FRP–brick masonry bond degradation under hygrothermal conditions

Accepted Manuscript FRP-brick masonry bond degradation under hygrothermal conditions Hamid Maljaee, Bahman Ghiassi, Paulo B. Lourenço, Daniel V. Oliveira PII: DOI: Reference:

S0263-8223(16)30202-1 http://dx.doi.org/10.1016/j.compstruct.2016.03.037 COST 7335

To appear in:

Composite Structures

Received Date: Revised Date: Accepted Date:

20 November 2015 10 March 2016 24 March 2016

Please cite this article as: Maljaee, H., Ghiassi, B., Lourenço, P.B., Oliveira, D.V., FRP-brick masonry bond degradation under hygrothermal conditions, Composite Structures (2016), doi: http://dx.doi.org/10.1016/ j.compstruct.2016.03.037

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FRP-brick masonry bond degradation under hygrothermal conditions Hamid Maljaee∗, Bahman Ghiassi1, Paulo B. Lourenço2, Daniel V. Oliveira3 ISISE, University of Minho, Department of Civil Engineering, Guimarães, Portugal

ABSTRACT Externally bonded reinforcement (EBR) of masonry structures with Fiber Reinforced Polymers (FRPs) has received extensive attention during the last years. Despite the vast literature on mechanics and short-term performance, the durability and long-term performance of these systems still remain insufficiently studied. Structures are subjected to harsh environments such as coupled temperature and moisture variations (hygrothermal conditions) during their service life. A clear understanding on the performance of the strengthening system under these conditions is critical at the design stage. This paper presents an experimental investigation on the effect of long-term hygrothermal conditions on FRP-strengthened masonry components. The focus is on the interfacial bond behavior and materials mechanical properties. The effect of mechanical surface treatment on the bond durability is also investigated. Keywords: FRP; Masonry; Bond; Durability; Environmental degradation; Mechanical testing.

∗

Corresponding author, ISISE, University of Minho, Department of Civil Engineering, Azurém, 4800-058

Guimarães, Portugal. Phone: +351 253 510 499, fax: +351 253 510 217, E-mail: [email protected] 1

Post-doctoral researcher, ISISE, University of Minho, Department of Civil Engineering, Azurém, 4800-058

Guimarães, Portugal. Phone: +351 253 510 499, fax: +351 253 510 217, E-mail: [email protected] 2

Professor, ISISE, University of Minho, Department of Civil Engineering, Azurém, 4800-058 Guimarães, Portugal.

Phone: +351 253 510 209, fax: +351 253 510 217, E-mail: [email protected] 3

Associate Professor, ISISE, University of Minho, Department of Civil Engineering, Azurém, 4800-058 Guimarães,

Portugal. Phone: +351 253 510 247, fax: +351 253 510 217, E-mail: [email protected] 1

1

Introduction

Externally bonded reinforcement (EBR) of masonry structures with Fiber Reinforced Polymers (FRPs) is found to be a promising technique. Several studies have focused on structural behavior, design parameters and failure modes. The effectiveness of EBR systems is known to be highly dependent on the bond performance between the strengthening material and the substrate [1–4]. The bond performance can be seriously affected under environmental conditions, which leads to premature debonding or change of expected failure modes. In addition to the short-term performance, understanding the long-term behavior and active degradation mechanisms under different environmental conditions is thus critical for service life predictions [5]. The dearth of available information on durability and long-term performance of externally bonded FRP systems applied to masonry walls demands a broader investigation. While several investigations can be found in the literature focused on durability of FRP-concrete systems [6– 12], the available information on FRP-strengthened masonry elements is still rare [13–16]. While most of the available studies focus on the effect of an isolated degrading agent on the bond behavior in FRP-strengthened systems, less attention has been given to the coupled effects (as happens in real condition) even in case of FRP-strengthened concrete elements. The combined effect of moisture and temperature, also called as hygrothermal conditions, is thus the main focus of this study. Moisture, after diffusion in epoxy resin and FRP-substrate interface, can lead to plasticization, degradation of mechanical properties and loss of integrity at the bond level through hydrolytic breakdown of the interface between matrix and FRP [17,18]. Exposure to elevated temperatures, near or above the glass transition temperature, leads to significant reduction of mechanical properties. While short-term exposure can be reversible, long-term exposure to high

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temperatures may induce irreversible changes in their properties [19]. On the other hand, the freeze-thaw cycling can lead to degradation of mechanical properties and of the bond performance [20,21]. Temperature cycles below glass transition temperature (Tg) can also affect the bond performance due to thermal fatigue [18]. Internal stresses due to thermal incompatibility (difference of thermal expansion coefficient in materials) may affect the mechanical properties, structural integrity and consequently exacerbate the bond degradation [22,23]. The moisture induced degradation can be accelerated when coupled with temperature [6] as the moisture absorption rate increases with increment of temperature [17,24]. On the other hand, moisture can affect thermal properties of materials and thus change the degradation rates. Ghiassi et al. [15] investigated the effect of 225 cycles of hygrothermal exposure (temperature cycles ranging from +10˚C to +50˚C and 90% R.H.) on the bond and material properties of GFRP-strengthened extruded solid clay bricks. Besides a significant reduction of bond strength, epoxy and GFRP mechanical properties, a clear degradation trend was not observed and the need for extension of the exposure period was reported. Extensive FRP delamination was also reported in the specimens after exposure. The observed delamination was reported to be due to thermal incompatibility between GFRP and masonry substrate. The failure mode also changed from cohesive to adhesive failure. In a recent study, the effect of hygrothermal conditions (constant temperature at +40˚C and 90% R.H.) on FRP-strengthened natural stones was reported [13]. The authors reported significant degradation of stones’ mechanical properties after 25 weeks of exposure. The failure mode was also changed from cohesive to adhesive failure. It was also reported that specimens prepared with Leccese stones showed higher degradation of bond in comparison to the specimens

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prepared with Neapolitan tuff. This difference was attributed to the differences in porosity of the substrates. This paper extends the previous research of Ghiassi et al. [15] to longer exposure periods. GFRPstrengthened bricks are exposed to temperature cycles ranging from +10˚C to +50˚C and relative humidity of 90% for 960 cycles (each cycle lasts for 6 hours). The influence of mechanical surface treatment on the durability of bond, not addressed before, is also assessed here. The effect of accelerated hygrothermal exposure on the material properties and bond behavior are deeply investigated. The bond degradation is investigated through performing pull-off and single-lap shear debonding tests. The mechanical tests are combined with thermal analysis of primer, epoxy (bulk adhesive) and epoxy taken from interface to deeply understand the degradation mechanisms. A decay model is also proposed for each property to be used for longterm predictions in hygrothermal conditions. Finally the design parameters presented in CNR-DT 200 are calibrated based on the experimental results. It should be noted that, throughout this study, the word “delamination” refers to the separation of FRP sheet from the masonry substrate due to the exposure before the test execution, and “debonding” describes the bond failure phenomenon in FRP-strengthened specimens after bond mechanical tests.

