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Composite Structures 85 (2008) 164–174

Degradation of bond between FRP and RC beams

Manuel A.G. Silva a,*, Hugo Biscaia b,1

a Research Centre for Structures and Construction UNIC, FCT-Universidade Nova de Lisboa, UNIC, FCT/UNL, 2829 516 Caparica, Portugalb Department of Civil Engineering, FCT-Universidade Nova de Lisboa, Portugal

Available online 17 October 2007

Abstract

Beams and slabs externally reinforced with FRP are often in contact with moisture and temperature cycles that reduce the expecteddurability of the system. Bond degradation is a frequent cause of premature failure of structural elements and environmental conditionsare known to relate to such failures. The study shows the effects of cycles of salt fog, temperature and moisture as well as immersion insalt water on the bending response of beams externally reinforced with GFRP or CFRP, especially on bond between FRP reinforcementand concrete. Temperature cycles (�10 �C; 10 �C) and moisture cycles were associated with failure in the concrete substrate, while saltfog cycles originated failure at the interface concrete–adhesive. Immersion in salt water and salt fog caused considerable degradation ofbond between the GFRP strips and concrete. However, immersion did not lower the load carrying capacity of beams, unlike temperaturecycles (�10 �C; 10 �C) that caused considerable loss. No significant differences were detected on the behavior of the systems strengthenedwith GFRP and CFRP, perhaps because the design of the tests impeded failure of the fibres.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Fibre reinforced polymers; Concrete; Bond; Durability; Environmental degradation

1. Introduction

The external strengthening of slabs and beams withstrips of fibre reinforced polymers (FRP) relies on bonddeveloped between concrete and the composite laminate.Bond depends on several parameters and has been the sub-ject of numerous studies, but, despite that wealth of infor-mation, the durability of bond under the effect ofenvironmental conditions is not sufficiently understood.

The durability of a structure reinforced with FRPreflects its ability to resist cracking, oxidation, chemicaldegradation, delamination, wear, and/or the effects of for-eign object damage for a specified period of time, underspecified load and environmental conditions [1].

Structures externally reinforced with FRP are often incontact with moisture and temperature cycles that reduce

0263-8223/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compstruct.2007.10.014

* Corresponding author. Fax: +351 21 2948398.E-mail addresses: mgs@fct.unl.pt (M.A.G. Silva), heccbiscaia@sapo.pt

(H. Biscaia).1 Engenharia Civil, Instituto Politecnico, Portalegre, Portugal.

their expected durability. Adhesives may deteriorate whenexposed to moisture and salt water cycles, ultraviolet radi-ation, alkalines, and temperature nearing transition vitre-ous values. Chlorides may be especially severe when theFRP externally strengthens structures in marine environ-ments or cold regions where salt deicing is used. Due toinsufficient knowledge of those effects, design rulesaccounting for the effect of aggressive environments arethought to be overly conservative.

Despite the lack of data on environmental degradationof bond, numerous studies of environmental effects onstrength and deformability of FRP laminates and struc-tural elements strengthened with FRP can be found andshed light on the phenomena affecting bond.

It is established that the deterioration of FRP is trig-gered by the diffusion of light atomic weight free ions,i.e. OH� and Cl� and water molecules into the poly-meric matrix [2]. Absorption accelerated by the cracksand voids in the matrix may cause hydrolysis and poly-merization that induce changes of the matrix. When dif-fused chemicals contact the surface of the fibres, their

M.A.G. Silva, H. Biscaia / Composite Structures 85 (2008) 164–174 165

ensuing degradation decreases the tensile strength andstiffness.

Physical degradation of the resin systems may be due tohigh temperature or moisture exposure. Temperatures,especially when close to the glass transition temperatureof the matrix Tg, may ‘free volumes’, which can be filledby other molecules as the polymer attempts to reach anequilibrium stage and may change mechanical propertiesof the materials.

Absorbed moisture can cause plasticization of thematrix, i.e. reduction of Tg and mechanical characteristicsdue to the interruption of Van der Waals bonds betweenthe polymer chains and be responsible for increasing theeffective free volumes in the resin system [3].

