This chapter discusses commercially available metallic pipes. The main disadvantage of the cast-iron pipe is that it is not ductile the pipe fractures under excessive loads or impact. Like the gray cast-iron pipe, the ductile-iron pipe is often lined and/or coated with cement mortar. The ductile-iron pipe has largely replaced the gray cast-iron pipe in years. Because concrete has high compressive strength, the plain concrete pipes can withstand high external pressure, such as that imposed on it by the earth and traffic above it. The weight of the pipe is given in schedule number (for steel, stainless steel pipes, and PVC), or class number (for cast-iron or ductile-iron pipes). The designations for stainless steel pipes and aluminum pipes are similar to that of steel pipe except that a suffix S is affixed to the schedule number, as for instance the designation schedule 40S.