Physical-Mechanical Properties of Stone Masonry of Gjirokastër, Albania

materials

Article Physical-Mechanical Properties of Stone Masonry of Gjirokastër, Albania

Enea Mustafaraj 1 , Erion Luga 1, Marco Corradi 2,3,* , Antonio Borri 3, Ylber Muceku 4 and Aleksandra Zharkalli 1

1 Department of Civil Engineering, EPOKA University, 1039 Tirana, Albania; [email protected] (E.M.); [email protected] (E.L.); [email protected] (A.Z.) 2 Department of Mechanical & Construction Engineering, Northumbria University, Newcastle upon Tyne NE1 8ST, UK 3 Department of Engineering, University of Perugia, 06125 Perugia, Italy; [email protected] 4 Institute of Geosciences, Energy, Water and Environment, 1024 Tirana, Albania; [email protected] * Correspondence: [email protected]; Tel.: +44-(0)191-227-3071

Abstract: In addition to reinforced concrete and steel buildings, a large part of the existing building stock in Europe is made of stone masonry. Prediction of the structural behavior requires the development of a systematic material characterization of the mechanical properties and structural details (units, arrangement, bonding, inter-connection). This study aims to analyze the mechanical and physical behavior of building stones in the historical city of Gjirokastër, Albania, known also as the Stone City. A thorough investigation of the regional stone quarries was performed, and the collected samples were cut into regular prismatic specimens for further analysis. The experimental campaign consisted of the determination of flexural strength and compressive strength, water absorption, porosity, specific gravity as well as structural analysis of the masonry material, using the Citation: Mustafaraj, E.; Luga, E.; MQI (Masonry Quality Index) method. The test results showed that there is a large scattering in the Corradi, M.; Borri, A.; Muceku, Y.; Zharkalli, A. Physical-Mechanical values of the mechanical and physical stone properties such as compressive strength varying from 20 Properties of Stone Masonry of to 115 MPa and flexural strength from 8 to 25 MPa. However, the analysis of the masonry material Gjirokastër, Albania. Materials 2021, revealed a satisfactory structural performance, based on a frequent, systematic respect of the good 14, 1127. https://doi.org/ construction practices (i.e., the rules of the art) in Gjirokastër. 10.3390/ma14051127 Keywords: masonry; historic construction materials; mechanical testing; earthquake engineering Academic Editors: Gabriele Milani and Bahman Ghiassi

Received: 29 January 2021 1. Introduction Accepted: 23 February 2021 The city of Gjirokastër is located in southern Albania, situated in a valley between Published: 27 February 2021 the Mali i Gjerë mountain and Drino river, at 300 meters above sea level (Figure1). Known also as the Stone City, in 2005, Gjirokastër was announced a UNESCO (The United Nations Publisher’s Note: MDPI stays neutral Educational, Scientific and Cultural Organization) protected site defined as “a unique with regard to jurisdictional claims in example of a well-preserved” Ottoman town [1] with a building stock mostly dating from published maps and institutional affil- iations. the 17th and 18th centuries. Nowadays, in Gjirokastër there are 590 monuments, which can be grouped into two main categories according to their importance. In the 1st category, there are about 56 monuments while at the 2nd category 540. Overall, near the city around 1200 stone buildings [2] are found. The typical buildings consist of a high stone block structure up Copyright: © 2021 by the authors. to five-story high (Figure2). Single family dwellings are generally smaller compared to Licensee MDPI, Basel, Switzerland. multi-family residential buildings which are of a considerable size. For their construction, This article is an open access article different building materials such as stone, wood, clay, glass, gypsum, plaster, binding distributed under the terms and conditions of the Creative Commons mortar, goat hair, etc., are used. In general, most of these materials have a local origin. Attribution (CC BY) license (https:// From the natural materials that are present in these buildings the most commonly used are creativecommons.org/licenses/by/ stone and wood. 4.0/).

Materials 2021, 14, 1127. https://doi.org/10.3390/ma14051127 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 21 Materials 2021, 14, 1127 2 of 23

Stone has not only been used for the construction of some of the most important monuments and structures but, at the same time, all the old streets are made of stone because of its strength, hardness, abrasion resistance, and durability. In general, in the whole range of constructions, there have been involved native highly skilled stonemasons, Materials 2021, 14, x FOR PEER REVIEW with the intent of preserving the characteristic architecture and building technology2 of 21 of the old town. Moreover, being so abundant in this region, stone has always been an affordable and cost-effective building material.

Figure 1. Location of the city (left) and aerial view of the old city center (right).

Stone has not only been used for the construction of some of the most important monuments and structures but, at the same time, all the old streets are made of stone because of its strength, hardness, abrasion resistance, and durability. In general, in the whole range of constructions, there have been involved native highly skilled stonemasons, with the intent of preserving the characteristic architecture and building technology

of the old town. Moreover, being so abundant in this region, stone has always been an affordable and Figurecost-effective 1. Location building of the city (left material.) and aerial view of the old city center (right). Figure 1. Locati on of the city (left) and aerial view of the old city center (right).

Stone has not only been used for the construction of some of the most important monuments and structures but, at the same time, all the old streets are made of stone because of its strength, hardness, abrasion resistance, and durability. In general, in the whole range of constructions, there have been involved native highly skilled stonemasons, with the intent of preserving the characteristic architecture and building technology of the old town. Moreover, being so abundant in this region, stone has always been an affordable and cost-effective building material.

Figure 2. Typical historic masonry constructions of Gjirokastër. Figure 2. Typical historic masonry constructions of Gjirokastër. An important past research [2] about the buildings of Gjirokastër, their classiﬁcation according to general type and building characteristics, was carried out several years after An important pastthe cityresearch was listed [2] about by the the Albanian build Heritageings of Gjirokastër, and Conservation their Authorities. classification This is a according to generalstudy type in and which building an effort characteristics, is made to provide was the characteristicscarried out several of the “fortiﬁed years after Gjirokastër the city was listed housing”by the Albanian considered asHeritage a type of and Albanian Conservation heritage building Authorities. stock, analysis This of is the aorigin study in which an effortand its is evolutionmade to [3provide]. the characteristics of the “fortified Gjirokastër housing" considered as a type of Albanian heritage building stock, analysis of the origin and its evolution [3]. At the same year, Kamberi [4] investigated the building techniques of Gjirokastër buildings of the 18-19th centuries. The main construction materials were also studied by Figure 2. Typical historic masonry constructions of Gjirokastër. [5,6], mostly by visual assessment and observations. Other studies were focused on the identification of the architectural features and restoration of important buildings [7–10] Anand important ethnographic past values research of the [2] city about [11,12], the buildingsbyzantine churchesof Gjirok andastër, monasteries their classification [13,14], accordingand other to general important type values. and building Nevertheless, characteristics, there is a lack was of carriedresearch out about several the mechanical years after the citycharacteristics was listed of by the the construction Albanian materials Heritage used and in ConservationGjirokastër. Authorities. This is a study in which an effort is made to provide the characteristics of the “fortified Gjirokastër housing" considered as a type of Albanian heritage building stock, analysis of the origin and its evolution [3]. At the same year, Kamberi [4] investigated the building techniques of Gjirokastër buildings of the 18-19th centuries. The main construction materials were also studied by [5,6], mostly by visual assessment and observations. Other studies were focused on the identification of the architectural features and restoration of important buildings [7–10] and ethnographic values of the city [11,12], byzantine churches and monasteries [13,14], and other important values. Nevertheless, there is a lack of research about the mechanical characteristics of the construction materials used in Gjirokastër.

Materials 2021, 14, 1127 3 of 23

At the same year, Kamberi [4] investigated the building techniques of Gjirokastër buildings of the 18-19th centuries. The main construction materials were also studied by [5,6], mostly by visual assessment and observations. Other studies were focused on the identification of the architectural features and restoration of important buildings [7–10] and ethnographic values of the city [11,12], byzantine churches and monasteries [13,14], and other important values. Nevertheless, there is a lack of research about the mechanical characteristics of the construction materials used in Gjirokastër. Over the past decades, several studies on Albanian heritage structures [15] and assessment and retrofitting of unreinforced masonry buildings have been conducted by the authors [16–18]. In Europe, on the other hand, several examples on characterization of physical and mechanical characteristics of construction materials can be mentioned: dry stone heritage materials and limestone from Portugal [19,20], freshwater limestones used in Bosnia and Herzegovina [21] and limestones from Italy [22]. Techniques involve on-site testing [23] and adoption of non-destruction techniques [24,25]. Moreover, in Italy, following 1998 and 2009 earthquakes, in Greece, in the Balkan countries and in several other territories where the seismic hazard is significant, a large amount of research has been devoted in academia to the use of composite materials as a method to increase the in-plane shear capacity of stone masonry wall panels [26]. These are typically epoxy-bonded to the masonry walls. Recently, in consideration of the unsatisfactory long-term behavior of the bonding and low compatibility with masonry material, epoxy resins have been replaced with inorganic matrices, i.e., mortars. In this latter method, composite materials (typically fiberglass meshes or steel cords) are embedded into a mortar coating to be applied on each wall faces [25,27,28]. However, the design of reinforcement interventions depends on the structural behavior of the unreinforced, original masonry, including not only the mechanical characteristics of the constituent materials (stone and mortar), but also the masonry texture and arrangement (stone shape and dimensions, type of wall, wall thickness, etc.,). As a consequence, many studies have been published to assess the structural response of unreinforced stone masonry [29–32], highlighting the frequent unsatisfactory response against the seismic action. Stone masonry buildings are particularly vulnerable to earthquakes, and it is well accepted that a preliminary assessment of the mechanical performance of stone masonry buildings is an important prerequisite for any future action. This information is critical not only for design, but also for the policymaker and governmental agencies dealing with the risk-analysis of the masonry building stock. Destructive in situ structural testing would be the best method to measure the mechanical properties of the stone masonry. This is typically carried out on wall panels cut off from the load-bearing walls of the buildings. Diagonal tension [33], compression [34], and shear-compression [35] tests can provide very useful data about the structural response of masonry (strengths, moduli, strains, yielding behavior, etc.,). However, these tests are highly invasive, expensive, and destructive: in many situations, conservation bodies do not permit such types of tests on the listed buildings. Nevertheless, structural engineers need to know these properties for their calculations, as mentioned above. To meet this need, we opted to estimate the masonry mechanical characteristics using the technique proposed by Borri et al. [36–38], known as the Masonry Quality Index (MQI). This is a visual method of assessment of the masonry mechanical properties based on the masonry mechanical given in the Italian Building Code, (NTC)- 2018 [39,40] and calibrated on a large database (over 120 test results) of destructive tests carried out by the authors in the last 25 years.

