Definitions:

Crown – inside top of pipe Invert – the lowest point of the inside of a pipe or manhole

Force main – A pipe that carries wastewater under pressure from the discharge side of the pump.

Pipe Construction and Materials

There are several different pipe materials available for wastewater collection systems, each with a unique characteristic used in different conditions. The four different pipe materials described below are:

* Ductile iron
* Concrete
* Plastic
* Vitrified clay

Pipe material selection considerations include:
* Trench conditions (geologic conditions)
* Corrosion
* Temperature
* Safety requirements
* Cost

Key pipe characteristics areas follows:
* Corrosion resistance (interior and exterior)
* Scouring factor
* Leak tightness
* Hydraulic characteristics

Pipe manufacturers follow requirements set by the American Society of Testing Materials (ASTM) or American Water Works Association (AWWA) for specific pipe materials. Specification standards cover the manufacture of pipes and specify parameters such as:
* Internal diameter
* Loadings (classes)
* Wall thickness (schedule)

The methods of pipe construction vary greatly with the pipe materials. Some new pipe materials and construction methods use the basic materials of concrete pipes with modifications (i.e., coatings). Other pipe manufacturing methods use newly developed resins which offer improvements in strength, flexibility, and resistance to certain chemicals. Construction methods may also allow for field modifications to adapt to unique conditions (i.e., river crossings, rocky trenches, etc.) or may allow for special, custom ordered diameters and lengths.

Ductile iron pipe (DIP) is an outgrowth of the cast iron pipe industry. Improvements in the metallurgy of cast iron in the 1940’s increased the strength of cast iron pipe and added ductility, an ability to slightly deform without cracking. This was a major advantage and ductile iron pipe quickly became the standard pipe material for high pressure service for various uses (water, gas, etc.)

Two types of concrete pipe commonly used today are Prestressed concrete cylinder pipe (PCCP) and reinforced concrete pipe (RCP). PCCP is used for force mains, while RCP is used primarily for gravity lines. PCCP may be of either embedded cylinder (EC) or lined-cylinder construction (LC).

The construction process for both the LC and EC begins by casting a concrete core in a steel cylinder. This single process produces the LC pipe. Once the cylinder cures, it is wrapped with a prestressed steel wire and coated with cement slurry and a dense mortar or concrete coating to produce the EC pipe.

The manufacturing process for reinforced concrete cylinder pipe (RCCP) is similar to embedded cylinder, however, a reinforcing cage and the steel cylinder are positioned within a reusable vertical form and the concrete is cast instead of using the prestressed wire. RCCP can be cured by using either water or steam.

Plastic pipe is made from either thermoplastic or thermoset plastics. Characteristics and construction vary, but new materials offer high strength and good rigidity. Fluorocarbon plastics are the most resistant to attack from acids, alkalies, and organic compounds, but other plastics also have high chemical resistance. Plastic pipe design must include stiffness, loading, and hydrostatic design stress requirements for pressure piping.

Thermoplastics are plastic materials which change shape when they are heated. Common plastics used in pipe manufacturing include Polyvinyl Chloride (PVC), Polyethylene (PE or HDPE for High- Density PE), Acrylonitrile-butadiene-styrene (ABS), and Polybutylene (PB). HDPE is commonly used with pipe bursting.

PVC is strong, lightweight, and somewhat flexible. PVC pipe is the most widely used plastic pipe material. Other plastic pipes or composites with plastics and other materials may be more rigid. Thermoset plastics are rigid after they have been manufactured and are not able to be reformed. Thermoset plastic pipes are composed of epoxy, polyester, and phenolic resins, and are usually reinforced with fiberglass. Resins may contain fillers to extend the resin and to provide specific characteristics to the final material. The glass fibers may be wound around the pipes spirally, in woven configurations, or they may be incorporated into the resin material as short strands. The pipes may be centrifugally cast. Stiffness may also be added in construction as external ribs or windings.

Reinforced Plastic Mortar (RPM) and Reinforced Thermosetting Resin (RTR) (or Fiberglass Reinforced Plastic Pipe (FRP)) are the two basic classes of these pipes. Another name is Fiberglass Reinforced Polymer Mortar (FRPM). Thermoset pipes are often manufactured according to the specific buyer requirements and may include liners of different composition for specific chemical uses.

For plastic pipes, resins composed of polymerized molecules are mixed with lubricants, stabilizers, fillers, and pigments, to produce mixtures with different characteristics. Plastic pipes are generally produced by extrusion.

Plastic pipe may be used for Sliplining or for rehabilitating existing pipes by inserting or pulling them through a smaller diameter pipe. HDPE pipes may also be used for bursting and upgrading. The smaller diameter pipe may be anchored into place with mortar or grout.