Alkaline solutions are one of the most severe causes ofdegradation of glass fibres. Concrete pore water is alka-line and a critical environment for glass fibres due tothe hydration of the cement [4]. The most importantreaction for the dissolution of the glass fibres in wateris ‘‘leaching”, the diffusion of the alkali ions out of theglass structure, a temperature dependent process [5], nor-mally more important for reinforcement rods than outerstrips.

Kootsookos et al. [6] studied CFRP and GFRP, withpolyester and vinylester matrices, immersed in seawaterat a temperature of 30 �C and showed that the carbon com-posites displayed better durability. The moisture absorbedby CFRP composites is lower compared with GFRP, theflexural properties of the glass composites declining slowlyfor long periods in seawater.

Chajes et al. [7] showed a 36% decrease in ultimatestrength for GFRP retrofitted specimens that were sub-jected to 100 wet/dry cycles, while a 19% reduction wasshown for CFRP bonded specimens.

Toutanji and Gomez [8] observed a strength reductionup to 33% on specimens made of different epoxies and sub-jected to 300 wet/dry cycles in salt water. Failure wasreported as a debonding mode that generally took placenear the FRP/concrete interface. Karbhari and Zhao [9]reported micro-cracking in the GFRP strip that strength-ened cement mortar beams. Specimens were subjected tofour-point bending mechanical tests until failure. A 40%reduction in bending capacity was noticed after 120 daysof moisture exposure.

Severe losses of pre-stress were found on GFRP sub-jected to dry/wet cycles with artificial sea water [10]. Theauthors showed that total immersion of structural compos-ites in water caused degradation of mechanical properties ifcombined with relatively high temperature.

A study of effects on GFRP laminates due to immersionin water, selected dry–wet (20%RH; 90%RH), thermal(20 �C; 50 �C) and salt fog spray (8 h at 96%RH; 16 h dry-ing) cycles, and cycles of UV radiation with flooding isdescribed in [11]. The results are correlated with thoseon the strength and deformability of RC cylindersconfined with the same GFRP material. Major conclusionsreported:

� Mass gain of GFRP plates immersed in distilled waterwas higher than for salt water. The results agree withAlmeida [12] who showed that the moisture uptake incomposites is quicker with deionized water, due to thesize of ions in the salt solution where large ions preventmoisture uptake. Although true for short-term degrada-tion the conclusion cannot be extended to long-termexposure when significant strength degradation due tothe presence of salt has been found [2].� The salt fog spray cycles were the most severe of the

actions considered.� Noticeable decay of tensile strength was detected on the

laminates under salt fog cycles and immersed in water at22 �C.� The results for ultra violet radiation combined with

flooding indicate that unprotected material should notbe used for applications requiring constancy of strength.� The effects of environmental conditioning on confine-

ment of concrete with GFRP jackets are much lesssevere than those found on the mechanical propertiesof the GFRP material alone.

Environmental effects, specifically, on bond have beenobject of fewer publications. A brief survey of suchresearch can be found in [13], comprising tests devised tomeasure bond on small scale physical models and on envi-ronmental degradation.

Temperature effects have been studied more extensivelythan those caused by moisture, especially due to the impor-tance of temperature on the curing process and, conse-quently, on the vitreous transition temperature Tg. Glasstransition temperature of polymer depends on the chemicalbonds in the polymer structure and also on the amount offree volumes and may be used to assess the effects of mois-ture absorption or chemical diffusion on the physical deg-radation that can damage the bond in the polymerstructure itself.

If Tg of the resin system decreases due to environmentaleffects, degradation takes place, originating a weaker cross-linked structure with reduced stiffness and strength of thematrix.

Wet lay up techniques used in structural civil engineer-ing are often associated with incomplete matrix cure atambient temperature. Exposure to elevated temperaturesfor short time increases the Tg, causing additional cross-linking between the polymer in a process called ‘post-cur-ing’. The fact that additional chemical reactions occurbetween the unreacted groups may also increase the Tg.Environmental operational conditions, namely with tem-perature increase, may contribute to the conclusion of theprocess of curing.

Walker and Karbhari [14] investigated the durability ofthree FRP composite systems through tests conducted onring-type specimens. Accelerated aging through the immer-sion of specimens in water at different temperatures wereused to predict long-term durability in terms of tensilestrength, modulus and ultimate strain. After correlation

166 M.A.G. Silva, H. Biscaia / Composite Structures 85 (2008) 164–174

with field data, materials properties are predicted and thenused for design of seismic retrofit.

