Construction and Building Materials 73 (2014) 740–753

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Construction and Building Materials

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Experimental in situ behaviour of unreinforced masonry elementsretrofitted by pre-tensioned stainless steel ribbons

http://dx.doi.org/10.1016/j.conbuildmat.2014.09.1160950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +39 090 397 7171; fax: +39 090 397 7480.E-mail addresses: nino.spinella@unime.it (N. Spinella), piero.colajanni@unipa.it

(P. Colajanni), antonino.recupero@unime.it (A. Recupero).

Nino Spinella a,⇑, Piero Colajanni b, Antonino Recupero a

a Dipartimento di Ingegneria Civile, Informatica, Edile, Ambientale e Matematica Applicata, Università di MESSINA, C. da di Dio 1, Vill. Sant’Agata 98166, ME, Italyb Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale e dei Materiali, Università di PALERMO, Viale delle Scienze, 90128 Palermo, Italy

h i g h l i g h t s

� Rubble masonry strengthened by three-dimensional pre-tensioned stainless steel ribbons.� In situ full-scale tests on masonry panels and an arch-wall in Messina earthquake prone area.� Discussion of the results that validate the retrofitting method.� The retrofitting method produces a noticeable increment in strength and ductility of ancient masonry.

a r t i c l e i n f o

Article history:Received 5 June 2014Received in revised form 1 September 2014Accepted 24 September 2014

Keywords:MasonryWallArchCAMStainless steelRibbonsReinforcementShear strengthIn-situ tests

a b s t r a c t

The results of in situ tests carried out on unreinforced and reinforced poor rubble masonry full-scale wallsin the earthquake prone Messina (Italy) area are presented and discussed. This experimental researchwas aimed at the assessment of the in plane shear behaviour of ancient masonry strengthened with aninnovative system for masonry and retrofitting of reinforced concrete element constituted by three-dimensional pre-tensioned stainless steel ribbons. A comparison between different strengthening config-urations was made in order to characterise the behaviour of masonry panels under shear-compressionload, focusing attention on the diagonal cracking failure mode. The effectiveness of retrofitting methodin enhancing strength and ductility of poor rubble masonry walls is proved by comparison of theresponse of the unreinforced and reinforced structural systems.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Unreinforced masonry (URM) structures are typical of manyseismic countries around the entire world. In Italy, many of the con-structions of the historical heritage are generally characterised bymasonry having low mechanical properties, both for its textureand for the bad mechanical characteristics of mortar. Masonry wallsare often consisted by irregularly shaped stones, although some-times minimally worked or selected for similar size, and rubblemasonry fill, without any transverse connections. Therefore, theearthquake performance of URM elements is often unsatisfactory.Under strong motions a brittle (shear) failure is frequently

expected. The failure mode is also strongly influenced by the ele-ment’s geometry and the existing vertical (compression) forceapplied.

Coupled with concerns over the maintenance of our architec-tural heritage, there is wide interest nowadays in studying newstrengthening techniques in greater detail. In the past few years,new reinforcement materials used in other applications, such asaeronautics, were introduced to the world of restoration. Thesematerials were, on the whole, presented in the form of Fibre Rein-forced Polymers (FRP) or Fibre Reinforced Cementitious Matrix(FRCM), which are made up of synthetic fibres embedded in resinsor mortar, respectively [1–6]. More recently, the potential of steelcords wrapping of masonry brick having different cross-sectiongeometry and subjected to uniaxial compression load was investi-gated [7].

Nomenclature

Es elastic modulus of stainless steelfys, fts stainless steel yield and failure strengthKa, Km corrective factors between pressure in the flat-jack and

the masonry stressp pressure applied by the flat-jackP, Py, Pm, Pu load, yield-cracking, maximum and ultimate loadt thickness of masonry walltp thickness of external plaster

d, dy, dm, du displacement, yield-cracking, maximum and failuredisplacement

ev vertical average strain of masonryes, et strain and failure strain of stainless steelU diameter of the circular sawrs stainless steel stressrv vertical average stress of masonry

Fig. 1. Stainless steel (a) plates; (b) angular elements; and (c) ribbons.

N. Spinella et al. / Construction and Building Materials 73 (2014) 740–753 741

The use of steel ribbons to improve the seismic performance ofURM walls was used in the past [8–11]. This method consists ofadding normal-strength steel rebar or ribbons along differentdirections on either one side or both sides of URM walls. The tiescomprise steel strips which are anchored to the walls using steelbolts, and they are either fixed or not fixed to the ground-foundation.

Within the framework of a research project at Università diMessina (Italy), the chance of improving the relatively poor perfor-mance of ancient masonry buildings, which characterise the regionof Sicily, by strengthening URM walls with an innovative systemwas investigated. The need for a compaction of masonry mass sug-gested the idea of using a three-dimensional system of tying, capa-ble to confine the masonry structure, giving a beneficial triaxialcompression stress state. On such concept the retrofitting method,namely Active Confinement of Manufacts (or Masonry) CAM sys-tem, is based [12,13]. These ribbons allow a larger stress field tobe formed inside the Reinforced Masonry (RM) elements, thusthe masonry can withstand higher levels of effort and large defor-mation are also attained.

