel.eusebio@lnec.pt mprodrigues@lnec.pt
ABSTRACT: This paper presents the results of experimental research on the physical, chemical,
mechanical and aesthetical changes suffered by glass fibre reinforced polyester (GFRP) profiles
under accelerated exposure to moisture, temperature and ultraviolet (UV) radiation. GFRP profiles
were submitted to four different exposure environments: (i) immersion in water at 20ºC, (ii)
condensation of water at 60ºC (iii) accelerated weathering QUV equipment and (iv) accelerated
weathering Xenon-arc equipment. Results are analysed regarding the sorption, tensile and flexural
behaviours, the chromatic and gloss variations and the chemical changes investigated by means of
infrared spectroscopy. Considerable chromatic changes were observed, especially due to UV
radiation. Regarding structural safety, although some reduction in the mechanical properties was
observed, especially in the immersion and condensation chambers, the durability tests proved the
generally good behaviour of this material under those aggressive conditions.
1.
INTRODUCTION
Glass fibre reinforced polymer (GFRP) pultruded profiles have great potential in the construction
industry, presenting several advantages comparing with traditional materials, among which, the
potentially improved durability under aggressive environments. The evidence of improved durability
performance when submitted to aggressive environments, can be perceived by the history of their
use in marine vessels, piping, storage tanks and in the above mentioned corrosive industries.
Paradoxically, one of the factors that is delaying the widespread acceptance of GFRP profiles as
load-carrying structural elements is the lack of comprehensive and validated data on durability [1],
which creates an obstacle for the construction agents, including owners, civil engineers and
contractors, as construction infrastructures service life is generally expected to exceed 50 years.
Furthermore, this factor has been recently identified by several authors as the most critical gap
between perceived need of information and available information, regarding future research [1, 2],
particularly for composites produced by large scale processing methods such as pultrusion.
Moisture, thermal effects and ultraviolet (UV) radiation play a primary role among the
environmental conditions identified as being of primary importance in terms of durability of FRP
composites in infrastructural applications. GFRP profiles are likely to be exposed to rain, humidity
or moisture and in some applications, such as bridge columns or in harbours, the material may
even be immersed. GFRP profiles used in outdoor are particularly exposed to UV radiation and
thermal effects, the last effect also influencing GFRP profiles used in industrial units. This paper
presents the results of experimental research on the physical, chemical, mechanical and
aesthetical changes suffered by GFRP pultruded profiles following accelerated exposure to
moisture, thermal effects and ultraviolet (UV) radiation. Specimens cut from commercial GFRP
pultruded profiles, currently being used for infrastructure applications, were submitted to four
different exposure environments: (i) immersion in water at 20ºC, (ii) condensation of water at 60ºC,
(iii) artificial accelerated weathering in a QUV equipment and (iv) artificial accelerated weathering
in a Xenon-arc equipment. After submitting the material to those aggressive environments, the
1
sorption behaviour was analysed, the tensile and flexural behaviours were studied, the chromatic
and gloss variations were measured and the chemical changes were investigated by means of
infrared spectroscopy.
2.
MATERIALS
The material studied flexion was obtained from two pultruded GFRP I-shaped profiles (Profile A –
200 mm × 100 mm x 10 mm; Profile B 150 mm × 75 mm x 8 mm). This material consists of
alternating layers of unidirectional E-glass fibre rovings and strand mats embedded in isopthalic
polyester resin. The fibre volume fraction of both profiles, determined from burn-off experiments
was 62%. Initial mechanical properties were determined through flexural and tensile tests in small
specimens according to ISO 14125 and ISO 527, respectively: u,flex = 627 MPa, Eflex = 26.9 GPa,
-3
(profile A); u,flex = 511 MPa, Eflex = 23.7 GPa, u,flex = 23.0 × 10-3; u,tens = 432
u,flex = 25.0 × 10
MPa, Etens = 34.2 GPa, u,flex = 13.0 × 10-3 (profile B). Specimens with dimensions
10 mm × 15 mm × 300 mm were cut from the original profile A in the longitudinal direction and
used in the different tests (3 to 5 specimens for each period and condition of exposure). From
profile B, specimens were cut in the longitudinal direction with several dimensions, according to the
nature of the test to be performed (5 specimens for each period and condition of exposure).
