Strengthening Masonry Arches with Composite MaterialsS. Briccoli Bati1; L. Rovero2; and U. Tonietti3

Abstract: The aim of this study was to compare the effects of strengthening masonry arches using two different composite materials. Tothis end, an experimental analysis was carried out on models of arches that were first damaged, then strengthened by applying compositematerial sheets to the surface of the intrados, and last, subjected to a loading process until the point of collapse. One arch was strengthenedwith carbon fiber-reinforced polymer, the other with glass fiber-reinforced cement matrix. Results collected during the experimentalanalysis were significant in assessing the capability to support horizontal load, in increasing the collapse load, stiffness, and ductility, andin assessing the different fracture patterns and collapse modes of the arches strengthened with different fiber-reinforced composites. Thecomparison will be useful for establishing the physical-mechanical and aesthetic compatibilities between the original construction and thestrengthening matrix �polymeric or cementitious�, particularly with reference to the safeguarding of historical buildings.

DOI: 10.1061/�ASCE�1090-0268�2007�11:1�33�

CE Database subject headings: Arches; Masonry; Composite materials; Fiber reinforced polymers.

Introduction

The safeguarding of historical buildings requires that appropriatemethodologies be defined for their restoration and consolidation.In the development of these methodologies, on the one hand,structural safety, including seismic hazard, must be accounted for,while on the other, the original construction must be preservedfrom both an aesthetic and a structural perspective. An importantissue, therefore, is the assessment of the effects on structural be-havior of advanced materials and technologies as applied to tra-ditional elements �Christensen et al. 1996�.

Strengthening based on the application of composites is cur-rently attracting particular interest. Composites are well suited fordeveloping connections between the structural elements of a ma-sonry building, even under conditions that could cause a connec-tion failure, such as a seismic load. In fact, their application canprovide masonry materials with tensile strength �though onlyunder the composite bond area�. In particular, the use of sheets ofcomposite material, bonded at the intrados or the extrados ofvaulted structures, could avoid some of the drawbacks inherent inthe consolidation techniques currently in use.

Schwegler �1994� was the first person to propose and study theuse of carbon laminates as strengthening elements for masonrystructures. The works of Saadatmanesh �1994� and Ehsani et al.�1999� were the first studies to deal with experimental investiga-tions of masonry specimens strengthened with epoxy-bonded

1Full Professor, Dipartimento di Costruzioni, Università degli Studi diFirenze, Florence, Italy. E-mail: silvia.briccolibati@unifi.it

2Researcher, Dipartimento di Costruzioni, Università degli Studi diFirenze, Florence, Italy. E-mail: luisa.rovero@unifi.it

3Associate Professor, Dipartimento di Costruzioni, Università degliStudi di Firenze, Florence, Italy. E-mail: ugo.tonietti@unifi.it

Note. Discussion open until July 1, 2007. Separate discussions mustbe submitted for individual papers. To extend the closing date by onemonth, a written request must be filed with the ASCE Managing Editor.The manuscript for this paper was submitted for review and possiblepublication on June 7, 2005; approved on May 16, 2006. This paper ispart of the Journal of Composites for Construction, Vol. 11, No. 1,

February 1, 2007. ©ASCE, ISSN 1090-0268/2007/1-33–41/$25.00.

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glass overlays. Kolsch �1998� reported the results of an experi-mental program on masonry walls reinforced with a carbon-fiber,cement-matrix overlay system subjected to bending loading. Tri-antafillou �1998� and Triantafillou and Fardis �1997� presented abasic theory of the mechanical behavior of masonry wallsstrengthened with fiber-reinforced polymer �FRP� laminates byusing simple models within the framework of modern designcodes �Eurocode 6�. In the work of Valluzzi et al. �2001�, theanalytical derivations proposed by Triantafillou were applied tothe study of masonry barrel vaults and were then compared withexperimental data obtained on scale models of reinforced vaults.Briccoli Bati and Rovero �2000a,b, 2001� reported and discussedthe results of an experimental investigation on masonry arch mod-els subjected to vertical loads and reinforced with carbon fiber-reinforced polymer �CFRP� sheets on the intrados or extradossurface. A comparison of the experimental results obtained forstructural elements reinforced with CFRP and with glass fiber-reinforced cement matrix �GFRCM� showed that the use ofGFRCM achieved greater mechanical compatibility between thestrengthening and the underlying masonry support �Briccoli Batiet al. 2003, 2004a,b�.

