On estimates of durability of FRP based on accelerated tests
Transcript of On estimates of durability of FRP based on accelerated tests
Accepted Manuscript
On estimates of durability of frp based on accelerated tests
Manuel A.G. Silva, B. Sena da Fonseca, Hugo Biscaia
PII: S0263-8223(14)00232-3
DOI: http://dx.doi.org/10.1016/j.compstruct.2014.05.022
Reference: COST 5703
To appear in: Composite Structures
Please cite this article as: Silva, M.A.G., Sena da Fonseca, B., Biscaia, H., On estimates of durability of frp based
on accelerated tests, Composite Structures (2014), doi: http://dx.doi.org/10.1016/j.compstruct.2014.05.022
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1
ON ESTIMATES OF DURABILITY OF FRP BASED ON ACCELERATED
TESTS
Manuel A. G. Silva1, B. Sena da Fonseca2, Hugo Biscaia3
Abstract
Structures externally rehabilitated with fiber reinforced plastics (FRP) are often exposed
to aggressive environmental conditions that dictate deterioration mechanisms and
shorten their structural life-cycle. That degradation and ensuing failure may be due to a
combination of factors and the environmental conditions leading to such failures include
cyclic temperature variation, sorption of saline moisture due to salt fogging or dry-wet
cycles and immersion in water.
This study seeks to contribute to better knowledge of the phenomena involved,
by studying degradation of GFRP laminates through tests of mechanical strength after
accelerated aging designed to estimate long term natural degradation. The measured
degradation is extrapolated to other environmental situations likely to occur for longer
periods using Arrhenius type of analysis. The validity of the generalizations based on
parameters derived from diffusion studies or from the tensile strength tests is examined.
It is preliminarily concluded that Arrhenius equations based on experiments
made after salt fogging at 30ºC, 45ºC and 55ºC are not applicable. The extrapolations
based on diffusion laws were found unsuccessful. However it was possible to extend the
results after accelerated aging to prototype conditions in the case of immersion in salt
water by application of Arrhenius methods.
[email protected] Tel: +351-21-2948580 Fax: +351-21-2948398 UNIC/Faculdade de Ciências e Tecnologia/Universidade Nova de Lisboa 2829-516 Caparica - Portugal 2 Faculdade de Ciências e Tecnologia/Universidade Nova de Lisboa 2829-516 Caparica - Portugal 3 UNIC/Faculdade de Ciências e Tecnologia/ Universidade Nova de Lisboa 2829-516 Caparica - Portugal
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Keywords: Composite laminates; Long term strength; Arrhenius method;
Accelerated aging; Strength degradation; Salt fog cycles.
1. Introduction
The physical and mechanical properties of composites of polymeric matrix reinforced
by fibres (FRP), and structures strengthened by such composites, e.g. the glass
transition temperature, the load carrying capacity and the ductility vary as a function of
time and ambient conditions, namely with the operational temperature. Application of
composites by the wet lay-up technique imply, in general, cold-curing of structural
epoxy adhesives with long post-curing that alters the initial value of the glass transition
temperature of the resin, Tg. Aggressive environmental conditions influence and, in
general, decrease the strength of both the matrix of the polymeric composite and the
adhesive that bonds the composite to concrete in the cases of external reinforcement by
FRP and design engineers need to know safe and economic values of the characteristics
of those materials to adequately dimension the structures for their life-cycle. Research
undertaken to know such values has been made comparing results obtained in
laboratory and after actual exposure, e.g. Moussa et al. [1], but there is still great
uncertainty on the long term evolution of important material parameters.
Glass fibre reinforced polymers (GFRP) have been fundamentally used to
strengthen structures either through rods "replacing" reinforcing steel bars, or strips
bonded to concrete as external strengthening of structural members, or confining
columns. The beneficial effects of their use in rehabilitation, based on ease of
installation, high chemical resistance, and reduced architectural impact [2] and/or
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increase of the mechanical or fatigue resistance are known and have been reported in the
technical literature [3-7].
However, GFRPs under aggressive environmental conditions are not a solution
free of problems. The action of moisture in glass fibres may induce damage and the
effect of moisture and temperature may reduce their expected durability [8]. Under the
influence of humidity or water the glass fibres form a water skin in which the alkali ions
(e.g. NaOH-) are leached, replaced by protons (H+), and such leaching of alkali oxides
(sodium and potassium oxide) from the surfaces of the glass fibres leads into formation
of microcracks [9]. The water around the glass fibres becomes an alkali solution as the
alkali ions dissolve out of the glass, and the glass fibres gradually decompose.
Some degradation mechanisms like etching, leaching, and embrittlement are
described by Chen et al. [10]. These authors utilized a procedure based on the Arrhenius
relation to predict the long-term behavior of GFRP bars in concrete structures from
results of their mechanical testing after accelerated aging in concrete pore solutions at
20, 40, and 60°C.
The degradation of polymeric laminates by moisture absorption is common,
associated with plasticization of the matrix, usually caused by local relative movement
of molecular chains due to reduced interaction between chains. The plasticization is
reversible before reaching a threshold [11, 12].
The structural integrity and the mechanical performance of GFRP depend also
on the stability of the interface fibre/matrix that may be affected by stresses caused by
differential volumetric expansion, and weakening bond [13]. Thermal expansion in
polymers due to temperature changes is reversible and in general does not significantly
affect their life expectancy. However, in polymer composites, the mismatch caused by
the difference of thermal expansion of the resin matrix and the reinforcing fibres may
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induce degradation especially during thermal cycling [14]. The estimate of the
consequences of these types of phenomena along the life-cycle of those systems has
been attempted by imposing accelerated aging followed by mechanical tests as already
mentioned. The extrapolation of those results to actual conditions raises objections from
some researchers and requires much attention.
