A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
Level Two (Advanced FAQs)
Part II: Cracking and Tests
1. What are the possible causes of longitudinal and circumferential
cracking in concrete pipes? (C1)
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
surface at the sides. As such, they are the potential locations of 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.
2. What is the importance of uniform support for precast concrete
pipes? (C1)
Concrete pipes are designed to be uniformly supported along the length to
carry vertical loads on its top. They are normally not intended to serve as a
beam to carry loads in longitudinal direction under poor ground supports
(i.e. high and low spots in bedding). Under cantilever beam action and
simply support beam action as shown in the diagram below, circumferential
cracks would develop in concrete pipes. Circumferential cracking develops
in concrete pipes only when pipe bedding becomes non-uniform. Therefore
in unstable ground conditions such as soft spot and hard foundations, care
should be taken to provide firm and even support to concrete pipes.
165