Although steel reinforced concrete is one of the most widely used construction materials around the world, it can suffer degradation over time due to the embedded steel corroding, causing the concrete to crack and “spall”.
In extreme situations, the integrity of the structure may be lost, resulting in the need for partial or complete demolition. Corrosion affects all reinforced concrete buildings and structures to some extent, with an estimated annual cost of billions of dollars to national economies. In addition, loose damaged pieces of spalled concrete falling from buildings and structures is a real safety risk.
However, if corrosion effects are considered in the design phase and the right decisions made prior to construction, structures can be built to last and protected for far longer. An asset can be optimised or extended and maintenance time and costs can be reduced. In addition, reduced maintenance requirements can increase the asset’s overall utilisation and can improve its environmental sustainability.
Warren Green and Dr Frédéric Blin are Adjunct Associate Professors at Melbourne's Deakin University, working as part of the Australian Centre for Infrastructure Durability. The centre is a one-stop national platform for industry to access the combined research capabilities and testing facilities of a number of leading Australian universities.
“The life-cycle cost of a structure might be greater than 50 per cent of the capital value of an asset,” Blin, also AECOM Associate Director – Strategic Asset Management and Advanced Materials, says. “The reason for building an asset is to support the delivery of the intended service to an organisation’s customers. It is important to plan and budget for operational costs to minimise the risk of services being affected and large funds required to undertake reactive maintenance.”
When chlorides, carbon dioxide gas and other aggressive agents penetrate concrete, they initiate corrosion of reinforcement that typically results in cracking, spalling and weakening of the concrete infrastructure. As reinforcing bars rust, the volume of the rust products can increase to many times that of the original steel, increasing pressure on the surrounding material which cracks the concrete. The cracks can then propagate to delamination and eventually spalling of the concrete.
Usually, the most exposed elements deteriorate first but because the active corrosion may take five to 15 years to initiate cracks in the concrete, much of the actual corroded reinforcement is not visible. Such corrosion is often called "concrete cancer", because it appears as if the structure was being eaten away from the inside.
Advances in technology
During the past 30 years, there has been a lot of research into replacing some of the Portland Cement used in concrete with alternative components such as "fly ash", polymers, recycled car tyres and fibres. Some of this research has been supported by the ACA. "Fly ash" is a by-product from burning coal at a power station and incorporating fibres into a mix is similar to the old practice of adding horse hair to wet plaster. One particular area of research is in the field of geopolymer concrete, utilising alkali-activated binding agents.
As a result of this research, construction companies and engineering consultancies have access to all the latest technologies that yield a suite of proactive and reactive processes and procedures to maximise the effectiveness of reinforced and pre-stressed concrete.
“If you have all the appropriate specialists involved at the design stage it is very possible to have a design life of 100 years or more,” Green, a Director and Corrosion Engineer at engineering consultancy Vinsi Partners, says.
By incorporating the by-products of other processes into the mix, it has been possible to get “green star” ratings for different types of concrete. There is the challenge of thinking outside the box as to what might be incorporated into concrete in order to enhance sustainability and durability.
In addition to new materials being incorporated into the concrete mix, other additives have created self-compacting and self-levelling concrete, which can save both time and money. Off-site construction of pre-stressed concrete panels, under factory conditions, permits a far greater degree of quality control. “Advances in admixtures means that we can build almost anything out of concrete these days,” Green says.
Concrete corrosion and Australian Standards
According to Blin, the Australian standards relating to concrete have not kept pace with the changing ways of making concrete.
Current Australian Standards do not make any distinction between the many different materials that can be used to create concrete. “It is like baking a cake and saying the type of flour used doesn't matter,” Blin says.
“You would expect the Standards to tell you to be aware of different durability outcomes resulting from the mix ratio, temperature and constituents. Unfortunately, this is not the case at the moment which may result in missing an opportunity to optimise the amount of concrete required for a particular job.”
Blin adds that some people in the industry may still have an attitude of, “She'll be right mate; concrete is just concrete”.
“We need to recognise what people do with concrete and educate the industry that it is possible to 'damage' concrete even before it is poured,” Blin says.
Green says current Australian Standards
“The Australian Standards for concrete work gives basic guidance for normal situations, but in aggressive environments such as tropical, coastal, acid-sulphate soils, etc, a structure will not necessarily achieve its design life if simply designed and constructed to comply with the Standards,” Green says.
Corrosion proofing concrete
It is possible to build a structure so that it 'never' corrodes, although there is inevitably a cost involved. For example, during construction, cathodic protection (prevention) systems can be installed and coatings applied to assist in maximising durability.
The Gateway Bridge in Brisbane has a design life of 300 years with the piers in the river utilising stainless steel rebar and good quality concrete that has been carefully compacted and cured. “For this structure, durability and other specialist engineers were involved in the project, which also included QC testing during every stage of construction,” Green says.
Early failure of a structure results in maintenance costs being incurred well before the time for which they were budgeted.
Many buildings and structures around Australia are likely to be threatened by spalling and degradation. According to a report in the Brisbane Times in June 2015, many of Australia's Gold Coast high rise apartment towers built in the 1970s face million-dollar concrete cancer repair jobs. In 2013 the 20-storey Iluka Surfers Paradise high-rise—built in 1972—was demolished after concrete cancer destroyed the hotel's structural integrity.
According to Green, the demolition of high-rise blocks on the Gold Coast is a drastic situation but probably indicates that the original design and construction had limitations.
It is not just the design and construction that affects durability. There are structures across the region that have degraded before they should have, due in part to inadequate design or poor construction practices. When some concrete bridges in remote parts of Australia were built, modern large B-Double trucks and massive ore trains did not exist. As a consequence, many bridges are deteriorating much faster than anticipated due to additional fatigue stresses.
However, it is not all bad news – concrete corrosion can be repaired once it is identified.
Owners of reinforced concrete structures have to commit to spend money on maintenance. Brisbane City Council closed Brisbane City Hall for two years between 2012 - 2013 and spent $215 million on repairing and restoring the building.
By utilising the latest technology and materials to design and build for durability, maintenance costs can be reduced and the buildings and structures that are integral to the modern world can be effectively and efficiently used and enjoyed well into the future.
The Australasian Corrosion Association
The Australasian Corrosion Association (ACA) works with industry and academia to research all aspects of corrosion in order to provide an extensive knowledge base that supports best practice in corrosion management, ensuring all impacts of corrosion are responsibly managed, the environment is protected, public safety enhanced and economies improved.
The ACA conducts regular training courses and hosts seminars across Australia and New Zealand throughout the year. Specialist technicians certified by the ACA—and other organisations—have the experience and understanding of the causes, effects and remediation of a wide variety of corrosion types which allows them to recommend mechanisms and procedures to consultants and asset owners.
To complement the Standards and support designing for maximum durability in specific situations, the ACA is developing a range of 'recommended practice' guidelines.