By Sam Lines
Concrete is one of the most economic and widely used building materials in the world. Its strength and durability, along with its ability to be molded into almost any shape, make concrete a leading construction material of choice for designers.
Still, concrete occasionally can deteriorate as a result of contact with chemicals, minerals and environmental conditions. Deterioration mechanisms include damage from freezing and thawing, salt attack and carbonation, among others.
One cause of deterioration is microbially induced corrosion of concrete (MICC).
Study of MICC
The root cause of MICC is well documented. After World War II, C.D. Parker discovered that a sulfur oxidizing bacterium – Acidithiobacillus thiooxidans – was used in converting hydrogen sulfide gas into sulfuric acid, and he wrote about it in “The Corrosion of Concrete.”
Parker originally called this bacterium “Thiobacillus concretivorus,” because it appeared to “eat” the concrete. The acid attacks concrete, causing the surface to erode or “corrode” (not to be confused with reinforcing steel corrosion).
Acidithiobacillus thiooxidans is the primary bacteria that causes MICC in pipes in sewer systems1. These bacteria live in very low pH environments, around 2-4. For reference, concrete generally has a pH of 12-13. The high initial pH of new concrete provides a period of immunity to most bacterial growth. As concrete surface pH is lowered by carbonation and sewer gases, it becomes more hospitable to hosting bacterial colonies.
The Three Phases of Corrosion
Based on work by Islander, et.al.2, and confirmed by House3, there are three distinct phases of the corrosion process. (see Figure 1)
- Phase 1 is carbonation of the concrete, a naturally occurring process. With a pH of 12-13, concrete is very alkaline. Pure water has a pH of around 7, and acids have a very low pH. Acids react with the calcium hydroxide (CH) and calcium silicate hydrate (CSH) concrete constituents that provide this high alkalinity. Carbon dioxide, thiosulphuric acid and other mild acids abiotically reduce the concrete pH to around 9. This process can take months, or even years, depending on the concrete quality.
- Phase 2 is the biological attachment phase. At pH 9, acid-producing bacteria of other species – for example, Thiomonas intermedia, Halothiobacillus neapolitanus and Thiobacillus thioparus – begin to colonize. These bacteria convert hydrogen sulfide into sulfuric acid. The weak sulfuric acid produced by this strain lowers the concrete pH until the bacteria dies off and another strain colonizes. Each strain of aerobic thiobacillus produces a stronger sulfuric acid than the previous one. Sand and Bock4 as well as Cho and Mori5 state that these neutrophilic sulfur oxidizing bacteria (NSOB) are required for Acidithiobacillus thiooxidans to colonize.
- Phase 3 is the acid corrosion phase. Acidithiobacillus thiooxidan, an acidophilic sulfur oxidizing bacteria (ASOB), produces a strong sulfuric acid, rapidly deteriorating the concrete. Under extreme conditions of high hydrogen sulfide gas concentrations above the wastewater liquid level, concrete structures can lose up to a half inch (12mm) of mass annually during Phase 3. It can take as little as two to three years or as long as 10 to 15 years for this chain of events to reach this level of destruction, depending on concrete quality and sewer conditions.
High Quality Concrete is First Line of Defense
In a harsh environment exposed to sulfates, chlorides or acids, it is important to use a high-quality concrete mix with a low water-to-cement ratio (w/c). According to the Portland Cement Association (PCA) book, “Design and Control of Concrete Mixtures,” “Decreased permeability improves concrete’s resistance to freezing and thawing, re-saturation, sulfate, and chloride-ion penetration, and other chemical attack.”
It is important to reduce concrete permeability to increase durability. A w/c of 0.45 is good for most concrete products that are not exposed to harsh conditions. If there is a potential that the concrete will be exposed to harsh conditions, the w/c should not exceed 0.40.
In addition to a low w/c, the use of pozzolanic and secondary cementitious materials (SCMs) can increase the concrete density and lower concrete permeability. Fly ash, slag and silica fume are options. Using one or more of these mineral admixtures in the concrete mix design can increase the concrete strength and density while lowering the porosity and improving chemical resistance.
Promising work with nanomaterials also indicates significant permeability reductions with additives such as colloidal silica. Colloidal silica-based admixtures used in concrete have reported increased compressive and flexural strengths in addition to reduced permeability of water under high hydrostatic pressure. They also are known to reduce bleed water channels in hardened concrete.
While concrete densification is important to increasing a concrete structure’s life, it will not stop the biological process that allows Thiobacillus bacteria to colonize. Antimicrobial concrete admixtures and surface-applied antimicrobial sealers are effective at reducing the effects of MICC6. Antimicrobial admixtures and sealers render the concrete uninhabitable to the growth and colonization of the neutrophilic bacteria in Phase 2, thus breaking the chain of events leading to acidophilic bacteria and Phase 3 of the MICC process.
Antimicrobial concrete admixtures are available in powder and liquid form. Regardless of the material type, any material that is marketed as a product preservative and labeled as an antimicrobial is considered a pesticide under the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA).
