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.

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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

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-

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. Heat ing is accomplished by connecting contracts to the bars in two points at the center of the bar, some dis tance from the ends. During the process the central part of the bar is heated up to 250-350oC ( 420- 600oF) while its ends remain cold. This permits the placement of the heated bars in the mould without any special equipment. Tight con tact of the electrode with the bar is ensured by a pneumatic press. Research has shown that the heat ing of the above mentioned types Fig. 9-Precast cored slab. of steel to temperatures of 460-500° C ( 780-870°F.) does not reduce a pointer indicating the elongation their strength. While heating is be of the bars during heating. One in ing carried out, special measures are stallation simultaneously heats from taken to prevent temperature rise .2 to 6 bars depending on the type above the predetermined degree. To of reinforced concrete construction accomplish this one contact on the that is being produced. Heating usu bar remains fixed during heating ally lasts from 2 to 4 minutes. while the other one moves together Heated bars are then placed into with the elongation of the bar (fig. the mould thrusts where they cool 7). When the predetermined elonga and acquire the predetermined ini tion is reached a switch cuts off the tial prestressing. Under usual work current and the heating automati shop conditions the cooling from cally stops. Some heating installa 250-350"C to 80-90oC lasts 12-15 tions have in addition a device with minutes. It may be accelerated by means of artificial cooling. Concret ing is allowee when the temperature is lower than 90°C. ( 130°F.). Heating may be applied not only , 11111111 I I I 11111111, to separate reinforcing bars but also to bars constituting elements of welded frames. Here part of the auxiliary reinforcement is being heat ed but during cooling only the pre I I stressing steel having end anchors \ a!Z utztensiol1ed ~az is stressed. Due to the flexibility of the assembly disturbance of the welds never occurs (fig. 8). The application of bar reinforce ~ 8 ment to mass produced prestressed Fig. 8-Diagram showing effect of electrothermic prestressing on reinforcing cage. concrete structural units has its own 62 PCI Journal structural and technological charac of the void-forming plungers. The teristics which must be taken into height of the slab is also reduced by consideration when organizing the 10 mm at the expense of the thick production processes. Let us take ness of the upper flange. This de some typical examples. creased the consumption of concrete In the USSR the most commonly by 18 per cent (fig. 9). Adequate used structural element for housing placing and compacting of the con is the cored floor slab. The electro crete is ensured by internal vibration thermic method of tensioning as pre of the plungers. The prestressed re viously described is used in manu inforcement is 12 and 14 mm in di facturing these members. ameter and is placed into every Formerly the practice was to second or third rib of the slab. make cavities of a circular cross sec When transferring the prestress tion; recently the cavity is increased force to the concrete the sectional by welding channels to the bottom reinforcement compresses the con-

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Fig. 10-Precast ribbed floor slabs. September, 1961 63 crete and, at the same time, tries to ess of striking the forms of the first split it. If special measures are not experimental slabs, cracking almost taken, cracks may appear and de always appeared at the intersections velop on the end faces of the slabs. of longitudinal and transverse ribs. In order to prevent cracking in Investigations have shown that the cored slabs at their end faces, weld striking of slabs after the transfer of ed meshes of cold-drawn wire 3 or prestressing to the concrete is effect 4 mm in diameter are laid. These ed by the angle of slope of the in meshes are only 0.3 m long and ternal facet of the transverse rib. have produced excellent results. When this internal facet has but As another example let us take a very slight slope prestressing as the ribbed floor slab. Dwelling sists in releasing the product from houses as well as civil buildings ob the form. viously require slabs with smooth The value of the critical angle surfaces for the ceilings and floors. which is called "the angle of self Such requirement is completely sat striking" depends upon the ratio of isfied by cored slabs. As to the floors prestressing force to the product's between basements and ground weight and upon the coefficient of floors here smooth surface is needed friction of concrete against the steel only for the floor. For such floors it form. This coefficient includes not is quite possible to use not the cored only friction but also the possible but the ribbed slabs which reduce cohesion of concrete with the sur the consumption of concrete by 28- face of the form. 38 per cent and which cost 25 per Calculations and experiments have cent less. shown that for ribbed slabs the an Due to prestressing the ribbed gle of slope of the internal facet of slab may have the same height as the end rib should be about 45°. the cored one ( 220-210 mm) which After the forms with the correct an ensures interchangeability. gle have been manufactured the Prestressed ribbed floor slabs are striking goes on unimpeded and no trough-like in shape with ribs locat cracks appear. ed along the perimeter (fig. 10). To prevent the splitting of con Each longitudinal rib is reinforced crete in narrow ribs, spirals about with prestressed reinforcement com 50 mm in dia. 300 mm long, of prised of 1 bar 14 mm in diameter, wire 3 mm in dia., spaced at 25-30 grade 30XC2S, and with non-pre mm, are put on the ends of the pre stressed reinforcement consisting of stressed reinforcement. l bar 10 mm in diameter, grade In addition to cored and ribbed 25C2S. slabs prestressed floor beams and gir End ribs are of the same height ders are also being produced. These as the longitudinal ones in order to girders are 6 m long with a 160 x ensure the direct transfer of pressure 600 mm cross section and are de from walls without additional work signed for a uniformly distributed on the job. But, as it is well known, load of 3000 kg/m. the transverse rib after transfer of By substituting prestressed steel the prestress force is pressed into 30X2S for ordinary reinforcing steel the mould projection which prevents which has yield point~ of 25-40 kg/ the free shortening of the concrete, mm2 the consumption of steel for and complicates the striking of the each girder is more than halved. formwork. And indeed in the proc- The main reinforcing of a girder 64 PCI Journal l1 I