2

Experimental program

The experimental program consisted of exposing FRP-strengthened bricks and material samples to hygrothermal exposure conditions in a climatic chamber. The specimens were taken from the climatic chamber after different periods of exposure to investigate possible changes in their mechanical performance. The main focus was on the changes of FRP-masonry interfacial bond and materials mechanical properties.

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2.1

Materials and specimens’ preparation

The specimens consisted of extruded solid clay bricks with dimensions of 200×100×50 mm3 strengthened with Glass Fiber Reinforced Polymer (GFRP) following the wet layup procedure. A unidirectional E-glass fiber and a two-part epoxy resin were used to prepare the composite material. The specimens were prepared with specific geometrical details for single-lap shear bond tests (a total of 70 specimens) and pull-off tests (a total of 28 specimens), as presented in Fig 1. The bricks were initially cleaned and dried in an oven at 100°C for 24 hours. After drying, a primer layer was applied on the bricks’ surfaces. A layer of epoxy resin was then applied followed by impregnation of the glass fibers with epoxy resin and then application to the bricks’ surfaces. A slight pressure was finally applied on the GFRP surface with a roller to remove any air voids at the interface. Two groups of specimens were prepared for single-lap shear bond tests to investigate the effect of surface mechanical preparation on the bond durability. The first group was prepared without application of any mechanical surface treatment on the bricks’ surfaces (denoted as ORGspecimens). In the second group, the bricks’ surfaces were grinded for about 5 mm with a mechanical saw before application of the GFRP (denoted as GR-specimens). In both specimen types, the GFRP sheets were applied over a 150×50 mm2 area, leaving a 40 mm unbonded length at the loaded end as shown in Fig 1a. The pull-off test specimens were prepared without application of any mechanical surface treatment. The GFRP sheets were applied over a 180×70 mm2 area in these specimens as shown in Fig 1b. A total number of 35 specimens (of each material) were also prepared, according to relevant codes, for materials’ mechanical characterization during the exposure. The specimens included

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40 mm cubic bricks, dog-bone shape epoxy resin and primer, and GFRP coupons with the geometrical details shown in Fig 2. 2.2

Material properties

Mechanical properties of materials were experimentally characterized according to relevant test standards. The tests included compressive tests on brick specimens, tensile tests on dog-bone shape epoxy resin and primer specimens, and tensile tests on GFRP coupons. Five specimens were tested for each material. Table 1 presents the mean value and coefficient of variation (CoV) of the experimental results. The obtained results were also used as reference for investigating the effect of exposure conditions on material properties. Compressive tests were performed on brick specimens according to ASTM C67 [25] and EN 772-1 [26]. The specimens were dried in an oven for 24 hours at 100°C before performing the tests. The tests were performed using a Lloyds testing machine under force control at a rate of 150 N/min, in flatwise direction. In each test, a pair of friction-reducing Teflon sheets with a layer of oil in between was placed between the specimen and the compression plates to reduce the friction and ensure a uniform distribution of stresses at the center [27], see Fig 3a. The compressive strength of brick was obtained as the maximum experimental compressive force per unit area. Tensile tests on epoxy resin and primer were performed according to standard ASTM D638 [28]. The specimens were cured in laboratory conditions (T=20°C and R.H=60%) for 100 days before performing the tests. The tests were conducted using a Lloyds testing machine at a displacement rate of 2 mm/min, see Fig 3b. The specimens were instrumented with a clip gauge at the middle to monitor the deformation during the test. The elastic modulus was obtained as the initial slope of the experimental stress-strain curve.

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Tensile tests on GFRP coupons were performed following the ASTM D3039 [29] and ISO 527-1 [30]. The specimens were cured in laboratory conditions (T=20°C and R.H=60%) for 100 days before performing the tests. The tests were conducted with an Instron testing machine at a displacement rate of 2 mm/min, see Fig 3c. Tensile stresses are determined based on the equivalent thickness concept according to ACI 440.3R [31]. This is a usual procedure when the composite material is prepared following the wet layup procedure [15,21]. The elastic modulus was obtained from the stress-strain curves as the slope of the chord between 20% and 60% of the maximum tensile stress (according to the code proposal). The initial slope of the stress-strain curves also provides a similar elastic modulus due to the linear behavior of GFRPs until tensile rupture. Differential Scanning Calorimetry (DSC) tests were also performed on epoxy and primer specimens (after 100 days of curing and just before starting the hygrothermal exposure) in order to quantify the glass transition temperature, Tg, and its changes during the environmental exposure. Three samples from each material (10~14 mg) were placed in the measuring pans to be tested in each exposure period. The pans were then heated between 20˚C and 200˚C at a constant heating rate of 10˚C/min under nitrogen atmosphere. Two heating scans were performed on epoxy resin specimens (taken from dog-bone shaped specimens) by cooling the samples with same rate after the initial heating and then heating again for a second time. The first heating scan represents the thermal history of the sample including aging and processing [32,33]. In the second heat scan, the thermal history is eliminated and thus the inherent properties of materials is captured [34]. Tg is determined as a midpoint temperature which is the point on the thermal curve corresponding to 1/2 of the heat flow difference between the extrapolated onset and extrapolated end [35]. The Tg obtained from the first and second heating scans was 54.2˚C and 66.1˚C for

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reference epoxy resin samples, and was 56.3 ˚C and 61.7 ˚C for reference primer samples, see Table 1. DSC tests were also performed on epoxy samples taken from the interfacial zone between FRP and brick after performing the debonding tests. The aim was to investigate the differences in the curing condition and degradation between bulk epoxy and interfacial epoxy. These tests were performed with only one heating scan. The Tg of the reference specimens taken from the interface was, 55.3˚C, very close to the tensile test specimens. 2.3

Bond characterization tests

Bond characterization tests included pull-off tests (only on ORG-specimens) and single-lap shear debonding tests (on both ORG- and GR-specimens). For performing the pull-off tests, two 50 mm diameter partial cores were initially drilled on each specimen with an approximate depth of 5 mm inside the brick, see Fig 4a. Aluminum disks were then glued on the GFRP surface using a high-strength epoxy paste adhesive. The specimens were firmly clamed to a steel frame, as shown in Fig 4a, to avoid any movements during the tests. A closed-loop servo-controlled testing machine was used to apply tensile load to the disks with a displacement rate of 0.2 mm/min. The load was monotonically applied until debonding of the disk from the specimens. For single-lap shear bond tests, the specimens were placed on a rigid supporting steel frame and firmly clamped to it in order to avoid any misalignment during load application, see Fig 4b. A closed-loop servo-controlled testing machine was again used for monotonically application of tensile displacements at a rate of 0.3 mm/min. The relative slip between GFRP sheet and masonry brick was monitored by means of four LVDTs placed on different locations along the

