Diversidad Funcional y las TIC

On the basis of strain gauges measurements, the strain variation along the reinforcement can be evaluated at different load levels up to the bond failure. The origin of axis x is always fixed at the loaded end of the reinforcement and the first value of strain is theoretically calculated dividing the applied load by the nominal value of the Young’s modulus and by the transversal area of the fibres.

In Figures 6.4(a) and 6.4(b) there are depicted the strain profiles along the sheet registered for two specimens made of tuff T1 and bonded with glass and carbon fibres, respectively, considering the bonded lengths of 200 and 300 mm. The strain profiles show that the effective bond length is not lower than 300 mm. This means that a further increase of the bonded length should not determine relevant increase of the debonding loads.

Moreover, the comparison between strain profiles in specimens with glass and carbon sheets having the same bonded length (300 mm, Figure 6.4(c)) at the same load levels (about 25%, 75% and 90% of F_max, being F_max comparable for glass and carbon fibres) shows that the distributions are quite similar due to the comparable axial stiffness of the two reinforcements.

In Figure 6.4(d) the strain distributions for two specimens bonded, respectively, with linen and carbon fibres without plaster are compared for Lb = 300

mm at two equal load levels. The comparison, clearly, shows a larger deformability of the linen fibres due to their lower axial stiffness; moreover, because the 90% of the failure load of the linen fibres corresponds to only 40% of the failure load of the carbon ones, these ones show a regular trend, while for the linen fibres a local debonding phenomenon is occurring close to the loaded end just before the final tensile failure of the fibres.

(a) (b)

(c) (d)

Figure 6.4. Strain profiles in pull-push bond tests in specimens made of tuff T1 with: (a) Glass fibers for Lb = 200 mm and Lb = 300 mm with plaster; (b) Carbon fibers for Lb = 200 mm and Lb

= 300 mm with plaster; (c) Carbon and glass fibers for Lb = 300 mm with plaster; (d) Linen and carbon fibers for Lb = 300 mm without plaster.

The effect of plastering on the strain distribution is evidenced in Figures 6.5(a) for two specimens bonded with carbon fibres over 300 mm and made of tuff T1. The same comparison is reported in Figure 6.5(b) for two specimens made of tuff T2. Both graphs show that for specimens without plastering the debonding starts at

lower loads; in particular, for tuff T1 (see Figure 6.5(a)) debonding starts at about 16-18 kN for specimens without plastering instead of about 20 kN, as evidenced by the strain trend that is almost constant in the first 150 mm and with mean values higher than in the case of plastering at the same load levels. These differences are, thus, significant of different interface bond laws with and without the plaster layer: in particular, it can be considered a stiffer bond law in the case of plastering.

The stiffening effect of the plastering is confirmed also by the global experimental load-displacement curves graphed in Figure 6.5(c) for specimens bonded with carbon and glass fibres made of tuff T2. The displacement at the loaded end is calculated by integrating the measured strains along each FRP reinforcement.

In Figure 6.5(c) also the global load-displacement curves of two specimens with basalt fibres (without plaster) are graphed. The comparison shows again that the curves of specimens bonded with carbon and glass fibres are comparable both in the case of presence or not of the plastering, while they are sensibly stiffer than the curves of specimens with basalt fibres due to their lower axial stiffness (about 1/3).

Moreover, the debonding process in the specimens with basalt fibres seems to be more ductile since larger ultimate displacements are attained. This peculiarity of the basalt fibres was already observed in bond tests on concrete elements externally bonded with different types of Near Surface Mounted bars (Ceroni et al., 2012) [70].

(a)

(b)

(c)

Figure 6.5. Effect of plastering in pull-push bond tests: (a) strain distributions for specimens made of T1 with carbon fibers; (b) strain distributions in specimens made of tuff T2 with carbon

fibers; (c) global load-displacement curves for specimens made of T2 with carbon, glass and basalt fibers with and without plaster.

(a)

(b)

Figure 6.6. Effect of tuff strength (T1 and T2) on the strain distribution in pull-push bond tests with carbon and glass fibers: (a) strain profiles for specimens with carbon fibers; (b) global

load-displacement curves for specimens with carbon, glass and linen.

In Figure 6.6(a) the strain profiles for specimens made of tuff T1 and T2 with carbon fibres are reported; the comparison does not evidence significant differences at the same load levels, unless the lower debonding load. This should indicate that the slope of the first branch of the interface bond law could be the same. The global experimental load-displacement curves graphed in Figure 6.6(b) confirm this assumption both for specimens bonded with carbon and glass fibres. In Figure 6.6(b) the global load-displacement curves of two specimens bonded with linen fibres are also graphed; the comparison with the curves of the specimens made of the same tuff T1 and bonded with carbon and glass fibres evidences, as already noted for the basalt fibres, the lower stiffness of the bond law in the case of linen.