3.
TESTING
3.1
Exposure environments
In order to study possible degradation from typical exposure environments in civil engineering
applications, test specimens were subjected to the following exposure conditions: (i) Immersion
chamber, where specimens from profile A were immersed in water, at a constant temperature of
20ºC for up to 6480 h. (ii) Condensation chamber, where specimens from profile A were exposed
to constant condensation of de-ionized water at a constant temperature of 60ºC for up to 6480 h.
(iii) Accelerated weathering fluorescent UV light apparatus – QUV, where specimens from both
profiles were exposed to repetitive cycles of light and moisture, produced by condensation of water
under controlled conditions. The exposure cycles were based on ISO 4892 - part 3 [3]
recommendations, consisting of 4 h of permanent exposure to solar radiation simulated with
fluorescent lamps at 60ºC, and 4 h of exposure to moisture caused by constant condensation of
de-ionized water at 50ºC for up to 6346 h for profile A and 2000 h for profile B. (iv) Accelerated
weathering Xenon arc light apparatus, where specimens from profile B were exposed, for up to
2000 h, to a predefined programme, consisting of a continuous irradiation with intermittent cycles
of water spray under controlled conditions. The relative humidity on the dry part of the cycle was
65% and the black standard temperature inside the chamber was 65°C. The duration of water
sprays was 18 min with 102 min of dry interval between spraying, according to ISO 4892 –
part 2 [4].
3.2
Experimental procedures
At predefined time intervals, a batch of test specimens placed in the different exposure
environments was removed, dried in a test chamber (at 20ºC and 50% relative humidity) until
specimens attained constant mass, and then submitted to physical, chemical, mechanical and
aesthetical characterization.
(i) Weight Changes (profile A): The specimens were removed from the test chambers after
2160 h, 4320 h and 6480 h exposure for specimens placed in immersion and condensation
chambers, and after 1642 h, 3304 h, 4839 h and 6346 h for specimens placed in QUV chamber, in
order to evaluate the weight changes under each exposure condition.
(ii) Tensile tests (profile B): Rectangular specimens (8 mm thick × 20 mm wide × 250 mm long
were tested in tension according to ISO 527 – parts 1 and 5 [5], without end-tabs, after 2000 h
exposure in QUV chamber and after 1000 h and 2000 h exposure in Xenon-arc chamber, and the
2
obtained data was analysed to determine the tensile properties (strength, Young’s modulus and
strain at failure).
(iii) Flexural tests (profiles A and B): Flexural tests (3-point) were conducted according to ISO
14125 [6] on specimens obtained from profiles A (span-to-depth ratio of 20) and B(span-to-depth
ratio of 16), in order to determine the effects of the degradation caused by the different exposure
environments on the flexural properties (strength, Young’s modulus and strain at failure).
(iv) Colour differences (profiles A and B): Colour differences caused by different environments
were measured with a spectrocolorimeter. For profile A, colour measurements were performed in
the exposed surface of specimens used for flexural tests, while for profile B, the colorimetric
determinations were performed using always the same plate, with dimensions 8 mm thick × 50 mm
wide × 100 mm long. Colour differences were determined in relation to the un-aged material. The
results were expressed in terms of CIE L*a*b* system according to ISO 7724 standard test method
for colorimetry of paints and varnishes [7]. In this system L* gives a measurement of the lightness
and a* and b* give a measurement of the colour in the red-green axis and in the yellow-blue axis,
respectively. The parameter E* gives a measurement of the overall colour change.
(v) Gloss variation (profile B): Gloss measurements were carried out using a glossmeter set at an
angle of incidence/reflection of 60°.
