Composites: Part B 43 (2012) 3239–3250

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Composites: Part B

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Influence of water on bond behavior between CFRP sheet and naturalcalcareous stones

Margherita Stefania Sciolti 1, Maria Antonietta Aiello ⇑, Mariaenrica Frigione 2

Department of Engineering for Innovation, University of Salento, Via per Arnesano, 73100 Lecce, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 October 2011Received in revised form 15 February 2012Accepted 2 March 2012Available online 13 March 2012

Keywords:A. Polymer–matrix composites (PMCs)B. DebondingB. Environmental degradationD. Mechanical testingCalcareous stones

1359-8368/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.compositesb.2012.03.002

⇑ Corresponding author. Tel.: +39 0832 297248.E-mail addresses: margherita.sciolti@unile.it (M.

unisalento.it (M.A. Aiello), mariaenrica.frigione@unile1 Tel.: +39 0832 297384.2 Tel.: +39 0832 297215.

In this paper the effect of a long term immersion in water on bond durability is analyzed when FRPs (FiberReinforced Plastic) are externally applied to a masonry substrate. In the performed research a substratemade by natural calcareous stones, strengthened by CFRP (Carbon Fiber Reinforced Plastic) sheets hasbeen analyzed. For a better comprehension of water effect on the adhesive bond between stone and CFRP,the same treatments were performed to the constituent materials, namely epoxy resins, CFRP sheets andstones. To this aim mechanical tests were carried out on stone, composite materials and epoxy resinsbefore and after their immersion in water, evaluating the effects of this agent on the properties of thematerials. The influence of the aging in water on the interface stone-reinforcement was analyzed in termsof bond strength, maximum bond stress, optimal bond length, slip-bond stress relationship and mode offailure. In addition the possibility of calibrating design relationships, taking into account the influence ofenvironmental conditions is discussed. Detailed results on adhesives and composites aged in water havebeen reported in a previous paper while in the present work the significant decay of the mechanical prop-erties of the stone is specifically investigated. With regard to the conditioning treatment a reduction ofthe bond strength has been observed (up to 26%) as well as a similar decrease of the maximum bondstress; in addition the aged specimens have shown a more fragile behavior. On the basis of the obtainedresults the empirical coefficient, reported in the available Italian Guidelines, to determine the FRP-masonry bond strength seems still effective when the system FRP-masonry is aged in water once theproperties of the aged materials are considered in the provided relationships.

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1. Introduction

The need of structural interventions on masonry constructionsis increasing in the recent years for several reasons: the low sus-tainability of demolition and reconstruction instead of repairing,the high vulnerability to seismic actions experienced during earth-quakes, the changed usage that requires structural upgrading, theimportance of conservation when referring to buildings of archi-tectural and historical value. Traditional techniques for both globaland local interventions are generally based on the use of steel andreinforced concrete: steel ties are considered for connecting struc-tural elements or for eliminating thrust of arches and vaults; injec-tions of mortar or reinforced grouted perforations are employed torepair damaged masonry or upgrading masonry structures underboth gravity and seismic loads; very common is also the applica-tion of single or double reinforced concrete leaves to improve

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S. Sciolti), antonietta.aiello@.it (M. Frigione).

stiffness and strength; finally the introduction of additional struc-tural elements (reinforced concrete walls or steel bracing) is con-sidered for bearing horizontal actions. The testing by time andthe deeper knowledge of the structural behavior, mostly underseismic actions, evidenced the main weakness of the traditionaltechniques: the durability aspects when using steel; the increasedmass when concrete or reinforced concrete is applied, involvinghigher gravity loads and, thus, rising seismic forces. Other draw-backs are related to aesthetic and functionality because of the pos-sible obstruction of new areas, long time of interventions andtherefore of activities interruption, conservation when referringto historical buildings and monuments. The last issue is of greatrelevance for constructions supervised by the National Offices forHistorical Heritage Preservation, for which the compatibility andreversibility of interventions are primarily addressed.

In the last decade, the use of FRP composites for repair and/orupgrading existing building has been proved an effective solution,able to overcome some of the drawbacks experienced with tradi-tional interventions. Several scientific works have been devotedto the use of FRP on existing concrete structures, the most recentfounds being reported in [1–4], as well as different codes orguidelines are available in many countries [5–10]. The application

