VIVIENDAS POR DISPONIBILIDAD DE SERVICIOS

MATERIAL: AN EXPERIMENTAL CAMPAIGN

The seismic behaviour of masonry can be reproduced and studied by means of the diagonal compression tests, regulated by the ASTM 519 standard. The study by (Faella et al., 2010) [6] presents the results of an experimental campaign carried out at the Structural Laboratory of the University of Salerno, on panels made by Neapolitan yellow tuff. In particular, 9 specimens were tested; among them 3 were unreinforced and 6 were reinforced on both sides by means of a layer of FRCM (fiber-reinforced cement matrix). This type of strengthening system is characterized by the use of a cement matrix, in place of the more common epoxy resin, in order to incorporate the composite material reinforcement. Standard dimensions defined by ASTM for the panels are 120 x 120 x 40 cm. The reinforcement is represented by a carbon fiber mesh embedded in between two layers of mortar. In order to evaluate the strength of each material, some compressive and flexural tests were carried out on specimens made of the same mortar and tuff employed for the construction of reinforced panels. The results obtained from tests are summarized in the following tables.

Table 3.3. Compressive tests on tuff specimens [6].

Table 3.4. Three-points bending tests on tuff specimens [6].

Table 3.5. Compressive tests on mortar specimens [6].

Table 3.6. Three-points bending tests on mortar specimens [6].

Furthermore, a compressive test on a 40 cm³ specimens cut out from a masonry panels was performed in order to evaluate the strength of the composite material tuff-mortar, obtaining a compressive strength of 1.31 MPa. The Eurocode 6 gives the following formula for the calculation of the same parameter using a combination of the strength of materials composing the masonry:

which gives a value very close to the experimental one.

Table 3.7 summarizes the mechanical characteristics of the strengthening system.

Table 3.7. Mechanical characteristics of carbon fiber net and mortar [6].

Figure 3.5 shows one panels tested with the diagonal compressive machine, while the characteristics of all the tested panels are reported in Table 3.8.

Figure 3.5. Diagonal compression test [6].

Table 3.8. Geometrical characteristics of tested panels [6].

Tests results are reported in terms of load-displacement diagram. In Figure 5.6 the result of the test on a unreinforced panel is reported. This typology of specimens featured a shear sliding rupture along a diagonal direction developing by the whole length of the panel at the interface between tuff and mortar. The collapse was attained at a value of load between 30 and 45 kN. In Figure 5.7 is shown the behaviour of a reinforced panel. In such case the collapse was attained for a value of the applied load from 4 to 6 times bigger than in the case of the unreinforced panel. In some cases a sudden change in the slope of the load-displacement curve, at a value near to the one corresponding to the collapse load of unreinforced panels, when the

crack forms inside the panel without having the possibility to develop due to the presence of the reinforcement applied on the specimen’s surfaces.

Figure 3.6. Load-displacement diagram, Specimen 1 [6].

Figure 3.7. Load-displacement diagram, Specimen 9 [6].

The results from experiments were also compared to the theoretical ones, obtained by means of different formulations found in the literature. The shear strength of the tested specimens can be obtained from the ultimate compressive load by means of the formulas given by the ASTM E519:

where An is the net area of the transversal cross-section of the specimens, depending on the dimensions of the panel and on the percentage of gross area n.

The formulations available in the literature consider the shear strength of reinforced panels as the sum of resistance of the unreinforced masonry panel and the contribution due to the strengthening system. For example, the Eurocode 6 considers the following formulation:

where:

fv0 is the shear strength of the unreinforced panel;

ρf is the ratio between the cross-section of reinforcement and the cross-section of the panel;

ffu is the tensile strength of reinforcing fibers employed for the strengthening.

Since it has been experimentally observed that the collapse occurs with the detachment of the strengthening system always before reaching the ultimate tensile strength of fibers, the formulation of the Eurocode 6 tends to overestimate the strength of the reinforced system. Thus it can be useful to calculate the value of the strength, based on the effective deformation that the fibers reach in correspondence of the ultimate load of the panel during the test (εf,eff):

.

In the following different formulations for calculation of εf,eff used in the comparison are reported:

I.C.B.O.-AC125

Tomažević et al.

Triantafillou

Triantafillou and Antonopoulos

dove con c1=0.015 CNR-DT 200/2004

In Figure 3.8 is reported a diagram with the comparison between the obtained results.

Figure 3.8. Comparison between experimental and theoretical results from different formulations [6].

As previously described, the formulation of the Eurocode 6 significantly overestimates the shear strength of reinforced masonry panels, since it is not able to catch the detachment of the strengthening system from the surface of the panel, but takes into account the strength of carbon fibers. It is noted how the formulations of both Triantafillou and Tomažević overestimate the strength of panels, even if not so much as the Eurocode 6, since they are obtained experimentally from tests carried out on clay brick masonry panels with better mechanical properties of tuff employed in this case. On the contrary, the formulation proposed by Triantafillou and Antonopoulos is able to catch the actual value of the shear strength also found in the experiments since it takes into account also the mechanical properties of the substrate material. Finally, the formulations of ACI-125 and CNR-DT 200 underestimate the experimental results, because the former one has a simplified formulation of εf,eff, while the latter one would require a better calibration of coefficient c1.

3.3 VALIDATION OF ANALYTICAL AND CONTINUUM NUMERICAL