Concrete Coatings For Waste Water Assets – A New Perspective
Waste water is a fascinating niche in the coatings sector because of the presence of chemicals, microbial reactions, climatic extremes, UV exposure and large surfaces.
It seems that because local council decision makers have strident budgetary constraints and are required to wear so many hats; engineer, administrator, coatings expert and contract negotiator that they look to coating manufacturers who claim to have specialist knowledge, as opposed to experienced experts who are able to provide a more balanced view. The downside is that manufacturers promote a one-side argument, shield their local council contacts from alternative coating options and they downplay the weaknesses of their own products.
On the other side of the equation, applicators often have challenging customer deadlines and they are applying specialty resins that have relatively small applications windows that rely on the weather. All too often we see coatings being applied as a matter of expediency with little regard for the climatic limitations or manufacturer warnings and the local council staff overseeing the project are none-the-wiser.
As a manufacturer of coatings, I have dealt with both local council employees and applicators. I believe that a successful, on-time and on-budget rehabilitation project takes the cooperation of all three, yet rarely, if ever, these 3 ‘interest groups’ get together and table all the various factors that need to be considered to get the best outcomes.
This article therefore has two primary objectives;
- to explain some of the coating considerations that are unique to the waste water sector; and
- to highlight a number of pertinent resin performance and application considerations so that local council decision makers and onsite engineers are better informed.
The 3 main coatings being applied in the waste water sector are epoxies, polyureas and resin coatings. Having worked with a polymer chemist for 10 years developing vinyl ester, polyester and epoxy resin formulations, I have a very good practical and theoretical understanding about them. I have also seen them being applied and I know how they age. However, I have only a limited practical knowledge about polyureas, though I have read about them and have seen how they age.
While most large councils have some capacity to take assets out of service for periodic maintenance and repairs, some of the smaller councils I have visited have very limited windows to carry out their recoating and maintenance. Fast installation routines are therefore essential if assets need to be put back into service as soon as possible, and factors such as high build, shrink, rapid cure and recoating intervals have added importance while material cost considerations are off lesser importance. For some reason rain, moisture and application temperature limitations are all too frequently ignored yet they are probably the most significant limitation of all!
With coatings, adhesion is everything. I say this because polymer relining systems have been applied internationally since the 1980’s to combat the increasingly acidic nature of sewer and waste water networks. They work well, though experience tells us that most coatings fail to last 10 years and most applicators provide only a 2-5 year warranty. They have a history of breaking down over time, mainly because the coating starts separating from the substrate due to:
- The surface not being properly prepared;
- Coatings being applied without a primer (!);
- The presence of moisture when they are being applied and while they are curing, some more so than others;
- Entrapped contaminants under the coating are absorbed into damp concrete surfaces. The mixture of hydrogen sulphide (H2S), ammonia, carbon dioxide and methane and other contaminants inevitably reacts with the available moisture, lowers the pH level of the concrete, and destroys the adhesive bond between the coating and the concrete substrate;
- Water vapour, CO2, O2, SO2, H2S and other small molecules permeate through the polymer over time, and when they get to the concrete surface they too react with the available moisture and destroy the adhesive bond;
- Epoxies are hydroscopic, which means that they continue to absorb moisture until they set hard so, if it rains or dew forms before it has fully cured, the coating’s long term performance will be impaired.
- Failing to test and reset the concrete surface to a PH level between PH7-11 (the higher the better) before applying the primer. This is an incredibly important issue because if a surface is acidic and is not ‘neutralised’ the entrapped acids will continue to break down the substrate, in which case it would not matter how good the protective coating was.
So to achieve the best outcomes you always need to:
- check the PH of the concrete before applying a concrete primer, and if need be apply an alkaline solution to lift the alkalinity of the surface; and
- apply a primer that sinks into the surface (the lower the viscosity the better) in order to seal and protect the substrate because primers eliminate the micro-spaces that would otherwise exist between the coating and the substrate, where entrapped moisture and contaminants would otherwise reside and react below the surface.
Products that cure rapidly in the presence of moisture (which is ever-present in porous concrete) will also have better adhesion.
Coating manufacturers and applicators place a major emphasis on coating thicknesses. Fortunately Sydney Water’s WSA 201 Manual for Selection and Application of Protective Coatings specifies a dry film thickness between 2000 microns and 4000 microns yet we frequently see 6mm coatings being specified on waste water projects. Not surprisingly coating thickness specifiers tend to follow the manufacturers’ recommendations and the perception is that thicker is better, which is only great for the manufacturers and applicators.
I just need to say this, polymer coatings themselves seldom break down before the adhesion bond, so if a local Council really wants to save money this is where it is possible to achieve tangible savings.
Other Product Variables
Having previously sourced and resold epoxy formulations I can tell you that the cost of epoxy Part B’s can vary by 400%, and only the more expensive Part B’s are water resistant, so how would a local council ever know the quality and properties of the epoxy Part B formulations they are receiving?
UV protection is another huge unknown when it comes to protective coatings. All polymers are affected by UV to varying degrees. A specific polymer may have good resistance to a particular weathering factor but then may be highly susceptible to degradation when exposed to other conditions. There is a mountain of research available online and most of it is quite complex. I will not bore you with UV science but I will make a few sweeping statements based on my industry experience:
- Polymer researchers have confirmed that a combination of hindered amine light stabilizers (HALS) and UV light stabilizers provides the best protection.
- UV inhibitors in vinyl esters and polyesters perform better than in epoxies and polyureas.
