Construction and Building Materials 23 (2009) 3337–3345

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Construction and Building Materials

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Experimental characterization of stone masonry in shear and compression

G. Vasconcelos *, P.B. LourençoISISE, Department of Civil Engineering, University of Minho, Guimarães, Portugal

a r t i c l e i n f o

Article history:Received 7 August 2008Received in revised form 2 June 2009Accepted 18 June 2009

Keywords:GraniteStoneShearCompressionBed jointTesting

0950-0618/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2009.06.045

* Corresponding author. Tel.: +351 253 510200; faxE-mail address: graca@civil.uminho.pt (G. Vasconc

a b s t r a c t

Shear and compressive mechanical properties are needed for the evaluation of the strength of masonryshear walls by means of simplified methods or numerical analysis. This, in turn, allows to design or assessmasonry buildings subjected to combined vertical and horizontal loading. Even if many results on themechanical properties of modern brick and block masonry are available in the literature, only a fewresults exist for stone masonry.

Here, the shear and compressive strength parameters of stone masonry using granite blocks are pro-vided. In addition, a first aspect addressed is the shape of the shear stress–displacement diagrams undermonotonic and cyclic loading. A second aspect addressed is the influence of the surface roughness and ofthe bed joint material on the compressive behavior of masonry.

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1. Introduction

Ancient masonry is a non-homogeneous material, composed byunits and mortar, which can be of different types, with distinctmechanical properties. Often in monumental buildings, and ifmaterials were available locally, bricks were used for vaults whilestone was used in the construction of walls.

The influence of mortar joints acting as a plan of weakness onthe composite behavior of masonry is particularly relevant in caseof strong unit–weak mortar joint combinations, which are charac-teristic of ancient stone masonry [1]. Two basic failure modes canoccur at the level of the unit–mortar interface: tensile failure(mode I) associated to stresses acting normal to joints and leadingto the separation of the interface, and shear failure (mode II) corre-sponding to a sliding mechanism of the units or shear failure of themortar joint. In terms of the composite behavior of masonry, fail-ure modes related to tensile splitting of the units and mortarcrushing need to be taken into account [2]. The preponderance ofone failure mode over another or the combination of various fail-ure modes is essentially related to the orientation of the bed jointswith respect to the principal stresses and to the ratio between theprincipal stresses [3].

Even if several results on the mechanical properties of brick andblock masonry assemblages are available in the literature [4–7]scarce experimental data is available for stone masonry [8,9]. Thepresent work deals firstly with the mechanical characterization

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of the masonry components: stone units and mortar, as well aswith the shear behavior of dry and mortar masonry joints (cohe-sion, friction angle and dilatancy). Then, the compressive proper-ties of the masonry composite (compressive strength andmodulus of elasticity) are addressed. In order to attain such a goal,an experimental program was defined, including direct shear testsconducted on dry and mortar masonry joints and uniaxial com-pression tests carried out on a set of stone masonry prisms withdistinct types of bed joints. Besides providing mechanical proper-ties for numerical simulations of the in-plane behavior of stonemasonry walls, the adopted testing program provides also the fun-damental parameters required for the seismic assessment of ma-sonry buildings based on simplified analytical models.

2. Shear behavior of stone masonry joints

Although several experimental studies have been carried out onthe bond shear strength of unit–mortar interfaces [4,5], limited re-search is available on the shear behavior of dry masonry joints [8].On the other hand, the knowledge gathered on rock joints undershear behavior can be partly extended to dry masonry joints. Theshear behavior of rock joints plays an important role in rockmechanics research, with several experimental and numericalstudies pointing out the role of the surface roughness on the cyclicshear behavior of natural rock joints [9,10].

The relation between normal and shear stresses plays a majorrole in the shear behavior of masonry joints, actually governingits failure mode [11]. For pre-compression stresses above a certainlevel, the shear strength decreases and a combined shear-splitting

a

3338 G. Vasconcelos, P.B. Lourenço / Construction and Building Materials 23 (2009) 3337–3345

failure or splitting of the units occur. In case of joint shear failureby slipping of the units, an increase of the normal stress leads toan increase of the shear strength. As has been widely reported[4,12], the shear strength of masonry under moderate normalstresses, for which the non-linear behavior of mortar is negligibleand the friction resistance takes the central role, can be given bythe Coulomb criterion:

s ¼ c þ lr ð1Þ

where c is the shear strength at zero vertical load stress (usually de-noted by cohesion) and l is the friction coefficient or tangent of thefriction angle. For dry joints, the cohesion is obviously zero.