8

bonded length. Two LVDTs were mounted at the loaded end, one at the middle of the bonded length, and another at the free end, as shown in Fig 4b. Four pull-off tests (on ORG-specimens) and five single-lap shear tests (on ORG- and GRspecimens) were performed on specimens before starting the hygrothermal exposure tests (untreated condition). 2.4

Hygrothermal exposure

Specimens were exposed to hygrothermal conditions in a climatic chamber after 100 days of curing in laboratory conditions. The exposure included 6-hours temperature cycles ranging from +10˚C to +50˚C with constant relative humidity of 90%, see Fig 5. The relative humidity inside the climatic chamber drops to 60% when the temperature reaches +10˚C and then goes back to 90% after a short period. This is a usual situation and it happens due to the complexity of controlling the relative humidity at low temperatures. Two ORG-specimens (from single-lap shear specimens), two bricks, two epoxy resin and two primer specimens were instrumented with strain gauges, according to the details shown in Fig 6, to monitor the deformation of the specimens during the exposure period. The strain gauges were carefully sealed with protective layers to avoid impregnation of relative humidity and thus affecting the obtained results. These specimens were kept inside the climatic chamber until the end of exposure period and the strains were monitored and recorded. Five specimens of each kind were taken from the climatic chamber every month (around 120 cycles and always when the temperature was stabilized at 10˚C) to perform post-exposure tests. The specimens were weighed and visually inspected, and stored in a controlled environment room (20˚C temperature and 60% R.H.) for 7 days before testing. The post-exposure tests included materials’ mechanical characterization, DSC tests and bond characterization tests. The

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DSC tests were performed on primer and epoxy resin samples taken from tensile test specimens and on epoxy resin samples taken from debonded FRP sheets after the shear tests. The test methods and procedures were as explained in sec. 2.2 and sec. 2.3.

3 3.1

Results and discussion Strain measurements

The variation of specimens’ surface strains along the exposure period are presented in Fig 7. Assuming that strain gauges worked properly during the tests, the changes of Coefficient of Thermal Expansion (CTE), α, can be obtained from the variation of strain in each cycle (∆ε=εmax – εmin) as ∆ε=α∆T. Looking at the figures, two different phenomena are recognized; change of CTE and change of the mean strain ((εmax + εmin) /2). The latter can be due to micro-cracking, delamination or swelling of the specimens due to moisture absorption. Thermal expansion of the brick seems to be similar in both longitudinal and transverse directions with a slight difference in CTE, being around 1.5×10-5/˚C and 1.36×10-5/˚C, respectively. A gradual increase in the mean strain can be observed in both curves, which can be due to swelling of the brick with moisture absorption. For the epoxy resin, the mean strain increases in the first 120 cycles and thereafter has negligible changes until the end of exposure period. This increment is coincident with maximum moisture absorption rate, see Fig 11, and can be due to moisture swelling [22,36]. The CTE of epoxy resin seems to be constant along the tests and is equal to 8.28×10-5/˚C. A similar change of mean strain can be observed in primer, which again can be attributed to moisture swelling, see also moisture absorption curves in Fig 12. In contrary, the CTE of primer decreases around 40% along the exposure period changing from α0=8.96×10-5/˚C to α480=5.52×10-5/˚C.

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The strain measurements on GFRP specimens show larger strain variations and thus larger CTE in transverse direction (α =9.28×10-5/˚C) in comparison to longitudinal direction (α =1.15×105

/˚C). The CTE and mean strain variation in transverse direction are in the same order of epoxy

resin, as expected. In FRPs prepared with unidirectional fibers, the properties are governed by fiber properties in fiber direction (longitudinal direction in this study) and by epoxy in transverse direction. This can be the reason why the effect of epoxy moisture swelling is not observed in the mean strain in longitudinal direction. In FRP-strengthened specimens, the strain variation captured by sg.1-2 does not show any specific changes during the exposure besides a slight increment of mean strain. However, the slope of the mean strain in sg.3, placed at the loaded end, increases at around 400 cycles, which can be due to FRP delamination from the substrate. ∆ε is similar to the one observed in GFRP in longitudinal direction, although the mean strain is slightly larger in bonded specimens. ∆ε is much larger in sg.4 and sg.5 as the behavior is governed by epoxy in transverse direction. The transverse CTE recorded by sg.4 and sg.5 is 1.16×10-5/˚C and 1.97×10-5/˚C at the beginning of exposure, both far from that of epoxy due to the FRP-brick bond. ∆ε and mean strain increase until around 240 cycles in sg.5 and then stabilize until the end of exposure. This change is due to progression of FRP delamination until the complete separation of FRP from the brick surface has occurred at around 240 cycles. After this point CTE is 7.98×10-5/˚C, which is very close to the one from epoxy (8.28×10-5/˚C) showing that FRP delamination has occurred. Both mean strain and ∆ε are smaller in sg.4 as this strain gauge was located far from the loaded end. ∆ε increases only after 480 cycles showing that FRP delamination or interfacial micro-cracking has occurred at this location. However, as the CTE is much less than that of epoxy (is 2.72×10-5/˚C at cycle 480) only micro-cracking has probably occurred at the interface and FRP is not completely

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debonded. Moisture induced swelling can also be responsible for a percentage of the observed changes in the mean variation in strain gauges bonded to the ORG-specimens. 3.2

Thermal properties

The variation of Tg obtained from DSC tests is presented in Fig 8. The DSC tests on bulk epoxy (taken from tensile tests specimens) included two heating scans. Both reference epoxy samples (taken from the interface and from tensile specimens) showed a similar Tg (around 55˚C for the 1-stage DSC tests), demonstrating a similar degree of cure at both conditions. The Tg of the bulk adhesive in both heating scans increased about 17% during the first 240 cycles of exposure and reached a plateau thereafter until the end of the tests (Fig 8). However, the Tg of epoxy at the interface is increased at a lower rate leading to a total 7% increase at the end. The Tg of primer, only measured at the beginning and at the end of exposure period, showed a 20% drop from 56˚C to 45˚C. The observed changes in the Tg are the result of several counteracting and interlinked mechanisms, which affect the network structure of epoxy resin. Moisture absorption causes plasticization in epoxy resin and thus reduction of its Tg. On the other hand, the epoxy is exposed to temperatures above its curing temperature (23˚C) during the hygrothermal cycles (when the temperature rises to 50˚C), which leads to further cross-linking of epoxy and thus increment of its Tg [37,38]. The curing time, by itself, also leads to improvement of the cross-link density and Tg increment. The effect of moisture absorption on Tg can be simulated as [39]:
 = ( +  + )


(1)

where Tgw is the Tg of wet epoxy, Tgd is the Tg of dry epoxy, ω is the moisture content in the epoxy in the percent weight, and A, B, and C are constant values obtained by curve fitting of the