2. Aim This experimental work aims to analyze the main mechanical and physical properties of the traditional building stones in the historic city of Gjirokastër. For this reason, a thor- Materials 2021, 14, 1127 4 of 23

ough investigation of the regional quarries was performed, and the collected samples were cut into regular prismatic specimens for further analysis. The experimental campaign consisted of the determination of flexural strength and compressive strength, water absorption, porosity, the specific gravity, as well geological and petrographic characteristics. Moreover, classification of the principal types of residences and masonry typologies is presented. Based on the stone load bearing wall characteristics, the mechanical properties and the expected structural response under seismic actions are investigated using the Masonry Quality Index (MQI). As a result, these data can be used for a detailed assessment of individual buildings depending upon their masonry typologies. The assessment procedure is presented in the

Figure3.

Traditional Constructions of Gjirokastër

Building Typologies Materials, Building Stones Structural Assessment - The perpendicular dimensions, - Geological & by MQI house arrangement & petrographic - Type 1 < 60 cm thick - Single-wing house texture properties - Type 2 > 60 cm thick - The Double-wing - Mechanical house properties - Physical properties

Figure 3. ExperimentalFigure 3. Experimental campaign flowchart. campaign ﬂowchart.

3. Gjirokastër case study 3.1. Materials, Dimensions, Arrangement, and Texture In Gjirokastër, stone was used from the foundations of the buildings till the roofs and can be identiﬁed in six primary groups (according to the names used locally by stone crafters): 1. White limestone, 2. Semi- crystalized white limestone, 3. Black stone – silicate, 4. Grey stone, 5. River stone, and 6. Porous stone of Peshkëpi. The white limestone was mainly used for structural walls in the buildings: this was taken either from the construction site, or from areas near the city in quarries around Gjirokastër, in Lazarat, Derviçan, Goranxi, and Grapsh. The white limestone can be found on-site in three different colors: (a) white matte (the color is absorbed from magnesium oxide), (b) red (the color is released by the oxide of iron), (c) Gushëpëllumbi (pigeon neck) (the color is released by the oxide of copper). White matte is a stone with many possible applications; this type of stone varies from 20 to 200 mm in thickness and it is considered as the most popular. This was mainly used for construction of the cobbled road due to its high strength and good abrasion resistance. The stone is dense with very low porosity and has a very good resistance against freezing and thawing. Moreover, since it is a material that can be easily cleft to particular shapes, it is used for decorations. The red stone is mainly used for the characteristic roofs of the city. It is extracted in 15–30 mm thick layers. In addition, because of its pleasant color, this stone has a large use for decoration in cobbled roads, arched doorway, or in inner design. The Gushëpëllumb stone is a mixture of yellow, white, red, and violet colors. This stone is widely used in architectural applications for walls, doors, as well as for decoration. The other group of the building stones of Gjirokastër is the dark stones. They are made of hydroxides, quarts manganese, and phosphates. In this group, there are two types

Materials 2021, 14, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/materials Materials 2021, 14, 1127 5 of 23

of stone: the black and the grey stones. Both of them are used for building, ﬂoors, and pavements. The black stone is often used in cobbled streets of the city where together with the white matte and red stone it creates the unique street pavements of the historical city of Gjirokastër. The black stone has a natural layering, which is formed in large blocks of varying thickness from 0.8 m to 1.2 m. It is either extracted from the area around the building, or from the surrounding area of the city, for example in Poliçan and Lunxhëri. The grey stone is used mostly by the people of Odrie. The river stone is extracted in relatively large dimensions from the Drinos river and near the village of Kardhiq. When compared to other stones, this type is used less. It is commonly used for shared walls between two buildings. Sometimes in stonemasons it is combined with the black stone or concrete and is used for pavement purpose. On the other hand, the Peshkëpi stone is extracted in irregular shapes and it is used for decoration purposes as it has a very beautiful natural shape.

3.2. Classification of the Principal Types of Residences The City Tower is one of the most developed dwellings typologies of the 18–19th centuries, only used in Gjirokastër. The early versions of these buildings were also applied in the cities of Berat, Krujë, and Shkodër. The most evident characteristic of this typology is the adaptation of the rational volume composition, development of mezzanine floor, unequal height distribution between stories, thick perimetral stone masonry walls (that served as a fortification), small openings, materials and positioning were closely connected with the folded terrain of the city. The main characteristic of the masonry buildings of Gjirokastër is the protective character: these are typically fortified constructions [2]. The principal constitutive parts of the typical buildings are the perimetral walls, "the outside Oda " (a separate structure from the building generally placed in the yard), Katoli (used for storing food), Stera (used as water tank, made of stones and plastered with impermeable mortar), the big Oda (or the guest room) etc. The main characteristic of the masonry buildings of Gjirokastër is the protective character: these are typically fortified constructions [2]. The development of the “City Tower” building typology is closely associated with the socio-economic changes occurring in the city of Gjirokastër. Based on the plan-volume composition criteria, the typical unreinforced stone masonry houses are categorized into three main groups [3]: 1. Perpendicular Building, 2. One-wing Building, 3. Two-wings Building.

3.2.1. The Perpendicular Building The base template for the other groups, is found in two-to-three story buildings often with a mezzanine floor. It is well-known for the enclosed volume and the defensive character, first seen during the 14th century and evolved to three-stories at the beginning of the 18th century. The base is a rectangle with the longer sides perpendicular to the terrain. The connection between the stories was done by stone-made stairs (Figure4). The volume of this dwelling is discontinuous with the upper floor, partly as ground floor lies of the back. On the ground floor there is the water deposit. The wider story accommodates the main building, and the guestrooms are located on top of it. The sanitary units are located by the side of the building. There is a clear differentiation of the functions of the stories. Over the years, the evolution of this building typology consisted of the addition of a third story. The new three-story building would be the base unit of further elaboration of this typology with distinctive functions: ground story serving for auxiliary services, the first floor for every-day living of family members, and the second story usually for guests. In the later evolved versions, this story would be used by the family members during the summertime as the spaces are bigger and the story height from 0.6 to 1. 6 m higher. Materials 2021, 14, x FOR PEER REVIEWMaterials 2021 , 14, 1127 6 of6 21 of 23

(a)

(b)

Figure 4.Figure Examples 4. Examples of the of theperp perpendicularendicular building:building: (a) ( twoa) two stories, stories, (b) three (b stories) three (adapted stories from (adapted [3]). from [3]). 3.2.2. The Single-Wing Building 3.2.2. The Single-Wing BuildingThe Single-wing building is the most common type of residence in Gjirokastër due to The Single-wingits versatilitybuilding in is terms the ofmost spaces, common and volume type adaptation. of residence This typology in Gjirokastër is found mostly due in the form of a three-story building. There are three sub-groups: the ﬁrst one is characterized to its versatility in termsby the of presence spaces, of and the stairs volume inside adaptation. of the buildings; This the typology second group is found is known mostly for the in the form of a three-storysimple plan; building. and the third There group, are more three irregular, sub-groups: is inﬂuenced the byfirst the one morphology is charac- of the terized by the presenceterrain of as the the volumesstairs inside of the buildings of the buildings; are not well distributedthe second (Figure group5). is known for the simple plan; 3.2.3.and Thethe Double-Wingthird group, Building more irregular, is influenced by the morphology of the terrain as the volumesThis typology of the buildings is one of the are most not featured well distributed versions of the (Figure buildings 5). of Gjirokastër. The wings are two-story, and the central main part is three-story in height. It is a timely evolution of the other typologies (Figure6).

(a)

(b)

Materials 2021, 14, x FOR PEER REVIEW 6 of 21

(a)

(b) Figure 4. Examples of the perpendicular building: (a) two stories, (b) three stories (adapted from [3]).

3.2.2. The Single-Wing Building The Single-wing building is the most common type of residence in Gjirokastër due to its versatility in terms of spaces, and volume adaptation. This typology is found mostly in the form of a three-story building. There are three sub-groups: the first one is charac- Materials 2021, 14, 1127terized by the presence of the stairs inside of the buildings; the second group is known 7 of 23 for the simple plan; and the third group, more irregular, is influenced by the morphology of the terrain as the volumes of the buildings are not well distributed (Figure 5).