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.12. Fig. 12-Eiectrothermic prestressing within the moulds (forms). September, 1961 65 consists of 3 bars, 14 mm in dia., ured" washer the current is cut off placed at different levels within the and the reinforcement becomes girder (fig. 11). stressed upon cooling. It is obvious In order to avoid splitting, a small that the ends of the reinforcing bars turned-up mesh is used at the ends which are in the thrusts have to be of the girders. isolated from the steel form. No con The girders are manufactured in tact between the central part of bars a multiple form consisting of 10-20 and the form is allowed either. sections and placed in steam curing The drawback of this method lies chambers. During the curing time in the increase of the labor for the the prestress forces are resisted by manufacture of the structural de the steel construction constituting tails. This increase is due to the ap the lower part of the form. (fig. 11) plication of the portable contacts, This report has dealt chiefly with insulation of thrusts, and use of ad the electrothermic method of pre ditional washers etc. stressing bar reinforcement with In mass production of prestressed heating accomplished before placing details of comparatively small di in the mould. However, heating re mensions ( 6-8 m long) it is advis inforcing in the moulds has also been able to heat reinforcing bars aside studied. The first experiments with and then carry them hot to the form. reinforcement heated in the moulds In case of long details, piles, masts were carried out in the USSR, in etc., when to carry long hot bars Cheliabinsk, by the Ural branch of presents some difficulty the heating the USSR Academy of Construction of bars on forms may be quite suc and Architecture. Later on in Mos cessfully applied. cow the method of tensioning with Up to the middle of 1959 the reinforcement heated outside the electrothermic method of tensioning form was worked out, and this meth reinforcement of prestressed con od proved itself as being simpler as crete structures has been used for well as cheaper and is now used manufacturing slabs of a length up everywhere. However in some cas to7m. es heating within the moulds ap Further development of this meth pears to be quite efficient. When od has shown that good results may using this method the bars are made be attained for longer structures. somewhat longer than the distance At present in the USSR various struc between thrusts for easy placement. tures are being manufactured: truss (fig. 12). The possible inequality in es and roof beams, large-size slabs, the length of the bars is then elimi piles, posts and others by tension nated by the driving in of thin "lev ing of the reinforcement by the elec elling" forked washers between the trothermic method. The tensioning anchors of the bar and the thrust. is accomplished by the wires cool After this operation the current is ing and is completed before the plac switched on, the bars freely elongate ing and hardening of the concrete and their anchors move off from the mix. In all cases heating of the rein thrusts of the mould. When the bars forcement outside of the moulds has have sufficiently elongated a "meas been found cheaper and reduces la ured" washer is inserted whose thick bor requirements. As an illustration ness corresponds to the predeter three examples of prestressed struc mined extent of the reinforcement tures are used: i.e. roof beams of elongation. After placing the "meas- industrial buildings with a 18 m 66 PCI Journal span, roof trusses with a 12 m span, (Figures 16, 17, and 18) and large size double !-section slabs. The moulds are designed so that The 18 m span roof beams have two trusses are manufactured simul a curved upper edge approaching a taneously. The tension forces of the parabola. This curve creates condi reinforcing bars are resisted by the tions for ideal moment resistance. 'fixed spacer placed between the The beam is of I -section with the lower flanges of the two moulded main stressed reinforcement in the trusses. This steel spacer is part of lower flange plus some in the web. the mould, which is freely support The rest of the reinforcement which ed on the bottom supports. During has not been prestressed, is made steam curing the mould is heated in the form of welded screens and simultaneously with the tensioned cages. (Figures 13, 14, and 15) bars and tensioning is not weakened Two beams are manufactured si as it is with concrete supports. multaneously, on a stand, in a hori The middle spacer also serves as zontal position. The prestress forces part of the moulds as it forms the are resisted by anchor plates in the lower surface of the truss. Due to reinforced concrete beam placed be this the bending forces in the spacer low the mould. are very low. A removable steel insert is placed The outer side of each half of the between the manufactured beams mould is made as an integral mem for performing simultaneous electro ber, and is pulled away from the thermic heating of two 14 mm bars. hardened concrete by four screw The installation consists of two con type hand-wheels. tacts with eccentric springs and in The spacer and outer sides of the sulated rolls, which prevent sagging mould have steam ducts for contact of the bars. The transformers are heating of the concrete plus the placed between the mould pairs. entire mould is placed in a steam From the side of one of the contacts chamber. is a terminal switch, automatically Large-size double-T-section panels. cutting off the current after the re When these structures were being quired lengthening of the reinforc made we took into account available ing bars during their heating. experience in U.S.A. where double Roof trusses 12 m long are de T -section slabs are usually made 1.2 signed for one-story industrial build m ( 4 ft.) wide. (Figures 19, 20, ings with a column spacing of 12x24 and 21) m. They support beams which span Taking into account the advantage 24 m and are spaced every 6 m. As of employing large-size precast the production of these roof trusses members and the availability of was started after mastering the man transport and assembly machinery in ufacture of the roof beams, several the USSR, double T-section panels improvements were made in their are made 3.0 m ( 10 ft) wide, 9, 12 production. and 15 m ( 30, 39, 49 ft) long and The bottom flange of the truss is 45 and 60 em ( 18 and 24 in) high. reinforced by 10 stressed bars of The long panels as are the previ low alloy steel with a 22 mm. dia. ously mentioned beams and roof The other parts of the truss are com trusses are made of concrete of a 2 pressed under load and are not pre strength of 350 kg/cm ( 5,000 psi). stressed; they are reinforced with The strength of the concrete during welded three-dimensional cages. transfer is not less than 225 kg/ cm2 September, 1961 67 Fig. 13-Forms for manufacturing curved roof beams.