Unlike the retrofitting methods based on the use of reinforce-ment bars or steel ties reported in the literature [8–11], the ret-rofitting method herein investigated is completely realised withstainless steel, to avoid any durability problem and get good duc-tility characteristics. In addition, the ties are realised with stain-less steel ribbons of different yield strength values to evaluatethe influence of this parameter on the efficiency of the retrofit-ting method. The ribbons runs all along the masonry wall, bothhorizontally and vertically, realising a continuous tying system,that is able to improve not only the shear resistance, but alsothe flexural resistance of masonry element in their single partsand as a whole [12,13]. Its effectiveness is enhanced by the useof special connection elements allowing to obtain pre-tensionedribbons able to apply a light pre-compression state to themasonry, which is particularly useful in the transverse direction.

Moreover, the advantage of the retrofitting method, comparedto the well known retrofitting technique with small diameter rebarembedded in the external plaster (i.e. concrete jacketing), lies inthe fact that the combination of masonry and concrete elementsin old building could be avoided, thus eliminating incompatibilityin deformation capacity masonry and concrete, without increasesof mass and/or stiffness that can determine variations in the attrac-tion of seismic forces. In addition, the installation of the retrofittingmethod, herein investigated to strength masonry elements, is littleintrusive and quasi-totally reversible. The ribbons can be easilyremoved and the hole of small diameter filled by mortar.

The retrofitting method was yet used to improve the seismicperformance of brick and ashlars masonry panels and structures[12], where the regularity of the masonry texture enables thedesign of regular ribbons system with large units. However, thereis a lack of knowledge of the effectiveness of this strengtheningsystem in enhancing mechanical properties of masonry walls facedwith irregularly shaped stones and rubble masonry fill.

In this context a research project was developed, aiming atinvestigating the effectiveness of this strengthening system onthe structural behaviour of masonry walls under shear-compres-sion loading by a series of in situ full-scale load tests [14,15].

At this aim, test results of six full-scale panels subjected toSheppard test [16] are reported, analysed and discussed. Some ofthe tested panels were also reinforced using a polymeric netimmersed in the external plaster, varying the setup and themechanical characteristics of the retrofitting method. Despite thenumber of panels investigated is not large, precious informationabout the increment of strength, deformation ultimate capacityand ductility, as well as modification of the original failure mode,resulted from the retrofitting method under shear compressivetests, were obtained.

Moreover, a full-scale wall with the opening that is strength-ened by the lintel in the arch form (for this reason and to distin-guish it by the others, hereinafter referred to as ‘‘arch-wall’’), cutfrom an ancient building, was first tested to characterise the crack-ing behaviour of URM elements and then re-tested after itsstrengthening and minimal repair, to evaluate the influence ofthe retrofitting method on the seismic behaviour of the arch-wallas it relates to the failure mode, resistive capacity and deformation.

2. The retrofitting method

The retrofitting method is mainly based on the use of stainlesssteel ribbons, to tie masonry with loops passing through transverseholes, as shown in Fig. 1. The loops are closed with a special device,

Fig. 2. Ribbons arrangement in a wall with a door and a reinforced concrete upper kerb.

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which is able to apply a calibrated prestress to the ribbon by meansof a seal shown in Fig. 2(a).

The system includes also drawpieces as connection elementsand steel angles as terminal elements, as shown in Fig. 2(b) and(c) respectively.

In current applications, the ribbon is 0.9–1 mm thick and19 mm wide, with yielding and failure strengths ranging from220 to 500 and from 540 to 850 MPa respectively. The drawpieces,which play the role of connection and force transmission elementsbetween adjacent ribbon loops as well as stress distribution ele-ments on masonry, are usually 125 � 125 mm and 4 mm thick(Fig. 2(b)). Similar sizes are used for steel angles in current applica-tions. The distance between holes is typically between 400 and800 mm.

The ribbons setup can be arranged in a squared, rectangular,rhombic, triangular or even irregular mesh, so that a horizontallyand vertical continuous sling is realised. Fig. 1 shows a typicalapplication on a wall faced and rubble masonry fill wall, with analternate arrangement of holes, to minimise their number. Theholes can be eventually injected with any kind - there being no cor-rosion problems – of mortar, to improve masonry characteristicsaround holes. Alternatively, diagonal arrangements of loops canbe more effective for regular brick masonry walls, as well as to con-nect floor kerbs to masonry walls to limit possible kerb-masonry

Fig. 3. Stainless steel tests: preparation (a) without and (b) wit

slipping. A more detailed description of the retrofitting method isavailable in Marnetto et al. [9].

3. Mechanical characterisation of stainless steel ribbons

The mechanical property and the efficiency of junction of thestainless steel ribbon can affect the efficiency of the retrofittingmethod. Therefore, two different types of stainless steel were usedto realise the reinforcement system of the masonry panels andarch-wall: CAM-1) stainless steel with minimum yield (fys) andminimum failure strength (fts) equal to 220 MPa and 540 MPa,respectively; and CAM-2) stainless steel with minimum yield (fys)and minimum failure strength (fts) equal to 500 MPa and850 MPa, respectively.