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of FRP on masonry structures, even if considered a promising solu-tion, has received less attention from researchers and only in the re-cent years the Italian Research Council published a guideline in thisfield [11]. Moreover it is well known that the assessment of designguidelines of general validity is even more difficult in this contextbecause of the great variability of masonry and construction typol-ogies worldwide as well as the different approaches in differentcountries for conservation. On the other hand it is well recognizedthat the reliability of the strengthening technique by using EBR(externally bonded reinforcement) made by FRP materials dependsin a large extent on the bond between the reinforcement and thesubstrate and therefore on the ability of stresses transfer at theinterface. Generally masonry wall strengthened by external FRPsheet may undergo the crisis by both local and global failure modes,namely the cracking of masonry in tension, the crushing of masonryin compression, the shear-sliding of masonry, the failure of the FRPreinforcement, and, finally, the delamination of FRP from masonrysubstrate. The last mechanism is considered very dangerous ascausing a brittle and premature collapse. In addition, under seismicactions the accomplishment of the capacity design principle re-quires that the debonding failure never takes place before the com-pression crushing of masonry. The debonding mechanism may beclassified in two main modes: plate end debonding (when debond-ing starts at the ends of the reinforcement) and intermediate crackdebonding (when debonding starts from mortar joints or masonrycracks). When the FRP reinforcement is subjected to tensile stresses(at its ends or in its area around cracks or mortar joints) the FRP –masonry interface undergoes high tangential stresses localized onvery short length, starting from the discontinuity section. Aimingto evaluate the maximum force that can be transferred by the rein-forcement (from a discontinuity section) prior to debonding specificbond analysis and testing should be assessed.

Some research works have been devoted to the analysis of thebond between FRP reinforcement and masonry. The bond behaviorand load transfer mechanisms at the FRP-masonry interface werefound basically similar to those of FRP-concrete joints. In fact bondtests evidenced the occurrence of a dangerous mechanism of fail-ure due to delamination, even more pronounced when FRP stripsare glued to historic masonry, characterized by poor surface prop-erties. [12–16]. The research work discussed in [15] emphasizes asthe failure delamination is caused by low values of the fracture en-ergy, varying in the range 0.21–0.52 N/mm; in addition experi-mental values of the ultimate slip have been found higher thanthose registered for FRP-concrete bonded joints. In [16] the effectsof a large amount of resin impregnating the porous masonry sub-strate has been also analyzed, evidencing as that parameter couldlimit the extension of existing bond models to the case of masonry.

Moreover the interface behavior can be affected by the environ-mental action or other aggressive agents, that could greatly com-promise the durability of the intervention. It is well recognizedas the performances in terms of durability is very difficult to eval-uate, since it depends on the intrinsic durability of not only the FPRsystem used to rehabilitate the structure, but also on the integrityof the adhesive bond between the FRP and the stone substrate, andon the durability of the substrate itself [17–37].

Few studies have been devoted to the analysis of the durabilityof masonry strengthened by FRPs materials and a deeper knowl-edge in this field is an important challenge of the scientificcommunity.

A finite-element modeling procedure for analyzing moisture-induced stresses in a multi-layered structure made by distinct per-meable materials is reported in [17–20]. The effect of a temperaturegradient on the moisture distribution has been also investigated,particularly referring to masonry elements made by concrete blocksand externally reinforced by FRP sheets.

The research study reported in Ouyang and Wan [21] concernsthe effect of the Interface Region Relative Humidity (IRRH) on thebond between CFRP and concrete. In particular, a relationship be-tween the residual thickness of concrete (RTC) and the IRRH hasbeen found, being RTC the concrete layer attached to the reinforce-ment after the FRP delamination. The moisture effects on bonddurability, referring to FRP-concrete joints, have been extensivelystudied using experimental methods [21–37]. The obtained resultsclearly evidence as moisture plays an important role in the durabil-ity of bond between FRP and concrete, affecting the whole struc-tural response of the strengthened elements. In fact, moisturereduces the fracture energy up to debonding; often causes the fail-ure mode changing from cohesive, within the concrete substrate,to adhesive, at the interface; involves a decrease of the ultimatecapacity of FRP strengthened reinforced concrete beams. As emerg-ing from the experimental researches performed, different param-eters affect the fracture energy value of the FRP-concrete joints,namely the environmental relative humidity, the specimen dimen-sion configuration, the material diffusion properties, the test ap-proach, and the surface treatment of the specimens. As aconsequence the assessment of a model of general validity, defin-ing the bond durability in presence of water/moisture, is still acomplex task.

In the present paper the effect of water on the durability of theinterface FRP-masonry has been analyzed. To this aim an investiga-tion on bond between FRP sheets and natural calcareous stones hasbeen carried out after immersion in water for different periods oftime. The utilized ‘‘Lecce stone’’ is traditionally employed in ma-sonry constructions of the Salentine Peninsula, in southern Italy,and is typically of the baroque architectural monuments of theregion. The commercial reinforcements used are made by unidirec-tional one-layer carbon fibers and an epoxy based matrix, appliedat the substrate by the hand lay-up technique. Each utilized mate-rial (stones, adhesive, composite sheets) has been also aged inwater following the same above-mentioned procedures in orderto investigate the influence of the durability of any single compo-nent on the bond durability. In particular, the effect of water onepoxy resins and FRP composites have been widely presentedand discussed in [38], while results regarding the mechanical deg-radation of the utilized stone following the treatment investigatedare reported here. As concerns the bond tests, the bond strengthand the kind of failure have been analyzed, as well as the strainsand stresses distribution at the interface. In order to estimate thedegradation caused by water on the bond performance and onmaterials properties, the results obtained after conditioning havebeen compared with those referring to standard conditions. Onthe basis of the provisions given by the Research National CouncilBulletin [11] the possibility of calibrating design relationships, ableto take into account the durability aspect, is discussed.

