own ideas. Displacement can re- sult from concrete being placed un- equally on different sides of the tubing, or from the vertical thrust of floatation. The tubes should be held at intervals along their length either by temporary locating de- vices suitable for withdrawal while the concrete is green, or by per- manent means such as wires attach- ed to the mild steel reinforcement.
The normal method of operation is for the tubing to be inserted and the ends fixed so as to allow suffici- ent slack within the mould. On in- flation the tube will assume correct tension, when it is located by the devices mentioned. The concrete is then poured and if vibrating is re- quired this can be done without affecting the rubber tube in any way. When the concrete has set the tubes are deflated and withdrawn and the concrete product is taken to a curing shed. The rubber tube is available for immediate re-use.
Pre-cast concrete lamp standards. have several features of interest which sometimes involve the use of duct formers. Cranked ends are adopted when the lower part of the
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Fig. 7. Pre-stressed concrete footbridge at Eel Pie Island, Twickenham, under construction. The ducts and the end of one of the tubes used are visible. (Photo by courtesy Cement and Concrete Association).
post has to avoid cables and drains in the ground. The central core throughout the height of the stand- ard, which provides a way for the lighting cable to the lighting unit at
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the top, is usually made by the use of duct tubes as they lend them- selves so easily to the changes of direction found in the design of these standards.
In recent years there has a growing use of pre- stressed concrete in building and civil engineering works. The system makes use of cables in the concrete made up of many fine wires which are stressed before the structure is subjected to the loading of its normal use. Ducts have to be pre-formed in the concrete to receive these cables, and the use of pneumatic rubber tubes dispenses with costly expendable tubes and avoids the inconvenience of tapered cores or withdrawable metal tubing. Many concrete designers prefer the ducts to be formed by removable cores, because when the cable is subsequently grouted it bonds well with the concrete walls of the duct. whereas the same bond cannot be anticipated where formers are left in as a permanent lining.
Pneumatic rubber tubes are shown to their best advantage in pre-stressed concrete work, where constant repetition of the process demands the use of a
non-expendable former. Today the Freysinnet. Lee McCall, Gifford-Udall, and Magnel Blaton systems all use rubber to deal with the accurate formation of ducts, sometimes in very close proximity inside the concrete member. The tubes are first positioned in the form-work and placed under tension to form a perfectly straight duct. The tension must be just sufficient to keep them accurately in position. Too much tension restricts the full expansion of the tubing, and is in- dicated by the diameter of the free end of the tubing outside the shuttering being appreciably greater than that inside. It would also impose a considerable un necessary strain on the shuttering.
THE HONG KONG & FAR EAST BUILDER VOLUME 15, NUMBER 1