Page 237 - Materials Chemistry, Second Edition

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Sustainable Industrial Design and Waste Management
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persons out. Manhole covers usually weigh more than 100 lb (50 kg), partly
because the weight keeps them in place when traffic passes over them, and
partly because they are often made out of cast iron, sometimes with infills
of concrete. This makes them expensive and strong, but heavy. They usually
feature pick holes in which a hook handle is inserted to lift them up.
Manhole covers and manholes are round, but could also be designed to
any shape according to the required dimensions. Round tubes are the strongest
and most material-efficient shape against the compression of the earth around
them. A circle is the simplest shape whereby the lid cannot fall into the hole.
Circular covers can be moved around by rolling and they need not be aligned to
put them back.
A manhole and its cover is a very important application to be produced
from rejects, from social, economical, and technical points of view. Rejects
can be used instead of ordinary cast iron with all the different geometries and
dimensions as shown in Figure 6.10.

FIGURE 6.10 Sand-plast manholes