Messa in sicurezza e interventi di risanamento del Ponte San Felice con FRP e malte tecniche Fibre Net

Fibre Net, the company that stands out in the field of manufacturers of such systems in terms of innovation, intense R&D activities, and certifications, has long dedicated a Business Unit entirely focused on the development of new technical solutions for infrastructure. The team is complemented by professionals who provide support and assistance to institutions, designers, and companies operating in the sector.

The San Felice bridge represents a significant case in the restoration, reinforcement, and protection of existing works of art through extraordinary maintenance interventions, where Fibre Net’s Infrastructure Division provided a complete range of intervention solutions. Their extensive expertise in FRP usage was integrated with guidance on the use of products and technical mortars specifically developed by the company to meet the most stringent requirements of road managers.

Geometric Characteristics of the Bridge: The bridge is located in the province of Belluno, connecting the municipalities of Borgo Valbelluna and Sedico. The structure, inaugurated in 1930, consists of 8 arch spans, each measuring approximately 38m x 8m, with a total length of 302m. The height varies from 9 to 13m. About 15 years ago, the structure underwent a significant renovation project, including resurfacing of the roadway, construction of a pedestrian-cycling lane, and initial structural reinforcement through the construction of walls and pillars between the arches. However, due to the progressing deterioration caused by severe environmental exposure and the high volume of daily traffic on this bridge, as well as the importance of this road artery, it has become necessary to intervene once again.

DAMAGE AND DEGRADATION OF THE STRUCTURE

All the recent diagnostic investigations conducted on the bridge have highlighted a degradation attributable to multiple causes, ranging from construction defects (such as insufficient concrete cover) to the poor management of rainwater, which facilitated the ingress of aggressive agents into the concrete. Typically, these include damage from exposure to carbon dioxide and chloride penetration. As Prof. Felitti points out in “Chloride Penetration in Reinforced Concrete Structures,” chlorides are also artificially present in deicing salts, which particularly expose all types of reinforced concrete structures, including highways and external pavements, to degradation. Chlorides, in addition to attacking the reinforcement bars, can also directly damage the concrete. The corrosion of reinforcement bars by carbon dioxide or chlorides is a highly complex electrochemical process that occurs in the presence of oxygen and water. Under these conditions, the metallic iron undergoes a chemical transformation into iron oxide or iron hydroxide, forming what is known as rust. It is important to distinguish the action of carbon dioxide, which completely destroys the protective film of the bars, causing widespread corrosion, from the action of chlorides, which cause localized corrosion. As highlighted by the same author in “How to Recognize and Counteract Carbonation in Reinforced Concrete Structures,” when carbon dioxide comes into contact with reinforced concrete structures, it neutralizes the alkaline components present in the concrete, causing the pH to drop from values >13 to values <9, reducing the pH of the concrete from its “physiologically” basic state. This process is known as carbonation. Carbonation does not directly damage the concrete itself, but by reducing the pH of the solution in the pores, it fails to provide the passivity conditions for the reinforcement bars, thus creating favorable chemical and physical conditions for their corrosion. When carbonation penetrates the entire cover thickness, the protective film (ferric oxide) of the reinforcement bars becomes porous and incoherent, no longer able to block the entry of oxygen and water to the metallic substrate. The iron transforms into rust, with an increase in volume of about four times the volume of the uncorroded iron, causing the cover to crack. Both of these well-known actions, as documented in the literature, have caused severe oxidation of the reinforcement bars on the San Felice bridge, to the extent that the stirrups have disappeared in some areas, and large portions of the concrete cover have been expelled. This suggests a comprehensive repair, structural reinforcement, and protection intervention for the reinforced concrete structures of the bridge.

INTEGRATED SOLUTIONS FOR RESTORATION INTERVENTION

The first design choice was to enhance the load-bearing capacity of the structural elements through the use of FRP systems. As known, the use of fiber-reinforced composite materials not only ensures excellent mechanical performance but also provides high corrosion resistance and ease of installation. As also mentioned in the article “Technologies and Targeted Interventions for Functional and Seismic Rehabilitation of Some Existing Reinforced Concrete Bridges” by Prof. Raffele Poluzzi, the use of carbon solutions for structural restoration, improvement, and static and seismic upgrading of bridges reduces strain and bending stresses, increases loads, and improves performance in the case of deteriorated or insufficient internal reinforcement, ensuring durability and long-term structural performance. Structural reinforcement was applied to beam-pillar nodes, arches, and the slab using high-strength carbon fiber fabrics and nets combined with thermosetting epoxy resins from Fibre Net’s BETONTEX line, which was the first in Italy to obtain the Technical Evaluation Certificate (CVT) from the CSLP. For the concrete repair, both in the areas to be reinforced with FRP and in any other degraded portion of the structure, products from the Structure and Integra technical lines developed by Fibre Net for large-scale projects have been selected and used. Specifically, a passivating mortar against reinforcement corrosion was applied, followed by the reconstruction with a tixotropic structural mortar of type R4, and subsequent application of a skim coat mortar and a protective finishing cycle. These products meet the requirements specified in the project and comply with the specifications of the main road and railway authorities for restoration and adaptation interventions on reinforced, pre-stressed concrete elements, as well as masonry structures.

