Basic principle and procedure

The basic principle of pretensioning involves the tensioning of the tendons to a predetermined level, after which the concrete is placed (see Fig. 3-1a). The resulting elongation of the tendons is maintained at a constant level while the concrete hardens. After the concrete has developed sufficient strength the tendons are released and, because they are now bonded to the concrete, their shortening is resisted by the concrete. In this way the concrete is prestressed by the action of bond when the tendons are released (see Fig. 3-1b).

It is important to ensure that the elongation of the tendons is maintained at a constant level while the concrete is allowed to harden, and this can be achieved by each of the following two methods (Refs. 3-1, 3-2 and 3-4):

• Pretensioning with individual moulds: According to this method the tendons are anchored directly to the individual steel moulds in which the concrete is cast. In this case, the moulds must be designed and constructed to withstand the additional forces induced by the tendons.

• Pretensioning on stressing beds: When pretensioning on a stressing bed, the tendons are tensioned between and subsequently anchored to the rigid vertical steel anchor columns, called uprights, placed at each end of the bed (see Fig. 3-1a). In this manner the tension is maintained in the

tendons while the concrete is placed and cured. The stressing bed also serves as a casting and curing bed.

With the exception of railway sleeper production, pretensioning with individual moulds is not commonly used (Refs. 3-1, 3-2 and 3-4). An apparent advantage of this method is that, in the case of small products, the individual moulds can be can be moved through the plant on a mass production line instead of having to move the materials and the process to the moulds, as is the case when stressing beds are used (Refs. 3-1 and 3-2). Because of its limited use, this method is not discussed here in any further detail.

Pretensioning on stressing beds is by far the most common method used today, and a typical arrangement is shown in Fig. 3-1a. This method, often referred to as the long-line or Hoyer method, lends itself to efficient mass production because a number of similar elements can be manufactured in a single tensioning operation if the bed is made long enough. The length of stressing beds varies between 25 m and 200 m, and long beds can be provided with removable intermediate uprights (see Fig. 3-1a) so that shorter tendons can also be tensioned (Refs. 3-2 and 3-5).

Tendons are tensioned by means of hydraulic jacks, and can either be stressed individually or simultaneously from one end of the stressing bed. Special jacks with a ram stroke of at least 750 to 1200 mm must be used if the strands are to be tensioned in a one-step operation (Ref. 3-1). After being tensioned, wires and strand are usually anchored by means of frictional split-cone wedges.

Efficient quick-release grips are also available for this purpose (see Fig. 3-2).

Tendons can be released individually or simultaneously. Tendons are released individually either by flame cutting, sawing or by hydraulic cutters, and a strict cutting sequence which minimises eccentric loading on the concrete, must be adhered to when carrying out this operation. It is also important to avoid the situation where too many tendons are cut at a single location because this can result in the failure of the remaining tendons at that location. Tendons must be cut gradually and as close to the ends of the members as possible to avoid large impact loads from being imparted to the concrete. These precautions will prevent excessive damage to the concrete at the ends of the members, and so will ensure that the bond between the concrete and the tendons in this zone is not impaired.

Stressing

jack Original length = L

L –∆

Removable intermediate

upright

Formwork Upright Upright

(a) Tendons tensioned between uprights

(b) Tendons detensioned (elastic shortening =D) Stressing bed

Figure 3-1: Pretensioning on a stressing bed (adapted from Ref. 3-5).

Tendons are released simultaneously by making use of hydraulic rams. The principle advantages of this method are that the prestressing force is gradually transferred to the concrete so that impact loading is avoided, and that the tendons can subsequently be cut between the members without following a strict cutting sequence. A disadvantage of this method is that the precast members close to the releasing end will experience relatively large movements away from the releasing end because all the strain is released at that end.

In pretensioned construction, the prestress is transferred to the concrete by bond and, therefore, particular care must be taken to ensure that the bond strength of the concrete is not exceeded. It can be shown that the bond stress induced by a tendon will decrease as its diameter decreases, for a given stress in the tendon. For this reason small-diameter wires and strand are used in pretensioning. Wire is often indented or crimped to improve its bond properties while, in the case of strand, 12.9 mm seven-wire strand is most commonly used.

