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Electrothermic Method of Pretensioning Bar Reinforcement of Precast Reinforced Concrete by B

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Electrothermic Method of Pretensioning Bar Reinforcement of Precast Reinforced Concrete by B PROCEEDINGS PAPER Electrothermic Method of Pretensioning Bar Reinforcement of Precast Reinforced Concrete by B. G. Skramtaev* Presented at the Sixth Annual Convention of the Prestressed Concrete Institute Precast standard prestressed rein­ quite easily laid into moulds. In cool­ forced concrete structural units are ing the bars try to shorten and thus being manufactured in the USSR in are tensioned to the given degree. large quantities on continuous pro­ The degree of prestressing de­ duction and conveyor system lines. pends upon the original difference The tensioning of bar reinforcement between the length of the bar end by means of jacks requires large ex­ anchors and the distance between penditures on equipment and much the supporting surfaces of mould labor. Therefore, under conditions anchorages. Consequently the exact­ of mechanical tensioning it is very ness of prestressing depends on the difficult to organize mass production accuracy of these dimensions. of prestressed reinforced concrete In order to provide accurate di­ structural units. mensions, the anchors on the rein­ Recently the situation has radi­ forcing bars as well as the anchorag­ cally changed due to broad appli­ es of the moulds are placed with the cation in the USSR of the electro­ help of special gauges. However, thermic method of prestressing the even in this case some deviations reinforcing bars. from the given degree of elongation This method employs heating of are inevitable. Therefore it became the reinforcement outside the molds necessary to investigate the effect and subsequent cooling in the produced by allowable tolerances of moulds. This procedure was worked the degree of prestressing. out in the Mosgorispolkom Munici­ In determining the allowable de­ pal Research Institute of Reinforced viation from the required initial pre­ Concrete Products. stressing use was made of the The theory of this method con­ specific features of the bar reinforce­ sists of prefabricating reinforcing ment which distinguish it from high bars which are shorter than the dis­ tensile wire. tance between mould anchorages In structures with high tensile (fig. 1). After being heated by elec­ wire the appearance of cracks is fol­ tric current the bars extend and be­ lowed by increasing deflections come longer than the distance be­ which may be explained by a very tween anchorages and are, therefore, low percentage of reinforcement. Consequently in members, where "Doctor of Technical Sciences USSR Academy of Construction and there is danger of corrosion this Architecture should be taken into consideration. Moscow, U.S.S.R. Bar reinforcement used in pre- September, 1961 57 possible to create conditions of large allowed steel stress deviations which are not usual for prestressed rein­ forced concrete. Stresses within these allowances are ensured by using special gauges: e.g. for ribbed floor slabs the accepted initial prestress­ ing force is no less than 36 and no a heated ;oeia{oecement: <1ae 2 ,et;;-ly more than 52 kg/mm . The checking of the degree of ini­ tial prestressing was carried out on a large number of bars. This was done by means of tensometers placed at the ends of reinforcing a motte'd bars not subjected to heating. Mass n n testing carried out by many plants J. has shown that with the accepted Fig. 1-Simplified diagram of electrothermic pre· methods of using accurate gauges stressing. for the placing of anchors on the stressed concrete has relatively large bar ends and for fixing the mould diameters ( 12-30 mm) ( .47-1.18 in) anchorages; the degree of initial pre­ and considering the danger of corro­ stressing never exceeds predeter­ sion, it is as susceptible as any kind mined limits. of hot rolled steel used in reinforced At the present moment the elec­ concrete without prestressing. More­ trothermic method of pretensioning over in constructions with bar rein­ the bar reinforcement is in use in forcement of 30XC2S type the ac­ many Soviet plants. In Moscow cumulation of deflections after the alone over 3 million square meters appearance of cracks increases with ( 32 million sq. ft.) of precast floor less intensity than does high ten­ slabs are being produced per an­ sile wire (fig. 2). Therefore in most num by means of this method. structures prestressed with bar rein­ forcement, cracks up to 0.2 mm wide are allowed to appear during loading. In such cases it is only nec­ essary to ensure sufficient rigidity of construction and to restrict the ex­ 1---- -- --- ----- M- pansion of cracks; which is why pre­ ~J - stressing is used. In many mass produced reinforced I A concrete units these requirements 1 7[ are satisfied during the initial pre­ 1/ 111 stressing of 30-40 kg/mm2 ( 42,500- v 57,000 psi). At the same time the w3 initial prestressing of reinforcement v / 7 may be brought up to 90 per cent ....... v l.....ef-' of the yield point which for streng­ thened steel of grade 25G2S and 2 30XG2S means 50-54 kg/mm2 (71,- Fig. 2-Graph of deflection versus moment for 000-77,000 psi). Thus it becomes bars (3) and strands (1 ). 