In terms of the mechanisms of attack of concrete, namely by sulphates and
chlorides, Table 1, excerpted from Buenfeld [15], gives a perspective of the types of
phenomena that may take place, observing that, except for abrasion, all deterioration
mechanisms involve transport of ions, gas or water and, except for abrasion and frost,
they all involve chemical reactions between the diffused fluid and concrete. In all cases
there are microstructural changes responsible for deterioration of physical properties of
concrete and mechanical strength of the structures. The document also summarizes
models for prediction of structural behaviour.
More recently, the same author wrote an encompassing article with his views on
the problems faced while studying and estimating deterioration of concrete structures
[16]. A major criticism he rises on accelerated aging is that each deterioration involves
several separate mechanisms, e.g. transport, chemical reactions and fracture, whereas
measures to accelerate one process do not accelerate the other processes to the same
extent and the resulting experimental global mechanism becomes distorted in relation to
the natural exposure. As a consequence, Buenfeld [16] recommends to accelerate the
aging process as little as possible.
One of the techniques more often considered to correlate accelerated
performance and actual behaviour is based on the denominated theory of Arrhenius that
is used to predict the effects of temperature and time. In the framework of the studies,
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the procedure is especially applied for estimating the effects of long-term exposures of
polymers from the results of tests after short-term exposure at elevated temperatures.
Table 1 – Deterioration mechanisms, after Buenfeld [15].
The basis of the application of this analysis in civil engineering was first described by
Litherland et al. [17] who studied changes in the strength of strands of glass fibre
embedded in a cimentitious matrix of Portland Cement, accelerated by ageing in hot
water at several temperatures and compared the results with strength changes in material
exposed to natural weathering.
Dejke [18] followed the procedures validated by those authors for glass fibre
reinforced concrete and described in his thesis the results of exposure of GFRP bars to
alkaline solutions at 20, 40, 60 and 80ºC, for a maximum of 568 days. Immersed
samples were periodically removed from the solutions and mechanically tested for
tensile strength. Actual life prediction was based in the concept of time shift factor
(TSF), showing that the tensile strength retention after 1.5 year in moisture saturated
concrete at 60ºC was approximately the same as for 50 years at a temperature of 7ºC.
Bank et al. [19] developed for the Federal US Highway Administration (FHWA)
a protocol to predict FRP long-term property retention when subjected to accelerated
aging based on the Arrhenius model. The authors specify four aging temperatures in
function of the glass transition temperature Tg of the composite and exposure times of
28, 56, 112 and 224 days.
Gonenc [20], also in his thesis work, studied glass, carbon and stainless steel
bars following the FHWA methodology. The samples were submitted to a conditioning
solution of 5% NaCl and tap water for 28, 56, 112 and 224 days at 44, 56, 68 and 80ºC.
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An estimate retrieved from his work indicates that the glass rods at 23ºC will have a
retention of 50% of bending capacity after 38 years of exposure.
In another thesis, GFRP bars used for reinforcement of concrete were encased in
a cement mortar paste to simulate an alkali environment [21]. The samples were placed
in a saturated calcium hydroxide solution at 30, 45 and 57ºC. Tensile tests were
performed after 10, 30, 60, 90, 180 and 300 days. Moisture absorption was also studied
using a Fickian model to predict long-term tensile strength retention. Having as
reference temperature 13ºC, the estimated strength reduction after 50 years was 45%.
This author also refers the method based on the concept of time shift (TSF),
concluding that there is not a single process that controls the kinetics of degradations in
such conditions and no Arrhenius type time-temperature relationship.
The predictions by Chen et al. [10] estimated excessively short lifetime periods.
As examples, at 20ºC, the tensile retention obtained after 3 years was 50%.and in
another solution the tensile strength was estimated to drop 50% after 6 months of
exposure.
Robert et al. [22] also used in their study GFRP reinforcing bars embedded in
concrete to simulate an aggressive alkaline environment of saturated concrete. The
embedded samples were immersed in tap water at three different temperatures 23, 40
and 50ºC during 60, 120, 180 and 240 days. The authors followed the procedure
proposed by Bank et al. [19] to estimate that, at 6ºC, after 100 and 200 years of
exposure, respectively, the strength would decrease 20 and 25%.
It is noted that the reported studies are mostly on GFRP bars and there is less
predictive information of long term behaviour of laminate composites. All considered,
there remain serious issues on the prediction of the life-cycle of these structures, and
their life cycle. The present work attempts at making a contribution to improve the
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knowledge on the degradation of GFRP laminates, in typical environmental conditions
of saline moisture and, taking into account the experimental results of tests after
accelerated aging, to illustrate the application of Arrhenius based models for long-term
predictions.
2. Long Term Prediction
2.1 Models based on Arrhenius equation
These models are often used when the temperature is the dominant factor of
acceleration in the aging process. The Arrhenius equation may be written in different
ways, namely
(1.a)
or
(1.b)
where k is the reaction rate or degradation rate, A is a material and environment related
constant, Ea is the energy of activation, R is the universal gas constant (8.3143 J/molK),
and T is the temperature in °K. Interpreting the degradation rate k as the inverse of the
time required for a material property to reach a given value, the logarithm of 1/k shows
that the time for a property to degrade to a chosen value is a linear function of 1/T with
the slope Ea /R.