Dohnalek [15] presents a brief summary of studies onfreeze–thaw effects and, additionally, shows results of lapshear and pull out tests that led to the conclusion thatthe durability of the adhesive bond depends essentially onthe freeze–thaw durability of the concrete. Conclusionsreported and drawn from previous freeze–thaw studiesinclude:

� CFRP has higher environmental durability than eitherGFRP or AFRP.� The ultimate strength of the adhesive bond deteriorates,

under freeze–thaw cycles in chloride solution.� The combined action of freeze–thaw cycles and chlo-

rides was the most damaging type of freeze–thaw cycles.

Effects of temperature on adhesion strength that devel-ops between an epoxy adhesive and concrete elements, inthe presence of water, are also described in [16]. Near thevitreous transition temperature Tg of the adhesive, thebond strength deteriorated, independently of the concretesubstrate. The mode of failure was also dependent on tem-perature, changing from a mixed failure to failure at theinterface between concrete and adhesive at higher temper-atures. The authors suggest that the Tg of the adhesiveshould be 5–10 �C higher than the operating temperature.

Studies similar to those reported in this article aredescribed in [17]. The authors tested plain concrete beamspecimens 90 cm long, reinforced with externally bondedwet-laid GFRP sheets. The beams were pre-cracked andsubjected to different environmental conditions includingelevated temperature/dry, and freeze/thaw cycles. Debond-ing of the GFRP sheet from the concrete was the morecommon cause of failure. The ultimate strength of theGFRP reinforced beams decreased for specimens subjectedto cycles of dry freezing and wet thawing.

Fava et al. carried out some recent experimental tests onenvironmentally conditioned FRP plates bonded to con-crete and deduced these conclusions [18]:

� Freeze–thaw cycles reduce bond shear strength and peakslip, so that the loading branch (in force-elongation dia-gram) shows highly non linear bond behaviour at alower applied force.� Beneficial effects of high humidity level characterizing

salt spray fog overcame damage due to presence of chlo-ride solution.� Higher deformability of interface behaviour (high peak

slip) appeared to be the only effect of aggressiveenvironment.

A broad presentation of the unanswered questions oflong-term performance and durability issues with respectto the effects of moisture, thermal cycles, freeze–thawactions, and coupling of environmental and mechanicalloading is offered by Buyukozturk [19]. The author

believes that the understanding of bond durabilityrequires the acceptance that moisture affected debondingmay involve material decohesion and/or interface separa-tion and requires explicit analytical approach, beyond theusual limited strength-based approaches. Physical changesin the bond as well as in the constituent materials areshown in moisture affected debonding. Fracture toughnessis shown to decrease by as much 60% and become asymp-totic for a certain moisture concentration value for CFRPstrengthening. Concrete delamination mode was observedfor dry specimens, while the epoxy/concrete interface sep-aration occurred in all wet fracture specimens tested.Interface fracture analysis indicates that this interfacialdebonding mode is attributed to an interfacial materialtoughening and an interface weakening mechanism dueto moisture diffusion.

Due to the relative paucity of data on degradation ofbond between FRP and concrete and insufficient knowl-edge of the corresponding mechanisms, design rulesaccounting for the effect of aggressive environment are cur-rently fairly conservative. These facts warrant studies onenvironmental aging as part of an evaluation of the long-term durability of adhesively bonded systems.

2. Experimental study – GFRP reinforcement

Environmental effects on GFRP laminates have beenreported [11]. GFRP laminates are vulnerable to moisturebecause of negative effects of absorption on their physicalproperties. Sorption is mostly caused by diffusion due toa concentration gradient and also to relaxation swellingof the composite. Tests on the effects of water immersionon plates of GFRP revealed lowering of the mechanicalproperties in the vicinity of 2000 h, with a recovery around180 days and a return to initial values after a year. The evo-lution of the Young modulus reflected the same trends andnegligible changes. From the overall data, water immersionat 22 �C causes only slight degradation of the GFRP lam-inates in the direction of fiber alignment. The salt fog spraycycles were the most severe action considered, loweringstrength of the laminates by 17% after 3000 h, while thereduction of strength after immersion in water was smallerthan due to salt fog cycles.

