Moisture exposure has been observed to be one of the degrading environmental agents which affects the durability of the FRP-strengthened elements by changing the constituent material or bond properties. This paper presents the experimental investigation on the effects of moisture on the pull-off bond strength of GFRP-strengthened brick specimens. The specimens have been prepared following the wet lay-up procedure and exposed to constant moisture level of 100% R.H. at 23̊C for 8 weeks. The degradation in the bond performance has been investigated by performing pull-off tests on the conditioned specimens after 4 and 8 weeks of exposure. The reversibility of the bond degradation has been also studied by storing some specimens in the laboratory conditions for 1 week after conditioning and before performing the pull-off tests. Comparative analysis has been performed and the main results are presented and discussed

Ghiassi, Bahman

Lourenço, Paulo B.

Marcari, Giancarlo

Oliveira, Daniel V.

Silva, Márcio M. M.

Universidade do Minho: RepositoriUM

Page 1 of 8Moisture effects on the bond strength of FRP-masonry elementsBahman GHIASSIPh.D. studentISISE, University of Minho, Dept. Civil Engineering, Guimarães, Portugal*bahmanghiassi@civil.uminho.ptMarcio M. M. SILVAM.Sc. studentUniversity of Minho, Dept. Civil Engineering, Guimarães, Portugala53486@alunos.uminho.ptGiancarlo MARCARIAssistant ResearcherISISE, University of Minho, Dept. Civil Engineering, Guimarães, Portugalmarcarigia@civil.uminho.ptDaniel V. OLIVEIRAAssistant ProfessorISISE, University of Minho, Dept. Civil Engineering, Guimarães, Portugaldanvco@civil.uminho.ptPaulo B. LOURENÇOProfessorISISE, University of Minho, Dept. Civil Engineering, Guimarães, Portugalpbl@civil.uminho.ptAbstractMoisture exposure has been observed to be one of the degrading environmental agents whichaffect the durability of the FRP-strengthened elements by changing the constituent material orbond properties. This paper presents the experimental investigation on the effects of moistureon the pull-off bond strength of GFRP-strengthened brick specimens. The specimens havebeen prepared following the wet lay-up procedure and exposed to constant moisture level of100% R.H. at 23 C for eight weeks. The degradation in the bond performance has beeninvestigated by performing pull-off tests on the conditioned specimens after four and eightweeks of exposure. The reversibility of the bond degradation has been also studied by storingsome specimens in the laboratory conditions for one week after conditioning and beforeperforming the pull-off tests. Comparative analysis has been performed and the main resultsare presented and discussed.Keywords: Bond, degradation, FRP composite, Masonry, Moisture, Pull-off.1. IntroductionExternal strengthening of masonry structures with fiber reinforced polymers (FRPs) hasbecome a popular method in the last years. The efficiency and reliability of this strengtheningPage 2 of 8technique depends intrinsically on the bond between the composite material and masonrysubstrate.Although previous studies have shown the advantages of using FRP composites instrengthening the structures, the long-term performance of the bond behavior between theFRP composite and masonry support is still unknown. Therefore, investigation of the long-term durability of the bond is a key issue in performance prediction of the strengthenedstructures in their service life [1-2]. Extensive experimental studies have been performed ondurability of bond in FRP strengthened concrete elements [3-6], while the available literatureon FRP-masonry elements is still few, see e.g. [7-9].The most common environmental factors which a strengthened element is exposed to duringits service life are moisture variations, temperature variations, and alkaline and acidicenvironments. Moisture has been observed to be one of the main deteriorating agents in thebonded specimens. On the other hand, most of the environmental factors and deteriorationprocesses are dependent on or coupled with moisture. Therefore a full understanding ofdeteriorating effects of moisture on the bond is a key step in durability modeling of FRP-strengthened masonry elements [10].In general, it has been observed that FRP materials can tolerate environmental conditions withsmall reductions in mechanical properties, while the substrate and adhesive properties maybehighly deteriorated. Temperature variations and moisture exposure conditions have alreadybeen found to reduce bond shear strength, fracture energy, and peak slip in FRP strengthenedconcrete elements. Moreover, the force-displacement diagrams of the bond behavior havebeen observed to show a non-linear trend at a lower applied force. The change of the failuremode from