Corrugated metal pipes (CMPs) are among the most widely used materials in underground stormwater infrastructure. Though often overlooked, they play a vital role in ensuring proper water drainage, preventing problems such as ponding, standing water on roads, and localized flooding.
CMPs serve multiple purposes in underground construction, including use in culverts, stormwater detention and retention systems, storm sewers, small bridge replacements, aeration pipes, and material conduits.
However, without sufficient protection—such as structural polymeric linings—CMPs may require frequent maintenance and repairs. Read on to learn more about CMPs, the three common problems they encounter, and effective ways to address them.
A corrugated metal pipe (CMP) is a type of pipe designed with parallel ridges and grooves that give it added strength and flexibility. It is primarily used in culverts and various underground infrastructure applications.
As noted, CMPs are most frequently used in drainage culverts, storm sewers, material conduits, and other systems that manage underground stormwater flow.
These pipes are typically made from aluminum or steel and are available with various metallic coatings and alloy compositions to suit different environmental and structural requirements.
Aluminum Alloy
Aluminum alloy CMPs are built to withstand corrosion and abrasion in underground environments. The pipe surface is coated with a high-grade aluminum alloy that enhances durability and extends service life.
Aluminized Steel
Aluminized steel CMPs are produced by hot-dipping steel pipes in pure aluminum. This process provides a strong barrier against corrosion, making these pipes especially effective in culverts and drainage systems exposed to mildly acidic water conditions.
Galvanized Steel
Galvanized steel CMPs are created through the galvanizing process, where a zinc coating is applied to protect the underlying steel from corrosion. Although generally less durable than aluminized steel, the zinc layer serves as a temporary shield against rust and deterioration by preventing direct contact between water and the steel core.
Asphalt-Coated Galvanized Steel
These pipes feature both zinc and asphalt coatings on their interior and exterior surfaces. The asphalt layer enhances corrosion resistance, providing longer protection against moisture, debris, and abrasive runoff. This dual-layer protection extends the service life of galvanized CMPs in demanding environments.
Polymer-Coated Steel
Polymer-coated steel CMPs use advanced polymeric coatings or structural linings to improve performance and longevity. These coatings provide superior resistance to abrasion and chemical damage from acids, salts, alkalis, and other corrosive materials commonly found in culverts and sewer systems.
#1. Open Joints
Joints are the connection points between adjacent sections of corrugated metal pipe. When these joints become misaligned or separate, gaps can form that allow surrounding backfill material to erode into the pipe. Over time, this can lead to structural weakening and eventual pipe failure if not properly addressed.
The solution is joint sealing, which involves sealing the joints to prevent soil and water infiltration. Polymeric linings are particularly effective for this purpose, as they provide both structural reinforcement and resistance against soil intrusion and hydrostatic pressure.
#2. Voids
Voids are empty spaces that develop between the CMP and the surrounding soil. These gaps can compromise structural integrity and cause ground settlement. They are typically repaired by injecting polyurethane grout to fill the voids, followed by applying polymeric linings to seal and strengthen the pipe’s surface, ensuring long-term stability.
#3. Deteriorated Inverts
The invert is the lowest interior point of a corrugated metal pipe where water flows. Because it experiences the highest levels of water contact, it is highly vulnerable to abrasion during heavy flow events and corrosion over time.
Without a durable protective barrier—such as a polymeric lining—the metal can corrode and eventually rust through. Once this occurs, it may lead to further complications like void formation or complete structural collapse. Protecting the invert with long-lasting, corrosion- and abrasion-resistant coatings is essential for extending the lifespan of CMP systems.
1. Fixing Open Joints
Open joints are typically repaired through joint sealing. This process involves cleaning the affected area and applying a polymeric or epoxy-based sealant to close the gap between pipe sections. In some cases, internal joint bands or repair sleeves are installed for additional structural support. Applying polymeric linings after sealing provides extra protection by reinforcing the pipe’s interior surface and preventing further soil or water intrusion.
2. Fixing Voids
Voids between the pipe and surrounding soil are repaired using polyurethane grouting. This method involves injecting high-strength polyurethane foam into the voided areas, which expands to fill gaps and restore soil contact around the pipe. After grouting, a polymeric lining is often applied inside the CMP to strengthen the pipe walls and prevent future deterioration.
3. Fixing Deteriorated Inverts
Deteriorated inverts are restored using structural polymeric linings or spray-applied polymer coatings. These solutions create a seamless, corrosion-resistant layer over the damaged metal, effectively sealing leaks and restoring hydraulic flow capacity. For severely corroded sections, invert replacement or trenchless rehabilitation (such as slip lining or cured-in-place pipe methods) may be used. Applying a durable polymeric liner ensures long-term protection against future abrasion and corrosion.
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