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experimental results. These parameters are calibrated with water immersion tests performed on the same epoxy resin in [40] and are obtained as A=0.0047, B=-0.056 and C=1. Eq.(1) is then used for estimating the effect of moisture absorption on Tg during hygrothermal exposure tests, see Fig 9a. According to the analytical formula, a total reduction of 10% in Tg is expected at the end of exposure period (corresponding to 1.9% moisture absorption). The difference between the experimental results and the analytical simulation, the shaded area in the graph, represents the counteractive effect of physical aging (further cross-linking) of epoxy resin due to both curing time and exposure to temperatures above the curing temperature. The effect of epoxy curing time on Tg is investigated by performing DSC tests on epoxy samples cured and kept in laboratory conditions for 2, 6 and 10 months. The results, presented in Fig 9b in comparison to hygrothermal exposure results, show a slow but continuous increment of Tg with time (starts from 53˚C and reaches 61 ˚C at the end of exposure period). Tg increases more rapidly in hygrothermal conditions in the first two months, but then it stabilizes at 61˚C until the end of the tests. The comparison of Fig 9a and Fig 9b shows that physical aging of epoxy resin due to exposure to temperatures higher than curing temperature is the governing mechanism (with the largest contribution) for increment of Tg. 3.3

Material properties

The changes in mechanical properties and moisture content of the specimens with exposure cycles are presented in this section. The moisture content is measured: (i) immediately upon removal of the specimens from the climatic chamber; and (ii) at the test moment (after seven days of storage in controlled environmental conditions). The variation of compressive strength and moisture content of bricks along the exposure is presented in Fig 10. The changes in the bricks’ compressive strength can be considered as

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negligible. The coefficient of variation of the results is at an acceptable range of 6 to 14%. The moisture absorption level (0.25% water mass after 960 cycles) is negligible in comparison to 12.5% saturation level reported in [40] for the same bricks. The results show suitable durability of the bricks used in this study under the considered hygrothermal conditions. The changes in mechanical properties and moisture content of epoxy resin with the number of exposure cycles (n) are presented in Fig 11. The coefficient of variation (CoV) of the results is at an acceptable range of 3-14% for elastic modulus and tensile strength [41], but larger variations were observed for ultimate strain. The largest reduction in tensile strength and elastic modulus occurred in the first 120 cycles, when also the largest moisture absorption was occurred. The rate of degradation in tensile strength is significantly reduced thereafter. Plasticization of epoxy resin, micro-cracking due to swelling or thermal cycles can be the responsible mechanisms for the observed degradation in mechanical properties. The elastic modulus has increased after the initial decrement and then it has decreased again until the end of exposure period. These fluctuations in the elastic modulus can be due to the counteractive effect of moisture plasticization and physical aging. The total degradation in tensile strength and elastic modulus is 26% and 18% at 960 cycles, respectively, corresponding to 1.96% water absorption. An exponential degradation model seems suitable for simulating the changes of tensile strength and elastic modulus, see Fig 11a,c. The moisture absorption curve shows that seven days storage before performing the tests has led to only a slight loss of the absorbed water. It also seems that epoxy is near saturation at the end of exposure period. A comparison with water immersion tests performed on the same epoxy resin in [40], see Fig 11b, shows that a similar degradation level is occurred in tensile strength at low moisture absorption levels (until 1.6%). However, the degradation is larger in hygrothermal

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conditions at higher moisture absorption levels in contrary to the larger Tg obtained from DSC tests. This can be attributed to micro cracking of epoxy resin due to thermal cycles. The changes in epoxy ultimate strain are presented in Fig 11d together with the typical stressstrain curves along the exposure. The ultimate strain increases with exposure time until 240 cycles and thereafter it reaches a plateau following an inverse exponential trend. The stress-strain curves show that the behavior of epoxy resin has changed from linear elastic (with a small nonlinearity near failure) to nonlinear (with a small linear region in low stress levels) along the exposure time. The variation of mechanical properties and moisture content of primer are depicted in Fig 12. The tensile strength of primer presented no changes during the first 240 cycles, although 2.4% moisture absorption was achieved, see Fig 12a. After this point, in which the primer seems to be saturated in this environment, the degradation starts and continues until a total drop of 42% at the end of the tests. The primer reached a similar level of moisture absorption during three months of water immersion with no significant degradation in tensile strength in [40], see Fig 12b. The effect of temperature cycles and exposure to high temperatures on the degradation of primer is clear. The large drop of Tg as a sign of extreme plasticization is another reason for the observed extensive degradation. In contrary to the epoxy resin, it seems that the tensile strength has not reach a plateau and the degradation trend even after the extensive 960 cycles of exposure is not clear. A chemical degradation analysis seems therefore necessary and is proposed for future investigations for interpretation of the observed behavior. A significant decrement of elastic modulus (around 57% at the end of the tests) and increment of ultimate strain (around 88% during the first 120 cycles and reaching 168% at the end of exposure) can be observed in Fig 12c,d). An exponential degradation model is used for each property and is presented on the

15

graphs. In case of tensile strength, no conclusion could be made for the degradation trend. However, following the consistency of fitting curves adopted for other material properties, a similar exponential model assuming that the degradation only starts after saturation is applied, see Fig 12a. Again, the stress-strain curves show a change of behavior from linear elastic to nonlinear with exposure time. The changes in mechanical properties and moisture content of GFRP coupons with the number of exposure cycles (n) are presented in Fig 13. The CoVs of experimental results were in the range of 4% to 21%, which is acceptable for wet lay-up manufactured specimens [14]. The water content curve shows that the maximum moisture absorption of GFRP is around 0.9% in this exposure condition. Again, most of the degradation occurs in the first 240 cycles and thereafter the tensile strength reaches a plateau. A 21% decrease in tensile strength is observed after 960 cycles, corresponding to 0.9% water absorption. Fig 13b shows that the degradation starts earlier than water immersion tests performed in [40], which can be due to the presence of thermal cycles in the present study. The changes in the elastic modulus were negligible during the exposure, see Fig 13c, while the ultimate strain showed a similar degradation trend as the tensile strength, see also Fig 13d. Mechanical performance of GFRP coupons is affected by mechanical performance of epoxy, fibers and epoxy/fiber interface [42]. The glass fibers show reasonable resistance against thermal and moisture exposure [43]. The degradation of GFRP coupons (including tensile strength and elastic modulus) is thus governed by degradation of epoxy resin and the fiber/matrix interface. Fiber/matrix interface is susceptible to moisture ingress and transverse cracking due to thermal incompatibility between epoxy and fibers, both of which can cause degradation in the performance of GFRPs. Again, it seems that an exponential degradation model is suitable for simulating the tensile strength and ultimate strain.