(vi) Infrared spectroscopy (profiles A and B): Infrared spectra were measured using FTIR
analysis performed on a spectrometer. Samples were scraped from the exposed surfaces of aged
specimens, mixed with dry spectroscopic grade potassium bromide and pressed into pellets. Thirty
two scans were collected and averaged at a spectral resolution of 4 cm-1. Spectra were rationed
against background spectrum previously measured to remove the effects of the background in the
tested pellets. The possibility of chemical reactions during different environments was investigated
by studying the relative intensities of the principal infrared absorption peaks susceptible to change
during aging. In order to compare the composite specimens subjected to different periods and type
of exposure, the absorbance of each peak has been referred to the absorption of the band at 1452
cm-1. This band can be attributed to the bending of C-H of CH2. This peak seamed to occur in all
specimens, which explained its choice as a reference band.
4.
RESULTS AND DISCUSSION
4.1
Weight changes (profile A)
Several researchers have reported a great influence of the equilibrium and rate of water absorption
on the thermophysical, mechanical and chemical properties of polymer composites [8-10]. The
amount of absorbed water depends greatly on the chemical composition, the morphology and the
degree of curing of the polymeric matrix of the composite. The water content may induce physical
and chemical aging, or a combination of both, which could be reversible, partially reversible or
irreversible, depending on the nature of the material and the period and conditions of exposure.
Usually, weight changes in moisture environments result from a balance between the weight gain
due to water absorption and the weight loss due to extraction of low molecular components. Fig. 1
shows the specimens weight changes as a function of the deterioration period for the different
exposure environments. In both immersion and condensation chambers overall weight gains of
0.34% and 0.31%, respectively, were observed and it appears that a quasi-equilibrium state was
reached, especially in the condensation chamber. A higher initial rate of water uptake was
observed in the condensation chamber and it is most likely due to the increased temperature of
such environment. This result is consistent with other results at elevated temperatures for different
types of polymeric composites [11, 12]. In the QUV chamber the weight variation was opposite, as
an overall weight loss of 0.29% was observed, showing that weight loss due to extraction of low
molecular components was more important than weight gain due to water absorption during
condensation cycles, which took place for only half of the total period of exposure.
3
4.2
Colour differences (profiles A and B)
Global chromatic variations measured in specimens from both profiles submitted to different
exposure periods and conditions are shown in Fig. 2. In the immersion chamber the chromatic
variations were not significant and consisted essentially of a slight surface brightening and a very
slight yellowing. In the condensation chamber the chromatic variations followed the same pattern
of surface brightening and yellowing but in an amplified scale. The most significant chromatic
variations were registered in the specimens aged in the UV radiation chambers (QUV and Xenonarc equipments). A slight increase in green colour, a brightness increase and, above all, a very
important yellowing were observed. This yellowness increase could be clearly noticed by the naked
eye. These important chromatic variations observed in specimens subjected to aging in the
condensation chamber and especially in the QUV and Xenon chambers present an obvious
esthetical concern regarding outdoor applications. However, in all aging conditions, chromatic
variations seem to have stabilized after a short period of exposure. This aspect may be considered
when structural elements of a construction have to be replaced, as the replacing elements will
rapidly acquire the same aspect.
4.3
Gloss variation (profile B)
Gloss variation of specimens obtained from profile B and placed in the QUV and Xenon
equipments are plotted in Fig. 3. After an initial increase in gloss for both UV exposure
environments, probably due to a mechanism of test specimens’ surface polishing, gloss started to
decrease. The highest decrease in gloss was found after 2000 h exposure to Xenon equipment,
with a gloss loss of approximately 87%, when compared with the un-aged test specimen.
0,30
0,20
0,10
0,00
-0,10 0
1000
2000
3000
4000
5000
6000
20
Variation of the coordinate E*
Weight change ratio (%)
0,40
7000
-0,20
-0,30
-0,40
16
12
8
4
0
0
Exposure period (hours)
Water 20C
Water 60C
1000
Water 20C (A)
QUV
Figure 1 – Weight change ratio (profile A)
2000
3000
4000
5000
Exposure period (hours)
Water 60C (A)
QUV (A)
QUV (B)
6000
7000
Xenon Arc (B)
Figure 2 – Colour differences (profiles A and B)
60 degree gloss (%)
50
40
30
20
10
0
0
500
1000
1500
2000
2500
3000
3500
Exposure period (hours)
Control Sample
QUV-A
Xenon-Arc
Figure 3 – Gloss variation (profile B)
4.4
Tensile properties (profile B)
The stress-strain curves for both un-aged (control specimens) and aged specimens were almost
perfectly linear up to failure. Results from tensile tests are plotted in Fig. 4.
