Ductile cast iron (cast iron with added magnesium) has been used for manhole covers and frames since the mid-20th Century due to its durability and high compressive strength. Engineers who design underground infrastructure have rarely considered the use of alternative materials. However, operators of localized multibuilding heating systems, such as those on college campuses, have become increasingly aware of the dangers posed by cast-iron manhole covers. Cast iron becomes very hot when exposed to internal steam and external sunshine, and it also conducts electricity — a concern when manhole covers are located in walkways where students wear sandals or go barefoot in warm weather. In addition, although cast iron stands up to heavy loads and severe impacts, it is very heavy — the density of a cast-iron manhole cover can be as high as 450 lb/ft3 (7,208 kg/m3). A small 32-inch/813-mm diameter cover can weigh as much as 250 lb/113.4 kg, which can lead to injuries to workers who have to move them.
These factors led the utilities department of a leading engineering university based in Cambridge, Mass., to replace traditional manhole covers with composite versions manufactured by Fibrelite Composites (Skipton, North Yorkshire, U.K., and Pawcatuck, Conn.). Many other universities in both the U.S. and Canada have since followed suit, says the company.
Fibrelite uses multiaxial and woven glass reinforcements to make a preform with a fiber architecture that maximizes bending stiffness and strength-to-weight ratio. The preform is then infused with polyester in a resin transfer molding process; for higher-temperature or highly corrosive applications, vinyl ester resin is used. The thermal gradient properties of Fibrelite’s composite covers significantly reduce heat transfer from the steam vault below — the surface temperature of the cover is typically only slightly higher than the ambient temperature. Extensive testing has shown that composite covers stay cool to the touch and support the same wheel loads as 32-inch cast-iron manhole covers, yet they weigh nearly 70 percent less. Fibrelite emphasizes that its cover eliminates the possibility of electrical shock and resists corrosion caused by salts, oils, water and steam and avoids theft of cast-iron covers, which have value as scrap metal. As an added incentive, Fibrelite can permanently mold into the cover’s top surface any style of school logo or other marking in single or multiple colors.