the cohesive failure to the substrate-adhesive interface failure has also beenobserved. These changes are usually attributed to thermal incompatibility, extensive moistureplasticization of the polymer adhesive, and additional breakage of the interfacial bonds. Sincethe available experimental results on durability and degradation mechanisms in FRPstrengthened masonry elements is still rare, performing comprehensive experimental andnumerical studies is mandatory in this field.The available information on FRP strengthened masonry elements shows that a significantreduction of bond strength is expected after wet-dry cycles (see e.g. [7] and [9]). Briccoli Batiand Rotunno [9] reported 17% reduction in shear strength in the CFRP strengthened masonryelements after 48 wet-dry cycles (10 hours of exposure in total). Aiello and Sciolti [7]reported 33% of reduction in shear bond strength in calcernite ashlar specimens strengthenedwith CFRP after 50 days of immersion in water.In this paper, the preliminary results of a comprehensive research attributed to the durabilityof FRP-masonry components being carried out at University of Minho are presented. The aimof the tests performed in this stage was to investigate the bond behavior in FRP strengthenedbrick specimens subjected to moist environment. In this regard, the GFRP strengthened brickspecimens were immersed in water and the changes in the bond strength was monitored afterdifferent periods of exposure. The effect of moisture conditioning was assessed usingstandard pull-off tests. Conclusions are presented regarding bond strength capacity, bondstrength degradation with time, and observed failure modes.2. Experimental tests2.1 Test outlineThe preliminary results of a comprehensive research attributed to durability of FRP-masonrycomponents being carried out at University of Minho are presented. The aim of the testsperformed in this stage was to investigate the bond behavior in GFRP-brick specimenssubjected to different moisture conditions. Two different moisture conditioning protocolsPage 3 of 8have been considered, in addition to laboratory-stored control specimens. The latter providedthe baseline against the results obtained from the conditioned specimens. The laboratorystorage conditions were typical ambient conditions averaging 23 C and 60% RH.The typical pull-off tests were used for investigating the tensile bond strength of thespecimens. The following paragraphs describe the specimens, the moisture conditioning andthe tests method considered in this study.2.2 Test specimensThe specimens were made of hand-made masonry clay bricks with dimensions of200x100x50 mm. Unidirectional glass fibers (MapeWrap G UNI-AX) were applied on thebrick surface following a wet lay-up procedure. The surface of the bricks was cleaned and twolayers of primer (MapeWrap Primer 1) were applied on the brick surface before application ofthe GFRP sheets. The GFRP sheets were glued on the prepared surface with a compatibleepoxy resin (MapeWrap 31). The specimens were cured in the laboratory conditions for 15days before exposure to moisture conditions.2.3 Material characterizationThe mechanical properties of masonry bricks were obtained according to the test standardsUNI EN 771-1[11] and UNI EN 8942-3[12] in terms of compressive strength, fcb, tensilestrength, ftb, and elastic modulus, Eb.Regarding the composite materials, the mechanical properties of GFRP coupons wereobtained according to ISO 527-1[13] in terms of tensile strength, ftf, and elastic modulus, Ef.Mechanical and thermal properties of the epoxy resin were investigated on samplespreviously cured for 15 days at room temperature. The basic tensile properties were obtainedfollowing the standard tensile tests, ISO 527-1[13]. The glass transition temperature (Tg) ofthe epoxy resin was obtained by means of the DSC (Differential Scanning Calorimetry)method. The epoxy samples were cured at room temperature 15 days before performing thetests. The thermal scans were carried out between 5°C and 200°C with a heating rate of10°C/min. The Tg was calculated as the mean value of four tests.Experimentally determined material properties of the bricks and composite material arepresented in Table 1. Here, C.o.V. is the coefficient of variation.(a) (b) (c)Figure 1. Material characterization tests; (a) Brick specimens; (b) GFRP coupons; (c) Epoxy resinspecimens.Page 4 of 8Table 1. Material properties.Masonry brick C.o.V.