At the aim to evaluate the effective mechanical properties ofstainless steel, six specimens for each types of stainless steel(CAM-1 and CAM-2) were subjected to uniaxial tensile: three with-out and three with junction. The specimens were obtained by cut-ting elements of length equal to 500 mm, and having width andthickness equal to 19 mm and 1 mm, respectively. A servo-hydrau-lic machine, with a 4000 kN load-carrying capacity, was used for thetensile tests of stainless ribbons, adopting a gauge length of 100 mmas shown in Fig. 3(a and b). The specimens without junction exhib-

h junction; failure mode (c) without and (d) with junction.

Fig. 4. Stress–strain curves for stainless steel ribbons with (R#) and without (R#J) junction: (a) CAM-1; and (b) CAM-2.

N. Spinella et al. / Construction and Building Materials 73 (2014) 740–753 743

ited a typical failure mode characterised by considerable elongationand subsequent necking of the cross-section (Fig. 3(c)); thespecimens with the junction at the middle height showed a failuremode characterised by successive losses of load corresponding tothe slips of the junction (Fig. 3(d)). In Fig. 4 the tensile stress–straincurves for each of the sixth tested specimens are plotted. The rib-bons of stainless steel of normal yield strength (CAM-1) showed agood capacity of deformation, with the maximum strain value about50 mm/m. The effective yield strength value was evaluated as sug-gested by the Italian code [17] as follows. When the slope of firstbranch of curve in not clearly identifiable, as for the stress straincurves of stainless steel, the elastic modulus (Es) is calculated limit-ing the linear part of curve between 0.1 fm and 0.3 fm, being fm thestress value at failure. Known the elastic modulus (Es), the effectiveyield strength (fys) is calculated intersecting the line having a slope

Table 1Summary of results of tensile tests carried out on each specimen for CAM-1.

Code Es (GPa) fys (MPa) fts (MPa) et (mm/m)

CAM-1R1 217 322.8 557.5 52.065R2 210 315.5 573.0 55.655R3 209 312.8 571.3 55.053Mean 212 317.0 567.3 54.257CoV 2% 2% 2% 4%

CAM-1 with jointR1s 234 320.6 487.9 19.560R2s 225 314.9 478.4 17.510R3s 228 309.4 477.6 17.568Mean 229 315.0 481.3 18.212CoV 2% 2% 1% 6%Mean 221 316.0CoV 5% 2%

Table 2Summary of results of tensile tests carried out on each specimen for CAM-2.

Code Es (GPa) fys (MPa) fts (MPa) et (mm/m)

CAM-2R4 200 669.4 811.1 17.097R5 198 671.3 815.4 18.818R6 199 678.2 822.2 17.379Mean 199 672.9 816.2 17.765CoV 0% 1% 1% 5%

CAM-2 with jointR4s 202 664.0 671.2 5.366R5s 200 641.0 645.4 4.833R6s 204 676.4 679.2 5.409Mean 202 660.5 665.2 5.203CoV 1% 3% 3% 6%Mean 200 666.7CoV 1% 2%

Es and a plastic strain of 2 mm/m with the experimental stress straincurve [17].

Tables 1 and 2 synthetically report the results of tensile testscarried out on each specimen, showing the value of elastic modu-lus (Es), yield strength (fys), failure strength (fts); and failure strain(et). In addition, the values of mean and Coefficient of Variation(CoV) are reported.

The constitutive behaviour of ribbons without junction is typi-cal of stainless steel (Fig. 4(a)). The specimens with the junctionin the middle are able to reach values of strength comparable withthe corresponding specimens without junction, but the deforma-tion capacity is reduced, as shown in Fig. 4(a). As summarized inTable 1, the average yield strength and the average elastic modulusfor all CAM-1 specimens are 316 MPa and 221 GPa, respectively.

The ribbons of stainless steel of high yield strength (CAM-2)without junction showed an experimental average yield strengthequal to 672.9 MPa with a capacity of deformation at failure of17.765 mm/m (Fig. 4(b)). These values are lower than correspond-ing one observed for specimens of stainless steel of normal yieldstrength (CAM-1). Moreover, for the sealed specimens the brittlecrisis of the junction was observed and the ultimate strainrecorded was around one third of the corresponding value for spec-imens without junction (Fig. 4(b)). As summarized in Table 2, theaverage yield strength and the average elastic modulus for allCAM-2 specimens are 666.7 MPa and 200 GPa, respectively.

4. In situ panels tests

The existing masonry wall was formed by a masonry of poorquality with stones of different size and crumbly lime mortar(Fig. 5). All panels were obtained cutting the wall and using thediamond-wire technique in order to leave a space between eachof them equal to 200 mm (Fig. 6(b)).

