Unreinforced masonry (URM) buildings are among the most common types of buildings in the seismically active zones of southern and central Europe. Some of them possess a high historical value and could therefore be classified as part of the architectural heritage, requiring special attention with regard to their preservation and retrofitting measures [29]. In order to achieve a better and standardized operation in Europe, on the recommendation of ENCoRE (European Network for Conservation—Restoration Educations), unique project provisions for the resistance of structures to earthquakes have been established, through codes that require checking the structural assembly of an object threatened by an earthquake in terms of strength and ductility [30]. According to Eurocode 8 (Design provisions for earthquake resistance of structures), the new structure of the building should be designed and constructed in a way to prevent its collapse and provide the necessary earthquake resistance [31].

Based on the analysis of seismic data in the last 120 years since earthquakes have been measured and monitored, the territory of Novi Pazar is not in the region of increased seismic activity. Regardless of these data, it is necessary to rehabilitate, reconstruct, and strengthen cultural heritage objects with appropriate methods in order to better respond to potential earthquakes. Parts of historical buildings have been demolished partly because of the time that has passed since their construction, partly because of earthquakes in the past, and often the cause is man (destruction by wars and the like). The epicenter of the earthquake most often occurs around Kopaonik mountain, which is near Novi Pazar, or in the part of central Serbia. Ground tremors of lower intensity (up to 2 degrees on the Richter scale) are present in the territory of Serbia almost every day throughout the year, but they do not leave any consequences on buildings [32].

The methods of seismic rehabilitation of buildings include various techniques for repairing the appearance and stability of structures, including effective parameters in improving seismic strengthening that can be applied to the buildings of the fortress. In order to preserve the stylistic integrity of the building and the longevity of the structure, a combination of old and new materials is most often introduced, as it has proven to be the least problematic and offers high quality. The performance of old materials is improved by new materials. The concept of building rehabilitation includes a set of measures to establish its stability in the event of an earthquake.

The technical measures necessary to establish the stability and resistance of the wall are [33]:

• Foundation consolidation,
• Connection of walls and floor structures,
• Strengthening the walls by grouting and/or injecting cracks,
• Reinforcement of masonry vaults,
• Renovation of existing floor structures or their replacement with new and solid structural elements,
• Reconstruction of the wooden roof structure, and
• Repair and strengthening of non-constructive elements.

There are different types of protection methods in practice from those that require strict adherence to traditional crafts and techniques to those that completely introduce new materials and constructive systems. The subject of many discussions is whether a better solution is to apply new techniques based on traditional materials whose long-term behavior is predictable, or to hurry with the application of materials that have not yet been sufficiently tested. The choice between traditional and innovative techniques should be weighed on a case-by-case basis and priority should be given to those that are least invasive and most compatible with heritage values, taking into account safety and durability requirements [34]. Defining an appropriate strategy for the rehabilitation of cultural heritage objects is an important step in overcoming the consequences of earthquakes. Wall reinforcement can be done by:

• injection mortars
• composite materials
• reinforced steel
• connecting mezzanine constructions and surrounding walls with various types of connections
• strengthening of floor beams

The design of temporary interventions for the safety of the historical building starts from the measurement of the damage and from the identification of the collapse mechanisms activated by the seismic effect. The proposed interventions refer to:

• Interventions to improve connections
• Interventions to increase wall strength
• Interventions to reduce the flexibility of floors and their consolidation

The general principles of wall strengthening refer to methods of repair, reconstruction, and strengthening [35].

• Repairs include operations that do not include the load-bearing structure, with the aim of improving the comfort of using the facility and repairing damage to non-load-bearing elements.
• Reconstruction includes the restoration of the supporting system of buildings to the level that existed before the damage occurred.
• Reinforcements include strengthening of the supporting structure (damaged or undamaged) to the level of the desired safety of the object.