In this paper, we report on an experimental study comparingthe effects of reinforcement by means of CFRP and GFRCM onmasonry arches subjected to loads. Real cases, such as earth-quakes and settlements of abutments, were reproduced. Experi-mental tests were carried out on scale models of masonry archeswith spandrel walls and with a soft abutment reinforced at theintrados and subjected both to distributed vertical loads and to anincreasingly concentrated horizontal load. Results collected dur-ing the experiment were compared in terms of mechanical param-eters, fracture patterns, and collapse modes, so as to highlight thedifferent physical-mechanical and constructive compatibilities be-tween the original building and the strengthening matrix �poly-meric or cementitious�.

Experimental Analysis

The mechanical behavior of arch models reinforced with CFRP

sheets has been investigated in previous experimental studies

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�Briccoli Bati and Rovero 2001, 2000a,b�. Up to now, experi-ments have been carried out on 1:2 scale models of segmentalcircular masonry arches reinforced with CFRP sheets, either inthe intrados or the extrados, and then subjected to a vertical con-centrated load at the keystone. Experimental results have pointedout a number of drawbacks in the use of FRP for the strengthen-ing of masonry. In particular, these include delamination withsignificant loss of material at the bonded surface and collapsemechanisms with masonry crushing or brittle failure under shearstresses, which alter the nature of masonry arch behavior. Thesedisadvantages appear to depend on the intrinsic difference be-tween the physical-mechanical characteristics of the polymericmatrix and those of the masonry. This difference has critical rel-evance in the case of historical structures, whose constructivefeatures must be preserved. These findings have motivated inves-tigations of the use of a composite matrix that would be morecompatible with the native characteristics of masonry, such as acomposite containing a cement matrix �Briccoli Bati et al. 2003;2004a; Katz and Bentur 1995, 1996; Kolsch 1998; Magalhaes etal. 1996; Mu et al. 2002�.

The very different characteristics of the two strengthening sys-tems, CFRP and GFRCM, derive from their different technologi-cal origins. On the one hand, CFRP consists of a unidirectionalcloth of high-strength carbon fibers and has a matrix with veryhigh adhesive capability as regards the fibers; however, it is notvery compatible with masonry �that is, very great differences be-tween stiffness, strengths, and thermal coefficients�. On the otherhand, GFRCM consists of a glass-fiber mesh with tensile strengthper unit width equal to one-sixth of the strength of the carboncloth, and has a cementitious matrix with good adhesion and com-patibility with masonry; however, it adheres imperfectly to glassnet. Consequently, the collapse modes of the two strengtheningsystems applied to masonry are also very different. In the case ofCFRP, the collapse modes always affect masonry by means ofdelamination or some different type of damage, while in the caseof GFRCM, the collapse modes are due either to exceeding thetensile strength of the glass fibers or to sliding between the glassfibers and the matrix �Briccoli Bati et al. 2004a,b�.

Mechanical Characteristics of Materials

The mechanical characteristics of the brick and lime-cement mor-tar employed in the arch models were determined by uniaxialcompression and by direct tensile and bending tests. The com-pression tests were performed on prismatic brick specimens,2.2�2.2�4.5 cm, made from brick blocks provided by the “Lat-erizi S. Marco” firm �Venezia, Italy�, as well as on 4�4�4 cmcubic specimens of lime-cement mortar. The mortar mixture con-sisted of one part hydrated “Fiore” type mortar, one part portlandcomposite cement, eight parts Ticino sand, and two parts water.Uniaxial compression tests were also performed on 10�10�25 cm masonry samples. The direct tensile tests wereconducted on 4�1�8 cm prismatic brick specimens with sym-

Table 1. Mechanical Characteristics of Brick, Mortar, and Masonry Emp

MaterialSpecific weight

�kg/m3�Elastic modulus

�MPa�

Brick 1,800 1,785

Mortar 2,000 133

Masonry — 830

metrically arranged notches approximately 1 cm in length. The

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bending stress tests were carried out on 11�25�5 cm prismaticbrick samples and 4�4�16 cm prismatic mortar samples. Themechanical parameters determined as a result of these tests arereported in Table 1.

