1. When designing a water storage tank, should movement joints be installed?
In designing water storage tanks, movement joints can be installed in parallel with steel
reinforcement. To control the movement of concrete due to seasonal variation of
temperature, hydration temperature drop and shrinkage etc. two principal methods in
design are used: to design closely spaced steel reinforcement to shorten the spacing of
cracks, thereby reducing the crack width of cracks; or to introduce movement joints to
allow a portion of movement to occur in the joints.
Let’s take an example to illustrate this. For 30m long tanks wall, for a seasonal variation of
35 degree plus the hydration temperature of 30oC, the amount of cracking is about 8.8mm.
It can either be reduced to 0.3mm with close spacing or can be absorbed by movement
joints. Anyway, the thermal movement associated with the seasonal variation of 35oC is
commonly accounted for by movement joints.
For water-retaining structure like pumping stations, the crack width requirement is even
more stringent in which 0.2mm for severe and very severe exposure is specified in BS8007.
It turns out to a difficult problem to designers who may choose to design a heavy
reinforced structure. Obviously, a better choice other than provision of bulky reinforcement
is to allow contraction movement by using the method of movement joints together with
sufficient amount of reinforcement. For instance, service reservoirs in Water Supplies
Department comprise grids of movement joints like expansion joints and contraction joints.
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