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3.4

Bond properties

3.4.1 Visual inspection The specimens were visually inspected after every 60 cycles of exposure (15 days). Due to the relative transparency of the epoxy resin, any possible delamination at the interface level was visible and could be easily detected, as also reported in [15,21]. A progressive FRP delamination was observed in ORG-specimens, prepared for both pull-off and shear debonding tests, prior to testing. A schematic diagram of the typical delamination progress in ORG-specimens is presented in Fig 14. The delamination generally initiated from the boundaries and propagated from loaded end to the free end. Although the debonded area varied from one to another specimen in each exposure period, its progression pattern was similar in most of the specimens. While the delamination was significant in ORG-specimens, no delamination was observed in GR-specimens showing the effect of surface preparation on improving the bond quality and durability. 3.4.2 Pull-off tests The variation of pull-off bond strength with exposure cycles is presented in Fig 15. The pull-off tests were only performed on ORG-specimens. A severe degradation in pull-off strength can be observed. The bonded length in the specimens decreased during the exposure, leading to the reduction of pull-off strength up to around 100% in some locations. The failure mode was initially cohesive with fracture inside the brick. Then it changed to cohesive-adhesive failure after 240 cycles and to adhesive failure after 480 cycles, see Fig 15.

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3.4.3 Shear debonding tests The changes in the debonding force and delaminated area in ORG- and GR-specimens are presented in Fig 16. In contrary to GR-specimens, that did not have any delamination, extensive delamination was observed in ORG-specimens (about 50% of the bonded area after 960 cycles). The debonding force in ORG-specimens showed a sudden decrease (about 50%) after 120 cycles. Thereafter, the degradation continued with a lower rate and reached 72% at the end of exposure period, see Fig 16b. A similar degradation trend was also reported by Ghiassi et al. [15,21] for 225 exposure cycles. In contrary, a slight increase was observed in the debonding force in GR-specimens, see Fig 16c. The results show clearly the significant effect of surface grinding on improving the bond performance and durability. The observed increment of the debonding force can be attributed to the variability of the specimens, curing time, and postcuring of epoxy adhesive at the interface level. As discussed in section 3.3, a 7% increase was observed in Tg of epoxy at the interface being the evidence of further cross-linking and bond improvement throughout the exposure period. The bond performance between epoxy and brick is the resultant of chemical and mechanical bond mechanisms [44]. The integrity of epoxysubstrate bond is believed to be primarily dependent on the mechanical interlocking. Post-curing of the epoxy resin leads to improvement of both chemical and mechanical bonding and consequently the bond strength [44]. The typical failure modes of the specimens before and after exposure are also presented in Fig 16. An adhesive failure mode combined with detachment of a thin layer of brick and tiny cracks on the brick surface was observed in the reference ORG-specimens. However, the detachment occurred only at the FRP/brick interface (adhesive failure) after 120 cycles and remained the same until the end of exposure. A similar change was also observed in [15] for the specimens 18

exposed to thermal cycles. In contrast, the failure mode in GR-specimens was cohesive with the fracture inside a thin layer of the brick in all the exposure period. This is another evidence of the improved quality of the bond in GR-specimens. It should also be noted that shear cracks are observable on the surface of the GR-specimens exposed to 960 cycles. These cracks, which are not present in the failure surface of the reference specimens, are the evidence of improved mechanical interlocking in these specimens due to the post-curing of the epoxy resins. The fracture-based approach is known as an appropriate approach at the interpretation of debonding problems [7]. Therefore, in order to evaluate the bond degradation, the degradation in the interfacial fracture energy is assessed. CNR DT 200 [43] proposes the following formula to correlate the debonding force to the fracture energy is shown:

 =

 

(2)

 .( . )

where Gf is the fracture energy, Pmax, bf, tf and Ef are the debonding force, FRP width and thickness, and the elastic modulus of FRP, respectively. It should be noted that this equation is applicable if the bond length is greater than effective bond length [43]. Having the variation of debonding force and FRP’s elastic modulus, the variation of fracture energy with hygrothermal cycles can be obtained in both sets of specimens. In ORG-specimens, FRP width varies due to delamination, which should be taken into consideration. The effective bond length is reported to be around 30 mm in [45]. Assuming that the effective bond length in ORG-specimens is constant during the tests, the remaining bonded length is more than the effective bond length until the end of exposure and thus Eq. (2) can be used. The variation of fracture energy in both sets of specimens is presented in Fig 17a. An exponential model is also presented for simulating the changes of fracture energy for each group. The fracture energy in ORG-specimens decreased significantly up to 68% during 240 cycles. Besides fluctuations, no further reduction was 19

observed in fracture energy after this point. In contrary, an increase of 15% is obtained for GRspecimens. As discussed before, no degradation was observed in the elastic modulus of GFRP and compressive strength of brick. Thus this slight increment of fracture energy can be attributed to the physical aging such as post-curing and additional cross-linking in epoxy resin due to the exposure to high temperatures [46]. Increment of joint strength due to physical aging after exposure to thermal cycles was also reported in [47]. The fracture energy can be also attributed to the mechanical properties of masonry substrate as [43]:

 =

.  

. !" . !"

(3)

where fbm and fbtm are the average compressive and tensile strength of masonry, kb accounts for the width effects (it can be obtained from CNR-DT200 [43]), kG is a corrective factor proposed to be equal to 0.031 mm for FRP systems applied to bricks following the wet layup procedures, and FC is a design confidence factor, which can be assumed equal to 1 for the purpose of this study. The tensile strength of the brick can be considered to be equal to 0.1fbm. Thus Eq (3) can be rewritten as: f = $ . $% . √0.1!"

(4)

It must be noted that this formula can only be applied to GR-specimens due to the main assumption behind this equation of having a cohesive failure inside the brick. Having the variation of masonry compressive strength from the experimental results and fracture energy from Eq. (2) and assuming that kb is constant during the tests (kb=1.29), the variation of parameter kG can be plotted versus hygrothermal cycles using Eq. (4), see Fig 17b. It can be observed that kG can have considerable changes after exposure to environmental conditions, which is contrary to the current assumption of being constant made in design codes. 20

4

Conclusions

The effect of hygrothermal conditions on durability of FRP-strengthened brick masonry was investigated by performing accelerated aging tests. The tests included exposing the specimens to concurrent temperature cycles (between 10˚C and 50˚C) and constant relative humidity (90%) in a climatic chamber. The main focus was on the changes of the bond performance at FRP-brick interface and the material properties. The use of strain gauges for monitoring the environmental induced strain on the specimens during the exposure helped in understanding the governing mechanisms in materials behavior and changes in the thermal expansion as well as detection of FRP delamination. The effect of hygrothermal exposure on the compressive strength of bricks, used in this study, was negligible. The behavior of epoxy resin and primer changed from linear elastic (with small nonlinearity near failure) to nonlinear showing reduction of both strength and stiffness with increment of exposure cycles. The observed degradation was due to the concurrent effect of moisture absorption (which leads to plasticization) and post-curing due to exposure to temperatures above the curing temperature. The DSC tests showed an increase of Tg in epoxy resin and a large drop of Tg in primer. The degradation in GFRP sheets was limited to the tensile strength and no specific changes were observed in the elastic modulus. The degradation in GFRP was probably due to both plasticization of epoxy resin and fiber/epoxy interface damage. The degradation trend in epoxy resin and GFRP was observed to be in direct relation with moisture absorption level following the exponential trend. The bond characterization tests showed that grinding the bricks’ surfaces before application of GFRP significantly improves the bond durability. While specimens with mechanical surface treatment did not show any degradation in the bond performance, extensive FRP delamination

21

and degradation in bond strength was observed in specimens prepared without any surface treatment. Increment of fracture energy in GR-specimens was correlated to the physical ageing (post-curing) of epoxy resin at the interface level. Post-curing of epoxy resin led to improvement of bond between epoxy and brick which was evident in the failure surface of the specimens after debonding tests. Predictive models for simulating the material properties, bond fracture energy and corrective design factors for hygrothermal environments were also discussed and proposed.