The uniaxial compression tests were also performed on 5�5�4 cm specimens of rubber. These were employed to make aright arch abutment that would make elastic displacements pos-sible �Fig. 1, number 7�. The elastic modulus of the rubber�E=4.72 MPa� was determined in order to select the height of therubber layer to be employed in the models so that the requireddisplacement was obtained.

The mechanical characteristics of both the matrix and the fi-bers employed �CFRP and GFRCM� are reported in Table 2. The“MBrace Fibers C-130” high-strength unidirectional carbon-fibersheets �0.16 mm thickness�, “MBrace” primer, and two-component epoxy-base adhesive that make up the MBrace FRPsystem were supplied by the MAC S.p.A. firm �Treviso, Italy�,which also provided the technical specifications. The GFRCMcomposite was designed and made with a glass-fiber net�5�5.9 mm mesh� produced by the GAVAZZI S.p.A. firm�Lecco, Italy� and an ad hoc cement matrix mixed in the labora-tory of the “Dipartimento di Costruzioni” of the University ofFlorence �Italy�. The composition of the cement matrix employedin the GFRCM was as follows: 42.5R cement, sand, water, silicaash, and a hyperfluidifying agent in the ratio 1:3:0.65:0.19:0.19.The corresponding mechanical parameters are reported in Table 2.

Homogenization techniques are normally used to determinethe overall mechanical parameters of the composites, since it isvery difficult to carry out experimental tests using composite ma-terials in the absence of a support. In the case of CFRP, themicromechanical structure of the composite can be correctly de-scribed in the homogenization process by means of analyticalsolutions. The elastic modulus of the CFRP composite was deter-mined by using the homogenization technique proposed in Zhao

in Making Model Arches

Compressivestrength�MPa�

Direct tensilestrength�MPa�

Bending tensilestrength�MPa�

17.39 1.7 3.53

0.78 — 0.23

8.6 — —

Fig. 1. Experimental trials: 1, 2, 3=horizontal displacementtransducers; 4, 5, 6=vertical displacement transducers; 7=layer ofrubber for elastic displacement; 8=load cell with capacity of KN;9=100 N weight

loyed

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and Weng �1990�. This is based upon the Eshelby theory of two-phase composites that contain homogeneously dispersed ellipsoi-dal inclusions. The composite is represented as a matrix withparallel, cylinder-shaped inclusions that are carbon yarns. It waspossible to use the Eshelby theory for ellipsoidal inclusions be-cause the cylinder-shaped inclusion is a particular ellipsoidal in-clusion in which the diameter is infinite. The elastic moduli of thecomposite are reported in Table 3, together with the modulus andvolume-fraction values of each component. On the other hand,GFRCM is an unusual kind of composite, in which the matrix-netbond is mainly a mechanical one. In fact, after a first short phasewith a complete bond between the components, a tensile crackoccurs in the cement matrix and the remaining bearing capacitydepends on the mechanical grip between the net and the matrix�in addition to the tensile strength of the net�. For this reason,homogenization techniques are not suitable for GFRCM.

Criteria for Determining Strengthening Dimensions

Because of the very different natures of the CFRP and GFRCMreinforcements, it is very difficult to make a direct comparison. Infact, as strengthenings are used to provide masonry materials withtensile strength, the possibility of utilizing �for comparison�sheets of different strengthening systems having identical dimen-sions was ruled out. This is because an epoxy matrix is 15 timesmore resistant than cementitious mortar, and the tensile strengthper unit width of carbon cloth is 6 times more resistant than glassnet �Table 2�.