4
Tensile Young's modulus (GPa)
500
Tensile strength (MPa)
450
400
350
300
250
200
150
100
50
0
Un-aged
QUV
Xenon 1000h Xenon 2000h
40
35
30
25
20
15
10
5
0
Un-aged
QUV
Xenon 1000h Xenon 2000h
Strain at failure (1E-3)
16
14
12
10
8
6
4
2
0
Un-aged
QUV
Xenon 1000h Xenon 2000h
Figure 4 - Tensile properties for different exposure environments and durations (profile B).
An overall decrease in the average tensile strength with the time of exposure was observed for all
environmental exposures. However, after 2000 h and 1000 h exposures in QUV and Xenon
equipments respectively, strength decrease was little (4% and 3%, respectively). A slightly more
significant strength decrease (13%) was observed after 2000 h exposure in Xenon equipment.
Nonetheless, regarding the magnitude of the error bars (representing one standard deviation) the
differences are not statistically considerable. The largest extent of degradation caused by Xenon
equipment comparing with that induced by QUV equipment, is not likely due to the UV radiation but
most probably due to the kinetic effect of the water sprays on the specimens’ surface which may
have accelerated material degradation. In fact, comparing UV radiation induced by both
equipments, QUV produces higher irradiance at 340 nm (which constitutes the most harmful part of
the sun’s spectrum for polymer debonding), while Xenon equipment provides a broader spectral
irradiance.
Strain at failure presents a similar pattern of variation, with almost no reduction after 2000 h and
1000 h exposures in QUV and Xenon equipments respectively, and a slight reduction (8%) after
2000 h exposure in Xenon equipment.
Young’s modulus remained largely unaffected for both types of exposure with a marginal increase
(2%) and a slight decrease (4%) after 2000 h exposure in QUV and Xenon equipments,
respectively.
4.5
Flexural properties (profiles A and B)
All specimens exhibited quite linear behaviour prior to failure with very similar load-displacement
experimental curves. Results from flexural tests performed after different aging conditions and
periods, for both profiles, are plotted in Fig. 5 (profile A) and Fig. 6 (profile B).
5
Flexural Young's modulus (GPa)
35
30
25
20
15
10
5
0
W
at Un
-a
er
W 20C ged
at
2
er
1
W 20C 60h
at
er -43
W 20C 20h
at
er -64
W 60C 80h
at
er -21
W 60C 60h
at
er -43
60 20
h
C
Q 648
U
0
h
V
Q 164
U
2
V- h
Q 330
U
V 4h
Q 483
U
V 9h
-6
34
6h
Flexural strength (MPa)
W
at Un
-a
er
W 20C ged
at
er -21
W 20C 60h
at
er -43
W 20C 20h
at
er -64
W 60C 80h
at
er -21
W 60C 60h
at
er -43
60 20
h
C
Q 648
U
0
h
V
Q 164
U
2
V- h
Q 330
U
V 4h
Q 483
U
V 9h
-6
34
6h
700
600
500
400
300
200
100
0
W
at Un
-a
er
W 20C ged
at
er 21
W 20C 60h
at
er -43
W 20C 20h
at
er -64
W 60C 80h
at
er -21
W 60C 60h
at
er -43
60 20
h
C
Q 648
U
0
h
V
Q 164
U
2
h
V
Q 330
U
4
h
V
Q 483
U
9
h
V
-6
34
6h
Strain at failure (1E-3)
35
30
25
20
15
10
5
0
Flexural Young's modulus (GPa)
Figure 5 - Flexural properties for different exposure environments and durations (profile A).
Flexural strength (MPa)
600
500
400
300
200
100
0
Un-aged
QUV
Xenon 1000h Xenon 2000h
30
25
20
15
10
5
0
Un-aged
QUV
Xenon 1000h Xenon 2000h
Strain at failure (1E-3)
30
25
20
15
10
5
0
Un-aged
QUV
Xenon 1000h Xenon 2000h
Figure 6 - Flexural properties for different exposure environments and durations (profile B).
