split and serrated conical wedges or other means, They may be gripped in groups by yokes, double cones, or multiple arrangements of wedges in steel plates. Fig. 18 shows a gripping plate used for the produc tion of railway sleepers. Each of the of the wedges grips two 5 mm, wires which are stretched together. The three heavy screws serve as releasing devices; the fast pitch of the screws ensures that little force is required during the releasing proces9.
The steel wires
may be stretched by a combination of levers, serew jarks or similar mechanical stretching apparatus, Hydraulic jarks are simpler in use than their mechanical counter. parts. Fig. 19 shews 1 hydraulic double-action jack at work and anchor- age cones which are used by Freyssinet for the stretching of
of parallel wire rables with from 12 to 32 wires.
In structures in which the steel is post-tensioned enre must be taken that no bond develops between the concrete and the prestressing members. Fig. 20 shews the manufacture of a bondless cable under rather primitive condi- tions. The required number of wires are laid side by side, cut to equal length, drawn through a bitumen bath and arranged parallel round a rentral spring. Then the cable is covered by a bitumen impreguate | tupe. In many
A
Fig. 20
eases the wire cable is covered by a tube of thin steel sheet instead of the tape.
If prestressing members are stretch- ed in groups eare must be taken that the initial tension of all the members is equal before the gripping block hold- ing the group is moved.
Ta control effectively all stages of the prestressing process extensive use should be made of pressure-gauges. dynamometers and extensometers, Whenever possible the applied prelimi- nary forces, the extension of the steel, and the contraction of the concrete should be measured and the measure- ments correlated.
Bonded prestressed eunerete nuits are usually produced in fine and with common ant continuous prestressing members.
each end of the long working platforms and prestressing beds there are solid anchor blocks which transfer the reactions of the stretching forces into the ground. Fig. 21 shows the first factory in which prestressed joists were produced on the long-line principle. The factory was designed and laid down by Hoyer, One end of all wires are held in fixed position at the far end of this 100 ya.
ong stretching bed.
other ends are clamped in movable steel yokea which are shown here. Each of the four screw threads serves one stretch
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Fig. 21 ing way over the full length. To ob tain the required length of the articles, patterns made of steel plate are insert- d in the mould. Two of these pat terus are placed at about 1 inch dis Tance, and the space between is filled with dry sand before concreting. After stripping the moulds, the sand flows out and the wires are burnt through between the plates,
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Photograph, Fig. 22, shews the first of three factories in England produc- ing railway sleepers on a similar prin- ciple. With one turn-over
week the Dow-Mac factory produces about 100,000 sleepers p.a. A special feature of this production process is that the moulds are filled at the centre of the factory and then slid along the com- pressor wires to their final position. There the concrete is revibrated to re-established bond.
Another plant for the out-put of railway sleepers on a mass production basis, is situated in Heidelberg, Ger- many, and was designed by Pistor. The steel moulds are here placed side by side and fixed to the prestressing bed. In each half of the work shop there ure 11 rows, with 21 sleepers in each POT. 231 sleepers are therefore cast in one process by pouring a continuous slab 170 ft. long and 10 ft. wide. The concrete is poured and comparted as in #1 road slab and the concreting equipment used including a heavy high frequency vibrator is very similar to a
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