No_1_May_and_June__1951 — Page 52

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wedges in

a gripping plate used for the produe- 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 process,

The steel wires

may be stretched by a combination of levers, screw jacks or similar mechanical stretching apparatus. Hydraulic jacks are simpler in use than their mechanical counter- parts. Fig. 19 sliews 1 hydraulic double-action jack at work and anchor- age coues which are used by Freyssinet for the stretching of parallel wire cables with from 12 to 32 wires.

In structures in which the steel is post-tensioned care must be taken that na 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, ent to equal length, drawn through a bitumen bath and arranged parallel round a central spring. Then the cable is covered by a bituumen-impregnated tape. In many

the initial tensión of all the members is equal before the gripping block hold- ing the group is moved.

To 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 conerete units are usually produced in line and with common and continuous prestressing inembers. On 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 shews 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 yil. long stretching bed. The other ends are clamped in movable steel yokes which are shewn here. Each of the four screw threads serves one

stretch-

J

stripping the mouins, rae sand flows out and the wires are burnt through between the plates.

Photograph, Fig. 22, shews the first of three factories in England produc- ing railway sleepers on a similar priu- ciple. With one turn-over

per 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 thera are 11 rows, with 21 sleepers in each row. 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 compacted 19 in # road slab and the conereting equipment used including a heavy high frequency vibrator is very similar to a

Fig. 22

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