The deterioration of concrete by heterothrophic bacteria and fungi

Abstract

The aim of this investigation was to design and develop a novel decontamination process for concrete, which could harness the deteriorative effect of various microorganisms found within the natural environment by a process known as biodecontamination. Whilst the majority of existing research into concrete deterioration has focused on the effect of sulphur or nitrogen bacteria, this study has investigated the effect of the lesser documented heterotrophic bacteria and fungi on concrete deterioration, and their possible use within a biodecontamination system.
A number of heterotrophic bacteria and fungi were isolated and characterised from samples of deteriorating concrete removed from four similar cellars. The deteriorative ability of ten of these bacterial and fungal isolates was established using weight loss analysis. A further evaluation of the deteriorative activity of two of the heterotrophic bacterial and fungal isolates, was investigated by means of Dionex ion exchange chromatography and atomic absorption spectrophotometry, which
traced the liberation of key cations as indicators of the deterioration of the calcite binary material which cemented the concrete particulate material together. The results obtained from this study indicated that the calcite binary material was deteriorated after a short period of time as a consequence of ion exchange reactions at the surface of the material between magnesium and calcium cations. Environmental scanning electron microscopy was used to visualise microorganisms at the concrete surface. This technique provided evidence of deterioration, as indicated by the expansion of pores at the material surface.
The microorganisms isolated were used to develop two mixed consortia (consortium 1 and consortium 2) in different media. The deteriorative activity of these organisms was compared with an axenic culture of Aspergillus carbonarius, since this fungus produces citric acid, a known deteriorative agent of concrete. Following the selection of these microorganisms, various conditions within the liquid culture medium were altered in an attempt to enhance the levels of deterioration obtained. As a consequence of results obtained from these investigations, it was recommended that the growth medium was supplemented with 5% (w/v) glucose or sucrose and adjusted to pH 6.5 prior to inoculation. It was found that the levels of weight loss, were not enhanced by the addition of sodium chloride or magnesium sulphate salts to the medium. Whilst
inoculum level was found to have had an initial effect on the rate at which deterioration occurred, this effect was felt less after a period of twelve weeks. Results obtained from a continuous culture study of consortium 1 and A.carbonarius indicated that biofllms consisting of these microorganisms could be grown on concrete under conditions of minimal surveillance and maintenance.
As part of this study, it was decided to develop a gel-based delivery system which consisted of the paint thickener, hydroxyethylcellulose. A gel system was found to confer the advantages of an 'artificial' biofllm environment, and improve the chances of colonisation by increasing the residence time of the microorganisms at the surface. The results obtained indicated that the gel did not inhibit the growth or deteriorative activity of the microorganisms even if the gel was dehydrated.