(a)

Materials 2021, 14, x FOR PEER REVIEW 7 of 21

(b)

(c)

(d)

(e)

Figure 5. ExamplesFigure 5. of Examples the single-wing of the single-wing building located building at: located (a) P. Xhixho at: (a) St,P. Xhixho n.15; (b St,) A. n.15; Toro (b St,) A. N.20; Toro ( cSt,) K.Bako St, n.17; (d) M. BakiriN.20; St, n.45; (c) K.Bako (e) A. Frasheri St, n.17; St,(d) n.6 M. (adaptedBakiri St, fromn.45; [(3e]).) A. Frasheri St, n.6 (adapted from [3]).

3.2.3. The Double-Wing Building This typology is one of the most featured versions of the buildings of Gjirokastër. The wings are two-story, and the central main part is three-story in height. It is a timely evolution of the other typologies (Figure 6).

(a)

(b)

Materials 2021, 14, x FOR PEER REVIEW 7 of 21

(c)

(d)

(e) Figure 5. Examples of the single-wing building located at: (a) P. Xhixho St, n.15; (b) A. Toro St, N.20; (c) K.Bako St, n.17; (d) M. Bakiri St, n.45; (e) A. Frasheri St, n.6 (adapted from [3]).

3.2.3. The Double-Wing Building This typology is one of the most featured versions of the buildings of Gjirokastër. Materials 2021,The14, 1127 wings are two-story, and the central main part is three-story in height. It is a timely 8 of 23 evolution of the other typologies (Figure 6).

(a)

Materials 2021, 14, x FOR PEER REVIEW 8 of 21

(b)

(c)

Figure 6.FigureExamples 6. Examples of the double-wing of the double-wing building: ( abuilding:) Babameto (a) Building Babameto (1885), Building (b) Zekate (1885), Building (b) Zekate (1812), Building (c) Skëndulaj Building(1812), (1823) ( (adaptedc) Skëndulaj from Building [3]). (1823) (adapted from [3]).

3.3. Material, Dimensions, Stone Arrangement, and Texture 3.3. Material, Dimensions, Stone Arrangement, and Texture The foundationsThe of the foundations buildings of in the Gjirokas buildingstër in are Gjirokastër dug up areto 130 dug cm up deep to 130 into cm deepthe into the ground. They areground. mostly Theymade are of mostly dry stacked made of stone dry stackedwith no stone mortar, with with no mortar, big-sized with stones big-sized stones or large pieces ofor rocks large piecesquarried of rocksaround quarried the city. around This the type city. of This system type allows of system for allows good for good drainage. On thedrainage. other hand, On the the other load hand, bearing the load walls bearing are made walls areof white made ofstone white with stone a withvar- a varying ying thickness ofthickness 60–75 cm. of 60–75 Whenever cm. Whenever this kind this kindof stone of stone was was not not available, available, the the blackblack stone was stone was used. used.The black The black stone stone is mainly is mainly found found at at PllakëPllakëand andPazari Pazari i Vjetër i Vjetërneighborhoods. neighbor- hoods. The construction of the stone masonry walls is often made of three wythes (triple-leaf): The constructionthe outer, of the middle,stone masonry and the outer walls wythe. is often The made binding of element three wythes for the stones (tri- of inner and outer wythe is either lime mortar or clay mortar. Regardless of the thickness of the ple-leaf): the outer, the middle, and the outer wythe. The binding element for the stones wall, in the inner wythe, no mortar was typically used. One of the main advantages of this of inner and outerdry wythe wall technique is either islime the mortar preventing or clay of capillary mortar. actions Regardless inside theof the wall, thickness thus, avoiding the of the wall, in therisk inner of humiditywythe, no and mortar water was inﬁltrations. typically According used. One to theof the construction main advantages methods, the stone of this dry wall masonrytechnique can is be the categorized preventing into twoof capillary main groups: actions inside the wall, thus, avoiding the risk of1. humidityThick Masonry and: withwater no in anti-seismicfiltrations. timber According ties, these to the buildings, construction generally single- methods, the stonestory, masonry are the can oldest be incategorized the city, and into are two made main of large groups: stone blocks. During the 18–19th 1. Thick Masonrycenturies,: with partially no anti-seismic dry-stone walls timber were ties, used these for construction. buildings, generally The key characteristic sin- is gle-story, are thethe oldest inner-to-outer in the city, leaf and transversal are made connection of large (knownstone blocks. as header During stone). the This 18– technique required a connection every four stones. When the cross-sectional thickness of the walls 19th centuries, partially dry-stone walls were used for construction. The key characteris- exceeds 60 cm, the wall is made of three wall leaves (triple-leaf wall). The outer and inner tic is the inner-to-outerleaves were leaf madetransversal of large co stonennection blocks (known and the as core header leaf was stone). typically This ﬁlled tech- with small nique required astones. connection These every walls four were stones. often reinforced When the with cross-sectional oak wood elements, thickness internally of the installed walls exceeds 60 cm, the wall is made of three wall leaves (triple-leaf wall). The outer and inner leaves were made of large stone blocks and the core leaf was typically filled with small stones. These walls were often reinforced with oak wood elements, internally installed during their assemblage in both longitudinal and orthogonal directions, creating a mesh that limits deformation and cracking. These walls were often reinforced with oak ties, on both faces of the walls, starting at 120 cm from the ground level, spaced every 80– 110 cm centers connected by nails, creating a mesh that limits deformation and cracking. 2. Thin Masonry: This is used mainly for the partition walls in the upper stories. These walls are built of small stones bonded with mortar and have a thickness of no more than 12 cm.

3.4. Geological and Petrographic Characteristics The stone quarries used for the construction of the buildings of Gjirokastër are found in the northern, western, and southern parts of the region. From the geological formation characteristics, the building stones are classified in the following types: i. The thin bedded limestones; ii. thick bedded to massive breccia’s limestones and conglobreccias limestones; iii. the bedded dolomites; iv. the thin bedded marls; v. the thin bedded sandstones. The pattern of each type of the limestones is shown in Figure 7.

Materials 2021, 14, 1127 9 of 23

during their assemblage in both longitudinal and orthogonal directions, creating a mesh that limits deformation and cracking. These walls were often reinforced with oak ties, on both faces of the walls, starting at 120 cm from the ground level, spaced every 80–110 cm centers connected by nails, creating a mesh that limits deformation and cracking. 2. Thin Masonry: This is used mainly for the partition walls in the upper stories. These walls are built of small stones bonded with mortar and have a thickness of no more than 12 cm.

3.4. Geological and Petrographic Characteristics The stone quarries used for the construction of the buildings of Gjirokastër are found in the northern, western, and southern parts of the region. From the geological formation characteristics, the building stones are classiﬁed in the following types: i. The thin bedded limestones; ii. thick bedded to massive breccia’s limestones and conglobreccias limestones; Materials 2021, 14, x FOR PEER REVIEW iii. the bedded dolomites; iv. the thin bedded marls; v. the thin bedded9 sandstones. of 21 The pattern of each type of the limestones is shown in Figure7.

(a) (b) (c) (d)

(e) (f) (g) (h)

(i)

Figure 7. ThinFigure bedded 7. limestone:Thin bedded (a) limestone: micritic, (b ()a medium-grained,) micritic, (b) medium-grained, (c) limestones ( withc) limestones rudist, (d with) macro-grained; rudist, (d) (e) conglobreccias andmacro-grained; breccias; (f) dolomites (e) conglobreccias (g) marl; ( hand) sandstones, breccias; (f ()i )dolomites breccies [units: (g) marl; cm]. (h) sandstones, (i) breccies [units: cm].

3.4.1. The Thin Bedded Limestones Buildings made with this stone are located in northern, western, and southern part of Gjirokastër. They consist of thin strata 3–5 cm up to 20–50 cm. Many thin sections of limestones samples taken from different buildings were done during last years, were observed using a polarized microscope. From these studies, the limestones are divided in: 1. Micritic limestones: compact and hard rocks, dense and mainly light gray and milky color; 2. Medium-grained limestones: compact and hard rocks, dense and mainly wite color; 3. Organogenic limestones with rudist: characterized by hard rocks, very compact and black color; 4. Macro-grained limestones: medium to hard rocks, compact and dark grey and black color. Considering the time of formation, the thin bedded limestones belong to Paleocene (Pg1) and Eocene (Pg2) series. They are generally characterized by micro fissures, randomly oriented and cemented with calcite. According to discontinuities the rock mass is generally affected by three system joints (cleavage), which are two vertical joints sets and a stratification joints with horizontal sets. The joints of vertical sets have a moderate spacing (1–2 mm to 5 mm). The limestones generally are composed of the calcite (90–

Materials 2021, 14, 1127 10 of 23

3.4.1. The Thin Bedded Limestones Buildings made with this stone are located in northern, western, and southern part of Gjirokastër. They consist of thin strata 3–5 cm up to 20–50 cm. Many thin sections of limestones samples taken from different buildings were done during last years, were observed using a polarized microscope. From these studies, the limestones are divided in: 1. Micritic limestones: compact and hard rocks, dense and mainly light gray and milky color; 2. Medium-grained limestones: compact and hard rocks, dense and mainly wite color; 3. Organogenic limestones with rudist: characterized by hard rocks, very compact and black color; 4. Macro-grained limestones: medium to hard rocks, compact and dark grey and black color. Considering the time of formation, the thin bedded limestones belong to Paleocene (Pg1) and Eocene (Pg2) series. They are generally characterized by micro ﬁssures, randomly oriented and cemented with calcite. According to discontinuities the rock mass is generally affected by three system joints (cleavage), which are two vertical joints sets and a stratiﬁcation joints with horizontal sets. The joints of vertical sets have a moderate spacing (1–2 mm to 5 mm). The limestones generally are composed of the calcite (90–99%) and dolomite impurity (1–10%). The other constituent’s minerals as clay, ancerites, aragonites, magnesites, and rodocrosites etc., are found in small quantities.

