Laboratory tests of fungal biocorrosion of unbonded lubricated post-tensioned tendons

In unbonded post tensioned concrete used in buildings, parking structures, and bridges utilizing steel cable, protection against stress corrosion is provided by grease. Microbiologically influenced corrosion (MIC) of prestressing steel mainly due to bacteria has been reported in literature while only few papers report on the role of fungi in the corrosion failure of unbonded lubricated tendons. The aim of this research was to evaluate to which extent three ubiquitous fungi (Aspergillus flavus, Penicillium commune and Fusarium solani) and an acidotolerant black yeast (Acidomyces acidophilum) could cause the corrosion of post tensioned lubricated tendons. The ability of the chosen fungi to degrade a commercial calcium based grease (OVOLINE 71C), used as lubricant in post-tensioning tendons was evaluated, followed by small scale sample experiments. The fungal colonization and extent of corrosion was studied by SEM microscopy and by FTIR spectra to analyze the organic acid produced. The results showed that all fungal strains were able to utilize the grease as sole carbon source and through the production of organic acids, they were able to produce diffused pits on steel surface. No Stress Corrosion Cracking (SCC), however, was noticed on the steel surface under tensile strength. Results showed that the fungi may cause an indirect steel corrosion through the release of organic acids following grease utilization as carbon source.