2.1. Test specimens and testing procedure

Although triplet tests have been adopted as the European stan-dard method [13] to perform shear tests in masonry joints, thestrength properties of dry and mortar joints were obtained hereby means of direct shear tests carried out on couplet specimens,see Fig. 1, [14]. In fact, the triplet test is rather complex to analyzeand control after peak displacement due to the fact that two jointsare tested simultaneously, see also [15]. The shear tests were car-ried out in a servocontrolled universal testing machine CS7400Scomposed by two independent hydraulic actuators used to trans-mit normal and shear loads, able to operate under force or dis-placement control. Both shear and normal stresses weremeasured and recorded by horizontal and vertical load cells of22 kN capacity. Due to the limited space between steel platensthe most suitable testing sample is composed by the two unitswith geometry and dimensions indicated in Fig. 1a, similarly to[6] and [16]. The surface of the dry stone masonry units is rela-tively smooth resulting from sawing, whereas the joint surface ofthe units of the mortar assemblages presents the typical hand-coursed roughness to achieve realistic bond conditions. The graniteused for the masonry units is a medium grained two-mica granite[17–19]. The shear specimens were placed between two thick steelplates and attached to the steel platens by steel bolts, so that shearforce could be transmitted, see Fig. 1b. Thin steel sheets were at-tached to the steel plates to concentrate the shear load as closeas possible of the bed joint, aiming at providing a more uniformshear stress distribution. In order to guarantee right angle surfaces,the dry specimens were suitably ground using a rectifying ma-chine. The specimens were properly attached for load reversal bymeans of adjustable steel plates on both sides of the specimen.

a

b

Fig. 1. Masonry specimens: (a) dry joints and mortar joints and (b) arrangement ofthe LVDTs for measuring the relative horizontal and vertical displacements.Dimensions are in mm.

The numerical assessment of test setup performed by Lourençoand Ramos [8] indicates the adequacy of the proposed approach.

In order to simulate typical normal stresses existing in ancientmasonry structures three distinct pre-compression stress levelswere considered, r = 0.5 N/mm2, r = 0.75 N/mm2 and r = 1.0 N/mm2. An additional pre-compression stress level equal tor = 1.25 N/mm2 was adopted for the monotonic tests in mortaredassemblages. Three specimens were tested for each level of pre-compression, and for dry and mortared joints. In addition, theinfluence of the moisture content on the shear response of dry ma-sonry joins was investigated by considering dry and saturated con-ditions. The relative horizontal displacement of the joint wasmeasured by the horizontal LVDTs placed at each side of the spec-imen, see Fig. 1b. The vertical displacement of the joint was mea-sured by the LVDTs placed at the opposite corners of thespecimen, which enabled the assessment of possible dilatantbehavior of the joints. The cyclic tests were carried out under dis-placement control following the time-displacement history used in[14].

2.2. Monotonic behavior of masonry joints

The shear load–displacement diagrams for distinct pre-com-pression stress levels resulting from the monotonic tests carriedout on dry and saturated specimens are displayed in Fig. 2. Theshear displacement is the result of averaging the measurements re-corded by the LVDTs placed at each side of the specimen. The shearstress s and the normal stress r are obtained by dividing the shear

b

Fig. 2. Shear stress–displacement diagrams in dry joints: (a) dry specimens and (b)saturated specimens.

G. Vasconcelos, P.B. Lourenço / Construction and Building Materials 23 (2009) 3337–3345 3339

force H and the normal force N, recorded in vertical and horizontalactuators, by the cross section of the joint A. It is observed that nosignificant differences were detected between dry and saturatedspecimens, apart from the slighter decrease on the peak stress insaturated specimens. Besides, higher scatter was found when themaximum pre-compression level (r = 1.0 N/mm2) was applied.

Three stages can be considered to describe the shear-displace-ment diagrams. The pre-peak behavior is characterized by a linearstretch for low levels of shear stress and by a clear non-linearstretch before peak stress. A plateau is found after peak stress, rep-resenting the considerable plastic deformations associated toinelastic sliding. Similarly to what has been reported in the litera-ture [10,20] no shear softening was recorded after peak stress, un-like rough rock joints that exhibit remarkable lowering of the shearresistance as the plastic shear displacement increases.

