Developing an Integrated Solution to Microbiologically Induced Deterioration in Concrete

Microbiologically induced deterioration (MID) in concrete, also referred to as microbial-induced corrosion (MIC) or concrete corrosion, is the result of acids from microbial activity degrading concrete components. MID negatively affects the structural integrity of sewer pipelines and other concrete structures (1). There are relatively few effective countermeasures to MID. In fact, even structures designed to withstand aggressive environments such as those with sulfates—concrete made with C₃A-free cement, fly ash, and a water to cement ratio of 0.35—can reach a corrosion rate of 1 cm/year within a decade of installation (2).

And it is not always practical to alter the C₃A (tricalcium aluminate) content of concrete. C₃A content in concrete, for instance, is associated with decreased setting time, accelerated hardening, and denser pore structure (3). Depending solely on novel concrete mixtures to promote MID resistance is insufficient. In fact, the latest research recommends an integrated approach that combines modifications to concrete mixtures, concrete protective coatings, and treatments with biocides (1).

In this article, we will provide an overview of MID, discuss the concept of an integrated MID/MIC solution, and explore existing concrete protection options.

An overview of microbiologically induced deterioration/corrosion

While there are currently 22 known microorganisms that affect MID, most of our knowledge is concentrated in Thiobacillus (see Table 1 for a list of known MID microorganisms).

For sewer systems, the MID/MIC process begins when aqueous H₂S is converted from sulfate by bacteria in a submerged slime layer. The gas form of H₂S is then released and is dissolved in a moisture film (also known as a biofilm) at the tunnel’s crown. In the last step, the dissolved H2S is converted to sulfuric acid (H₂SO₄) by sulfur-oxidizing bacteria such as Thiobacillus (see Figure 1 for a visual schematic of the process) (4).

Mitigating the MID/MIC process depends largely on preventing bacterial adhesion and the development of biofilms. By preventing the development of biofilms, key steps in the MID/MIC process are inhibited and therefore biocorrosion can be prevented. The application of polymer coatings has been leveraged as a corrosion protection strategy against MID/MIC for decades (5). Polymer coatings, however, can suffer from poor adhesion to the base materials and can quickly degrade (6). While innovations such as conductive polymer coatings and graphene coating show significant promise, other factors such as high backpressure still pose a problem (5, 6).

Protecting concrete from MID/MIC using an integrated solution

The annual cost of corrosion is approaching US$1 trillion, with MID/MIC accounting for nearly half of the cost (6). Much of these costs are associated with frequent rehabilitation. For instance, it is possible to stop MID/MIC through regular high-pressure washing of the concrete surface, which removes the biofilm. However, it takes between 60 and 140 days for the bacteria to recover the biofilm to prewash levels (5).

Instead of focusing on temporary answers, a better approach relies on combining concrete mixtures, chemical intervention, and protective coatings into an integrated solution. The solution takes a multi-pronged approach to inhibit biofilm growth and prevent concrete biodeterioration (1).

The first layer defense is altering the concrete composition to decrease the rate of corrosion should initial biofilm inhibiting efforts fail. Decreasing the rate of corrosion is a mitigating factor that helps reduce potential rehabilitation costs. Effective mixtures include incorporating silica fume concrete, polymer-modified mortar, and C₃A. These mixtures can increase the strength, durability, and pore density of concrete structures to lower the rate of corrosion (1). It is also possible to incorporate antimicrobial aggregates within the concrete mixture through low-cost sorbents and metal solutions such as copper and copper/cobalt (7).

The second layer of defense uses chemical intervention to control MIC by raising the pH of the sewage (e.g., magnesium hydroxide), inhibiting growth (biocides e.g., sodium bromide, sodium hydroxide), or interrupting the process by oxidizing sulfide directly (e.g., hydrogen peroxide) (4).

The last layer of defense, which was described earlier, uses a protective coating to inhibit biofilm growth (5).

In systems where known MID/MIC microorganisms are present, these layers of defense are very effective in slowing down the development of biofilms and preventing concrete corrosion. However, the full implementation of this integrated solution is not always possible.

Chemical intervention, for instance, carries a significant, recurring cost. Additionally, chemical intervention can lead to environmental problems as a result of unintended chemical reactions. For existing systems, replacing concrete structures with concrete using enhanced mixtures is unlikely. And protective coatings have an even shorter service life in concrete systems with high backpressure. Using a concrete waterproofing membrane—also known as a concrete protective liner (CPL)—in place of a polymer coating offers an alternative approach.