Simic improvement: the parameter ζ AND
Differently from punctual interventions (par. 8.4.1 NTC 2018) for which a static evaluation limited only to the element or structural system being repaired is required, i seismic improvement works instead they must satisfy an increase in the seismic safety of the entire structureevaluated with the parameter ζ AND what is the ratio between the maximum seismic action that the structure can bear and the maximum seismic action that would be used in the design of a new building.
Since these are existing buildings, ζ AND will be less than 1 since the adaptation to seismic safety equal to a newly built building can be difficult due to the high invasiveness and economic impact. The NTC 2018 provide different levels of seismic improvement depending on the intended use: to give some examples, ζ AND + 0.1 for a residential building (a 10% increase in seismic safety compared to the current situation) and ζ AND at least equal to 0.6 for an existing school building (i.e. it is resistant to at least 60% of the expected ground acceleration).
What technologies should be used to seismically reinforce walls? The reinforced plaster
One of the traditional processes consists inreinforced plastera solution widely tested over the decades and today improved with the use of innovative and more compatible materials with reduced massive impact. The intervention is applicable on already plastered surfacesby removing the existing plaster and remaking a similar one with lime mortar but reinforced with a mesh, generally in fiberglass (Fig. 1) or basalt.
The networks of the internal and external wall surface must be connected using transversal connectors consisting of bars made of the same material as the networks, otherwise the system is not effective. This creates a sort of “jacketing” of the masonrywhich significantly improves its mechanical performance in compression and shear.
The reinforcement can be light with a limited plaster thickness (maximum 1.5 cm) according to FRCM (Fiber Reinforced Cementitius Matrix) technology, or with a more traditional thickness of at least 3 cm according to CRM (Composite Reinforced Mortars) technology.
The installation of steel fiber bands
A variant provides the installation of steel fiber bands arranged to create a mesh in two directions, vertical and horizontal on both surfaces of the wall in order to create a “discrete” grid to replace the application of a diffused reinforcement mesh (Fig. 2).
Spreading the plaster will make the laid fabrics invisible. If the external surface is with exposed bricks, it is possible to create a styling reinforced with small diameter strands (Fig. 3) connected transversally, useful for reinforcing slender wall structures or if the beloved plaster is not possible for aesthetic reasons.
Injections with lime mortar coli
An alternative to the use of reinforced plaster lies in the technique of injections with lime mortar inside the masonry (Fig. 4). The purpose is to fill the voids present within poorly cohesive wall textures, such as sack walls. Since not all wall textures can be injected equally with the same effectiveness, it is advisable to test the technique on a limited surface by combining a sonic test before and after the intervention.
If there is a clear improvement in sonic speeds after the application of the injections, it will be guaranteed that the cracks have managed to fill the voids and restore continuity to the resistant section of the masonry. The designer must pay close attention to verify that the multiplication coefficient foreseen in the table C8.5.II from the Circular 21 January 2019 n. 7 for the use of injections is actually achievable and applicable, depending on the real effectiveness of the technique with respect to wall texture.
The techniques can possibly be also applied simultaneouslyif the quality of the wall texture is poor and seriously compromised. It is also possible to combine them with the insertion of artificial diatons, preferably of a metallic nature, to improve the resistance to shear and bending in the plane of the masonry walls.
Which reinforcing material to choose?
A further point of reflection concerns the choice of reinforcing material. «(…) These interventions are aimed both at the rehabilitation and repair of deteriorated and damaged masonry, and at the improvement of the mechanical properties of the masonry; the technical solution to be applied will work also evaluated based on the type and quality of the masonry. The interventions must use materials with similar physical-chemical and mechanical characteristics and, in any case, as compatible as possible with those of the materials in place. The intervention must aim to ensure that the wall regains substantially uniform resistance and continuity in rigidity, also creating the appropriate grips, if missing. (…)» (DPCM 02/09/2011 – Guidelines for the assessment and reduction of seismic risk of cultural heritage).