3.4.2. Thick Bedded to Massive Conglobreccias and Breccias Limestones They are mainly thick bedded to massive with varied and multicolored gravels and rubbles grains, having the mosaic view after the sawing. They are commonly hard rocks and have reddish color. The age of these formations is Lower Cretaceous (Cr1).

3.4.3. The Bedded Dolomites This type of building stones extend along the northern part of Gjirokastër. They are commonly thin to thick bedded limestones, grey and beige color. These rocks are softer than limestones, therefore they are easily seen. These deposits are found in Ftera village, Mali Gjërë. They consist of thin strata, from 5–10 cm to 20–40 cm. The age of these formations is Lower to Upper Cretaceous (Cr1-Cr2).

3.4.4. The Bedded Marl’s Limestones These formation are represented by medium to thick layers with glass textures, compact and reddish in color. The marl’s limestones are medium to hard rocks. The age of these formations is Paleocene (Pg1) and Eocene (Pg2) series.

3.4.5. The Thin Bedded Sandstones They are found in ﬂysch’s formations of Lower Oligocene-Pg13, between claystones and siltstones layers. These deposit are stratigraphic break over Eocene and upper Creta- ceous limestones. The sandstones are thin layers (3–5 cm to 10 cm thick), micro-granular, compact, and grey color. They are included in weak to medium group.

4. Material Characterization and In Situ Test Results 4.1. Building Stones From a field investigation, seven traditional quarry areas were identified within the region of Gjirokastër and some representative specimens were collected for laboratory testing. The collected specimens were shortly identified according to the following letter designation: D—Red Limestone collected from Dropulli valley, J—White limestone collected from Jorgucat village, G—Gushëpëllumbi (pigeon neck) limestone found also in Dropulli valley, L—Black silicate stone found in Lunxhëri and Poliçan, O—Grey sandstone collected in Odrie village, P—Porous stone of Peshkëpi, and K—Riverstone of Kardhiq and Drino river (Figure8). Materials 2021, 14, x FOR PEER REVIEW 10 of 21

99%) and dolomite impurity (1–10%). The other constituent’s minerals as clay, ancerites, aragonites, magnesites, and rodocrosites etc., are found in small quantities.

3.4.4. The Bedded Marl’s Limestones These formation are represented by medium to thick layers with glass textures, compact and reddish in color. The marl’s limestones are medium to hard rocks. The age of these formations is Paleocene (Pg1) and Eocene (Pg2) series.

3.4.5. The Thin Bedded Sandstones They are found in flysch’s formations of Lower Oligocene-Pg13, between claystones and siltstones layers. These deposit are stratigraphic break over Eocene and upper Cretaceous limestones. The sandstones are thin layers (3–5 cm to 10 cm thick), micro-granular, compact, and grey color. They are included in weak to medium group.

4. Material Characterization and In Situ Test Results 4.1. Building Stones From a field investigation, seven traditional quarry areas were identified within the region of Gjirokastër and some representative specimens were collected for laboratory testing. The collected specimens were shortly identified according to the following letter designation: D—Red Limestone collected from Dropulli valley, J—White limestone collected from Jorgucat village, G—Gushëpëllumbi (pigeon neck) limestone found also in Dropulli valley, L—Black silicate stone found in Lunxhëri and Poliçan, O—Grey sand- Materials 2021, 14, 1127 stone collected in Odrie village, P—Porous stone of Peshkëpi, and11 ofK—Riverstone 23 of Kardhiq and Drino river (Figure 8).

Materials 2021, 14, x FOR PEER REVIEW 11 of 21

Figure 8. LocationsFigure of Gjirokastër 8. Locations in ofAlba Gjirokastërnia and the in Albaniaquarries andof stones. the quarries of stones.

The raw samplesThe were raw samplescut into regular were cut prismatic into regular or cubic prismatic shape or cubicin accordance shape in accordancewith with the the respective ENrespective standard EN such standard as EN such 12372 as EN[41] 12372 for the [41 flexural] for the strength ﬂexural strength and EN and1926 EN 1926 [42] for [42] for the compressivethe compressive strength. strength. The prod Theuced produced specimens specimens were wereused usedalso alsofor the for thede- determination of the physical properties. Table1 and Figure9 show the coding and preparation of some termination of the physical properties. Table 1 and Figure 9 show the coding and prepa- representative samples for testing. ration of some representative samples for testing.

Table 1. Table 1. The building stoneThe coding building according stone coding to their according origin. to their origin.

Acronym AcronymStone Type Stone Type Origin Origin D D Red stone Red stone Dropulli Valley Dropulli Valley J White limestone Jorgucat J White limestone Jorgucat G Gushëpëllumb (Pigeon neck) Dropulli Valley G Gushëpëllumb (Pigeon neck) Dropulli Valley L Black stone Lunxhëri & Poliçan L Black stone Lunxhëri & Poliçan O Grey stone Odrie O Grey stone Odrie P Porous stone of Peshkepi Peshkëpi P Porous stone of Peshkepi Peshkëpi K River stone Kardhiq & Drino K River stone Kardhiq & Drino

Figure 9. Representative stone prisms before testing. Figure 9. Representative stone prisms before testing.

4.2. Results Compressive and flexural strength tests are the most common tests performed for the determination of the mechanical behavior of the building stones.

4.2.1. Flexural Strength The flexural strength test was performed in accordance with EN 12372 [41] using equation (1). According to this the thickness of the stone sample h should be between 25 mm and 100 mm, the total length l should be equal to six times the thickness, the width b should be between 50 mm and three times the thickness (50 mm ≤ b ≤ 3 h), and the distance between the supporting rollers l should be equal to five times the thickness. 𝜎 = 𝑙, (1)

Figure 10 shows the schematic view of the testing procedure. In this study, the cross section of the specimen was 50 × 50 mm. Three samples were tested for each stone type and the average flexural strength value was calculated.

Materials 2021, 14, 1127 12 of 23

4.2. Results Compressive and ﬂexural strength tests are the most common tests performed for the determination of the mechanical behavior of the building stones.

4.2.1. Flexural Strength The ﬂexural strength test was performed in accordance with EN 12372 [41] using equation (1). According to this the thickness of the stone sample h should be between 25 mm and 100 mm, the total length l should be equal to six times the thickness, the width b should be between 50 mm and three times the thickness (50 mm ≤ b ≤ 3 h), and the distance between the supporting rollers l should be equal to ﬁve times the thickness.

3Ff σu = l , (1) 2bh2 Materials 2021, 14, x FOR PEER REVIEW 12 of 21 Figure 10 shows the schematic view of the testing procedure. In this study, the cross section of the specimen was 50 × 50 mm. Three samples were tested for each stone type and the average ﬂexural strength value was calculated.

Bending Load F f

80 diameter 10 Stone Prism 50x50x160

30 100

diameter 10

160

FigureFigure 10. 10. The schematic schematic representation representati of the testingon of procedure the testing (dimensions procedure in mm). (dimensions in mm). 4.2.2. Compressive Strength 4.2.2. CompressiveThe compressive strengthStrength test was performed in accordance with EN 1926 [42]. It is deﬁned that the test specimens were cut in cubes with (70 ± 5) mm or (50 ± 5) mm edges or rightThe circularcompressive cylinders with strength diameter test and heightwas equalperformed to (70 ± 5) in mm accordance or (50 ± 5) mm. with EN 1926 [42]. It is definedIn Figure that9 are presentedthe test thespecimens stone prisms beforewere testing. cut in For cubes this study, with the specimen(70 ± 5) cubes mm or (50 ± 5) mm edges were (70 ± 5) mm in size. Three samples were tested for each stone type and the average or rightcompressive circular strength cylinders value was calculated.with diameter The compressive and height strength equal is calculated to (70 by the ± 5) mm or (50 ± 5) mm. In formulaFigure in 9 equation are presented 2, where F is thethe applied stone load prisms in N and be A isfore the resistingtesting. area For in mm this2. study, the specimen

cubes were (70 ± 5) mm in size. ThreeFc samples were tested for each stone type and the σc = , (2) average compressive strength valuebh was calculated. The compressive strength is calculated byIn Figurethe formula 11 are plotted in theequation compressive 2, where strength andF is ﬂexural the applied strength ofload the tested in N and A is the resisting specimens. According to the test results the ﬂexural strength values vary from 8 to 33 MPa. 2 areaThe in compressive mm . strength test results showed that there is a large range in the values varying from 25 to 115 MPa. Physical properties test like open porosity and apparent density were performed according to EN 14617-1 [43], whereas𝜎 = the, water absorption of the (2) specimens was measured in compliance with EN 13755 standard [44].

