Introduction

Nondestructive and instrumental investigation methods are currently employed to measure and check the evolution of adverse structural phenomena, such as damage and cracking, and to predict their subsequent 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 characteristics, 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) detecting 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 structural 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 effective (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 formation 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 dissipation 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 theory is employed to monitor masonry structures by means of the AE technique. The fractal theory takes into account the multiscale character of energy dissipation 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 in a structure and the corresponding activity recorded on masonry elements of different sizes, tested to failure by means of double flat-jacks.

2 NONDESTRUCTIVE EVALUATION TESTS 2.1 Flat-jack tests

The single flat-jack test concerns the measurements of in-situ compressive stress in existing masonry structures by use of a thin flat-jack device that is installed in a saw cut mortar joint of the masonry wall (ASTM 1991a). The method is relatively non-destructive. After the slot is formed in the masonry, compressive stress at that point causes the masonry above and below the slot to get closer. Inserting the flat-jack into the slot and increasing its internal pressure until the original distance between points above and below the slot is restored, can thus measure the compressive stress in the masonry. The slots in the masonry are prepared by removing the mortar from masonry bed joints, avoiding disturbing the masonry. Care must be taken in order to remove all mortar in the bed joint, so that pressure exerted by the flat-jack can be directly applied against the cleaned surface of the masonry units. The state of compressive stress in the masonry is approximately equal to the flat-jack pressure multiplied by factors which account for the ratio Ka of the bearing area of the jack in contact with the masonry to the bearing area of the slot, and for the physical characteristic of the jack Km. In fact, the flat-jack has an inherent stiffness which opposes expansion when the jack is pressurized. Therefore, the fluid pressure in the flat-jack is greater than the stress that the flat-jack applies to masonry, and a conversion factor Km is necessary to relate the internal fluid pressure to the stress really applied. The average compressive stress in the masonry, fm, can be calculated as:

where, p is the flat-jack pressure required to restore the gage points to the distance initially measured between them. We performed the tests using rectangular flat-jack 240 mm x 120 mm wide and 7 mm thick (by BOVIAR s.r.l., Italy). Their calibration factor was Km = 0.90-0.92. The loading procedure was synchronized and the pressure was applied with a manual equipment (pressure range between zero and 60 bar). The usual coefficient of variation of this test method can be estimated equal to 20%; therefore, at least three tests have been carried out on each area of interest.

The double flat-jack test provides a relatively non-destructive method for determining the deformation properties of existing unreinforced solid-unit masonry(ASTM 1991b). The test is carried out inserting two flat-jacks into parallel slots, one above the

Figure 1. Typical set-up for in situ flat-jack test. The dimensions given are those of the specimen referred to as Vol. 1. (Reprinted from Gregorczyk and Lourengo 2000).

Figure 1. Typical set-up for in situ flat-jack test. The dimensions given are those of the specimen referred to as Vol. 1. (Reprinted from Gregorczyk and Lourengo 2000).

other, in a solid-unit masonry wall (Fig. 1). By gradually increasing the flat-jack pressure, a compres-sive stress is induced on the masonry comprised in between. The stress-strain relation can thus be obtained measuring the deformation of the masonry. In addition, the compressive strength can be obtained, if the test is continued to local failure. However, this may also cause damage to the masonry in the area adjacent to the flat-jacks. The tangent stiffness modulus at any stress interval can be obtained as follows:

where, 8om is the increment of stress, and Ssm is the increment of strain. On the other hand, the secant modulus is given by:

where, om and em are the actual stress and strain in the masonry.

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