## 2 Planar Deformation Measurement Methods in Masonry Specimens  
### Classical Instrumentation  
The study of the structural behavior of masonry walls--composed by solid and hollow ceramic bricks and air lime bedding mortars--can be supported in multiple experimental activities, namely, mechanical testing in order to characterize this type of old building material when subjected to compressive or flexion actions. In general, testing machines and dedicated devices such as reaction frames and hydraulic actuators are used in the assembly and testing of specimens representing the structural element in study. In addition to the force and displacement measurement chains that compose the testing equipment, the instrumentation of masonry specimens can include (see Figure 1):  
* Unidimensional deformeters;
* Electrical strain gauges;
* LVDT sensors.  
In general, these contact sensors are connected to a data acquisition system that provides electrical power and is responsible for the conditioning and processing of the electrical signals received. The existence of multiple measurement chains and the dynamic nature of the testing activities also imply the use of a synchronization signal generator. The collected data allows quantifying the relation between stress and the dimensional quantities (displacement or strain) in regions of interest in the tested specimen, namely in its central area, from which several mechanical properties can be obtained and used for its characterization, namely, the compressive strength, \(\sigma\), the modulus of elasticity, \(E\), and the Poisson ratio, \(\upsilon\), which are, respectively, determined by the following expressions:  
\[\sigma=F_{\text{max}}/A, \tag{1}\]  
\[E=F_{\text{max}}/(3\times\varepsilon_{\text{v}}\times A), \tag{2}\]  
\[\upsilon=\varepsilon_{\text{h}}/\varepsilon_{\text{v}}, \tag{3}\]  
where \(F_{\text{max}}\) is the maximum load force, \(A\) is the specimen's cross-section area (defined by the product between its length, \(l\), and width, \(w\)), and \(\varepsilon_{\text{h}}\) and \(\varepsilon_{\text{v}}\) are the horizontal and vertical strains measured when the maximum stress is achieved. Both the horizontal and vertical strains are obtained from the ratio between the deformation, \(\Delta L\), observed between two points at the maximum stress and the initial length, \(L_{0}\), between the same two points before load application, i.e.,  
\[\varepsilon=\Delta L/L_{0} \tag{4}\]  
\[\Delta L=L-L_{0} \tag{5}\]  
where \(L\) is the measured length between the two points at the maximum stress.  
The overall calculation process related to the compressive mechanical testing of masonry specimens is shown in Figure 2.  
Figure 1: Classical instrumentation of masonry specimens: (**a**) unidimensional deformeter; (**b**) electrical strain gauge; (**c**) linear variable displacement transformer (LVDT) sensor.  
The determination of compressive strength requires the destructive testing of the specimen, which can seriously damage the above-mentioned sensors and, in the case of specimens with large dimensions retrieved from the old buildings, can significantly increase the economic cost of the performed testing activities, namely, when dealing with unique specimens with a high asset value. The loss of instrumentation during the collapse of the specimen also invalidates any measurement performed in that stage, thus not allowing any behavior analysis.  
In the destructive compressive testing, LVDT sensors are usually placed on each frontal and rear surface of the specimen, aiming strain measurement in two directions (vertical and horizontal) for determination of the Poisson ratio. When the specimen begins to show the first signs of permanent deformation, the LDVT sensors are removed in order to avoid any damage, and the test progresses to the specimen collapse.