No_2_1960 — Page 47

Figure 6.

But the use of rubber duct form- ers is by no means confined to new buildings. They can be used for restoring chimney stacks in old period houses when the structure can be dangerous due to the break- down of old pargings. These stacks are often

often too large by modern standards and are usually renovated by forming some sort of new lining. Rubber tubing is ideal for shaping and forming the core of the new concrete lining, particularly where the flue is cranked. The concrete is, of course, poured from the top. High-aluminous cement is often used for the lining in such cases.

THE HONG KONG & FAR EAST BUILDER

In certain countries where deep winters are experienced, particularly in Canada and Sweden. use has been made of duct-forming tubes to create networks of heating ducts under concrete drives. These prevent the settlement of snow on the paved surface.

Hot air systems have the advan tage over copper tube hot water systems since they are immune from freezing.

Ducts formed by withdrawable rubber tubes are also most useful in the construction of ventilation sys- tems. One system makes use of

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forced air inlet and extraction ducts as in mains and branch ductings in multi-storey blocks of offices and flats.

This is a dual purpose application since the air is also warmed before circulation. The system can also be adapted for the circulation of cold air in hot weather,

The system relies on mechanical means of forcing the circulation while other systems work by gravity. An example is afforded by a multi- storey block of flats in Sweden, where the party walls are of con- crete 12 in. thick and contain 9-in. diameter ducts to provide a means of natural ventilation for the kit- chens and bathrooms.

Ventilating ducts which are in- tegral with the structure have several advantages. The first is quietness. The drumming which is sometimes met with in more traditional methods of construction is absent. Also, provided the system is proper- ly designed, there is none of the "whistling" which is such a problem in free standing ducts and which is either transmitted from the fans or is due to sympathetic resonance in box ducts.

Large diameter rubber tubes are used to form ventilating shafts to mines and other subterranean work- ings. In the case of Fig. 4 the 3-ft. diameter shaft was constructed for the purpose of ventilating an under- ground chamber in a gypsum sul- phate mine in France. The work was done in relatively small sections of from 6 ft. to 9 ft. at a time due to the weak nature of the soil.

Working from ground level, an excavation is made, the tube lowered into position and inflated, concrete poured around it. When the con- crete is set the tube is withdrawn and the process repeated below.

One great advantage of rubber formers for such work is their ease of withdrawal. which is much greater than traditional segmental shaped metal pieces. In the latter case, ample time must be allowed for the concrete to be hard enough to resist damage when the liners are withdrawn. Rubber can be removed in a fraction of the time because it does not injure materials with which it is brought into accidental contact.

There are several problems in the construction of dams which give

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