## 1 Introduction  
In many European countries, a large proportion of the building stock constructed after the end of WWII is multi-story reinforced concrete (RC) structural formations composed of slabs, beams, columns, and shear walls cast in place. The ease offered by this type of construction and the relatively low cost compared to alternative structural forms led to RC building becoming dominant in many countries. Unfortunately, a large number of these buildings were designed and constructed to resist low levels of seismic actions as shown by the development of serious structural damage when subjected to strong earthquake excitations [1]. During the last thirty years, a large volume of research on the seismic response of RC structural components and structural systems has provided a strong basis for upgrading the seismic provisions of RC structural design. Therefore, RC buildings that are currently designed and constructed according to such upgraded seismic code provisions can meet future earthquake excitations, as defined from probabilistic studies for each country, developing controlled structural damage that ensures that such RC buildings will not collapse. Moreover, the structural damage which will develop for the \"design\" earthquake is repairable. In this way, upgraded design seismic codes safeguard against the loss of life as well as against excessive repair costs for contemporary RC buildings. In contrast, old existing buildings are quite vulnerable to severe structural damage for such \"design\" earthquakes that could lead to collapse and the loss of life, as was the case during many strong past earthquakes around the world. Confronted with such a seismic risk,research efforts have focused on devising retrofitting techniques capable of upgrading the seismic resistance of such relatively \"weak\" RC structural formations. This was a difficult task because, whereas reinforced concrete construction offers very substantial flexibility when cast in place, it offers no such flexibility when it is hardened. Therefore, it is quite difficult to introduce any alterations or additions to the structural elements themselves or to the structural system as a whole. A typical retrofitting technique devised for strengthening such \"weak\" RC structures is to upgrade the capacity of selected critical structural elements of the structural system (e.g., weak columns) with RC jackets, providing in this way the required upgrade in strength and ductility (Figure 1a,b). A well-known construction technique for strengthening existing reinforced concrete (RC) structural elements is to apply such RC jacketing surrounding the old structural elements with these new RC jackets. RC jackets are constructed using concrete of high bond strength and low shrinkage characteristics together with additional steel reinforcement and occasionally fiber-reinforced polymer composites. A variety of techniques have been investigated in the past to strengthening under-designed RC beams, RC columns, and RC joints employing such RC jacketing techniques. Published research [2; 3; 4; 5; 6; 7; 8; 9; 10; 11] has shown that such external RC jacketing improves both the flexural and shear capacity, increases stiffness and ductility together, and provides for higher axial load capacity to under-designed structural members than prior to such RC jacketing. Code provisions [12] include guidelines for the design of RC jackets. RC jacketing was utilized in the past either by strengthening structural members being damaged after a strong intensity earthquake sequence or as a preventing measure for \"weak\" structures due to the outcome of a relevant inspection prior to an earthquake. Such RC jacketing during the last decades has been the most favorable choice for structural engineers in seismic-prone areas. However, there are also certain difficulties in employing such RC jacketing techniques.