The shear stress–displacement diagrams of mortared joints areshown in Fig. 3a. The general shape of the shear stress–shear dis-placement is characterized by a sharp initial linear stretch. Thepeak load is rapidly attained for very small shear displacements.Similarly to what was reported for dry masonry joints, non-lineardeformations develop in the pre-peak regime. After peak load is at-tained there is a softening branch corresponding to progressivereduction of the cohesion, until reaching a constant dry-friction va-lue. This stabilization is followed by the development of large plas-tic deformations. The evolution of the vertical displacement withthe shear displacement is displayed in Fig. 3b, where in the major-ity of the tests two distinct phases can be distinguished. Firstly, theuplift of the joint is expressed by increasing positive vertical dis-

a

b

Fig. 3. Shear behavior of mortared joints: (a) shear stress–displacement diagramand (b) relation between vertical displacement and shear displacement.

placement. It is observed that the non-linear evolution of the ver-tical displacement provides variable dilatancy assumingdecreasing values as the shear displacement increases up to theshear displacement corresponding to the stabilization of the shearstress. During the subsequent regime of pure friction the verticaldisplacement remains constant or progressively decreases, partic-ularly when the level of pre-compression increases. The verticaldisplacement exhibits even negative values in some specimenssubmitted to pre-compression levels of r = 0.875, 1.00 and1.25 N/mm2).

The dilatant behavior reflects, to great extent, the distinct shearfailure modes obtained in the specimens submitted to differentnormal stresses. For low to intermediate levels of pre-compression,shear failure occurs at the unit–mortar interface along one unitface, or divided between two unit faces. For the larger normalstress level (r = 1.25 N/mm2), failure is localized in the mortar.

2.3. Cyclic behavior of dry masonry joints

The typical shear stress–displacement diagrams obtained in di-rect cyclic shear tests conducted in dry masonry joints, using dryand saturated specimens, are displayed in Fig. 4 for the level ofpre-compression r = 0.75 N/mm2. Apart from the small differenceof the peak shear stress, no significant differences in the shape ofthe diagrams were found. Minor differences were found in theshear strength during the reversal cycles, see Fig. 5a, despite wear-ing of the surface and degradation of rock forming minerals withthe accumulation of degraded material, similarly to what has beenpointed out by Lee et al. [9]. A very minor trend for compaction canbe seen from the normal displacement–shear displacementdiagrams indicated in Fig. 5b. A maximum value of vertical

a

b

Fig. 4. Shear stress–displacement diagrams for dry joints under cyclic loading(r = 0.75 N/mm2): (a) dry specimens and (b) saturated specimens.

a

b

Fig. 5. Characteristic aspects of the shear behavior of dry joints: (a) evolution of theshear stress–displacement diagrams between the first and the last cycle of reversalloading and (b) compaction due to the wearing of the joint surface.

Fig. 6. Correction of the measured shear displacement for a saturated specimensubmitted to a normal stress r = 1.0 N/mm2.

3340 G. Vasconcelos, P.B. Lourenço / Construction and Building Materials 23 (2009) 3337–3345

displacement of ±0.06 mm was measured for the dry and saturatedtested specimens, indicating that dry granitic joints are non-dilat-ant, which is in agreement with the findings of Lourenço and Ra-mos [8] for sandstone dry joints and of Homand et al. [21] forhammered granitic joints.

Other relevant aspect in the discussion is the comparison be-tween the loading–reloading stiffness. The stiffness of the unload-ing branches exhibits always higher stiffness than the stiffnessobtained in the reloading cycles. The effect of the elastic deforma-tion of stone unit is assessed by removing the elastic deformationof the unit:

ujoint ¼ umeas �sku

ð2Þ

where umeas is the shear displacement given by the horizontalLVDTs, s is the shear stress for a given displacement and ku is thestiffness calculated in the unloading branches. It is possible to con-firm that the elastic deformation of the units has a minor role in thetotal shear displacement of dry joint, see Fig. 6.

Finally, it is observed that the shear cyclic behavior of dry jointsis characterized by non-linear deformations in the pre-peak stageand perfect plastic deformations after peak stress resulting fromthe characteristic sliding failure mode.