A Reinforced Concrete (RC) beam was cast over the masonrywall in order to achieve a top stiffened constraint, while a rigidsteel beam was used to link the panels together on the bottom sideand with the ground. At the aim of spreading the horizontal load ina uniform manner, two steel elements were inserted in the middleheight of each panel (Fig. 7).

In Fig. 6, the layout of all panels is shown. All panels had anaverage thickness (t) of 500 mm and they were identified by aindex code P#: upper half of the panel P1, with total dimensionsof 1000 � 2150 mm, was tested under compression; and panelsfrom P2 to P7, with dimensions of 1100 � 2150 mm, were testedunder shear-compression.

With the purpose of investigating the efficiency of the retrofit-ting method, the two different types of stainless steel previouslytested for the ribbons were used: CAM-1); and CAM-2). In details,the CAM-2 reinforcement was just used for the horizontal ribbons

Fig. 5. Detail of masonry of existing wall.

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in panels P6 and P7. Moreover, all panels were coated with exter-nal plaster having an average thickness (tp) of 40 mm and an aver-age compressive strength of 5 MPa (data provided by themanufacturer), while a polymeric net with a rectangular mesh51 � 71 mm and an average tensile strength equal to 15 kN/mand 22 kN/m for unity of thickness along longitudinal and trans-versal direction (data provided by the manufacturer), respectively,was immersed in the external plaster for some panels, as it isdetailed reported in Table 3. The use of the polymeric net was

Fig. 6. Panels’ layout an

due to two reasons: (1) to evaluate the efficiency of the polymericnet immersed in the external plaster as a traditional reinforcementsystem; and (2) to evaluate the chance to further improve the per-formance of the retrofitting method with the external applicationof the polymeric net as a device for spreading the confining actionof the ribbons.

4.1. Masonry panel compression test

The compression test was carried out on upper half of the panelof 1000 mm wide (P1). The test has consisted in several cycles ofloading and unloading with increasing the maximum values ofthe vertical compression load. A series of steel elements wereinterposed in the middle of the panel, and connected on both faceof the wall at two metallic plates, positioned over the RC beam on abed of mortar, by four dywidag bars (Fig. 8). Two hydraulic jackswere interposed between two metallic plates of 40 mm thick(Fig. 8). During the loading, the two jacks compress the two platesand pulling the dywidag bars upwards, so as to push the steel ele-ments against the upper half of the panel and cause the compres-sion. This technique was also used to apply the constantcompression load during the shear-compression tests along thetotal height of the other panels (P2–P7).

Each side of the upper half of the panel P1 was instrumentedwith three vertical transducers. A horizontal transducer was posi-tioned at the centre line of both face of the upper half of the panel(Fig. 8).

In Fig. 9, the vertical average compression stress (rv) versus thevertical average compression strain (ev) curve is plotted. Theresults show a very low maximum value of compressive stress(rv,max = 0.89 MPa), highlighting the bad quality of masonry. More-over, the initial branch of curve is characterised by a high stiffnessprobably due to an adjustment of the contrast device, followed by aregular branch of the curve up to failure. The values of the verticaland the horizontal average strain, corresponding to the maximumvalue of compression stress, were 3.709 mm/m and 3.148 mm/m,respectively. Unfortunately, the horizontal transducer has mea-sured strain values of similar magnitude of the vertical strain

d cutting scheme.

Fig. 7. Details of steel elements inserted in the middle of each panel.

Table 3Summary of reinforcement system for each panel.

Code P2 P3 P4 P5 P6 P7

Polymeric net X X XCAM-1 X X X XCAM-2* X X

* Only for horizontal ribbons.

Fig. 8. Test setup for panel P1 in compression.

Fig. 9. Compression stress versus compression vertical strain curve.

N. Spinella et al. / Construction and Building Materials 73 (2014) 740–753 745

values, which is mechanically questionable and would result in anunrealistic Poisson’s ratio. Therefore, only the vertical stress–strainrelationship was considered.

4.2. Description and analysis of the results

In the last decades, several in situ tests were performed todetermine the mechanical properties of masonry, but the literatureconcerning experimental studies on in situ poor masonry walls israther sparse [8–11,18,19]. The shear-compression test technique[16] allows to obtain reliable information about the mechanicalproperties of the masonry, especially regarding the interactionbetween the shear strength and the compressive stress.

The shear-compression tests were carried out on the P2–P7panels. The same test apparatus (plates, dywidag bars, jacks)employed for compression test on panel P1 was used to give thecompression stress to each panel. A constant vertical compressionstress equal to 0.36 MPa (about half of compressive strength ofmasonry supposed in the design phase of tests) was applied onthe tested panel for the entire duration of test. As it shows inFig. 10, the hydraulic jacks were positioned over a high stiffness

RC beam in correspondence of vertical axis of the panel. Therefore,the reactions of the adjacent panels to the applied vertical loadwere considered negligible.

Fig. 10. Shear-compression test setup: (a) panels P2, P3, P4 and P5; and (b) panels P6 and P7.

