Design Considerations

• Can I use indigenous material for bedding and backfill?
• Are there non-standard fittings available?
• Do you have alternatives to hydrostatically testing the completed pipeline?

Only one pipeline industry has consistently been able to answer yes to these and many other questions regarding the adaptability of its pipe products. That industry is the concrete pressure pipe industry. It’s this design adaptability that principally sets concrete pressure pipe apart from all other pipeline materials.

Concrete pressure pipe is designed for your project, not the other way around. Unsurpassed in strength, design and corrosion resistance, concrete pressure pipe beats the competition in every way. There is no other material available with more configurations than concrete pressure pipe.

If it’s sometimes difficult to compare and understand the attributes of different pipeline materials, then perhaps the following information will help.

Joints

Concrete pressure pipe is produced with flexible rubber-gasketed push-on joints. The joints are made with steel bell and spigot rings, each sized to permit no greater a tolerance than 1/32 of an inch between the outside diameter of the spigot and the inside diameter of the bell. The spigot has a groove on its circumference to hold an O-Ring rubber gasket. Once the spigot is inserted into the bell, the gasket is compressed on all four sides, creating a watertight seal between the rings. Bell and spigot, O-ring-sealed joints permit deflection for long radius curves and have ample flexibility to permit future ground settlement, expansion and contraction. Such joints are all routinely designed and tested for high pressures.

Unlike concrete pressure pipe, steel pipe joints are typically welded in the field, which limits the pipe’s ability to move with the soil. Moreover, field welding has significant quality control issues compared with factory welding in a controlled environment. Alternatively, steel pipe is sometimes produced with a roll-formed, O-ring sealed joint that remains as flexible and dimensionally unstable as the flexible steel pipe itself and therefore becomes difficult to join and make leak-free.

Mechanically restrained joints to resist pipeline thrust are available for concrete pressure pipe and produce a certain amount of flexibility in the pipeline. Concrete pressure pipe mechanically restrained joint types such as the C-Clamp, Snap-Ring and the Bell Bolt have provided the contractor and owner with an ease of harnessing for over four decades. However, steel pipe is typically welded and therefore cannot adequately transfer settlement loads back to the earth. They all transfer axial thrust forces while providing a degree of flexibility.

Unlike the joints for ductile iron pipe and steel pipe, concrete pressure pipe manufacturers provide an air testable joint. These double gasket air testable joints allow the installation contractor to pressure test individual joints with air immediately after they are field assembled to verify the gasket seal. This kind of design adaptability is of particular importance when the pipe is laid in areas where leaks would be difficult to discover or the pipe is otherwise inaccessible, such as subaqueous installations, deep cuts and pipe in tunnels. They are also valuable tools when connecting a new pipe section to an existing pipeline, such as during relocations, when it is difficult to isolate the area to be tested. In these cases, shut down time is usually extremely limited and it is important to know that the joint was properly made - before it is put into service.

Fittings and Specials

Concrete pressure pipe fittings are custom fabricated from steel plate on a project by project basis. Unlike when using ductile iron pipe, the designer is not forced to make do with “standard” fittings, or to require the contractor to install 2, 3 or 4 different fittings to accomplish a special section of the pipeline alignment.

All concrete pressure pipe specials and fittings are designed and manufactured in accordance with AWWA Standards and the AWWA M9 Manual to meet the same pressure and loading conditions as the adjacent pipe. Fittings (bends, reducers, tees, etc.) are fabricated from steel plate, joint rings or other end connections are attached and the assembly is lined and coated with concrete or cement mortar. Specials like outlets for lateral connections, air-release valves manholes, etc., can be incorporated into full lengths of pipe. These openings are reinforced with steel wrappers, saddles or sometimes, crotch plates.

Actually, the word “special” is misleading as it applies to concrete pressure pipe. There is nothing special about concrete pressure pipe fittings except the extraordinary capability to manufacture virtually any configuration or geometry or length or angle. A 34 degree bend, for example, is as common as a 45 degree bend. Reducing bends, wyes, tees and pipe with multiple connections are routinely produced. While this design adaptability is helpful to the designer on all projects, it is invaluable in the piping layout for Water and Wastewater Treatment Plants, Pumping Stations, Power Plants or any other congested area where it is virtually impossible to achieve the desired result with standard fittings produced by a pipeline supplier who is ridged in its approach.

Range of Sizes and Lengths

Concrete pressure pipe is offered in a wide range of diameters up to and sometimes exceeding 144 inches. Ductile iron pipe is only available only up to 64 inches.

