Making the Case for Composite Manhole Covers

How composite manhole covers can alleviate I&I and help the environment.

Making the Case for Composite Manhole Covers

Compression-molded composite manhole covers and frames are manufactured using the same mold every time, resulting in a product that is engineered to precise dimensions with little part-to-part variation.

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Over the years, a number of accessories have been designed to alleviate issues with cast iron manhole covers — issues like corrosion, theft, heavy weight, stormwater infiltration or their significant carbon footprint. Today, there’s a comprehensive option in the form of composite manhole covers.

Composite manhole covers were originally developed to prevent theft of iron manhole covers for scrap metal content, which cost cities millions and left gaping holes in streets for people, vehicles and wildlife to fall into. But municipalities found they’d also gained another benefit: not having to deal with the corrosion of iron covers and frames.

A variety of bacteria in sanitary sewers emit hydrogen sulfide gas, which gets converted by a second bacteria, Thiobacillus, into sulfuric acid. The acid corrodes iron covers and rings, degrading the mechanical strength and sometimes fusing the cover to the frames. Covers can become nearly impossible to open without slamming the cover with a sledgehammer, further damaging the unit.

Composite covers solved those issues. They’re not recycled in the same manner and there’s no market for the scrap, which eliminates the root cause of cover theft. Also, they’re resistant to corrosion and don’t fuse, allowing inspectors critical access for reviews and preventive system maintenance.

Stormwater issues

Some 32 trillion gallons of floodwaters are polluted annually by sewage, chemicals and waste, according to the Summer 2019 issue of Nature Conservancy. When stormwater enters sanitary sewer lines through water pathways at the junction of the manhole cover and frame, it results in inflow and infiltration as pumps are overworked, sewer systems are overburdened and municipalities have to pay for unnecessary wastewater treatment costs.

When systems overflow, hazardous waste finds it exit into streets, creeks, rivers, lakes, ponds and oceans. Recent increases in flooding have only exacerbated the issue of sanitary sewer overflows.

Iron covers and frames are molded in individual sand casts, and a different cast is used for each cover and frame, which can lead to part variation. Iron covers are also designed with larger gaps between cover and frame than composites, and utility workers sometimes replace damaged iron covers without properly mating them to the frame. All of this can lead to stormwater infiltration.

Corroded covers are even worse, as sections of the cover or frame can break over time. And using a sledgehammer to combat cover fusion also chips away at the seal.

Composite covers and frames are a watertight solution. For instance, Composite Access Products (CAP) covers and frames have been shown to be 100% watertight in 20 inches of water, as 0.00 gpm infiltration was measured and verified by a third party.

Unlike iron castings, compression-molded composite covers and frames are formed by the same mold every time. The molds are highly engineered to precise dimensions (ten thousandths of an inch tolerances), and the part-to-part variation is greatly reduced with single molds.

Furthermore, because of the properties of polyester/vinyl ester thermosetting resins, which make up the composite resin matrix, the resulting parts maintain a tight dimensional precision and stability over a range of conditions. This includes dimension stability from 60 degrees below zero to 360 degrees F.

In an effort to monitor and prevent SSOs, smart city technologies are being developed to detect irregularities inside the manhole, transmitting data to utility operators for a quick response. Iron covers block the transmission signals, and it is necessary to drill holes in the cover for antennae, thus creating an additional water pathway.

Meanwhile, composite covers are transparent to transmission signals and don’t require antenna holes. Furthermore, technology can be encapsulated into the substrate of the composite to protect sensitive instruments from chemicals and impacts.

Environmental impacts

The iron forging process is performed at a significantly higher temperature than that which is used for composite molding, which is approximately 275 to 300 degrees F. The much higher temperature of iron forging incurs higher energy consumption and a larger carbon footprint for the production of parts. Also, in iron forging, dust and fumes can be exhausted into the air, and slag must be disposed of.

To slow the corrosion process on iron manhole covers, some municipalities require them to be dipped in chemical coatings, which can eventually wear off. Some use a coal tar sealant, which the U.S. Geological Survey found is linked to several forms of cancer and is toxic to aquatic life when runoff occurs. While coal tar coatings are banned in several states and cities, they’re still in use.

Meanwhile, the compression molding of thermoset composites produces little waste and emissions. Less than 5 ppm of styrene is released during the process, and the Occupational Safety and Health Administration sets human exposure limits for styrene at 100 ppm for an eight-hour time-weighted average.

No runners or sprues are produced for scrap, and only about 200 grams of excess molded fiberglass is produced per part in composite manhole manufacturing. The molds used are steel and are used for millions of cycles.

For years, industry outsiders have claimed that thermosets can’t be recycled. Though not recycled in the usual method like plastics (melting and reforming), thermosets have been recycled and blended back into compounds starting with applications in the ’90s. There is no “market” for recycled thermosets, so thieves will not have an incentive for stealing these manhole covers. At the end of a composite cover’s useful life, however, it will be possible to regrind and reuse this material back into the raw material mix.

Finally, many technologies are being developed that use biodegradable resins and reinforcements, but research is required to see how this would work in a manhole cover application.

Environmental awareness and gauging how products impact the environment will continue to grow in importance to society. While the fast-growing composite manhole cover technology presents significant benefits to infrastructure, those benefits also extend to the environment.


Chad Nunnery is the president of Composite Access Products (CAP). 


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