Buildings and structures that incorporate concrete and exposed metal are subject to corrosion. The nature of corrosion varies based on the type and location of the structure. Globally, corrosion-related catastrophic failure of infrastructure has been most visible in the collapse of bridges. There is a history of bridges that have failed due to corrosion, frequently with a loss of life. Examples include the Mianus River Bridge (Connecticut, USA, 1983), the I35 Mississippi River Bridge (Minneapolis, Minnesota, USA, 2007), and the Ponte Morandi (Geneva, Italy, 2018). Fortunately, in Australia and New Zealand there have been no examples of catastrophic corrosion-caused infrastructure failure. 

The impact of corrosion is not limited to major infrastructure. It also impacts on residential buildings throughout both countries. While many investigators have researched the causes, impacts, remediation and prevention strategies for corrosion in the construction industry, there are no comprehensive details associated with the costs associated with corrosion. The cost of corrosion-related maintenance of infrastructure (e.g. bridges) in Australia is currently estimated to be $A8 billion.

Geographic Influence on Corrosion

The primary factor influencing corrosion of structures in Australia and New Zealand is salt. As a result, the distance from saltwater is a factor in the rate of corrosion. Both Australia and New Zealand have been mapped to measure the likely rate of corrosion. Factors that impact include distance from the ocean, prevailing wind direction and rainfall intensity. 

Latitude has a significant effect, with lower rates of corrosion in tropical areas, while corrosion increases in higher latitudes. Salinity levels and corrosion rates are much higher on Australia’s southern coasts than in the northern coasts. Incidence of corrosion is higher in areas close to saltwater, while high rainfall has an effect of washing the salt residue, reducing the rate of corrosion. Coastal landforms, humidity and vegetation also impact on the transport of aerosol salt. In northern Australia, the effect of high intensity rainfall effectively cleans exposed metal surfaces, reducing corrosion.

New Zealand has differentiated building codes based on the corrosion risk of the building site. In New Zealand, geothermal areas with high levels of atmospheric sulphur also experience higher rates of corrosion. In these areas, corrosion of mild steel was found to increase rapidly in the first six to nine months, then decrease slowly. However, the corrosion rate measured after one year was still higher than that measured after one month. Similarly, when exposed to sulphur-containing geothermal emissions, zinc corrodes quickly. Sulphide-rich clusters form on its surface, but can be removed easily by gentle rubbing, and potentially by rainwater. Claddings made of copper and copper alloys have been used in many circumstances mainly due to their high corrosion resistance in many environments. However, copper was found to corrode severely even in areas with very low concentrations of airborne hydrogen sulphide. 

Causes of Corrosion

Concrete/ Steel Reinforced Concrete

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”. The chemicals involved include chlorides, carbon dioxide and other aggressive agents. These chemicals penetrate concrete, which initiate corrosion of reinforcement that typically results in cracking, spalling and weakening of the concrete infrastructure. The reinforcing bars will rust, and in doing so, the volume of the rust increases to many times that of the original steel. This in turn increases pressure on the surrounding material which cracks the concrete. The cracks can then propagate to delamination and eventually spalling of the concrete.

Bridges

Degradation of bridges is caused by many different factors including corrosion and other stresses from both the environment and heavy vehicles passing over them. In 2018, the Australian Government committed funding for 186 additional projects to the 201 bridges already being remediated. The cost of the current remediation is $A216 million39

New Zealand’s NZTA is responsible for maintaining approximately 2,300 bridges. The largest and most iconic, the Auckland Harbour Bridge, handles 160,000 vehicles per day. The bridge is a steel truss and box girder design. The corrosion prevention process for the bridge has been a continuous painting process using a cured urethane paint that provides 20-year protection. Many older timber rail bridges nearing the end of their useful life are being replaced by ‘weathering steel’ girder bridges which should provide a longer operational lifespan. Weathering steel is a high strength, low alloy steel that, when used in environments not exposed to high levels of salinity and pollutants can be left unpainted. This allows a protective rust “patina” to form and minimise further corrosion.

