ANALYSIS AND ASSESSMENT OF MICROBIAL BIOFILM- MEDIATED CONCRETE DETERIORATION

Inspections of bridge substructures in Texas identified surface deterioration of reinforced concrete columns on bridges continuously exposed to water. Initial hypothesis was that the surface deterioration was a result of the acidity of the water in which the columns were exposed. However, evaluation of the water acidity indicated that the surrounding waters were only very slightly acidic and near neutral. Discussions between engineers from the Texas Department of Transportation (TxDOT) and researchers at Texas A&M University and the Texas Transportation Institute (TTI) hypothesized that the damage could be a result of microbial attack. Microbial attack is often identified as an acid attack because some microbes can produce sulfuric acid. This research investigated whether microbes were present at areas on the bridge that were exhibiting attack, determined if there was a correlation between degree of damage and number of microbes present, determined if these microbes were acid producing microbes, and identified the microbes present at the field sites. Results indicate that microbes are present at the bridge columns experiencing surface deterioration, that the number of microbes present is directly correlated with the degree of damage, and that these microbes are acid producing. The research identified five genera: these included Bacillus, Brachybacterium, Flavobacterium, Lysinibacillus and Thiomonas. The group with the largest numbers of representatives was Bacillus, which was composed of 17 strains. The second largest group was identified as Thiomonas perometabolis, which consisted of seven strains. The researchers concluded that the damage to the concrete bridge columns is due to microbial attack. Because some bridge structures are exhibiting significant microbial attack of the concrete cover and because the long-term performance of the columns (and hence bridges) are most sensitive to concrete cover, further research is needed on how to prevent and mitigate this attack.