## 2 Experimental Scheme
### Construction of the Squat Masonry Walls  
Three squat walls were constructed using the above-mentioned materials and labeled by W1, W2, and W3 respectively. The W1 was constructed without any strengthening measure, the W2 was constructed and strengthened by a UHPC layer, and the W3 was constructed and strengthened by a reinforced polymer mortar layer. The authors applied in-plane lateral load cyclically on W1 under constant vertical compression until the wall failed, and then strengthened the failed W1 in-situ with a UHPC layer. The strengthened wall was labeled as W1S, and it was loaded to failure again. To limit the variety of structural details, two senior technicians constructed the masonry walls in three consecutive days and then cured the bare walls for two weeks. After curing the walls, the strengthening was finished in three consecutive days. On day 1, the mixing of the UHPC and strengthening with the UHPC layer was completed. On day 2, the grid of steel bars was formed. On day 3, strengthening with the reinforced polymer mortar layer was completed. After the strengthening and before the tests, the authors cured the strengthened walls for 28 days in a stable lab environment. The curing temperature was between 17 \({}^{\circ}\)C and 23 \({}^{\circ}\)C, and the environmental humidity was greater than 90%. The profile of the walls is shown in Figure 2. Thickness of the walls was 240 mm and thickness of the mortar joints was 10 mm, like those measured in the historical building. Thickness of the UHPC layers on W2 and W1S was 40 mm, like that of the general strengthening layers used in practices (in China). The UHPC layers are thinner than other strengthening layers that work with inner reinforcement. Therefore, it is possible to apply extra strengthening measures on top of the UHPC layers without changing the profile of the historical walls. The authors controlled the thickness of the UHPC layers using a wooden mold, as shown in Figure 3a. They injected the UHPC batch from a \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & **Compressive** & **Tensile** & **Elastic** & **Strain at** & **Ultimate** \\  & **Strength** & **Strength** & **Mmodulus** & **Elastic Limit** & **Tensile Strain** \\  & **(N/mm\({}^{2}\))** & **(N/mm\({}^{2}\))** & **(\(\times\)10\({}^{5}\) N/mm\({}^{2}\))** & **(\(\mu\varepsilon\))** & **(\(\mu\varepsilon\))** \\ \hline UHPC & 137.60 & 5.80 & 0.49 & 200 & 1100 \\ Polymer mortar & 19.10 & 3.10 & 0.30 & — & — \\ Reinforcing bar & 345.00 & 345.00 & 2.10 & 1700 & 110000 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of the strengthening materials. UPHC: ultra-high performance concrete. hole on the bottom of the mold, until the space between the mold and the wall surfaces was filled. During the procedure, the UHPC batch also immersed and filled the existing damages in the walls. Such a strengthening procedure had slight intervention on the damaged masonry walls, as required by most rehabilitation practices. Moreover, it potentially helped to improve the integrity of the masonry substrates, the elastic modulus of the target walls, and the bondage between the UHPC layers and the masonry substrates. Comparatively, strengthening with the reinforced polymer mortar layer was more complicated. First, the surface of the masonry substrate was cleared, and then anchoring short reinforcing bars were planted into the masonry, as shown in Figure 3b. As the following step, the reinforced grid was hung on the anchoring short bars and then was fasten by binding. The polymer mortar was then spread onto the cleared surface of the masonry substrate by two times. After the first time, the polymer mortar layer was just thick enough to cover the reinforced grid; after the second time, the polymer mortar layer reached its design thickness of 50 mm. Planting of anchoring short bars and spreading of polymer mortar had obvious intervention on the target wall.