These results may help the interpretation of tests onbeams reinforced with FRP, but the present study is basedon design that precludes tensile failure of the FRP.

The experimental study was part of a research programpartially reported in [20]. RC specimens externally rein-forced with GFRP were prepared. Geometrically, the spec-imens are formed by two independent prismatic concreteblocks, 100 mm wide, connected through a stainless steelhinge device with remaining geometric data shown inFig. 1. All specimens were pre-treated with jet sandingand externally reinforced with two layered GFRP stripsbonded by the wet lay up technique to the tension-side sur-face of the beams. The GFRP reinforcement was 1.3 mmthick, 520 mm long and 80 mm wide. GFRP strips were

Fig. 1. Geometry of specimens to be submitted to four point bendingtests.

M.A.G. Silva, H. Biscaia / Composite Structures 85 (2008) 164–174 167

made of unidirectional SEH51/Tyfo GFRP fabric, withsparse Kevlar fibres orthogonal to the glass fibre. The resinused was Tyfo S epoxy as advised by the supplier. Thesebeams were submitted to four point bending tests, asdescribed further on.

Standard tests of concrete cubes, at reference dates, ledto an average value of compressive strength 47.0 MPa andstandard deviation 2.4 MPa. Table 1 shows values of fc

converted from the experimental values through (1) toestimate values of fc at the same times as the duration ofartificially aging of specimens tested for bending strength.Differences introduced by Eq. (1) were not significant

fcðtÞ ¼ 30:852 � t0:036; t in days: ð1ÞThe average tensile strength, evaluated in cylindrical cou-pons, was fctm = 2.3 MPa.

Properties of Tyfo S epoxy resin obtained from the sup-plier were Young modulus 3.18 GPa, tensile strength72.4 MPa, ultimate strain 5.0% and vitreous transition tem-perature of 82 �C. Flat standard GFRP coupons weretested on a Zwick machine, at a rate of 2 mm/min, at theonset of artificial aging. The data generated led to an aver-age Young modulus of 20.4 GPa, tensile strength of500 MPa and an ultimate strain of 2.5%. Matrix contribu-tion to the tensile strength of the GFRP is negligible whencompared with the glass fibre, but higher than the tensilestrength of concrete (2.7 MPa), essential requirement fortransfer of load through bond.

The specimens were instrumented with strain gaugesbonded to the GFRP strip, spaced at intervals of 40 mmto measure the strain distribution along the GFRP lami-nate at different loading levels, Fig. 2. The strains were con-tinuously recorded using a data-logger.

The application of loading in the four point bendingtests was made through an actuator that supported againsta steel frame, Fig. 3. Two pressure cells placed under thesupports of the specimen control the applied load.

Table 1Cube strength of concrete at selected reference times

Time from aging start (h) Age (days) fc (MPa)

0 486 38.551000 516 38.635000 692 39.0410,000 914 39.44

For pull out tests, rectangular concrete slabs 10 cmthick, 20 � 60 cm on plan view, were designed with circularcylindrical incisions adequate to use dollies of 50 mm diam-eter and cause comparable rupture surfaces. Acceleratedaging corresponded either to total immersion in 5% saltwater, or temperature and salt fog cycles.

Salt fog cycles were imposed with the same salinity(50 g/l water) as in the immersion tests. Temperature waskept in the vicinity of 35 �C and moisture level defined by16 h to dry the specimens, followed by 8h of fog.

Temperature cycles consisted on the imposition of anenvironment at �10 �C for 12 h, followed by +10 �C foranother 12 h.

The specimens were tested after 1000, 5000 and 10,000 hof exposure. At the beginning of artificial aging (0 h ofexposure) control or reference tests were performed. Theaverage of the results at each time ti is designated hereafteras the reference value at ti.

2.1. Bending tests

Failure of reference specimens R1, R2 and R3, as well asin specimens subjected to temperature cycles, was due toconcrete tensile failure in the concrete substrate near thesurface and chunks of concrete could be seen adherent tothe GFRP strips after rupture. Typical configurations afterfailure are represented in Figs. 4 and 5. Fig. 4 shows failuresurfaces of a reference beam and of beams subjected to1000 h and 5000 h of the (�10 �C; +10 �C) cycles.