The fundamental assumption to use this approach for prediction of long term
behavior is that there is a single dominant mechanism of degradation which does not
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change with time, nor within the temperature range of the accelerated aging, while the
rate of degradation will increase with temperature. In composites of polymeric matrix,
the assumption precludes the use of temperatures too close to Tg. Those conditions
respected, the tensile strength retention (SR) of composite laminates of GFRP, i.e. the
percentage of residual strength divided by the original tensile strength, is calculated
from experimental data obtained for each temperature of exposure T. The points so
obtained are plotted against logarithm of time and allow procedures estimating the loss
of strength for longer periods and different temperatures. Following the procedure
adopted by Chen et al. [10], it is accepted that the percentual stress retention in the tests
along the accelerated aging, SR, follows the expression
(2)
where t is the time of exposure. The expression implies total loss of strength if time t
increases beyond limit.
Plotting experimental values of stress retention versus corresponding time of
aging allows the determination of the rate of degradation k by regression analysis valid
if the correlation coefficient is above 0.8 [19]. The time to reach a selected value of
tensile strength retention for a different temperature can be obtained from Eq. (2). All
temperature regression curves should have approximately equal slopes, otherwise the
degradation mechanism is temperature dependent.
An operationally different approach is used by Dejke [18] based on the concept
of time shift factor, TSF, defined as the ratio between the times required for a certain
decrease in a mechanical property at two different temperatures. The time for a certain
reaction to take place is proportional to the inverse of the reaction rate k and the ratio
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of t1, time required for a certain decrease of a property at temperature T1, and t2, time
required for the same decrease at temperature T2, can, then, be written as
(3)
TSF allows the calculation of the degradation under a selected service condition from
the data obtained along the accelerated aging [23] by shifting the latter data obtained in
laboratory conditions (e.g. strength retention) along the time axis to find the time t1 at
which, under a lower temperature, the same degradation is estimated to take place. Once
obtained TSF from the experimental data, (Ea/R) is calculated i.e. the activation energy
becomes known and it is possible to apply the regression curves to predict strength
retention for site conditions and a prescribed time, always within the temperature
interval on which the Arrhenius model is expected to be valid.
2.2 Prediction based on diffusion
The simulation of the moisture diffusion through the epoxy matrix is presented with a
single free-phase model. Assuming that Fick's law adequately represents GFRP
moisture absorption [24], the governing equation is:
(4)
where c is the moisture or ionic concentration (mol/l) measured in the direction normal
to the surface at a distance x, and D is the diffusion coefficient in the x direction. The
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solution to the problem as given by [25] can be represented by the following expression
[26], for an infinitely large plate of thickness h, at time t:
(5)
where M% is the percent moisture absorbed at time t and M∞ is the maximum moisture
content. The initial part of the M% versus t1/2 is linear if the nature of the diffusion is
indeed Fickian. The diffusion coefficient (D) can be estimated by the slope of the linear
portion of the curve
(6)
Assuming moisture content and diffusivity as parameters governed by the Arrhenius
equation, a relationship between the maximum moisture content or the diffusion
constant and temperature, for accelerated ageing, can be expressed as
(7)
(8)
where is the activation energy, the subscript of E identifying M or D (x=m,d). From
(7) it is derived
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= + (9)
Plotting the data into a graph of ln(1/M∞) versus 1/T, with T in Kelvin degrees, the
linear relationship allows the obtaining the activation energy Em from the slope of the
straight line, (Em/R), and a similar procedure applies to (8).
Since a linear direct relation has been observed between moisture absorption and
retained tensile strength, the latter can, thus, be obtained from moisture content
prediction [27].
This Fickian model has prevailed on the analyses of diffusion in civil
engineering applications of FRP, but has shown poor adequacy on the lower range of
temperatures (25 to 50ºC) and there are studies pointing to other modelling techniques
[28] that may be more adequate for diffusion of moisture through epoxy matrices,
leading to the so-designated Langmuir models that provide better fit in the mentioned
interval of temperatures.
Non-Fickian behaviour may result from irreversible reactions between polymer
and moisture and formation of hydrogen bonds. Two-stage diffusion has been detected
and reported in polymers, for instance on sorption of salt water by an epoxy resin [29],
and is referred to as anomalous absorption [30]. The Langmuir model was contrasted
with Fickian based models by Walker and Karbhari [31] who compared the results of
tests for immersion in deionised water, relative humidity (RH) cycles and different
temperatures at various fixed RH. Some differences were registered, but the study was
made for E-glass vinylester composite specimens and not for epoxy. In the present
study the Fickian model was applied.
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3. Experimental Program
The composite laminates of GFRP were composed of two layers, each made of one-
directional E-glass fibre reinforcing fabric (Tyfo SEH-51) and an epoxy resin
recommended by the supplier.
The tensile strength of the dry fibre, the elasticity modulus and the ultimate
elongation indicated by the supplier are 3.24 GPa, 72.4 GPa and 4.5%, respectively. For
the epoxy, the tensile strength is 72.4 MPa, the longitudinal modulus E=3.18GPa, and
the ultimate elongation 5.0% [32]. The given glass vitreous transition temperature, Tg, is
82ºC after curing for 24 hours at 22ºC. The dry mass density of the fibre is 0.915kg/m2,
per ply. The composite laminate has a design ultimate tensile strength 460 MPa, an
elongation at break 2.2% and a tensile modulus 20.9 GPa [33], all according to the
supplier.
The coupons of 300×25mm for tensile tests were cut from 650×550mm plates.
Tabs from the same plates, 40×25 mm, slightly chamfered, were glued on both sides of
the coupons ends, using a two component adhesive designated commercially as Epoxy
AMS 222. When applicable, the lateral cut surfaces of the coupons were sealed with
resin to avoid solution uptake through them.