While several specimens have been tested for materials charac-terization, few bond tests have been investigated at this stage ofthe research work. On the other hand, the main scope of this workhas been the understanding of aging effect on materials involved(substrate and reinforcement) and the detailed analysis of its influ-ence on the bond behavior under the most severe conditions. Theresults obtained could contribute to furnish useful design indica-tions, even if lying on the safe side, and to address the future devel-opment of the research.

2. Experimental investigation

2.1. Materials test

In this study the investigated substrate was a natural stonewidely utilized in the South of Italy and called ‘‘Lecce stone’’. The

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Lecce stone is calcareous in nature, characterized by high porosity,easy workability, good aesthetic and satisfactory mechanical andphysical properties, even if highly dependent on the quarry loca-tion. In particular, it is characterized by a sedimentary successionof regularly stratified limestone, constituted by a fine-grained bio-calcarenite, basically made of calcareous microfossils as foramini-fers, in an abundant matrix of fine calcareous detritus, oftenmixed with little particles of clay minerals. Silicatic minerals andiron oxides are present in very little amount [39]. Lecce stone ishighly packed but poorly cemented, because of the low amountof microcrystalline calcite, with cementing function, dispersedwithin the matrix. As a result it shows widespread porosity andsmall pore size, smaller of 50–60 l [40]. In this paper, among thedifferent varieties of local stone, that extracted from the quarriesof Cursi (Lecce) was used.

As reinforcing system one layer of a commercial Carbon FiberReinforced Polymer (CFRP) sheet was employed. That unidirec-tional carbon sheet was applied at the substrate by the hand-layuptechnique, using two different epoxy resins and by following thesupplier’s indications. The first resin is a preparatory coating (atwo part epoxy primer) able to ensure a better adhesion betweenthe reinforcement and the substrate. A layer of epoxy adhesive isnext applied before the carbon sheet; after finishing with a secondlayer of epoxy adhesive a roller is used to remove air voids and al-low better impregnation of the resin. Even though the curing timesuggested by the supplier for the resins was of about 7–14 days atambient temperature (�23 �C), a higher curing time was employed(about 10 months), following the results of previous studies per-formed on cold-curing epoxy resins used as adhesives and matricesfor composites [41,42]. An appropriate curing time is mostlyneeded when the influence of environmental or other aggressiveagents on the mechanical performances are addressed, in order toguarantee the attainment of a ‘‘stable’’ system and, allowing to sep-arate and analyze the effects that can be totally attributed to theperformed treatments [38].Mechanical properties of materials havebeen experimentally evaluated. The compressive strength of stoneshas been determined by compression test on cubes of 71 mm high,according to [43], while the flexural strength has been evaluated bythree-points bending test on prisms of 20 mm � 30 mm � 120mm,according to [44]. The Young’s modulus of stones has been deter-mined by compression test on prisms of 50mm � 50mm � 200mm[45]. It has been evaluated in terms of secant elastic modulus, be-tween zero and 50% of the ultimate stress. The possible materialanisotropy has been also considered for mechanical characteriza-tion; therefore the test were performed both on specimens loadedin the direction perpendicular to the stratification layers (\) andin that parallel to the stratification layers (//) of the limestone, refer-ring to the evaluation of the flexural strength and the elastic mod-ulus. The effect of anisotropy on compressive strength has beenalready investigated by authors [46] and its influence resultedalmost negligible.

Cured samples of epoxy resins (primer and adhesive) were real-ized in order to evaluate the effect of water on each component ofthe strengthening/repairing system. Once identified eventual vul-nerabilities of any component, their effects on the performanceof the whole system can be investigated. The tensile properties(modulus of elasticity and tensile strength) of the primer and adhe-sive resins were evaluated following the appropriate code [47] andcalculated by averaging the results of at least five specimens. The‘‘in-plane’’ tensile properties of the CFRP (ultimate tensile strainand modulus of elasticity) were evaluated on composite’s speci-mens made by the wet lay-up technique [48]; results were referredto the net area of the FRP system, namely the area of the fibersexcluding that of the resin. The net area is typically used for wetlay-up FRP systems, as suggested by available guidelines [5,7]. At

least five specimens of composite were tested and the resultsaveraged.