Therefore, the problem was to establish a criterion for deter-mining the strengthening dimensions in the two cases. As far asapplicative purpose are concerned, a simple design criterion couldconsist of providing, by means of different reinforcements, acomparable tensile strength capability for the masonry structure.A possible criterion for the design of the dimensions of the dif-

Table 2. Technical Specification for MBrace System Components

Components

Specificweight

�kg/m3�

Tensileelastic

modulus�MPa�

Compressionelastic

modulus�MPa�

Carbon fibera 1,820 230,000 —

Epoxy matrixa 1,020 3,000 —

Glass fiberb 2,490 87,000 —

Glass net — — —

Cement matrixc 2,000 — 15,000aMAC S.p.A., Treviso, Italy.bGAVAZZI S.p.A., Lecco, Italy.cProduced in laboratory.

Table 3. Elastic Modulus of CFRP Composite Determined UsingHomogenization Techniques

Epoxy matrixvolume fraction

Carbon fibervolume fraction

Composite elasticmodulus �E�

�MPa�

0.8 0.2 48,577

0.825 0.175 42,899

0.85 0.15 37,222

0.875 0.125 31,5450.9 0.1 25,868

Note: Homogenization techniques proposed by Zhao and Weng.

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ferent strengthenings could be based on the fiber tensile strengthper unit width, in order to obtain the same total tensile force bythe different kinds of fibers.

The choice of the fibers’ tensile strength to be used for com-parison was supported by the fact that tensile strength is the onlyreally available value for both strengthening systems �as providedby the manufacturer�. The values of strength and stiffness of thecomposites �matrix and fibers� were less reliable, since thevolume-fraction values of each composite component used in themodels could not be precisely determined.

The effectiveness of the proposed criterion, as far as confer-ring carrying capacity comparable to that of masonry structuremodels reinforced with composites of different nature was con-cerned, was tested by means of an adhesion test campaign. Withthis objective in mind, direct tensile tests �adhesion tests� onspecimens made up of two bricks connected by composite sheets�Fig. 2� were performed.

In both the traction tests and the tests on the arch models, thecomposite sheets used were 1.25 cm wide in the case of CFRPand 8 cm wide in the case of GFRCM. In fact, since the strengthof the carbon fibers was 5,660 N/cm, while that of the glass meshwas 900 N/cm, the ratio of the two values of strength per unitwidth �i.e., glass net strength/carbon fiber strength� for the twotypes of composite material employed was of about 1 /6. Conse-quently, a 1.25 cm wide CFRP sheet and an 8 cm wide GFRCMsheet offered the same total tensile force �about 7,100 N�. Thesheet thicknesses used, which were about 0.2 cm in the case of

irect tensilestrength�MPa�

Tensilestrength perunit width

�N/cm�

Heatexpansioncoefficient

�10−6�1/ °C��

Ultimatestrain�%�

3,500 5,660 — 1.5

50 — 60 2.5

— — 5 5.5

— 900 — —

3.3 — 6 0.01

Fig. 2. Setup of adhesive test �S=displacements transducer,C=anchorage for load cell�

D

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CFRP and 1 cm in the case of GFRCM, depended on the differentnature of the two strengthening systems utilized and on the tech-nology of the applications �Fig. 3�.

The axial stiffness of the two strengthening-sheet fiberswas comparable. In fact, CFRP exhibited axial stiffnessEcAc=460,000 N, with the elastic modulus of the carbon fiberbeing Ec=230,000 MPa and the surface of the section of carbonfiber being Ac=0.02 cmq �Ac= tc ·wc, with tc=0.016 cm thicknessand wc=1.25 cm width�. The GFRCM exhibited axial stiffnessEg ·Ag=370,000 N, with the elastic modulus of the glass fiberbeing Eg=80,000 MPa and the surface of the section of glass fiberbeing Ag=0.046 cmq �Ag=Agy ·ngy, with Agy =0.0034 cmq beingthe surface of a glass yarn and ngy the number of yarns strands ina width of 8 cm�.

As regards the axial stiffness of the composites �matrix andfibers�, comparison of the two sheets applied was difficult be-cause, while the axial stiffness of CFRP could be calculated�ECFRP·ACFRP=787,500 N, with the elastic modulus of CFRPbeing ECFRP=31,500 MPa and the surface of the section of CFRPbeing ACFRP=0.25 cmq�, the value of the tensile elastic modulusof the composite in the case of GFRCM was not available sincethe cementitious mortar was characterized only by the compres-sion elastic modulus. Moreover, it is important to note that theaxial stiffness of the mortar matrix was present only during theinitial phase of the load process, when the modest tensile strengthhad not yet been exceeded �a phase that was rapidly exceededwhen the matrix fractured� �Briccoli Bati et al. 2004a,b,c�.