In the case of salt fog cycles and immersion in salt water,failure occurred with separation of the GFRP strip close tothe interface with concrete, as shown in Fig. 5, and theaverage bond stress was always greater than at the onsetof aging (0 h).

Load capacity decreased continuously with increasingnumber of freeze–thaw temperature cycles. On the con-trary, salt fog cycles caused an initial increase of strength,by comparison with the reference values. The load capacityafter 10,000 h was found only marginally below the refer-ence value (�3%).

Total immersion improved the overall strength still fur-ther than salt fog, leading to higher values of the ultimateload that reached 20% at 10,000 h, whereas it decreasedabout 31% in the case of freeze–thaw temperature cycles.Fig. 6 depicts the described behavior.

The behavior under immersion is partially due toimprovement of characteristics of concrete. Tests of exter-nally bonded GFRP sheets on concrete showed that theconcrete gained substantial strength due to acceleratedconditioning in water at 60 �C with the compressivestrength gaining 22% while the split tensile strengthincreased by 85% [21].

The average bond stresses increased 25% in the GFRPbeams submitted to total immersion, and decreased 18%for the �10 �C and +10 �C temperature cycles. Theseresults agree with [13] on that beneficial effects of highhumidity level may overcome detrimental aspects of the

Fig. 2. Bottom view of location of strain gauges in four point bending tests.

Fig. 3. Post-failure of reference specimen – GFRP.

168 M.A.G. Silva, H. Biscaia / Composite Structures 85 (2008) 164–174

accelerated aging, at least for a time period, due to post-curing of the polymers and possible improvement ofconcrete tensile strength due to wet curing of the concrete

Fig. 4. Post-failure surfaces (GFRP): (i) referenc

Fig. 5. Separation surfaces (GFRP): (i) after

substrate, as reported also in [22]. It is known thatunder-cured polymer matrix systems have higher diffusionrates for moisture and ions than the completely cured spec-imens [23]. At later times the beneficial effects of immersiontend to be overshadowed by mechanisms of degradation.

Average bond stress greater than that found on theonset of aging may explain the more irregular distributionand lower bond stress encountered in the control or refer-ence specimens. The effective bond length did not allow aclear understanding of the modifications. It decreased25% in the salt fog cycles tests, while it increased 19% forthe specimens totally immersed. The results are summa-rized in Table 2.

Fig. 6 represents typical curves of development of thebond stress s and the GFRP stress r = Ef � e, withEf = GFRP Young’s modulus, e = strain at gauge, alongthe span of the beams, for different levels of external force,and s is approximated by

e specimen and (ii) after temperature cycles.

immersion and (ii) after salt fog cycles.

Laminate stresses - GR-NS-1Test at 1000h

0

50

100

150

200

250

0 155 310 465 620

Position of gauges (mm)

Str

ess

on

GF

RP

(MP

a) P=5kN

P=10kN

P=15kN

P=20kN

P=25kN

Pmax=30,29kN

Fig. 6. Typical stress diagram as loading of specimen increases (case of salt fog –1000 h).

Table 2Average responses from four point bending tests

Specimen Maximum deflection(mm)

External loadcapacity (kN)

Force on GFRP(kN)

Bond stress(MPa)

Effective bond length(mm)

Remarks

References 4.7 28.7 25.4 3.71 150 0 hSalt fog 4.9 30.3 26.8 4.62 157 Slight increase of

capacity4.6 30.0 26.4 4.73 1204.0 28.1 24.8 4.09 113

(�10 �C;10 �C)

4.1 25.3 22.3 3.45 128 Decrease of capacity3.7 23.0 20.3 3.17 1313.6 22.0 19.4 3.14 176

Immersion 4.4 33.8 29.9 4.92 178 Larger increases ofcapacity5.2 35.5 31.4 4.55 170

4.3 36.3 32.0 4.32 151

M.A.G. Silva, H. Biscaia / Composite Structures 85 (2008) 164–174 169

s ¼ De � Ef � tf

DL; ð2Þ

with tf is the thickness, e is the strain at gauge, and DL isthe length of segment of influence.

Under the load, in the first stage, the behaviour is totallyelastic, with concrete resisting low tensile stresses. At theselow values of the load, the stresses on GFRP are also lowand transferring the load takes place essentially in the cen-tral region. Increasing the external force, the stresses rise,concrete tensile strength is exceeded and cracks and non-negligible GFRP stresses appear near the extreme sides ofthe GFRP strip.

