EFFECTS OF MICROBES IN WASTEWATER ON SOME CONCRETE PROPERTIES

Abstract

Structural reliability of many civil engineering structures has been affected by microbial processes. This research presents effects of microbes in wastewater on some concrete properties. Wastewater samples in concrete environment were collected from three different sources. Microorganisms in the wastewater samples were isolated using pour plate method to determine the microbial load. Morphological and biochemical characterization of the pure isolates were carried out in order to identify the microorganisms present. A particular bacterium present in all the wastewater samples as the most predominant namely Staphylococcus epidermidis based on the microbial load was selected. Concrete cubes of mix ratio 1:2:4 were used for the experiment. The cubes were cured in different media namely; potable water, the broth culture of S. epidermidis, broth culture of S. pidermidis and Bacillus subtilis and nutrient broth alone. Also S. epidermidis and the consortium containing S. epidermidis plus B. subtilis were introduced into the concrete matrix and cured in both potable water and nutrient broth. The weight loss, modulus of elasticity and the compressive strength of the concrete were determined over the period of 28 days. Also, the result at 28 days of curing showed that the greatest reduction in weight (4.2%) and compressive strength (16.69%) compared with control was observed in concrete inoculated with S. epidermidis; while the least decrease in weight (0.8%) and an increase in compressive strength (1.79%) was observed with concrete cast with potable water and cured in nutrient broth medium. Healing effect of B. subtilis on the concrete was also considered and analysed. The result of Scanning Electron Microscope (SEM) analysis obtained was used to explain the mechanism of deterioration induced by the microorganism. The SEM analysis result showed that the presence of microorganisms increase the pores within the concrete thereby leading to reduced compressive strength.