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Mic 2013
- 1. A A R O N A U S E N D A L M A R A Y C O N C R E T E P R O D U C T S S A M L I N E S C O N C R E T E S E A L A N T S MIC 2013
- 4. MIC Attributes ๏ Bacteria / Biofilm ๏ Low dissolved oxygen in wastewater ๏ Sulfates in the effluent ๏ Warm Temperatures ๏ Turbulence ๏ Moisture on the walls above water line ๏ Reactive compounds in concrete ๏ Low effluent flow
- 10. Link 1 โ Hydrogen Sulfide Wastewater containing sulfates Anaerobic bacteria reduce sulfate to sulfide Sulfide mixes with water to form hydrogen sulfide
- 11. Link 2 โ pH Reduction Carbon Dioxide and water form carbonic acid Carbonic Acid reacts with concrete above water level pH of Concrete is reduced to about 6.5
- 12. Link 3 โ Bacteria Colonization At ~ pH 6-6.5, thiobacillus bacteria colonize Turbulence causes H2S to rise into airspace H2S provides nutrients for aerobic bacteria Different strains of Thiobacillus live and die off Thiobacillus consume H2S and excrete sulfuric acid
- 13. Link 4 - MIC Sulfuric acid reacts with calcium hydroxide This reaction forms gypsum Gypsum further reacts with the aluminates This reaction forms ettringite Ettringite is a gel that expands with water contact
- 14. People often wonder why MIC is more of a problem today than it was 30 years ago.
- 16. 1972 - CWA 1987 - WQA
- 22. Unconsolidated sand and gravel aquifers at or near the land surface. Semiconsolidated sand and gravel aquifers. Sand and gravel aquifers of alluvial and glacial origin are north of the line of continental glaciation.
- 24. Iron Bacteria
- 25. Manganese (Mn)
Editor's Notes
- Microbial induced corrosion, unlike the other causes of deterioration, can affect high quality as well as low quality concrete. The primary difference is the length of time it takes for the deterioration to occur. MIC is caused by bacteria, specifically, the species Thiobacillus. The thiobacillus bacteria consume sulfur from hydrogen sulfide as food and secrete sulfuric acid.
- There are dozens of strains of thiobacillus, non are more destructive than the strain named Concretivorous. This strain was named by C.D. Parker in 1945 and is derived from the Latin โconcrete eatingโ. The name was later changed to acidothiobacillus.
- For MIC to occur, most researchers agree that certain attributes must be present. (read the list) Remove any one of these and the chance of having MIC is reduced or eliminated.
- MIC is the product of a chain of events. In the first link, the non-air breathing bacteria convert sulfates into sulfides. These sulfides form hydrogen sulfide gas in the wastewater.
- Above the water line, carbon dioxide in the environment mixes with condensation inside the tank to form carbonic acid. The mild acid reacts with the concrete above the water line reducing the pH to around 6.5.
- Thiobacillus can survive on concrete with a pH of 6.5 or lower. Turbulence releases hydrogen sulfide from the wastewater providing a source of food for the thiobacillus. The thiobacillus bacteria consume sulfur and secrete sulfuric acid. Different strains of bacteria survive at different pH levels.
- This link is where the damage occurs. Sulfuric acid reacts with the free lime or calcium hydroxide forming calcium sulfate, also called gypsum. The gypsum reacts with the alumina in the concrete to form ettringite which can expand in the presence of moisture. This expansion causes the concrete to crack and spall. This leads to deeper penetration of acid, more cracking and spalling, and the cycle continues.