Durability of silica fume concrete in Aursundet Bridge
Along the Norwegian coastline there are more than 300 concrete bridges, of which more than half are suffering from chloride-induced steel corrosion (Østmoen et al., 1993; Gjørv, 1994). Most of these bridges have been built during the last 30 years, and one of them was so heavily corroded that it was demolished after a service period of 25 years (Hasselø, 1997). In spite of the extensive experience on the beneficial effect of both pozzolanic additions and blast furnace slag cements on the resistance of concrete against chloride penetration, most of the bridges along the Norwegian coastline have been produced with a concrete based on pure portland cements. Only in more recent years have additions of silica fume and partly also blended portland cements with fly ash been more commonly applied. When the Aursundet Bridge was constructed in 1993-1995, special efforts were made by the Highway Authorities to obtain an improved long-term performance, which included both an improved structural design and selection of a concrete quality with better resistance against chloride penetration. This bridge is a four-span cantilever bridge with a post-tensioned box girder having a total length of 486 m. For the most exposed parts of the bridge, a 55 MPa type of concrete (C55 SA) with 12.5% silica fume by weight of the portland cement was specified. Since such a silica fume content was much higher than that previously used for bridge construction along the Norwegian coastline, it was decided to closely follow up how this type of concrete would resist the future penetration of chlorides from the local marine environment.