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Conduit can be joined by a variety of heat fusion and mechanical methods. Since conduit does not experience any long-term internal pressure and acts only as a pathway for power or other cables, the owner of the system may be tempted to neglect the importance of specifying effective couplings.

However, an integral part of any conduit system is the type and quality of joining method used. Proper engineering design of a system will consider the type and effectiveness of these joining techniques.

Joint Selection Considerations

The owner of the conduit system should be aware that there are joint performance considerations that affect the system’s reliability well beyond initial installation. Some of those might include:

• Pull-out resistance, both at installation and over time due to thermal contraction/ expansion and ground movement must be considered. This is critical for “blow-in” cable installations, which will exert an outward force at joints, but less so for pulling installations, which will tend to exert the opposite force. Once conduit is installed, temperature fluctuations are moderated by the surrounding soil, therefore pull-out forces due to temperature expansion/contraction are mitigated. Pull-out resistance due to expansion and contraction should not be confused with the safe pull strength required in HDD (Horizontal Directional Drilling). For HDD, either butt- or electro- fusing the joint are acceptable methods for joining the conduit if the joint is going to pulled into the bore.
• Internal pressures for “blow-in” installations are typically between 125 to 200 psi for a short period. Consider how much leakage can be tolerated without reducing the distance the cable can consistently be moved through the conduit.
• Infiltration leakage, allowing water and/or silt to enter the conduit over time, can create obstacles for cable installation and repair or in above ground installations can cause the water to freeze and compress the fiber optic cables.
• Corrosion resistance is important as conduit systems are often buried in soils exposed to and containing alkali, fertilizers, and ice-thawing chemicals, insecticides, herbicides and acids.
• Cold temperature brittleness resistance is required to avoid problems with installation and long-term performance in colder climates during installation.

General Joining Provisions

HDPE-to-HDPE joints may be made using heat fusion, electrofusion or mechanical fittings. The use of mechanical couplings are often preferred over fusion joints for diameters up to 6”. Mechanical couplings allow HDPE conduit to be joined to other junction boxes or other hardware utilized by the communication and electrical industries.

The user may choose from many available types and styles of joining methods, each with its own particular advantages and limitations. Contacting the various manufacturers is advisable for guidance in the proper applications and available styles for joining.

Mechanical Fittings

HDPE conduit can be joined by a variety of available styles of mechanical fittings, each with its own particular advantages and limitations. This section will not address these advantages or limitations but will only offer general descriptions of many of these fitting types and how they might be utilized.

Experts recommend that the user be well informed of the manufacturer’s intended applications, performance limits, and joining procedure for the particular mechanical fitting being used.

Barbed (i.e. Hydraulic Press-on) Mechanical Fittings

Barbed fittings are available in various materials and configurations for joining conduit sizes 2 in. and smaller. Installation involves pressing the fitting over the ends of the conduit to be joined using a special tool.

The inside of these fittings contains sharp, inward-facing barbs that allow the conduit to be pressed-in, yet dig into the outer surface of the conduit and resist removal when pulled. They offer excellent tensile strength for plowing operations, and, if installed correctly, have very good internal pressure characteristics for cable blowing applications.

Threaded Mechanical Fittings

Threaded mechanical fittings are available in various materials and configurations for conduit sizes 2 inch and smaller. Some are designed with sealing capabilities while others are not. The internal thread designs of these fittings are typically tapered similar to pipe threads, with a left-hand thread on one end and a right-hand thread on the other to cut thread paths on the conduit’s outer surface.

This thread design allows the operator to thread the fitting onto the ends of both conduit sections simultaneously. Some variations of threaded fittings may also be pressed on the conduit ends and used as barbed fittings. The user should consult the fitting manufacturer to determine if this alternate installation method is recommended.

Compression Coupler

Compression couplers apply compression to the outside wall of the conduit to retain the conduit and create the joint, as shown in Figure 25. As with the other mechanical fittings, compression fittings are also available in numerous designs and for different size ranges.

While compression fittings used in the HDPE pressure piping industries, such as water or gas, typically require internal stiffeners, conduit systems typically do not as stiffeners may create obstacles for cable being blown through the conduit.

Expansion Joints

Expansion joints are designed primarily for aerial conduit installations. The primary purpose of this fitting design is to absorb thermal expansion and contraction in the conduit system created by ambient temperature changes, which can be extreme in above ground installations.

System designers should determine the number of expansion joints required based on the expansion length provided by the fitting and a calculation of the conduit’s overall thermal expansion factor for the length of above ground section being joined.

Heat Fusion

The principle of heat fusion is to heat two surfaces to a designated temperature and fuse them together by application of a force sufficient to cause the materials to flow together and mix. When fused in accordance with the manufacturer’s recommended procedure and allowed to cool to nearly ambient temperatures, the joint becomes as strong or stronger than the conduit itself in both tensile and pressure properties.

