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AGRU America representative Cody Miles sat down with AGRU engineer Ernie Heins to discuss the problem of microbiologically induced deterioration, or MID, in concrete and how concrete protective liners can provide an all-in-one solution to help protect against this costly outcome.
Cody: First, what is MIC and how does it typically affect a project? In other words, why is it a problem?
Ernie: In simple terms, MIC is corrosion resulting from chemicals produced as a consequence of microbiological activity. For instance, in anaerobic conditions in a municipal wastewater environment, hydrogen sulfide is readily produced in sewer systems. The hydrogen sulfide (H2S) generated rises and interacts with aerobic bacteria residing in a slime layer which develops on the concrete walls of sewer structures. In this interaction the H2S is oxidized into sulfuric acid (H2SO4) which as an acid can readily attack the calcium carbonate constituents of the concrete structure ultimately resulting in a total structural failure. Obviously, the requirement for replacement and/or repair of these structures comes at a significant cost.
Cody: There is a growing library of literature regarding the development of a gold standard that counteracts the MID problem. What is the gold standard?
Ernie: The gold standard is a three-pronged approach with multiple levels of attack against corrosive microorganisms. I think of it as “a belt and two pair of suspenders scenario,” and it would most likely be considered overkill or cost prohibitive in most circumstances.
Cody: That’s an interesting way of putting it. What are these three layers?
Ernie: The first layer of defense uses concrete admixtures to alter the concrete composition to decrease the rate of corrosion. The second layer of defense involves chemical intervention to control MIC by raising the pH of the sewage, and thereby inhibiting microbiological growth. The last layer of defense involves a protective lining (such as AGRU’s concrete protective liner products) or coating to act as a barrier layer for the concrete and inhibit biofilm growth.
Cody: Are these three solutions equal in their effectiveness? How do they interact with one another?
Ernie: While each of these methods may prevent corrosion to some level by themselves, they are not all equal in their effectiveness. Also, as I noted earlier, the full implementation of this integrated solution is not always possible. In fact, it is quite unlikely to do so. Therefore, since it is unlikely that all three or even two of the three would be used, it should be noted that the order in which they are presented as “lines of defense” should in no way to be considered an order of effectiveness.
Cody: So their order in the list is random and their ranking in terms of effectiveness actually differs. How would you rank them?
Ernie: I’d say the least effective among the three is chemical intervention because it carries a significant, recurring cost and requires diligence in ongoing applications. Additionally, chemical intervention can lead to environmental problems as a result of unintended chemical reactions. Therefore, I would consider this a last line of defense to be used only in the most critical applications.
Cody: What about concrete admixtures and concrete protective liners?
Ernie: Concrete admixtures would be the next most effective even though, in this scenario, the concrete is still accessible by microorganisms to a lesser degree. However, we have seen critical projects such as sewer tunnels where admixtures were used as a second line of defense behind concrete protective liners and they can offer a sort of “back-up plan.”
Concrete protective liners undoubtedly provide the most effective line of defense, even beyond coatings. This solution acts as a barrier to prevent access of the microorganisms to the concrete. Concrete protective liners are manufactured from thermoplastic resins that are resistant to the attack of the acids produced by MIC. Therefore, they should always be considered the first step in a multi-staged defense.
Cody: Regarding protective coatings, concrete protective liner is a viable alternative. What is concrete protective liner and what makes it different from typical coatings used to protect concrete surfaces?
Ernie: Concrete protective liners are produced from thermoplastic materials and utilize locking extensions such as anchors, studs, or ribs to mechanically anchor the liner into the concrete. Due to this mechanical attachment concrete protective liners are typically more resistant to hydrostatic backpressure than a coating. In fact, AGRU’s Ultra Grip offers the highest backpressure resistance in the industry. In contrast coatings are typically only tested for pull-out resistance. For this reason, these products also offer superior protection against infiltration and inflow in addition to the corrosion resistance being primarily discussed in this article. Furthermore, the durability and life expectancy of a concrete protective liner far exceeds that of a typical coating, and this solution does this with little to no maintenance.
Cody: The benefits of concrete protective liner seem to stem from its longevity and ability to protect the concrete for more than 50 years with few maintenance requirements.
Ernie: Yes. HDPE geomembranes used in non-exposed conditions have been estimated by the Geosynthetics Research Institute to have a half-life of up to roughly 450 years. And they may perform much longer beyond their half-life depending on the applicable field conditions. Similarly, HDPE concrete protective liners used in tunnel applications are often assigned a design life in excess of 100 years as this is the typical design life of a tunnel. As noted above, this product is practically maintenance free, and is also repairable at any point in their life should damage occur.
Cody: AGRU has produced a concrete protective liner product, Sure-Grip, for decades. How has this product performed over the years? What are some metrics of its success?
