So

Figure 14. Comparison among plain masonry and three strengthening configuration by using experimental elaborations and elasto-plastic law for FRP strips.

The comparison with models where fracture energy parameters are assumed as in the CNR standards revealed minor performances, thus confirming the proposed models being highly conservative (Figure 15). This is surely safety oriented, but if not controlled in its boundaries could induce overesti-mation of FRP design, and consequent utilization of unnecessary reinforcement. Moreover, the sensitivity of CNR assumptions to the elastic-plastic or elastic-brittle behaviour of FRP are less evident in comparison with experimental elaboration of basic parameters.

Finally, it is worth to remark that experimental results on bond are available by tests where FRP is glued only on brick surface, neglecting the significant influence of mortar joints in the complex adhesion phenomenon involving the masonry.

4 FEASIBILITY IN HISTORICAL BUILDINGS 4.1 Reference criteria

Interventions to perform in historical environment cannot disregard to satisfy the specific requirements on which preservation is based.

Figure 15. Comparison among plain masonry and three strengthening configuration by using CNR DT-200 formulation and elastic-brittle or elasto-plastic law for FRP strips.

It is worth to remark that, even in very hazardous conditions, as mainly occur in seismic zone, a proper compromise has to be taken among safety and preservation, keeping priority the safeguard of human life. This enable to reduce and control upgrading, in favour of alternative measures of improvement, specifically targeted to the large complexity of historic masonry and constructions (ICOMOS/ISCARSAH). Recent updating national codes (e.g., OPCM 3431/2005 seismic code in Italy) finally provide specialized sections for existing masonry, recognizing the differences in typologies and materials, and by pointing out the great importance of preliminary knowledge, supported by the suitable application of in-situ non-destructive (ND) and minor-destructive (MD) tests.

Nevertheless, the Charter of Venice (1964), the Declaration of Amsterdam (1995) and the Charter of Cracovia (2000), could be considered reference documents for the definition of criteria and actions devoted to Cultural Heritage. Minimum interventions, having characters of "reversibility" (intended as substitutabil-ity or removability); use of materials and techniques compatible with the original ones, able to guarantee durability to the intervention itself and, consequently, to the building; respect to the original functions (both structural and of utilization), distinguishability of the intervention; all these criteria should be considered when proposing repair or strengthening of historic structures.

FRP has a great potential to improve the brittle behaviour of masonry components, but many aspects related to its interaction with traditional materials and durability are still under experimental study, especially due to the use of resins as bonding system.

More recent studies are focused on the use of organic matrixes, as FRCM (Fiber Reinforced Cemen-ticious Matrix) or TRM (Textiles Reinforced Mortar) (De Lorenzis et al. 2004, Protaetal. 2006, Papanicolau et al. 2007), and could represent significant steps for

Figure 16. Repair intervention on masonry vault of Villa Bruni (Padova): position and view of depressed structures propped before intervention (a), design and assessment with contribution of FRP sheets (b), scheme of global intervention and detail of application of CFRP strips at extrados in severely cracked zones (c).

Figure 16. Repair intervention on masonry vault of Villa Bruni (Padova): position and view of depressed structures propped before intervention (a), design and assessment with contribution of FRP sheets (b), scheme of global intervention and detail of application of CFRP strips at extrados in severely cracked zones (c).

the identification of compatible binders with original masonry.

Cautious approach should dominate, as well as the basic principle that intervention now has not to pretend to be definitive, as further more appropriate measures can be more reliable in the future.

The CNR DT-200 itself declare that interventions with FRP on monuments and historical architecture have to be justified as indispensable for the building, and the respect of the above-mentioned principles of restoration has to be guaranteed.

4.2 Some cases study and applications

The current use of FRPs in restoration work, should be particularly cautious when dealing with historical constructions, as many restoration principles cannot be fully satisfied. On the contrary, their high mechanical performances can be usefully exploited, where other systems fails or are more invasive. Especially textiles and bars have the main advantage of not increase dimensions of strengthened sections, thus their use, if properly designed to solve specific problems, can be targeted to large elements, but also to solve very peculiar weaknesses, not only in standard bearing masonry constructions. Some examples are given in the following.

Even if in large assemblages, FRP can be very versatile to repair urgent local cracks, as part of combined

Figure 17. Repair intervention on masonry vaults of S.ta Corona Church in Vicenza: view of church and detail of cracked cross vault (a), scheme of intervention at extrados and view of FRP application combined with injections (b).

intervention. As an example, the consolidation of the very depressed thin vault of the central hall of Villa Bruni in Megliadino S. Vitale (Padova), has included the repair with FRP strips of the main cracks at the intrados and the extrados, together with other tying measures acted by the timber beams of the floor. FRP strips were designed and assessed by using a specific method applied to vaults, taking into account the modified failure mechanisms of the strengthened structure (Valluzzi et al. 2002 and 2004).

Other applications on vaults can be combined with injections, as well as ties, to improve the global behaviour, as in the church of S.ta Corona of Vicenza, where cross vaults have been reinforced at their extrados with strips 20 cm wide in their longitudinal direction (Figure 17).

In the church of S.ta Maria in Organo of Verona, the intervention with FRP has been limited to the inplane reinforcement of the walls stabilizing the vaults from the sides and to existent transversal ribs, as well as new ones, provided in the longitudinal direction (Figure 18).

Finally, specific applications where the high versatility of FRP laminates can hardly even the performances of other materials, are the local repair of weaknesses in stone elements, being part of structures or monuments. As an example, the confining of capitals can be performed, to substitute or integrate actual metal rings, as executed at the top of the columns of the 'Palazzo della Ragione' in Padova. CFRP strips have been positioned after restoring of the capitals and successively hidden beneath the historical original metal confining elements (Figure 19). Another peculiar example is the reinforcement of the basement of

Figure 18. Intervention on church of S.ta Maria in Organo (Verona): view of church and position of longitudinal cracks on main vault (a), application of FRPs on ribs and side walls (b), scheme of intervention and final view after protection of strips (c).
Figure 19. Intervention with CFRP in capital of 'Palazzo della Ragione' of Padova: original confining rings (left) and FRP strengthening (right).

a equestrian statue characterizing the marble monumental tomb of Cansignorio in Verona, particularly deteriorated by metal oxidation and environmental aggression. A system of superimposed layers of CFRP has allowed to rebuild the loose parts of the bearing elements, re-establishing the aesthetics of the statue (Gaudini et al. 2008).

Therefore, in monuments, where the aesthetics could be the most important requirement, the

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