Q. Mild steel and high yield steel in water-retaining structure?

In designing water-retaining structures, movement joints may be installed in parallel with steel reinforcement. To control the movement of concrete because of seasonal variation of temperature, hydration temperature drop as well as shrinkage etc. two major methods in design are used: to design closely spaced steel reinforcement to reduce the spacing of cracks, thus reducing the crack width of cracks; or to introduce movement joints to enable a part of movement to take place in the joints. 

For the choice of steel reinforcement in water-retaining structures, mild steel and high yield steel can both be accepted as reinforcement. With the limitation of crack width, stresses in reinforcement in service condition are usually below that of normal reinforced concrete structures and henceforth the use of mild steel reinforcement in water-retaining structure will suffice. Furthermore the use of mild steel restricts the development of maximum steel stresses so as to decrease tensile strains and cracks in concrete. 

Though critical steel ratio of high yield steel is much smaller than mild steel since critical steel ratio is inversely proportional to yield strength of steel. Thus the use of high yield steel has the potential benefit of using smaller amount of steel reinforcement. Conversely though cost of high yield steel is slightly higher than that of mild steel, little cost difference is offset by better bond performance and higher strength associated with high yield steel.