Novel conservation engineering techniques,restoration and strengthening

Structural Analysis of Historic Construction – D’Ayala & Fodde (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-46872-5

FRP-strengthening of masonry structures: Effect ofdebonding phenomenon

E. Grande, M. Imbimbo & E. SaccoDepartment of Mechanics, Structures and Environment, University of Cassino, Italy

ABSTRACT: The present paper shows and discusses some aspects concerning the delamination phenomenonof Fibre Reinforced Plastic (FRP) materials fixed on masonry support. With this aim, in the first part of thepaper the results of an experimental campaign conducted by the authors concerning the effect of the bond lengthon the delamination phenomenon of CFRP glued on clay bricks are discussed. Moreover, in order to underlinefurther aspects concerning the decohesion mechanism of FRP-reinforcements, in the second part some numericalanalyses are also presented.

1 INTRODUCTION

Masonry structures constitute a significant part ofhistorical constructions in Europe and particularly inItaly. Many of these structures are structurally defi-cient for current or safe use in the light of the newseismic code regulations. This involves the need toretrofit and upgrade the masonry structures throughstrengthening techniques.

A quite recent strengthening approach consistsin applying FRP strips on the external surface ofthe structure adopting different configurations andanchorage modalities; the FRP strips are arrangedin order to give an external resisting system whichenhances the global capacity of the structure. Thistechnique is used particularly to preserve historicbuildings and monuments because the lightweightFRP strips or sheets are easy to handle and can bealso applied into restricted spaces with little risk ofdamaging the support material. In addition, compos-ites can be made to fit irregular geometries and can becut or trimmed in-situ.

In recent years, large investments have beenconcentrated in order to investigate both the modali-ties of application and the efficacy of FRP-reinforcingsystems. Several developments have been performedand some guidelines for the strengthened and the con-servation of existing structures have been formulated(CNR-DT200/2006).

Several aspects concerning the behaviour of FRP-strengthened structures have emerged from theperformed studies.Among these, the decohoesion phe-nomenon between the support and the FRP sheetsrepresents the most important one because it isresponsible of the efficacy of the reinforcing system.

This aspect has been widely investigated by severalauthors particularly with reference to RC structures.Only in the last few years some experimental tests havebeen developed also to study the nature of the bondbetween composite and masonry supports (Aiello andSciolti, 2005; Casareto et al., 2003; Barbi et al.,2004). These tests have evidenced several differencesbetween the FRP-strengthened masonry and the FRP-strengthened concrete structures, particularly in termsof bond strength and detachment mechanisms. Thisinvolves the need to further analyze the experimentalbehaviour of masonry elements strengthened by FRPand provide the possible improvement for the standardcode formulation.

The present paper shows and discusses some aspectsconcerning the delamination phenomenon of fibrereinforced plastic (FRP) materials attached to masonrysupport. To this purpose, in the first part of the paperare discussed the results of an experimental cam-paign conducted by the authors concerning the effectof the bond length on the delamination phenomenonof CFRP glued on clay bricks. In addition, to betterunderstand further aspects concerning the decohe-sion mechanism of FRP-reinforcements, in the secondpart of the paper are also presented some numericalanalyses.

2 EXPERIMENTAL PROGRAM

2.1 Test specimens

The program involves eight bonding tests (single shearpushing tests) performed on clay bricks reinforcedby carbon-fibre-reinforced-plastic laminates (CFRP)

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80 m

m

120

mm

160

mm

40 m

m

250

mm

120 mm

legend: strain-gauge on the unbonded part

strain-gauge on the bonded part

Figure 1. Geometry of the tested specimens.

Figure 2. (a) Test for determining the compressive strength,(b) Test for determining the tensile strength, (c) Test for deter-mining the elastic modulus and (d) Test for determining theshear strength.

with different bond lengths. The specimens were char-acterized by four different values of the bond length Lband the same type of support. Figure 1 shows the detailsof the specimens (scheme 1: Lb = 40 mm; scheme2: Lb = 80 mm; scheme 3: Lb = 120 mm; scheme 4:Lb = 160 mm).

