The fact which makes it possible to combine steel and concrete is that concrete contracts on setting in the air and if a steel rod is embedded in a mass of wet concrete, it will be found that considerable force is necessary to pull out when the concrete is set. If the steel section is in the form of a plate, although it will resist removal when the concrete is set, yet it can be knocked off by sharp blows.
In the first case, the concrete grips the still, while in the second it only adheres. The grip depends upon the strength of concrete used in the work as well as the perfection with which the concrete has been mixed, placed in position, compacted and cured. Besides this, the grip also depends upon the condition of the surface of the rod (whether it is smooth or rough). Specially shaped bars (ribbed bars etc.) have therefore been introduced from time to time with the object of increasing the grip.
It is on account of the similar coefficient of expansion of the two materials, the superior bond value of high tensile strength and less cost of steel that’s why steel rods are used in reinforcement in R.C.C work.