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.

1mgs@fct.unl.pt 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

4

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

7

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

8

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

9

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

16

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

17

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].

19

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