Mostly due to concrete cracking, the load transfer has tobe extended further out by the GFRP strip. In the laststage, GFRP alone resists the load increase, and does itup to structural failure that occurs in a sudden mannerpulling chunks of concrete. The bond stresses on the spec-imens subjected to freeze–thaw type of cycles showed avery irregular spatial distribution and caused failure atthe lowest loads. They correspond to the only aged speci-mens where rupture took place exclusively in the concrete.The developments of the average bond stresses for the threecases of artificial aging are depicted on Fig. 7. The timeevolution of the load capacity is depicted in Fig. 8 for eachtype of aging.

From measurements on the strain gauges, bond slip wasmodeled. Bond slip curves were obtained for each specimen

and the fracture energy Gf was calculated by integrationbased on approximate exponential laws for stress-slip.Environmental effects on the capacity of the GFRP stripPmax is introduced by inserting a factor, aE, on the usualexpression

P max ¼ aE � bf �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2Gf � Ef � tf

p; ð3Þ

where bf is the width of strip, Ef is the Young’s modulusand tf is the thickness.

The coefficient aE, as proposed in [17], obtained fromcomparison between the experimental values and (3),ranges from 0.58 to 0.65 as displayed in Table 3. Theexpression proposed by the fib bulletin 14 [24] leads to

P max ¼a � Kb � Kc � b

cf

�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiEf � tf � fctm

p; ð4Þ

where Kb is a geometric factor, Kc equals 1, for the studiedspecimens, fctm is the concrete tensile strength and a = 0.9.The values of cf evaluated from the experimental data arealso shown in Table 3.

2.2. Pull out tests

The utilization of pull out tests to assess the effects ofaccelerated degradation on bond between FRP andconcrete is known to have drawbacks [24]. The problem isrelated to bond friction caused by swelling, more

0

1

2

3

4

5

6

0 2000 4000 6000 8000 10000

Aging Hours

Ave

rage

Bon

d St

ress

(MPa

)

Salt fog cycles

Temperature cycles

Total immersion

Fig. 7. Evolution of the maximum average bond stress for each artificial aging considered.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 2000 4000 6000 8000 10000

Aging Hours

Load

max

/ L

oad

Ref

max

Temperature cycles

Total immersion

"Salt fog cycles"

Fig. 8. Dimensionless load capacity of specimens vs. aging time for temperature and salt fog cycles, and total immersion.

Table 3Factors aE to account for environmental exposure effects on Pmax

Type of environmental exposure aE cf

Salt fog cycles (50 g/l of salt/water) 0.62 1.50Temperature cycles (�10 �C to +10 �C) 0.58 1.85Total immersion (50 g/l of salt/water) 0.65 1.25

Table 4Compressive strength of concrete cubes – CFRP study

Cubes Age (days) fc (MPa)

Control 395 32.13000 H 402 32.26000 H 557 33.010,000 H 700 33.6

170 M.A.G. Silva, H. Biscaia / Composite Structures 85 (2008) 164–174

pronounced for rods, but the results of pull out tests are onlyto be interpreted as providing additional comparative data.

Pull out tests were performed according to Europeanstandard EN 1542. The average results for pull out stress,both after immersion in salt water and after the salt fogcycles, were, respectively, 3.37 MPa and 3.58 MPa, lowerthan for the reference tests that showed 4.09 MPa.

Failure took place in the concrete except for one casethat corresponded to full immersion in salt water, after10,000 h. In this case the separation occurred at the inter-face and is thought to have been caused by voids presentbetween the adhesive and concrete.

3. Experimental study – CFRP reinforcement

The specimens for bending tests and the slabs for pull-off were geometrically equal to those described earlier,

except for the replacement of GFRP by CFRP and theirstudy part of another MSc thesis presented elsewhere [25].

The compressive strength of the cubes, extrapolated forthe time of each set of bending tests, is shown in Table 4and it is seen that the resistance is lower than for the GFRPtests.

The average tensile strength, evaluated in cylindricalcoupons, was fctm = 2.3 MPa.