- White pigmented coatings are better at reflecting heat.
- Heat is one of the main reasons why polymers age. With our rapidly increasing summer temperatures this is going to become an even bigger issue in the future.
So, how would you know what UV protection you are actually getting when manufacturers do not describe their UV packages? This is a significant issue because the International Bureau of Biometeorology has advised, “UV exposures in Australia have been increasing between 2 and 6 percent in each of the last 20 years.”
All the more reason why local councils need to ask a few more questions before selecting their coatings.
Selecting an experienced applicator (who understands the material he is applying and how to apply it) is fundamental if you want to apply a coating that will last, yet frequently we see price driven decisions that result in the use of inexperienced applicators who have little or no industry experience and are not familiar with the materials they are applying. I also have a problem with tender processes where local applicators are winning projects with their 20% local pricing ‘discount’, even though they may be applying an inferior product and are using people who have little or no practical experience.
Speaking as a coating manufacturer it does not make any sense whatsoever because the risk of failure and defects drastically increases when you are relying on inexperienced applicators with little knowledge of the product they are applying.
Substrate Condition and Bond Strength
Waste water assets are continuously exposed to cocktail of chemicals that attack the surface. It seems that most people think that if they apply a protective topcoat they are guaranteed to get the right outcome. Nothing could be further from the truth! Let me explain why:
- Substrates that are already in poor condition will probably continue to deteriorate unless something is done to restore their structural properties;
- Entrapped contaminants within the concrete will continue to react with the available moisture in the concrete;
- The protective coating will probably pull itself off the surface if the surface is unable to support the protective coating.
Local councils therefore need to assess and specify what they want done to protect, seal and maintain the strength of their concrete substrates. This is critically important because of the chemical exposures in the waste water sector. Specifying PH testing, adhesion tests and the use of a primer that sinks into the concrete should be standard practice.
It is also worth looking into how applicators store their resins onsite because the viscosity of most polymers will halve or double with every 5⁰C change in temperature, which affects how well a primer will penetrate the concrete. Experienced applicators are probably used to the idea of properly storing their resins in order to get the best and most consistent results!
Remember, adhesion is everything when it comes to how well protective coatings perform.
Epoxies don’t smell so everyone thinks they are OK. Polyesters and vinyl esters on the other hand contain styrene, a small and pungent molecule which is quite noticeable. People automatically think that because it smells it must be more harmful than epoxies, yet nothing could be further from the truth. Extensive international studies have concluded that styrene is “non-toxic”, which is backed up by our Australian regulatory standards. Conversely, 2-pack epoxies give off carcinogens and have been banned as floor coatings in North America and Europe, yet they are still widely used here in Australia, and people still think they are safe to use!
Here are a few other unknown facts about styrene;
- it has a half-life ranging between 7 and 16 hours;
- you can smell styrene when concentrations are less than 1 part per million;
- styrene is found in every piece of plastic in your kitchen and also occurs naturally in foods such as cinnamon, beef, coffee beans, peanuts, wheat, oats, strawberries, and peaches;
- it smells while it is curing, but the smell is temporary.
All this means is that sometimes things are not what they seem.
Moisture at the time of application affects all polymers to different degrees. Depending on the product being applied moisture on the surface and adhesion numbers can go from awesome to deplorable in one brush stroke.
Moisture certainly affects epoxies, because epoxies are hydroscopic and they take a long time to cure. Let’s face it, most waste water assets are either wet, damp, moisture saturated, highly humid, or wet from the surface preparation so this is a big issue. There are epoxies that claim moisture tolerance, or even the ability to be applied underwater to various substrates. Unfortunately, the performance of the so called ‘moisture tolerant epoxies’ varies greatly and relies on the applicator being really diligent so local councils should be wary.
Polyureas are also affected by surface moisture because the moisture is entrapped between the epoxy primers they use and the impervious polyurea coating.
Grouts and Patching
I am surprised how often cement-based grouts and patches are recommended when light weight, moisture tolerant, trowelable vinyl ester and epoxy grouts are available. Cement-based grouts bond poorly to existing cements, take way too long to dry and tend to suffer the same problems experienced by the original damaged substrate. In this area it appears that price or tradition is guiding the specifiers.
Vinyl esters and epoxy coatings can also be used with fiberglass cloth to create structural linings that are capable of bridging large gaps and providing substantial reinforcement.
Pinholing is more of an issue for epoxies and polyureas. It is caused when air is trapped below the coating. The air expands because of the differential heat between the concrete and the coating and it expands and makes its way to the outer surface before the coating has cured. Vinyl esters and polyesters do not tend to experience the same issue because of the nature of their curing process.
One of the other ways pinholes arise is when coatings are brushed on and the brush strokes create pockets of entrapped air. This is less of an issue when products are sprayed on or when multiple brushed coats are applied.
Pinholing is largely solved by applying primers that seal the concrete before topcoats are applied.
There are many different products and approaches to concrete protection and much or most of the information available is predisposed toward a particular product or system. All too often the selection of a particular coating or use of a particular applicator is based purely on pricing considerations and yet critical factors are overlooked, such as:
- applicator experience;
- material selection considerations (such as UV stability, heat tolerance, cure characteristics, primer viscosity);
- surface preparation;
- the presence of moisture during application;
- substrate and coating material temperatures;
- QC adhesion testing;
- minimum coating thickness.
At times it seems that people fail to appreciate that a deficiency with regard to any one of the above factors will ultimately determine whether a coating application is going to be successful or not.