The future of concrete bridges
Bridge research has declined significantly over the last decade and there is a real need for establishing an active and focused bridge research programme if we are going to be able to face future challenges. The success of such a programme will depend on the support from asset owners, designers, suppliers, universities and research providers, says RUDOLPH KOTZE from the NZ Transport Agency.
With the merger of Transit and Land Transport New Zealand, the New Zealand Transport Agency (NZTA) is now responsible for maintenance and operations of the existing state highway network as well as delivering a substantial capital investment programme for new infrastructure.
Currently the state highway network is made up of about 11,000 kilometres of roads, more than 4000 bridges and large culverts. The combined length of bridges on the state highway network is over 140 kilometres. A breakdown of material and type of structure on the state highway network is: 35 percent in-situ concrete; 18 percent precast concrete: 34 percent culverts; and 12 percent steel. The rest is made up of masonry, timber and other bridges.
The George Street Bridge in Dunedin, completed in 1903, is thought to have been the first reinforced concrete arch bridge constructed in New Zealand, while Grafton Bridge in Auckland when it was completed in 1910 was the longest (97.6 metres)
reinforced concrete arch span bridge in the world. Prestressed concrete was introduced in early 1950 and after 10 years established itself as one of the most versatile bridge engineering materials. The Hutt Estuary Bridge (1954) was the first major prestressed concrete bridge designed and constructed in New Zealand.
In terms of age profile, it is interesting to note that the average age of the bridge stock is about 50 years with approximately 150 bridges older then 80 years still in service.
While there has been a dramatic increase in expenditure on roading projects, and while the pressure on value-for-money solutions has not changed over the last 50 years, there is an increased focus on the environment and sustainability. Over the past decade the impact of the Resource Management Act has resulted in the delivery of projects taking into consideration not only sensitive environments, but in the case of urban areas, the ‘wellbeing’ of the local communities.
From an asset owner perspective, lowest cost options are not good enough. Whole-of-life principles are critical in delivering structures – lasting at least 100 years and requiring minimum ongoing maintenance. The increased focus on durability is a challenge for all suppliers of materials and it will become increasingly important to be able to demonstrate sustainability of products and solutions.
There are many examples of recent projects that have already delivered value-for-money solutions in sensitive environments. One example is the Northern Gateway Toll Road – the extension of State Highway 1 from Orewa to Puhoi, involving the construction of 7.5 kilometres of dual lane motorway traversing a series of ridges and gullies, with a combination of viaducts, tunnels and earthworks. This project recently won the Infratrain Industry Best Practice Award for Environmental Sustainability.
Concrete has played an important part in delivering this complex project, particularly in the construction of the Otanerua Eco-Viaduct spanning the natural habitat of the Otanerua Valley. The project team used pulverised-fly ash in the concrete mix, which is a waste material from the Huntly power station. Health monitoring was required during construction and the use of precast concrete elements made sense when it came to ensuring protection of the biodiversity the waterways.
The Johnstone’s Hill tunnels eliminated the need for large-scale cutting into the environment and provided concrete mix design challenges in terms of construction of the concrete linings as well as protection of the structure against potential fire damage. In many ways this project has set the standard against which future projects will be measured.
In 2001 Transit embarked on a series of major projects to address connectivity and congestion issues in Auckland. The initial objective was to connect existing motorway networks with the central business district and port, reducing congestion by separating through and local traffic.
Delivery was undertaken through the Freeflow Alliance and what started out as the Grafton Gully Project turned into a wider Central Motorway Stage project. The alliance embraced the vision of leaving a legacy through delivering sustainable urban design, centred on the project structures and their place in the landscape. This involved paying respect to the area’s rich history and incorporating this into an integrated urban design.
According to Freeflow, “Concrete was selected as the preferred material for the structures because of its ability to accept urban design without incurring a significant cost premium”. The flexibility of concrete was demonstrated through the visible patterns, texture and forms produced, while the invisible technical application of self-compacting concrete in confined spaces made it possible to undertake difficult strengthening of Khyber Pass Viaduct while minimising traffic disruption.
In the same way that Alpurt set new benchmark standards for rural infrastructure, this project has set the benchmark for urban design. The issue of ‘sustainability’ in future bridge solutions will only become more involved, and while ‘whole-of-life’ costs will remain crucial in selecting solutions, the suppliers of materials will have to include carbon costs as part of total costs of production and manufacturing of materials.
There will also be an increased focus on durability and the need to minimise maintenance of bridges. Products and materials will be required to deliver far greater performance than at present.
The delivery of infrastructure projects in rural or urban environments will require increased sensitivity to the needs of communities they will ultimately serve. This will require more than just technical skills but an increased ability to engage with a wide range of people and professionals.
In order to embrace this future it is critical, Kotze believes, that research and development in the area of bridge engineering be undertaken now. The success story of bridge engineering in New Zealand, and for that matter the concrete industry, in the past has been dependent on a vibrant and active research programme. During the last 10 to 15 years research in bridging has decreased significantly and there is a real need to establish an effective and focused research programme.
Available funding for research has been very limited and difficult to secure. In order to ensure that effective research is undertaken, the broader engineering industry, including, major asset owners, universities, suppliers and practitioners will need to get together and develop a sustainable programme of focused research.
Contractor Vol.33 No.1 February 2009
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