No_2_1960 — Page 45

THE FORMATION OF DUCTS IN CONCRETE

HEAVY cables such as those used

by electricity authorities and the Post Office have to be conducted through ground, in sub-soil of varied characteristics. They must be given mechanical protection and safe- guarded from trouble due to settlement. water penetration. frost attack and corrosion.

Rubber core formers are a con- venient and efficient means of form- ing ducts, fulfilling all the necessary requirements and proving especially useful where it is necessary to deviate from straight lines. change level.

or

Mains cables are usually run on one side of a roadway with branches taken into the various premises, crossing under the road where neces- sary. The cable ducts must be constructed to allow for these branches. permit change of direc tion. and give access at predeter- mined positions.

The technique involves digging a trench, placing concrete on the bot- tom; positioning the tubes so that they will remain in position during pouring: placing the concrete: and withdrawing the tubes on setting. The fines in the concrete find their way to the face of the rubber and

By F. E. Jones, F. R. I. B. A.

give a smooth internal bore to the duct.

Fig. 1 shows an arrangement of 12 ducts for telephone cables for the G.P.O, at Shepherd's Bush. The fanning out of the ducts is dis cernible.

A convenient way of enabling electrical supply service mains to enter blocks of flats is through 3-in. or 4-in. ducts preformed in, or below, the site concrete and brought to a central position. From here 1. in. diameter unmetered distributors are taken to consumer's intake units in each flat.

Semi-detached houses require on- ly one underground service main for each pair of houses. This can be taken through a duct formed in the site concrete, to the meter position in the second house, thus halving the length of cable needed. Similarly, formers are often used to make ducts for the cable used in pedestrian crossings and to take lengths of cable in concrete runways on aerodromes. The Air Ministry has made provisions in its Specifica- tion for the use of pneumatic rubber tubing in this type of work.

Rubber formers are used to make small bore ducts through which

approved types of wiring for in- ternal electrical systems may be installed. The ducting gives con- tinuous and effective protection and dispenses with the

use of metal conduit.

The ducts are formed either in the thickness of the structural floor itself or in the cement screed (mini- mum 1-in. thick) on top

of the floor and changes of direction from the horizontal to vertical planes are of concrete achieved by the use ceiling boxes and junction boxes.

Rubber duct formers can be used in conjunction with non-expendable

switchbase ceiling and

formers effecting considerable economy. The box used is, in appearance, similar to the standard type made to con- form to the British Standard Speci- fication, but it has a tapered side for easy removal.

Advantages of the system are: (1) Speed of execution.

(a) 10-point flat lay-outs can be positioned in less than one hour.

(b) Cables can be drawn into position as the tubing is extracted.

Top, figure 1.

Above, fig. 3. Right, fig. 2.

THE HONG KONG & FAR EAST BUILDER VOLUME 15, NUMBER 2

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