2.4. Shear strength parameters

Fig. 7 shows the linear relationships between the values of theshear strength obtained in the monotonic tests and in the firstcycle of the cyclic tests for dry and saturated conditions and the

values of the normal stress, which confirm the assumption thatthe shear strength is well described by Coulomb’s friction law.

The slight decrease in the shear strength obtained on saturatedspecimens is reflected by a decrease of 5% on the friction coefficientin saturated joints, being of 0.65 in dry joints and of 0.6 in satu-rated joints. A range for the friction angle between 10� and 22.4�for mudstone was pointed out by Geerstsema [22]. The frictioncoefficient of dry granitic joints is similar to the values found byLourenço and Ramos [8] for sandstone units (l = 0.63), and byLee et al. [9] for granitic joints (l = 0.69). The narrow range of val-ues found for the friction angle seems to indicate that no signifi-cant differences are expected among distinct types of naturalstone under similar roughness surface conditions (sawn-cutsurfaces).

It is observed that, particularly in the case of dry specimens,there is a small increase of the frictional coefficient with wearing.Significant linear correlation coefficients between normal and peakand residual shear stresses were also found in case of mortaredmasonry joints with coefficients of correlation of r2 = 0.88, andr2 = 0.81, respectively. A value of cohesion about 0.36 N/mm2 andthe tangent of the friction angle, tan /, equal to 0.63, correspondingto a friction angle of 32.2�, were attained for the peak strength. Theresidual shear strength can be calculated with reasonable accuracyfrom a friction coefficient of 0.78.

Table 1 summarizes selected results published in the literaturereferring to the shear strength properties, where very different val-ues of the cohesion and friction angle are pointed out for distinctunit–mortar assemblages. The shear strength parameters aregreatly dependent on the moisture content, porosity of the units,on the strength and composition of mortar and on the nature ofthe interface [23]. From the results of direct shear tests carriedout by Pluijm [6], the coefficient of internal friction ranges between0.61 and 1.17, whereas cohesion varies from 0.28 up to 4.76,depending on different types of units and mortar.

It is seen that the values of the shear strength parameters ob-tained for assemblages of granitic stone and lime mortar presentedin this study are of the same order of the shear parameters pointedout for assemblages of lime mortar and old bricks [4].

3. Compressive behavior of stone masonry

Several experimental, numerical and simplified analytical stud-ies have been carried out in order to increase the knowledge aboutthe compressive behavior of masonry [7,27,28]. Being masonry acomposite material usually made of units and mortar, it has beenlargely accepted that its failure mechanism is governed by theinteraction between the components.

a

b

c

Fig. 7. Relationship between normal and shear stress: (a) dry specimens, (b)saturated specimens and (c) mortared joints.

G. Vasconcelos, P.B. Lourenço / Construction and Building Materials 23 (2009) 3337–3345 3341

The experimental characterization of masonry under compres-sion requires representative wallets, with geometry and dimen-sions described in, e.g. the European Standard EN1052-1 [29],including at least one head joint and three masonry courses. Dueto the very high strength of granite and the maximum capacityof actuators available for testing, the compressive features of stonemasonry were obtained using prisms. According to Page and Shrive[30], the prisms are adequate masonry assemblages that includesimultaneously the effect of bedding type, unit–mortar interactionand workmanship.

3.1. Experimental details

The present experimental program aims at obtaining data onmasonry compression for dry and low strength mortar, being thelatter more representative of ancient masonry structures. In addi-tion, the tests aim also at providing insight about the influence ofthe surface roughness and bed joint material on the compressivestrength and deformation characteristics of masonry. With respectto dry masonry, both sawn (PR_S) and rough surface (PR_SR) jointsare considered. The rough surface is obtained by sand-blasting. Formortared specimens, two types of interlayer material representa-tive of ancient buildings are adopted, namely low strength limemortar (PR_SM) and dry clay from sieving granitic soil (PR_SS).The compressive strength of masonry prisms is known to be af-fected by several factors, such as size, height and end conditions[30–32]. In order to reduce the platens confining effect, a heightto length ratio of three was adopted according to ASTM E447[33], see Fig. 8.

