This was the backdrop painted by ISSF’s Tim Collins during a recent webinar entitled ‘Coastal structures: protecting our people’. Fortunately, Mr. Collins was able to quickly enthuse those taking part by presenting a more optimistic scenario, one based on the premise of taking a long-term viewpoint, stopping the use of fossil fuels, making much more use of sustainable materials and promoting zero-maintenance options.
“Our current approaches to coastal defences are generally simplistic and based on ‘old thinking’. Nor are the corrosive effects of the sea properly understood. Short-term defence thinking still dominates, with many decisions based on material and installation costs. However, I am glad to say that the importance of life-cycle costing (LCC) is slowly becoming more important,” stated Mr. Collins.
Giving an example of LCC, Mr. Collins discussed the recent repair of a traditional breakwater in Bayonne, France, where both surface cracks in the reinforced concrete as well as sub-surface cavities in the embankment had been found. Looking for a longterm solution, a repair strategy was called for, which would extend the service life of the breakwater by at least fi fty years. Based on experiences elsewhere, 2304 duplex stainless steel rebar was selected for the project. Thanks to its high strength and excellent corrosion resistance, a much lower tonnage of 2304 was needed than would have been the case with carbon steel.
Stainless steels are also ideal materials when looking for extreme protection, noted Mr. Collins. “In March 2011, Miyako City in Japan experienced a tsunami around ten metres high. After the disaster, the authorities installed a nine-metre-high seawall, plus twenty lock gates of the same height.” Duplex stainless steel was chosen for the lock gates, continued Mr. Collins. “Benefi ts of using duplex include the fact it is three times stronger than the aluminium alloy previously used, it enables an overall weight reduction per gate, the construction costs are reduced compared to previous designs, it offers excellent corrosion resistance, and it is able to withstand the pressure of ten metre high tsunami waves.”
2304 duplex stainless steel was also a clear choice during the renovation of the Samuel de Champlain Bridge over the St. Lawrence seaway in Montreal, Canada. Dating from 1962, the original bridge was suffering from corrosion caused not just by seawater corrosion, but also by de-icing salts. As the steel rebar rusted, the volume expansion caused the concrete to degrade. Following life-cycle costing analysis plus the requirement for construction materials that could withstands the
harsh environment as well as the extensive use of de-icing salts, 15,000 tons of 2304 lean duplex were used for rebar. With renovation work now complete, the bridge is expected to deliver over 125 years of useful life. Incidentally, as a knock-on saving, the use of duplex rebar meant that the weight of concrete required could be reduced by 20 percent!
Mr. Collins gave a quick course in etymology by way of introducing the next topic. “Derived from the Italian language, gabions are those wire cages which, when filled with rocks, can be piled up to create walls for protection and retention. In many cases gabions are a versatile and cost-eff ective building technique. However, the traditional material – galvanized steel wire – quickly loses the zinc coating in the coastal environment. Stainless steel wire is the optimum material choice in such cases. The same applies to wire mesh used to secure cliff faces. With a stainless-steel mesh plus stainless-steel bolts you will have a real ‘fit and forget’ coastal defence solution.”
Summing up, Mr. Collins said that the coming eighty years would be critical for our coastlines. Confirming that hard coastal protection will be necessary, he encouraged the use of life-cyclecosting to better assess the overall project costs from cradle to grave, to select the optimum materials and to design with longevity in mind. “In coastal infrastructure applications, stainless steels – if properly specified in terms of grades and product forms – do off er the lowest lifecycle cost of all materials,” he concluded.