South-east Queensland is the fastest growing region in Australia. To meet the region’s future demands and to secure its economic success, Brisbane’s six-lane Gateway Bridge will be duplicated as part of the Gateway Upgrade Project. This is the largest road and bridge project in Queensland’s history, delivered by Queensland Motorways with design and construction by the Leighton Abigroup Joint Venture.

Looking to the future, the new bridge will have a design life of 300 years. To ensure such a long lifespan, the bridge design specifies stainless steel reinforcement bar (rebar for short) in the most critical bridge structures: the splash zones of the two main river pylons. Leighton Abigroup Joint Venture approached Outokumpu as they were searching for a stainless rebar supplier. The Outokumpu Group is one of the leading producers of stainless steel long products in the world and manufactures stainless rebar at its ASR Rod Mill in the United Kingdom.

Artist’s impression: Gateway Bridges. Courtesy of Leighton Abigroup Joint Venture

Based on state-of-the-art knowledge of stainless rebar, the Joint Venture were looking for a product in the austenitic grade EN 1.4404 (ASTM 316L). Outokumpu had a better idea: the Group’s Australian sales company suggested using duplex stainless steel of grade LDX 2101® instead. The corrosion resistance of this proprietary Outokumpu duplex grade is close to that of 1.4404. However, LDX 2101® does the task with very low nickel, making the duplex grade highly competitive to austenitic grades.

LDX 2101® had never been used in rebar before. So the bridge engineers had the question, would that grade have the required corrosion resistance for the extended period of time? Outokumpu’s in-house research metallurgists had made extensive corrosion tests on LDX 2101® rebar in the event that concrete is permeated by seawater. The results prove that LDX 2101® withstands such a high corrosion environment.

For additional proof of the advisability of the Outokumpu concept, ASR Rod Mill sent a trial rebar coil to Atlas Specialty Metals in Melbourne for further processing to confirm that LDX 2101® coil could be easily straightened.

As a result of the research and tests, the second Gateway Bridge is now under construction with LDX 2101® rebar. Outokumpu has supplied 28 tons for the project to date, and further deliveries are scheduled for early 2008. The Gateway Upgrade Project is scheduled for completion in 2011.

Why and how to use stainless rebar

The Progresso Bridge in New Mexico, USA, was built in 1937-41 using stainless rebar. The bridge has not required maintenance to date. A parallel bridge, built with carbon steel in the 1960s, was closed in 1982 beyond repair. In the United States, corrosion is estimated to cost close to US$ 300 billion per year, eating away 3-4 percent of GDP. As much as 40 percent of this is preventable.

Stainless steel rebar has been around since the 1930s, but an overwhelming majority of concrete reinforcement is still made using carbon steel. Instead of reinforcing concrete, carbon steel rebar can do exactly the opposite: if humidity manages to ingress the structure, the rebar swells as it corrodes resulting in cracking in the concrete. The corrosion process is made far worse by chlorides from either atmospheric sources or de-icing salts on roads. In recent years, a large number of reinforced concrete structures have shown early signs of deterioration. Bridges have been closed for maintenance after as little as three or four years in service.

Lyndon Morgan, representing Outokumpu’s stainless-rebar technical expertise, speaks about the proper use of stainless steel in bridge construction. He does not advocate building all-stainless structures, saying, “Stainless steel should be specified selectively for parts where it makes a positive contribution.” These parts are in splash zones and the bridge deck. If carbon steel rebar is used, the bridge deck needs a water-proof membrane, and concrete must be of high quality. If stainless rebar is used, reduced concrete cover can be specified, and it is also possible to relax the design criteria with respect to maximum crack width. Mr. Morgan states, “With stainless rebar, bridges can be built either with no extra cost or for a lower cost than by using carbon steel reinforcement.”

Today LDX 2101® offers the most cost-effective alternative for durable reinforced concrete structures. Due to its good price stability, LDX 2101® offers construction projects vitally important predictability. The win-win outcome from the use of LDX 2101® rebar is much improved sustainability in our constructed environment.

The foundations of the Spire of Dublin. Photo courtesy of Anaco Trading Ltd