Achieving sustainable and durable bridges

Sustainability, limited resources and environmental concerns are increasingly front of mind for bridge design and optimised maintenance of bridges. It’s important to look at whole of life costs as well as alternative materials to improve in service life and to minimise greenhouse gas emissions. We spoke to Dr Richard Yeo, Program Manager, Assets, at Austroads, to find out about some of the innovative research that Austroads has been undertaking on bridges and how the results could change the way bridges are designed and constructed.

Austroads is the peak organisation for Australasian road transport and traffic agencies, with its members collectively responsible for the management of over 900,000km of roads valued at more than $200 billion.

In order to support member agencies to deliver an improved road transport network, Austroads undertakes leading road and transport research to support policy developments, and provide guidance in the design, construction and management of roads and associated infrastructure.

Bridge Task Force on site at the Darlington Upgrade Project south of Adelaide Feb 2018.

In the area of bridge technology, Austroads has brought together representatives from state and territory road agencies, the National Transport Commission and the Australian Road Research Board, to form the Bridge Task Force. The Task Force is responsible for maintaining the Austroads Guide to Bridge Technology, preparing specialist reports, and arranging for research and testing. It considers all aspects of bridge planning, design, construction, operation and maintenance.

The Bridge Task Force coordinates research that is working towards:

  • Developing enhanced bridge design guidelines
  • Improving evaluation methods for bridge load capacity and associated traffic demand
  • Ensuring the Austroads Guides are up-to-date and better integrated with jurisdictional supplements
  • The revision of Bridge Design Standard AS5100

Two research reports have been released by the Task Force to help pave the way forward for achieving longer service life and increasing the sustainability of bridges.

Achieving a 100 year design life

Improving service life by examining different methods for concrete deterioration protection was the focus of the reports delivered through the project, Realising 100-year Design Life of Bridge Structures in an Aggressive Environment.

Dr Richard Yeo, Program Manager, Assets, at Austroads said the research looked at structures like the Gateway Bridge in Brisbane and the mechanisms available that will ensure a 100 year design life is achieved.

“The project involved a very thorough literature review and critical assessment of available means to ensure that the durability of those particular concrete structures would achieve that outcome,” Dr Yeo said.

“Usually bridge designers prepare a durability plan for concrete structures based on a common approach with specific reinforcement cover requirements and making sure all steel reinforcement is connected to allow for future cathodic protection if needed.”

Dr Yeo said the recent research found that it can’t be guaranteed that construction on-site will be as planned.

“You can’t always guarantee that the cover requirements will be met for the reinforcement and that impacts on the durability,” Dr Yeo said.

“At the end of the day, you might get critical zones of the structure, such as the piles, pile caps or interfaces, which are typically either underwater or buried and very difficult to access for later repair or replacement should early deterioration issues occur down the track. Typically that’s where we have problems, particularly in aggressive environments.”

The research looked at a wide range of options to prevent concrete deterioration, and the whole of life implications and costs, and came up with a different solution.

The study found that the most cost-effective long-term solution was the replacement of around 20 per cent of steel reinforcement with stainless steel reinforcement in those critical zones. It found that even if the construction is not quite right or the environment adversely impacts the structure, the stainless steel will not be affected, mitigating the issues that are often seen in aggressive environments.

Dr Yeo gave an example of a bridge in Cairns in Far North Queensland that was built 30 years ago which had significant issues with corrosion of the reinforcement in the piers just below the waterline.

“If 30 years ago they had used this type of approach, they would not have an issue, it would not be deteriorating in that way. Of course, to inspect and repair that structure in that environment, you have problems with crocodiles. Now just to go and look at the problem is a challenge, because it’s not safe.”

An in-depth look at durability

The findings are split into two related reports – experimental work and literature review.

The experimental work was informed by the extensive literature review and focused on concrete durability. The work comprised investigations into concrete deterioration mechanisms and their mitigation, including corrosion of steel reinforcement as influenced by chloride diffusion into concrete, carbonation of concrete, corrosion resistance of stainless steel, alkali-aggregate reaction and sulfate resistance of concrete.

New bridge at Darlington in place.

“The research team did a whole range of laboratory tests to validate that you could buy the stainless steel reinforcement, and what type of stainless steel you should use. That in an aggressive environment it would not deteriorate, compared with say a conventional reinforcement,” Dr Yeo said.

The literature review took a more in-depth look at the durability issues that affect the service life of reinforced concrete structures, and considered information from sources within Australia and overseas.

It looks at the range of options available for improving durability, including the use of epoxy to coat the reinforcement. The research considered the pros and cons of the different alternatives but found that there were issues that could compromise their suitability. For example, if an epoxy coating is scratched or there is a breakdown in the barrier, corrosion will be able to take hold where these defects occur.

“Everybody I talk to says yes, we’ve done a durability plan, and yes we’ve defaulted to what we always do. Yet, this report is more or less challenging that, and saying have you really thought about all of the options, and the whole of life cost implications of those? Because things like cathodic protection can be expensive and difficult to maintain over long periods,” Dr Yeo said.

