Structural rehabilitation of a railway overpass using FRP (Fiber Reinforced Polymer)

In the infrastructure sector, the current state of conservation of artistic, road, and railway works increasingly requires us to monitor the structures and elements that compose them in order to design and implement maintenance interventions, to adapt their load-bearing capacity to current traffic volumes, and to increase their durability and safety.

For an intervention to be effective and long-lasting, it is not sufficient to restore the structures to their original conditions before deterioration. It is necessary to intervene with appropriate techniques and methodologies, including the use of composite materials such as Fiber Reinforced Polymers (FRP).

This article presents a recent case study in which FRP solutions with carbon fibers were used.

Structural rehabilitation of a railway overpass using FRP The Porta Milano railway overpass represents a crucial point in the flow of traffic entering and exiting the city of Novara towards Milan in the northeast direction.

The structure is characterized by a geometry that clearly distinguishes two parts: a straight portion on the Novara side, consisting of a continuous deck with a hyperstatic structure, and a section on the Milan side, which features a sequence of isostatic spans that create a curved ramp.

The overpass allows for the crossing of the Turin-Milan railway line and, as mentioned earlier, is located on a fundamental road artery with a high volume of continuous vehicular and pedestrian traffic.

Structural rehabilitation of a railway overpass using FRP For several years, the structure of the viaduct has shown clear signs of deterioration, mainly caused by its age (the structure dates back to 1930), impacts and various damages, as well as significant water infiltration due to the lack of proper waterproofing.

Given the severity of the situation, the Municipal Administration of Novara initially banned heavy vehicle traffic and later carried out emergency safety measures to support the walls of the ramp that delimits the maintenance facility for Trenitalia rolling stock.

In 2018, the worsening overall conditions led to the development of a restoration and reinforcement project for the structure, which was subsequently assigned to Mercitalia Shunting & Terminal srl, an engineering company within the FS Group, with a design by engineer Paolo Clemente. This company operates in the railway services sector and, particularly through its Construction Operating Structure, performs Technical Entity activities in the fields of Design, Works Supervision, and Safety for both FS Group companies and private entities.

Damage and intervention The reinforced concrete structure of the viaduct exhibited all the signs of degradation associated with aging and intense wear, such as detachment and expulsion of coverings, carbonation of concrete, rusting of reinforcement bars, cement disintegration, and cracking.

Moreover, the load-bearing structures showed losses of section in several points, requiring urgent structural rehabilitation to restore the original strength of the elements. The intervention also involved the adaptation of lateral containment elements, such as bridge edge barriers and parapets, in compliance with regulations.

Fibre Net solutions The structural reinforcement intervention aimed to increase the load-bearing capacity by installing additional reinforcement bars and, most importantly, by using Carbon Fiber Reinforced Polymer (CFRP) composite systems. These systems were applied to the deck beams of the viaduct to enhance both flexural and shear strength.

Fibre Net’s BETONTEX-EPOXY system was employed in the form of CFRP laminates and through the bonding of CFRP fabric. This was applied to all the spans of the viaduct, totaling 14 spans, each consisting of 6 out-of-thickness beams. The intervention utilized approximately 1000 linear meters of carbon fiber laminate with a width of 100 mm and 350 square meters of single-layer fabric.

The flexural strengthening intervention involved the installation of FB-G14L-HM10 laminates by Fibre Net Spa (width 100 mm): 2 laminates were placed on the intrados and 2+2 laminates on the lateral faces of the beams, on all beams, near the intrados, after preparing the substrate, applying FB-RC01 primer, and using FB-RC30/3 adhesive.

For shear reinforcement of the beams and to improve the bond stress between the laminate and the substrate, transverse wrapping was performed using unidirectional carbon FABRIC.

The carbon strips (width 10 cm) were applied in U shape at regular and progressive intervals, following the project’s specifications, with a higher concentration towards the beam’s ends compared to the central portion.

To prevent premature detachment of the carbon fiber fabrics at the beam support zones, an anchoring system for the fibers was developed, as described in CNR DT200R1/2013 guidelines (Section B-B image).

It is recommended, as indicated in CNR DT200R1/2013 guidelines (paragraph 4.3.3.2), to use mechanical anchoring systems (e.g., for cantilever beams) and to limit the detachment stress to the ffed strength, applying expression 4.21 from the guidelines.

The installation of BETONTEX FB-GV320U-HM fabric followed a predetermined cycle, including the application of FB-RC01 epoxy primer, the application of a first layer of thixotropic bonding resin, placing the fabric on the beam according to the project specifications, and impregnating it with FB-RC02 impregnating resin using a roller.

The materials used for consolidation with the BETONTEX-EPOXY system from Fibre Net are:

• High Modulus FRP laminate, width 100 mm: FB-G14L-HM10; FB-RC01 primer; FB-RC30/3 adhesive.
• High Modulus FRP fabric, width 10 cm: BETONTEX FB-GV320U-HM 300 g/m2; FB-RC01 primer; FB-RC02 impregnating resin.

The composite materials used, carbon fiber fabrics, are now accompanied by a CVT (Certificate of Technical Validation) instead of the previous CIT (Certificate of Technical Suitability).

Construction Site Information:

• PROJECT: Structural and conservative restoration of the Porta Milano railway overpass.
• STRUCTURE CHARACTERISTICS: Length approximately 300 m.
• LOCATION: Novara, Italy.
• CLIENT: Trenitalia S.p.A., Ferrovie dello Stato Group.
• DESIGNER: Ing. Paolo Clemente_Mercitalia Shunting & Terminal Srl, FS Group.
• CONTRACTOR: Notari Srl Special Construction.
• FIBRE NET SYSTEMS APPLICATOR/INSTALLER: F.lli B. System Srl.
• YEAR OF REALIZATION: 2019.