Acknowledgements The second author acknowledges the financial support of the Ministério da Ciência, Tecnologia e Ensino Superior, FCT, Portugal, under the grant SFRH/BPD/92614/2013.

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2015;78:112–25. [10] Benzarti K, Freddi F, Frémond M. A damage model to predict the durability of bonded assemblies. Part I: Debonding behavior of FRP strengthened concrete structures. Constr Build Mater 2011;25(2):547–55. [11] Lau D, Büyüköztürk O. Fracture characterization of concrete/epoxy interface affected by moisture. Mech Mater 2010;42(12):1031–42. [12] Won J-P, Yoon Y-N, Hong B-T, Choi T-J, Lee S-J. Durability characteristics of nanoGFRP composite reinforcing bars for concrete structures in moist and alkaline environments. Compos Struct 2012;94(3):1236–42. [13] Sciolti MS, Aiello MA, Frigione M. Effect of thermo-hygrometric exposure on frp, natural stone and their adhesive interface. Compos Part B Eng 2015;80:162–76. [14] Ghiassi B, Marcari G, Oliveira DV, Lourenço PB. Water degrading effects on the bond behavior in FRP-strengthened masonry. Compos Part B Eng 2013;54:11–9. [15] Ghiassi B, Lourenço PB, Oliveira D V. Accelerated hygrothermal aging of bond in FRP– masonry systems. J Compos Constr 2015;19(3):04014051. [16] Sciolti MS, Aiello MA, Frigione M. Influence of water on bond behavior between CFRP sheet and natural calcareous stones. Compos Part B Eng 2012;43(8):3239–50. [17] Maxwell AS, Broughton WR, Dean G, Sims GD. Review of accelerated ageing methods and lifetime prediction techniques for polymeric materials. NPL Rep DEPC MPR 016 2005. [18] Karbhari VM, Chin JW, Hunston D, Benmokrane B, Juska T, Morgan R, et al. Durability gap analysis for Fiber-Reinforced Polymer composites in civil infrastructure. J Compos Constr 2003;7(3):238–47. [19] Hollaway LC. A review of the present and future utilisation of FRP composites in the civil infrastructure with reference to their important in-service properties. Constr Build Mater 2010;24(12):2419–45. [20] Cromwell JR, Harries K a., Shahrooz BM. Environmental durability of externally bonded FRP materials intended for repair of concrete structures. Constr Build Mater 2011;25(5):2528–39. [21] Ghiassi B, Oliveira DV, Lourenco PB. Hygrothermal durability of bond in FRPstrengthened masonry. Mater Struct 2014;47(12):2039–50. [22] Vaddadi P, Nakamura T, Singh RP. Transient hygrothermal stresses in fiber reinforced composites: A heterogeneous characterization approach. Compos Part A Appl Sci Manuf 2003;34(8):719–30. [23] Ray BC. Temperature effect during humid ageing on interfaces of glass and carbon fibers reinforced epoxy composites. J Colloid Interface Sci 2006;298(1):111–7. [24] Bao L-R, Yee AF. Effect of temperature on moisture absorption in a bismaleimide resin and its carbon fiber composites. Polymer (Guildf) 2002;43(14):3987–97. [25] ASTM C67. Standard test methods for sampling and testing brick and structural clay tile 2014;04. 23

[26] EN 772-1. Methods of test for masonry units. Determination of compressive strength 2011. [27] Lourenço PB, Fernandes FM, Castro F. Handmade clay bricks: chemical, physical and mechanical properties. Int J Archit Herit 2009;4(August 2013):38–58. [28] ASTM D638-03. Standard test method for tensile properties of plastics 2014. [29] ASTM D3039/D 3039M. Standard test method for tensile properties of polymer matrix composite materials 2014. [30] ISO TC 71/SC 6 N. Non-traditional reinforcing materials for concrete structures-Test methods-part 1: FRP bars and grids 2015. [31] ACI 440.3R. Guide test methods for Fibre-Reinforced Polymers (FRPs) for reinforcing or strengthening concrete structures 2012. [32] Sharma KS, Mudhoo A, editors. A handbook of applied biopolymer technology: synthesis, degradation and application. Royal Society of Chemistry (RSC); 2011. [33] Bengoechea C, Arrachid A, Guerrero A, Hill SE, Mitchell JR. Relationship between the glass transition temperature and the melt flow behavior for gluten, casein and soya. J Cereal Sci 2007;45(3):275–84. [34] Soenen H, Besamusca J, Poulikakos LD, Planche J-P, Das PK, Kringos N, et al. Differential Scanning Calorimetry applied to bitumen: Results of the RILEM NBM TG1 Round Robin Test. In: Kringos N, Birgisson B, Frost D, Wang L, editors. Proc. Int. RILEM Symp., Stockholm, June 2013: Springer; 2013, p. 311–23. [35] ASTM E1356-08. Standard test method for assignment of the glass transition temperatures by Differential Scanning Calorimetry 2014. [36] Robert RJ, Kural MH, Macke GB. Thermal expansion properties of composite materials 1981. [37] Zhou J, Lucas JP. Hygrothermal effects of epoxy resin. Part II: variations of glass transition temperature. Polymer (Guildf) 1999;40(20):5513–22. [38] Wondraczek K, Adams J, Fuhrmann J. Effect of thermal degradation on glass transition temperature of PMMA. Macromol Chem Phys 2004;205(14):1858–62. [39] Chamis CC, Murthy PLN. Simplified adhesively procedures for designing bonded composite joints. J Reinf Plast Compos 1991;10:29–41. [40] Maljaee H, Ghiassi B, Lourenço PB, Oliveira DV. Moisture-induced degradation of interfacial bond in FRP-strengthened masonry. Compos Part B Eng 2016;87:47–58. [41] Haldar A, Mahadevan S. Probability, reliability, and statistical methods in engineering design. John Wiley; 2000. [42] Schutte CL. Environmental durability of glass-fiber composites. Mater Sci Eng R 1994;13(7):265–322. [43] CNR-DT 200 R1. Guide for the design and construction of externally bonded FRP systems for strengthening existing structures 2013. [44] Blackburn BP, Tatar J, Douglas EP, Hamilton HR. Effects of hygrothermal conditioning on epoxy adhesives used in FRP composites. Constr Build Mater 2015;96:679–89. 24

[45] Ghiassi B, Xavier J, Oliveira DV, Lourenço PB. Application of digital image correlation in investigating the bond between FRP and masonry. Compos Struct 2013;106:340–9. [46] Benzarti K, Chataigner S, Quiertant M, Marty C, Aubagnac C. Accelerated ageing behaviour of the adhesive bond between concrete specimens and CFRP overlays. Constr Build Mater 2011;25(2):523–38. [47] Hu P, Han X, Da Silva LFM, Li WD. Strength prediction of adhesively bonded joints under cyclic thermal loading using a cohesive zone model. Int J Adhes Adhes 2013;41:6– 15.

