Of the more prominent challenges facing the construction industry are those related to
deterioration of material and structures by actions of microorganisms. The process is
loosely known as Microbial Induced Corrosion (MIC), which belies an incredible range of processes occurring as a result of the growth of microorganism, principally bacteria, on our steel and concrete infrastructure. This article will focus on concrete but keep in mind that equally interesting and costly corrosion is occurring on metal based materials and infrastructure as well.
In the simplest sense, corrosion induced by microorganisms are a range of chemical reactions driven by microorganism metabolism (consumption of nutrients) that changes the environment on and around a concrete material resulting in a subsequent chemical reaction with the concrete that causes its deterioration. Starting at a very small scale, the progression leads to the deterioration of the concrete that accelerates both in size and chemical rate, and over time, allowing the reaction to proceed more and more quickly as the number of organisms continues to grow.
Overall the process occurs fairly slowly with a time scale of months to years, but it can lead to potentially catastrophic failure of very large scale materials. Given the scale and high costs associated with the repair or replacement these materials it’s no surprise that producers are looking into laboratory methods to both test and control the problems that are caused by microorganisms.
The Micro-environment – when we think of microorganisms causing corrosion, we generally consider their presence as a ‘blob-like’ or ‘slime – like’ material on the concrete. This blob or slime is actually a biofilm, and although it may look like a uniform brownish-grey mass, it is actually a highly stratified layering of many types of microorganisms and their byproducts. The nature of the biofilm is such that within its mass different types of organisms can reside and thrive by virtue of the microenvironment created by the growing film.
The most notorious microorganisms - Good examples are those organisms that prefer low pH or acidic conditions. Thiobacillus bacteria species (spp), which are more recently known as Acidithiobacillus spp., are the most commonly identified. But there are a range of these organisms with different preferences/requirements for acidity some preferring acidic conditions with a pH as low as 1. Examples of these acid-loving Thiobacillus spp. are T. concretivorus, and T. ferrooxidans,
But concrete has a pH of 12!! - People in the concrete industry know that concrete is highly alkaline (high pH) ~11.5 to 12 by virtue of its mineral composition. Overtime though this changes and the local environment will either mask or neutralize the outer most surface of the concrete to a lower pH level consistent with the environmental materials. Generally the surface of concrete must be ~pH 8 for organisms to adhere to it and survive but even at pH 8, acid loving bacterial will still not grow and likely die..so something else must also happen to progressively lower the surface pH.
In the biofilm, the earliest stages of organisms will begin growth on the surface of the concrete provided that there is enough water, nutrient and air (for aerobic types). Overtime, the environment changes and other organisms can take advantage of this new environment and begin to grow and thrive. The process is in part known as succession. By which the residence of one organism changes the environment in a favorable manner that allows another organisms to establish and grow.
Initially, the process is quite slow, concrete has an immense alkaline capacity; so the organisms first established don’t directly cause corrosion, but through their natural respiration and accumulation of biomaterials, they continue to lower the pH and, as this progresses, thiobacillus and other acid loving bacteria become a significant part of the biofilm. Unlike neutral pH bacteria, acid loving bacteria actually utilize the pH as part of their system for acquiring and consuming nutrients. They become less and less reliant on oxygen, and less reliant on carbon based nutrient sources. And most significantly, their waste products are no longer organic acids (weak acids) but mineral acids such as sulfuric acid (strong acid). Once at this stage, the rate of corrosion increases rapidly and overall destruction of the concrete accelerates.
Breaking the Cycle – The challenges posed by microbial induced corrosion are well recognized. As research into this problem continues it is becoming more apparent that an adaptive process will be needed. If human based antimicrobial challenges are a guide (ex. bacterial resistance of antibiotics), the path to inhibition of MIC will require a range of strategies that must be suited for different environments and applications.
Promising testing has been done to demonstrate some levels of microbial control for the treatment of concrete but these are still early stages of development. While modest success can be demonstrated it is the results of this work that will provide better solutions moving forward.
Key steps in this process are as follows:
- Establish as relevant ‘as possible ‘test system for laboratory testing and development of promising applications.
- The test system must provide meaningful simulation of the concrete environment and one that will allow the organism to become established on untreated control samples to mimic the actual corrosion process.
- Extend test parameters to include environmental inocula (mixed populations of organisms).
- Challenge the test condition by testing different environmental over time. (several parameters can be evaluates such as nutrient load, pH, oxidation and other parameters.
Bulk chemical additives as well as surface treatment strategies should be considered due the nature of the material, the size, and scope of the environment that they may be used. Simply adding an antimicrobial chemical my work in the short term, but the material will need a very large chemical capacity if the concrete is intended to reside submerged for long periods of time (years). Coatings offer more cost effective application strategies provided that loss of material due to the environment (chemical mobility or leaching) can be appropriately controlled with continued development and understanding of the additives and their respective benefits. It is likely that given an appropriate application, that a meaningful solution to this problem can be provided.