Lighter composite manhole covers with polyurethane

In fact, the composite covers, used by municipalities and plant sites, are significantly lighter with a 24-inch cover weighing only 25 pounds compared with a cast-iron equivalent weighing more than 150 pounds. Yet, they meet rigorous standards for cyclical fatigue, proof-load and durability that the market requires. Additionally, they are corrosion and slip resistant and non-conductive, providing added safety to pedestrians and utility workers who may come in contact with them.

A customised formulation of Baydur RTM 902 polyurethane, developed collaboratively by Bayer and GMI, is used to mould the composite manhole covers through GMI's proprietary liquid resin injection process.

"We were on-site, providing hands-on support to the GMI team running tests to decide the best production process for the composite covers," said Harry George, manager, New Applications, Polyurethanes, Bayer MaterialScience LLC. "It is always rewarding to work from start-to-finish on a product like the GMI manhole covers and see a concept turn into reality."

GMI selected the Baydur polyurethane system for its strength, durability and ability to facilitate the creation of unique designs, according to Bob Brady, president, GMI Composites, Inc.

"After testing many different materials, we chose Bayer's polyurethane system to produce the manhole covers because of its high strength and durability," Brady said. "The material suited our cover well, but we also elected to work with Bayer MaterialScience because of its overall reputation. Bayer offered tremendous technical assistance to achieve the material requirements necessary to meet the highest standards for manhole covers."

GMI composite manhole covers and frames have an average proof load in excess of 100,000 pounds – more than five times the H-20 loading standard per AASHTO (American Association of State Highway and Transportation Officials) Standard Specifications for Highway Bridges.

GMI maintains testing equipment on premises, allowing for production to be randomly sampled and tested during the manufacturing process. Independent certification and test witnessing is available for the following internationally accepted test standards:

• United States Standard: AASHTO M 306 Standard Specification for Drainage, Sewer, Utility, and Related Castings, with a 40,000-pound proof load concentrated on a 9-inch-by-9-inch square area and held for one minute.
• H-25 & HS-25: A 50,000-pound proof load concentrated on a 9-inch-by-2- inch square area and held for one minute. The 50,000-pound proof load represents a safety factor of 2.5 for H-25 & HS-25 loading.
• European Standard: EN-124 for Gully Tops and Manhole Tops for Vehicular and Pedestrian Areas.

The composite covers are customisable and are available in a variety of shapes – round, square and rectangular – In sizes from 12 inches to 50 inches in diameter. GMI works with its customers to determine what attributes are needed for each application, and offers the ability to include a logo or lettering on each cover.

Additionally, the composite covers complement the bourgeoning smart grid market, allowing automation signals and wireless communications to pass freely through them. This also makes them ideal for electrical switching, water metering, and sanitary waste water and storm sewer monitoring applications.

Unlike cast-iron covers, GMI's manhole covers have zero scrap value and are therefore unattractive to thieves, helping municipalities avoid the cost and labour associated with cover replacement. GMI covers are compatible with a variety of retention and locking solutions for increased infrastructure security.

GMI Composites, Inc. located in Muskegon, MI, USA, was established in 1923 as George Morrell Industries. GMI made its first fibreglass-reinforced composite manhole cover more than 19 years ago – these initial covers are still performing exceptionally in the field today, says the company. Through continued innovation GMI has made a variety of unique underground access covers.