11 - ericssonhistory.com

Laying
The corrugated duct may have to be protected in different ways. When
laid under pavement at a normal depth and under normal conditions no extra
protection is necessary. But if the duct is laid under streets and/or if obstructions
necessitate a shallower depth of laying, protection must be provided
either in the form of embedment in lean concrete or of some form of protective
tiles. Where there is a risk of rodents or insects attacking the plastic,
the ducts must likewise be embedded in lean concrete.
To facilitate jointing, one end of the duct is tapered. The ends are first
coated with a sealing compound NTV 60021. They are then jointed together
by pushing the straight end of one duct into the tapered end of the next. The
joint is thus gastight and watertight.
For embedment in a manhole wall or other concrete structure the outside
of the duct is coated with resin NTV 610 mixed with hardener NTV 611.
Heated sand is then sprinkled over the resin. The resin solidifies and binds
the grains of sand so that the duct is covered with a firmly adhering layer of
sand. The duct is then embedded in the concrete and a gastight and watertight
inlet is obtained.
Split Ducts
If cables are already installed in the conduit, a longitudinal slot is made in
the duct with a metal saw. The split portion is laid around the cable. Owing
to the corrugation the duct reassumes its original form. For alterations to
cable plant the cables can be temporarily protected in the same way.
Summary
Non-corrugated plastic ducts have been used for several years in conduit
and have many excellent properties. The result is still better, however, if the
duct is corrugated. Corrugation increases the radial rigidity and the lateral
and longitudinal flexibility. It results in a better and cheaper conduit.
The basic material is non-corrugated PVC tubing, which is made in many
countries.
To reduce the cost of transport to the construction site the non-corrugated
ducts are packed in sets of nine ducts 6 m in length and of different diameters,
telescoped into one another. Corrugation is done on site in a machine
specially developed for the purpose. A cheap plastic conduit can thus be
constructed even at places where there is no manufacture of non-corrugated
duct.
Details concerning the construction and operation of the corrugating
machine LVA 29901 and the dimensions and properties of the non-corrugated
plastic ducts NPB 30101 will be found in Catalogue 742, Section 23, Supplement
1, and Section 12, Supplement 1. A detailed description of the laying,
jointing and embedment of the corrugated ducts will be found in instruction
N 1531-073.
36