PSI - Issue 64

Marco Carlo Rampini et al. / Procedia Structural Integrity 64 (2024) 2141–2148

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Author name / Structural Integrity Procedia 00 (2019) 000–000

Fig. 8. Electrical resistivity mapping, corrosion potential results and identification of the main defects for tie rod S6.

3.3 Preliminary discussion The limited knowledge of the interventions carried out in the past years makes it difficult to properly assess the conservative state of the tie rods, particularly regarding their corrosion conditions. The available documents are mainly related to the construction design, while little is known in relation to any previous survey campaigns and interventions. It is reasonable to assume that corrosive phenomena occurred in the past and that, during the intervention carried out before 2005, a protective paint was applied to the bars together with the encapsulation of the rebar system in a cementitious material to remedy at the ongoing deterioration. The metallic covers currently present were used as in-place formwork during the cast. In such type of intervention, the protection against corrosion is mainly entrusted to the barrier effect given by the paint, that should be intact and continuous, rather than to the bar re-passivation due to the contact with an alkaline material. The diagnostic campaign showed that the stainless steel case prevented the penetration of carbon dioxide, since the carbonation depth was almost null in all the samples, keeping high the alkalinity of the cementitious material. In this case, even in correspondence of possible defects of the protective paint, corrosion would be prevented. However, when half of the metallic cover was removed, defects such as macroscopic voids in the cementitious material were observed, in correspondence of which corrosion products were also present (Figure 7). This highlighted the inhomogeneity of the environment the bars were in contact with, which is usually associated to critical points where corrosion is triggered. Moreover, it is likely to assume that not all the core bars (Figure 3b) were in the same conditions. In fact, it is reasonable to assume that the paint was applied only to the external, accessible, surfaces of the most external bars, and that those surfaces were also the only ones in contact with the cementitious material, after it was poured in the external formworks. It is, therefore, very difficult to assess the local environmental conditions inside the cavities among the core bars, which may be very different from those on the external part of the bars themselves. The metallic case also contributed to keep the cementitious material dry. In all the analyzed tie rods, resistivity values were never lower than 100 Ω∙m, and in the most cases resulted higher than 500 Ω∙m, suggesting that, at the moment of the inspection, the cementitious material was almost dry, at least in the central part of the tie rod. It is worth noting that the extremities might be in different moisture