Extensive use was made of inflatable rubber tubing to form the ducts in the structural members of this pre-cast, pre- stressed concrete footbridge at Eel Pie Island, Twickenham. The bridge replaces a centuries old ferry. (See Fig. 7).
THE FORMATION OF DUCTS IN CONCRETE
Fig. 1. Inflated rubber tubes held in position by removable steel bridges before the concrete is poured.
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By F. E. Jones, F.R.I.B.A.
N the past ten years a completely new piece of equipment has been introduced to fulfil an important function in the builder or engineers' plant. This is the rubber duct for- mer, a device pre-eminently suitable for producing continuous cores in concrete structures of all sizes.
The success of this equipment is due to the toughness and elasticity of the natural rubber used. its ability to withstand repeated contact with green concrete and the smooth- ness and uniformity of the bore it produces both in precast and in situ work.
The formers employed may be either pneumatic tubes or solid rub- ber of rectangular section.
The pneumatic type of duct former is made of rubber reinforc- ed with a special cotton braid woven a diagonal mesh. This rein- forcement is responsible for the de-
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pendable manner in which the tube will contract in diameter and in- crease in length upon deflation. thereby breaking any bond with the concrete. The power of contraction is aided because the fabric mesh is made with one set of the diagonal fibres in greater tension than the other, which results in twisting of the fabric tube.
Fig. 2. Inflatable rubber tubes used in the manufacture of 70-ft. pre-cast concrete piles. In the foreground the tube is shown in position in the mould.
THE HONG KONG & FAR EAST BUILDER · VOLUME 15, NUMBER 1
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