The tensile strength of the CFRP strips was obtained bytesting ten two-layered, longitudinal flat coupons, made ofMBrace C1-30 and MBrace Saturate epoxy resin, 25 mmwide and 250 mm long and a design thickness of0.176 mm. Average values obtained were eftm = 1.54%,rfm = 3937 MPa and Efm = 241GPa. The resin supplierindicated Tg = 71 �C, er = 2.5%, Er = 3034 MPa andrr = 54 MPa.

Fig. 9. CFRP – Rupture surfaces at 3000, 6000 and 10,000 h of salt fogcycles.

M.A.G. Silva, H. Biscaia / Composite Structures 85 (2008) 164–174 171

Aging of the prismatic specimens followed the programsummarized in Table 5. Differently from the GFRP beams,cycles of moisture with relative humidity RH = 20%, for12 h, followed by 90%, another 12 h, at a temperature40 �C were selected and imposed on six specimens, whiletemperature cycles were not repeated here.

Salt fogging cycles followed the same specifications asused for GFRP, also with salinity at 50 g/l. The fact thatan intermediate step for bending tests was selected at6000 h rather than 5000 h was due to malfunctioning ofequipment for the bending tests, when 5000 h were reachedand it was thought advisable to keep the conditioning whilethe equipment was repaired.

3.1. Bending tests

The modes of failure were affected by the type of envi-ronmental action that subjected the specimens. Figs. 9,10through Fig. 11 illustrate the observed behavior. Failuresof CC1/CC2 after 3000 h of moisture cycles were associ-ated with debonding of CFRP following tensile failure ofconcrete, pulling adherent thin chunks of concrete. TheCC3/CC4 beams showed bigger chunks of concrete adher-ent to the CFTP strip, yet bigger for CC5/CC6 showingprogressive increase of the tensile strength of concrete.

Specimens subjected to salt fogging showed debondingalong the interface concrete–adhesive as the cause of fail-ure. It was noticed for 6000 and 10,000 h some pulling ofsmall chunks of concrete.

The minimum load capacity of the reference beams was34.0 kN and the decrease resulting from the moisture andsalt fog tests is shown in Table 6. The differences of theresults are small and can be partially due to scattering ofexperimental data, showing the need for further testing.It can be stated that salt fog tests show a trend for causinga continued decay of capacity.

The force transmitted to the CFRP strip was also eval-uated from the measurements in the tests. The average val-ues obtained are tabulated in Table 7. The decrease ofcarried load was more pronounced, globally, under saltfog cycles, but not very significant and a recovery of capac-ity was found from 6000 h to 10,000 h.

Table 5Program of artificial aging for beams reinforced with CFRP

Specimen label Aging time (h)

Control specimensR1, R2, R3 0

Moisture cyclesCC1, CC2 3000CC3, CC4 6000CC5, CC6 10,000

Salt fog cyclesNS1, NS2 3000NS23, NS4 3000NS5, NS6 6000

The generated data allowed the determination of theeffects of aging on the rigidity of the beams, henceforthcalled stiffness and measured by the ratio of the maximumexternal load by fmax, fmax being the maximum deflectionunder the load, read from two deflectometers CDP-100(TML). Results obtained are summarized in Table 8. Theaverage values tabulated when compared with the referencespecimens showed that the effects were small, with salt fogcycles associated consistently with higher values than thehumidity cycles.

Table 9 summarizes the maximum bond stresses evalu-ated with (2) from the readings of the strain gauges. It isnoticed that the specimens under salt fog cycles developedhigher maximum bond stresses, by comparison with thosesubmitted to moisture tests.

Patterns of bond stresses along the span for specimenssubjected to moisture cycles showed similarities with thosefound for the beams loaded prior to aging, namely they are

Fig. 10. CFRP – Rupture surfaces for reference specimens and after moisture or salt fog cycles. Reference specimen (R), moisture cycles (CC), and saltfogging (NS).

Fig. 11. CFRP – Failure surfaces after 10,000 h of moisture cycles.

172 M.A.G. Silva, H. Biscaia / Composite Structures 85 (2008) 164–174

fairly distributed along the bonded length. In the case ofsalt fog cycles the distribution is less homogeneous, reveal-ing a lesser capacity of redistributing stresses in correspon-dence with local ruptures. This aspect is related to thedifferences mentioned on the failure surfaces. They initiated

on the interface adhesive–concrete for the salt fog cycles,instead of being due to tensile failure in the concrete sub-strate. In fact, since the local failures are associated withmuch smoother surfaces for salt fog cycles, the contribu-tion of friction for bond is also much smaller.