“We’re talking 100 plus years. So if you have problems at year 30 and you put that in place, you’ve got 70 years of maintaining a system. Or you could have stainless steel built from the outset, and you won’t have a problem.”

Breaking down the science of geopolymer

Another key problem that the Task Force is working to find a solution to is greenhouse gas emissions. One of the concerns with using concrete for components is the large amount of greenhouse gases that are produced, and the energy required for cement manufacture.

Because of this, Dr Yeo said that people have been working to create alternative cement mixes and types of cement for decades, with one of these being geopolymer cement.

“Geopolymer concrete does not use ordinary Portland cement. It uses alternative materials, often by-products of other processes, which have a cementitious effect. The interesting part is it’s been around for some decades. But what was tending to happen in the market was that it wasn’t readily available, and what was available was proprietary product with limited information on the active ingredients in the mix,” Dr Yeo said.

The led to the Task Force releasing a report, Specification and Use of Geopolymer Concrete in the Manufacture of Structural and Non-Structural Components, that looked into the properties of geopolymer and how it works in concrete. As geopolymer is not typically used as a structural concrete, the research focused on the parameters around using it for structures.

Offsite assembly of steel bridge components.

“Importantly we wanted to look at what the active constituents and reactions are, to provide an explanation of how it actually works. The research sought to open up the chemical nature of geopolymer cements so that asset owners considering this alternative material can better understand what ingredients it should have, what properties they should be looking for, and indeed being able to make their own mixes without having to require or rely on a particular supplier saying, ‘Trust us, it’s good’, Dr Yeo said.

The Austroads funded research took place over five years and involved the Australian Road Research Board working in collaboration with Swinburne University. Dr Yeo said the challenge of the project was breaking down the chemistry of the product and trying to understand it, as geopolymer manufacturers were not willing to divulge this information.

The research team then undertook a thorough literature review to see what information had been published, and then started to formulate their own mixes using ingredients that they knew worked. The team was able to focus in on the types of mixes and materials used to manufacture geopolymer and were able to create and assess small scale samples up to full structural beams to prove that it could be used as a structural concrete.

Dr Yeo said while research and experiments can predict the performance and long-term durability of geopolymer concrete, the problem with new technology such as this is that you can’t wait 100 years to see how well it lasts compared to conventional concretes.

“We were able to do the accelerated deterioration tests, which take 12 months to two years, and look at how this concrete performs, relative to the standard Portland cement concrete.

“In fact, you can look at all the test data, and the trends show that this was a better product in durability than conventional concrete. So in addressing the problem of predicting the long-term durability, the observed positive trends give you some confidence,” Dr Yeo said.

“We followed up the research with a survey and asked who is using this as early adopters across our member road agencies? Are there some good examples of its use? The survey looked at road agency adoption of geopolymer concrete, specifications that might allow its use and barriers to adoption as well as experiences to date.

“The champions I think were VicRoads, they were absolute leaders. They had modified their standard specifications in five different areas to allow for geopolymer concrete used for, in this case, non-structural applications. We found that they had a wide range of concrete applications where they allow for geopolymer concrete mixes. The door is open for other road agencies to adopt or adapt the VicRoads specifications.”

Dr Yeo said estimates of the amount of greenhouse gas emissions savings that could be achieved by using geopolymer instead of Portland cement had been developed.

“If you were to build a dual carriageway, four-lane highway with geopolymer concrete as the pavement, as opposed to Portland cement concrete, around 2,000 tons of greenhouse gas emissions would be saved per kilometre.”

“So every time someone proposes a concrete road, I say, ‘Are you going to use geopolymer concrete?’ Because we’ve shown how it can be used,” Dr Yeo said.

The next challenge

Dr Yeo said that while research and specifications for new bridges is going well, the biggest challenge is managing Australia’s bridge stock to facilitate greater freight productivity.

“There’s a lot of work on assessment of bridges, and a lot of angst, I suppose, around the ability of older bridges to sustainably carry heavier trucks, particularly emerging high productivity freight vehicles, over mass super loads and large mobile cranes which develop concentrated loads, and allowing them access.

“We have certainly changed our views from ‘we can’t let them go because the bridge will be damaged’, to ‘what can we let go? How far can we push things? What is the real capacity and associated risk’,” Dr Yeo said.

“When you look at the bridge stock, a state road agency may manage 5,000 bridges and major culverts. You refine it down and you might have around 200 bridges that assessment indicates structural inadequacy for a certain vehicle mass and axle spacing. Means to work beyond the current AS5100 assessment provisions while managing the risks become a key consideration.”

An ongoing challenge connected to this is getting the funding needed to maintain or strengthen these bridges.

“There’s a lot of discussion at the Task Force around bridge assessment and what live load factors are required. What access we can provide, and when we can’t, we’ve really got to draw the line. Optimised asset management for bridges is a current focus of the Task Force.” 

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