In order to evaluate the effect of water on the mechanical prop-erties of the materials under analysis, mechanical tests were per-formed on specimens of composites, both epoxy resins and Leccestone at different time of immersion in water up to the saturationcondition.

For the two epoxy resins and the composite a proper standard[49] was followed to evaluate the water absorption at differenttimes. However it is worth to evidence that a different procedurewith respect to that provided by the code was used to dry the spec-imens before the immersion in water. It consisted of storing thesamples in a desiccator with silica gel at ambient temperature untiltheir weight reached a constant value. More details about theexperimental investigation regarding the reinforcement systemare reported in [38].

The content of water absorption was evaluated also for the Leccestone specimens; they were dried in an oven for 24 h at 105 ± 5 �Cand under vacuum conditions, thus immersed in distilled water, at23 �C ± 2 �C. Specimens were periodically taken from water, wipedwith a dry cloth and weighed in order to calculate the percentageof water absorption. The procedure stopped at a percentage differ-ence in weight between two successive measurements of almost0.1%.

The water absorption, for all the involved materials (resins, FRPand stone) was expressed as the increase in weight percent, i.e.:

Percent Water Absorption ¼Wet weight� Dry weightDry weight

� 100 ð1Þ

The number of stone specimen’s tested under standard condi-tions and left in water for different periods of time was at least five.

2.2. Bond test

Bond tests have been assessed as a type of double lap shear test,in which the reinforcing sheet has been bonded at two oppositesides of the lecce’s ashlars, measuring 100 mm � 100 mm �250 mm. The specimens have been inserted into a steel box fixedat the bottom crosshead of a 300 kN capacity universal testing ma-chine. The free end portions of the sheet have been clamped at thetop crosshead by a special gripping device, able to transmit the ten-sile load to the reinforcement via a steel pin, inserted thorough adrilled hole in the composite sheets; premature damage in that re-gion has been avoided providing an adequate bond of the sheetswithin two steel plates. The tests have been carried out under dis-placement control, with a displacement rate of 0.2 mm/min. Thetest set-up (Fig. 1) and the test procedure have been specifically as-sessed and more details reported in previous papers [12,46,53]. Thestone specimens were reinforced with CFRP sheets providing abond length of 150 mm, while a 50 mm length of the sheet was leftunbonded in order to limit specimens imperfections [46]. Sevenelectrical strain gauges have been glued on both sides of the speci-mens in the load direction, corresponding to the fibers direction ofthe sheets (Fig. 2). Strain gages were also glued on the unbonded re-gion to determine the tensile modulus of elasticity of the FRPlaminate.

The interface FRP-masonry has been analyzed in terms of dura-bility after two different period of immersion in distilled water,namely 8 and 25 weeks. Similar specimens tested in standard con-ditions have been also analyzed for comparison. Two identicalspecimens were tested for each analyzed case, as better specifiedin Table 1. The aim of the present research work has been theanalysis of bond in extreme conditions under the effect of water,

Fig. 1. Test set-up.

Fig. 2. Details of the applied sheet and scheme of electrical strain gauges.

Table 1Tested specimens.

Immersiontime (weeks)

Specimens label

0 1–0w2–0w

8 1–8w2–8w

25 1–25w2–25w

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corresponding to the complete saturation of materials. The resultsobtained in these conditions could allow to define a upper boundindication of bond degradation. In real applications the structuresare not completely immersed in water, however the water capil-lary absorption, very common in old masonry structures, couldcause a saturation condition for some parts of the structures, asthat achieved for laboratory samples.

3. Result and discussion

3.1. Materials

Results obtained on resins and composites have been widelypresented and discussed in [38]; in the present paper a synthesisof main results obtained from tensile tests is reported (Table 2).In Table 2 it can be observed as the yield strength of the primershowed a decrease of about 50%, after 24 weeks of immersion inwater, corresponding to the saturation time. The tensile modulusdecreased of about 50% with respect to the value in standard con-dition, under the same treatment. In the adhesive, the tensilestrength decreased of 23% and 45%, after 60 days and 200 days ofimmersion in water, respectively. The tensile modulus decreasedby about 60% after 200 days of immersion, while the decay wasof 36% after 60 days. Referring to the CFRP, both tensile strengthand tensile modulus remained almost constant. In fact, as reportedin [38], the presence of water has an insignificant effect on the ten-sile mechanical properties of unidirectional composites, in the fi-bers direction, mostly when made by a single ply.

Table 2Tensile mechanical properties of the resins and the composite before and during their immersion in water (curing time = 45 weeks).