3.1 Salt fog cycles
Dimensions of coupons and tabs were the same as described for salt water immersion. A
group of samples used had been left at room temperature for 4 years, while the
remaining underwent a much shorter 30 day post-curing. Reference specimens, so
designated when tested at the onset of the aging experimental program, led to the results
shown in Table 2.
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Table 2 – Average GFRP properties of reference specimens - batch for salt fog cycles.
The 4-year cured samples showed significant improvement in mechanical behavior
attributed to stronger bond of matrix and fibres.
The accelerated cycles of salt fog spray were imposed in three chambers (two
ASCOTT S-120 and one ASCOTT S-450) and a 5% NaCl solution. The spray cycles
were programmed for 6h spraying followed by 18h drying, and each set of coupons was
submitted to a different temperature of 30ºC, 40ºC and 55ºC, the latter being the highest
temperature that could be imposed on the available chambers. Tests for tensile strength
were made at selected intermediate time stages, similarly to those described before for
the immersed coupons.
3.2 Salt water immersion
Laboratorial characterization of the material was made according to ASTM D-570 [34]
for water absorption and, according to ISO 1172:1996 [35], for textile-glass content.
The ultimate tensile strength, elongation at break and tensile modulus were obtained
according to ASTM D3039 [33]. The average results are shown in Table 3.
Table 3 – Average GFRP properties of reference specimens- batch for immersion.
Accelerated aging was imposed by immersing the coupons in a 5% NaCl solution in
small containers. The salinity was frequently measured and maintained by controlled
addition of water. The solution was kept at three different temperatures, 35, 50 and 65ºC
in constant temperature chambers (WTB binder). The temperatures were selected based
on two criteria: the maximum temperature (65ºC) was sufficiently below the value of Tg
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and the minimum temperature (35ºC) was at a level still causing accelerated aging. The
temperature (50ºC) was selected as an intermediate average value.
The evaluation of the mass gain due to moisture absorption during immersion
was made by periodically inspecting the samples, drying their surface and weighing
them on a scale with a resolution of 0.001g.
After 750, 1500 and 2500 hours of immersion, 5 samples of each type of
conditioning were removed from the solutions and tested for their mechanical
properties. The mechanical tensile tests were performed in a Zwick universal machine
according to ASTM D3039 [33].
4. Degradation after salt fog cycles
4.1 Coupons with 4 years post curing
The laminates were naturally aged for 4 years at room temperature before being
submitted to salt fog cycling. Fig. 1 and Table 4 display the average ultimate
mechanical properties obtained along the aging processes, with no data available at
2500h for 40ºC.
After 750 hours of contamination it were registered decreases in ultimate
strenght of 2.5% at 30ºC, 6.4% at 40ºC and 6.0% at 55ºC. The salt fog cycles at 55ºC
caused strength reduction until 1500 hours, and that decrease remained until 5,000h. In
terms of strength, the laminates aged at 40ºC reveal a quite similar evolution to that
shown by 55ºC laminates. Contrarily, for the specimens exposed at 30ºC there was an
increase of the state of degradation throughout the entire period of aging.
Between 2,500h and 5,000h, the lowers temperatures are more damaging than
higher temperatures, this evidence the action of two distinct processes: i) higher
temperature in the chamber makes diffusion faster and deeper during the spray phase; ii)
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along the drying phase the higher chamber temperature lowers the humidity which
cause a decrease on the rate of moisture sorption, promoting salt cristalization.
It is known that salt moisture by chemical action leads to a weakening of the
adhesion or even a separation between matrix and fibre at the interface [13], but such
effect may be “...compensated by the relaxation of residual stresses, swelling of the
matrix...” Mourad et al. [36].
Until 1500 hours and in the three cases (30ºC, 40ºC and 55ºC) the modulus
behavior suggests that the prevailing damage mechanism is swelling. Then, between
1500h and 2500h plasticization processes were predominant especially for specimens
aged under 30ºC.
For 40ºC and 55ºC aging, some recovery in strength was recorded after 1500h of
exposure. Mourad et al. [36] explain a similar recovery due to the release of residual
stresses since, when the moisture absorption reaches equilibrium, plasticization and
swelling occur in matrix and interface. The same justification is advanced by Merah et
al. [37] to explain the absence of strength reduction after 3000 hours of immersion in a
salt solution. At 30ºC the strength decreased linearly, leading to the conclusion that the
previously mentioned equilibrium had not been reached until 5000 hours of exposure.
Fig. 1 – Salt fog cycles - GFRP naturally aged for 4 years: (a) Average tensile strength; and (b)
longitudinal modulus along time.
Table 4 – Tensile tests after salt fog cycles under 30ºC, 40ºC and 55ºC.
On the opposite side, even though a most intense diffusivity occurs at higher
temperatures, the crystalization phenomena is also modified by temperature and, due to
evaporation, a larger number of halite crystals could be seen on the surface of the
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coupons samples at 55ºC. Similar formation of crystals is expected to take place in
microvoids of the matrix. This increase of density of halite crystals delayed the moisture
uptake in the following spray cycles. Is believed that the repeated cycles of salt fog
caused a larger concentration of Na+ and Cl- inside the laminates and their presence is
known to have a slight retarding influence on damage [9]. Only a few crystals could be
seen in laminates aged at 30ºC but with different morphology when compared to the
crystals formed at higher temperatures, Fig. 2.
Fig. 2 – After 5000 h of salt fog cycles (a) morphology of crystals at 55ºC; and (b) at 30ºC.
The major conclusion is that, in the case of salt fog cycles, temperature and drying time
determine the rate of the degradation mechanisms and long term predictions based on
Arrhenius principle are not valid.
