Question
While designing sewer pipes, why vitrified clay pipes commonly is used for pipe size less than 600mm still as precast concrete pipes with PVC lining is used for pipe size more than 600mm?

Answer
Market price of vitrified clay pipes is usually less than that of precast concrete pipes with PVC coating. So, for little size of pipes as pipe diameter less than 600mm, it is more cost-effective to use vitrified clay pipes. Though, vitrified clay pipes do experience from problem of brittleness and its effect is still severe for outsized size of pipes. Furthermore, it is somewhat time consuming to send clay pipes products since majority of them is manufactured in Europe. Therefore, for larger size of sewer pipes as diameter additional than 600mm, it is usual to use precast concrete pipes with PVC coating.