# Effect of Thin Cement-Based Renders on the Structural Response of Masonry Wall Panels
## 6 Conclusions  
URM masonry buildings tend to perform poorly in earthquakes owing to the low shear strength of the masonry material and the inconsistent lime-based mortar used for construction in the past. The use of rubble or barely cut stone, the absence of adequate connections between wall leaves, the lack of skilled masons and adequate resources were the main causes for the low quality of historic constructions. Knowledge of the mechanical characteristics of URM masonry is of critical importance for any type of seismic analysis or design of a retrofitting intervention. Several researchers have studied and attempted to measure the mechanical properties of historic masonry, by considering different constituent materials (mortar and blocks), arrangements, dimensions and shapes of the blocks (typically bricks or stones), levels of connection between the wall leaves. Historic walls are often rendered with cement-based renders. However, the effect of these renders was not studied in the past. Mechanical on-site tests were conducted on twelve wall panels subjected to in-plane shear or compressive loading. All wall panels were made of a lime-based putty and irregular stone masonry. These were tested with two different configurations: before removal of the cement-based render or after. Only static tests have been performed to date to estimate the static ultimate capacity and deformation response of the wall panels. Compressive and in-plane shear loads were applied in cycles to make sure that the transient phase during which the wall panels are transferred between the elastic and plastic strain zone was accurately recorded and the effect of the cement-based render adequately analyzed. In conclusion, it has been demonstrated that:  
1. The lateral load-capacity and shear stiffness of historic wall panels can increase considerably as the walls are rendered with a cement-based mortar; significant shear strength and stiffness increments were observed for barely-cut (irregular) double-leaf stone and brickwork panels;
2. The application of a cement-based render did not significantly alter the failure mode of the wall panels, both under shear and compressive loading;
3. Limited effect was produced by the render when the wall specimens were tested in compression. For these tests, the behavior of the wall panels was governed by the bond between the masonry and the render. As cracking and debonding of the cement render occurred, the positive effect of the render, both in terms of load capacity and stiffness, became negligible;
4. With regard to the shear tests, the bonding between renders and masonry substrate remained effective for the entire duration of the test. Shear failure occurred with characteristic diagonal cracks along the diagonal in compression. If maintaining the bond is critical for the performance of a wall panel, shear modulus of the render material should not be substantially different from the one of the masonry material. 5. A main limitation of this study is that masonry specimens were cut from different buildings, making a direct comparison difficult, given the intrinsic variability in mechanical properties of historic masonry and the different conditions of conservation of the buildings. However, an  
Figure 11: Effect of non-perfect plastic behavior: (**a**) typical case for high-strength (cement-based) render; (**b**) typical case of a lime-based render; (**c**) typical case of low-strength (i.e., soft thermal-insulating coating). emerging trend was noted, indicating the effects of a cement-based render applied to historic wall panels. 6. Finally, more systematic tests are needed to clarify the force transfer from the masonry substrate to the render under different test configurations, and a more in-depth study should be conducted. Tests should be carried out on purposely constructed masonry specimens, having controlled and reproducible properties, to obtain more conclusive data.