Concrete Manhole Threats
Biogenic Sulfide Corrosion
The most prevalent source of manhole or wet well corrosion is known as Biogenic Sulfide Corrosion, or BSC. BSC is the process by which bacteria in the sewer system metabolize the sewage and excrete hydrogen sulfide gas- H2S. The hydrogen sulfide itself is not detrimental to the structure's integrity, although it does have the characteristic "rotten egg" smell. The issue arises when the H2S condenses on the concrete walls, often starting at the top of the structure and moving down over time, and oxidizes to form Sulfuric Acid- H2SO4. That sulfuric acid steadily eats away at the concrete and thins out the walls over time.
Wet wells and manholes with slow flow are most susceptible to BSC, as the bacteria can linger for longer. In these structures, an estimated 0.1" to 0.2" of the concrete wall is lost every year. This can put a manhole or wet well out of service in under a decade. Even manholes with fast moving flow will suffer from BSC over time. Their life will just be extended to 20-30 years at best.
Polymer Concrete and BSC
Our Polymer Concrete is made with a high strength, thermosetting resin that contains no styrene or Volatile Organic Chemicals (VOCs) along with a custom blend of inert mineral aggregates. That means that not only is it safe for both people and the environment, it is also extremely corrosion resistant. It would take several times the concentration of sulfuric acid found in a manhole over 300 years to even make a measurable impact on the structure. And,
unlike coatings, our structures have the same corrosion resistance throughout the entire wall and base. That means that even if there were a surface defect or scratch that resulted from installation or a hard object flowing through, the structure will remain just as strong for generations to come. We back this claim with a 50-year Corrosion warranty on all of our products.
One of the threats to all buried concrete structures, whether they'll be carrying wastewater or not, is sulfate attack. Sulfate attack is a reaction that is both chemical and physical in nature, and is very detrimental to the integrity and durability of concrete. There is external and internal sulfate attack, which both operate slightly differently. Internal sulfate attack can be prevented with good quality control at the precast plant, but external sulfate attack is more difficult to avoid.
External sulfate attack is a type of sulfate attack that occurs when sulfates from external sources, such as soil or groundwater, come into contact with the concrete. Sulfate ions react with the calcium and aluminum components of the cement paste in the concrete. The reaction can result in the formation of new compounds, including calcium sulfate and ettringite. These compounds can cause cracking, expansion, and weakening of the material, leading to significant damage to the concrete structure over time.
Unfortunately, internal manhole liners or coatings are not an effective solution to prevent external sulfate attack. This is because the attack is coming from the soil outside the manhole, and the liners or coatings will not protect the concrete from exposure to sulfates. External manhole liners often only consist of a thin coat of acrylic paint, which is easily damaged during installation or delaminates from the concrete once the manhole is in the ground. Any damage to the coating provides an opening for sulfate attack to begin. In fact, liners or coatings may even exacerbate the problem by trapping moisture inside the manhole walls, creating a favorable environment for sulfate attack to occur.
Polymer Concrete and Sulfate Attack
Polymer concrete is immune to both the physical and chemical elements of sulfate attack. To begin with, polymer concrete in nonporous, which means that there is no way for the sulfates in the soil or ground water to penetrate past the surface of the manhole wall. Without being able to enter into the wall, there is no opportunity to expand and cause cracking. However, even if there were a way for the sulfate to enter into the structure, the aggregate blend is completely inert, meaning there can be no reaction to form ettringite or any other harmful compound.
Ion penetration occurs when certain ions, such as chloride, calcium, and magnesium, are able to penetrate the surface of concrete and migrate through the concrete matrix. This can happen due to various factors, such as exposure to seawater, deicing salts, or contaminated aggregates. Once these ions penetrate the concrete, they can cause a variety of problems, including:
Corrosion of Reinforcing Steel: The presence of chloride ions in concrete can cause corrosion of the reinforcing steel, which can weaken the structure and ultimately lead to structural failure. This is a major concern for manholes located in coastal or snowy regions where deicing salts are commonly used.
Cracking and Spalling: Calcium and magnesium ions can also cause damage to concrete by reacting with the cement paste and creating expansion. This can result in cracking and spalling of the concrete, which can compromise the structural integrity of the manhole.
To mitigate the effects of ion penetration in manholes, various techniques can be used. One technique is to use high-performance concrete mixes that are specifically designed to resist ion penetration. These mixes typically have a lower water-to-cement ratio, which makes the concrete more dense and less permeable to ions. However, while this option may cost more, it only slows down ion penetration without doing anything to truly prevent it.
Another common option is to use epoxy coated rebar. Epoxy coated rebar is an effective way to truly prevent the effects of ion penetration, but it might not be right for every application. There is an ongoing debate as to the viability of epoxy coated rebar, partially due to bonding issues between the bar and concrete.
Polymer Concrete and Ion Penetration
To understand how polymer concrete mitigates any risk of ion penetration, it might be helpful to consider why epoxy coated rebar works. The epoxy coating forms a nonporous barrier that completely shields the steel rebar from any contact with the corroding ions. Epoxy is a thermosetting resin just like the vinyl ester we use in our polymer concrete. The difference is that instead of coating just the rebar with a layer of resin, we coat the rebar along with every single grain and rock of aggregate in a high-strength, corrosion resistant resin to form one solid and nonporous structure.