4. 2 Coupons with 30 day post cure
The average ultimate mechanical properties of the laminates subjected to accelerated
aging by salt fog cycles, after 30 days of post curing, are displayed in Fig. 3 and in
Table 5. After 750h the ultimate tensile strength increased due to post-cure, especially
in the case of aging at 30ºC. The same finding was described by Robert et al. [22] when
GFRP bars were subjectd to higher temperatures.
The results are influenced by two opposite effects, post cure and aging effect due
to the previously mentioned phenomena. However, in the case of 40ºC and 55ºC
degradation prevails in relation to the laminates submitted to 30ºC, which is somehow
contraditory to the findings described for coupons with 4 years of cure. Futhermore, the
overall degradation along the total period of exposure is higher for the cases of 40ºC
and 55ºC. The results seem to suggest that the physical effect of a more intense salt
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crystalization due to higher temperatures of drying is more damaging in this case than in
the previous situation, possibly due to some matrix softening at the beginning and along
the aging time. Regarding the evolution of the tensile modulus, it is coherent with that
interpretation, since after an initial increase (at 750h) all the values kept approximately
constant until 5,000h.
In terms of direct applicability of Arrhenius equations there is an initial
difficulty since the ultimate stress decreased only after 750h, having increased till then.
This is due to the fact that the curing process of the material was far from complete in
these specimens and the higher temperature acted as an accelerator of curing that
preevailed over the mechanisms of degradation.
The importance of (complete) curing is seen by observing Figs. 1 and 3. The
strength at 0h is higher for the fully cured laminates and decays continuously whereas
the partially cured coupons at 0h have their curing accelerated by the higher
temperatures and that effect leads to higher strength in the early stages, exceeding the
decay caused by moisture sorption and associated phenomena.
Fig. 3 – Salt fog cycles - 30 days post cure coupons: (a) Average tensile strength and (b) longitudinal
modulus along time.
One might have to consider, then, the applicability of Arrhenius type modeling
restricted to values measured at times elapsed from “fairly complete” curing of the
specimens.
For the evolution of the longitudinal modulus the observed trend is for its
increase rather than decrease with time, meaning that the reduction of the ultimate strain
is more influential than the modification of the ultimate strength, and the Arrhenius
equation is not applicable.
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Table 5 – Tensile tests after salt fog cycles under 30ºC, 40ºC and 55ºC in coupons with 30 day post cure.
5. Degradation after salt water immersion
5.1 Moisture absorption
Fig. 4 shows the curves of mass gain along the aging time. All four curves display
similar behaviour, with a first linear segment that defines D, followed by stabilization in
mass gain designated as M∞. In the cases of 35ºC and 65ºC curves, a second stage
reveals a slight increase in mass gain interpreted as further diffusion in the interface
fibre/matrix. Based on the Fickian sorption, the values of D and M∞ were calculated and
are displayed in Table 6.
Fig. 4 – Immersion - maximum mass gain for salt water at different temperatures, and for deionised
water.
The samples immersed in deionised water at 23ºC had a larger diffusion coefficient D,
higher than all the values obtained for samples immersed in a 5% of NaCl solution even
at higher temperatures. The same applies to the values of M∞ as previously found by
Silva and Biscaia [8]. This fact can be explained by the low water activity aw in a saline
solution Römhild et al. [38]. Water activity in composite laminates is related to
chemical and physical trapping of water in the interface fibre/matrix. The water activity
modifies the water transport kinetics Lekatou et al. [39] and it increases the osmotic
pressures. Thus, high water activities produce an increase of the interfacial gap, which
leads to an increase in the voids and more incoming water.
The formation of delamination and debonding at interface has been shown to be
a function of water concentration [40].
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The values of samples immersed in a solution of 5% NaCl also demonstrate a
close relation between the temperature and the moisture kinetics uptake and constants
M∞ and D depend on temperature of immersion [41].
Fig. 5 shows the relation between D and M∞ and temperature. Confirming the
strong correlation between temperature of the solution and uptake, the regression lines
present correlation coefficients of R2=0.997 and R2=0.947 for diffusion coefficient and
maximum moisture content, respectively. It can be anticipated that higher temperature
also causes more severe degradation of the tensile strength in GFRP, since it is
associated with higher moisture uptake.
Table 6 – Parameters from diffusion analysis.
Fig. 5 – Salt water immersion - Maximum moisture and diffusion coefficient variation with temperature
of solution.
5.2 Tensile tests
The thickness of the laminates considered for calculations was 1.27 mm per layer. The
tests of the reference coupons, i.e. non artificially aged, led to an average tensile
strength 484 MPa, with a standard deviation 36 MPa, and an average ultimate strain
1.901%, with a standard deviation 0.164%. The corresponding average elasticity
modulus was 25.61 GPa with a standard deviation of 2.60 GPa.
The evolution of the tensile strength along the process of immersion in salt water
at different temperatures is presented in Fig. 6. Main preliminary conclusions are that
strength decreases with increasing temperature of the solution and with time. The often
cited initial increase of strength attributed to the completion of curing of the resin along
the aging process was not registered here since the coupons had been curing for over 2
20
years at room temperature, prior to the immersion in salt water. The first tests of the
coupons aged at 35ºC for 750 hours led to unreliable results due to poor gripping and
are ignored.
Fig. 6 – Salt water immersion - tensile strength at different temperatures.
Water and NaCl molecules reach the fibres around t=750h as shown by Fig. 4 and as
has been reported their interaction with the fibres may cause fibre swelling or leaching
of some of their constituents [9]. However, these alterations could not be observed.
The principal role of the resin matrix is to ensure force distribution by the fibres,
a function that chemical reactions, resin plasticization and possibly hydrolysis may
disturb [12], but these possible situations could not be observed in the tests. In any case,
the loss of mechanical strength due to debond at the fibre-matrix interfaces has high
probability of having occurred.