Table 6Minimum load capacity due to moisture and salt fog cycles – CFRP

Aging period (h) Moisture cycles (kN) Salt spray fog (kN)

0 34.0 34.03000 31.7 33.46000 31.0 29.210,000 34.9 32.2

Table 7Maximum force transmitted to the CFRP strip

Aging period (h) Moisture cycles (kN) Salt spray fog (kN)

0 39.8 39.83000 32.2 37.66000 34.1 32.810,000 39.3 37.0

Table 8Effects of aging on ‘‘stiffness” parameter of beams (kN/mm)

0 h 3000 h 6000 h 10,000 h

Reference 8.6 –

Moisture cycles 6.7 6.7 8.0Salt fog cycles 10.6 8.0 12.1

Table 9Effects of aging on average bond stresses (MPa)

0 h 3000 h 6000 h 10,000 h

Reference 2.1 –

Moisture cycles 1.7 1.8 2.0Salt fog cycles 1.9 1.7 1.9

M.A.G. Silva, H. Biscaia / Composite Structures 85 (2008) 164–174 173

4. Conclusions

4.1. GFRP

– In terms of ultimate capacity of beams:� The temperature cycles were most detrimental, with a

loss of 31% at 10,000 h.� The immersion tests showed a gain of 21% at 10,000 h,

due to the conjugated effect of increase in tensilestrength of concrete and post-curing of the polymers,the decrease in strength of the laminate beingnegligible.

– In terms of bond:� Aging associated with salt water, i.e. immersion and

salt fog, caused considerable degradation of bondbetween the GFRP strips and concrete.� Salt fog cycles, showed gains in load capacity in the

earlier stages, perhaps due to the temperature effecton curing and plasticization of the epoxy resin.

– In terms of failure mechanisms:� For the temperature cycles and for reference speci-

mens, failure initiated in the concrete substrate layernear the surface of adherence.� Salt fog cycles and immersion in salt water caused fail-

ure on the interface between the adhesive and theconcrete.

4.2. CFRP

– In terms of ultimate capacity of beams:� Cycles of moisture at fixed temperature and cycles of

salt fog caused approximately the same degradationof capacity. Highest reduction of load capacity neared20%.

– In terms of failure mechanisms:� Failure surfaces differed, depending on type of aging.� Moisture cycles associated with surfaces similar to

those obtained for the control specimens, with failurein the concrete near surface substrate.� Salt fog cycles were associated with smoother surfaces

and failure along the concrete–adhesive interface.

– Bond stress distributions were different for each type of

action:� For reference specimens, as well as for those subjected

to moisture cycles, there was a fairly continuous distri-bution of stress along the bonded surface.� For specimens aged by salt fog cycles the length mobi-

lized was shorter and there were heterogeneities. As aconsequence, bond stresses attained higher peaks forsalt fog cycles.

– Pull out tests showed an increase of strength for salt fogcycles, attributed to higher tensile strength of concretedue to wet curing associated with the artificially highmoisture environment created at 35 �C.

4.3. Overall

– Temperature (�10 �C; 10 �C) and moisture cycles wereassociated with failure in the concrete substrate.

– Salt fog cycles were associated with failure at the inter-face concrete–adhesive.

– Temperature cycles (�10 �C; +10 �C) caused significantloss of capacity of beams, unlike immersion (GFRP).

– Although CFRP composites are known to behave betterunder environmental action than GFRP laminates, nosignificant differences were detected in the present study,seemingly because failure was not due to rupture of thefibres.

– More data is necessary to consolidate proposals forinsertion in Codes.

– Computational models are being developed and resultscompatible with experiments expected to be publishedrelatively soon.

Acknowledgements

The studies were partially financed by Fundac�ao para aCiencia e Tecnologia through Project POCTI/36059/ECM/2000 – Behavior and Design of Concrete StructuresStrengthened with FRP considering Ageing Effects. Mostof the experimental results on CFRP were obtained by aformer student, R. Marreiros, now engaged on differentwork.

174 M.A.G. Silva, H. Biscaia / Composite Structures 85 (2008) 164–174

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