Adhesion Tests

Direct tensile tests on specimens made up of two bricks con-nected to each other, in an end-to-end arrangement, by two com-posite sheets �Fig. 2� were performed to assess the bond forcebetween the brick and the CFRP or GFRCM sheet. In particular,

Fig. 3. Two different strengthening systems used in comparison:�a�=GFRCM; �b�=CFRP

Fig. 4. Two kinds of failures for adhesion test specimens:delamination for CFRP, fiber breakage for GFRCM

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in accordance with the comparison criteria of the preceding sec-tion, the brick samples were reinforced with a 1.25�10 cm wideCFRP sheet �0.2 cm thick� or an 8�10 cm wide GFRCM sheet�1 cm thick�. The tests were carried out using a displacement-controlled device, and the overall strain was deduced from theglobal displacements measured by four transducers applied to theupper surface of the specimen and capable of assessing possiblebending deflections �Fig. 2�.

During these experiments we observed two different kinds ofmechanical behavior and collapse mode for the two types ofstrengthening �Fig. 4�. In the case of GFRCM, the load-displacement diagrams recorded during the tests exhibited a firstlinear phase, a subsequent transition phase that was characterizedby load drop, and a third phase in which the load increased �Fig.5�. The first phase ended with the tensile strength of the cementmatrix being exceeded and was the only phase in which the ma-trix and the fibers were intact and the adhesion was effective. Inthe following phases, the strength was fully assigned to the fiber-matrix interface, and collapse occurred mainly in a breaking ofthe glass fibers. On the other hand, in the case of the CFRP, theload-displacement diagram exhibited two ascending brancheswith different slope until the collapse suddenly occurred, due todelamination and resulting in the removal of a layer of the bricksurface. The average collapse loads corresponding to the bondforce were 4,800 and 5,800 N, respectively, for the CFRP- andGFRCM-reinforced specimens �the difference was 17%�.

Fig. 5. Load-displacement diagrams recorded during adhesion test

Fig. 6. Photo of arch model and relative instrumentation

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Tests on Arch Models

The case study under consideration consisted of arch models witha spandrel, which was made of bricks to a 1:2 scale with respectto a real structure �Fig. 6�. In particular, two segmental circularmasonry arches were prepared with double-layered brick andlime-cement mortar, with a span of 150 cm and a rise of 43.5 cm,with both a height and a thickness of 10 cm. The models werecompleted with a masonry spandrel having the same width as thearch and a height of 10 cm on the keystone and 60 cm on theabutments. Fig. 1 shows the arch model and the experimentaltrials performed at the Official Laboratory for the Testing of Ma-terials and Structures at the “Dipartimento di Costruzioni” of theUniversity of Florence. The testing apparatus consisted of a steelframe that was rigid enough to make it possible to impart hori-zontal loads using a hydraulic jackscrew �Fig. 1, number 8�. Thehorizontal load simulated the seismic action by means of an al-most equivalent static load and was located so that it could affectthe arch structure rather than the spandrel. The horizontal load,which was increased step by step, was measured by means of aload cell �Fig. 1, number 8� having a capacity of 20 kN. Verticalloads were applied by imposing weights of 100 and 10 N on theupper surface �Fig. 1, number 9�. The total weight was 5,600 N,supported by poles positioned so as to be symmetrical with re-spect to the greater surface of the model. The right abutment wasarranged with a rubber layer capable of permitting elastic settle-

Fig. 7. Phases

Fig. 8. Fracture pattern and collapse mechanism of

Fig. 9. Fracture pattern and collapse mechanism of fi

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ments �Fig. 1, number 7�. Three vertical displacement transducerswere placed on the upper surface of the models �Fig. 1, numbers4, 5, 6�, and three horizontal displacement transducers wereplaced on a lateral surface of the models �Fig. 1, numbers 1, 2, 3�.

