## 6 Sustainable Strengthening and Seismic Improvement Techniques
Defining an appropriate strategy for the rehabilitation of culturalheritage 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.