Damage of the buildings depends on the layout of the load-bearing walls at the base, the connection between vertical elements and ceiling, and ductility. Depending on the place that needs to be strengthened, technical solutions are applied that increase the bearing capacity of the structure, increase the ductility, or increase both factors. Due to the effect of the earthquake, the wall may fail diagonally, when the wall is loaded with vertical and horizontal loads at the same time. Vertical load is constantly present, while horizontal load occurs occasionally, due to the effect of earthquakes or strong winds. Compressive strength depends on the strength of the masonry elements. Fracture most often occurs at the element itself and not at the mortar joint (whose compressive strength is much lower) due to the incompatibility of the characteristics of the masonry element and the mortar, as well as their non-adhesion to each other. Adhesion depends on the strength, consistency, and composition of the mortar, the quality of aggregates, the strength of the contact surfaces, their roughness, climatic conditions, etc. In practice, it is recommended to wet the wall elements before placing them in the mortar, due to better adhesion [36].

Injection mortars can have different ingredients, depending on the dimensions of the cracks and fissures, that is, on the dimensions of the opening that is filled with the injection mass. The following can be used for grouting: epoxy resins, cement suspensions, cement mortars, polymer-cement mortars, and epoxy mortars. Quartz flour is most often used as a filler in the mentioned mortars. Finely divided mineral materials (bentonite—a kind of clay of volcanic ash, natural and artificial pozzolans, used as a filler) are used as additives for making injection mixtures, and plasticizers and aerants (air entraining admixture) are used as chemical additives. Epoxy resins are used for injecting cracks up to 0.3 mm wide. Other types of injection mortars are used for injection of cracks wider than 0.3 mm, cavities, injection of prestressing cables, etc. (Figure 16a). Cracks wider than 0.3 mm can be filled by injecting liquid cement mortar, and in special cases epoxy materials are used. Injection of cracks can be omitted if the reinforcement of the shotcrete wall planned to be done by injection of water-cement mortar in nozzles, under pressure. Injection can also be used for larger cracks up to a size of 10 mm. The injection procedure is carried out step by step: first, the plaster around the crack is removed, using a jet of water or pressurized air. Then along the crack, at lengths of 300 to 600 mm, holes are drilled into which plastic tubes are placed at a depth of 50 mm, fixed with cement mortar. Cracks are filled with cement mortar along the entire length. The tubes are plugged and then, by opening the plugs in pairs, the crack between the adjacent tubes is washed or blown out. Injecting is done under low pressure, from the bottom up, and the emulsion starts to flow on the first upper tube [36]. Instead of tubes, the grouted wall can be strengthened by inserting clamps at the grouted places (Figure 16b).

Composite materials are materials composed of two or more components, where the mechanical characteristics of the material depend on the mechanical characteristics of the components. In practice, most composite materials are composed of a base material (matrix) and reinforcement (a material that provides strength). In construction practice, epoxy resins are most often used as a matrix, and glass, aramid, or carbon fibers as reinforcements. Steel clamps, reinforcement, or reinforced concrete beams can be installed directly on the cracks (Figure 17a). Cavities can also be filled with concrete, thus strengthening the entire wall. In the case of significantly damaged walls, it is often necessary to replace entire parts of the wall mass, where it is first necessary to temporarily support the structure located above the damaged wall. In the case of visible dislocations of a part of the wall, the existing wall should be removed and a new one made of the same material should be rebuilt. One of the ways to rehabilitate and strengthen walls damaged by an earthquake is reinforcement. The reinforcement bar is attached to the pre-cleaned wall that has open joints and cracks, by inserting vertical and horizontal ring beams, while injecting the filling mass. Cement mortar M30 is applied to the reinforced wall in a thickness of 3 to 5 cm. Another way is to install concrete “niches” (Figure 17b). In order to increase the rigidity of the existing floor structure, reinforcement is recommended, with the use of steel clamps on both sides of the walls (attached to the wooden floor structure). Damaged and dislocated walls should be rebuilt with the same material but of better quality or reinforced with ring beams [48]. Part of the facade can be connected to the rest of the building, using wooden beams—tensioners and iron anchors, so-called “keys” (Figure 17b). In this way, during an earthquake, due to stress concentration, the creation of cracks and the collapse of the walls would be prevented. Blocks of stone can be connected to each other with metal anchors, in order to act on the principle of a chain [37].