Immersion time Maximum strength, rmax (MPa) Elastic Young modulus, E (GPa) Poisson coefficient, m

(weeks) ½ffiffiffihp�

Primer 0 0 48.8 ± 2.4 3.02 ± 0.69 0.35 ± 0.118 37 25.2 ± 1.3 1.24 ± 0.11 0.36 ± 0.03

13 47 24.8 ± 1.3 1.49 ± 0.12 0.38 ± 0.0227 67 17.6 ± 3.0 0.89 ± 0.23 0.37 ± 0.07

Adhesive 0 0 44.3 ± 4.7 2.95 ± 0.26 0.31 ± 0.048 37 33.8 ± 3.6 1.88 ± 0.35 0.33 ± 0.01

14 48 28.0 ± 6.2 2.04 ± 0.34 0.41 ± 0.0629 70 24.0 ± 2.8 1.20 ± 0.14 0.39 ± 0.03

CFRP 0 0 2319a ± 216 198.84 ± 27.10 –10 41 1879 ± 711 306.45 ± 113.00 –14 48 1685 ± 636 234.93 ± 144.78 –25 65 2034 ± 539 221.74 ± 20.83 –29 70 1630 ± 713 262.92 ± 44.84 –

a Curing time = 36 weeks.

Table 3Mechanical properties of the stones (Lecce’s stone) after immersion in water; thepercentage reduction of the mechanical properties after the treatment is reportedwithin the parentheses.

Immersion time Compressive strength (MPa)

(weeks) ½ffiffiffihp�

0 0 31 ± 214 48 14 ± 1 (�54.8)25 65 13 ± 2 (�57.4)

Secant elastic modulus (MPa)

// \0 0 22,542 ± 3102 18,144 ± 17738 37 12,299 ± 798(�45.4) 9816 ± 1017(�45.9)17 54 13,615 ± 3615(�39.6) 9850 ± 1295(�46.7)

Flexural strength (MPa)

// \0 0 8.2 ± 0.8 7.3 ± 1.08 37 4.9 ± 0.5 (�40.1) 4.8 ± 0.5 (�34.6)17 53 4.6 ± 0.2 (�44.0) 3.9 ± 0.4 (�47.0)23 62 4.8 ± 0.5 (�42.1) 4.8 ± 0.4 (�34.2)

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In Fig. 3, the value of water uptake for all types of the stonespecimens is reported as a function of the square root of immersiontime. The weight gain shown in the Fig. 3 is averaged on more than80 specimens utilized for compression test and 160 specimensrealized for bending test. After the first measures, performed after24 h, the increment of the water uptake remains always limited(almost 0.1% with respect to the initial measured increment),therefore it can be argued that the most relevant amount of waterwas absorbed during the first day of immersion. A similar trend isreported in [50], where the investigation concerned a type of cal-carenite stone from a Spanish quarry. In that study the variationof water adsorbed appears very low since few hours of immersion.However after a prolonged time of investigation, a small amount ofwater is still absorbed by the stones, as can be observed fromFig. 3; this phenomenon can be justified by the presence of verysmall pores which slowly saturate [51].

The stone properties were drastically affected by the aging pro-cess. In fact the strength and the elastic modulus reduced up to 50%(Table 3) with respect to those measured in standard conditions.The main reason of the observed mechanical degradation can belinked to the high stone porosity; in fact the water seeped intothe stone causes internal damage, due to the pressure of the liquidwithin the pores. In particular the compression strength is almosthalved respect to that corresponding to the dry condition. Increas-ing the days of immersion in water the variations of the mechani-cal properties becomes irrelevant. The relevant reduction of themechanical performances for the utilized type of stone has beenfound also in other studies, where the decrease of the uniaxialcompressive strength in presence of water is reported [52].

Fig. 3. Moisture absorption curves of the compression and bending specimen.

Analyzing the flexural strength and the secant elastic modulusobtained for all tested specimens it is evident as the behavior ofmaterial can be considered isotropic in relation to these mechani-cal property, as already found for compressive strength. In fact, thescatters between specimens loaded in direction parallel and per-pendicular to the stratification layers of the limestone are withinthe experimental variation (see Table 3). As already observed refer-ring to the compressive strength, a significant reduction of the flex-ural strength and of the secant elastic modulus was obtained afteraging in water, up to 47%. Comparing the decrease of flexuralstrength and of the secant elastic modulus at the different periodsof immersion in water, it appears almost constant, accordingly tothe result obtained for the compressive strength.

3.2. Bond

All test results are summarized in Table 4, where smax is thebond strength; smax is the maximum bond stress; Fsmax is the loadvalue corresponding to smax; le is the optimal bond length.

The optimal bond length refers to the length of the sheet wherestresses are effectively transferred at the interface under servicecondition, namely before the delamination process starts. The lelength is measured from the loaded end to the distance corre-sponding to negligible strain values; its determination has been

Table 4Bond test results.