Table 1a. Tests results (compressive strength).

Dimensions (mm)

Specimen L B H Section (mm2) fb (MPa)

1 55 56 55 3080 41.52 55 55 54 3025 35.73 56 55 55 3080 40.64 55 55 55 3025 34.75 55 55 54 3025 36.66 55 55 55 3025 41.7

Average value 38.5

Table 1b. Tests results (tensile strength).

Dimensions (mm)

Specimen L B H Section (mm2) fv (MPa)

1 55 255 117 14025 3.842 55 255 117 14025 3.153 55 255 117 14025 4.454 55 255 117 14025 2.455 55 255 117 14025 3.756 55 255 117 14025 3.02

Average value 3.44

Commercial available clay bricks, of dimensionsequal to 250 × 120 × 55 mm3, are used. Standardtests for determining the secant elastic modulus andthe compressive and tensile strengths are performedaccording to UNI EN 8942/3, UNI 8942/3 and UNI6556 respectively (Figure 2a, b, c). In particular, thecompressive tests have been performed consideringspecimens of dimensions 55 × 55 × 55 mm3 obtainedby cutting the brick; the tests for the elastic modu-lus have been performed on specimens of dimensions50 × 50 × 150 mm3, obtained by cutting the brick; thetests for the tensile strength have been performed con-sidering the entire brick; the tests for the shear strengthhave been performed on the brick providing four ver-tical cuts. The tests were performed using a universaltesting machine (Gabaldini SUN) and deducing theapplied load (for all the tests) and the strain values(only for the specimens used for evaluating the secantmodulus).

The results are reported in Tables 1a, b and c. Addi-tional shear tests (Figure 2d) were performed and theresults are also reported in Table 1d.

The CFRP composite used is a unidirectional lam-inate strip of type Sika Carbodour S512 characterizedby the properties reported in Table 2. The adhesiveused is of type Sikadur 30.

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Table 1c. Test results (elastic modulus).

Dimensions (mm)

Specimen L B H Section (mm2) Eb (MPa)

1 50 50 150 2500 138502 50 50 150 2500 166183 50 50 150 2500 15369

Average value 15279

Table 1d. Tests results (shear strength).

Dimensions (mm)

LoadSpecimen Cut height (mm) (N) τb (MPa)

1 36.0 4524 1.142 36.5 2100 0.523 37.0 3679 0.904 36.5 3878 0.97

Average value 0.88

Table 2. Characteristics of the strengthening system.

FRP Laminates (Sika Carbodur S512)

Average value of the tensile strength ftfm 3100 MPaMinimum value of the elastic modulus Ef 160000 MPaUltimate deformation εf 1.7%Thickness tf 1.2 mmWidth bf 25 mm

Adhesive material (sikadur 30)

Average value of the tensile strength fta 25 MPa(+15◦)Average value of the shear’s modulus Ga 2694 MPaAverage value of the elastic modulus Ea 12800 MPa

2.2 Experimental procedure

The specimens were tested by using the test devicereported in Figure 3.

In the test set up the bricks are fixed on the steelplate while the free end portion of the CFRP lami-nate is gripped to the special device connected to thehydraulic jack. All the specimens were instrumentedby electrical strain gauges applied along the middlelongitudinal section of the reinforcement. In partic-ular, five strain gauges were used in the case of thescheme 1 and eight strain gauges were used in theothers schemes (Figure 1).

The signals from the gauges and the load cell wererecorded by a National Instrument data acquisitionsystem with 16 channels and processed through thelabview software (2006).

Figure 3. Test set-up.

Table 3. Bond strength and failure mode.

