FAILURE MECHANISMS OF PROTECTIVE COATINGS FOR CONCRETE
Michael Wheatland, Business Development Manager, Calix Limited
Regardless of the technology chosen, every protective coating will eventually come to the end of its life. Selecting the right technology gives a better opportunity to do this in a planned way. When selecting a technology to protect a concrete surface, it is critical to understand how and why they fail, and more importantly, the cost that will be incurred to repair or replace the coating.
Mechanisms of failure can range from blistering, increased porosity, cracking and sheet collapse due to wall adhesion failure. Some cracking failures can exacerbate acid attack by allowing accumulation of acid inside the failure. While sheet collapse, there is a risk of water flow blockage in channels at the bottom of the asset which can result in high emergency repair costs.
Some surface coatings can be very sensitive to surface water, humidity and dust on the surface during the application process, risking early life failure. Salt content that has penetrated a surface, called white metal, has a strong negative effect on the adhesion of polymer coatings, but assists in the adhesion of some chemical barrier coatings such as PROTECTA-Mag™.
Repair or replacement of a failed protective coating ranges from simply water washing through to an expensive confined space entry with hot work and manual handling to remove the coating. This can be particularly difficult when the coating has collapsed inwards and blocked the outlet of the asset. This whitepaper investigates the end of life decommissioning costs of the protective coatings available in the market to determine the whole of life cost of application of a type of coating.
Sulphide acid corrosion is the primary cause of concrete asset failure within waste water collection and treatment networks. The sulphuric acid is concentrated on all surfaces above the water line by the combination of condensation on the cold pipe with hydrogen sulphide gas that is generated from the wastewater.
There are a number of factors which contribute to how quickly acid forms:
Figure 1 – A pump station inlet well showing typical hydrogen sulphide corrosion
As there is no ‘Silver Bullet’ protection solution, understanding the whole product lifecycle of a protection system is essential in making an informed decision about the type of protection that would work best in a specific situation.
When Calix started performing demonstrations for local councils along the east coast of Australia a common theme that was repeated regularly was the unhappiness with premature failures of protective coating products, and the lack of support or solutions if there was an issue with the product. Often when the question of warrantee claim was asked after a failure, it was met with blame shifting by both the manufacturer and the applicator.
During a demonstration run in Mullumbimby, NSW we encountered our first major failure of a physical barrier where the adhesion between the coating and the wall had failed, resulting in the coating peeling off the wall like a banana peel and collapsing into the manhole, blocking the flow. We have since discovered that this is quite a common problem with epoxy and polymer coatings.
Since this initial experience at Tweed Heads we have come across many different types of coatings that have failed within waste water collection and treatment networks across Australia, New Zealand and the USA.
As a company, Calix has a culture of being open and honest about shortcomings, and we prefer to focus on the improvements that we have made along the way and how we have rectified any problems that have arisen. So, amongst these observations you will find examples of issues and failures of our own PROTECTA-Mag™ product.
This investigation is based on visual inspection of coating failures, investigated through anecdotes and discussion with asset managers and operators across the Australia-Pacific region and do not reflect the statistical results of any individual product or operator. But using these experiences trying to identify strengths and weaknesses of the three common coatings and demystifying the end of life behaviour.
Figure 2 – Example of a typical polymer liner failure
Although many wastewater systems may visually look good for many years after installation, if hydrogen sulphide is present the acidification process has already begun. A scratch test on the surface gives a better indication as to the depth of the acid penetration and concrete mass loss
Figure 3 - Without protection some concrete assets degrade quickly
Photos in Figure 3 show concrete surfaces which have been blasted with water or scratched to expose the extent of the concrete mass loss.
Figure 4 - Epoxy manholes showing initial signs of peeling near top lip (left) and blistering that traps acid behind (right)
The Photos in Figure 4 show epoxy polymer coating in the initial stages of failure where the edges peel away from the edges of the surface, and later in the failure stage where acid has been trapped behind the coating against the surface, concentrating the acid attack on the concrete. All physical barrier polymer coatings have a similar failure mechanism of peeling and bubbling as seen with the polyurethane in Figure 5 which can be very costly to remove at end of life.
Figure 5 – Lower polyurethane coating peeling away from surface while upper epoxy blisters due to bubbles in application.
The Calcium Aluminate Cement coating in Figure 6 has come to the end of the life, having been consumed by the acid within this rising main discharge manhole.
Figure 6 - Calcium Aluminate Cement consumed by strong acid attack
Magnesium Hydroxide coatings like PROTECTA-Mag can suffer from early life failure if the product quality and application process are not adhered to. The photos in Figure 7 show the issues that can be faced if the manhole lid is not sealed to prevent water ingress through the lid.
Figure 8 shows a PROTECTA-Mag coating that was fully in tact and neutralising the acid, but had been discoloured by a nearby gum tree releasing tannins into the stormwater which resulted in streaks down the hole.
If an appropriate procedure of Magnesium Hydroxide coating is not followed, coatings applied at the wrong thickness can result in the material cracking and falling off the concrete surface.
Figure 7 - MHL Coating taken off by storm water coming through lid.
Figure 8 - PROTECTA-Mag showing discolouration (left) and cracking (right)
When it comes to sewer corrosion coating protection system, owners of assets have a right to be cynical when assessing new products and technologies. Often the first experience that people have with coatings is dealing with a failed liner which is blocking the wastewater flow or dealing with an applicator that doesn’t want to honour a warrantee claim.
By understanding the technologies, asset managers can assess a pump station, inlet works or manholes based on their conditions, and assess full lifecycle costs of each product.
In all cases, doing something to protect the concrete assets is better than doing nothing. The alternative is uncontrolled failure of the asset resulting in unpredictable costs due to blockage, injury or other damage to vehicles or impact on businesses.
When looking to apply a product, the project manager should consider the consequences of early failure or end of life removal and reapplication of the product, as costs of removing a polymer coating can vastly exceed the initial application cost.
The best overall solution is to find a supplier you can trust to correct arising problems, continually improve their product and honour the warrantee.
Acknowledgement must go to the water authorities whom were able to assist us with the early testing and development of the PROTECTA-Mag product.
Tweed Heads Council, Byron Bay Council and Goldcoast City Council were instrumental to giving Calix the first full run of manholes.
Western Water was a great help to let us test and develop the product and application method within the network using manholes and pump stations.
Tauranga City Council in New Zealand were very understanding when we had an early failure of the coating due to the application process, allowing us to iterate the coating rectification process.
Perhaps the activity that resulted in the most progress in the shortest time was the performance of a Technical Assessment Group Study which involved Yarra Valley Water, Coliban Water, South East Water. Being some of the earliest trials of the PROTECTA-Mag product, the product and the process was not yet robust enough to cope in that environment. However, using the learnings from the application process and the inspections that followed allowed the Calix team to pinpoint the key failure mechanisms and adjust the product formulation and application procedures to mitigate these risks.
Gunderson, B. L. (1997). Process for protecting concrete sewer pipes. US Patent US5683748
Miller, T.M. (1998). Corrosion protection in concrete sanitary sewers US Patent US5834075