25

List of Figures Fig 1. Geometry of the specimens prepared for: (a) single-lap shear tests; (b) pull-off tests (dimensions are in mm). Fig 2. Geometry of the specimens prepared for material characterization: (a) brick cubes; (b) epoxy resin and primer; (c) GFRP coupon (dimensions are in mm). Fig 3. Schematic of test setups for material characterization: (a) compressive test on bricks; (b) tensile test on epoxy resin and primer; (c) tensile test on GFRP coupons. Fig 4. Schematic of test setups for bond characterization: (a) pull-off test; (b) single-lap shear test. Fig 5. Hygrothermal exposure tests condition. Fig 6. Schematic position of strain gauges on the specimens. Fig 7. Strain changes in different specimens with exposure cycles. Fig 8. Variation of Tg with exposure cycles. Fig 9. (a) Tg depression with moisture absorption; (b) changes of Tg in specimens exposed to hygrothermal and laboratory conditions. Fig 10. Changes of bricks’ compressive strength with exposure. Fig 11. Changes of epoxy resin properties with exposure cycles (a) tensile strength; (b) comparison with water immersion tests; (c) elastic modulus; (d) ultimate strain. Fig 12. Changes of primer properties with exposure cycles (a) tensile strength; (b) comparison with water immersion tests; (c) elastic modulus; (d) ultimate strain. Fig 13. Changes of GFRP properties with exposure cycles (a) tensile strength; (b) comparison with water immersion tests; (c) elastic modulus; (d) ultimate strain. Fig 14. Progressive delamination in ORG-specimens. Fig 15. Variation of pull-off strength and failure mode with exposure cycles. Fig 16. Variation of debonding force and failure modes in (a) ORG-specimens; (b) GRspecimens. Fig 17. Variation of: (a) fracture energy; (b) KG in GR-specimens.

26

Table 1. Material properties (CoV is given inside brackets). Compressive strength (MPa)

Elastic modulus

Peak strain

(GPa)

(%)

_

_

_ 2.83

Tg2: 66.1

1.98

Tg1: 56.3 Tg2: 61.7

Tensile strength (MPa)

Masonry brick

15.38

Epoxy resin

_

58.84

2.86

(4%)

(8%)

Primer

_

42.04

2.85

(18%)

(10%)

GFRP coupons

_

1185.14

58.84

(8%)

(8%) (5%) ‫ ٭‬Tg1 and Tg2 are obtained from the first and second heating scans, respectively.

27

Tg‫٭‬ (˚C)

_ Tg1: 54.2

2

_

40

150

10

50

Brick GFRP sheet

450

Aluminum plate

Unbonded area

Bonded area

(a) 200 180

GFRP sheet

70

100

Brick

(b) Fig 1. Geometry of the specimens prepared for: (a) single-lap shear tests; (b) pull-off tests (dimensions are in mm).

28

190

40

20

10

40

40

t=4 90

(a)

(b)

15

200 100 35

35

50

50

(c) Fig 2. Geometry of the specimens prepared for material characterization: (a) brick cubes; (b) epoxy resin and primer; (c) GFRP coupon (dimensions are in mm).

29

(a)

(b)

(c)

Fig 3. Schematic of test setups for material characterization: (a) compressive test on bricks; (b) tensile test on epoxy resin and primer; (c) tensile test on GFRP coupons.

30

50 mm

50 mm

Top LVDTs

Bottom LVDT

Drilled cores GFRP sheet

Brick

LVDT supports

F

Aluminum disks

F

5 mm

GFRP sheet

(a)

Middle LVDT

Brick

(b)

Fig 4. Schematic of test setups for bond characterization: (a) pull-off test; (b) single-lap shear test.

31

100

Temperature [° C]

140 80

120 100

60

RH (real condition) RH (planned condition) 40 Temperature

80 60 40

20

20 0

0

6

12

18

24

Relative Humidity, RH [%]

160

0

Time [hrs]

Fig 5. Hygrothermal exposure tests condition.

32

Fiber direction

sg.3

sg.2

sg.1

Brick

L

T L

GFRP coupons

sg.5

sg.4 Epoxy resin and primer

T Fiber direction

Fig 6. Schematic position of strain gauges on the specimens.

33

10

6 4 2 0

1

0

0

240

4

2 3 Cycles

720

480

8

-3

-3

8

10

Strain [× × 10 ]

Brick-L

Strain [× × 10 ]

-3

Strain [× ×10 ]

10

6 4 2

Brick-T

0

960

240

0

Cycles

720

4 2 0

960

-3

6

GFRP-L 240

0

480

720

0

960

240

480

720

960

-3

-3

sg.1

Strain [× 10 ]

8

Strain [× × 10 ]

8 6 4 2 0

sg.2 240

0

Cycles

10

8

8

-3

6 4 2 0

GFRP-T 240

0

sg.4 0

240

480

720

960

480

720

960

2 0

sg.3 240

0

480

Cycles

4 2

Cycles

sg.5 0

240

480

720

960

Cycles

Fig 7. Strain changes in different specimens with exposure cycles.

34

960

4

6

0

720

6

Cycles Strain [× 10 ]

-3

480

10

Strain [× 10 ]

Strain [× 10 ]

8

2

960

Cycles 10

4

720

2

Cycles

6

480

4

10

0

240

0

6

10

0

Epoxy resin

Cycles 8

Cycles

-3

960

8

Primer 480

0

Strain [× 10 ]

4

240

2

10

-3

6

0

4

10

Strain [× 10 ]

-3

Strain [× 10 ]

8

0

720

6

Cycles

10

2

480

8

720

960

90

Tg[° C]

75 60 45 st

Epoxy resin (1 heat scan) nd Epoxy resin (2 heat scan) Epoxy at bond interface Primer

30 15 0 0

240

480

720

960

Cycles

Fig 8. Variation of Tg with exposure cycles.