746 N. Spinella et al. / Construction and Building Materials 73 (2014) 740–753

To distribute the horizontal force throughout the panel thick-ness, two steel elements having C shapes, coupled whit plateswelded to the webs, were connected each other across the middleheight of the wall. The shear load was applied in different way ateach panel: (1) for panels P2, P3, P4 and P5, the horizontal loadwas directly applied on the metal element at the middle heightof the panel, using the adjacent panels, among which was inter-posed a steel element as contrast; (2) as detailed below, the valuesof load and displacement achieved during the test of panels P4 andP5 caused a sign of damage in the contrast system, therefore forpanels P6 and P7, the horizontal load was applied by two dywidagbars pulling on the metal element at the middle height of thepanel. The panels P4 and P5, previously tested, were reinforcedand re-connected to each other with concrete, thus a monolithicwall was obtained and it was used as a contrast element. A hydrau-lic jack was interposed between the reinforced wall and the panelto be tested. During the loading, the jack acts on a plates positionedon the reinforced wall and then on the two dywidag bars (Fig. 10).

Eight inductive transducers were positioned along the diagonalsof the four halves obtained by subdividing each of the two verticalsides of the panel in two equal parts. Four other transducers werepositioned on each side of panel and along the vertical middle line.

Fig. 11. Horizontal load–displacement curve for pane

Two more transducers were placed on the other side of the jackand along the line of the horizontal load (P) to measure the hori-zontal displacement (d).

The panel P2 was tested without any kind of reinforcement,while in panel P3 only the polymeric net immersed in the externalplaster was used. The comparison of the test results shows that theinfluence of the polymeric net in the unreinforced panel was neg-ligible both in terms of strength and ductility. In details, theobserved load-capacity (Pm) and the ultimate displacement (du)of panel P2 were equal to 179.8 kN and 13.4 mm, respectively;while for panel P3 the recorded values were Pm = 193.5 kN anddu = 14.6 mm (Fig. 11(a and d)). The crack pattern at failure wasthe same for both panels, with the principal tensile stress in thecentre of upper half panel equal to the tensile strength of themasonry and the consequent formation of the shear critical crack(Fig. 12(a and d)).

The panels P4 and P5 were reinforced with CAM-1 system, andfor the P5 panel the polymeric net immersed in the external plasterwas also used. The beneficial effects of the retrofitting method onthe shear behaviour of masonry are visible in Fig. 11(b and e),where the observed maximum loads (Pm) were 261.6 kN and353.8 kN for P4 and P5 walls, respectively. By comparison of the

ls (a) P2; (b) P4; (c) P6; (d) P3; (e) P5; and (f) P7.

Fig. 12. Crack pattern at failure for panels (a) P2; (b) P4; (c) P6; (d) P3; (e) P5; and (f) P7.

N. Spinella et al. / Construction and Building Materials 73 (2014) 740–753 747

aforementioned values to the URM panels P2 and P3, the maxi-mum loads of RM panels were incremented of 45% and 83%,respectively, showing that the three-dimensional confinementeffect due to the stainless steel ribbons was improved by the poly-meric net distributing the confining stress through the entire faceof the panel. It has to be emphasized that test for panel P4 wasstopped at a displacement value of 17.4 mm, (only 30% greaterthan the one recorded for the unreinforced specimen P2) sincethe test setup did not allowed to apply greater displacement, sincethe contrast system begun to move; thus, to avoid any damage onthe adjacent panel P5, the increment of displacement capacityensured by the retrofitting method until the failure could not bemeasured. However, the post-peak plateau of load–displacementcurve for panel P4 (Fig. 11(b)) and the complete formation of crit-ical crack (Fig. 12(b)) allow to reasonably consider that the regis-tered maximum load was representative of the effective ultimateload of reinforced panel (Pm � Pu). The panel P5 showed a differentcrack pattern, characterised by a principal diagonal crack and sev-eral secondary cracks (Fig. 12(e)). The maximum displacementimposed to the panel P5 was of 33.9 mm, (Fig. 11(e)), that was132% greater than the displacement capacity of the panel P3. Onceagain, the test was interrupted since the displacement capacity ofthe testing setup was exceeded.

The panels P6 and P7 were reinforced with CAM-1 along thevertical direction and with CAM-2 along the horizontal direction,as shear reinforcement. As panels P3 and P5, for the panel P7 thepolymeric net immersed in the external plaster was also used.The maximum load (Pm) registered for the panel P6 and P7 wereequal to 452.4 kN and 486.6 kN respectively, corresponding to an

increment of maximum load to 152% and 151% respect to thestrength of panels P2 and P3, respectively.

It has to be emphasized that, as showed in Fig. 12(c and f), thecrack pattern at failure was different for the two panels. The panelP6 showed a principal diagonal crack similar to panel P4, while thepanel P7 showed several secondary cracks parallel to the principalone similar to panel P5. As for panel P4 and P5, also for panels P6and P7 the retrofitting method provided a considerable increasingof strength and ductility, which is strictly related to the mechanicalpercentage of stainless steel ribbons used. In addition, the poly-meric net allowed distribution of the stress through the entire faceof the panel, as shown by the widespread cracking.

