The standard length of the tubes is 60 ft. and the range of diameters available is in. to 12 in. It is possible to join several standard lengths together to a maximum of six giving a total working length when inflated of about 330 ft. Great- er total lengths than this create ex- cessive friction on withdrawal, but it is possible to have two lengths butted together and drawn in opposite directions. If this procedure is employed it is neces- sary to provide a short metal
330-ft.
with-
OT
similar sheath to cover the connec tion where true expansion is pre. vented by the through connector. or plugs.
Each tube is fitted with a plug and at the other end an ordinary Schrader tyre valve for inflation by hand or foot pump. or compressor. as required. Typical pressures are: Up to 85 lb./sq. in. for the small.
er sizes
25-33 lb./sq. in. for medium sizes
and
25 lb. sq. in. and below for the
larger sizes.
When inflated the length is decreas ed by about 10 per cent. and the cross sectional area increased be. tween 33-1 3 per cent, and 60 per
cent.
re-
The tubes should be kept clean and free from mineral oil. apart from which no special care is quired to preserve them for use over a long period. The makers guarantee the tubing for 100 uses but it is common to find instances where 300 uses are regularly ob- tained. The author has met sever- al contractors who have had well over a thousand uses from one length. Incidentally, no mould oil lubricant is required and it is a sim- ple matter to brush off any cement dust that may adhere to the tube.
The tubes are used for forming straight or curved ducts in any type of concrete--the result could only
be
achieved otherwise
by using
metal or fabric formers and leaving them in position.
When laid to bends the inside radius should be greater than the minimum specified by the manufac turers. For sizes up to 4-in. dia- meter this is not less than four times the inflated diameter, e.g. a tube of 3-in. diameter should be laid to a minimum inside radius of
12 in.
Inflation of the tube should be
governed not by pressure but by the required diameter. since after continuous usage less pressure
is required to obtain the same diame
ter.
After the concrete has set, usual ly after about 12 hours, the tube is deflated and withdrawn. The de. flation should not be too sudden. If, occasionally, the tube "sticks” it should be inflated again, after which it will not fail to release itself.
When tubes have been unused for a time it is usual to inflate them before use to test for loss of air through the valve. The
The Schrader valve cores
should be checked to ensure that they are fully screwed home.
The method of using the tube is
common to some of which scribed below.
all its applications,
are listed and de- Other uses will form the subject of an article in a sub- sequent issue of this journal.
Rubber pneumatic tubing is used to form continuous ducts in hollow concrete slabs and beams for floors and roofs to save weight and in- crease thermal insulating capacity, also for piles and many other pre- cast elements.
These products are usually made in factories or in sheds on the site, in prepared moulds. Various methods of keeping the tubes in position are suggested by the mak- ers, but most designers develop their
Opposite (top to bottom): Fig. 3. Pre-cast lighting columns cranked at the bottom to avoid underground obstacles,
Fig. 4.
Pre-cast concrete roof trusses for factory building at Urdingen, The Magnel Blaton system was used to form 3-in," ducts for post
Germany. tensioning.
Fig. 5. Inflatable tubes were used to form the ducts in the Udall beams of this bridge erected to Mr. E.W.H. Gifford's design by the School of Military Engineering at Chatham.
Fig. 6.
This 87-in. diameter tubing is the largest size yet produced for duct forming. It was made for a siphon contract in Swaziland. The inflation pressure is only 2 p.s.i. approximately.
THE HONG KONG & FAR EAST BUILDER VOLUME 15, NUMBER 1
51