Scheme Lb (mm) Fu (kN) Failure mode

Series 1 1 40 6.32 a+b2 80 6.90 a+b3 120 10.28 a4 160 8.34 a

Series 2 1 40 6.57 a+b2 80 7.09 a+b3 120 10.40 a4 160 9.57 a

2.3 Test results

The results of the tests in terms of bond strength andfailure modes are summarised in Table 3.

The first, indicated as “a” in the table, is shown inFigure 4.a and is characterised by the detachment of athin and uniform layer of the brick material. The sec-ond, indicated as “b” in the table, is shown in Figure 4.band occurs with the removal of a considerable part ofthe brick material near the unloaded end of the rein-forcement. It was observed that the type of the failuremode was particularly influenced by the length Lb ofthe bonded part of the FRP-reinforcement. In fact, theschemes with the smallest dimensions of Lb (schemes1 and 2) showed the failure mode “b” whilst the oth-ers (schemes 3 and 4) primarily showed the failuremode “a”.

The experimental values of the bond strengthreported in Table 3 point out the following consider-ations. First, the variation of the bond strength fromscheme 1 (Lb = 40 mm) to scheme 2 (Lb = 80 mm)is small. Second, the bond strength increases by 50%

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Figure 4. (a) Decohesion mechanisms of specimens and(b) Decohesion mechanisms of specimens.

if the bond length increases from 40 mm (scheme 1)to 120 mm (scheme 3). These first results suggest thatthe bond length of scheme 1 could be lower than theoptimal length of the reinforcement since an increaseof the bond length (from 40 mm to 120 mm) enhancesthe bond strength.Third, the bond strength and the fail-ure mode do not significantly change by increasingthe bond length from 120 mm (scheme 3) to 160 mm(scheme 4). This implies that the bond length of thescheme 3 could be close to the optimal bond length ofthe reinforcement; in fact, further increase of the bondlength, for example scheme 4, does not affect neitherthe bond strength nor the failure debonding mode.

In order to analyse the transfer mechanism of theapplied tensile load from the FRP reinforcement tothe brick support, in Figure 5 are reported the strainpaths occurred along the reinforcement of the speci-mens (series 2) at various load levels. From the plotsit is possible to notice a greater slope of the curves inthe vicinity of the loaded end of the plate. This couldsuggest the fact that the load transfer mechanism pri-marily occurs along a limited length of the plate nextto the loaded end.

3 NUMERICAL ANALYSES

In order to propose a simple numerical model basedon the experimental data and able to capture the globalbehaviour of CFRP-laminates glued on masonryblocks, some numerical analyses have been performed.

3.1 Modelling approach

Regarding the modelling approach adopted by theauthors for simulating the response of the examinedspecimens, a 2D finite element model (Figure 6) hasbeen developed for all the examined specimens (i.e.varying the bond length Lb of the reinforcement)using the code DIANA9.1 (2000). In the models, both

Figure 5. Strain profiles of FRP laminates.

the brick, the FRP-laminate and the FRP/brick layerhave been modelled by adopting the following finiteelements:

• clay bricks: four-node quadrilateral isoparamet-ric plane stress elements (labelled in the code asQ8MEM) based on linear interpolation and Gaussintegration.

• FRP-laminates: beam elements characterized onlyby the axial deformation (also called truss-elementand labelled in the code as L2TRU).

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Figure 6. Numerical model.

• FRP/brick layer: interface elements (labelled in thecode as L8IF) placed between L2TRU and Q8MEMelements. The basic variables for the interfaces ele-ments are the nodal displacements. The derivedvalues are the relative displacements and the trac-tions used by code to describe a relation betweenthe tractions and the relative displacements acrossthe interface.