The three primary heat fusion methods used in joining PE conduit are butt, socket, and electrofusion. Butt and socket fusion joints are made using “hot irons” designed specifically for PE joining, while with electrofusion fittings heat is internally generated by electrical current applied to embedded wires within the fitting.

We recommend that users precisely follow the qualified fusion procedures established by the manufacturer of the particular heat fusion joining equipment and fittings being used.

Butt Fusion Joining

Butt fusion joints are produced without need of special fittings, using specially developed butt fusion machines, that secure, face and precisely align the conduit for the flat face hot iron fusion process. In the process, the ends of the two conduits to be joined are heated until molten and then brought together and allowed to cool creating a strong joint.

It should be noted that the butt fusion process produces an internal bead of equal or larger size than the visible outer bead that can cause restrictions and interfere with cable placement. If internal restrictions are a concern for the cable installation, alternative-joining methods may be more appropriate.

The use of a metal end cap with a rounded end will mitigate potential damage of the fiber, and normally “deflect” off any internal bead during cable installation.

Socket Fusion Joining

This technique requires the use of specially designed hot irons to simultaneously heat both the external surface of the conduit and the internal surface of the socket coupling. Specially designed hand tools are available to maintain alignment and the stab depth of the hot irons until the materials reach fusion temperature.

These tools also help secure the heated conduit end and coupling as the joint is made. Design requirements for socket fusion are provided in ASTM D2683 for fittings and in ASTM F1056 for socket fusion tools. As with butt fusion, socket-fused joints may have an internal bead that can interfere with cable placement.

Electrofusion Joining

Electrofusion is somewhat different from the hot iron fusion method described previously, the main difference being the method by which heat is applied and the location of the resulting fusion. Electrofusion involves the use of a special electrofusion socket fitting that fits over the ends of the conduit.

The fitting has an embedded wire coil. Using a specialized external electrofusion control box, electrical current is supplied to the wire coil heating and melting the inside of the socket and the outside surface of the conduit until they fuse together.

The resulting joint is water tight and as strong as the conduit. No internal bead is generated with electrofusion.

Repair Operations

Repair fittings, as the name implies, are often designed specifically for use in repair situations. The nature of the damage will often dictate what types of joints are needed for repairs. For example, of one type of design, a clamp-on style may be preferred when damage is limited and removal of the cable for repair is not necessary.

There is a PVC tongue-and-groove semi-circle product that can be made water tight with caulking or two-part epoxy glue. However, in more severe damage situations, where new cable and conduit sections must be installed, many of the joining methods described earlier in this section may be suitable.

Ultimately, the type of repair fitting or joint installed should maintain the integrity of the conduit system, prevent infiltration and provide sufficient resistance pull-out from thermal expansion/contraction.

Special Applications

The following subsections provide an introduction to some specialized products and applications for conduit. For more information, consult with your conduit supplier.

Corrugated Duct

Corrugated conduit has properties that generally make it easier to work with in difficult and confined environments. The corrugated profile of the wall provides a greater degree of flexibility and the lack-of-memory that makes it easier to handle when used in confined spaces and other restricted environments, such as “inside plant- premise” applications.

Corrugation design (or profile) greatly affects the properties of the conduit such as compressive resistance and tensile strength. The ASTM standards that cover SIDR and SDR conduit designs do not apply to corrugated duct.

All corrugated conduit specifications are specific to the producer. Generally, a minimum ID and a maximum OD is specified. Products vary from manufacturer to manufacturer so check with the source of supply for detailed dimensional and performance specifications.

Corrugated conduit is not appropriate for use in direct buried applications because of its limited crush resistance and the difficulty of laying it in a straight path. It should not be installed using directional drilling equipment due to limited tensile strength and the fact that the corrugations will create significant friction and stretch during the pullback causing the conduit to separate.

Corrugated conduit is also not appropriate for use when cables are to be installed using air-assisted placement as it is relatively thin-walled and may not be able to handle the air pressure of air-assisted placement. The corrugations also create significant air turbulence that is counterproductive to the air-assisted placement systems and reduce the distance cables can be blown.

Multi-Cell Conduit

Multi-cell conduits are designed to meet special needs and unique job situations. There are a number of designs available to meet most of these special needs.

Multi-cell conduit can be a product that is installed as an innerduct inside of existing conduits designed to maximize the available space in a vacant or occupied conduit, or it can be a fully assembled conduit with internal conduits that, when installed, provides a multi-channel conduit without the need to install any other innerducts.

Some multi-cell designs can be direct-buried like HDPE conduit using standard installation methods (plowing or open trenching).