In Table 4, the results of test panels are summarized in terms ofyield-cracking (Py, dy), maximum (Pm, dm) and ultimate (Pu, du) loadand displacement. The ratio du/dy provides a measure of displace-ment ductility, which results in a large improvement due to theuse of the retrofitting method.

5. In situ arch-wall tests

The masonry arch-wall chosen for the investigative campaignwas part of an ancient masonry building destined to demolition(Fig. 13(a)). It was a real masonry wall, not just a specimen pre-pared in laboratory, thus the geometrical shape was not perfectlysmooth. The arch-wall was formed by a masonry of poor qualitywith stones of different size and crumbly lime mortar, similar tothat of the panels.

Table 4Summary of panels test results in terms of yield-cracking, maximum and ultimateload (P) and displacement (d).

P2 P3 P4 P5 P6 P7

Py kN 130.7 137.5 186.7 0.0 308.2 303.1Pm kN 179.8 193.5 261.6 353.8 452.4 486.6Pu kN 147.1 156.5 243.7 353.8 400.6 431.4dy mm 2.4 2.5 1.8 3.4 1.7 3.2dm mm 6.9 6.9 14.1 33.9 15.4 14.5du mm 13.4 14.6 17.4 33.9 41.2 66.2du/dy 5.5 5.8 9.7 9.8 23.9 20.5

748 N. Spinella et al. / Construction and Building Materials 73 (2014) 740–753

The arch-wall had a nearly square prospectus: the basis was4450 mm width; the height was 4600 mm; and the average thick-ness (t) was 700 mm. The abutments had a tetragonal cross-sectionshape: the left was around 950 mm in width; the right was around1200 mm in width; and the width of arch was 2300 mm. Along theopening, a curved lintel was present (Fig. 13(b)).

The preparation phase of testing was characterised by the fol-lowing steps: (1) the existing external plaster was eliminated; (2)the railing on the top was demolished; (3) the masonry arch-wallwas isolated by the rest of building with a diamond circular saw;and (4) it was coated with new external plaster without anyreinforcement.

With the aim to obtain a fixed reference for measurements, alight steel frame was built close to the front face of the arch-wall;then eight Linear Voltage Displacement Transducers (LVTDs) werepositioned in different points of arch-wall (Fig. 13(b)). The LVDTsS1 and S2 measured the displacements on the left abutment alongthe vertical and horizontal direction, respectively; on the rightabutment, the corresponding LVDTs were S4, vertical, and S5, hor-izontal; along the horizontal load line, the LVDT S3 was positionedat the middle of the arch-wall, above the keystone, while the LVDTS7 was positioned on the opposite side with respect to the hydrau-lic jack to measure the global horizontal displacement; the LVDTsS8 and S6 measured the horizontal displacement and were posi-tioned at the top-left and top-right of the arch-wall, respectively.In addition, to check eventual dangerous out-of-plane displace-ments, two LVDTs orthogonal to the internal arch-wall face andon the same level of LVDTs S8 and S6 were used.

In the second phase of tests, the reinforcement of arch-wall wasrealised using two overlapped ribbons having CAM-1 mechanicalcharacteristics.

Fig. 13. URM arch-wall: (a) original

5.1. Masonry double flat-jack test

A test with double flat-jacks was performed on the ancientmasonry of the building at a wall orthogonal to the arch-wall.The objective was to estimate the compression strength in themasonry. The flat-jack test technique is a relatively well knownnon-destructive one and its efficiency was largely investigated byGregorczyk and Lourenço [20], which observed as the flat-jacktechnique can as well over or underestimate actual stress ofaround 20%. After the creation of two horizontal slots in themasonry, the compressive stresses cause the closure of themasonry above and below each slot. Before cutting the wall, origi-nal dimensions were taken between gage points. Once the cutswere made, the flat-jacks were loaded in the cuts and readingsbetween the gage points are taken at various pressures. From thisdata, it can back calculate the stress present in the wall before thecuts were made as well as the modulus of elasticity of the masonry.After data for both tests are obtained, the cuts were restored to itsoriginal conditions with mortar.

Taking into account some corrective factors, the measure of thepressure p applied by the flat-jack approximately corresponds tothe local pressure in masonry. In details, the average compressivestress in the masonry, rv, could be calculated as follows:

rv ¼ Ka � Km � p ð1Þ

where Ka is the factor that accounts for the ratio between the bear-ing area of the jack in contact with the masonry and the bearingarea of the slot; Km is the factor accounting for the physical charac-teristic of the jack, and p is the pressure required to restore thegauge points to their original distance. Both the single and doubleflat-jack tests were conducted with respect to the recommenda-tions issued by the American Society for Testing and Materials[21,22]. For the execution of the in situ tests the following equip-ment was used: semi-oval flat-jack (dimensions:350 � 260 � 4 mm with Km = 0.84 and Ka = 0.90); hydraulic circularsaw (U 350 mm and thickness 3.5 mm); hydraulic hand pump(manometer with ranges of 20 and 60 bar); and mechanical straingauges. Fig. 14 reports the scheme of the test (acquired geometryand position of the bases measurement of the removable strain-gauge), and Fig. 15 reports the outcome of the double flat-jack testwhere vertical strains are plotted as functions of the stress in thewall (mean stress–strain curves).

conditions and (b) test design .