Strengthening of floor structures is performed by placing diagonal connecting beams if the floor structure is wooden, with anchoring of wooden beams in the walls. For such interventions, a static calculation is necessary. Renovation of the roof structure involves replacing damaged parts of the structure with new ones and installing a new roof covering (tile, in the case of Tower Motrilja).

It should be mentioned that there are two conflicting opinions of researchers, considered to be two schools: on the choice of materials for the parts to be added: French and Italian. According to the French school, new parts of the building should be made of the same material and processed in an identical way as the original parts—a replica, a “fake”. The Italian school advocates a completely opposite opinion: that the new parts are embedded into the original with a general impression, but should differ in processing or construction in order to make these additions visually distinguishable on the building itself [38].

This issue was resolved in the Athens Charter (adopted in 1933, contributed to the internationalization of cultural heritage) as follows: “The restoration of the original parts to their former place is recommended when the situation allows; the necessary new materials in that case must be such that they can always be recognized”.

The Venice Charter (International charter for the conservation and restoration of monuments and sites, 1964) states: “The additions that are supposed to replace the missing parts must fit aesthetically into the whole, but still differ from the original parts, so that the restoration does not represent a falsification of the documents of art and history.”

Building conservation and preservation techniques should be directly related to researched materials and technologies for construction, repair and restoration of built heritage. The chosen intervention should respect the original function and ensure compatibility with existing materials, structures, and architectural values. All new materials and technologies should be precisely tested before use because their properties and behavior can change over time and threaten the structure of cultural heritage. In the past, there have been many missteps in the use of materials and restoration techniques that have irreversibly compromised the appearance (or even the continued existence) of many works of art. The use of modern materials, as a substitute for original materials, is suitable only if it meets several prerequisites:

• provides a significant, identifiable advantage;
• their use has a solid scientific basis;
• their use is supported by experience.

In the case of using new materials, it is necessary to choose those that are compatible with their expression, appearance, texture, and shape of the original pieces. New materials should also meet the requirements of the physical and geographical characteristics of the climate, as well as the local way of life of the population [4].

Epoxy-based materials are most often used. When using these materials, it is necessary to determine the compatibility of their physical/mechanical properties (density, linear thermal expansion) during bonding, the volume, the size of the joint to be filled and the temperature around the joint to prevent stress between the resin and the surrounding structures. Emerging building materials that can be applied for cultural heritage interventions are smart materials, fiber-reinforced composites (FRP) suitable for repairing and strengthening structures, geomaterials for improving terrain properties, aluminum materials suitable for protective structure construction, often combined with glass or transparent plastic, polymer-modified mineral materials (mortars, concrete), and materials for wall modification mainly based on polymers [33]. In recent years, innovative techniques have been developed consisting of the use of fiber-reinforced polymers (FRP) as well as fabric-reinforced cement matrices (FRCM). Both methods have proven to be effective in terms of increasing the capacity and ductility of masonry elements without overloading the structure with additional weight. The use of FRP fibers is almost irreplaceable in the restoration of cultural assets from the consequences of earthquakes. FRP composite materials are available as ready-made factory products in the form of strips, fabrics, and rods that are joined with suitable adhesives (most often epoxy-based) for the external surfaces of the structural elements to be strengthened. FRP composites can be used to strengthen wall units or only individual wall segments. Reinforcements include FRP strips placed horizontally, vertically, and diagonally, as well as a combination of vertically placed FRP strips and horizontally placed FRP bars [39]. The advantage of strengthening masonry structures using FRP products, compared to traditional reinforcement techniques, is minimal additional weight, quick application, increased resistance to pressure, and impossibility of breakage by cracking.