It is noted that the polymers chemical structure does not suffer any significant
change due to immersion as concluded by some authors [42, 43] after FTIR analysis.
Figs. 7, 8 display the time history of ultimate strain and elasticity modulus
respectively. Strains follow a trend apparently similar to stress, with higher temperature
causing higher strain decrease, a result more pronounced for 65ºC. The elasticity
modulus E is more difficult to characterize, since it results from the quotient of those
quantities. For specimens aged at 35ºC and 50ºC, E decreases markedly until 1500 h. At
2500h, the absorbed moisture creates hydrostatic pressure and E remains essentially
stationary as found elsewhere [44].
In the case of 65ºC the elasticity modulus first increases and then decays. This
different behaviour may be linked to the higher diffusion coefficient, leading initially to
a considerable uptake in a short period of time which affects the interface matrix/fibre
21
and consequently the elasticity modulus. In the second phase, the first plateau of mass
intake may be depassed by an aforementioned slight increase in mass gain associated
with diffusion in the interface fibre/matrix further weakening the respective bond and,
thus, decreasing E.
Fig. 7 – Effect of salt water on strain at different temperatures.
Fig. 8 – Effect of salt water on elasticity modulus at different temperatures.
6. Prediction of long-term effects for immersion
6.1 Application of Arrhenius equation
In agreement with methodology summarized in Section 2, estimates of long term
behaviour for salt water immersion were calculated. Fig. 9 shows curves of strength
retention versus time represented in a log scale with the R2 regression values close to 1.
Similar slopes for the different temperatures indicate that this type of degradation is
predictable by Arrhenius law. The retention of tensile strength illustrated earlier in Fig.
6 is numerically displayed in Table 7 for easier reference hereafter.
Table 8 shows the values of TSF obtained by averaging the values corresponding
to the three degrees of degradation and each temperature. The calculation of the
tabulated values is illustrated for ageing at 50ºC. The expression found by linear
regression, Fig. 9, is SR=-0.148ln(time)+1.8556 and leads to t=1252 h for a strength
retention of 80%. Similarly, for the specimens aged under 35ºC, the time required to
degrade the strength by 20% is 3254 h. Then the TSF to move from 35ºC to 50ºC is
given by TSF=3254/1252=2.599. The procedure was repeated for three different values
of SR and the three temperatures of water and an averaged TSF calculated, TSF=2.545.
22
Table 7 – Strength retention (SR) for immersion in salt water.
Fig. 10 shows an interpolated approximation of TSF versus temperature of immersion
taking TSF=1 for 35ºC. The activation energy can be found based on the interpolated
curves of Fig. 10 and Eq. (3), leading to Ea/R =5278ºK.
Fig. 9 – Salt water immersion - Tensile strength retention versus logarithm of time.
Table 8 – Time (hours) to reach given values of strength retention and TSF (w/respect to values at 35ºC).
Aging differs with the mean temperature of water at the site where the degradation of
strength needs to be known. Consider, then, for example, 10ºC as the water temperature;
the activation energy found as explained earlier and TSF obtained from Equation(3),
e.g. generating the values 4.54, 10.05 and 20.74, for shifts from 10ºC to 35ªC, 50ºC and
65ºC, respectively. Multiplying the 2500h of accelerated aging at 65ºC by the TSF, i.e.
20.74×2500 h, the approximately 6 years obtained represent the time required for a
strength retention SR=0.61. In an analogous manner, a loss of strength of 29% can be
predicted after (20.74×1500 h) i.e. approximately 3.6 years.
Fig. 10 – Salt water immersion - Relation between TSF and temperature.
In order to predict the retention after 25 years, for aging at 10ºC, based on accelerated
aging at 65ºC, the ratio 25/20.74 indicates that data should be recorded for 1.2 years.
The estimate of the SR, at 65ºC, after 1.2 years of aging, more precisely 10,560h is
made from the linear regression in Fig. 9, SR=-0.166ln(10560)+1.9091=0.37 that is also
23
the retention predicted after 25 years of immersion in salt water with 5% salinity and
10ºC local mean water temperature.
Fig. 11 depicts curves for prediction of tensile strength retention of GFRP
immersed in salt water at 10ºC, 14ºC and 18ºC.
The durability of FRP composite systems used for seismic retrofit of columns
and immersed in water, based in the Arrhenius method and compared with ACI 440
requirements is estimated in Walker and Karbhaei [31].
Saadatmanesh et al. [45], in a study that lasted more than 3 years, submitted
GFRP laminates to seawater immersion and concluded that the strength decayed 30%, a
percentage close to the predicted in the present work.
Fig. 11 – Immersion - Estimated retention of GFRP tensile strength (TSF approach).
Bank et al. [19] proposed a slightly different methodology to the Federal Highway
Administration (FHWA), still based on Arrhenius equation, designated below as FHWA
methodology. Considering four aging temperatures in function of the Tg of the
composite and four exposure periods of 28, 56, 112 and 224 days, a plot of retention
property SR versus log(time) is made and a R2 factor larger than 0.8 required for all four
curves. Then, for each imposed temperature, service lifetime predictions can be made
for selected values of SR, based on a plot of log(time) versus (1000/T), as shown in Fig.
12, with T in Kelvin degrees, where regression lines are also shown. These curves, for a
selected temperature on the x-axis, provide the estimate of time taken to reach the
selected retention values Gonenc [20]. Fig. 13 was drawn following those steps to show
the values of GFRP tensile strength retention, corresponding to immersion in salt water
with 5% salinity, at 10ºC, 14ºC and 18ºC and does not significantly differ from Fig. 11.
