Structural Analysis of Historic Construction – D’Ayala & Fodde (eds) © 2008Taylor & Francis Group, London, ISBN 978-0-415-46872-5 Cracking simulation of brick-masonry elements subjected to the double flat-jack test A. Carpinteri, S. Invernizzi & G. Lacidogna Dipartimento di Ingegneria Strutturale e Geotecnica, Politecnico diTorino,Torino, Italy ABSTRACT: In the present paper, we describe the results obtained from double flat-jack tests performed varying the size of the masonry prism involved in the test. In addition, not only the deformations have been acquired, but also the acoustic emissions (AE), in order to get information about local cracking during the test. We present a meso-scale numerical model of the test, where every brick of the masonry is modeled in the details. Discrete cracks can arise both in the mortar joints and in the brick units. A good correlation is found between the amount of cracking simulated numerically and the experimental acoustic emissions for different prism sizes. The model is also able to catch the decrease in the compressive strength with increasing size. It is not possible to obtain an easy direct relation between the acoustic emission and the amount of cracking; nevertheless, it is possible to state that the two quantities are proportional to each other when increasing sizes are considered. 1 INTRODUCTION Nondestructive and instrumental investigation meth- ods are currently employed to measure and check the evolution of adverse structural phenomena, such as damage and cracking, and to predict their subse- quent developments. The choice of a technique for controlling and monitoring reinforced concrete and masonry structures is strictly correlated with the kind of structure to be analyzed and the data to be extracted (Carpinteri & Bocca 1991; Anzani et al. 2000). For historical buildings, nondestructive evaluation (NDE) techniques are used for several purposes: (1) detecting hidden structural elements, such as floor structures, arches, piers, etc.; (2) determining masonry character- istics, mapping the nonhomogeneity of the materials used in the walls (e.g., use of different bricks during the life of a building); (3) evaluating the extent of the mechanical damage in cracked structures; (4) detect- ing voids and flaws; (5) determining moisture content and rising by capillary action; (6) detecting surface decay phenomena; and (7) evaluating the mechanical and physical properties of mortar and brick, or stone. This study addresses some of the aforementioned problems deemed of special significance. The struc- tural geometry was defined through the customary survey methods. Damage, cracking, and the evolution of these phenomena over time were assessed through a number of nondestructive techniques: tests with flat- jacks were conducted in order to evaluate the range of stresses affecting the structures; and at the same time, the cracking processes taking place in some portions of the masonry structures were monitored using the acoustic emission (AE) technique. The AE technique has proved particularly effec- tive (Carpinteri & Lacidogna 2002, 2003, 2006), in that it makes it possible to estimate the amount of energy released during the fracture process and to obtain information on the criticality of the process underway. Strictly connected to the energy detected by AE is the energy dissipated by the structure being monitored. The energy dissipated during crack forma- tion in structures made of quasibrittle materials plays a fundamental role in the behavior throughout their life. Strong size effects are clearly observed in the energy density dissipated during fragmentation. Recently, a multiscale energy dissipation process has been shown to take place in fragmentation, from a theoretical and fractal viewpoint (Carpinteri & Pugno 2002a,b, 2003). Based on Griffith’s assumption of local energy dissi- pation being proportional to the newly created crack surface area, fractal theory shows that the energy will be globally dissipated in a fractal domain comprised between a surface and a volume in the Euclidean space. According to fractal concepts, an ad hoc the- ory is employed to monitor masonry structures by means of the AE technique. The fractal theory takes into account the multiscale character of energy dis- sipation and the strong size effects associated with it. With this energetic approach it becomes possible to introduce a useful damage parameter for structural assessment based on a correlation between AE activity 367