35

Tg[° C]

Normalized value [Tg/Tg0]

80

es cl Cy

1.0

0 96

100

es cl Cy 0 48 es cl Cy es cl Cy

1.2

0 24

120

0

1.4

0.8 2

0.6

Tgw/Tgd=0.0047ω −0.056ω+1

60 40

0.4 20

Experimental data Simulated data

0.2 0.0 0.0

0.4

0.8

1.2

1.6

Hygrothermal condition Laboratory condition

0 0

2.0

2

4

6

8

10

12

Curing time [months]

Moisture content [%]

(a)

(b)

Fig 9. (a) Tg depression with moisture absorption; (b) changes of Tg in specimens exposed to hygrothermal and laboratory conditions.

36

0.5

16

0.4

12

0.3

8

0.2

4

Compressive strength 0.1 MC after exposure MC at test moment 0.0

0 0

240

480

720

Moisture content, MC [%]

Compressive strength [MPa]

20

960

Cycles

Fig 10. Changes of bricks’ compressive strength with exposure.

37

3.0

50

2.5

40

2.0

30

1.5

20

1.0

Tensile strength MC after exposure MC at test moment

10 0 0

240

480

720

0.5 0.0

1.2

4 months of exposure

1.0 0.8 0.6

8 months of exposure

0.4 0.2

Water immersion Hygrothermal exposure

0.0 0.0

960

0.5

Cycles

(a)

2.5

2.0

2.0

1.5

1.5

Elastic modulus MC after exposure MC at test moment

0.0 0

240

480

720

1.0 0.5 0.0

4.5

Ultimate strain [%]

3.0

2.5

0.5

2.0

2.5

4.0 3.5

3.6

εe=3.7-exp(-0.003n)

3.2 2.8

3.0 2.5

2.4 2.0

2.0 1.5

1.6 1.2

Stress

3.5

Moisture content, MC [%]

Elastic modulus [GPa]

Ete=2.29+0.57exp(-0.006n)

1.0

1.5

(b)

3.5 3.0

1.0

Moisture content, MC [%]

960 cycles 0.8

1.0 0.5

360 cycles Reference Strain

0.0

960

0

240

480

Cycles

Cycles

(c)

(d)

720

0.4 0.0

960

Fig 11. Changes of epoxy resin properties with exposure cycles (a) tensile strength; (b) comparison with water immersion tests; (c) elastic modulus; (d) ultimate strain.

38

Moisture content, MC [%]

60

Normalized strength [Ft /Ft0]

3.5

fte=44.5+14.48exp(-0.004n)

Moisture content, MC [%]

Tensile strength [MPa]

70

5

40

4

30

3

20

2

Tensile strength MC after exposure MC at test moment

0 0

240

480

720

1 0

1.2

3 months of exposure

1.0 0.8 0.6

8 months of exposure

0.4 0.2

Water immersion Hygrothermal exposure

0.0 0.0

960

0.5

1.0

Cycles

(a)

5

2.0

4

1.5

3

1.0

2

Elastic modulus MC after exposure MC at test moment

0.5 0.0 0

240

480

2.5

3.0

720

1 0

5

5

4

4

3

3

2

2

480 cycles 960 cylces

1

1 Reference Strain

0

960

6

εp= 4.44-2.34exp(-0.005n)

Stress

6

Ultimate strain [%]

Etp=1.5+1.42exp(-0.0065n)

Moisture content, MC [%]

Elastic modulus [GPa]

6

7

2.5

2.0

(b)

3.5 3.0

1.5

Moisture content, MC [%]

0

240

480

Cycles

Cycles

(c)

(d)

720

0

Moisture content, MC [%]

10

Normalized strength [Ft /Ft0]

fte=13+37exp(-0.001n)

Moisture content, MC [%]

Tensile strength [MPa]

50

960

Fig 12. Changes of primer properties with exposure cycles (a) tensile strength; (b) comparison with water immersion tests; (c) elastic modulus; (d) ultimate strain.

39

1.8

1000

1.5

800

1.2

600

0.9

400

0.6

Tensile strength MC after exposure 0.3 MC at test moment 0.0

0 0

240

480

720

1.2

2 months of exposure

1.0 0.8 0.6

8 months of exposure 0.4 0.2

Water immersion Hygrothermal exposure

0.0 0.0

960

0.5

Cycles

70

1.4

60

1.2

50

1.0

40

0.8

30

0.6

20

0.4

Elastic modulus MC after exposure 0.2 MC at test moment 0.0 0

240

480

720

2.4

2.0

1.8

εf=1.6+0.41exp(-0.01n)

2.0

1.5

1.6

1.2

1.2

Stress

1.6

Strain at peak load [%]

80

0

1.5

(b)

Moisture content, MC [%]

Elastic modulus [GPa]

(a)

10

1.0

Moisture content, MC [%]

0.8

0.6

480 cylces 120 cylces

0.4

0.3

Strain 0.0

0.0

960

0.9

Reference

Moisture content, MC [%]

200

Normalized strength [Ft /Ft0]

1200

Tensile strength [MPa]

2.1

ftf=956.97+229.9exp(-0.015n)

Moisture content, MC [%]

1400

0

240

480

Cycles

Cycles

(c)

(d)

720

960

Fig 13. Changes of GFRP properties with exposure cycles (a) tensile strength; (b) comparison with water immersion tests; (c) elastic modulus; (d) ultimate strain.

40

Delaminated area [%]

Delamination process

Delaminated area

1.0

F

0.8 0.6 0.4 0.2 0.0 0

120 240 360 480 600

Cycles

0

60

120

240

480

Cycles

Fig 14. Progressive delamination in ORG-specimens.

41

600

Pull-off strength [MPa]

1.2

Zone II

1.0

Zone III

0.8 0.6 0.4 0.2 Zone I 0.0 0

240

480

720

960

Cycles

Zone I Cohesive

Zone II Cohesiveadhesive

Zone III Adhesive

Fig 15. Variation of pull-off strength and failure mode with exposure cycles.

42

70

60

12

60

10

50

8

40

6

30

4

20

2

10

0

0

50

8

40

6

30

4

20

2

10

0

0 0

240

480

720

960

0

240

Cycles

Zone I Adhesive

480

720

960

Cycles

Zone II Adhesive

0 cycles Cohesive

(a)

960 cycles Cohesive

(b)

Fig 16. Variation of debonding force and failure modes in (a) ORG-specimens; (b) GRspecimens.

43

Debonded area [%]

14

Debonding force [kN]

70

Debonded area [%]

10

Zone II

12

Zone I

Debonding force [kN]

14

0.20

Gf=0.8-0.15exp(-0.0024n)

0.8

0.15

0.6

ORG GR

0.4

KG

Fracture energy [N/mm]

1.0

0.10

KG=0.099+0.00003n

0.05

0.2

Gf=0.13+0.3exp(-0.011n)

0.0 0

240

480

720

0.00 960

0

Cycles

240

480

Cycles

(a)

(b)

Fig 17. Variation of: (a) fracture energy; (b) KG in GR-specimens.

44

720

960