Many bridges in the New Zealand highway network are constructed from precast pre-tensioned concrete. These were built between 1950 and 1980. They are at risk from chlorine-induced pre-tensioned reinforcement corrosion. While the deterioration may be difficult to detect, it has structural implications, so prediction or early detection of at-risk structure is crucial to ensure the bridges achieve their required service lives. Because of difficulties specific to pre-tensioned concrete, corrosion can lead to the replacement of the entire bridge superstructure. 

Corrosion was discovered in the Hamanatua Stream Bridge in Poverty Bay, New Zealand in 2004. The chloride ions had penetrated the cover concrete and their concentration at the steel’s surface was high enough to cause the steel to corrode. Following this discovery, an investigation was conducted into bridges using the same design throughout New Zealand. In the case of Tiwai Point Bridge in Southland, the entire superstructure of the bridge was replaced due to the difficulty and cost associated with restoring the lost capacity and arresting severe, widespread corrosion within the pre-tensioned beams. A further 137 bridges have been identified as being at risk. Most are located within 1Km of the coast. Inspections were carried out on 30 bridges, and all but one exhibited signs of corrosion. The implications involve costly repair, and the risk of structural failure with minimal warning. Two were identified as needing prompt assessment and remediation.

In another corrosion scenario, two Southland bridges were found to have an alkali silica reaction, with extensive cracking, spalling, and surface erosion below the water line. Chemical reactions in the concrete were causing it to expand and contract. 

Examples of Corrosion Impacts

Ohau Diversion Wall, Lake Rotoiti, New Zealand

A diversion wall was built across Lake Rotoiti with the aim of diverting nutrient-rich water from the lake into a diversion channel, which in turn would improve water quality, reduce algal blooms, and restore the lake fishery. The construction was completed in 2008, and consisted of a 1,275m sheet pile wall, 75 metres offshore. The wall was designed for a 50-year life, based on an assumed corrosion allowance of 0.04mm/year of the uncoated steelwork in the freshwater and 0.015mm/year for the steelwork embedded in the soil. The steel in the water was found to be corroding at the rate of 0.5mm/year in 2014. As a result, the wall was expected to collapse within seven years. Options for remediation were investigated and costed, with costs ranging from $NZ1.3 to $18.1 million.

TransGrid Transmission Line, Sydney

TransGrid provides infrastructure to deliver electricity in NSW. Line 959/92Z was constructed in 1965. It includes 61 transmission structures and extends for 23.7Km through parts of Sydney including national parks and urban areas. A significant proportion of towers are corroded, and there is a risk of conductor drop. The risks associated with this include a significant electricity outage through parts of Sydney, and the risk of bushfire. The cost of remediation has a budget of $A7.13 million.

Catagunya Dam, Tasmania

Hydro Tasmania has several dams which were designed and constructed in the 1950-70s with fully grouted, post-tensioned anchors. While the method used was leading edge in its day, it does not achieve the cable protection of modern methods which provide two barriers against corrosion and are monitorable. Inspection of the anchors in the 1990s identified that corrosion of the grouting meant remediation and re-anchoring would be required, with significant re-construction costs.

Water Storage Tanks, South Australia

SA Water has responsibility for 700 concrete water storage tanks. Many of these tanks are approaching 100 years of age, having been commissioned in 1920s. SA Water is now using Remotely Operated Vehicles, submersibles and drones to spot the beginnings of corrosion. The cost benefit is significant, as for each tank whose life can be extended, there is a saving up to $A1 million a year.