The masonry prisms were made with three cubic stone units of150 mm length and two bed joint courses, as shown in Fig. 8. Thethickness of mortar or granitic soil placed in the bed joints was10 mm. The dry stacked masonry prisms, PR_S and PR_SR, were builtat the testing location by simply laying the second and third unit ontop of the bottom unit in order to obtain vertical alignment of thespecimen. The contact dry bed joints were properly selected so thatmaximum contact area between units could be attained. When mor-tar bed joints were considered, the stone units were carefully cleanedand wet to provide enough adhesion between units and mortar.

The testing equipment consisted of the three-dimensional stiffsteel frame used in [19]. Besides the large load capacity of the actu-ator, its remarkable stiffness is useful when stable displacementcontrolled failure is required. In order to induce uniform load dis-tribution, a thick steel plate connected to a steel spherical seat waslocated at the top of the specimen. The deformation of the speci-men during the test was recorded by means of four LVDTs locatedbetween steel platens at each side of the specimen. The verticaldisplacement of masonry prisms is defined by averaging the dis-placements measured by the LVDTs. In a first stage, two specimensof each masonry type were submitted to monotonic compressionunder displacement control. In a second stage more five specimenswere tested under loading control and for cyclic conditions in thepre-peak regime at three different load stages of 25%, 55% and75% of the average compressive strength obtained in the two pre-vious monotonic tests. After this, the test control was switched todisplacement control at a rate of 3 lm/s. The adoption of thisvelocity aimed to follow the possible stable failure of the masonryspecimens and to obtain and characterize the post-peak behaviorof stone masonry under uniaxial compression.

3.2. Experimental results

3.2.1. Failure modesThe failure pattern of dry masonry prisms was difficult to mon-

itor due to the encasing added to avoid injuries. Given its brittlebehavior, most of the failures of the specimens PR_S were abruptand explosive, see Fig. 9a. No signs of micro-cracks were recordedin the pre-peak regime. Micro-cracks become visible only afterpeak load has been reached. For dry masonry specimens, failurecracks usually develop along continuous shear bands that crossthe joints, with some specimens exhibiting even double shearcracks. This suggests that after the initial adjustment of dry bedjoints, the prism behavior is homogeneous and appears to bemostly determined by the compressive behavior of the graniticunits. No differences were introduced in the failure modes byincreasing the roughness of the bed joint surface. Abrupt failure oc-curred also in two specimens of rough dry masonry prisms tested

Table 1Shear strength properties for different unit–mortar assemblages.

Source Units Mortar c (N/mm2) l

Atkinson et al. [4] Old clay units 1:2:9 (13) 0.127 0.695Old clay units 1:2:9 (7) 0.213 0.640New clay units 1:1.5:4.5 0.811 0.745

Amadio and Rajgeli [23] Solid bricks Cement mortar 0.65 0.723Lime–cement mortar

Magenes [24] Solid bricks Hydraulic lime mortar 0.206 0.813Lime mortar 0.081 0.652

Binda et al. [25] Sandstone Hydraulic lime mortar 0.33 0.74Calcareous stone Hydraulic lime mortar 0.58 0.58

Roberti et al. [26] Bricks Hydraulic lime mortar 0.23 0.57Lourenço et al. [15] Hollow bricks Micro-concrete 1.39 1.03This study Granitic units Lime mortar 0.359 0.630

Fig. 8. Typical geometry of the masonry specimens. Dimentions are in mm.

3342 G. Vasconcelos, P.B. Lourenço / Construction and Building Materials 23 (2009) 3337–3345

under monotonic compressive loading. For the remaining five dryrough specimens, typical failures as the one exhibited in Fig. 9bdeveloped.

The failure of mortared masonry prisms was characterized byvertical splitting cracks appearing firstly in the central unit and ex-tended to the other stones as the compressive stress increases, seeFig. 9c. This failure mode is mostly due to the lateral tensile stres-ses of the granitic units induced by the composite behavior of unitsand mortar with different elastic mechanical properties. Similar re-sults have been reported by several authors [34]. On the otherhand, the failure of the specimens PR_SS with granitic soil in thebed joints is more similar to the crack patterns exhibited by thedry masonry specimens. This can be explained by the peculiartrend for compaction exhibited by the granitic soil joints in con-trast with the expansion of mortar [14].

