# Effect of Thin Cement-Based Renders on the Structural Response of Masonry Wall Panels
## 1 Introduction  
From medieval times to the modern era, the use of stone for private, public and religious buildings and infrastructures was very common in many parts of Europe. Especially in areas where it was abundant, perfectly cut, roughly squared or irregular stone became the material of choice for important houses and other domestic buildings [1; 2]. Rubble or straight cut stone, sometimes with only minimal dressing and often rendered or flush-pointed, was not only used for humbler buildings, but often for standard houses and religious buildings in many rural areas [3; 4; 5]. Brickwork masonry is relatively more recent in Europe: solid hand-made bricks were used with increasing diffusion and frequency from the 1600s. Initially, these artificial blocks were used for public buildings or infrastructures, but they became extremely common in the 19 th century in many parts of Europe in the housing construction industry [6; 7]. Today, most of these buildings belong to the cultural heritage and their conservation is often a priority for local governments and conservation bodies. However, historic masonry buildings have been designed in the past with little or no regard for the effects of the seismic action. Unreinforced masonry (URM) walls are prone to failure when subjected to out-of-plane or in-plane seismic loads [8; 9; 10; 11; 12]. Several retrofitting techniques have been studied with the aim of improving the in-plane [13; 14; 15; 16; 17] or the out-of-plane behavior [18; 19]. With regard to the techniques for the in-plane reinforcement, several retrofitting methods have been proposed: grout injections [20; 21], joint repointing [22], ferro-cement coatings [23], application of FRP (Fiber Reinforced materials) grids or fabrics [24; 25; 26; 27; 28; 29]. In this area, a standard method to \"measure\" the effectiveness of a new retrofitting method is to compare the load-capacity and the overall structural behavior before and after the application of the reinforcement. Control, or un-reinforced walls, are often tested on-site or in the laboratory without the pre-existing plaster or the render. However many rubble stone walls were originally lime-rendered for essential protection, with the exception of perfectly-cut stone buildings, for which the exterior walls were often left exposed (fair-faced masonry) and would have gradually weathered. This was particularly true in the case of public buildings, where an overhanging roof provided protection, and infrastructures (bridges, stations, etc.). Rubble or pebble stone walls, which were more exposed to driving rain, were more likely to be plastered with a lime slurry. However, starting from the 1930-1940s when the use of cement became frequent, cement-based mortars have been commonly used for rendering and replaced the pre-existing lime based soft mortars. In 1824, [PERSON], a Leeds bricklayer, patented Portland cement and by the end of 19 th century, a dramatic increase in production occurred. Cement-based mortars have compressive and tensile strengths that are roughly 10-30 times higher compared to the corresponding strengths for a lime-based mortar [30]. However, this depends strongly on the ratios between the main mortar components (cement, sand, water and air). A significant difference in mechanical properties can be also noted by comparing cement-based mortars and historic masonry material. By considering masonry as an assemblage of blocks (bricks or stones) with explicitly defined geometry and lime-based mortar, its mechanical properties are dependent on the properties of its components and are strongly governed by the weakest (usually the mortar). Today, it is very common to find historic masonry constructions rendered with strong, high-strength cement-based mortars. When a building is under the protection of a Conservation body, the re-application of lime-based renderings for rubble or pebble stone walls is often suggested or imposed. However, the removal of a cement-based render is sometimes a difficult task, which can damage the integrity of the walls.