In Figure 11 are plotted the compressive strength and flexural strength of the tested specimens. According to the test results the flexural strength values vary from 8 to 33 MPa. The compressive strength test results showed that there is a large range in the values varying from 25 to 115 MPa. Physical properties test like open porosity and apparent density were performed according to EN 14617-1 [43], whereas the water absorption of the specimens was measured in compliance with EN 13755 standard [44].

D − Dropull valley (2) J − Jorgucat G − Dropull valley (1) L − Polican & Lunxheri O − Odrie & Lunxheri P − Peshkepi K − Kardhiq 130

120 115.45 110 [31.41] 102.82 100 [28.35] 90 80 80.69 70 [13.06] 67.02 60 [8.67]

50 42.08 40 28.97 [33.15] 30 [8.56] CompressiveStrength (MPa) 20 22.73 10 [8.26] 0 024681012141618202224262830323436 Flexural Strength (MPa)

Materials 2021, 14, x FOR PEER REVIEW 12 of 21

Bending Load F f

80 diameter 10 Stone Prism 50x50x160

30 100

diameter 10

160

Figure 10. The schematic representation of the testing procedure (dimensions in mm).

4.2.2. Compressive Strength The compressive strength test was performed in accordance with EN 1926 [42]. It is defined that the test specimens were cut in cubes with (70 ± 5) mm or (50 ± 5) mm edges or right circular cylinders with diameter and height equal to (70 ± 5) mm or (50 ± 5) mm. In Figure 9 are presented the stone prisms before testing. For this study, the specimen cubes were (70 ± 5) mm in size. Three samples were tested for each stone type and the average compressive strength value was calculated. The compressive strength is calculated by the formula in equation 2, where F is the applied load in N and A is the resisting area in mm2. 𝜎 = , (2)

In Figure 11 are plotted the compressive strength and flexural strength of the tested specimens. According to the test results the flexural strength values vary from 8 to 33 MPa. The compressive strength test results showed that there is a large range in the values varying from 25 to 115 MPa. Physical properties test like open porosity and apparent density were performed according to EN 14617-1 [43], whereas the water absorption of Materials 2021the, 14 specimens, 1127 was measured in compliance with EN 13755 standard [44]. 13 of 23

D − Dropull valley (2) J − Jorgucat G − Dropull valley (1) L − Polican & Lunxheri O − Odrie & Lunxheri P − Peshkepi K − Kardhiq 130

120 115.45 110 [31.41] 102.82 100 [28.35] 90 80 80.69 70 [13.06] 67.02 60 [8.67]

50 42.08 40 28.97 [33.15] 30 [8.56] CompressiveStrength (MPa) 20 22.73 10 [8.26] 0 024681012141618202224262830323436 Flexural Strength (MPa)

Figure 11. Compressive and ﬂexural strengths of the tested specimens.

4.2.3. Physical Properties Physical properties test like open porosity and apparent density were performed according to EN 14617-1 [34], whereas the water absorption of the specimens was measured in compliance with EN 13755 [35]. The following equations show the calculation of the physical properties. Open Porosity: Mssd − Mdry p(%) = 100, (3) Mssd − Mw Apparent Density: Mdry ρ∗ = , (4) Mssd − Mdry Water absorption: Mssd − Mdry Wa(%) = 100, (5) Mdry

where Mw is the mass of the sample fully immersed in water, Mssd is the saturated surface dry mass of the sample, Mdry is the completely dry mass of the sample. In Figure 12 are plotted the physical properties of the tested specimens. The porosity values ranged from 0.30 to 11.11%. On the other hand, it was observed that, as expected, the water absorption values were closely related to porosity, ranging from 0.11 to 4.68%. Materials 2021, 14, x FOR PEER REVIEW 13 of 21

Figure 11. Compressive and flexural strengths of the tested specimens.

Open Porosity:

𝑝% = 100, (3)

Apparent Density:

𝜌∗= , (4)

Water absorption:

𝑊% = 100, (5)

where Mw is the mass of the sample fully immersed in water, Mssd is the saturated surface dry mass of the sample, Mdry is the completely dry mass of the sample. In Figure 12 are plotted the physical properties of the tested specimens. The porosity Materials 2021, 14, 1127 14 of 23 values ranged from 0.30 to 11.11%. On the other hand, it was observed that, as expected, the water absorption values were closely related to porosity, ranging from 0.11 to 4.68%.

(%) 12 11.11

10 9.66

4.68 4.46 3.9 4

2.7 2.7 2.69 2.67 2.72 2.69 2.42

2 1.49 1.05 1.11 0.49 0.3 0.42 0.11 0.18 0.39 0 D − Dropull valley J − Jorgucat G − Dropull valley L − Polican & O − Odrie & P − Peshkepi K − Kardhiq (1) (2) Lunxheri Lunxheri Water Absorption (%) Porosity (%) Apparent density

Figure 12. Physical propertiesFigure of 12. thePhysical tested properties specimens. of the tested specimens.

5. Structural Assessment5. Structural of the Assessment Masonry of the Masonry In this section, the mechanical properties and the expected structural response of the historic masonry constructions of Gjirokastër under the action of an earthquake have been evaluated. These properties are typically employed in commercially available modeling software (Ansys, Algor, SAP, etc.,) for numerical analyses and represent the fundamental parameters governing the structural behavior of a masonry building. In Figure 13 are presented some typical stone masonry buildings of Gjirokastër. Structural assessment of masonry is done using the MQI method which consists of a visual survey of the faces and the cross section of a wall. It considers seven critical parameters (mechanical characteristics and quality of the masonry units (acronym SM), dimensions of the masonry units (SD), shape of the masonry units (SS), level of connection between adjacent wall leaves (WC), horizontality of mortar bed joints (HJ), staggering of vertical mortar joints (VJ), quality of the mortar/interaction between masonry units (MM)). For each parameter, the visual analysis may result in three different outcomes: Fulfilled—F, Partially Fulfilled—PF, Not Fulfilled—NF. A chart of the method is sum- marized in Figure14. Materials 2021, 14, x FOR PEER REVIEW 2 of 3

Materials 2021, 14, 1127 15 of 23

Figure 13. Several typicalFigure stone 13. Several masonry typical buildings stone masonry of Gjirokastër. buildings of Gjirokastër.

The analysis of each parameter leads to a numerical value (for a total of 7 numerical results) based on its fulﬁllment category. The combination of the seven numerical values gives the value of MQI [40]. This is based on a wall panel and it is aimed at verifying if a wall complies with the “rules of the art” for the Gjirokastër historic walls (Figure 15). Based on this analysis, it is possible to calculate three numerical values. Because the structural response of a wall also depends on the loading condition, the values assigned to the seven

Materials 2021, 14, 1127 16 of 23

parameters depend on the loading condition acting on the wall under consideration. Three loading conditions are considered in the MQI method: V (vertical loads), O (out-of-plane horizontal loads), and I (in-plane horizontal loads). Consequently, three different MQI values (MQIV, MQIO, and MQII), can be calculated. The approach is to attribute different weights to the above parameters (between 0 and 3) based on the evidence that they affect Materials 2021, 14, x FOR PEER REVIEW the quality of the masonry with different degrees depending on the loading condition15 of 21 (Table2). In case of fulﬁlment of all parameters of quality, the MQI index is 10 irrespective of the loading condition.

Materials 2021, 14, x FOR PEER REVIEW 3 of 3

FigureFigure 14. Summary 14. Summary of ofthe the MQI MQI (Masonry (Masonry Quality Index)Index) method. method.

The analysis of each parameter leads to a numerical value (for a total of 7 numerical results) based on its fulfillment category. The combination of the seven numerical values gives the value of MQI [40]. This is based on a wall panel and it is aimed at verifying if a wall complies with the “rules of the art” for the Gjirokastër historic walls (Figure 15). Based on this analysis, it is possible to calculate three numerical values. Because the structural response of a wall also depends on the loading condition, the values assigned to the seven parameters depend on the loading condition acting on the wall under consideration. Three loading conditions are considered in the MQI method: V (vertical loads), O (out-of-plane horizontal loads), and I (in-plane horizontal loads). Consequently, three different MQI values (MQIV, MQIO, and MQII), can be calculated. The approach is to attribute different weights to the above parameters (between 0 and 3) based on the evidence that they affect the quality of the masonry with different degrees depending on the loading condition (Table 2). In case of fulfilment of all parameters of quality, the MQI index is 10 irrespective of the loading condition. The different masonry typologies can be compared with the mechanical characteristics suggested by the Italian Building Code (NTC 2018) [39]. Numerous tests, carried out on-site by the authors to validate the method, have demonstrated that the index is able to provide useful information about the mechanical characteristics, and structural response in general, of the analyzed wall panel.

Figure 15. Typical stone walls of Gjirokastër. Figure 15. Typical stone walls of Gjirokastër.

Materials 2021, 14, x FOR PEER REVIEW 16 of 21

Figure 15. Typical stone walls of Gjirokastër.

Table 2. Numerical values of the seven parameters for the calculation of the MQI.