Fig. 14-Reinforcing in place in roof beam forms.

Fig. IS-Completed roof beams.

68 PCI Journal Fig. 16-Roof truss forms.

Fig. 17-Completed trusses being stripped from the forms. Note wheel on screw device for pulling top form away from concrete.

Fig. IS-Completed truss being lifted from form.

September, 1961 69 Fig. 19-Completed double tee's. An end diaphragm is cast with the tee to stiffen the slab edges.

( 3,200 psi). is used for several moulds. The average concrete thickness The hot bars are carried from the is 8 em for a length of 9 m, and heating installation to the mould by 10 em for 12 and 15 meters. The 3 or 4 workers, two holding the bars panels are designed for live loads by hooks. Heating and placing of 2 of 350 and 600 kg/ cm ( 123 psf). one pair of reinforcing bars into the Taking into account the width of mould takes about 4 minutes. the panel, low transverse ribs are cast In ·our building practice we also at the ends to prevent damage to post-tension the reinforcing bars in the thin slab during transportation set concrete. The reinforcement is and erection. laid, without coating, in opened or The stressed working reinforce closed channels. ment of each of the two ribs of the The main characteristic of the So panel consists of 4-8 bars of low viet method of electrothermic post alloy steel, 14 mm india. tensioning is that tensioning without The panels are manufactured in coating has been worked out and is separate steel integral moulds, widely used as distinct from post which also resist the stressing forces. tensioning the reinforcement with The moulds are placed in steam coating as suggested by Germany chambers. The entire cycle of panel and USA but not used in practice. manufacture, including steam curing In conclusion we should like to does not exceed 20 hours. say that no new method of prestress We consider it expedient to have ing was ever mastered with such a special mould for each standard speed by numerous enterprises and size, without using inserts, as done in construction jobs in different in the U.S.A. This is explained by parts of the Soviet Union, as that our guaranteed sale of products and of electrothermic tensioning during obtained economy. the last 18 months. The machinery for electric heat In the USSR electrical heating is ing of the reinforcing bars is porta now beginning to be used in preten ble so that one heating installation sioning high tensile wire also. 70 PCI Journal Fig. 20-Transporting tee's to the job site.

Fig. 21-Typical erection procedures. September, 1961 71