## 3 Experimental Results  
### Deformation and Failure Mode  
There were three typical states of the walls during they deformed under the lateral loading. At state 1, major cracks in the walls became visible; at state 2, lateral forces on the walls reached their peak values; and at state 3, the forces of the actuator dropped by 15% or more after reaching peak values and the walls failed. The lateral displacement of the walls along their height at the three typical states is shown in Figure 6, in which \(\Delta\) represents the displacement.  
The displacement of the lower part of the W1 remained small until the wall failed, while the displacement of the lower parts of all the strengthened walls increased obviously with the load.  
The W1 and W1S deformed similarly at state 1, indicating the UHPC layer had recovered the stiffness of the failed W1. However, the displacement of W1S at state 2 and state 3 is much larger than that of W1, indicating the UHPC layer had postponed the cracking and changed the failure mode. Because of the favorable cooperation between the UHPC layer and the masonry substrate, the ultimate deformation of W1S-which was recovered from the severely damaged W1-increased by 113% than that of W1. Similar result was observed for W2, whose ultimate deformation increased by 109% than that of W1. The difference between the increase of the ultimate deformation of W1S and W2 was less than 4%, indicating the improvement of ultimate deformation was similar for the two walls. The similarity can be explained by the walls' failure mode. Both W1S and W2 failed in a bed-joint slide mode that was premature and the strengthening layers had not taken full effect. Therefore, the ultimate deformation of W1S and W2 relied much on the resistance of the masonry in their bottom, as explained in Section 4.3. The resistance of the bottom masonry is similar for W1S and W2, because the W1 had previously failed in a typical shear-compressive mode and its bottom masonry was still in good condition after the failure. The results indicate the UHPC layers significantly improved the deformation ability of the masonry walls.  
While, the ultimate deformation of W3 increased by 96% than that of W1. The increase is less than that of W1S and W2 because the UPCP layers deformed with the masonry substrates better than the reinforced polymer layer did, as explained in Section 4.2 and Figure 14. Besides, the W3 had a large displacement at the bottom when it failed, indicating a severe bed-joint slide.  
There were two failure modes of the walls, as shown in Figure 7. The unstrengthened W1 failed in a typical shear-compressive mode, like most load-bearing walls with weak mortar joints. Cracks originated when the lateral force was only 18 kN, and crossing cracks formed when the lateral force was only 35 kN. In the subsequent load cycles, lateral deformation of the wall increased significantly. It failed suddenly when the lateral force was 41.4 kN, and then the force of the actuator dropped.  
Figure 6: Lateral displacement of the walls: (**a**) W1; (**b**) W1S; (**c**) W2; (**d**) W3.  
For the strengthened walls W1S, W2, and W3, horizontal cracks formed and developed at their bottom along with the increase of the lateral forces, and the walls failed through bed-joint slide finally. The horizontal cracks of the W1S, W2, and W3 formed at their bottom when the lateral forces were 32.4 kN, 40.8 kN, and 31.0 kN respectively, which were much higher than that of the cracking load of W1. No severe cracks formed in other parts of the walls after the horizontal cracking, until the walls failed through bed-joint slide suddenly. Such a failure mode of strengthened historical masonry walls hardly happens for strengthened regular masonry walls, for which debonding of the externally bonded layers or strips is the typical failure mode.