DESCRIPTION OF INTERVENTIONS AND WORKING PHASES

Following the investigations to identify the causes of degradation phenomena and with the aim of extending the service life of the structure while ensuring maximum durability of the restoration intervention, the areas to be addressed and the thicknesses of inconsistent or contaminated concrete to be removed were identified. The intervention techniques, determined based on the type of structural element (horizontal or vertical), the thicknesses, and the extent of the intervention, along with the material requirements for performance and durability, were divided into appropriate phases. This includes the preparation and removal of inconsistent or detached concrete, followed by the restoration and structural reinforcement phases with FRP, and concluding with the application of finishing mortar and protective coating.

RESTORATION AND RECONSTRUCTION OF CONCRETE

All deteriorated concrete in the structure, including reinforced concrete pillars, beams, and arches, underwent a process involving the removal of damaged parts, cleaning of exposed or discovered reinforcements, and the application of INTEGRA FERRO – FR 718. This passivating mortar meets the performance requirements specified by the European standard EN 1504-7 and was specially developed by Fibre Net to counteract reinforcement corrosion.

Subsequently, as designed, the portions of the concrete cover on the elements were reconstructed using STRUTTURA TIXO – TX 500, a high-performance tixotropic structural cementitious mortar with expansive properties. It is classified as CC and falls under class R4 according to UNI-EN 1504-3. The choice to use an expansive fiber-reinforced mortar of class R4 was guided by the need to ensure durable and reliable interventions with a rapid commissioning of the structures, while also guaranteeing resistance to weathering and freeze-thaw cycles. In particular, STRUTTURA TIXO – TX 500 mortar, with its content of synthetic micro-fibers and long inorganic macro-fibers, allowed for the elimination of the reinforcing mesh due to its highly effective dispersed reinforcement matrix, which controls plastic expansion and increases flexural strength.

STRUCTURAL REINFORCEMENT OF ELEMENTS USING FRP

Following the concrete repair, the reinforced concrete nodes of the bridge underwent structural reinforcement using high-strength carbon fiber fabrics and grids, combined with thermosetting epoxy resins from the BETONTEX line. The fiber-reinforced plating intervention allowed for an increase in both the mechanical strength of the reinforced element, particularly against tensile and shear stresses, and its ductility, thus achieving effective confinement. The solution for reinforcing the arches, in the designated portions indicated by the designer, involved the use of carbon unidirectional fabric to enhance their shear resistance. For the intrados of the deck slab, a bi-axial carbon fiber grid and connectors were applied as reinforcement, all utilizing elements from the BETONTEX system by FIBRE NET. In particular, this latter solution provides an effective alternative to the traditional method of installing embedded reinforcement bars around the perimeter of the slab, followed by the application of a cementitious mortar on the intrados. To complete the reinforcement works, carbon connectors and high-strength carbon fiber patches were also used to mechanically anchor the carbon fiber reinforcement systems to the structure, as specified by the designer. The careful evaluation of the fiber quantities and their arrangement during the design phase allowed for the optimization of the mechanical properties of the reinforcement according to the desired improvement needs. Furthermore, the choice of using carbon fabrics to be impregnated on-site facilitated the easy and rapid adaptation to the complex and irregular geometries of the structure, enabling convenient application.

FINISHING AND PROTECTION

To further extend the service life of the structure by enhancing its protection against aggressive environmental actions, the intervention concluded with the application of STRUTTURA RASO FINO – RF 114, a mortar used for repair, skim coating, and finishing of concrete elements, providing surface protection against penetration risks (PI). Additionally, the primer INTEGRA PROTECTION P 202 and the anticarbonation acrylic protective finish INTEGRA PROTECTION P 407 AC, in RAL 7032 coloration, were applied to all surfaces, both restored and non-restored. To date, the completed interventions cover the first 3 spans of the bridge, with work soon to resume on the remaining portion of the structure. Upon completion, the bridge will resemble the rendering shown below.

Technical Data: Contracting

Authority: Veneto Strade S.p.A. – Viale Paolucci, 34 – 30175 Marghera (VE)

Designer: Proto Studi – Piazza San Valentino, 54/7 – 33031 Basiliano (UD)

Works Supervision: Veneto Strade S.p.A. – Belluno Branch Office

Executing Company: Deon SPA – Via degli Agricoltori, 13 – 32100 Belluno (BL)