- Corrosion Experts
- Technical Groups
Monitoring the impact of corrosion on concrete infrastructure such as storage sheds, wharves and bridges is a critical aspect of ensuring structural integrity and durability performance.
A key way of minimising corrosion is to design for durability and employ the most appropriate technologies and prevention techniques.
Corrosion continues to impose a massive cost on asset owners and industry. This has been estimated, in a report issued by NACE (USA), to be more than three per cent—or many billions of dollars—of global GDP each year. Owners of high-value infrastructure assets must understand the cost implications of ignoring the effects of corrosion.
Concrete reinforcing steel corrosion is a worldwide problem that causes a range of economic, aesthetic and utilisation issues. Asset owners and managers operating and maintaining concrete infrastructure face different corrosion challenges depending on the industry sector in which they operate. The concrete degradation in the football pitch-sized storage sheds operated by Queensland Sugar differs from that of the bridges maintained by VicRoads.
Harsh environments—especially coastal, tropical or desert ones with high salt levels or extreme temperatures—can accelerate the rate of corrosion of steel in concrete. Usually, the most exposed elements deteriorate first but it may take 5 to 15 years for the effects of reinforcing steel corrosion to become visibly noticeable.
The two commonest causes of concrete corrosion are carbonation and chloride or ‘salt attack’.
The alkaline (high pH) conditions in concrete forms a passive film on the surface of the steel reinforcing bars, thus preventing or minimising corrosion. Reduction of the pH caused by “carbonation” or ingress of chloride (salt) causes the passive film to degrade, allowing the reinforcement to corrode in the presence of oxygen and moisture. As reinforcing bars rust, the volume of the rust products can increase up to six times that of the original steel, thus increasing pressure on the surrounding material which slowly cracks the concrete. Over the course of many years, the cracks eventually appear on the surface and concrete starts to flake off or spall.
Fred Andrews-Phaedonos, Principal Engineer – Concrete Technology at VicRoads, said the government road authority has a range of assets throughout the State that face degradation from a range of sources.
“The iconic Westgate Bridge carries massive loads in addition to being subject to high winds and salt spray,” he said.
Inspection of the many concrete culverts and low road bridges along the hundreds of kilometres of country highway has shown that their effective operational life is being reduced as the size of interstate road trucks increases. Many structures were designed for vehicles half the size and weight of modern trucks.
“Current estimates suggest that a proportion of Australia’s bridges are structurally or functionally deficient and require major rehabilitation, strengthening, improvement or replacement to bring them to current design standards,”
Queensland Sugar Limited (QSL) operates and maintains a range of assets, the major ones being its storage sheds and wharves from where raw sugar is loaded onto ships. According to David Edelman, Project Engineer at QSL, the company’s massive storage sheds—some of which are 45 metres wide and 400 metres long—also face a slow acting but pervasive threat.
“Sugar makes a mildly acidic solution that can slowly eat away at the concrete floors and walls of the sheds,” Edelman said. “This damage leads to a pot-holed, uneven surface and breaking of the concrete at joints, which adds to the difficulty of washing the floors in addition to presenting hazards to workers. The sugar forms a sticky, unsafe coating that builds up over time and makes work inside the sheds difficult meaning the floors have to be washed periodically.”
To minimise the damage caused by the sugar-attack, the walls and floors of the company’s storage sheds are coated with a sealer. Deeper holes and cracks are filled with epoxy, and joints are kept maintained to prevent sugar attacking deeper into the slabs. In addition to this chemical attack, the continual operation of large, front-end loaders moving hundreds of thousands of tonnes of raw sugar through the sheds and onto conveyor belts and ship loaders damages the concrete surfaces.
Similarly to VicRoads’ coastal concrete structures, QSL’s port assets degrade in the aggressive maritime environment.
For QSL this is exacerbated by them being located in the Tropics as well. The wharves and associated infrastructure at Lucinda, Bundaberg and Cairns are under threat of chloride attack, in addition to damage from tropical storms and cyclones.
“Correct interpretation of observations and testing is essential to a correct diagnosis and prognosis of the problem, and thus enable appropriate corrective measures to be taken,” Andrews-Phaedonos said.
The traditional method of concrete repair is to remove the cracked, delaminated and spalling concrete to a depth of 20-30mm behind the reinforcing bars to fully expose the rusted material and remove the contaminated concrete from the steel. All the corrosion affected material is then removed and the steel treated or replaced, after which specialist repair concrete mortars are applied and the surface made good. A modern development is for the repair mortars to be polymer modified to improve adhesion and resist further ingress of contaminants. Coatings are commonly used in combination with patch repairs to reduce further entry of carbonation or chlorides.