For profile A, a noticeable flexural strength decrease was observed in immersion and condensation
exposure conditions, where flexural strength retentions of 84% and 81% respectively, were
observed after 6480 h exposure. This flexural strength decrease occurred especially after the first
period of exposure, with 2160 h exposure (87% and 84% retention, respectively), while the
reduction in the final period (2160h to 6480 h) was only of 3% in those exposure environments. For
specimens aged in the QUV chamber, flexural strength seems to have been unaffected. After a 6%
drop, occurred in the first 1642 h, the flexural strength continuously increased up to 98% retention
after 6346 h exposure. Also for profile B, the flexural strength after 2000 h of exposure in QUV
appears to have been unaffected (it even increased by 1%). A slightly more significant decrease in
flexural strength was observed in specimens aged in Xenon equipment, with 93% retention after
2000 h exposure. Nonetheless, considering the error bars, this reduction is not statistically
6
noteworthy. Additionally, a 4% increase in flexural strength occurred from 1000 h to 2000 h
exposure.
The strain at failure reduction, presents a similar pattern to that of flexural strength. Strain at failure
retention values of 87% and 82% were observed after 6480 h exposure in immersion and
condensation exposure conditions, respectively, for profile A. In the environments with UV radiation
exposure (QUV and Xenon equipments), strain at failure was practically not affected for both
profiles.
The flexural modulus of specimens from both profiles was not significantly affected. For profile A, a
stiffness increase of about 10% was observed after 2160 h exposure for all the aging environments
and remained almost constant in the subsequent measurements. For profile B, a 2% flexural
modulus decrease was observed for both QUV and Xenon equipments after 2000 h exposure.
These results are in agreement with the experiments performed by Liao et al [11] and
Kajorncheappunngam et al [13] with glass fibre reinforced vinylester and epoxy, respectively. This
behaviour is due to the fact that flexural modulus of glass fibre reinforced polymers is only affected
by temperatures approaching polymer glass transition temperature. Moreover, according to
Apicella et al [14], it seems to exist a competitive effect between matrix plasticization as a result of
water absorption and stiffness increase due the loss of low molecular weight substance.
The reductions observed in strain at failure for test specimens of profile A in the immersion and
condensation chambers, are a clear indicator of polymer degradation due to water absorption,
which may have caused polymer hydrolysis and/or plasticization, weakened the fibre-resin
adhesion or even attacked the glass fibres. Water absorption seems to have had also a noticeable
influence on the flexural strength reduction and this effect seems to have been accelerated by
temperature. Similar results were obtained by Nishizaki and Meiarashi [12] and Van de Velde et al
[15] with pultruded glass fibre reinforced polyester. Regarding UV radiation, results seem to
confirm that the effects of this exposure agent are usually confined to the top few microns of the
surface, consequently having limited influence on the mechanical properties of the material [16,
17]. Although UV radiation may cause surface deterioration, therefore increasing the degradation
potential of moisture and other aggressive agents, the increased thickness of GFRP profiles
currently used in construction will hardly be directly affected by UV radiation.
4.6
Infrared spectroscopy (profiles A and B)
Figs. 7 and 8 show the spectra of specimens obtained from profile A and submitted to immersion
exposure and QUV exposure, respectively.
(i) Immersion and condensation environments: The obtained spectra revealed changes in
some absorptions bands during aging, namely modifications in the intensities of the peaks,
especially for the longer aging duration. In both environments, the intensities of carbonyl peak
(1730 cm-1) decreased, more clearly in immersion (13%) than in condensation (5%), probably due
to slight leaching of low molecular weight carboxylic constituents soluble in water. Changing during
aging in those environments was also observed in the height ratio of the peaks corresponding to
styrene (701 cm-1) and phthalate (732 cm-1). This fact is confirmed by the decrease on intensity of
another styrene characteristic band at 3027 cm-1. Such trend over the time can be explained by the
loss of styrene, most likely residual monomer. This behaviour was already observed in previous
studies [18]. Despite the occurrence of chemical attack has been described by some authors,
namely on the ester linkage of unsaturated polyester resin of composite matrix after long periods of
water exposition at elevated temperature, in the current experiments there are no evidence of such
type of changes.