It is observed that at 10ºC and after 25, 50 and 75 years of exposure, the GFRP would,
24
then, present, respectively, 38, 28 and 22% of strength retention. The predictions made
may be conservative for FRPs whose equilibrium moisture condition is significantly
below saturation [20].
The same author predicted a retention of 50% at 23ºC for GFRP rods immersed
in a 5% NaCl after 38 years using the FHWA method.
Chen et al. [10] also considered the Arrhenius relation to estimate the long-term
behavior of GFRP rods in concrete structures, based on data from accelerated aging
tests with the rods exposed to concrete pore solutions at 20, 40, and 60°C. Under such
conditions, the authors reported very high degradation rates after only 178 days of
exposure at 20°C.
Fig. 12 – Arrhenius plot for service life as function of temperature and percent retention.
Fig. 13 – Immersion - Estimated retention of GFRP tensile strength (FHWA approach).
The reported values, notwithstanding the fact that a longer experimental period and
more specimens are required for design conclusions, confirm that this type of
degradation is severe and demands attention.
6.2 Fickian Diffusion
The technique based on diffusion was also applied and the previously calculated
diffusion coefficient and maximum moisture absorption utilized to generate Fig. 13. The
regressions show high R2 values, revealing a good correlation between the temperature
and these two variables.
25
Fig. 14 – Salt water immersion - Arrhenius analysis for maximum moisture absorption and diffusion
coefficient versus inverse of temperature.
Treatment of the linear regression curves led to D0 and M∞0, as shown in Table 9, and
the functional relations from (7-8) allow to write
(11)
(12)
Table 9 – Parameters for Arrhenius analysis based in moisture diffusion.
The determination of D and M∞ for any aging temperature is made by application of (6).
For water temperature of immersion 10ºC, D=D(283.5)=4.61×10-8 mm2/s and
M∞=M∞(283.5) =0.64%. Once predicted the mass gain over time at 10ºC, and using the
relation between strength retention and maximum moisture absorption obtained from
the accelerated aging tests, Fig. 15 was drawn, showing high scattering and R2 below
admissible. Predictions would lead to unreliable results and a high almost constant
retention of strength after a relatively short time of exposure. After the composite
reaches values near M∞, the prediction ceases to be valid, a difficulty also encountered
by Bhise [21] who presented results just for one retention value, thus obscuring this
problem with the method. This model does not take into account the relaxation
processes that often occur in composites and affect the moisture uptake parameters and
appears to be inferior to the time-shift, or similar, procedure already presented for
systems as those studied here.
26
Fig. 15 – Relation between strength retention and mass gain.
Conclusions
Major conclusions derived from the reported study are the following:
- The immersion of GFRP laminates in a salt water solution produced significant
alterations in mechanical properties, namely, a decrease on their tensile strength.
- The composites fully cured showed significant improvement in mechanical behavior at
the onset of accelerated salt fog cycles, a fact attributed to stronger bond of matrix and
fibres.The trends of degradation along the salt fog cycles were similar.
- The solution uptake by the composites increased with water temperature as well as
their degradation.
- The prediction of long term strength retention based on the Arrhenius equation is not
directly applicable in the case of accelerated salt fogging cycles, especially in cases on
which the initial cure of the specimens is very incomplete.
- The prediction of long term strength retention based on the Arrhenius equation in the
case of salt water immersion led to fairly low retention of tensile strength.
- The evolution of the longitudinal modulus of elasticity does not fit Arrhenius type of
behaviour, in either type of the aging imposed to the GFRP specimens.
- Long-term prediction from data obtained in diffusion studies and based on Fickian
laws led to erroneous estimates of strength.
- The obtained results indicate that the codes have to introduce time into environmental
reduction factors for salt water immersion of GFRP composites. This recommendation
is stronger when GFRPs are used for column wrapping, rather than for external
reinforcement of beams [24].
27
- The studies advise further tests for several values of water salinity, a finer analysis of
the importance of degree of curing as well as modified modelling based on non Fickian
laws of diffusion for different polymeric matrices.
ACKNOWLEDGEMENT
The partial funding of the studies by the Portuguese Fundação da Ciência e Tecnologia,
via Project PTDC-ECM100538/2008, is thanked.
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47
LEGENDS
Fig. 1 – Salt fog cycles - GFRP naturally aged for 4 years: (a) Average tensile strength;
and (b) longitudinal modulus along time.
Fig. 2 – After 5000 h of salt fog cycles (a) morphology of crystals at 55ºC; and (b) at 30ºC.
Fig. 3 – Salt fog cycles - 30 days post cure coupons: (a) Average tensile strength and
(b) longitudinal modulus along time.
Fig. 4 – Immersion - maximum mass gain for salt water at different temperatures, and for deionised water.
Fig. 5 – Salt water immersion - Maximum moisture and diffusion coefficient variation
with temperature of solution.
Fig. 6 – Salt water immersion - tensile strength at different temperatures.
Fig. 7 – Effect of salt water on strain at different temperatures.
Fig. 8 – Effect of salt water on elasticity modulus at different temperatures.
Fig. 9 – Salt water immersion - Tensile strength retention versus logarithm of time.
Fig. 10 – Salt water immersion - Relation between TSF and temperature.
Fig. 11 – Immersion - Estimated retention of GFRP tensile strength (TSF approach).
Fig. 12 – Arrhenius plot for service life as function of temperature and percent retention.
Fig. 13 – Immersion - Estimated retention of GFRP tensile strength (FHWA approach).