3.2.2. Stress–strain relationshipFor each specimen the corresponding value of the compressive

stress was obtained by dividing the compressive load by the aver-age cross section. The deformation at a given stress level, e, wascalculated by dividing the vertical displacement by the height ofthe specimen, since the vertical displacement was taken as theaverage of the displacements recorded by the four vertical LVDTs.

For a comparative analysis, all stress–strain diagrams obtainedunder monotonic loading are depicted in Fig. 10. The shape of thestress–strain diagrams of sawn dry joint prisms PR_S is character-ized by an initial upward concavity. Although sawn surfaces arerather smooth and good adjustment of the stones was achievedduring the construction of the masonry prisms, the upward concav-ity is in part due to the initial setting of the bed joints. After full con-tact of the bed joints upon increasing compressive stress, thebehavior of masonry prisms depends mostly on the behavior ofthe stone. All prisms exhibit reasonable linear behavior until a com-pressive stress close to the peak strength. After the peak load isreached, it is still possible to record a considerable stretch of the

descending branch of the stress–strain diagrams in some speci-mens, even if most of them failed in a brittle manner. The stress–strain diagrams of rough dry joint prisms PR_SR show also an up-ward concavity for low stress level but over a larger part of thecurve, leading to higher levels of deformation at peak load. As thecompressive load increases, the roughness of the contact surfacetends to break, leading progressively to partial leveling of the bedjoint and to the improvement of the surface contact. In this case,the total deformation is the result of the deformation of the unitsand the local deformation at the bed joints. On the contrary, limemortared masonry prisms PR_SM exhibit an initial stretch withdownward concavity due to the composite behavior of units andmortar since early stages of loading. The role of the bed joint mate-rial on the deformational behavior of the masonry prism is furtherconfirmed by the stress–strain diagrams obtained for the soil mor-tared specimens PR_SS. These diagrams are characterized by a largeextent of the upward concavity from low to medium stress levels.The large initial deformation is related to the compaction of the gra-nitic clay at the bed joints. The bed joints with an initial thickness ofnearly 10 mm is at the end of the test a thin layer of compacted soil.

3.2.3. Pre-peak cyclic behaviorIt is observed that significant higher stiffness in the reloading

cycles in the pre-peak regime was recorded comparatively to thestiffness of the virgin stretch for dry masonry prisms PR_S andPR_SR, see Fig. 11. This feature can be attributed to the fact thatthe stiffness of the unloading–reloading branches involves perma-nent deformations corresponding to a given stress level. Thisbehavior is also found in the compressive behavior of rocks [35].Besides, higher permanent deformations are recorded in roughspecimens PR_SR with respect to the sawn prisms PR_S. This dis-tinct behavior is the result of the continuous wearing of the higherasperities during the loading process. For lime mortared prismsPR_SM, unrecoverable deformations corresponding to the unload-ing–reloading are even more remarkable than deformations ofdry rough specimens PR_SR, which should be attributed to themortar deformability, see Fig. 12. Unlike dry masonry prisms, sig-nificant permanent deformations were recorded in the pre-peakunloading–reloading cycles in soil mortared prisms PR_SS, due tothe compaction nature of the interlayer material [14].

3.2.4. Compressive strength parametersA summary of average compressive parameters derived from

the stress–strain diagrams, namely the compressive strength, fc,and the modulus of elasticity, Ec, is indicated in Table 2. Similarlyto what was indicated by Binda et al. [5], the modulus of elasticity,Ec, was determined as the secant modulus in the range of 30% and60% of the ultimate compressive strength in the ascending branchof the stress–strain diagram.

Fig. 10. Stress–strain diagrams for monotonic tests.

a

b

Fig. 11. Typical stress–strain diagrams for cyclic tests: (a) specimen PR_S10 and (b)specimen PR_SR7.

Fig. 9. Typical failure patterns of dry masonry prisms: (a) sawn dry joint specimenPR_S, (b) rough dry joint specimen PR_SR and (c) mortared joint specimen PR_SM.