(%)Average compressive strength fcb (MPa) 19.8 2.5Average tensile strength ftb (MPa) 1.95 4.0Average elastic modulus Eb (GPa) 1.12 3.8GFRP couponsAverage tensile strength ftf (MPa) 1350 11.9Average elastic modulus Ef (GPa) 77.1 5.3Ultimate deformation ε (%) 1.86 20.2Thickness t (mm) 0.17 -Epoxy resinAverage tensile strength ftm (MPa) 31.2 5.4Average elastic modulus Em (GPa) 1.89 5.85Average shear modulus Gm (GPa) 1.64 6.1Glass transition temperature Tg ( C) 70 3.22.4 Moisture conditioningThe selected exposure is intended to investigate the influence of moisture on the bondbehavior in FRP-strengthened masonry components. The test specimens were exposed toconstant condition of 100% R.H. at 23 C for duration of 4 and 8 weeks. The 100% R.H. at23 C was provided by immersing the specimens in a water tab in a temperature controlledenvironment.For each exposure condition, five specimens were tested immediately after the exposure andanother five were stored in laboratory conditions for one week before performing the pull-offtests. In the latter case, the aim was to investigate the reversibility of the observed degradationafter moisture conditioning. Two pull-off tests were performed on each specimen resulting inten tests in total for each moisture conditioning. Five specimens were also tested without anymoisture conditioning to provide a baseline for the moisture exposed specimens.The complete conditioning plan is shown in Table 2.Table 2. Conditioning program.No.ofspecimensConditioning5 No conditioning5 100% R.H., 23°C (4 weeks)5 100% R.H., 23°C (4 weeks)+laboratory conditions (1 week)5 100% R.H., 23°C (8 weeks)5 100% R.H., 23°C (8 weeks)+laboratory conditions (1 week)2.5 Pull-off testsThe pull-off tests were performed based on a partial coring technique. In this method thetensile load is applied to the partial core through an aluminum disk bonded to the overlay withPage 5 of 8a structural adhesive, see Figure 2. A loading device with a reaction frame applies the tensileforce at a constant rate to the aluminum disk.In this study, 50 mm cores with an approximate depth of 10 mm were drilled on the bricksurfaces by means of a rotary core cutting drill with diamond bits. To avoid damage in thebricks, special care has been devoted to ensure uniform pressure when the core is beingdrilled. Afterwards, the aluminum disks were glued on the core surfaces by means of astructural adhesive.The loading were applied with a DYNA Z15 instrument at a rate of 0.05±0.01 N/mm2S-1. Thisequipment has a capacity of 16 KN and an accuracy of 2%.(a) (b)(c)Figure 2. (a) Test specimen; (b) schematic of the pull-off test; (c) test setup.3. Test results and discussionAverage bond strengths obtained from 10 pull-off tests for each conditioning type arenormalized by the baseline specimens in Figure 3. It can be seen that a relatively largereduction is found in the bond strength after four and eight weeks. On the other hand, thebond strength was not recovered significantly after one week laboratory storage, which can bedue to the irreversibility of the observed degradation in the bond strength.The mean values of the bond strengths and their corresponding standard deviation andcoefficient of variation (C.o.V.) are presented in Table 3. A relatively large scatter is found onthe bond strength values which is common in this type of experimental test [14]. The strengthreductions comparing to the baseline specimens are also presented in this table. In particularthe reduction of the strength was 15% and 23% for four and eight weeks of immersion inPull-off forceAluminum disk20030 50 40 50 30Drilled corePage 6 of 8water, respectively. It can be seen that the reduction rate has been decreased in the period offour to eight weeks of exposure comparing to the period between commencement of the testand four weeks. Moreover, the bond strength was recovered about 3% in both exposureperiods, after one week of laboratory storage.As it can be observed in Figure 4, the reduction in the bond strength during the investigatedperiod can be expressed with a linear formula. However, since the reduction rate has beenreduced with time increment using an exponential decay curve with respect to the immersiontime seems more reasonable, Eq. (1). Although, performing experimental tests with longerimmersion times is necessary for a better understanding of the decay trend.tEbaseePP )41.2(  (1)where P is the bond strength after t hours of immersion in water and baseP is the baselinebond strength.A typical failure mode of fracture in the upper layer of the bricks was observed in all the dryspecimens, see Figure 5. However, the failure was mainly observed in primer-brick interfacein the moisture conditioned specimens.Figure 3. Average pull-off bond strength of specimens exposed to different moisture conditions.Table 3. Average values of pull-off strengths and their variation after moisture conditioning.Condition Non-conditionedWater(4 w)Water(4 w)+Lab. (1 w)Water(8 w)Water(8 w)+Lab. (1 w)Average 0.90 0.75 0.78 0.68 0.71Std. Dev. 0.11 0.10 0.12 0.15 0.13C.o.V. 7.8 13.3 15.4 21.8 