# Application of Sonic Tests and Modal Identification in the Characterization of Masonry Walls
## 1 Introduction  
To perform an assessment of the existing structures, typically, numerical models are developed to represent the structural behavior, namely through finite-element techniques, allowing the accurate representation of the static and dynamic structural behaviors. The fast development of the computational tools, with user friendly packages available to all the engineering community, introduces a great instrument to perform the assessment of existing structures. However, to apply these powerful tools, the accuracy of the computer models becomes essential, and any uncertainty in the obtained results should be minimized. Considering that for complex structures, the use of complex numerical models is undeniably the most correct method, the user must be assured of how accurately the model represents the real behavior of the structure. To improve the accuracy of the numerical models to represent the structural behavior, a detailed investigation should be performed, i.e., verifying the geometry, assessing the mechanical properties of the materials, identifying zones where the deterioration or damage is present, and also checking the information in the design documents and drawings. For all these reasons, the use of nondestructive testing (NDT) techniques has become more common in the condition assessment of existing structures, as it enables to gather data with minimum damage to the structure in study [1, 2]. The aim of this work is, on the one hand, to characterize specimens of reinforced masonry walls of soil-cement compressed earth blocks and understand their structural behavior using NDT techniques, such as sonic tests and environmental vibration tests, while, on the other hand, to develop a numerical model of the walls and calibrate it withthe information obtained from an analysis of the data collected during the experimental campaign. This work will make it possible to verify the reliability of the application of sonic tests in the characterization of masonry, and to test a methodology of combined tests for a structural assessment by carrying out a comparative study. Among the NDT methods, sonic tests are commonly applied to existing masonry structures as it has several applications, namely for masonry characterization and the detection of voids, delamination, and weak areas [3,4]. In fact, the velocity of propagation of sonic waves through the material is related to its physical and elastic mechanical properties, making sonic tests a solution for estimating the elastic properties of masonry and to identify unknown conditions inside the material [5, 6, 7]. The impact of an object on a material generateS-waves that propagate inside the material (body waves) and waves that propagate on the surface of the material (surface waves). Thus, in terms of mechanical waves, it is possible to encounter: (i) longitudinal waves (P-waves), which propagate by motion in the direction of propagation and are the fastest, (ii) transverse waves (S-waves), which propagate by motion perpendicular to the direction of propagation, and (iii) Rayleigh waves (R-waves), which are surface waves with retrograde elliptical motion and are the ones with the highest percentage of energy. Depending on the configuration adopted in the test (the relative position of the emitter and receiver), it is possible to identify different types of waves (Figure 1a). Direct tests aim to measure the velocity (VP) of primary waves (P-waves) by placing the hammer and accelerometer in line at opposite sides of the wall. Indirect tests can measure both the velocity of P- and R-waves (VR) [8]. Figure 1: (**a**) Sonic tests configuration and (**b**) wave identification scheme. Considering a particle at the surface, it is known that it will first experience the arrival of P-waves, followed by S-waves, after a period of quietness.