## 5 Conclusions  
The strengthening effect of applying UHPC layers and reinforced polymer mortar layer on historical squat masonry walls is investigated through pseudo-static experiments. The experimental results highlight the strengthening effect of the UHPC layers, but indicate the premature failure by bed-joint slide of the strengthened walls. The conclusions as follows can be drawn,  
1. The UHPC layer improved a typical historical squat wall's resistance by 193%, cracking load by 127%, and ultimate deformation by 109% simultaneously. The increase of resistance is favorable for ensuring the safety of the historical walls under static load, the increase of cracking load is favorable for ensuring the serviceability, and the increase of ultimate deformation is favorable for ensuring the safety of the historical squat walls under seismic load. The UHPC layer also increased the stiffness of the wall, indicating an obvious effect of integrity rehabilitation. Its strengthening effect was more obvious than that of the reinforced polymer mortar layer. Therefore, the UHPC layer is appropriate for both static and seismic strengthening of historical squat masonry walls, and it is favorable for restoring the serviceability of the walls.
2. The UHPC layer rehabilitated a severely damaged historical squat wall and improved its resistance by 147% and ultimate deformation by 113% simultaneously. It also increased the stiffness of the wall, indicating an obvious effect of integrity rehabilitation. The UHPC can fill the existing damages in the wall. Such a mechanism rehabilitated the masonry substrate before strengthening, and made the UHPC deformed with the masonry well. The mechanism is specifically favorable for strengthening historical masonry walls that have been severely damaged. Because the mechanism has minor relation to the walls' aspect ratio, similar strengthening effect could be expected when the UHPC layers are applied on slender masonry walls.
3. The historical squat masonry walls strengthened by the UHPC layers failed through bed-joint slide at their bottom, and their energy dissipation ability was not excellent. The bed-joint slide was governed by the strength of the weak bottom parts of the walls. These parts resisted the maximum shear forces and bending moments, but they were not effectively covered by the strengthening layers. For the slender walls, there is significant compressive stress caused by the in-plane bending deformation and normal to the bed-joints. The tendency to bed-joint slide could be relieved.
4. The premature failure by bed-joint slide of individual strengthened walls should be prevented in the strengthening of historical structures. Therefore, the strengthening scheme should be designed at structural level. The interactions among adjacent walls should be well considered, and the joints among the strengthened walls should be strengthened simultaneously.  
The UHPC layers are easy to apply and they have slight intervention on the vulnerable historical masonry walls, and they will not prevent the realization of better strengthening strategies in the future. With appropriate strengthening scheme, the UPHC layers are potentially effective and efficient for strengthening the historical squat masonry walls.  
**Author Contributions**: Conceptualization, B.P.; methodology, L.L.; validation, S.W.; formal analysis, S.W.; investigation, L.L.; resources, Q.Z.; writing--original draft preparation, L.C.; writing--review, editing, and revising, B.P.; visualization, Y.M.; supervision, B.P. and Q.Z.; project administration, Y.M.; funding acquisition, B.P. and Q.Z.  
**Funding**: This research was founded by the National Natural Science Foundation of China (NNSFC), grant numbers 51208300.  
**Conflicts of Interest**: The authors declare no conflicts of interest.