Horizontal Out-of-Plane Horizontal In-Plane Parameters Vertical Loading (V) Loading (O) Loading (I) NF PF F NF PF F NF PF F

MaterialsHJ2021 , 14, 1127 0 1 2 0 1 2 0 0.5 1 17 of 23 WC 0 1 1 0 1.5 3 0 1 2 SS 0 1.5 3 0 1 2 0 1 2 VJ 0 Table0.5 2. Numerical1 values 0of the seven0.5 parameters for1 the calculation 0 of the MQI. 1 2 Horizontal Out-of-Plane Horizontal In-Plane SD 0 0.5Parameters 1 Vertical Loading 0 (V) 0.5 1 0 0.5 1 Loading (O) Loading (I) MM 0 0.5 NF2 PF 0 F0.5 NF1 PF F 0 NF 1 PF 2 F SM 0.3 0.7HJ 1 0 10.5 20.7 01 10.3 2 00.7 0.51 1 WC 0 1 1 0 1.5 3 0 1 2 SS 0 1.5 3 0 1 2 0 1 2 Most parts of the masonryVJ 0 buildings 0.5 in 1 Gjirokastër 0 0.5have been 1 constructed 0 1 using 2 white limestone (letter designationSD 0 J, Table 0.5 1) 1and soft 0 stone 0.5 (letter 1 designation 0 0.5L). In the 1 MM 0 0.5 2 0 0.5 1 0 1 2 following paragraphs theSM MQI has 0.3 been 0.7 calculat 1ed for 0.5 these 0.7 two types 1 of 0.3stone materials. 0.7 1

5.1. Wall panels Type 1 (CrossThe Sectional different masonry Thickness typologies < 60 cancm) be compared with the mechanical characteristics suggested by the Italian Building Code (NTC 2018) [39]. Numerous tests, carried out In this section, weon-site consider by the authorsmasonry to validate stone thewalls method, having have demonstrateda cross-sectional that the indexthickness is able to up to 60 cm (Figure 16).provide Only useful load-bearing information about walls the mechanicalare used characteristics,in the analysis. and structural The MQI response as- in sessment of the walls gavegeneral, the of theresults analyzed shown wall panel.in Table 3. This is based on the assumption Most parts of the masonry buildings in Gjirokastër have been constructed using white of the use of a lime mortarlimestone for (letter all studied designation wall J, Tables. It1 )is and worth soft stone noting (letter designationthat masonry L). In thewalls following of Gjirokastër are properlyparagraphs assembled the MQI with has beenregard calculated to three for these critical two types “rules of stone of the materials. art”: 1. the

horizontality of the bed joints5.1. Wall (HJ Panels parameter), Type 1 (Cross 2. Sectional the staggering Thickness <60of the cm) head joints (VJ parameter) and 3. the worked shaped (parallelepiped)In this section, we of considerthe stone masonry units stone(SS parameter). walls having a cross-sectionalIt is also important thickness up to highlight that headerto 60 stones cm (Figure (stone 16). Only bloc load-bearingks placed walls transversally are used in the analysis. to the The plane MQI assessmentof the walls) were used in Gjirokastërof the walls gavein historic the results constructions. shown in Table3. ThisTheir is based frequency on the assumption in the walls of the is use of a lime mortar for all studied walls. It is worth noting that masonry walls of Gjirokastër not high, but sufficientlyare properlyenough assembled to provide with a regard connection to three criticalbetween “rules the of thewall art”: leaves. 1. the horizontality This is an importantof the bedgeneral joints (HJ observatio parameter),n, 2. notthe staggeringcommon of thein headhistoric joints (VJ masonry parameter) con- and 3. structions in Southern theEurope. worked shapedAs a (parallelepiped)consequence, of thethe stone estimated units (SS parameter).values of Itthe is alsomechanical important to highlight that header stones (stone blocks placed transversally to the plane of the walls) parameters (compressivewere and used shear in Gjirokastër strength, in historic elastic constructions. moduli) Their are frequencygenerally in thehigh walls with is not de- high, cent compressive and seismicbut sufﬁciently capa enoughcity of to the provide investigated a connection walls. between the wall leaves.

header

(a) (b) (c)

FigureFigure 16. 16. SketchesSketches of of a walla wall panel panel Type 1Type (cross 1 sectional (cross thickness sectional < 60 thickness cm): (a) axonometric < 60 cm): view, (a) (baxonometric) front view, (c) wallview, (b) frontcross section. view, (c) wall cross section.

5.2. Type 2 (Cross Sectional Thickness ≥ 60 cm) The second type of masonry walls noted in Gjirokastër is characterized by a cross sectional thickness ≥60 cm (Figure 17). Again, two types of stone materials have been

Materials 2021, 14, 1127 18 of 23

Table 3. The building stone coding according to their origin.

Wall Type Stone HJ WC SS VJ SD MM SM MQIV MQIO MQII Stone J F PF F PF PF PF F 7.5 7 6.5 Type 1 Stone L F PF F PF PF PF PF 5.25 4.9 4.55 Stone J F NF F PF PF PF F 6.5 5.5 5.5 Type 2 Stone L F NF F PF PF PF PF 4.55 3.85 3.85

This is an important general observation, not common in historic masonry constructions in Southern Europe. As a consequence, the estimated values of the mechanical parameters (compressive and shear strength, elastic moduli) are generally high with decent Materials 2021, 14, x FOR PEER REVIEW compressive and seismic capacity of the investigated walls. 17 of 21

5.2. Type 2 (Cross Sectional Thickness ≥60 cm) The second type of masonry walls noted in Gjirokastër is characterized by a cross observed: the whitesectional limestone thickness (letter≥60 cmdesignat (Figure ion17). Again,J, Table two 1) types and of stonethe soft materials stone have (letter been observed: the white limestone (letter designation J, Table1) and the soft stone (letter designation L). Consideringdesignation these L). Considering two variable these twos (wall variables thickness (wall thickness and andstone stone material), material), inin total, the main macro-typologiestotal, the main macro-typologies of walls of Gjirokastër of walls of Gjirokastër used in used this in investigation this investigation are are 4.4.

(a) (b) (c) FigureFigure 17. 17.Sketches Sketches of a wallof a panel wall Type panel 2 (cross Type sectional 2 (cross thickness sectional≥60 cm): thickness (a) axonometric ≥ 60 cm): view, ((ba)) front axonometric view, (c) wall view, (crossb) front section. view, (c) wall cross section. For stone wall having a cross sectional thickness ≥60 cm, the analysis of the cross For stone wall sectionhaving revealed a cross a structure sectional of three thickness wall leaves: ≥ 60 two cm, outer the parts analysis typically of made the ofcross cut section revealed a structurestones, and anof innerthree core wall made leaves: of peddles, two small outer stones parts with typically a large percentage made of of voids. cut There are no transversal connections (headers) between the wall leaves and this highly stones, and an innerreduces core made the wall’s of capacitypeddles, against smal seismicl stones actions. with a large percentage of voids. There are no transversalTypically, connections head joints (headers) are properly between staggered the (outcome wallPF), leaves the mortar and isthis lime-based, highly reduces the wall’s capacitythe horizontal against bed joints seismic are continuous actions. (outcome F), the stones are worked, and their shape is parallelepiped (outcome F). It is worth noting that timber ties or beams sometimes used Typically, headin joints the past are to properly connect the staggered wall leaves have (outcome not been PF), considered the mortar in the assessment. is lime-based, Their the horizontal bed jointspresence are could continuous highly improve (outcome the structural F), responsethe stones of a wall are panel, worked, and a case-by-case and their shape is parallelepipedanalysis (outcome is needed forF). thoseIt is buildingsworth noting where these that timber timber elements ties areor noted.beams For some- walls having a cross sectional thickness ≥60 cm, we have concluded that transversal connection times used in the past(WC parameter)to connect between the wall leaves leaves is weak have (outcome: not been NF). considered in the assessment. Their presenceTables could3 and highly4 show theimprove results of the the visualstructural analysis response in terms of of MQI a wall indices panel, and and a case-by-case analysismechanical is characteristics: needed for forthos wallse buildings having a cross where sectional these thickness timber≥60 elements cm, results are noted. For walls having a cross sectional thickness ≥ 60 cm, we have concluded that transversal connection (WC parameter) between wall leaves is weak (outcome: NF). Tables 3 and 4 show the results of the visual analysis in terms of MQI indices and mechanical characteristics: for walls having a cross sectional thickness ≥ 60 cm, results are given for both J and L stone types. It can be noted that the use of a soft stone (Type L) highly affects the structural assessment, with a value of MQI index of 3.85 for horizontal in-plane and out-of-plane loading. Regardless of the thickness size, it is observed that the type of the stone has a higher influence on the structural behavior of the wall. The MQI values varies from 6.5-7.5 to 5.5-6.5 for stone J, whereas 4.55-5.25 to 3.85-4.55 for stone L.

Table 3. The building stone coding according to their origin.

Materials 2021, 14, 1127 19 of 23

given for both J and L stone types. It can be noted that the use of a soft stone (Type L) highly affects the structural assessment, with a value of MQI index of 3.85 for horizontal in-plane and out-of-plane loading. Regardless of the thickness size, it is observed that the type of the stone has a higher inﬂuence on the structural behavior of the wall. The MQI values varies from 6.5–7.5 to 5.5–6.5 for stone J, whereas 4.55–5.25 to 3.85–4.55 for stone L.

Table 4. Results of the assessment of the mechanical properties of the masonry materials.

