In Italy masonry structures
are mainly historical-monumental ones and deserve to be preserved
and restored from any type of degradation phenomena, from structural
to material durability issue.
Technological solutions adopted for restoring and consolidating
such structures need to follow the logic that, even if keeping
in mind the need of solving safety issues generated by structural
problems, still follow the concept of preserving the architectural
and historical characteristics of the structure.
On the other hand, almost recent seismic events have proven
in the field the poor performance of such retrofit solutions
that used to transform the structure with an hybrid behaviour
mixed between the one of masonry and that one of concrete and
steel structures, with all the issues generated by heavier dead
loads, incompatibility and incoherencies with the existing structure.
Understanding such issues and problems has brought, particularly
when reconstructing or retrofitting in seismic areas, to a new
engineering approach. Having abandoned retrofitting techniques
that used to transform the structure in RC frame with infill
masonry walls, are today preferred much lighter but yet very
high strength retrofit solutions that are able to conjugate
the need of strength with the respect of the structure, avoiding
invasive irreversible solutions or such ones that do not assure
a good durable performance.
It is such evolution of retrofit techniques that Fiber
Reinforced Materials, known with the acronym FRP
(Fiber Reinforced Polymer), have entered the civil engineering
market. Such innovative materials allow to retrofit/strengthen
the structure without compromising the structural integrity
and respecting its characteristics through the following principles:
- high strength and stiffness of the materials implemented;
- lightness and reduced invasivity of the installation;
- short time and ease of installation;
- possibility of future removability.
Next to such consolidated retrofitting technology, is nowadays
available also another family of composite materials similar
to FRP, made of fine ultra-high tensile steel filaments, twisted
to form cords and tailored similarly to unidirectional (carbon,
aramid or glass) fiber sheets in order to then be easily installed
on site with the manual lay up technique. The great advantage
of such materials is that they can be either impregnated using
epoxies, then called SRP
(Steel Reinforced Polymer), or using mortars, then being called
SRG (Steel
Reinforced Grout). Such materials present the same advantages
of FRP in terms of lightness, strength and ease of installation,
but with several additional advantages that have made them very
attractive for masonry retrofitting. The possibility of impregnating
them with cement or hydraulic mortars allows a much easier installation
for skilled workers and also a considerable reduction of installation
costs with respect to FRP. Secondly, being such sheets made
of steel cords, they have a considerable shear strength, not
present in FRP, making them particularly attractive for uneven
surfaces and also for mechanical anchors, prohibitive with FRP.
Thanks to the recent publication of the CNR (National Research
Italian Council), “Guide for the Design and Construction
of Externally Bonded FRP Systems for Strengthening Existing
Structures” (CNR-DT
200/2004) the use of composite materials in the civil industry
has had a great increment. |
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