Standard lengths vary by material. Concrete pressure pipe is available in lengths of 16 to 24 feet, depending on the type and diameter of the pipe. Ductile iron pipe lengths vary from 18 to 20 feet; Steel is generally produced in lengths of approximately 40 to 50 feet. While 20 feet may be an optimum length for most projects, there are conditions that demand longer or shorter pipe. Only the concrete pressure pipe industry has shown a willingness or even capability to provide non-standard lengths.

Shoring struts or utilities sometimes do not permit the use of full-length pipe, especially when it’s 50 feet long. Obstructions arise during the pipe’s installation that necessitates the use of non-standard pipe sections, or “short” pipe. While all pipes can be manufactured in shorter lengths, steel and ductile iron pipe encourage contractors to purchase full lengths and cut it to fit their needs, sometimes resulting in inferior joints and certainly increasing waste. Concrete pressure pipe manufacturers, on the other hand, regularly produce the lengths that the conditions call for - and provide a pipe layout schedule instructing the contractor where these pipe sections should be installed.

Unlike ductile iron pipe, two or more sections of concrete pressure pipe may be permanently joined together at the plant to provide a 40 or 60-foot pipe length. While not relevant for most projects, this additional example of concrete pressure pipe’s design adaptability can greatly reduce the amount of a contractor’s underwater work during the installation of outfalls and river crossings or permit a designer to create an aerial pipeline with a reduced number of supports.

Strength

Flexible pipe, including ductile iron pipe and steel, are greatly dependent on the support of the surrounding ground to provide strength to the pipe-soil structure. In fact, the flexible pipe itself may provide as little as 15% of the necessary strength, relying on the contractor to construct the balance, a sometimes elaborate, structural soil envelope. This generally requires strict control of the trench width, special imported bedding and backfill material and compaction of the soil in the pipe zone, all of which adds to the cost of construction and transfers the onus of performance from the pipe to the contractor and field inspection.

Concrete pressure pipe is designed to withstand extreme earth loads and live loads because of its rigid structural wall. Concrete pressure pipe’s load carrying capability is built in, providing up to 70% of the strength of the pipe-soil structure. Unlike flexible ductile iron of steel, the inherent strength of Concrete pressure pipe often allows the designer to use native bedding and backfill material. The designer can be confident that varying soil conditions along the pipeline alignment, whether existing or as constructed, will not have a major influence the pipe's performance. When concrete pressure pipe arrives on the project, the structure arrives in it. However, when steel or ductile iron pipe arrives on the project, the contractor must create the structure around the pipe. Unfortunately, the only way to ensure that the proper structure is created around flexible pipe is to provide for continuous inspection and testing of the contractor’s work. And that is an unnecessarily costly addition to the project.

Rigid concrete pressure pipe also provides an additional defense against hazards experienced by flexible pipe such as protection against vacuum collapse. Unlike ductile iron pipe and steel pipe, rigid concrete pressure pipe has a natural resistance to buckling.

Special Applications

Generally the concrete pressure pipe designs can be accomplished with “standard” walls and dimensions, but sometimes, due to extreme conditions, non-standard walls and special designs are necessary.

Ductile iron pipe is available in a number of standard thicknesses, suitable for specific external loads with specific beddings. Concrete pressure pipe, on the other hand, can be manufactured with a special increased wall thickness permitting it to be designed for substantial earth loads with virtually any bedding. This design adaptability allows concrete pressure pipe to be used in special applications, such as a spillway beneath a dam with earth covers of 100 feet or more.

Increasing the wall thickness also provides the designer with the ability of increasing the weight creating a non-buoyant pipe. The weight of concrete pressure pipe generally precludes it from the floatation that might occur with lighter pipe materials, both during construction and when installed with shallow earth cover. Some subaqueous applications, however, require that the pipe to be negatively buoyant, even when completely empty and not backfilled. Steel pipe is occasionally “weighted” to render it non-float. Concrete pressure pipe can be manufactured to be negatively buoyant.

The heavy wall and straight barrel of reformed concrete cylinder pipe, RCCP (AWWA C300), renders it a superior pipe for jacking and microtunneling. There is no ductile iron choice for large diameter direct jacking pipe. Steel pipe requires substantial welding or expensive machined joints to be jacked and very often, increased wall thickness to prevent buckling. RCCP is manufactured with thick, high strength concrete walls, providing an inherent axial strength and buckling is not a consideration. RCCP for jacking is provided with the same a flexible rubber gasketed push-on joint as in all concrete pressure pipe.

2008 Copyright American Concrete Pressure Pipe Association
11800 Sunrise Valley Drive, Suite 309 Reston, Virginia 20191
(P) (703) 391-9135   -   (F) (703) 391-9136