Fig. 14 – Salt water immersion - Arrhenius analysis for maximum moisture absorption
and diffusion coefficient versus inverse of temperature.
Fig. 15 – Relation between strength retention and mass gain.
48
TABLES Table 1. Deterioration mechanisms, after Buenfeld [15].
Chloride-induced corrosion
Carbonatation-induced corrosion
Sulphate attack Alkali aggregate reaction
Frost action
Leaching
Abrasion Conventio
nal Thaumasite
DEF
Above ground buildings
Bridges
Foundations
Marine/maritime structures
Dams
Tunnels
Tanks and pipes
Industrial floors
Table 2. Average GFRP properties of reference specimens - batch for salt fog cycles.
30-days cure 4-years cure Ultimate tensile strength (ASTM D3039) 420 MPa 446 MPa Elongation at break (ASTM D3039) 2.22% 2.30% Tensile modulus (ASTM D3039) 19.3 GPa 18.6 GPa Water Absorption (ASTM D-570) 1.12 % -
Table 3. Average GFRP properties of reference specimens- batch for immersion. Average reference values Average Standard Deviation Water Absorption (ASTM D-570) 1.12 % 0.03% Textile-glass content (ISO 1172) 37.8% 1.7%Ultimate tensile strength (ASTM D3039) 484.37 MPa 36.10 MPa Elongation at break ( ASTM D3039) 1.90% 0.16% Water Absorption [34] 1.12 % 0.03%Textile-glass content [35] 37.8% 1.7% Ultimate tensile strength [33] 484.37 MPa 36.10 MPa Elongation at break [33] 1.90% 0.16% Tensile modulus [33] 25.61GPa 2.60MPa
Table 4. Tensile tests after salt fog cycles under 30ºC, 40ºC and 55ºC.
30ºC
Time (h) σult (MPa) Δσult (%) εult (%) Δεult (%) E (GPa) ΔE (%)
0 446 – 2.40 – 18.6 –
750 435 -2.5 2.34 -2.53 18.6 -0.0(4)
1500 421 -5.7 1.98 -17.52 22.4 20.0
2500 414 -7.3 2.08 -13.22 20.0 7.3
5000 383 -14.1 1.80 -24.74 21.2 13.9
49
40ºC
Time (h) σult (MPa) Δσult (%) εult (%) Δεult (%) E (GPa) ΔE (%)
0 446 – 2.40 – 18.6 –
750 418 -6.4 2.21 -7.61 18.9 1.2
1500 406 -9.0 1.90 -20.62 21.4 14.7
5000 416 -6.8 2..22 -7.46 18.8 0.65
55ºC
Time (h) σult (MPa) Δσult (%) εult (%) Δεult (%) E (GPa) ΔE (%)
0 446 – 2.40 – 18.6 –
750 419 -6.0 2.28 -5.02 18.5 -0.8
1500 408 -8.5 1.92 -20.09 21.3 14.5
2500 418 -6.3 2.05 -14.42 20.4 9.4
5000 419 -6.2 2.06 -14.04 20.3 9.1
Table 5. Tensile tests after salt fog cycles under 30ºC, 40ºC and 55ºC in coupons with 30 day
post cure.
30ºC Time (h) σult (MPa) Δσult (%) εult (%) Δεult (%) E (GPa) ΔE (%) 0 420 – 2.22 – 19.3 – 750 466 11.0 2.20 -0.77 21.2 10.1 1500 429 2.3 1.99 -10.29 21.6 12.0 2500 435 3.7 1.99 -10.20 21.9 13.9 5000 427 1.8 1.94 -12.28 22.0 14.1 40ºC Time (h) σult (MPa) Δσult (%) εult (%) Δεult (%) E (GPa) ΔE (%) 0 420 – 2.22 – 19.3 – 750 416 -0.8 2.01 -9.5 20.9 8.6 1500 387 -7.7 1.81 -18.4 20.7 7.3 2500 402 -4.11 1.97 -11.1 20.8 6.2 5000 403 -3.84 1.89 -14.8 21.6 12.2 55ºC Time (h) σult (MPa) Δσult (%) εult (%) Δεult (%) E (GPa) ΔE (%) 0 420 – 2.22 – 19.3 – 750 441 5.1 2.03 -8.4 21.7 12.8 1500 394 -6.2 1.86 -16.1 21.2 10.12500 387 -7.7 1.86 -15.9 20.8 8.2 5000 358 -14.7 1.74 -21.55 20.7 7.5
Table 6. Parameters from diffusion analysis.
M∞ (%) D (mm2/s)×105
23ºC – Deionised water 1.030 0.75300 35ºC – 5% NaCl 0.850 0.01389 50ºC – 5% NaCl 0.930 0.03056 65ºC – 5% NaCl 1.121 0.04443
Table 7. Strength retention (SR) for immersion in salt water.
Time (h) 35°C 50°C 65°C
0 1 1 1
750 - 0.88 0.81
50
1500 0.92 0.78 0.71
2500 0.84 0.70 0.61
Table 8. Time (hours) to reach given values of strength retention and TSF (w/respect to values
at 35ºC).
35ºC 50ºC 65ºC
80% SR (h) 3,254 1,252 797
70% SR (h) 6,261 2,460 1,456
60% SR (h) 12,047 4,836 2,660
TSF for SR=0.80 1.000 2.599 4.081
TSF for SR=0.70 1.000 2.545 4.299
TSF for SR=0.60 1.000 2.491 4.529
TSF (average) 1.000 2.545 4.303
Table 9. Parameters for Arrhenius analysis based in moisture diffusion.
Ed (D) [cal/mol] Em (M) [cal/mol] D0 M∞0 [%]
8128.16 1912.95 290.61 18.72