Concrete surround is generally adopted for rigid drainage pipes to resist high traffic loads (for instance under shallow covers) and to allow for using pipes with lower strength. Furthermore the use of concrete surround can minimize settlement of adjacent structures. Additionally the highest possible accuracy in levels and gradient can be achieved by using concrete surround as considerable settlement is expected in other categories of beddings such as granular bedding. 

The distribution of reinforcement in concrete pipes can't be uniform due to the occurrence of tensile stresses in different locations around the circumference of pipes. For illustration tensile stresses are highest at the inner face of pipes at invert and crown levels and at outer face of two sides of pipes. An elliptical cage can be designed in order to optimize the usage of steel reinforcement.

Fig: Concrete surround of drains