Compressive Shear Shear Strength, Shear Strength, f , Young’s Modulus, Wall Type Stone Strength, f, 0 Modulus, G, τ (MPa) [44] (MPa) [44] E, (MPa) (MPa) 0 (MPa) 5.69 0.097 0.237 647 2316 Type 1 Stone J (4.53–6.68) (0.070–0.121) (0.167–0.306) (538–756) (538–756) 4.74 0.082 0.207 561 2011 Stone L (3.72–5.74) (0.060–0.102) (0.145–0.269) (466–656) (1668–2353) 3.77 0.069 0.178 490 1659 Type 2 Stone J (2.92–4.60) (0.052–0.069) (0.124–0.232) (407–573) (1374–1944) 3.31 0.060 0.156 443 1490 Stone L (2.55–4.07) (0.045–0.074) (0.108–0.203) (368–519) (1232–1746)

Table4 reports the values of the most important mechanical parameters resulting from the application of the MQI method for the four macro-typologies of walls noted in Gjirokastër. These parameters are given in terms of an average value: however, to take into account the intrinsic variability of a material like masonry, an “interval of confidence” is also given in parentheses in Table4 for each mechanical parameter. The use of an interval is consistent with the method suggested by the Italian Seismic Code [39] and attached Guidelines [40]: in this code, the masonry mechanical parameters are given in terms of an interval of confidence, where the average value is typically the mean. A MQI value smaller than 4 is an indication of a likely masonry failure mode by disaggregation (The failure mode of a wall panel under the action of an earthquake can be categorized in two classes: masonry disaggregation and the development of a local or global mechanism of wall elements (macroelements). Under the action of an earthquake, some types of masonry are typically unable to deform and to split in macroelements and fail by “masonry disaggregation” or “masonry crumbling”. This failure is typical of some types of low-quality masonry: it is worth noting that the seismic capacity of a building where the governing failure mode is disaggregation is often low). This failure mode should always be avoided in masonry buildings and interventions are normally needed to reinforce the wall when this failure mode is likely to occur during a seismic event. For walls made of stone material type J, the outcomes of 6 out of the 7 parameters remained unchanged. However, the use of a stone of higher compressive and flexural strengths has a positive effect on the structural response of the walls: the MQIV, MQIO, MQII increased to 6.5, 5.5, and 5.5, respectively, and better results for the mechanical parameters have been estimated. Figure 18 shows the correlation curves used to estimate the mechanical properties of the 4 wall typologies of Gjirokastër. These curves have been calibrated using the experimental data available in the scientific literature and suggested by the Italian Building Code (NTC 2018) [41]. Materials 2021, 14, x FOR PEER REVIEW 18 of 21

attached Guidelines [40]: in this code, the masonry mechanical parameters are given in terms of an interval of confidence, where the average value is typically the mean.

Table 4. Results of the assessment of the mechanical properties of the masonry materials.

Compressive Shear Shear Shear Young’s Wall Type Stone Strength, f, Strength, τ0, Strength, f0, Modulus, G, Modulus, E, (MPa) (MPa) [44] (MPa) [44] (MPa) (MPa) 5.69 0.097 0.237 647 2316 Type 1 Stone J (4.53–6.68) (0.070–0.121) (0.167–0.306) (538–756) (538–756) 4.74 0.082 0.207 561 2011 Stone L (3.72–5.74) (0.060–0.102) (0.145–0.269) (466–656) (1668–2353) 3.77 0.069 0.178 490 1659 Type 2 Stone J (2.92–4.60) (0.052–0.069) (0.124–0.232) (407–573) (1374–1944) 3.31 0.060 0.156 443 1490 Stone L (2.55–4.07) (0.045–0.074) (0.108–0.203) (368–519) (1232–1746)

A MQI value smaller than 4 is an indication of a likely masonry failure mode by disaggregation*. This failure mode should always be avoided in masonry buildings and interventions are normally needed to reinforce the wall when this failure mode is likely to occur during a seismic event. For walls made of stone material type J, the outcomes of 6 out of the 7 parameters remained unchanged. However, the use of a stone of higher compressive and flexural strengths has a positive effect on the structural response of the walls: the MQIV, MQIO, MQII increased to 6.5, 5.5, and 5.5, respectively, and better results for the mechanical parameters have been estimated. Figure 18 shows the correlation curves used to estimate the mechanical properties of the 4 wall typologies of Gjirokastër. These curves have been calibrated using the exper- Materialsimental2021, 14, 1127data available in the scientific literature and suggested by the Italian Building 20 of 23 Code (NTC 2018) [41].

6000

0.18 • average values of E (NTC 2018) y = 740.08e0.1538x 5000 R² = 0.7225 0.16 ...... trend line • average values of τ0 (NTC 2018) 0.14 ...... trend line 2 y = 0.0005x + 0.0086x + 0.0199 4000 R² = 0.922 0.12

0.1 3000 (MPa) E (MPa)

0 0.08 τ

0.06 2000

0.04 1000 0.02

0 012345678910 0 012345678910 Materials 2021, 14, x FOR PEER REVIEW MQII 19 of 21 MQIV (a) (b)

1400

10 • average valuesy = of 1.4357e f (NTC0.1837x 2018) 1200 • average values of G (NTC 2018) R² = 0.8355 9 ...... trend line y...... = 256.13e trend0.1426x line R² = 0.6963 1000 8

7 800 6 G (MPa) f (MPa) 5 600

3 400

1 200

0 012345678910 0 MQIV 012345678910 MQII (c) (d) 0.35 y = 0.0528x0.8024 • R² average = 0.9187 values of fV0 (NTC 2018) ...... trend line 0.3

0.25

0.2 (MPa) V0

f 0.15

0.1

0.05

0 012345678910 MQI I (e)

FigureFigure 18. 18.MQI MQI correlations correlations with with masonry masonry mechanical mechanical parameters: parameters: (a) Masonry (a shear) Masonry strength shearτ0 vs. strength MQII (NTC τ0 2018, Italianvs. MQI SeismicI (NTC Code, 2018, [40 ]),Italian (b) Masonry Seismic Young’s Code, modulus [40]), (b E) Masonry vs. MQIV,( Young’sc) Masonry modulus compressive E vs. strength MQIV, f ( vs.c) MQIV, (Masonryd) Masonry compressive shear modulus strength G vs. MQI fI ,(vs.e) MQI MasonryV, (d shear) Masonry strength shear fV0 vs. modulus MQII. G vs. MQII, (e) Masonry shear strength fV0 vs. MQII.

Materials 2021, 14, 1127 21 of 23

6. Conclusions In this paper the main characteristics of the traditional masonry constructions of the UNESCO heritage site of Gjirokastër, Albania, which is located in a high-risk seismic area on the seismogenic zone of Ionian Coast are discussed and presented. The construction materials used in Gjirokastër are roughly cut stones which although look similar in shape, color, and dimensions, exhibited significant difference in terms of physical and mechanical properties. Identification of material types of these stone typologies resulted in specimen col- lection of samples in traditional stone quarries nearby. The stones were cut into regular prismatic shape as of relevant international standards and flexural, compressive strength, as well as porosity, water absorption, and apparent densities were computed. From the test results it was observed that there is a significant scattering of the mechanical properties of the building stones. The compressive strength varies from 22.7 to 115 MPa, whereas flexural strength from 8 to 33 MPa. This fact is also supported by the different use of various stones in construction. There was also found a close relationship between the physical and mechanical properties. The assessment of the structural behavior of the masonry buildings demonstrated that the shear walls used in Gjirokastër for construction were of acceptable quality: the stones, mainly stones with medium-to-very high mechanical properties, were typically roughly reduced and cut to prismatic shape. Unlike the stone masonry often used in Italy, Greece, and other neighboring countries, rubble or pebble stone masonry is very rare in Gjirokastër buildings. The structural response of four stone masonry typologies was investigated using the Masonry Quality Index (MQI). The results showed that, in general, the failure of masonry is considered to be at an acceptable level. The MQI provided an estimation of the main mechanical parameters (compressive and shear strength, elastic moduli). However, only for one masonry typology (the thick masonry made of a soft stone), the MQI results were below the threshold and a risk of failure by disaggregation under the seismic action was observed. For all other investigated masonry typologies, the MQI analysis provided an index result varying between 5.25–7.5 (MQIV), 4.9–7 (MQIO), and 4.55–6.5 (MQII). Of particular interest are the results in terms of masonry shear strength, τ0, also in consideration of the high seismic hazard of the territory of Gjirokastër. The average masonry shear strength τ0, estimated using the correlation curves, resulted in 0.097 MPa (Type 1 masonry, J stone), 0.082 MPa (Type 1 masonry, L stone), and 0.069 MPa (Type 2 masonry, J stone). Based on the authors’ experience, these values are slightly higher compared to the average typical shear strength of historic masonry of other common masonry typologies studied and tested by the authors in the last two decades.