Fig. 14. Double flat-jack test: (a) setup and (b) preparation (dimensions in mm).

Fig. 15. Double flat-jack test: average stress–strain curves.

N. Spinella et al. / Construction and Building Materials 73 (2014) 740–753 749

The results show a weak masonry with a compression strengthequal to 1.90 MPa, while the first crack appeared at a level of com-pression stress equal to 1.30 MPa.

5.2. Description and analysis of the results

The pseudo-static horizontal load was applied by a hydraulicjack governed by a hand pump linked to a pressure transducer.At the aim to uniformly distribute the horizontal force throughoutthe wall thickness, a metal element was positioned between thehydraulic jack and the masonry wall.

For the first test on the URM arch-wall, a hydraulic jack with acapacity of 200 kN and a maximum displacement of 50 mm wasused. For the second test on the arch-wall reinforced with thestainless steel ribbons, a hydraulic jack with a capacity of1000 kN and a maximum displacement of 100 mm was used. Both

Fig. 16. First cracks on URM arch-wall: (a) left haunch of the arch; (b) long horizont

load and displacements data were acquired by an electronic con-trol unit.

Moreover, a constant vertical compression load equal to 7 kN/mwas applied for the entire duration of both tests by positioningcement sacks along the top of the masonry wall. This distributedload was necessary to partially reproduce the action of the verticalloads transmitted from the slab originally connected to the top ofthe masonry arch, and its value was chosen compatibly with the con-ditions of the test field and ensuring the minimum safety level insitu.

5.2.1. URM arch-wall testThe first test was carried out on the URM arch-wall. The load

was applied by several cycles and incremented until the first cracksappeared. The first cracks were formed: (a) in correspondence ofthe left haunch of the arch; (b) close to the top of the wall (a hor-izontal one); and (c) close to the point load (Fig. 16). In Fig. 17, theload–displacement curves show the capacity of URM arch-wallboth in terms of crack strength and ductility. The crack load (Py)was equal to 50.6 kN, it was reached at a corresponding horizontaldisplacement (S7) of 5.4 mm exhibiting an almost linear behaviour(Fig. 17(h)). The left abutment was subjected to a tensilestress, while the right abutment was compressed as expected(Fig. 17(c and d)). In the part of arch-wall over the keystone themeasured horizontal displacement was about the same for theLVDTs S8, S6 and S3 (Fig. 17(e–g)), because just few parts ofmasonry were cracked.

During the unload cycles, the first cracks were all closed, withnegligible residual plastic deformations. However, the load incre-ment was stopped to avoid wide plastic deformations and the col-lapse of the URM arch-wall.

5.2.2. RM arch-wall testIn the second phase of the test, two overlapped ribbons of CAM-

1 stainless steel was used to confine the masonry along the face of

al crack closes to the left top of the wall; and (c) crack closes to the point load.

750 N. Spinella et al. / Construction and Building Materials 73 (2014) 740–753

the wall and in the orthogonal direction to the arch-wall plane,with ribbons passing through the holes drilled in the masonry.The ribbons scheme adopted is shown in Fig. 18, with a squaredarrangement of stainless steel ribbons along the abutments, while

Fig. 17. Horizontal load–displacement (P–S#) curves for URM and RM arc

a radial arrangement was adopted for the masonry over thekeystone. The radial stainless steel ribbons was positioned orthog-onal to the haunch of arch-wall to confine the masonry along in-plane and out-plane directions when subjected to horizontal load.

h-wall: (a) S1; (b) S4; (c) S2; (d) S5; (e) S8; (f) S6; (g) S3; and (h) S7.

N. Spinella et al. / Construction and Building Materials 73 (2014) 740–753 751

Moreover, four steel angles (40 � 40 � 4 mm) were used to rein-force the inner edges of each abutments increasing their flexuralstrength (Fig. 18).

Then, horizontal load cycles having an increasing displacementamplitude variable on the basis of the slop of P � d curve at eachcycle were applied. The maximum load (Pm) of 111.1 kN wasreached at the end of the 6th cycle, when the horizontal displace-ment (dm) reached about 50 mm. The observed displacementcapacity of the arch-wall reinforced with the retrofitting methodwas unexpected and higher than the elongation capacity of thehydraulic jack, as shown in Fig. 19(a); thus the collapse of the

Fig. 18. RM arch-wall: (a) original co

Fig. 19. RM arch-wall: crack patterns, before the failure, of (

structure was not reached. Aiming to evaluate the displacementcapacity of the RM arch-wall, the structure was unloaded, and anhydraulic jack with a larger stroke was used to impose the dis-placement. A new branch of the load curve was realised by impos-ing displacement up to the capacity of the new hydraulic jack; alsothis time the collapse was not reached and the attained displace-ment was about 70 mm. The arch-wall must be unloaded againdue to the attainment of the full displacement capacity of the test-ing jack.

