Issue 46

F. Bazzucchi et alii, Frattura ed Integrità Strutturale, 46 (2018) 400-421; DOI: 10.3221/IGF-ESIS.46.37 408 traffic), many other maintenance and monitoring activities have been performed over the viaduct [10]. Most of the monitoring regarded the health condition of the strands of the remaining towers [11], while the repairing affected the carriageway slabs. When Tower 9 collapsed, a replacement intervention of its strands was planned for the following two months, and ongoing maintenance was present on the deck. M AIN BRIDGE VULNERABILITIES AND CONTINGENCY SOLUTIONS any bridges of Italian road infrastructural network are now between 50 and 100 years old. Throughout this period of time, materials, technologies and traffic loads significantly changed, together with the community necessities. The challenge for a new record span or a lightweight design mutated, in the scientific and technical community, in favor of the challenge of diagnosing and securing the inherited infrastructural heritage. Italy counts around 60000 bridges (a detailed census does not exist), more than half in high seismic areas [12]. Failure collapse probability assumed by European Building Codes [13] for a new structure is mandatory to be less than 1E-6 per year. With a failure ratio of 1.25 bridge/year, the situation produced by the vulnerability of the existing buildings has a detrimental factor of 13. The vulnerabilities, as evidenced in first part, are essentially connected to the unawareness of the material degradation or to the lack of devices in avoiding fragile collapse. Durability in concrete bridge engineering The first long span concrete bridges have been erected at the beginning of the last century. Examples like the Ponte Risorgimento (Hennebique, Rome, 1911) or Saint Pierre du Vauvray Bridge (Freyssinet, 1923) were considered pioneering works and stands still today as symbols of innovative conceptions. It is, however, after the second World War that under the continuous push of the economic revolution the concrete industry had its widespread diffusion. The sole construction of the A1 Highway Milan-Napoli, started on 1956, brought to the country 800 km of heavy roadway and 400 bridges in less than eight years [14]. Between 1954 and 1970 more than 20 bridges were erected over the Po river [15]. The structural scheme was commonly the multi-span simply supported beam or the Gerber cantilever. This choice relied on the availability of precast prestressed concrete beam. The introduction of prestressing, around 1950, drastically increased the achievable spans and the better use of the concrete compressive strength. Moreover, the lack of numerical methods, especially for prestressing loss, did not allowed the use of continuous beams or statically undetermined framed schemes. Italian Building Codes, drafted in 1960 (c. n. 94 7/03/1960), regulated the use of prestressing, but, with today’s knowledge about concrete chemistry and long-term properties decay, they appear tremendously inadequate in fulfilling the durability requirements. An example extracted by the codes states that “ […] the water content must be the least possible to guarantee the workability of the mixture without exceeding the 0.45 weight ratio with cement and aggregates” or “ […] to execute frequent grain size checks to fulfill the maximum compactness”. These are all qualitative recommendations that demonstrates the lack of a tangible knowledge of the time-dependent behavior of the concrete in its crucial years of widespread diffusion. One of the main factors that influences corrosion of reinforcement is the porosity of the mixture. This allows the aggression of the iron by the external chlorides and promote carbonation. Porosity has a direct correlation with water content and increasing aggregated grain sizes [16]. The promoted studies in the early 1970 [17, 18] set a milestone in concrete technology, evidencing the carbonation phenomenon and the quantitative role of porosity. Furthermore, they facilitated the introduction of super-fluidifying solutions, which exploited the power of chemical additives in reducing drastically the water content with an appreciable workability. At the same time, minimum concrete cover prescription and efficient detailing for diverting running water were introduced in the upcoming building codes [19]. In 1979, Morandi published an update [20] on the Polcevera bridge behavior after twelve years of service. It was then clear that the wall cracking represented a problem for long-term performance of the structure, especially for an aggressive environment like the one where the bridge was situated. Polcevera viaduct is 1 km distant from the sea and in the underlying industrial hub there was, until 2005, a steel production plant. This means that the wind that climbs back the city coast brought not only chlorides, but also sulphur dioxide in contact with the structure. Sulphur dioxide, when combined with carbon dioxide, becomes sulphuric acid that reacts with cement transforming lime in salt composites that can be washed out by rain. As suggested by Morandi in [20], and as illustrated in section 1, in the 80s the concrete of the Polcevera towers had a surface polishing and restoration treatment. Literature does not report any details about this repair intervention, but we know from [20] that the Author suggested the exposure of reinforcement, rust removal, epoxy covering and the addition of a protective layer with a wire mesh. Form picture of this intervention (Fig. 12(a)) it is possible to see that, in some parts, an actual wire mesh was embedded in the new surface concrete layer. From Italian building magazines [20,21] we know that M