Precast manufactures should verify that any product they purchase has a U.S. EPA registration number when sold in the United States. Registration in the country of import may be required when the product is sold outside of the United States. Additionally, each U.S. state requires registration for all pesticides sold and shipped into the specific state. If the pesticide is used as a material preservative (e.g., when the antimicrobial is added to the concrete), the treated article does not usually need to be registered. This is called the “treated article exemption.”
Antimicrobial concrete admixtures are classified as Type S under ASTM C494, “Standard Specification for Chemical Admixtures for Concrete,” which is applicable to both liquid and non-liquid concrete admixtures.
Section 2.1.5 of the NPCA Quality Control Manual states that: “Admixtures shall be products from manufacturers from whom test data are available to establish their effects on concrete and compatibility with other materials in the mix.” Certificates of conformance for chemical admixtures need to be provided from the product distributor or manufacturer and must be updated annually.
Batching an admixture into a concrete mix will depend on the type of product and the volume being used. If the admixture is a liquid, the dosage is in ounces per 100 pounds of cement, also referred to as CWT or in gallons per cubic yard. A dry powder is dosed by weight as a percent of cement content. A liquid also can be batched as a percent by weight if the specific weight of the admixture is known.
When batching a liquid, the amount can be measured manually or automatically and added to the batch during the part of the batch cycle recommended by the admixture supplier. Most powdered admixtures used for MICC protection are batched manually. A dry material can be supplied in a water-soluble bag with a pre-measured volume for one yard of concrete.
Antimicrobial admixtures in concrete are intended to stop the transition of the MICC process in Phase 2. They typically are not effective in preventing corrosion from chemical attacks such as sulfuric acid immersion.
A concrete densifier may provide some protection at a pH as low as 2 or 3. The appropriate test method for assuring the efficacy of the product is a bacterial colonization test (ISO 22196 – Measurement Of Antibacterial Activity
On Plastics And Other Non-Porous Surfaces, modified for concrete) or a biogenic acid immersion test (ASTM C1904 – Standard Test Methods for Determination of the Effects of Biogenic Acidification on Concrete Antimicrobial Additives and/or Concrete Products).
The modified ISO 22196 test first creates a condition in which bacteria grow and survive. As developed by Situ Biosciences, the concrete is first carbonated to having a surface pH of 6.0 to 6.5. At this pH, two of the predominate NSOB grow and multiply on reference concrete. A successful antimicrobial admixture results in a reduction in the bacteria count from the initial colony forming units (CFU) applied to the sample in contrast to the significant multiplication in bacteria count on the reference sample. This test can take three to six months to ru.
ASTM Subcommittee C13.03 – Determining the Effects of Biogenic Sulfuric Acid on Concrete Pipe and Structures developed a simple, safe and realistic test for antimicrobial admixtures and for alternative concrete mix designs. ASTM C1904-20 is a test method for the effectiveness of determination of the results of biogenic acidification on concrete products and/or efficacy of antimicrobial products to resist microbially-induced corrosion (MIC) of concrete. This test builds on research and test methods developed during the past few decades to accelerate the process of MICC for product evaluations.
The standard has three methods to use depending on the additive being tested.
- Method A is used to test the efficacy of liquid antimicrobial admixtures (See Figure 2).
- Method B is a biogenic acid immersion test of a mortar wafer containing an antimicrobial admixture.
- Method C is a phase three acid immersion test of a mortar wafer using ASOB to create the acidic conditions.
In addition to research by NPCA and ASTM, MICC is now a topic of discussion in American Concrete Institute (ACI) Committee 201 on Durability. This committee has formed a task group with a goal of developing a chapter on MICC for its ACI 201.2R Guide to Durable Concrete. The committee has representatives from around the world bringing new research and information to the industry.
Virtually every material has a mechanism by which it can be damaged. Having the knowledge and understanding of MICC is useful in designing products using concrete that will outlast our lifetimes. Solutions are available to protect concrete and/or reduce the risk of MICC in sewerage infrastructure products. PI]
Sam Lines is the engineering manager at Concrete Sealants Inc.
1 E. B. N. V. W. Vincke, “Analysis of the microbial communities on corroded concrete sewer pipes – a case study,” Appl Microbiol Biotechnol, pp. 776-785, 2001.
2 R. L. Islander, J. S. Divinny, F. Mansfield, A. Posyn and H. Shih, “Microbial ecology of crown corrosion in sewers,” Journal of Environmental Engineering,, pp. 751-770, 1991.
3 M. W. House, “Using biological and physico-chemical test methods to assess the role of concrete mixture design in resistance to microbially induced corrosion,” Purdue University, Civil Engineering, West Lafayette, 2013.
4 W. B. E. Sand, “Concrete corrosion in the Hamburg sewer system,” Envir Tech Lett, pp. 517-528, 1984.
5 K.-S. Cho and T. Mori, “A newly isolated fungus participates in the corrosion of concrete sewer pipes,” Water Science Technology, pp. 263-271, 1995.
6 A. R. Erbektas, O. B. Isgor and W. J. Weiss, “Evaluating the efficacy of antimicrobial additives against biogenic acidification in simulated wastewater exposure solutions,” RILEM Technical Letters, pp. 49-56, 2019.