3.2 Modelling of material behaviour

From the experimental tests it was observed that thedelamination phenomenon involved only the brick,whilst it did not affect neither the FRP laminates northe FRP/brick interface. On the basis of this observa-tion and in order to simplify the numerical model, ithas been adopted a nonlinear behaviour only for thebrick. On the contrary, a linear-elastic behaviour hasbeen assumed for the FRP laminates and the FRP/brickinterface. The mechanical properties of the bricks, theFRP and the adhesive material have been describedin the previous section 2. The mechanical propertiesof the FRP/brick layer (i.e. the tangential stiffnesskt and the normal stiffness kn) have been obtainedthrough an homogenization of the brick and adhesiveproperties using the following relationships:

where:Ea(see table 2), Eb (see table 1.c) are the elastic’s

moduli of the adhesive and the brick respectively; hi isthe thickness of the interface layer between the brickand the FRP, assumed equal to 1.0 mm, as observed bytests; Ga (see table 2), Gb (see table 4) are the shear’smoduli of the adhesive and the brick respectively. Theproperties of the elements used in numerical model aresummarised in Table 4.

In order to model the nonlinear behaviour of the claybrick, an orthotropic elasto-plastic continuum modelwith the Hill (1947) yield condition and characterizedby an elastic-perfectly plastic behaviour in tension andcompression has been considered.

Table 4. Material properties used for the numerical models.

Element Mechanical properties

FRP-laminates Young’s modulus: EF = 160000 MPa(L2TRU) Poisson’s ratio: νF = 0.3clay brick Young’s modulus: Eb = 15279 MPa(Q8MEM) Poisson’s ratio: νb = 0.2

Shear modulus: Gb = 6366 MPa

interface layer Tangential stiffness: kt = 4670 N/mm3

(L8IF) Normal stiffness: kn = 78891 N/mm3

Table 5. Parameters characterizing the yield domain.

Mechanical parameters Values

Compressive strength in x (y) direction 38.5 MPaTensile strength in x (y) direction 3.44 MPaShear strength 0.88 MPa

The adopted yield condition is an extension of thevon Mises failure criterion and is characterized by thefollowing formulation (see also DIANA User’s Guide;DIANA, 2000):

where σ is the stress vector, σ∗(κ) is the referenceyield strength as a function of the internal state vari-able κ, and P is the projection matrix which dependson the yield strengths in the x and y directions, andon the yield strength in shear. An associate flow ruleis adopted for the yield criterion. The parametersreported in Table 5, selected for characterizing theyield domain, have been deduced by the experimen-tal tests performed on the clay bricks and described insection 2.

3.3 Results

Two types of analyses have been performed: a linear-elastic analysis where all the elements are elastic andan incremental nonlinear elastic analysis where the nonlinearity has been adopted only for the brick elements.

The first type of analysis has been performed withthe aim of capturing the elastic behaviour of speci-mens. In Figure 7 have been compared the displace-ments of the FRP deduced by the experimental tests(circular symbols) and those obtained by the numer-ical analyses (continue line). In particular, the figurerefers to the scheme 4 of the examined specimens con-sidering a low value of the applied force (assumedequal to 20% of the maximum force sustained by thereinforcement) in order to examine the behaviour of

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Figure 7. Comparison between numerical and experimentalvalues of FRP-displacements.

the specimen before delamination or damage of thesupport, which could be considered elastic.

From the Figure it is possible to observe a goodagreement between the numerical and the experimen-tal values of the FRP displacements particularly in thezone of FRP-laminate near to the unloaded end. On thecontrary, the experimental values of the part of stripsnear to the loaded end are greater than the numeri-cal ones. This effect is probably due to the presenceof some damages which arise for low external loadsalready and local effects orthogonal to the FRP stripswhich arise near the vertical edge of bricks, do notcaptured by a 2D model.

The second type of analysis has been performedin order to examine the capability of the numericalmodel to provide the maximum load sustained by thereinforcement.

In Figure 8 are reported the curves in terms ofapplied force vs. horizontal displacement of the FRPobtained from the numerical analyses. In the same fig-ures are reported the results obtained from the tests(triangular symbols). In particular, the displacementhas been measured at the section located at 65 mmfrom the free edge of the brick. The choice of the sec-tion is due to the fact that it represents the location ofa strain gauge for all the specimens.