G. Vasconcelos, P.B. Lourenço / Construction and Building Materials 23 (2009) 3337–3345 3343

Dry masonry prisms exhibit the highest compressive strengthand modulus of elasticity and the lowest deformation at peakstress, being the compressive strength similar to the compressivestrength of the units, which is equal to 69.2 N/m2 according toVasconcelos [14]. According to Hendry [36], the strength of stonemasonry built from dimensioned blocks with thin joints wouldbe close to stone strength, irrespectively of the mortar strength,which is not clearly confirmed in the present study. The compres-

sive strength of rough prisms PR_SR undergoes a reduction ofapproximately 29% with respect to prisms PR_S, whereas the mod-ulus of elasticity is 46% lower. Since the units are of the same gran-ite of specimen PR_S, the increase of vertical strain at peak stress ofapproximately 27% is directly connected to the lower stiffness ofthe bed joints. The higher scatter found in sawn prisms PR_SR isattributed to the variability of the distribution of the roughnessat the bed joint surfaces.

a

b

Fig. 12. Typical stress–strain diagrams for cyclic tests: (a) specimen PR_SM7 and(b) specimen PR_SS3.

Table 2Mean values of the mechanical compressive properties of masonry prisms (sevenspecimens). Coefficient of variation is indicated inside brackets (%).

Masonry prism fc (N/mm2) Ec (N/mm2)

PR_S (sawn) 73.0 (9.1) 14,722 (19.0)PR_SR (rough) 51.9 (16.2) 7934 (37.1)PR_SM (lime mortar) 37.0 (11.8) 4629 (17.5)PR_SS (soil mortar) 64.2 (13.5) 8920 (12.6)

3344 G. Vasconcelos, P.B. Lourenço / Construction and Building Materials 23 (2009) 3337–3345

The reduction on the compressive mechanical properties withrespect to the reference masonry specimens (PR_S) is even moresignificant in mortared specimens (PR_SM), with a reduction ofnearly 50% and 70% in the compressive strength and in the modu-lus of elasticity, respectively. The reduction of the compressivestrength can be attributed to composite behavior of masonry withthe development of a compression–tension–tension triaxial stressstate for the units and a triaxial compression state for the mortar[24,37]. The introduction of granitic soil instead of mortar in thebed joints leads to an increase of 73% on the compressive strengthand an increase of 92% on the modulus of elasticity concerningmortared prisms PR_SM. This is due to the compaction of the gra-nitic soil and dilation of the mortar.

4. Conclusions

Aiming at obtaining reliable insight on the shear and compres-sive strength properties of stone masonry an experimentalcampaign for the characterization of stone masonry under shear

and compressive loading is addressed, namely with respect to theshear behavior of dry and mortar masonry joints and to the uniaxialcompressive behavior of masonry prisms with distinct bed jointmaterials and joint surfaces. It should be stressed that the shearstrength of the mortar–stone interface as well as the compressivestrength of masonry stone are basic mechanical properties whendesign or numerical modeling of stone shear walls is required.

Concerning shear behavior of masonry joints, it is observedthat: (a) an elastic perfectly plastic shear stress–shear-displace-ment diagram was found to characterize the monotonic and thecyclic envelope of dry masonry joints, whereas post-peak strengthdegradation and a consequent stabilization characterizes themonotonic behavior of mortar joints; (b) the moisture conditionon the joints, from dry to saturated, seems to have a negligible ef-fect on the friction coefficient; (c) no dilatancy was found to char-acterize the shear behavior of dry masonry joints, whereas dilatantbehavior was found on mortared joints and (d) the shear strengthproperties of masonry joints enables the assessment of the in-plane lateral strength of masonry walls under in-plane loading.

With respect the compressive behavior of masonry it was ob-served that: (a) the surface condition (smooth or rough) influencesthe composite behavior of masonry under compression, mostlywith respect to the compressive strength, modulus of elasticityand deformation at peak stress; (b) the material of the bed joints(lime mortar or clay material) influences considerably the failuremode and the compressive strength of stone masonry. The differ-ent bed joint materials lead to clear distinct pre-peak regime(stress–strain diagrams) resulting in very different modulus ofelasticity, compressive strength and permanent deformations afterloading-reloading cycles. The materials of the bed joints play a cen-tral role on the deformation behavior of stone masonry under com-pressive loading. (c) as the shear strength properties of masonryjoints, also the compressive strength of stone masonry representsa valuable property for the assessment of the in-plane and out-of-plane strength of masonry walls.

Finally, it should be stressed that the experimental results andmechanical data on stone masonry pointed out in this workcontribute for the improvement of the knowledge on the mechan-ics of stone masonry.

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