Ernie: AGRU concrete protective liners have indeed been successfully used globally for decades. In the United States, we have Sure-Grip that has been installed in active manholes for over 25 years with no significant maintenance or performance issues. However, I would say that some of the more impressive metrics can be witnessed in the large diameter sewer tunnel projects that have been completed. These tunnel projects often present some of the most complex installations coupled with the most demanding environments.
Cody: Could you describe a project like this?
Ernie: Yes, there’s the Deep Tunnel Sewer System (DTSS) project in Singapore. It has been in service since 2005 and is one of the first projects of its kind for AGRU Sure-Grip. The tunnel is approximately 20 feet in diameter and up to 150 feet deep and 120 feet below the water table. The system was designed with a life expectancy of over 100 years. Several similar projects have followed suite since using AGRU concrete protective liners.
Cody: I know in recent years your department announced an innovation to Sure-Grip’s design and have been calling it Ultra Grip. What are some design differences between Sure-Grip and Ultra Grip?
Ernie: Both Sure Grip and Ultra Grip have AGRU’s iconic “V” shaped anchors that are 13mm in height. However, while not always noticeable at first glance, AGRU has redesigned the anchors on Ultra Grip with a couple of slight engineering modifications which results in significant performance improvements. These modifications have almost doubled the pull-out resistance and have significantly increased the long-term backpressure resistance. In the past year AGRU has fully completed the transition from Sure Grip to Ultra Grip in all polyethylene and polypropylene concrete protective liners.
Cody: In the context of MID/MIC, how is Ultra Grip an effective solution?
Ernie: The critical stage of MIC is when the pH of the concrete has been neutralized to point that a biofilm develops on the concrete surface. At this point, the microorganisms are free to thrive and accelerate the generation of acids that corrodes the concrete. A properly installed concrete protective liner creates a barrier so that the microorganisms are never in contact with the concrete, thereby eliminating this condition in its entirety. Ultra Grip’s high-performance anchor design ensures that the concrete protective liner performs in the toughest environments.
Cody: It is pretty clear from the literature that there is some lack of awareness surrounding concrete protective liner, which means it is likely many individuals are not experienced with incorporating such a solution. How does a project lead or engineer typically circumvent this roadblock?
Ernie: Too often engineers and agencies responsible for MIC protection rely on and are complacent with whatever systems they have used in the past. In addition, the number of products and amount of information can also sometimes be overwhelming to digest. AGRU’s technical staff and sales engineers do their best to reach out to design engineers and municipal districts (as well as industrial clients) to detail the benefits and design methods of concrete protective liner through lunch and learn presentations, webinars, one-on-one meetings, trade shows, and other avenues. However, although certain theme parks may want you to believe otherwise, it is NOT a small world after all! If someone is interested in learning more I suggest that they contact AGRU to discuss their needs and to develop a solution that best fits their situation. And if we do not believe AGRU is a proper fit for your specific application we will always advise accordingly.
Cody: What about legacy projects in need of rehabilitation or projects that have previously used a spray-on protective coating? How would concrete protective liner be used in these situations? Is it economically viable?
Ernie: Certainly. It is fairly well documented that structures with coatings will occasionally need to have the coatings reapplied. In order to properly reapply coatings a substantial amount of surface preparation is typically required and the appropriate conditions necessary for reapplication may be difficult to achieve in a structure that is/has been in service. Bypass pumping will likely be required and traffic patterns may require alterations which can significantly add to the cost.
Existing structures (whether previously lined with coatings or not) can be rehabilitated using concrete protective liner in most instances. And rehabilitation with concrete protective liner will ensure a long-term fix over coatings. However, there are sometimes limitations that make this task much more difficult such as when a manhole cone section cannot be removed to allow adequate access. In either instance rehabilitation costs far exceed those costs associated with adding appropriate protection on the front end in the initial construction whether coatings or concrete protective liner is applied. And it makes sense to choose concrete protective liner at this time due to the long-life expectancy. Therefore, it becomes more important and fiscally responsible to take appropriate measures for MIC protection in the initial construction and to make the appropriate decisions when doing so.
Cody: Last question: How do you envision we will deal with MID in the future and what are some likely innovations you believe will change the way we use concrete protective liner?
Ernie: I sure wish I had a crystal ball to see exact the innovations. Innovation and technology always seem to find a way to meet increasing expectations and demands. I expect that resin manufacturers will continue to improve the performance of concrete protective liners. I also expect that resin converters such as AGRU will continue to innovate as they did with the development of Ultra Grip. For over 70 years AGRU has always been a leader in R&D and proactive in striving to constantly improve. I certainly do not see this trend changing. But I feel that true innovation will be the result of a blending of improvements throughout all facets of the industry including concrete mixes, resin designs, welding technology, etc. to provide the best possible solutions.
There is certainly already an extremely wide variety of projects that can benefit from the use of concrete protective liner. The best we can hope for is that this generation of engineers and owners learns from the previous efforts of others that a little extra money spent for prevention can save a whole lot of money in repairs. I know we are already see this trend gaining ground as aging infrastructure rehabilitation costs become more and more prevalent.