NYAB was the lead contractor for the project. Built by Steel Group Pohjanmaa Oy, the bridge is an excellent example of how SSAB Weathering steel can contribute to revolutionizing bridgebuilding. Made with SSAB Weathering 420 ML, the bridge is one of the first of its kind in Finland. This weather-resistant high-strength steel has turned out to be an excellent choice for bridgebuilding since it is very low maintenance and provides excellent resistance to challenging conditions.
SSAB Weathering 420 ML – outlasting, weather-resistant steel
SSAB Weathering 420 ML was chosen as the material for Jokikylä bridge because of its excellent weather resistance and low maintenance.
"Workshop time is saved if steel is not painted, especially in large projects. SSAB Weathering does not need painting, which in turn also significantly reduces maintenance costs and environmental impacts,” says CEO Henrik Kiviniemi at Steel Group Pohjanmaa Oy.
Erkki Krankkala, Technical Development Manager at SSAB, emphasizes that whereas the maintenance frequency of conventional painted steel is for example 10 years, it’s double that with SSAB Weathering 420 ML. Krankkala points out that painting conventional steel also causes major environmental damage since the old paint has to be removed and destroyed. SSAB’s weather-resistant steel is an eco-friendlier alternative as it doesn’t need painting.
Strength and durability in cold conditions
According to Erkki Krankkala, 2019 saw changes to the EN10025-5 material standard for weather-resistant steels, with the addition of strength classes S420 and S460. These changes subject steel to stricter requirements. SSAB Weathering 420 ML has excellent impact toughness, especially in cold conditions. The impact toughness value of conventional structural steel is normally around either 27 J or 40 J at-20°C. The value for SSAB Weathering 420 ML is 27 J at -50°C.
“This means that SSAB Weathering 420 ML retains its toughness and better withstands impacts also in low temperatures,” Krankkala explains.
Impact toughness value measures the ability of steel to resist fracture caused by impact. This is important because it indicates the toughness of the material and its ability to absorb energy before fracture, which is particularly important in structures subjected to dynamic loads, such as bridges.
Role of patina and freedom of maintenance
SSAB Weathering forms a dense and tightly attached oxide layer, a patina, on the surface of the steel in 2-4 years, which protects the steel from corrosion. The steel must be exposed to alternating wet and dry conditions for the protective layer to form.
“The patina is self-healing, which means that scratches and other surface damage do not result in the spread of corrosion, as a new protective layer of patina is formed on the damaged area. We have 32 years of bridge research by VTT Technical Research Centre of Finland on the corrosion resistance of steel in bridges," Krankkala says.
According to Krankkala, paint adheres considerably better to weather-resistant steel than to conventional steel. A six-year study in Sweden found that when surfaces of weather-resistant and conventional steel were scratched, only the conventional steel scratch rusted and damaged the paint surface around the damage site. On the weather-resistant steel, however, the scratch patinated and the paint remained intact.
“Many bridges built from Cor-Ten steel in the 1970s are still in good condition. They are repaired of course, but not because of the steel,” Krankkala says.
“A problem with old steel bridges is their deficient load-bearing capacity. It is precisely for this reason, for example, that old bridges have to be replaced with new ones. Demands on bridges are continuously increasing,” Kiviniemi says.
As the decks of bridges get to the stage where they need to be repaired, the load-bearing capacity of bridges is also reviewed and the bridges are updated to take account of today's traffic conditions.