The two arch models were first subjected to a static monotonicloading process, in the absence of any strengthening, until frac-tures opened that reduced the arches to a statically determinatestructure. After unloading, the arches were strengthened with acomposite material sheet on the surface of the intrados and sub-sequently subjected to the same loading process. To be more spe-cific, the first arch was strengthened with CFRP and the secondwith GFRCM. As in the adhesion test, different sheet widths wereutilized: a width of 1.25 cm for CFRP, versus a width of 8 cm forGFRCM, in order to bestow a tensile strength capability thatwould be comparable in the two arch models.

Models with or without strengthening were loaded following athree-phase process �Fig. 7�: �1� first, a vertical, statically increas-ing load was applied to the overall upper surface until it reached5,600 N, which simulated the dead weight and the overloads typi-cal of a two-storey building; �2� next, while the constant verticalload was maintained, an increasing horizontal load �so as to simu-late a seismic effect� was applied to a 10�10 cm area of thelateral surface, until the arches were transformed into mecha-nisms; and �3� last, the vertical load placed on the right half of theupper surface was statically removed so as to obtain an asymmet-ric loading condition.

ading process

odel without strengthening after last loading phase

del with CFRP strengthening after last loading phase

of lo

first m

rst mo

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During this loading process, a fundamental role was played bythe rubber layer on the right abutment, which simulated the fre-quent occurrence of differential displacements of the supports.Figs. 8–11 show the fracture patterns and collapse mechanisms ofthe models observed, which correspond to the last phase of theloading process.

First Arch Model

After the last loading phase, the fracture pattern of the first arch�without strengthening� was characterized by three hinges �Fig.8�: one was in correspondence is the left abutment �at the intra-dos�, another was near the keystone �at the extrados�, and a thirdwas in correspondence to the right abutment �at the intrados� �Fig.12�. In addition, a detachment between the wall and the archoccurred near the right abutment.

In the first arch with a CFRP sheet, detachment of the strength-ening near the right abutment had already occurred during thefirst loading phase �corresponding to 2,000 N vertical load�,whereas the detachment of the CFRP sheet near the left abutmentoccurred during the second loading phase �corresponding to avertical load of 5,600 and a 4,700 N horizontal load�. The occur-rence of these detachments caused damage to the masonry sup-port �delamination�. After the final loading phase, the collapsemechanism was characterized by only two fractures �Figs. 9 and13�, in correspondence to the abutments, because the strengthen-ing prevented the third hinge from opening near the keystone.Nevertheless, the collapse mechanism was activated due to thehorizontal load, which produced horizontal translation. It is im-portant to emphasize that the CFRP detachment had already oc-curred during the first phase of the loading process, because thecomposite material was not compatible with the compressive ma-sonry strain.

Fig. 10. Fracture pattern and collapse mechanism of

Fig. 11. Fracture pattern and collapse mechanism of seco

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Second Arch Model

The fracture pattern of the second arch without strengtheningafter the last loading phase differed from the first pattern �Fig.10�: the keystone hinge shifted over to the point of application ofthe horizontal load, whereas the other fractures remained un-changed �Fig. 14�. It is important to emphasize that it is verydifficult to construct identical models that exhibit the same char-acteristics. Consequently, a substantial evaluation of the varioustypes of strengthening effectiveness can be deduced only by com-paring the performances of each model with the initial unrein-forced one.

In the GFRCM-reinforced second arch, the first loading phaseproduced only slight fractures in the wall. During the secondloading phase, however, one tensile fracture in the matrix of thestrengthening occurred near the surface of the horizontal loadapplication �corresponding to 8,200 N horizontal load and5,600 N vertical load�. The detachment of the strengthening nearthe left abutment, without any damage to the masonry support,occurred in correspondence to the maximum horizontal load�9,200 N�. After the final loading phase, the detachment betweenthe arch and the spandrel occurred near the right abutment,whereas the strengthening remained attached �Figs. 11 and 15�.