One of the directions in the reconstruction of the architectural heritage after the earthquake is the use of old, traditional materials significantly improved by new technologies. Their application in the reconstruction of architectural heritage enables the implementation of one of the basic principles of preservation—reversibility. However, when FRP systems are used with epoxy resins, despite their advantages in overall structural performance, they are considered less viable due to low fire resistance, high sensitivity to ultraviolet light when exposed to open air, high toxicity, and unsatisfactory durability. The FRCMs, on the other hand, combine a fiber-reinforced mesh embedded in a high-performance, sprayable cement matrix and overcome most of the disadvantages arising from the use of epoxy resins as a bonding agent [53]. The FRCM technique is often considered a “green solution” because it involves the use of fiber-based meshes embedded in an inorganic (cement or lime) coating, instead of the epoxy resin used as a matrix for FRP systems. In addition to improving structural safety in the event of seismic events, FRCMs enable the reuse of existing material, which is an advantage in terms of sustainability and reduction of raw material use. The use of an inorganic matrix (lime or cement) has very positive characteristics in terms of “compatibility” between old masonry materials (lime, stone, brick) and new ones. The compatibility of new and old materials is the most important prerequisite for obtaining good and lasting results in the rehabilitation of cultural heritage buildings. Similar to improved old materials, brand new nano-materials enable delicate interventions that can maximally preserve the original fabric of buildings that have remained undamaged or slightly damaged in earthquakes. The latest materials research based on nanoparticles and the study of physical and chemical phenomena at the nano scale create new approaches to the science of preservation of historical monuments, which leads to new methodologies that restore the original appearance of cultural heritage buildings. The combination of different nanotechnologies allows today’s conservators to provide interventions that respect the physical and chemical characteristics of the materials at each step of the restoration. They are used in the restoration process [54]. The future will show whether it has moved in the right direction.

The Venice Charter (1964) addressed the problem of use of modern techniques in its Article 10: ”Where traditional techniques prove inadequate, the consolidation of a monument can be achieved by the use of any modem technique for conservation and construction, the efficacy of which has been shown by scientific data and proved by experience”.

The type of measures and methods that will be applied in order to increase the resistance of the specific object depends on the building configuration, the type and quality of the material from which it was built, the degree of damage, as well as the level of resistance that should be achieved. The final decision on the method of rehabilitation, i.e., strengthening, should always be based on the appropriate calculation, engineering decision, data related to the assessed condition of the object, including its damage, type, and quality of the material from which it was made.

Strengthening and rehabilitation of the existing supporting system includes various interventions [40]:

• strengthening by walling up individual walls in the construction system;
• introducing new walls into the basic construction system;
• connecting the construction of the walls at the level of the inter-floor constructions.

Considering the degree of damage, the literature recommends the application of three levels of reconstruction [55]. Each of the proposed levels has its own specific characteristics that are reflected in the details. By applying these levels, the structural assembly, purpose, or area of the space does not change. Each of the proposed levels can be applied separately, with a time distance from the previous level.

The reconstruction and rehabilitation process itself should be the least invasive and the most cost-effective, with the use of light materials and simple techniques. By applying one level or combining several at the same time, the overall earthquake resistance of the building increases.

Level 1 is the simplest and includes restoring the building to its original state, before the earthquake, while strengthening all parts of the building that showed particular vulnerability to the earthquake. Construction calculations are not necessary for this level of rehabilitation. The measures include the following procedures:

• Repair of observed cracks and wall damage (filling with mortar and FRP protection)
• Rehabilitation and stabilization of the roof structure and roof covering
• Connecting the floor structure and the surrounding walls
• Strengthening walls (grouting and filling cracks)
• Connecting the walls (the estimated value of the work performed at this level is 90–95 Euros/m² [26]).

Level 2 is more complex than level 1 and its main objective is to strengthen the building in order to increase its resistance to earthquakes by a minimum of 50% of the resistance required for new buildings. This level involves the analysis of the entire building by an authorized construction expert, with the goal to make a static calculation of all damaged places on the building. Reinforcements are most often made with FRP products. Depending on the scope of the planned works, the estimated value is 70–165 Euros/m² (the price varies if the works are performed on this level or in combination with level 1) [26].

Level 3 is a superstructure of level 2 because it requires even greater resistance and, therefore, more extensive works with the preparation of project documentation of the construction project and seismic classification of the building, with the requirement to achieve 75–100% resistance to the effects of earthquakes of the new building. The estimated value of the works on this level is 230 Euros/m², and if the works are combined with the works performed on level 1 and level 2, the estimated value of the works goes up to 400 Euros/m².

In case of implementation of all three proposed levels, the total saving is about 15% compared to the case where each level is done separately [26].

The most commonly applied structural restoration techniques are:

• non-invasive technique
• invasive technique