manhole covers?

Question

I have an homework about manhole covers. The main goal is to obtain the cheapest one which can work out effectively. I have selected two types of materials. One of them is the classical one, cast iron. The other one is reinforced concrete which may be a solution to thefts of manhole covers. I have some questions about details.
1. Which kinds of cast iron are most suitable, economical, safe etc.?
2. The possible disadvantages or any other advantages of reinforced concrete?
3. Cost comparisons between cast iron types and reinforced concrete?
Thanks a lot for any helpful answer.

Answer 1

When I used to interview potential employees, I used to ask them, "Why are manholes round?". The object was to test their ability to think on the fly as well as to see how they went about resolving a question. Some of the potential answers were as follows:

A round manhole cover cannot fall through the hole since the diameter of the collar is smaller than the diameter of the manhole cover.

A round manhole cover can be snatched out of it's resting place by one man using a steel hook with a long handle and then moved by one person by rolling it on it's edge like a tire.

A round manhole cover that is slightly domed, offers equal compressive resistance over it's surface compared to a square or rectangular manhole cover.

As to your questions, the only advantage I see to concrete manhole covers is that they are probably initially cheaper to manufacture than cast iron manhole covers. Disadvantages to concrete manhole covers include:

They could be damaged of dropped and/or the edges could be chipped and damaged if rolled.

They are subject to deterioration due to contact with de-icing chemicals such as road salt.

Spalling occurs when water soaked concrete freezes. The water expands as it freezes and pops off the surface of the concrete. Over time, this seriously weakens the concrete.

To reach the same compressive strength as cast iron, concrete would need to be much thicker and reinforced with prestressed steel rebar.

While one man can easily use a hook to jerk a cast iron manhole cover out of position and slide to to the side, the same would not be possible with a much thicker concrete manhole cover. It would have to be lifted straight up until it cleared the collar and then moved laterally before being set down. This might even require the help of a winch, backhoe, or other mechanical device.

As to the actual cost of each, I can't offer an opinion. My guess is that the concrete would be cheaper initially, but cost more over time due to the inherent disadvantages as outlined above.

Good luck with your homework.

Answer 2

The type ou use depends on what is under the cover. Will the space under it need to ba accessed on a regular basis for maintenance? This wold apply to covers on vaults for electrical and telephone system wiring.

Cast iron is cast iron. There really isn't a "best type" as it is pretty much all the same. It is the manufacturers tat may have better 'styles'. If the space needs to be accessed on a regualr basis, cast iron is the way to go. These covers are meant to be lifted fairly easily using a cover tool.

If the space under the cover does not require access, then a good concrete cover could be the best. This would apply to areas where the hole was needed to allow a one time access for application of coatings in the space then could be sealed for the rest of the life of the vault.