United States Department of Transportation
i
Characterization and Mediation of Microbial Deterioration of Concrete Bridge Structures
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2013-04-01
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[PDF-2.30 MB]
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Edition:Final Report
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Abstract:Samples obtained from deteriorated bridge structures in Texas were cultured in growth medium containing thiosulfate as an energy source and investigated for acid production, type of acid produced by microbes and the bio-deterioration of concrete cylinders. Enriched cultures decreased the pH to 3.5. Whereas in culture containing concrete the pH remained stable, although there was 97% oxidation of thiosulfate to sulfate. Bio-deterioration was also evident from the 2-fold higher amount of calcium leached out from concrete compared to controls. Stereo microscope and scanning electron microscopy (SEM) images of the concrete cylinders attacked by acid revealed cracks, which could be due to the formation of expandable products gypsum and ettringite. The microbe responsible for acid production was isolated. The 16S rRNA-gene of microbe was genetically sequenced, analyzed with the use of basic local alignment search tool (BLAST) and was identified as Streptomyces sp. This strain was capable of reducing the culture pH to 3.4 in absence of concrete. Whereas in presence of concrete a drop in pH was observed , when there was sufficient amount of thiosulfate for oxidation reaction. There was 98 % oxidation of thiosulfate to sulfate and the amount of calcium leached out from concrete was 6 fold higher than control. SEM images revealed cracks on concrete exposed to Streptomtces sp. culture. Approximately, 400 to 600 16S rRNA-gene sequences representing microbial communities on concrete surfaces from three different bridge structures were analyzed. Highly diverse bacterial and archael communities with only a few known acid producers existed at the time of sampling at all three sites. However, our laboratory studies revealed that the community composition dramatically shifted from a highly diverse to a highly rich dominated by mainly sulfur oxidizing acid-producers such as Thiobacillus thioparus, Alicyclobacillus ferrooxydans, Alicyclobacillus pomorum, Alicyclobacillus acidocaldarius, Alicyclobacillus sp. and Bacillus sp., when thiosulfate was provided. The acid producers were able to oxidize 40 mM thiosulfate to roughly 75 mM sulfate within 5 weeks suggesting almost stoichiometric conversion (94%) of the added energy source. The pH of the culture decreased from 6.7 to 2.8. These results clearly demonstrate the role of sulfur oxidizing microorganisms in concrete corrosion and the availability of reduced sulfur compounds in the environment is important for corrosion to occur.
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