PSI - Issue 84

Anna Bontempi et al. / Procedia Structural Integrity 84 (2026) 1039–1046

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It should be noted that the materials choice for the strengthening was driven by the need to protect the cross-section against corrosion. Indeed, the environment in which the beam was located was aggressive with respect to exposure conditions. The use of polymer-based materials, which are inherently resistant to corrosion, can enhance the durability of the structural element. Furthermore, the mortar is designed to be applied using a spray technique, without the need for formwork. In fact, casting would have been difficult to implement in the environment where the beams were located. Figure 3 shows the location of the specimens in 1927, during the casting phase, and in 2021, during the dismantling process.

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Figure 3: pictures of the bridge location at the moment of casting (a) and during the dismantling process (b).

The strengthening intervention was carried out through a sequence of preparatory and executive phases aimed at ensuring adequate adhesion and structural effectiveness of the retrofit system. Initially, the exposed concrete surface was cleaned by high-pressure water jetting in order to remove unsound and delaminated concrete and surface dust, thereby promoting proper bond with the strengthening layer. In real-case applications, existing reinforcement passivation should be carried out; however, in the present case, this procedure was not performed. Holes were then drilled for the installation of connectors (dowels, see Figure 4a) for transferring the stresses from the overlay to the existing flange; they were bonded using epoxy resin. Subsequently, local repairs of the concrete cross-section were performed using a thixotropic repair mortar (cylindrical compressive strength 62 MPa) to restore damaged cover concrete and seal pre-existing cracks resulting from the previous loading test (Figure 4b). Before the repair of existing cracks and concrete cover loss, the longitudinal reinforcement was instrumented with strain gauges. The beam surface was mechanically roughened, with a demolition hammer, to achieve adequate interface roughness for stress transfer between the existing concrete and the strengthening overlay. The target surface roughness was approximately ±5 mm.

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Figure 4: position of dowels to connect the overlay to the flange (a) and restored cracks and concrete cover loss (b).