Predicting life expectancy of concrete septic tanks exposed to sulphate and biogenic sulphuric acid attack

The prediction of the expected long-term performance of concrete exposed to sewage and similar materials can be difficult as it is affected by a large number of parameters. In addition, the deterioration process in concrete is generally slow. The focus of the study was to ascertain the life expectancy of concrete septic tanks located in rural Victoria. In developing the accelerated test method, ASTM C 192, ASTM C 452, ASTM C 1293 and ASTM C 109 standard procedures were adopted wherever possible. From the analysis of mass change data after 350 days, it was found that the mass change rate for concrete in sulphate solutions increases with the increase of concentration of Na2SO4 solution. The weights of the samples in Na2SO4 solution increased with time, whereas the control specimens lost weight as a result of heating cycles. The probable reason for weight increase in Na2SO4 solution was hypothesized as the formation of gypsum (CaSO4.2H2O) and ettringite (3CaO.Al2O.3CaSO4.32H2O), which is confirmed from microstructural analysis. The rate of weight gain was higher at the beginning and reduced with time. The stronger the concentration of Na2SO4 the stronger was the weight gain or expansion of mass. All the samples in sulphate solutions attained their maximum weight at around 250 days. For the specimens in sulphuric acid solutions weight loss was observed to be higher for higher concentrations. The lower the pH of the acidic solutions, the larger was the weight loss. The weight loss of specimens in acidic solutions exceeded the control specimen after 250 days. The reason for the loss of weight of the samples in sulphuric acid may be the decalcification of C-S-H gel within the concrete, and as a consequence the loss of cementitious structure. Comparison of the corrosion of concrete and also microstructural examination of field samples confirmed that the deterioration mechanism is similar to that observed in the laboratory. The accelerated testing adopted here offers a realistic method of predicting the deterioration of septic tanks under biogenic sulphuric acid corrosion. Two equations have been proposed to predict deterioration due to sulphate attack and sulphuric acid attack as mass loss (or gain) with time