17.8Strength reduction (%) 15 12 23 20Page 7 of 8Figure 4. Bond strength decay with water immersion time.Figure 5. Typical failure mode in the specimens.4. ConclusionsThe results of the preliminary tests performed on GFRP strengthened masonry elementsexposed to moisture environment presented. The specimens were exposed to 100% R.H. at23 C for four and eight weeks. The non-conditioned specimens provided the baseline againstthe moisture exposed specimens. The specimens were tested immediately after the exposure.The reversibility of the bond strength degradation has also been studied by storing somespecimens in the laboratory conditions before performing pull-off tests.In particular a large reduction of bond strength was observed in the conditioned specimensbeing 15% and 23% for the specimens immersed in water for 4 and 8 weeks, respectively. Thebond strength recovered 3% for the specimens that were stored in the laboratory conditionsbefore performing the pull-off test. Based on the observed degradation of the bond strength intime, an exponential decay relation was proposed.A typical failure mode of fracture in the upper layer of the bricks was observed in all the dryspecimens. However, the failure was mainly observed in primer-brick interface in themoisture conditioned specimens.The results obtained showed that moisture can reduce the bond strength of the FRP-masonryelements largely within a short period of exposure (two months). However, furtherPage 8 of 8investigation is required to validate the obtained results.5. AcknowledgementsThis work was partly funded by project FP7-ENV-2009-1-244123-NIKER of the 7thFramework Programme of the European Commission Project, which is gratefullyacknowledged.6. References[1] Hollaway, L.C., “A review of the present and future utilisation of FRP composites in thecivil infrastructure with reference to their important in-service properties.”,Construction and Building Materials, Vol 24, No 12, December 2010, pp. 2419-45.[2] Karbhari, V.M., Chin, J.W., Hunston, D., Benmokrane, B., Juska, T., Morgan, R.,Lesko, J.J., Sorathia, U., Reynaud, D., “Durability gap analysis for Fiber-ReinforcedPolymer composites in civil infrastructure.”, ASCE Journal of Composites forConstruction, Vol. 7, No. 3, August 2003, pp. 238-47.[3] Dai, J.G., Yokota, H., Iwanami, M., Kato, E., “Experimental investigation of theinfluence of moisture on the bond behavior of FRP to concrete interfaces.” Journal ofComposites for Construction, Vol. 14, No. 6, November/December 2010, pp. 834-44.[4] Silva, M.A.G., and Biscaia, H.C., “Effects of exposure to saline humidity on bondbetween GFRP and concrete.”, Composite Structures, Vol. 93, No. 1, December 2010,pp. 216-24.[5] Benzarti, K., Chatatigner, S., Quiertant, M., Marty, C., Aubagnac, C., “Acceleratedageing behavior of the adhesive bond between concrete specimens and CFRP overlays.”Construction and Building Materials, Vol. 25, No. 2, February 2010, pp 523-38.[6] Tuakta, C., and Buyukozturk, O.,“Deterioration of FRP/concrete bond system undervariable moisture conditions quantified by fracture mechanics.” Composites: Part B,Vol. 42, No. 2, March 2011, pp. 145-54.[7] Aiello, M.A., and Sciolti, M.S., “Influence of environmental agents on bond betweenFRP reinforcement and calcarenite ashlars.” 4th International conference of StructuralAnalysis of Historical Constructions, November 2004, pp. 875-81.[8] Desiderio, P., and Feo., L., “Durability evaluation of EBR CFRP strengthened masonrystructures.” in Proceedings of BBFS, 2005, pp. 481-88.[9] Briccoli Bati, S. and Rotunno, T., “Environmental durability of the bond between theCFRP composite materials and masonry structures.” Historical Constructions, 2001, pp.1039-46.[10] Ouyang, Z. and Wan, B., “Experimental and numerical study of moisture effects on thebond fracture energy of FRP/concrete joints.”, Journal of Reinforced Plastics nadComposites, Vol. 27, No. 2, January 2008, 205-23.[11] UNI EN 771-1, “Specification for masonry units. Part1: clay masonry units.”, EnteNazionale Italiano di unificazione, 2005, (In Italian).[12] UNI EN 8942-3, “Clay bricks and blocks. Test methods.”, Ente Nazionale Italiano diunificazione, 1986, (In Italian).[13] EN ISO 527-1, “Plastics-determination of tensile properties- Part 1: generalprinciples.”, ISO 527, 1993.[14] Bonaldo, E., Barros, J.O., Lourenço, P.B., “Bond characterization between concretesubstrate and repairing SFRC using pull-off testing.”, International Journal of Adhesionand Adhesives, Vol. 25, No. 6, December 2005, pp. 463-74.

Moisture effects on the bond strength of FRP-masonry elements

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Moisture effects on the bond strength of FRP-masonry elements

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