Name’s specimens Fmax (kN) smax (MPa) Fsmax (kN) le (mm)

0w 9.83 2.87 6.75 1241–8w 7.67 2.72 5.61 962–8w 7.49 1.95 4.71 1211–25w 7.10 1.95 5.03 1102–25w 7.46 2.18 5.72 118

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performed here considering as negligible the strain values equal to0.001 e0, being e0 the strain of the unbounded sheet, and at loadlevels less than 50% Fmax.

Whit regard to the maximum bond stress and the optimal bondlength, the corresponding average values registered on both theapplied sheets are reported in Table 4 for each specimen.

The ultimate load has been determined by halving the appliedload, in the hypothesis of symmetric behavior at the two oppositesides of the specimen. For the unaged specimens the average valuefor two identical specimens are reported.

Results reported in Table 4 show that the ultimate load, re-corded in standard conditions, is higher than that correspondingto aged specimens. The average decay of bond strength is of 23%and 26% for specimens tested after 8 weeks and 25 weeks ofimmersion in water, respectively; on the other hand its degrada-tion seems to be independent on the immersion period, at leastreferring to the performed analysis. This result appears basicallyrelated to the decay of mechanical properties of the substrate, asfurther confirmed in the following. In fact, for both aged and un-aged specimens, the failure occurred by debonding of the FRP rein-forcement from the substrate at the masonry face-level, with a thinlayer of calcarenite stone remaining attached to the delaminatedsheet (Fig. 4). Such kind of failure is expected when the tensilestrength of the adhesive at the interface is higher than that ofthe substrate [11,46]. Analyzing the properties of materials ob-tained in the present investigation and reported in Tables 2 and3, it is evident as the mechanical performances of the substrateis very much lower than those of the resins, although the relevantdecay of the strength and the elastic modulus registered afterimmersion in water for both the primer and the adhesive.

With reference to the almost negligible decay of bond strengthobserved between 8 weeks and 25 weeks of immersion in water it

Fig. 4. Typical failure for both a

can be justified by the similar trend observed for the decay ofmaterials properties.

A similar result is observed for the maximum bond stress andthe corresponding applied load. In fact, the average maximumbond stress evaluated considering all aged specimens is2.23 ± 0.37, corresponding to an average load value of 5.11 ± 0.49,with a decrease of 22% with respect to the maximum bond stressof control specimens.

On the other hand the interface stiffness seems unaffected bythe conditioning treatment, in agreement with results reportedin [24,25], in spite of the plasticization effect expected in the resin[24,25,38]. In fact, the optimal bond length is almost the same foraged and unaged specimens, the average value determined for allspecimens is 112 mm ± 11 mm (COV = 10%). Furthermore, compar-ing the strain values measured along the sheet, at load levels below60% of the ultimate load, for aged and unaged specimens, the scat-ters appears negligible, confirming again the slight variability ofthe stiffness under service condition. Typical strain paths are re-ported in the Fig. 5, where the strains distribution is fairly expo-nential regardless of the aging regime.

Specifically in Fig. 5a and b the comparison between the strainspath along the reinforcing sheet recorded for unaged specimensand specimens tested after 8 weeks and 25 weeks of immersionin water, is drawn respectively: The load levels analyzed in the fig-ures correspond to 35% and 50% of the ultimate load.

In the Fig. 6, the strain values versus applied load are plotted atdifferent strain gauges (ei, i = 0 � � � 7) positions, for both aged andunaged specimens. Results are reported for clarity referring tothe average value of those recorded at the two sides of each spec-imen. The curves show that at low load level the stress is almostcompletely transferred to a small length of the sheet, near theloaded end; when the applied load increases and the de-bondingat the loaded end starts, the optimal bond length is shifted alongthe sheet. This occurrence is enough clear analyzing the curves re-ported in the Fig. 6, where a sudden increase of strain is registeredat the delamination starting by strain gauges applied near theloaded end; at the same time the strain values rise in otherpositions, where an almost zero value of strain was maintained be-fore the delamination starting.

In the Figs. 7 and 8 results obtained for specimens after immer-sion in water are compared with those of specimens in standardconditions. The mechanisms of stresses and strains transfer at

ged and unaged specimens.

Fig. 5. Strains values along the sheet at different load levels varying the time of immersion in water: (a) control specimens and specimens tested after 8 weeks of immersionin water. (b) control specimens and specimens tested after 25 weeks of immersion in water.

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the interface are similar, as before explained. However the loadlevel corresponding to the debonding initiation (marked in theFigs. 6–8) is lower for aged specimens. In addition a more fragilebehavior can be evidenced after the immersion in water, due tothe short extension of the non-linear stage. In fact, the load valuecorresponding to the attainment of the maximum bond stress isequal to 70% and 60% of the ultimate load for aged and unagedspecimens, respectively.In order to evaluate the slip at the inter-face the strain values registered along the sheet are utilized. In par-ticular, referring to a small length of the sheet, dx, and imposingthe strains compatibility the following relationship is obtained:

dsdx¼ esðxÞ � ebðxÞ ð2Þ

where s is the slip between stone and reinforcement, es the strainvalue within the sheet, eb is the strain value within the stone block.

