Structural Consolidation

The use of composite materials

 

The structural consolidation world has been revolutionised in the last few years by the introduction of innovative materials, the so called FRP-Fiber Reinforced Polymer composite materials, composed of a matrix and a reinforcement fibre.
The matrix is usually of organic nature, composed of thermosetting resins of various types, whilst the reinforcement is composed of long fibres: carbon (CFRP - Carbon Fibre Reinforced Polymer), glass (GFRP - Glass Fibre Reinforced Polymer) and aramids (AFRP - Aramid Fibre Reinforced Polymer).
In addition to these fibres, recognised and covered by Italian norms through dedicated guidelines, other types of reinforcement with different performances are currently being studied, especially as far as the mechanical resistance and characteristics of durability with time are concerned.
The development in Italy of these alternative materials was encouraged by the presence of a vast, existing historical heritage, on which interventions were carried out with traditional techniques utilising materials which were often heavy, invasive, with low compatibility and little coherence with the structures, resulting in vetoes by the institutions responsible for their protection.

The Italian norm identifies the different levels of controls on the existing buildings, dictated by their characteristics and age, but the principals, with which one approaches an historical building or a building/artefact appertaining to an archaeological area, are commonly shared.
There are four principals which must be referred to during the design and execution of a restoration project: compatibility, reversibility, durability and limited invasiveness.
Faced with the necessity of a consolidation intervention, which traditional building methods carry out with steel mesh and cement mortar, innovative materials like FRP composites are the best solution, as they fully satisfy such interventions.
A steel mesh installed with cement mortar does not satisfy compatibility requests when used on historical walls - traditionally in stone and brick - due to the intrinsically rigid characteristics of the mortar's composition. Such consolidation is hardly reversible: an intervention involving the removal of high performance materials, like the cementitious ones, characterised by an elevated capacity of grip, inevitably lends itself to an invasiveness which damages the existing structure.
A composite FRP material generally unites its capacity to increase the performance of the structural elements relative to the static or seismic nature of the solicitations, with the characteristics of light weight, compatibility, reversibility (the contact/adhesion surface remains unchanged) and limited invasiveness.

The need to confront consolidation issues for the historical/architectural heritage emerged strongly in Italy, starting from 2009 after the earthquake in L'Aquila and subsequent seismic events which involved the centre of Italy, probably being the territory with the highest concentration of artistic and architectural heritage of elevated value.
Since then, the need arose to repair damages and intervene urgently, involving interventions for ensuring immediate safety, over and above the need for compatible and safe repairs and partial reconstruction. In parallel and simultaneously, the need also arose for ample and complex type interventions, in terms of solutions for risk situations, namely: seismic resistance improvement, adaptation, the reconstruction with adequate criteria and new building with seismic resistance characteristics.

The intrinsic seismic characteristics of the Lebanese earthquake, in a manner similar to Italy's, induced certain reflections relative to the opportunity to evaluate adequate preventative actions.
To intervene in terms of consolidation signifies operating in an efficient manner with materials and systems which are capable of reinforcing the existing structures, after having considered the fragilities caused from different mechanical issues, which cause a multitude of repurcussions in relation to the type of masonry structure on which they act (stone, brick, etc.) and in relation to the way they have been built.
In an historical wall, the absence of structural integrity, the inadequate distribution of the reinforcement elements, the scarce resistance of the materials, the inadequate anchorage or an insufficient foundation system, are stresses which promote damages.
Once the appropriate characteristics have been defined in context, utilising composite materials for vertical walls or intervening on domed surfaces, there is the preventative need to identify the most adequate method considering the specific characteristics of the structure.
The reinforced plaster with GFRP reinforcement and lime mortars for the reinforcement of masonry walls, the reinforced re-pointing for the consolidation of face brick wall joints, the localised, fibre-reinforced plate for local structural reinforcement and composite pultruded profiles for the construction of light structures, are all consolidation techniques which utilise composite materials with the scope of confronting the specific problems in proportion to the different types of masonry structures, intrinsic fragilities and damage causing mechanisms.

 

REINFORCED PLASTER (RI-STRUTTURA system), Villa Adriana - Tivoli (figure 1a,b,c,d.)
Utilising mesh, connectors and accessories in GFRP (fibre glass composites) together with lime mortars, the technique of "reinforced plaster" provides for the carrying out of compatible and reversible reinforced plastering, which significantly improve the shear, flexural and compression resistance of masonry walls, keeping the weights and thicknesses as low as possible. This system was used on the infamous Roman villa called Villa Adriana in Tivoli, for the consolidation intervention on the vaults.

REINFORCED POINTING (RETICOLA system), Sheik Süleyman Mosque - Istanbul (figure 2a,b,c)
Utilizing stainless steel cables and connectors, the technique of "reinforced pointing" of the mortar joints was identified in Istanbul as the most appropriate for a consolidation intervention carried out at the Sheik Süleyman Mosque, a religious building of Byzantine origins of elevated historical and symbolic significance. The system was developed for consolidation interventions on masonry walls where the "face brick" feature needed to remain, and which consents the carrying out of an efficient and diffused reinforcement and confinement, maintaining the original aesthetic aspect.
Specifically, the project foresaw an intervention which was capable of guaranteeing, as much as possible, a boxed behaviour to the masonry structure of the building, extending the reinforcement in two directions, one in a central position and the other close to the top of the building's apex, whilst a particular consolidation solution was studied for the dome.

FIBRE-REINFORCED PLATE (BETONTEX-EPOXY system), Duomo di Carpi and Basilica di San Petronio in Bologna (figure 3a,b,c,d.)
This technique utilises glass fibre or carbon fibre fabrics, which need to be glued with polymeric, thermosetting resins which increases the shear force resistance in masonry walls and the compression resistance in beams and columns, as well as the use of local reinforcement and enveloping of irregular geometric shapes and in the areas which are solicited the most, like curbs and apexes.
Bandaging in carbon fibre was also used on the Duomo di Carpi, damaged by earthquake on 2012, damage which was located close to the dome and the lateral aisles, both zones having suffered a series of cracks and detachments.
A localised intervention with CFRP materials was carried out on the Basilica di San Petronio in Bologna for the consolidation of a breakage found on the architrave of the left door, which in turn caused the rotation of the one part with respect to the other, probably imputable to the original installation or due to material defect. The intervention was carried out using CFRP bars, cords, laminates and fabrics.

COMPOSITE PULTRUDED PROFILES IN GFRP, Catacombe of San Callisto - Rome (figure 4a,b,c,d.)
Characterised by an elevated level of resistance, reliability and safety, profiles and light structures in GFRP possess particular peculiarities such as low weight, compatibility, reversibility and are scarcely invasive relative to the structures they will act on.
Such profiles have been used on an intervention in Rome in the San Callisto Catacombs in the form of light infrastructure for the construction of pedestrian handrails and the consolidation of some protections on crypts.
The particular ambient conditions and the specific context, primarily in consideration of the geological characteristics of the location, were a determining factor for the definition of the materials to be used: the elevated versatility of the composite materials, combined with the possibility of controlling and calibrating in advance the performance offered, together with the durability against the aggressive external agents combined with the benefit of reduced weight, as well as the optimal weight/performance ratio and the ease of workability on site with normal cutting and drilling tools.

[by Marco Volpi, Sara Gabriele]