Author Contributions: Conceptualization, A.B. and E.M.; methodology, E.L.; validation, A.B., M.C., and A.Z.; formal analysis, E.M.; investigation, Y.M., and A.Z.; data curation, Y.M. and E.L.; writing— original draft preparation, M.C.; writing—review and editing, M.C.; visualization, E.M.; supervision, A.B.; project administration, E.M.; All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Date Availability Statement: Data is contained within the article. Acknowledgments: The authors would like to thank Arch. Kreshnik Merxhani for providing some of the archive documents cited in this study. Conﬂicts of Interest: The authors declare no conﬂict of interest. Materials 2021, 14, 1127 22 of 23

References 1. The World Heritage Committee. UNESCO convention concerning the protection of the world cultural and natural heritage. In Proceedings of the World Heritage Committee, Thirtieth Session, Vilnius, Lithuania, 8–16 July 2006. 2. Riza, E. Banesa e fortifikuar Gjirokastrite, (in Albanian). Monumentet 1971, 1, 127–148. 3. Bace, A.; Meksi, A.; Riza, E.; Karaiskaj, G.j.; Thomo, P. Historia e Arkitektures Shqiptare; Institute of Monuments of Culture: Tirana, Albania, 1979. 4. Kamberi, T. Disa të dhëna mbi teknikën e ndërtimit të banesës gjirokastrite. Monumentet 1971, 2, 157–164. (In Albanian) 5. Merxhani, K.; Mamani, E. Materialet ndertimore ne monumentet historike—Gjirokaster. Monumentet 2014, 52, 127. (In Albanian) 6. Merxhani, K.; Mamani, E. Construction materials in historical and monumental buildings—Gjirokaster. In Proceedings of the International Students’ Conference of Civil Engineerin ISCCE 2012, Tirana, Albania, 10–11 May 2012. 7. Riza, E. Arkitektura dhe restaurimi i banesës së Zekatëve—Gjirokastër. Monumentet 1977, 13, 107–132. 8. Riza, E. Nje shembull i zhvilluar i baneses gjirokastrite (Banesa e Skendulajve). Monumentet 1984, 2. 9. Doraci, A. Restaurimi i banesës Kadare, Gjirokastër. Monumentet 2015, 235–241. (In Albanian) 10. Hadzic, L.; Mamani, E.; Merxhani, K. Restaurimi i banesës Babameto, monument kulture i kategorisë I, Gjirokastër. Monumentet 2015, 53, 201–214. (In Albanian) 11. Shkupi, F. Zbukurimorja e disa banesave gjirokastrite. Monumentet 1986, 12, 99–116. (In Albanian) 12. Strazimiri, G. Gjirokastra dhe vlerat e saj kulturore. Monumentet 1972, 2, 85–102. (In Albanian) 13. Meksi, A. Dy kisha bizantine në rrethin e Gjirokastrës. Monumentet 1975, 9, 77–106. (In Albanian) 14. Thomo, P. Manastiret e krahinave të Gjirokastrës. Monumentet 2015, 53, 147–169. (In Albanian) 15. Mustafaraj, E. Assessment of Historical Structures, A Case Study of Five Ottoman Mosques in Albania; LAP Lambert Academic Publishing: Tirana, Albania, 2014. 16. Barroso, C.E.; Oliveira, D.V.; Ramos, L.F. Physical and mechanical characterization of vernacular dry stone heritage materials: Schist and granite from Northwest Portugal. Constr. Build. Mater. 2020, 259, 119705. [CrossRef] 17. Pinheiro, A.C.; Mesquita, N.; Trovão, J.; Soares, F.; Tiago, I.; Coelho, C.; Paiva de Carvalho, H.; Gil, F.; Catarino, L.; Piñar, G.; et al. Limestone biodeterioration: A review on the Portuguese cultural heritage scenario. J. Cult. Herit. 2019, 36, 275–285. [CrossRef] 18. Ademović,N.; Kurtović,A. Influence of planes of anisotropy on physical and mechanical properties of freshwater limestone (Mudstone). Constr. Build. Mater. 2021, 268, 121174. [CrossRef] 19. Fratini, F.; Cantisani, E.; Pecchioni, E.; Pandeli, E.; Vettori, S. Pietra Alberese: Building material and stone for lime in the Florentine Territory (Tuscany, Italy). Heritage 2020, 3, 84. [CrossRef] 20. Ding, S.; Jia, H.; Zi, F.; Dong, Y.; Yao, Y. Frost damage in tight sandstone: Experimental evaluation and interpretation of damage mechanisms. Materials 2020, 13, 4617. [CrossRef][PubMed] 21. Salvatici, T.; Calandra, S.; Centauro, I.; Pecchioni, E.; Intrieri, E.; Garzonio, C.A. Monitoring and evaluation of sandstone decay adopting non-destructive techniques: On-site application on building stones. Heritage 2020, 3, 71. [CrossRef] 22. Yin, D.; Xu, Q. Comparison of sandstone damage measurements based on non-destructive testing. Materials 2020, 13, 5154. [CrossRef] 23. Mustafaraj, E.; Yardim, Y.; Corradi, M.; Borri, A. Polypropylene as a retrofitting material for shear walls. Materials 2020, 13, 2503. [CrossRef][PubMed] 24. Mustafaraj, E.; Yardim, Y. Retrofitting damaged unreinforced masonry using external shear strengthening techniques. J. Build. Eng. 2019, 26, 100913. [CrossRef] 25. Mustafaraj, E.; Yardim, Y. In-plane shear strengthening of unreinforced masonry walls using GFRP jacketing. Period. Polytech. Civ. Eng. 2018, 62, 330–336. [CrossRef] 26. Capozucca, R. Experimental analysis of historic masonry walls reinforced by CFRP under in-plane cyclic loading. Compos. Struct. 2011, 94, 277–289. [CrossRef] 27. Corradi, M.; Borri, A.; Castori, G.; Sisti, R. Shear strengthening of wall panels through jacketing with cement mortar reinforced by GFRP grids. Compos. Part B Eng. 2014, 64, 33–42. [CrossRef] 28. Del Zoppo, M.; Di Ludovico, M.; Balsamo, A.; Prota, A. Experimental in-plane shear capacity of clay brick masonry panels strengthened with FRCM and FRM composites. J. Compos. Constr. 2019, 23, 04019038. [CrossRef] 29. Sorrentino, L.; Cattari, S.; da Porto, F.; Magenes, G.; Penna, A. Seismic behaviour of ordinary masonry buildings during the 2016 central Italy earthquakes. Bull. Earthq. Eng. 2018, 1–25. [CrossRef] 30. De Felice, G.; De Santis, S.; Lourenço, P.B.; Mendes, N. Methods and challenges for the seismic assessment of historic masonry structures. Int. J. Archit. Herit. 2017, 11, 143–160. [CrossRef] 31. Corradi, M.; Borri, A. A database of the structural behavior of masonry in shear. Bull. Earthq. Eng. 2018, 16, 3905–3930. [CrossRef] 32. Kita, A.; Cavalagli, N.; Masciotta, M.G.; Lourenço, P.B.; Ubertini, F. Rapid post-earthquake damage localization and quantification in masonry structures through multidimensional non-linear seismic IDA. Eng. Struct. 2020, 219, 110841. [CrossRef] 33. Brignola, A.; Frumento, S.; Lagomarsino, S.; Podesta, S. Identification of shear parameters of masonry panels through the in-situ diagonal compression test. Int. J. Archit. Herit. 2008, 3, 52–73. [CrossRef] 34. Corradi, M.; Borri, A.; Vignoli, A. Strengthening techniques tested on masonry structures struck by the Umbria-Marche earthquake of 1997–1998. Constr. Build. Mater. 2002, 16, 229–239. [CrossRef] Materials 2021, 14, 1127 23 of 23

35. Chiostrini, S.; Galano, L.; Vignoli, A. On the determination of strength of ancient masonry walls via experimental tests. In Proceedings of the 12th World Conference on Earthquake Engineering, Auckland, New Zealand, 30 January–4 February 2000. 36. Borri, A.; Castori, G.; Corradi, M.; De Maria, A. A method for the analysis and classiﬁcation of historic masonry. Bull. Earthq. Eng. 2015, 13, 2647–2665. [CrossRef] 37. Borri, A.; Corradi, M.; De Maria, A.; Sisti, R. Calibration of a visual method for the analysis of the mechanical properties of historic masonry. Procedia Struct. Integr. 2018, 11, 418–427. [CrossRef] 38. Borri, A.; Corradi, M.; De Maria, A. The failure of masonry walls by disaggregation and the masonry quality index. Heritage 2020, 3, 65. [CrossRef] 39. Italian Seismic Code 2018, Act (D.M.) 17 Jan 2018, Aggiornamento Delle Norme Tecniche per le Costruzioni. G.U. n. 42, 20 February 2018. Available online: https://www.gazzettaufﬁciale.it/eli/gu/2018/02/20/42/so/8/sg/pdf/ (accessed on 22 December 2020). 40. Italian Seismic Code, Guidelines, 21 Jan 2019, n. 7 CSLL. Istruzioni per l’applicazione dell’ Aggiornamento delle “Norme Tecniche per le Costruzioni”. Available online: http://www.cngeologi.it/wp-content/uploads/2019/02/Circolare-21-gennaio-2019-n.-7 .pdf (accessed on 22 December 2020). 41. EN 12372—Natural Stone Test Methods. Determination of Flexural Strength under Concentrated Load; British Standards Institution (BSI): London, UK, 2006. 42. EN 1926—Natural Stone Test Methods. Determination of Uniaxial Compressive Strength; British Standards Institution (BSI): London, UK, 2006. 43. EN 14617-1—Agglomerated Stone. Test Methods. Determination of Apparent Density and Water Absorption; British Standards Institution (BSI): London, UK, 2013. 44. EN 13755—Natural Stone Test Methods. Determination of Water Absorption at Atmospheric Pressure; British Standards Institution (BSI): London, UK, 2008.