(ii) QUV and Xenon arc exposures: In the IR spectra performed on the surface of test specimens
exposed to environments with UV radiation, there are evidences of chemical changes, revealed by
modifications in the intensity and in the shape of some peaks, as well as in the appearance of new
ones, especially for the longer aging durations. During the QUV exposure, the carbonyl band
(1730 cm-1) intensity increased, and after 6480 hours it was 42% higher than that of the control
7
sample. In the Xenon camera, in spite of the shorter duration of aging, the intensity of carbonyl
band was increased by 49%. In both cases, the intensity increase of the carbonyl peak was
followed by its widening. As in previously analysed environments, in QUV and Xenon aging there
was also a decrease in styrene content, revealed by a decrease in the intensity of the
characteristic bands. New bands were also observed, namely at 1630 cm-1 which could be related
to the formation of conjugated double bonds C=C. These bonds can explain the yellowness
observed in samples subjected to UV radiation [19]. The observed chemical alterations result from
the photo degradation of the unsaturated polyester, that has been studied and has proved to
involve chain scission, cross-linking and yellowing [20], which consist in a complex set of
mechanisms difficult to quantify only by FTIR. Results prove and confirm that the effects of UV
exposure are usually confined to the top few microns of the surface, leading only to a minor
change in mechanical properties, as previously described. This is particularly true for thick sections
used in Construction, in which degradation affects only a thin part of the material’s section.
0.8 [1452] Un-aged profile
[1452] Un-aged profile
Abs
Abs
0.6
0.4
0
0.2
[1452] Profile A- QUV- 1642 hours
Abs
0.0
0.8 [1452] Profile A- Immersion - 2160 hours
0.6
Abs
1
0.4
1
0
[1452] Profile A- QUV- 3304 hours
0.2
Abs
0.0
0.8 [1452] Profile A- Immersion - 4320 hours
Abs
0.6
1
0
[1452] Profile A- QUV- 4839 hours
0.4
Abs
0.2
0.0
0.8 [1452] Profile A- Immersion - 6480 hours
1
0
[1452] Profile A- QUV- 6480 hours
0.4
Abs
Abs
0.6
1
0.2
0
0.0
3500
3000
2500
2000
1500
1000
3500
500
Wavenumbers (cm-1)
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Figure 7 - Infrared spectra - Profile A: immersion.
5.
3000
Figure 8 - Infrared spectra - Profile A: QUV.
CONCLUSIONS
Based on the findings of this study, the following conclusions can be addressed:
1. Immersion and condensation environments had a noticeable effect on the flexural
properties of GFRP profiles. Strength and strain at failure decreases due to moisture were
observed and these effects were accelerated by increased temperature. Nevertheless the
degradation is mainly due to physical phenomena, as plasticization of polymeric matrix,
since no appreciable chemical degradation was detected by FTIR analysis.
2. For the QUV and Xenon arc experiments, although FTIR analysis shows important
chemical degradation in the surface of the material, the synergetic effects of simultaneous
exposure to UV radiation, moisture and temperature on the flexural and tensile properties of
GFRP profiles were not significant throughout the experiment period. These results seem to
confirm that UV radiation has limited influence on the mechanical properties of the material,
affecting, above all, the material’s surface.
3. The considerable chromatic and gloss changes observed, especially due to UV radiation,
cause an aesthetical concern for outdoor applications, where the use of protective coatings
can hardly be avoided.
Although some reduction in the mechanical properties was observed in the durability tests (up to
19%), especially in the immersion and condensation chambers (what means that it may occur in
structures placed underwater or in high moisture environment), the present study proved the
generally good mechanical behaviour of glass fibre reinforced polyester pultruded profiles,
especially under the other environmental conditions used in the experiments. The study confirms
that pultruded profiles, besides presenting excellent structural performance and lightness, offer
improved durability when compared with traditional materials.
8
6.
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