PSI - Issue 64

Marco di Prisco et al. / Procedia Structural Integrity 64 (2024) 2133–2140 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 8. Location of control points

6. Discussion and final remarks To control the load-bearing capacity of the infrastructure over time, that could be reduced due to the ongoing carbonation phenomenon, a load test with a 44t truck was suggested to be scheduled every 5 years: the truck will be placed in the centre of the central span of the bridge, with an eccentricity of 0.8 m (it has to be placed at a distance of 50 cm from the external edge of the roadway). The test must be repeated for both directions. The threshold of the relative displacement measured at the lower edge in the centre, on a basis of 1 m, at the intrados of the outer beam, must be less than 120 μm. This value corresponds to the increase of positive strain at the intrados starting from a zero stress value, without tension stiffening, and assuming a strands loss corresponding to section C, which guarantees the minimum safety coefficient (Table 2). It is worth to note that the situation of the dead load only, in fact, even in this condition, implies a compressive strain at the intrados of about 5E-5: this means that the threshold selected ( 120 μm ) could indicate the proximity to cracking condition, neglecting the initial compressive strain induced by the strands. If this threshold will be exceeded, the full oxidation of the two last rows of strands should be expected and, therefore, it will be necessary to check the state of the reinforcement and, hence, to set a new load capacity limit up to a new retrofitting intervention. The paper discusses the case of a retrofitted bridge and proposes a monitoring correlated to a specific loading case corresponding to the maximum load expected on that infrastructure. First of all, the strengthening introduced by the application of CFRP corresponds to a significantly lower contribution with respect to what computed by the designer at the time of the retrofitting application. Then, due to the carbonation revealed, the main suspect is correlated to a possible progressive initiation of oxidation process of the prestressed steel of the beams in the time. To control this phenomenon, a check carried out with the same procedure aimed at identifying the effectiveness of the strands closer to the intrados external surface every five years is proposed.