ABSTRACT

Microbiologically Induced Corrosion (MIC) is a complex problem facing global concrete sewer structures. Despite the substantial efforts made, MIC of concrete sewers remains a significant challenge. Concrete is susceptible to corrosion induced by microbial species which convert the main binding agent Ca(OH)2 to CaSO4, leading to the disintegration of concrete, loss of strength and structure failure short of its predicted life. Concrete specimens were prepared with corrosion inhibitors and immersed in sodium sulfide and sulfuric acid solutions for more than 400 days. The concrete samples without inhibitor or admixtures showed more than 33% loss of compressive strength, roughly 6.2 mm layer of sulfate attack and concrete disintegration. The concrete samples treated with admixture and surface applied corrosion inhibitors demonstrated no loss of compressive strength and less than 0.12 mm layer of the sulfate-attack after 400 days immersion in the aggressive solutions. A combination of admixture and highly hydrophobic surface applied corrosion inhibitor is recommended for aggressive waste water systems such as manholes, channels and pipes to assure a satisfactory performance for these concrete structures.

INTRODUCTION

The life-cycle of concrete structures used in waste water systems should consider all factors that might cause a structural system to perform unacceptably at any point during its lifetime. This includes the progressive and sustained loss of load capacity caused by operational or environmental factors. In general terms, deterioration can be defined as a loss of structural load capacity with time as a result of the action of external agents causing chemical attacks or material weakening due to these environmental interactions. Concrete is the most extensively utilized building material in structures designed to collect, store, transport and treat municipal and industrial wastewater because of its ease of use, availability, structural capabilities, and cost effectiveness. However, at highly acidic conditions (pH< 3.0), the physical and chemical qualities of concrete formed with hydrated Portland cement make it susceptible to deterioration and breakdown. As a result, a considerable portion of the concrete wastewater infrastructure may be vulnerable to microbially induced corrosion (MIC), a complex multistage deteriorating process. Biodeterioration is caused by organisms that grow on concrete surfaces with dissolved oxygen, high sulphate concentration, low pH, high COD (chemical oxygen demand) and BOD (biological oxygen demand). These organisms can induce corrosion in various places like fire sprinkler pipes, chiller pipes, sewer systems, potable water, oil and gas pipes [1]. Conducive environments include elevated relative humidity around 60% to 98%, long cycles of humidification and drying, freezing and defrosting, high carbon dioxide concentrations, high concentrations of chloride ions, high concentrations of sulfates and small amounts of acids [2]. Increased temperature and sulphate containing detergents are also considered to be contributors for MIC [3].

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