At the maximum level load, a side of the bottom parts of boththe left and right abutments were subjected to tensile stress and

nditions and (b) design of test.

a) arch-wall; (b) left abutment; and (c) right abutment.

752 N. Spinella et al. / Construction and Building Materials 73 (2014) 740–753

wide cracks were formed (Fig. 19(b and c)). Several cracksappeared across the top part of the arch-wall and a widespreadcracking was observed over the entire RM arch-wall. It was dueto the capacity of the retrofitting method to confine the masonry,allowing the stress transfer across cracks and to spread stressesall over the arch-wall. Moreover, during the unload cycles, thewide cracks were always closed also for high load levels and limit-ing the plastic deformations.

Some ribbons in the region over the left abutment have reachedits yield strength, showing a plastic behaviour in the unloadingphase.

The load–displacement curves obtained in the first (URM arch-wall) and second phase (RM arch-wall) of tests are plotted togetherin the Fig. 17 for a comparison of results. The maximum load wasequal to 111.1 kN, which was reached at a corresponding horizon-tal displacement (S7) of 50 mm (Fig. 17(h)) with a percentageincrement of 120% of RM arch-wall failure load respect to theURM arch-wall cracking load. The percentage increase of displace-ment was not calculated because the data corresponding to theURM arch-wall is related to the cracking condition. The initial slopeof curves obtained for URM and RM arch-wall are very similar, con-firming that the retrofitting method does not modify the stiffnessof wall, as expected. The stainless steel ribbons induce a first con-finement effect due to their prestressing, but their effectivenessincreases with the increase of the external load.

6. Conclusions

The results of an experimental in situ campaign about the retro-fitting method based on pre-tensioned stainless steel ribbons onancient masonry of poor quality with stones of different size andcrumbly lime mortar typical of Messina area (Italy) were presentedand discussed.

The masonry structure was confined, along three orthogonaldirections, by pre-tensioned stainless ribbons. The compressionstress state induced by such retrofitting method, allows to improvethe strength and ductility of the masonry elements.

In details, seven full-scale panels were cut from an ancientmasonry wall and a full-scale wall was isolated from an existingmasonry building. Some of specimens were reinforced with theretrofitting method, adopting different schemes and mechanicalcharacteristics for stainless steel ribbons. The specimens were sub-jected to in situ shear-compression tests, providing precious infor-mation about the capacity of the retrofitting method investigatedto improve the mechanical behaviour of URM elements.

Using the retrofitting method herein investigated, the increaseof strength of tested masonry panels ranging between 45% and152%; the remarkable increases in the displacement capacityensured by the retrofitting method caused the attainment of thedisplacement capacity of the testing setup for some of the RM pan-els and the RM arch-wall before the collapse occurred. Moreover,the retrofitting method ensures a large displacement ductility forRM elements, which was observed to be about four times ofURM panels.

When reinforcing of incoherent masonry is concerned, as inthe cases of tests presented, the pre-tensioned stainless steel rib-bons, coupled to the polymeric net immersed in the external plas-ter, allows a distribution of the stress across the entire face of thewall, strongly enhancing the displacement capacity of masonryelement.

The retrofitting method contributes to improve the transverselink between masonry layers, the increase of the in-plane strengthand ductility, and the connections between intersecting walls.

The presented strengthening elements are easy to install on siteand integrate well in the overall concept, especially in unlisted

buildings where the realisation of the holes for the passage ofthe ribbons is acceptable. This retrofitting method uses modernmaterials and is based on a little invasive technology, then itsapplicability on important heritage buildings must be evaluatedcase-by-case.

The experimental results obtained by the tensile tests per-formed on the steel ribbons have highlighted the slight loss ofeffectiveness of the same at the junction. Consequently, in thedesign phase of the strengthening system it needs to take accountof this strength reduction.

Even though more tests are needed, it can be concluded that thespecimens retrofitted or repaired with the retrofitting methodherein investigated behaved satisfactorily.

Moreover, the experimental results show as the retrofittingmethod needs an optimize design to obtain the desiderate ductilityand strength, avoiding waste of material and additional costs. Theanalytical modelling of the URM and RM masonry tested elementsbehaviour is the natural topic of future research.

Acknowledgements

This experimental tests have been made possible thanks tofinancing from the Sicilian Government within the research projectPO. FESR 2007-2013 – Sicilia – Linea di Intervento 4.1.1.2 involvingABI S.r.l. (Italy), Chimetec S.a.s. (Italy) and Università di Messina(Italy). A special acknowledgment goes also to Engineer SebastianoD’Andrea for the contribution in performing the tests. The dataelaboration was carried out within the 2010–2013 Research Pro-ject ‘‘DPC-ReLUIS (Dipartimento Protezione Civile – Rete dei Labor-atori Universitari di Ingegneria Sismica)’’. The related financialsupport was greatly appreciated.

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