Assessment of chemo-mechanical degradation of concrete sewer pipes through an integrated experimental approach

The chemo-mechanical degradation of unreinforced concrete sewer pipes applied in domestic service locations is assessed through a systematic, integrated experimental approach, considering 18 new and 35 used sewer pipes. The characteristics and environmental conditions of the sewer pipes are reported, and data obtained from surface condition classification, residual alkalinity tests and XRD analyses are combined to identify the type and degree of chemical attack of the used sewer pipes. Concrete material properties are determined by material tests on sewer pipe samples, providing quantitative insight into the age dependency of the Young’s modulus, compressive strength, tensile strength and mode I toughness. All relatively old pipes (installed in the 1920s and 1950s) show substantial chemical attack by biogenic sulphide corrosion (at the inner side of the pipe) and carbonation (at the outer side of the pipe). The time development of the corresponding corrosion depths on average follows a linear trend, whereby the corrosion rate for biogenic sulphide corrosion is about a factor of 1.3 larger than for carbonation. Due to these chemical processes, the mechanical properties of concrete may significantly depend on the age of the sewer pipe. In particular, the average compressive strength and average tensile strength decrease approximately linearly with the age of the pipe, in correspondence with relative reductions of, respectively, a factor of 1.7 and 1.5 over a period of almost 100 years. The values and time-dependent trends found for the concrete properties and corrosion depths of sewer pipes can serve as input for practical analyses and advanced numerical simulations on their bearing capacity and time-dependent degradation. The experimental results also emphasize the importance of regularly assessing the amount of chemical degradation of (especially older) in-situ sewer pipes, by determining the (decrease in) effective wall thickness via core sampling and laser profiling.