In water retaining structure a dense impermeable concrete is required
therefore, proportion of fine and course aggregates to cement should be
such as to give high quality concrete.
Concrete mix weaker than M20 is not used. The minimum quantity of
cement in the concrete mix shall be not less than 30 kN/m3
.
The design of the concrete mix shall be such that the resultant concrete is
sufficiently impervious. Efficient compaction preferably by vibration is
essential. The permeability of the thoroughly compacted concrete is
dependent on water cement ratio. Increase in water cement ratio increases
permeability, while concrete with low water cement ratio is difficult to
compact. Other causes of leakage in concrete are defects such as
segregation and honey combing. All joints should be made water-tight as
these are potential sources of leakage.
Design of liquid retaining structure is different from ordinary R.C.C,
structures as it requires that concrete should not crack and hence tensile
stresses in concrete should be within permissible limits.
A reinforced concrete member of liquid retaining structure is designed on
the usual principles ignoring tensile resistance of concrete in bending.
Additionally it should be ensured that tensile stress on the liquid retaining
face of the equivalent concrete section does not exceed the permissible
tensile strength of concrete as given in table 1. For calculation purposes
the cover is also taken into concrete area.
Cracking may be caused due to restraint to shrinkage, expansion and
contraction of concrete due to temperature or shrinkage and swelling due
to moisture effects. Such restraint may be caused by
(i) The interaction between reinforcement and concrete during shrinkage
due to drying.
(ii) The boundary conditions.
(iii) The differential conditions prevailing through the large thickness of
massive concrete.