## 1 Introduction  
Strengthening of historical masonry walls is critical for heritage reservation, because these walls are the major load-bearing members in many architectural heritages around the world. The topic has attracted intensive research [1; 2; 3; 4; 5]. Masonry walls can be strengthened by applying externally bonded layers or strips. The former covers the walls' surface entirely while the latter covers partially. Given appropriate strengthening designs, they would not intervene the target buildings significantly and would not influence the buildings' aesthetics. Therefore, they are also applicable for strengthening the historical buildings. The two methods can significantly reduce the working stress in regular masonry walls under different load cases, and then comprehensively rehabilitate the walls' performances [6; 7; 8]. However, special considerations deserve when they are used for the historical brick masonry walls [9]. First, it is difficult for the strengthening layers or strips to deform well with the masonry substrates, especially under high in-plane compression. Because the elastic modulus of the historical masonry substrate is extraordinary lower than that of the strengthening layers or strips. Under the circumstance, the layers or strips do not activate much of their strength, and then the resistance of the strengthened walls can still be low [10]. In addition, the stiffness of the strengthened walls can also be low, resulting in risks of unacceptable lateral deformation or even collapse during earthquakes. Although the issues can be relieved by prestressing the strengthening strips, it requires a large anchor length of the strips on the weak historical masonry substrates. Anchorage relaxation caused by large deformation and creep of the soft historical masonry can reverse the effect of prestressing [11; 12]. Second, surface preparation is restricted, or even prohibited, when strengthening the historical masonry walls. Because the historical masonry substrate is vulnerable and sensitive. Under the circumstance, the bondage of the strengthening layers or strips is difficult to guarantee. In addition, unfavorable stress concentrations can take place within masonry substrates, the externally bonded layers or strips, and their interfaces. If the strengthening layers or strips are made of FRP with anisotropic and brittle nature, the stress concentration can result in piercing or threading effects on them [13]. Third, it is difficult to achieve favorable integrity of the strengthened historical masonry walls, if the walls have been damaged significantly before strengthening. When applying strengthening layers on these walls, the integrity of the damaged substrates and the layers are unreliable because it is subject to many uncertain factors. For example, the walls' aspect ratio, the damage distribution and extent, the properties of the bricks and mortar, the properties of the strengthening materials, and so on [14; 15; 16; 17]. When applying strengthening strips on these walls, the integrity of the strengthened walls may relate to the strip configuration [18]. For example, damaged parts of the walls that are not covered by the strengthening strips can undergo large deformation under high stress, and result in collapse of the walls potentially [14]. Besides, an important requirement for strengthening historical masonry walls is the reversibility of the intervention [19; 20; 21], which keeps the possibility to remove and replace the retrofit when the advances in technology will allow better strategies. Considering the characteristics of the historical masonry walls, to cover their surfaces entirely using strengthening layers is preferable. The materials used to construct the strengthening layers should deform well with the soft masonry substrates, have favorable strength, and have good ductility. The ultra-high performance concrete (UHPC) is a potential candidate because of its favorable features [22].