DESIGN OF LIQUID RETAINING STRUCTURES IS DIFFERENT FROM THE ORDINARY RCC STRUCTURES

Design of liquid retaining structures is different from the ordinary RCC 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 doesn't exceed the permissible tensile strength of concrete and 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 :--(1) The interaction between reinforcement and concrete during shrinkage due to drying. (2) The boundary conditions (3) The differential conditions prevailing through the large thickness of massive concrete. Use of small size bars placed properly, lead to closer cracks but of smaller width. The risk of cracking due to temperature and shrinkage effects may be minimised by limiting the changes in moisture content and temperature to which the structure as a whole is subjected. The risk of cracking can also be minimized by reducing the restraint on the free expansion of the structure with long walls or slab founded at or below group level, restraint can be minimised by the provision of a sliding layer. This can be provided by founding the structure on a flat layer of concrete with interposition of some material to break the bond and facilitate movement. In case length of structure is large, it should be sub divided into suitable lengths separated by movement joints, specially where sections are changed the movement joints should be provided. Where structures have to store hot liquids, stresses caused by difference in temperature between inside and outside of the reservoir should be taken into account. In water retaining structures a dense unpermeable concrete is required, therefore, proportion of fine and coarse aggregates to cement should be such as to give high quality concrete. Hence concrete mix weaker than M20 is not used. The minimum quantity of cement in concrete mix shall not be less than 300N/cubic metre . Design of liquid retaining structures is different from ordinary RCC structures as it requires that concrete should not crack and hence tensile stresses in concrete should be within permissible limits.