Flatjack And Ae Tests

Flat-jack testing is a versatile and powerful technique that provides significant information on the mechanical properties of historical constructions. The first applications of this technique on some historical

Figure 6. Combined flat-jack test and AE monitoring.

monuments (Rossi 1982) clearly showed its great potential. The test is only slightly destructive, and this is why it is now widely accepted and used by monument monitoring and rehabilitation experts (Binda & Tiraboschi, 1999; Gregorczyk & Lourengo, 2000). When double jacks are used, this test works according to the same principle as a standard compressive test. The difference is that it is performed in situ and the load is applied by means of two flat-jacks instead of the loading platens. The test method is based on the following assumptions: the masonry surrounding the slot notches is homogenous; the stress applied to the masonry by the flat-jacks is uniform and the state of stress in the test prism is uniaxial.

In order to assess the extent of damage in the zone monitored using the AE technique, a compres-sive test was conducted on the masonry through the combined use of double jacks and AE sensors (Fig. 6). The tests were carried out with flat-jacks measuring 24 x 12 cm2. The cuts made into the masonry wall to obtain a smaller-sized specimen were made into two horizontal mortar joints spaced about 30 cm apart.

The minimum slenderness ratio of the specimens was hit = 2.5, where h is the height of the prism comprised between the two flat-jacks and t = 120 mm the deepness of each flat-jack. This made it possible to reduce the friction effects on masonry behavior arising from the action of the flat-jacks.

During the tests, the stress-strain relationship of the masonry was determined by gradually increasing the pressure applied by the flat-jacks in the course of three loading-unloading cycles. Peak compressive strength was obtained from the load-displacement diagram, when the latter became highly nonlinear, denoting imminent failure. Compressive tests were performed on three different masonry portions. The prismatic masonry volumes tested in compression were delimited crosswise by vertical cuts (Fig. 7). Consequently, the in-situ test is equivalent to a compression test performed on specimens with different sizes, as shown in Figure 8. The tests were performed in keeping

Figure 7. Schemes of the double flat-jack tests performed on different wall sizes.
Figure 8. Equivalent masonry prisms tested in compression by means of double flat-jacks.

with the procedures specified in ASTM (1991b), other than for the vertical cuts produced in order to eliminate, in the cracked element, the influence of the adjacent masonry portions.

Figure 9. Double flat-jack test on Volume 2: cumulative number of AE events (2) versus cyclic loading (1).

Specimen a. 1

Figure 9. Double flat-jack test on Volume 2: cumulative number of AE events (2) versus cyclic loading (1).

Table 1. Experimental values obtained from flat-jack tests and AE measurements.

Specimen

Volume (cm3)

Peak stress (MPa)

Nmax at au

Vol. 1

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