How to Produce Concrete Pipe Since the beginning of civilization, conduits have been used for the transportation of water, industrial wastes and sewage. Perhaps the most permanent conduits were formed of reinforced concrete pipe. Modern sanitation was made possible in large part through the use of concrete sanitary sewer pipe. Concrete is one of the most versatile, durable and economical conduit materials available for storm sewers, sanitary sewers and irrigation lines. As a result of advanced technology and design, the use of concrete pipe continues to increase throughout the world. Engineers and specifying agencies have recognized it as a permanent, cost-effective system for transporting liquid and waste materials. Once in place, properly designed underground pipelines require no upkeep or operational expense and retain their serviceability for many years. Three Common Processes Today, there are three commonly used processes for producing precast concrete pipe: dry cast, packerhead, and wet cast. The first two, dry cast and packerhead (conventional or bidirectional) are classified as immediate strip methods. (The word "strip" refers to the removal of the form from the pipe.) "Immediate strip" is characterized by the use of no-slump concrete, which is sufficiently compacted during the pipe-making cycle to permit removal of the inner core or outer form as soon as the pipe has been produced. Thus, the form is available for re-use any number of times during the day. Dry Cast The dry cast process conforms to the basic principle of making a pipe by consolidating relatively dry concrete between an inner core and an outer form. Vibration is used to distribute and densely compact the mix. Lengthwise compression of the pipe is often employed to supplement the vibration. This vibration may be external electric, hydraulic (on the form), internal hydraulic or pneumatic (inside the core). Dry cast pipe machines may be either one, two or three station models and these various models can generally cover a size range from 12" to 156" (300 mm - 4000 mm) in diameter in lay lengths of 8’, 12’, and 16’ (2.5 m, 3.5 m, and 5 m). These machines are unique because they can make a wide variety of shapes and types of concrete products such as round pipe, elliptical pipe, arch pipe, pre-bed, box culvert, and PVC-lined pipe. Packerhead The other immediate strip method is the packerhead process. Packerhead models can make round pipe from 6" - 84" (150 mm - 2100 mm) in diameter and in lengths up to 1’ - 12’ (.3 m - 3.5 m). This type of pipe is used for storm and sanitary sewers as well as irrigation and drainage applications. In this production process, dry (no-slump) concrete is fed into an outer form to make the exterior of the pipe. The interior of the pipe is shaped by a rapidly rising and revolving mechanism called a rollerhead, which compacts the concrete radially outward against the inside of the form. On conventional pipe machines, all rollerhead elements rotate in the same direction. On bidirectional pipe machines, the rollerhead and trowel rotate in opposite directions, ensuring that torsional stresses are eliminated in the green pipe. The rollerhead is mounted on the end of a long spinning shaft and turns within the form to roll and pack the pipe to the proper density while the form itself remains stationary. Right: BiDi has eliminated cage twist problems while producing a pipe with excellent wire-to-concrete bond. Above: BiDi Concrete Pipe Machine Wet Cast The wet cast manufacturing process uses a relatively wet concrete mix. The pipe is made vertically in the annular space between an inner core and an outer form. This process is most commonly used for large diameter pipe production where the pipe sections are manufactured, cured and stripped at a single location. Depending on the pipe diameters and lengths, various numbers of external vibrators are attached to the forms for consolidating the concrete. The forms are freestanding and no machine is involved in production. Usually, several forms are set up at the same time so a pouring line can be established. Wet casting is a very versatile process. Because there are no constraints of a machine frame, pipe may be cast in several lengths. Pipe of many lengths and diameters may be intermixed in the casting line. Lengths of 24’ (7 m) are common, especially for use in wide-open terrain and outfall lines. Above: Pre-cast applications for the concrete industry. Choosing a Process Many conditions dictate which concrete pipe making process is best for the individual job or producer. These include such factors as pipe diameters and lengths to be made, quantities of pipe, availability of equipment and labor, and actual application of the end product. A specifying agency is responsible for making sure that job specifications are clear and properly reflect what is needed and expected. It is then up to the producer to determine the best way to comply with these pipe specifications and requirements.