Neglecting the stone deformability and the slip value at the un-loaded end of the sheet, the slip at the loaded end is evaluated as:

s ¼Z L

0esðxÞdx ð3Þ

Using strains measured at discrete points along the sheet, theapproximate slip value can be determined as follows:

s ¼P

iesi� Dxi ð4Þ

being esithe recorded strain and Dxi the sheet length between two

contiguous strain gages.The bond stresses distribution within the reinforcement have

been evaluated imposing the translational equilibrium conditionof the sheet length Dx:

sðxÞ ¼ ts � Es �esi� esiþ1

xiþ1 � xið5Þ

where s(x) is the bond stress in position x measured from theloaded end of the sheet, esi

; esiþ1 is the strain values in correspon-dence of two contiguous strain gages; xi, xi+1 is the position of twocontiguous strain gages measured from the loaded end of the sheet;ts is the thickness of the sheet glued at the stones surface; Es is theelastic modulus of the sheet, determined on the basis of strainvalues registered on the unbonded sheet of the tested specimens,as reported in more details in previous works [2,3,36,45].

In the Fig. 9 the average bond stress–slip curves are drawn foraged and unaged specimens. In the first stage a linear trend canbe observed in all cases, up to the maximum bond stress is at-tained. When analyzing the system stiffness Ga/s (Ga = shear mod-ulus of adhesive, s = adhesive thickness) the difference betweenaged and unaged specimens are negligible, as already discussedabove. On the other hand, a different behavior after the attainmentof the maximum bond stress can be observed; in fact, for speci-mens left in standard conditions, a greater extension of the post-peak curve is obtained with an ultimate slip value of about0.12 mm, indeed almost 30% higher than that found for aged spec-imens. This last result further confirms the more fragile failure ofthe interface caused by the aging in water.

4. Design relationships

As well known, when strengthening stone blocks by FRP com-posites, a premature mechanism of failure may occur by debonding(loss of bond). The delamination, as for concrete substrate, maytake place within the adhesive, between stone and adhesive, with-in the stone itself, or within the FRP reinforcement. [11]. When

Fig. 6. Strain values versus applied load at different strain gauges (ei, i = 0 � � � 7) positions: (a) specimens in standard conditions; (b) and (c) specimens tested after 8 weeks ofimmersion in water. (d) and (e) specimens tested after 25 weeks of immersion in water.

3246 M.S. Sciolti et al. / Composites: Part B 43 (2012) 3239–3250

proper installation is performed, debonding is expected within thesubstrate, because the adhesive strength is typically much higherthan the stone tensile strength. Therefore the sheet delaminatesremoving a thin layer of the substrate, whose thickness may rangefrom few millimeters to more relevant stone portions [11].

Experimental bond tests, performed on masonry elements, showthat the bond capacity, namely the ultimate value of the force trans-ferred from FRP reinforcement to the support prior debonding, de-pends on the bond length, lb, growing with lb up to a maximumbond length, le. In fact a further increase of the bond length doesnot involve a higher bonding force to be transferred. The length leis the so called ‘‘optimal bond length’’ and corresponds to the min-imal bond length able to carry out the maximum anchorage force.

In addition, the bond strength varies depending on the mechanicalproperties of the stone and FRP [11]; therefore, a decay of the bondcapacity is expected while a mechanical degradation of the materi-als occurs. Some available codes and guidelines [11,7] suggest theintroduction of environmental coefficients to take into accountthe material degradation while evaluating the reinforcementmechanical properties; however, these environmental coefficientsare not considered when determining the bond strength. In othercases [5] the protection of the strengthened structural element isonly advised.

In order to evaluate the influence of water immersion on bondcapacity, the relationships provided by the CNR-DT 200/2004 Bul-letin [11] are calibrated on the basis of experimental results.When

Fig. 6 (continued)

Fig. 7. Comparison between strains distribution of specimens immersed in water for 8 weeks and specimens left in standard condition: (a) specimen 1–8w and (b) specimen2–8w.

M.S. Sciolti et al. / Composites: Part B 43 (2012) 3239–3250 3247

Fig. 8. Comparison between strains distribution of specimens immersed in water for 25 weeks and specimens left in standard conditions: (a) specimen 1–25w and(b) specimen 2–25w.

Fig. 9. Experimental bond stress–slip curves at loaded ends for different specimens.

Table 5c1a, c1b values.

