Effect of biochemical attack on the mechanical performance of used concrete sewer pipes

The structural response and failure mechanisms of 35 used concrete sewer pipes are investigated by means of a combined experimental–numerical approach, whereby specific attention focuses on determining the effect by biochemical attack. Accordingly, the degree of degradation of the inner and outer surfaces of the pipes is carefully analysed by visual inspection, and subsequently categorized in 6 different surface condition classes. The type and degree of biochemical attack are determined by respectively performing X-ray diffraction analyses
and phenolphthalein tests. The sewer pipes, which vary in age, size and geometry, are subjected to biaxial loading conditions in a full-scale test set-up. The experiments show that the material degradation of relatively old sewer pipes can be considerable, and for a large part may be attributed to the process of biogenic sulphide corrosion. This process typically induces a weak, corroded layer at the inside of the pipe, which is characterized by the appearance of exposed granulates and porous mortar between the granulates. The experimental failure responses of the used sewer pipes are compared against numerical results obtained from detailed Finite Element Method (FEM) simulations. The failure response of each sewer pipe type is well predicted by the FEM model if the negligible mechanical properties of the biogenic sulphide corrosion layer at the inside of the pipe are accounted for in the simulations. The relatively low concrete tensile strength of old sewer pipes and the reduced effective wall thickness due to biogenic sulphide corrosion cause that the ultimate bearing strength and structural stiffness of these pipes may be more than a factor of two lower than those of completely new sewer pipes. The results of the present work provide a scientific basis for the decision-making process on sewer rehabilitation and replacement, in particular by revealing and quantifying the influence on the structural bearing strength by biogenic sulphide corrosion.