58 PC! Journal Fig. 3-Five different types of anchors used at the ends of the prestress bars. /'t:Zed a mova6'i'e at'am "'i/7~ -i·eaa-.:f'oz-mt/7.9 et'ect'~lcat' ctl;ztacts ee'ectzic ct1dac.t a diSk' 4. Fig. 4-Bar heading machine. a) ,--~~~--+-- 6 .3 :1 5. Fig. 5-Hinged self locking form. September, 1961 59 Depending on local conditions dif­ 18 mm in diameter. At some plants ferent anchors at the ends of the equipped with mould-cars and dow­ bars and different mould anchorag­ els for continuous wire winding, the es are being used (fig. 3). Anchors bar anchors are made in the form in the form of heads and fixed of welded loops made of Hat steel. anchorages of the fork type are most This makes it possible to use the commonly used. Heads at the ends same cars for winding high tensile of reinforcing bars are accomplished wires and tensioned bar steel. by machines usually used for butt The "Barricade" factory in Lenin­ welding reinforcing, with additional grad uses multi-purpose stock clamps clamping and up-setting pneumatic of the wedge type. (fig. 3) cylinders (fig. 4). In all the cases described above, During a 7-hour shift two workers with moulds having fixed anchors, can head, by means of a machine, after the concrete has acquired the from 800 to 1000 bars. Before head­ predetermined strength of 150-200 ing the bar is usually capped with a kg em 2 ( 2100-2800 psi) the ends of washer which insures uniform distri­ the bars are cut off thus transferring bution of pressure on the mould an­ the prestressing to the concrete. chorages. In order to have adequate There are some other solutions strength the heading has to be ex­ where additional economy is actly symmetrical to the bar axis. achieved by eliminating the rein­ Where the need for reinforcing forcement cutting. bars is comparatively low the anchor Here are two examples. is made by welding to each end of For manufacturing cored floor the bar two short pieces of reinforc­ slabs one plant uses trays which take ing waste, 40-45 mm long, and 14- up the prestressing stress and also 6. Fig. 6-Use weld plates to fix prestressed reinforcing to the form. 60 PCI Journal allow the upper part of the mould When the members are pre­ to be taken off directly after plac­ stressed with reinforcing bars the ing and compacting the concrete. insert angles are used as anchors to The tray is provided with thrusts fix the bars temporary on the mould in the form of segments which are without any projection of prestress placed in the concrete (fig. 5). reinforcing from the concrete. To When the concrete has acquired suf­ accomplish this a hoop is attached ficient strength the tray with the to each end of the prestress rods. precast product is put on the form­ (fig. 6-b ). work post and the projections are Therefore a space is formed be­ rested upon special thrusts. Due to tween the angle, the reinforcement the weight of the tray with the slab, and the hoop bar and into this space the projections open and the mov­ a wedge is inserted which transfers able end of the tray rotates in rela­ the prestress force to the form tion to the fixed part of the tray. while the concrete is acquiring The thrust then comes out from the the predetermined release strength. body of concrete and the slab tears These wedges are inserted after away from the tray as well as from placing the heated reinforcement the fixed thrusts placed at the oppo­ into the mould. site end of the tray. The slab is then Upon cooling and shorting of the taken from the tray by a crane. reinforcement the supporting angles Another example takes advantage set against the wedges and pre­ of the insert details used for fixing stress forces develop in the reinforce­ ment. The tension forces are trans­ the slabs to roof trusses or beams. ferred through the wedges of the When there is no prestressing, longi­ steel mould. After the concrete has tudinal reinforcement and addition­ acquired its predetermined strength al anchoring bars are welded to the the wedges are knocked out and the angles (fig. 6-a). prestressing is transferred to the con- 3 Fig. 7-Methods used to turn off the current used in heating the prestressed reinforcing. September, 1961 61 crete. Reinforcing bars are heated on special installations equipped with : Z'he- anttslteaZinj' zetn.:fozce.men~ caj'e transformers which are similar to those used for electric welding.
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