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Concrete pipes are designed to crack in tensile zone so that steel reinforcement could take up the tensile stress. Flexural stresses are developed at the top and bottom inside surfaces and on the outside longitudinal cracks.
Longitudinal cracks are formed as a result of excessive soil and traffic loads or inadequate pipe bedding. Visible longitudinal cracks observed at top and invert inside the pipe should be more severe than those on the outside because tensile stress occur at top and invert portion of the inside of the pipes. On the contrary, longitudinal cracks formed outside the pipe at the sides of concrete pipe should be more severe than those on the inside.
This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.