“New water tank design doesn’t include guttering nowadays, which means water can pond at the base of the tank and depending on the surrounding soils, can accelerate concrete corrosion,” Jonathan Morris a senior asset management consultant explained. Concrete sewers are also subject to corrosion from chlorides, sulphates, thermal cracking, and other challenges that pipes carrying organic waste need to withstand. Exhumed sewer pipes are often found to be very thin at the crown, where acidic condensates formed by microbial action on hydrogen sulphides have eaten into the concrete. In extreme cases, this can result in complete loss of the pipe wall. “Acidic and high sulphate conditions are very bad for concrete,” Morris explained. “A slime layer forms below the surface level of the wastewater, which houses bacteria that convert sulphates into hydrogen sulphide. When the hydrogen sulphide escapes from the water, it can be converted into sulphates in the above-water slime layers, which are converted into powerful acids by other bacteria.”

Housing

Where construction is from timber, metal is used as a fastening tool. Nails and nail plates are subject to corrosion. The rate of corrosion also varies based on the nature of the roof structure. Houses with concrete tile rooves were found to have a higher rate of nail plate corrosion than those where alternate roof material was used. The inclusion of building paper in the concrete tile roofs mitigated the effect of the corrosion. 

While corrosion of metal fasteners is partly as a result of salt, it is also caused by the action of the arsenic used to treat the timber. Specifically, three compounds were tested. Alkaline Copper Quaternary (ACQ) and Copper Azole (CuAz) were contrasted with Copper Chrome Arsenic (CCA). Test results showed that the two alternate treatments resulted in higher levels of corrosion of the metal fasteners than the traditional CCA treated timber. Subfloor spaces were also identified as being a risk-site for corrosion in coastal areas, with the possibility that sea salt could be transported into the sub-floor cavity.

The impact of corrosion on housing stock can affect other surfaces including metal roofs, metal wall cladding, aluminium joinery and metal fixings,. When two metals are in electrical contact while in the presence of moisture or another corrosive electrolyte, there is enhanced aggressive corrosion at the joint area. For example, when galvanic zinc coating is damaged and steel is exposed, there will be aggressive corrosion of the steel. Similarly, when aluminium alloys are joined to steel or copper, the aluminium will corrode more quickly than would otherwise have been the case. Other corrosive electrolytes can include fluid waste and bird guano. 

In aluminium joinery, corrosion can occur when crevices in the aluminium expose the aluminium under the barrier of aluminium oxide. This rarely occurs due to the rapid oxidation of the exposed aluminium. However, in the presence of mortar, bricks and concrete, a chemical reaction may occur between the aluminium oxide and the matrix. The result is the formation of aluminium hydroxide and hydrochloric acid. The acid can then corrode the metal.

In aluminium joinery, corrosion can occur when crevices in the aluminium expose the aluminium under the barrier of aluminium oxide. This rarely occurs due to the rapid oxidation of the exposed aluminium. However, in the presence of mortar, bricks and concrete, a chemical reaction may occur between the aluminium oxide and the matrix. The result is the formation of aluminium hydroxide and hydrochloric acid. The acid can then corrode the metal.

Corrosion Prevention and Management

When corrosion effects are considered in the design stage of an infrastructure basis, structures can be built to be protected and to last longer60

A change in the constituents of cement can have a positive impact on corrosion. The commonly used ‘Portland Cement’ can be replaced with alternative components such as ‘fly ash’, polymers, recycled car tyres and fibres43. Incorporation of these products can reduce the rate of corrosion, with a corresponding increase in the life of the structure.

Further enhancements can be made to corrosion protection in reinforced concrete by using a two-stage corrosion protection system that may combine cathodic protection, galvanic protection and electrochemical treatments.

Historically, testing has been conducted to measure electrical resistivity (the inverse of electrical conductivity) and a chloride and sulphate measurement to measure the degree of corrosion hazard. For concrete testing, laboratories are now offering a service that measures a much broader range of potential risks including the following:

Soil Tests: pH and EC, Texture, Permeability Class, Sulphate ion, Chloride ion, ResistivityWater: pH and EC, Sodium ion, Magnesium ion, Calcium ion, Ammonium ion, Sulphate ion, Chloride ion, Co32 and HCO3, Calculated CaCo3 saturation index.

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