Armored Conduit (Rodent and Mechanical Protection)

When placing cables underground, there are occasional concerns about the need for greater mechanical protection of the cable(s) inside from the cutting and gnawing by animals. Armored Conduit is HDPE conduit that has a layer of metal for additional protection against animal attack.

Armored conduit also provides greater mechanical protection against cuts and abrasions from accidental strikes during excavation of adjacent utilities.

Conduit Building Entry Electrical Applications

Electrical/Building Code regulations vary greatly regarding the placement of conduit into a building. Codes require the use of conduit constructed of a material that is listed for use in specific building areas, and these codes limit the use of HDPE conduit beyond a specific distance after entry through an exterior wall.

The greatest variation in local code is the location of the transition from HDPE conduit to a conduit that meets the code requirement (distance from the exterior of the wall). Check your local codes for local amendments.

Premise (Flame Retardant) Comm/Data Applications

In addition to using conduit for installing fiber optic/communication cables in the underground, there are a few other very specialized applications for conduit type products. With the growing market for data communications systems and IOT (internet of things) within buildings, there has been a concurrent growth in the use of fiber optic/communication cables in buildings as well.

As these installations typically place fiber optic/communication cables in the same cable trays and vertical risers as other communications cables and electrical cables, designers and installers have been concerned about identifying and protecting these fiber optic/communication cables.

Manufacturers have responded with the development of several types of conduit for building use, or as it is known in the industry, premise wiring. Premise wiring generally uses plenum air spaces, vertical riser shafts and general- purpose areas to run cables throughout buildings.

The types of conduit developed are specific to these environments. Because premise wiring falls into areas generally thought of as locations having greater human health and safety concerns, the NFPA 70 and Underwriters Laboratories have addressed the characteristics needed by conduits to be safely used in premise wiring.

These conduit products are generally not made of HDPE due to the flame spread and smoke requirements; however, future innovations may bring acceptable HDPE-based products to this market in the future. Initial development produced the Plenum Raceway, a specialized conduit that meets more stringent Underwriters Laboratories UL 2024 requirements for minimum flame spread and smoke generation than riser rated conduit.

Plenum Raceway can be a corrugated conduit or microduct made from plastic materials that do not support flame and produces very low smoke. At this time, PVDF (polyvinylidene fluoride) is the material of choice for Plenum Raceway.

Similarly, Riser Raceway was developed for premise wiring applications to be installed in specific locations such as in riser shafts. Riser Raceway meets the Underwriters Laboratories UL 2024 requirements for vertical flame spread. The Riser Raceway requirements are lower than Plenum Raceway. Riser Raceway can also be a corrugated conduit or microduct.

General-Purpose Raceway was developed for premise wiring applications in general purpose applications. General-purpose raceway meets the Underwriters Laboratories (UL 2024) requirements for flame spread and has lower requirements than Riser Raceway.

General-purpose raceway is typically a corrugated conduit or microduct, which is currently produced from either PVC or polyamide materials. All conduit products for use in riser, plenum, or general raceway applications are required to be marked with the application specific product standard and a Listing

Mark from a nationally recognized independent certification laboratory (e.g. UL, ETL20, CSA, etc.,) to demonstrate that the product has been tested and meets the requirements for installation in the specific application. Plenum Raceways are permitted to be placed in a riser application. Plenum and Riser Raceways are permitted to be placed in a general-purpose application.

The uses of Plenum or Riser Raceways do not eliminate the use of a Plenum- or Riser-rated cable. As the use of fiber optic/communication cables in premise wiring increases there will likely be other specific needs that may generate other types of conduit for use in building wiring systems.

Inside plant microducts are now produced for premise and raceway product meeting UL 2024 standards. Fiber cable can be air blown into microduct.

Underwater Applications

The term underwater is used to describe marine, or submarine, applications. The three basic methods of placing an underwater conduit are laying the conduit on the bottom, plowing and jetting the conduit into the sub-aqueous terrain, or horizontal directional drilling under the waterway. Each method has its own unique requirements based on the type of waterway, length, environmental issues, and federal, state and local requirements.

There may be instances when all three methods will be required within the same installation. Conduit placed on the bottom of waterways should be black to prevent UV damage.

Bridge Crossing Applications

Bridge structures can range from a simple conduit placed in the bridge structure when the bridge is built to a major retrofit of an existing bridge that does not already contain a conduit or structure in place to secure a single conduit or conduits.

Bridge structures, new or old, require specially designed support systems to ensure structural integrity and meet all federal, state and local requirements. The expansion/contraction considerations with HDPE make it a less than ideal product for exposed bridge crossings.

Generally, FRP conduit (Fiberglass Reinforced Pipe) is a better alternative for bridge conduit as it has beam strength, is produced to “low smoke zero halogen” standards, and can be designed to be bullet proof.

Conduit Specific Industry Terminology & Abbreviations