Figure 8 shows that the theoretical curves are ingood agreement with the experimental results. Thismeans that since the numerical model is based only onthe nonlinear behaviour of brick, it could be assumedthat the support is the main responsible of the nonlinearresponse of the tested specimens. Moreover from theplots it is also evidenced an increase of the maximumload sustained by the reinforcement by increasing thebond length.

In Figure 9 is shown the comparison between themaximum load obtained by the numerical analyses(triangular symbols) and the corresponding valuesobtained by experimental tests (vertical bars). Fromthe figure it is possible to notice that the adopted modelfurnishes a good estimation of the bond resistance of

Figure 8. (a) Push-over analysis of the specimens:scheme 1, (b) Push-over analysis of the specimens:scheme 2, (c) Push-over analysis of the specimens: scheme 3and (d) Push-over analysis of the specimens: scheme 4.

the examined specimens particularly for the schemes2 and 3.

It is also important to notice that, the bond strengthis not linear dependent on the bond length.

In Figure 10 are reported the variation of the bondstrength Fmax deduced from the experimental testsand that obtained from the numerical analyses. In thesame figure the dotted line indicates a variation of

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Figure 9. Maximum force sustained by the reinforcement.

Figure 10. Bond strength variation.

the bond strength linearly proportional to the bondlength of the reinforcement. From the figure it is clearthat the variation of the bond strength with the bondlength is strongly far from the linear variation and ischaracterised by a smaller slope.

This is a further aspect that confirms the presenceof different interaction mechanisms between the FRP-reinforcing and the support depending on the bondlength. Indeed, the increase of the bond length of theFRP not only produces an increase of the zone whichresists to external loads but produces a variation interms of stress transfer between the reinforcement andthe support through the interface layer.

4 CONCLUSIONS

In this paper are discussed the results obtained by anexperimental campaign and a numerical study devotedto analyse the delamination mechanisms of CFRPlaminates glued on clay bricks.

From the experimental results, the influence of thebond length on the bond strength and the delaminationmechanism has been examined in detail. In particular,it was observed that after the detachment of the FRPlaminate only the brick support was damaged.

On the basis of the experimental evidence, a 2Dnumerical model of the tested specimens has beendeveloped adopting an elasto-plastic behaviour for thebrick response.

The good agreement between the numerical and theexperimental results both in terms of strength and non-linear behaviour have confirmed the reliability of theproposed model.

It is clear that a more comprehensive study of thebonding response would require the evaluation of afracture energy as defined in the theoretical formu-lations reported in the codes. This means that first,the numerical model would also provide the maximumdeformation in the brick and the post-peak behaviour,and second, the experimental tests would be performedby a displacement control procedure.

REFERENCES

Aiello, M.A. & Sciolti, S.M. 2005. Bond analysis of masonrystructures strengthened with CFRP sheets. Constructionand Building Materials. Vol.20: pp. 90–100

Casareto, M., Olivieri, A., Romelli, A. & Lagomarsino,S. 2003. Bond behaviour of FRP laminates adherent tomasonry. In: Proceedings of the international conferenceadvancing with composites, Milano, Italy

Barbi, L., Briccoli Bati, S. & Ranocchiai, G. 2004. Anal-isi sperimentale di campioni in mura-tura fasciati concomposito CFRP. inAtti del II Convegno Nazionale“Mec-canica delle struttu-re in muratura rinforzate con FRP –materials”, Venezia, Italy (in Italian).

CNR-DT200/2006. 2006 Guide for the design andconstruction of externally bonded FRP systems forstrengthening existing structures. Materials, RC andPC structures, masonry structures. National ResearchCouncil, Rome-CNR

National Instruments LabVIEW 2006. Graphical Develop-ment Platform for Design, Control and Test.

DIANA, 2000. Displacement analysis finite elementsoftware. Version 9.1, TNO-Building Division, Delf,The Netherlands.

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