Edelman said that when the QSL jetty at Lucinda was built there was an issue with alkali silica reaction (ASR) causing cracking of the concrete. Chlorides have penetrated the concrete and caused premature corrosion of the reinforcing steel on parts of the structure. In some highly exposed parts of the structure this corrosion has caused extensive damage where elements have had to be repaired or replaced. However, in large sections of the jetty structure, the chlorides in the concrete have not yet reached a concentration where corrosion has initiated.
“The chloride concentrations have been monitored over many years and they are slowly increasing,” Edelman said. To counter this, QSL has started a program to apply an impregnating silane coating to the underside of the 5.7 kilometre length of the jetty to prevent further ingress of chlorides. “By putting this relatively inexpensive protection in place now, we can extend the life of the structure,” stated Edelman. “If we wait another 10 or 15 years the chlorides levels will have increased, corrosion will have started and it will be too late.” “The square-metre cost of a simple protective coating like silane is as little as 1/100th the cost of a concrete patch repair, but it is only effective before corrosion starts”.
Monitoring chloride levels, through core sample testing, allows a proactive approach.
All asset managers should get to know the chloride and carbonation profile of their concrete better, particularly if that concrete is aging and located in coastal environments. “Without a proactive approach, the first sign of a problem with a structure is typically when a piece of concrete falls off due to corrosion,” said Edelman. “At that point it may be too late for a coating to protect the remainder of the structure, and you may be up for some very large repair bills.”
A number of QSL’s assets have experienced significant corrosion and spalling of concrete over the years due to chloride ingress. Traditional patch repair, in many cases with replacement of corroded reinforcement, has been used, but with inconsistent results. “We have some patch repairs that are pushing 30 years old and remain in great condition,” Edelman said, “but others are beginning to crack and fail after less than 10 years.” However, one of the limitations of patch repairs is that it is often necessary to remove large quantities of sound concrete to solve the problem, causing extensive disruption and costing approximately $3000 per square metre.
One of the alternative methods of protection used on concrete, especially in marine environments, is Cathodic Protection. One type, Impressed Current Cathodic Protection (ICCP), is a technique whereby a small, permanent current is passed through the concrete to the reinforcement in order to virtually stop the corrosion of the steel.
Cathodic protection is relatively simple in theory. Anodes are inserted into the concrete at set spacing attached to the positive terminal of a DC power supply and connect the negative terminal to the reinforcing steel. Large amounts of cabling and permanent power supplies are required, making the technology really only suitable for commercial infrastructure. The initial CP current totally passivates the steel reinforcement, migrating chloride away from the bars and restoring an alkaline (high pH) environment in the concrete around the steel reinforcement.
Well designed and installed CP systems can achieve a 30 year or longer operational life.
One of the QSL conveyor tunnels has already had an ICCP system installed and the company is preparing to add the technology to a particular section of the Lucinda Jetty that is subject to near-constant wetting from waves.
“In this section, chlorides have reached a level where corrosion has begun and some spalling has occurred. Cathodic protection is a more cost-effective option compared to allowing the corrosion to continue and having to carry out constant repairs,”
Edelman stated. “There will be long term cost savings, which helps a lot – with the total annual spend for concrete repair and protection of around $1 million across the six terminals.”
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’, ‘blastfurnace slag’, ‘silica fume’, polymers, recycled car tyres and fibres. Some of this research has been published through 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.
According to Andrews-Phaedonos, the enhanced characteristics of fibre reinforced polymer (FRP) concrete include increased flexural and shear capacity of beams and slabs. FRP concrete is now regularly specified by VicRoads for repair and strengthening works.
“The material is thinner, lighter, non-corrosive and easier and quicker to install,” he said. “It also has increased axial load, bending, shear and confinement capacities.”
As a result of the research into concrete additives, 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,”
said Warren Green, a Director and Corrosion Engineer at engineering consultancy, Vinsi Partners.
By incorporating the by-products of other processes into the concrete 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,”
“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,”
To complement the Standards and support designing for maximum durability in specific situations, the Concrete Institute of Australia is developing a range of ‘recommended practice’ guidelines.
“VicRoads was the first State Road Authority in Australia to publish standard specifications for concrete maintenance work and has made a significant contribution to the preparation of Standards such as AS 5100 Part 8,”
As concrete infrastructure ages, corrosion prevention has to be as cost effective as practical. Owners and operators are being challenged to find better ways to maintain the integrity of their assets. Some of the factors that need to be considered include how long the asset has to remain in operation and would a shorter life extension be acceptable if maintenance has to be repeated more frequently.