Composite materials are the combination of at least two components, chemically different one from another, characterised by chemical-physical properties which cannot be found in the individual materials making them up. In particular, compared to traditional materials, composite materials are at the same time sturdy and lightweight, have ideal resistance to corrosion and chemical agents and a high thermal and electric insulation capacity.
Glass fibre is one of the most well known reinforcing materials, widely used for the production of composite materials. Common experience tells us that monolithic glass is very fragile due to the great number of crystallisation defects which act as micro fractures and stress concentration areas. Whereas if glass is spun to diameters lower than microns it loses its fragile property becoming a material with high mechanical resistance and resilience: these features are maintained even at very high temperatures.
The family of composite manhole covers includes, for example, fibreglass manhole covers, while plastic manhole covers do not. Composite manhole covers have countless advantages over manhole covers already on the market made of cast iron or concrete.
The main features are:
One of the main characteristics of composite manhole covers is their light weight.
The weight difference is on average beyond 70% compared to covers in grey cast iron and beyond 65% compared to covers in ductile cast iron.
The low weight considerably reduces operators’ risks during handling, installation and subsequent maintenance of the manhole covers.
The manhole cover in composite material is not subject to corrosion or to the attack of chemical substances: in fact the superficial part of the cover is fully built in thermosetting resin, resistant to acid and alkaline substances, oil, grease and hydrocarbons.
This feature, in conjunction with the impossibility of generating sparks or condensate, make the manhole cover in composite material especially suitable to be used in fuel dispensing plants. By now it is common practice worldwide to use manhole covers in composite material instead of cast iron in these plants.
Corrosion resistance is also particularly important in industrial and civil sewerage networks, especially in the case of weak slopes: the gases generated inside pipes and manholes do not corrode the lower part of the manhole cover.
Lastly, we emphasise the resistance of the thermosetting resin to salt water, a characteristic which makes the use of manhole covers in composite material especially suitable near the sea.
All of these features make it unnecessary to paint the manhole cover in composite material, unlike those in cast iron.
Thanks to the property of the raw material used, use of a cover and frame in composite material reduces the noise of traffic or pedestrians passing over, typical of cast iron systems and due to an imperfect coupling between the cover and frame or caused by installation errors.
The outer surface in thermosetting resin makes the manhole cover a perfect insulating system, making stray voltage impossible, thus protecting pedestrians from the possibility of accidental electric shocks due to uncovered electric wires.
The manhole cover is also a low heat conductor: for practical purposes, this behaviour can be important where for example there are hot steam leaks in remote heating networks.
Manhole covers made of composite materials are more permeable to electromagnetic fields than those made of both spheroidal and lamellar cast iron. Thanks to this feature, multiple applications are possible:
– Sewer systems: a level sensor placed below a composite manhole can monitor and control the level of effluent inside the manhole. An anti-intrusion sensor can also be combined with this technology in order to detect tampering, forcing or removal of the manhole cover;
– aqueduct networks;
– transport systems: the manhole cover can be used as a cover for electronic devices that control and manage the circulation of trams, trains and metros.
Two main factors weigh in favour of manhole covers in composite material compared to cast iron manhole covers as to the emission of CO2 into the air:
– energy necessary to reach the fusion temperature of the raw material to be cast inside the moulds of cast iron manhole covers (beyond 1200°C) and the operating temperature of the moulds for composite materials (around 60°C)
– reduced weight of each individual piece making it possible to load 3-4 times more manhole covers than normal cast iron manhole covers on a trip thus reducing pollution due to fewer number of transports.
The theft of manhole covers is aimed at selling them for metal remelting. Aside from the economical damage, these thefts have extremely dangerous consequences as they leave dangerous openings on the road which are not signalled.
The theft of composite manhole covers (wrongly called plastic manhole covers) is unattractive since there is no illicit secondary market.