Name’s specimens c1a c1b

0w 0.017 0.0171–8w 0.010 0.0182–8w 0.010 0.0171–25w 0.009 0.0162–25w 0.009 0.017

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debonding involves the first masonry layers and the bond length islonger or equal to the optimal bond length, the maximum value ofthe transferred force, Fmax, shall be expressed as follows [11]:

Fmax ¼ bs

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2tsEsCF

pð6Þ

where bs is width of the sheet glued at the stones surface and thevalue, CF, of the specific fracture energy is given as:

CF ¼ c1

ffiffiffiffiffiffiffiffiffiffiffiffiffifmfmtm

pð7Þ

where c1 is an experimentally determined coefficient, fm is the aver-age strength of calcareous stone, fmtm is the average tensile strengthof calcareous stone.

From Eq. (6) the specific fracture energy, CF, can be evaluated:

CF ¼F2

max

b2s 2tsEs

ð8Þ

M.S. Sciolti et al. / Composites: Part B 43 (2012) 3239–3250 3249

By using Eqs. (7) and (8) the c1 value can be expressed as:

c1 ¼F2

max

b2s 2tsEsffiffiffiffiffiffiffiffiffiffiffiffiffifmfmtm

p ð9Þ

On the basis of experimental results two values of c1 were cal-ibrated. The former (c1a) was obtained introducing in Eq. (9) thematerials properties, referred to standard conditions while the sec-ond coefficient (c1b) was calibrated by employing in Eq. (9) theproperties of aged materials. Obtained results are reported in Table5.

From Table 5 it can be observed that the c1b coefficient is almostconstant and equal to 0.017 with a coefficient of variation of 6%,while the c1a coefficient for aged specimens resulted 43% lowerthan that evaluated for unaged ones. In standard condition, thesuggested value for c1 is 0.015 [11]; therefore for design purposesavailable relationships for the evaluation of bond strength seemstill effective when the interface substrate reinforcement is ex-posed to the adverse effect of water, once the decay of the mechan-ical properties of materials are taken into account. It also evident asa significant reduction of the c1 (almost 40%) coefficient should beintroduced in the Eq. (7) when only the materials properties instandard conditions are available, at least for the kind of substrateand treatment investigated.

5. Conclusion

The bond behavior of calcareous stones strengthened by FRPsheets was analyzed by a double lap shear test after aging in water;the treatment in water was extended to each utilized material,namely substrate, epoxy and composite. The mechanical behaviorof the substrate after the aging in water is presented and discussedin this paper while the effects of the water on epoxy and compositewas detailed in a previous paper. The bond between the stone sub-strate and the CFRP sheet has been investigated in terms of bondstrength, maximum bond stress, optimal bond length, strains pathat the interface and bond stress–slip law. Some considerationsabout the possible extension of available relationships to the bondstrength evaluation in presence of water have been also given.

On the basis of results obtained the following considerationscan be remarked:

– The mechanical properties of the utilized natural stones are sig-nificantly affected by the presence of water, with a reductions instiffness and strength of about 50%.

– A decay of the maximum bond stress and of the bond strengthhas been observed (up to 26%), as well as a more fragile bondbehavior in the case of conditioned specimens. In all cases thedebonding involved the first masonry layers, due to the weak-ness of the substrate with respect to the reinforcing system,both in standard conditions and after the aging in water.

– The optimal bond length resulted almost unaffected by theaging in water, its value averaged on all specimens is112 mm. Therefore the water seems to have little influence onthe stiffness of the interface, as confirmed also through theanalysis of the strains path along the sheet during loading. Infact, the strain values measured along the sheet, at load levelsbelow 60% of the ultimate load, for aged and unaged specimens,were enough similar.

– The bond stress–slip curves have been evaluated for aged andunaged specimens. In the first stage an almost linear trend, upto the maximum bond stress was attained in all cases; the dif-ference between aged and unaged specimens resulted negligi-ble in terms of initial stiffness, as already discussed above. Onthe other hand a different behavior after the attainment of the

maximum bond stress has been observed; in fact for specimensleft in standard conditions a greater extension of the post-peakcurve was obtained.

– The available relationship, provided by the Italian Technicaldocument [11] for evaluating the bond strength in standardconditions, seems still effective in the case of aged specimens,once the decay of the mechanical properties of the utilizedmaterials is considered. Otherwise an appropriate environmen-tal coefficient should be added for taking into account thereduction of the bond strength.

In conclusion, the obtained results clearly evidence the need ofan accurate evaluation of the bond behavior when adverse envi-ronmental condition, as the presence of water/moisture, are fore-seen during the service life of FRP strengthened masonrystructures. Even if further investigations are needed and interest-ing, the performed research furnishes a first contribute to theunderstanding of the durability of bond in presence of water whenthe substrate is made by calcareous natural stones. In addition ashort term analysis is suggested, namely after a period of immer-sion in water for few hours, in order to accurately define the satu-ration period of the analyzed stone and the influence of water onbond behavior when the weakest component of the system (thestone) is still unsaturated.

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