Analysis of Experimental Results

Figs. 16 and 17 contain the load-displacement diagrams recordedduring the tests of both models with and without strengthening. Inparticular, the load-displacement diagram corresponding to theload of the first loading phase and the vertical displacement re-corded by Transducer 5 �Fig. 1� is shown in Fig. 16, and theload-displacement diagram corresponding to the load of the sec-

model without strengthening after last loading phase

odel with GFRCM strengthening after last loading phase

second

nd m

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ond loading process and the horizontal displacement recorded bytransducers 2 and 3 �Fig. 1� is shown in Fig. 17. Table 4 reportsthe values of the mechanical parameters determined from the datarecorded during the tests. The following characteristic points wereidentified in each load-displacement diagram: li, start of linearsegment; l, end of linear segment; and l�, intersection between thelinear branch and the ordinate corresponding to the peak load m�Fig. 18�. The values of the following mechanical parameterswere calculated by using the abscissa and ordinate values of thecharacteristic points recorded during the tests: tangent stiffnessK= �yl-yli� / �xl-xli� and kinematic ductility �c= �xm /xl��, whereyli and yl are, respectively, the loads corresponding to the startand the end of the linear segment; xli and xl are, respectively, thedisplacements corresponding to the start and the end of the linearsegment; xm is the displacement at peak load m; and xl� is thedisplacement corresponding to point l� �Fig. 18�.

During the first loading phase �vertical load�, the CFRPstrengthening produced an increase in stiffness equal to 16%,whereas the GFRCM strengthening resulted in an increase equalto 131%. This unbalanced result also depended on the differentdimensions of the strengthening sheets �1.25 cm width and0.2 cm thickness for CFRP, and 8 cm width and 1 cm thicknessfor GFRCM�. The ultimate tensile force of the two kinds of fiberswas the same, but GFRCM produced greater stiffness, partly dueto an increase in the dimensions of the model section �in theamount of 8%�. In this respect, it is important to emphasize thescale effect intrinsically linked to the tests carried out. In fact, for

Fig. 16. Load-displacement diagram: vertical displacement�Transducer 5� due to first load phase

Fig. 17. Load-displacement diagram: horizontal displacement�Transducers 2 and 3� due to second loading phase

Fig. 12. Fracture in first model without strengthening after lastloading phase

Fig. 13. Fractures in model with CFRP strengthening after lastloading phase

Fig. 14. Fractures in second model without strengthening after lastloading phase

Fig. 15. Fractures in model with GFRCM strengthening after lastloading phase

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technological reasons, it was not feasible to scale the thickness ofthe GFRC sheets, and this thickness thus caused a substantial risein the area and inertia in the arch scale section.

The two models without strengthening exhibited the same ca-pability to support the horizontal load. This capability was greatlymodified in the reinforced models. In fact, in the CFRP-reinforcedarch, the increase in capability for the horizontal load was equalto 21%, whereas in the GFRCM-reinforced arch the increase wasequal to 78%. This result was consistent with the different frac-ture patterns in the two cases of strengthening. In the CFRP-reinforced model, the detachment of the composite sheet occurredin correspondence to the abutment covered with the rubber layer.This was due to compressive stress already during the first load-ing phase �vertical load only�. The detachment in correspondenceto the left abutment occurred during the second loading phase�5,600 N vertical load and 4,700 N horizontal load�. In the caseof the GFRCM-reinforced arch, detachment of the left abutmentoccurred only during the second loading phase �5,600 N verticalload and 9200 N horizontal load�, whereas detachment of thestrengthening sheet did not occur in correspondence to the rightabutment covered with the rubber layer.

The increase in stiffness of the arch reinforced with CFRPsheet was about 9%, while the same increase in the arch rein-forced with GFRCM sheet was about 38%. The two modelsshowed the same increase in kinematic ductility. Moreover, in theCFRP-reinforced arch, the extrados hinge near the keystone couldnot open; hence, the collapse mechanism was different from thatof the model without strengthening. In the case of the GFRCM-reinforced model, the strengthening did not inhibit the secondhinge from opening, but limited the opening of its mouth. Indeed,while the cement matrix �in the intrados� broke, the glass fiber netresisted; therefore, the general behavior remained similar to thatof the model without strengthening.

