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.
Multiple longitudinal cracks with small crack width (e.g. 0.15mm) is acceptable which indicate effective transfer of stress from concrete to steel. Care should be taken when discovering a single wide longitudinal crack. Circumferential cracks may occur owing to loads imposed during
construction and uneven bedding. It may also be caused by the relative movement of another drainage structure connecting to the concrete pipe. Circumferential cracks do not generally affect the load-carrying capacity of concrete pipes.
This question is taken from book named β A Closer Look at Prevailing Civil Engineering Practice β What, Why and How by Vincent T. H. CHU.
If you have a query, you can ask a question here.