A few years ago, I purchased a red car. I didn't really notcie how many red cars were on the road until after I owned one. Now I seem to find them everywhere. There must be a red car epidemic in our society. When I hear people talk about the issue of this biogenic corrosion caused by the Thiobacillus bacteria, I wonder if the same thing isn't occuring. A septic tank has the signs of corrosive activity in one locality, and it heightens our awareness. We now become more sensitized to installations where this might be occuring and think it must be an epedimic.
While I do not want to minimize the impact that Microbial Induced Corrosion has, both financial and environmental, in our society, I want to point out that in a decade and a half of research on this topic, the number of actual cases is small compared to the number of precast systems installed and funtioning for many, many years. The cases of MIC, as uncovered in the current research, tend to be concentrated in specific regions. The area affected is possibly only 1% -2% of the land in the United States. This may be true in other parts of the world as well.
The research has pinpointed several characteristics that seem to be common at sites where septic tanks are known to be corroding. The factors include, high amounts of sulfur in the water, extremely hard water, the presence of gas or oil wells in the area, iron content, venting, lack of venting, and chemicals in the waste stream. The conditions that are necessary for the microbial induced concrete to occur are also difficult to replicate. This make the process of designing resistent materials very difficult.
As this topic becomes more widely known, it will improve the information that is available for research. The data from a larger population will lead to more information about the sites which will eventually isolate a common set of characteristics that are always present when concrete corrosion due to MIC is found.
Friday, March 25, 2011
Friday, March 4, 2011
Microbial Induced Corrosion: Part 3
LOW WATER TO CEMENT RATIO
In a harsh environment exposed to sulfates, chlorides, or acids, it is very important to use a high quality concrete mix with a low water to cement ratio. According to the book Design and Control of Concrete Mixtures, "Decreased permeability improves concrete's resistance to freezing and thawing, resaturation, sulfate, and chloride-ion penetration, and other chemical attack." It is very important to reduce the permeability to increase durability. A water to cement ratio 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 these harsh conditions, the water to cement ratio should not exceed 0.40. The diagram below shows how important a low w/c is to reducing the permeability.
The water to cement ratio is simply the weight of the water divided by the weight of the cement. The amount of water comes from two places: the water added to the mix and free water in the aggregates. It is very important to calculate the free water on the aggregates in this calculation. The amount of water on the aggregates contributing to the batch water can be 70 pounds or more. This is enough water to change a 0.45 w/c ratio to a 0.57 w/c ratio.
SECONDARY CEMENTITIOUS MATERIALS
In addition to a low water to cement ratio, the use of pozzolanic and secondary cementitious materials can increase the density and lower the porosity. Fly ash, slag, silica fume, and calcium aluminate cement are just a few of the options. Using one or more of these mineral admixtures in the concrete mix design will increase the strength and density while lowering the porosity and improving chemical resistance. There are also various liquid admixtures, such as silica, that can be added to the concrete mix or applied to the concrete after the pour to improve its mechanical properties.
ANTIMICROBIAL TREATMENT
While concrete densification is important to increasing the life of the concrete structure, maybe even by 100% or more, it will not stop the biological process that causes the Thiobacillus bacteria to secrete sulfuric acid. This chain is broken by the addition and/or application of a material with an antimicrobial bound to a silane molecule. The silane provide a mechanical bond with the concrete that will not leach out or wear out. The antimicrobial portion is also chemically and mechanically bound to the concrete structure. The technology name is QuaternaryAmmonium Silane, or Quat Silane. The chemistry is considered a pesticide by the US EPA and by law can only be distributed by organizations having an US EPA registration and a pesticide registration in the state where it is sold. One such product is ConBlock MIC from Concrete Sealants, Inc.
The positively charged molecule is not destroyed by this process, and it is present for future antibacterial needs. Since the bacteria cannot survive and colonize on the surface, the acid that is produced by them cannot be created biogenically. If there is no acid, then there is no corrosion. For this protection to completely protect the concrete structure, the antibacterial must be incorporated into the concrete as an admixture, and it must also be applied to the surface after all other embedments, attachments, or accessories are installed.
The Quat Silane will also provide some level of hydrophobic protection to the concrete as well, helping to reduce the absorption of water (as well as water soluble chemicals) into the concrete. The result is that the concrete life will be extended. Dense concrete, treated with an antimicrobial additive and coating, should last in excess of 100 years. The longevity of this protection is under review, but if the products are used according to the manufactures label, there is no reason to expect anything different.
So in conclusion, a dense concrete is critical for chemical resistance and reduced permeability. There are several products on the market which can and do enhance the product density. In addition, for full protection against the bacteria that causes Microbial Induced Corrosion, an antibacterial called a Quat Silane, must be incorporated into the concrete design. Following these procedure will increase the longevity of concrete structures for generations to come.
In a harsh environment exposed to sulfates, chlorides, or acids, it is very important to use a high quality concrete mix with a low water to cement ratio. According to the book Design and Control of Concrete Mixtures, "Decreased permeability improves concrete's resistance to freezing and thawing, resaturation, sulfate, and chloride-ion penetration, and other chemical attack." It is very important to reduce the permeability to increase durability. A water to cement ratio 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 these harsh conditions, the water to cement ratio should not exceed 0.40. The diagram below shows how important a low w/c is to reducing the permeability.
SECONDARY CEMENTITIOUS MATERIALS
In addition to a low water to cement ratio, the use of pozzolanic and secondary cementitious materials can increase the density and lower the porosity. Fly ash, slag, silica fume, and calcium aluminate cement are just a few of the options. Using one or more of these mineral admixtures in the concrete mix design will increase the strength and density while lowering the porosity and improving chemical resistance. There are also various liquid admixtures, such as silica, that can be added to the concrete mix or applied to the concrete after the pour to improve its mechanical properties.
ANTIMICROBIAL TREATMENT
While concrete densification is important to increasing the life of the concrete structure, maybe even by 100% or more, it will not stop the biological process that causes the Thiobacillus bacteria to secrete sulfuric acid. This chain is broken by the addition and/or application of a material with an antimicrobial bound to a silane molecule. The silane provide a mechanical bond with the concrete that will not leach out or wear out. The antimicrobial portion is also chemically and mechanically bound to the concrete structure. The technology name is QuaternaryAmmonium Silane, or Quat Silane. The chemistry is considered a pesticide by the US EPA and by law can only be distributed by organizations having an US EPA registration and a pesticide registration in the state where it is sold. One such product is ConBlock MIC from Concrete Sealants, Inc.
The way this technology works is not completely known, even though the chemistry was invented and first patented by Dow Corning in the late 1960's. The most common explanation is that the Quat Silane is present on the surface of the concrete structure. Through a positive charge present in the molecular structure, bacteria are attracted to the silane molecule where a carbon chain extends outward from the silane. The carbon chain is sometimes compared to a sword that punctures the membrane of the cell wall.
The Quat Silane will also provide some level of hydrophobic protection to the concrete as well, helping to reduce the absorption of water (as well as water soluble chemicals) into the concrete. The result is that the concrete life will be extended. Dense concrete, treated with an antimicrobial additive and coating, should last in excess of 100 years. The longevity of this protection is under review, but if the products are used according to the manufactures label, there is no reason to expect anything different.
So in conclusion, a dense concrete is critical for chemical resistance and reduced permeability. There are several products on the market which can and do enhance the product density. In addition, for full protection against the bacteria that causes Microbial Induced Corrosion, an antibacterial called a Quat Silane, must be incorporated into the concrete design. Following these procedure will increase the longevity of concrete structures for generations to come.
Subscribe to:
Posts (Atom)