Table 4. Mechanical Parameters Derived by Tests on Arch Models

Models

Tangent stiffness forvertical load

�N/�m�

Maximum horload �with 5,6

vertical lo�N�

Arch 1 0.56 5,040

Arch 1 CFRP 0.65 6,100

Arch 2 1.3 5,170

Arch 2 GFRCM 3.0 9,200

Fig. 18. Characteristic points of load-displacement diagram

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Conclusions

The experimental data obtained concerned the capability to sup-port horizontal load, an increase in stiffness, kinematic ductility,and the fracture pattern and collapse mode of masonry archeswhen strengthened with two types of fiber-reinforced composite.Significant results emerged from comparisons made between re-sults of strengthening with polymeric or cement matrices withregard to mechanical behavior. In particular, the following obser-vations can be made:• Strengthening with a polymer matrix presented an unforeseen

weakness when applied under compressive stress. The detach-ment observed under vertical load made for a poor strengthcontribution and affected the effectiveness of the strengtheningin the case of seismic load, which alternatively produced ten-sile and compressive stress. Conversely, the GFRCM exhibitedstrengthening under both traction and compression, thanks tothe cement matrix. It was thus able to make its strength con-tribution in spite of previous loading events.

• It is important to point out that the two strengthenings weredesigned to confer a theoretically comparable tensile strengthcapability on the arch models. In the light of the tests carriedout, the GFRCM strengthening, which has not yet been suffi-ciently considered in the literature, conferred a significantlybetter mechanical behavior on the structure under horizontalloads. Although this result may have depended on the increasein the arch section area that was gained with the strengthening,it is important to note that this increase in area �and inertia�was much less than the increase in mechanical performance.

• The two kinds of strengthening showed different collapsemechanisms. In fact, the GFRCM-reinforced model had a frac-ture pattern that affected the wall, while the basic collapsemechanism was similar to that of the traditional arch. On thecontrary, the CFRP strengthening radically changed the behav-ior of the original structure. At this point, it should be ques-tioned whether a radical change in structural behavior �apartfrom the performance achieved by the different kinds ofstrengthening� is necessary to obtain a small increase in carry-ing capacity.

• With reference to problems of safeguarding, the choice of thestrengthening needs to take into account the fact that innova-tive materials must necessarily be placed next to traditionalmaterials, and therefore the strengthening material has to be ascompatible as possible with the masonry support. This charac-teristic, which is peculiar to the GFRCM composite, enablesbetter mechanical behavior thanks to more effective adhesion.Moreover, the better compatibility of the cement matrix avoidsthe delamination of masonry that occurs in polymeric compos-

Tangent stiffness forhorizontal load

�with 5,600 N vertical load��N/�m�

Kinematic ductilityfor horizontal load

�with 5,600 N verticalload�

1.12 1.75

1.2 3.16

3.2 6.47

4.42 9.75

izontal00 N

ad�

ites. The subjects discussed are very important within the con-

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text of restoration and the safeguarding of historical buildings.Frequently, repair procedures involve monumental buildingsmade of masonry. Therefore, in addition to mechanical andphysical benefits, the aesthetic and historical characteristics�material compatibility and the need to respect the structuraland constructive identity of the building� are of fundamentalimportance.

• With regard to the design of strengthening with GFRCM, it isquite natural to propose a criterion based on glass net tensilestrength. In fact, it has been ascertained that first-collapsemode occurs when glass net fibers reach their ultimate tensilestrength. This is an easy and very practical criterion, and more-over, this type of strengthening is close to masonry buildingtechnology and so is also economical in its use. On the otherhand, the design of CFRP needs to avoid delamination, andtherefore, if the limit strength of the masonry to which it isapplied is considered, it is not advisable to count on overlyhigh performance. Furthermore, determination of the strength-ening dimensions requires an investigation of the local bond-slip relationship for the masonry-composite adhesion in orderto establish a simple criterion based on limit-debonding tan-gential stress.

References

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Briccoli Bati, S., and Rovero, L. �2000a�. “Consolidamento di archi inmuratura con nastri di composito a fibre lunghe di carbonio.” Proc.,Meccanica delle Structure in Muratura Rinforzate con FRP-Materials; Modellazione, Sperimentazione, Progetto Controllo, A. DiTommaso, ed., Edizioni Libreria Cortina, Padova, Italy, 53–64.

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