It is often necessary to deflect some of the tendons to obtain the desired cable profile, particularly in the case of long span members (see Fig. 3-3). These deflected tendons are often referred to as draped or harped tendons, and are held in their deflected position by special hold-down devices at the lower deflection points (also called hold-down or draping points) and by hold-up devices at the high positions. Depending on the design requirements, deflected tendons can be provided with either one or two hold-down points, as shown in Fig. 3-3. Tendons can only be deflected if the stressing bed has been properly reinforced to sustain the vertical forces imposed by the hold-down devices.

Chuck

Seven-wire strand

Retaining ring

Jaw assembly

Spring

Cap

Body

Figure 3-2: Typical quick release grip (adapted from Ref. 3-2).

Hold-up

device Double hold-down point Single hold-down point

Figure 3-3: Pretensioning with deflected tendons (Ref. 3-5).

Deflected tendons are usually tensioned straight and then deflected by a hydraulic ram, after which the hold-down devices are installed to keep them in their deflected shape. Two methods of deflecting tendons in this way are shown in Figs. 3-4 and 3-5. In the method shown in Fig. 3-4, which is commonly used for the manufacture of double-tee beams, the deflected tendons are pushed down to their lower position by means of a temporarily installed hydraulic ram. These tendons are subsequently held in position by the hold-down pins which bear against the hold-down reaction beam. After the concrete has developed the required strength, the hold-down pins are removed and the tendons are released. The method shown in Fig. 3-5 makes use of a centre hole jack to deflect the tendons, while a strand chuck bearing against the hold-down anchors is used to anchor the tendons in their deflected positions. In this procedure, the strand chuck and the hold-down anchors cannot be recovered (Ref. 3-1).

The actual length of a deflected tendon, measured along its deflected path, can be significantly greater than the horizontal distance between its ends, particularly in the case of deeper members such as bridge girders. If such a tendon is initially tensioned straight, the subsequent deflecting operation will increase the tension in the tendon and, hence, the prestressing force. It is important to consider this effect when determining the initial tension to be applied to the tendons, particularly if the increase in tension is significant.

Deflected tendons can also be tensioned in their deflected shape, in which case the hold-down devices must be capable of permitting the tendons to move longitudinally during the tensioning operation and provision must be made to reduce the friction between the tendons and the hold-up and hold-down devices. The various techniques which have been used to reduce the effects of friction include tensioning the tendons from both ends, using rollers with needle bearings at the hold-up and hold-down points, and vibrating the tendons while they are being tensioned (Ref. 3-1).

As previously mentioned, the precast members will move longitudinally when the tendons are released so that it is essential to release the hold-down devices before releasing the tendons.

However, when the hold-down devices are released before releasing the tendons, the undesirable situation arises in which concentrated upward vertical forces are imposed on the beam at the positions of the hold-down points before any prestress has been transferred to the beam. If these effects are not properly accounted for in the design or in the releasing procedure (e.g. by partially releasing the tendons to transfer some prestress to the beams before releasing the hold-down devices) cracks can develop in the top of the beam.

Tendons in beams are deflected to reduce the cable eccentricity in the support regions which, in turn, prevents flexural cracks from developing at the top of the beam in these regions. This objective can also be achieved by debonding some of the tendons over a distance at the ends of the beam.

Such tendons are referred to as blanketed tendons (see Fig. 3-6).

Hydraulic ram pushes pin down Hold-down pin (removed from hardened concrete)

Hold-down reaction beam Double-tee form Hold-down

device Ratchet

adjustment

Figure 3-4: Deflecting tendons in a double-tee beam (Ref. 3-5).

12.5 mmdiameter strand Strand chuck

Strand chuck Center hole hydraulic jack

Hold-down anchors

Deflected strand group

Strand chuck

Figure 3-5: Tendon hold-down device for use with a centre hole jack (Ref. 3-1).

Plastic tube over strands

in bottom

Blanketed strand length (debonded by plastic tubes)

Figure 3-6: Blanketed strands (Ref. 3-5).

Typically, a stressing bed must allow a daily production cycle so that members can be produced in large numbers. Under such conditions the use of steel forms or moulds is preferable for the following reasons: Steel forms are durable and perform well under repeated use; they can be manufactured to a high degree of precision; they are easy to handle when being erected or stripped; they can be made adjustable to easily accommodate variations in member shape; and they can easily be made strong enough to allow form vibration (Refs. 3-1 and 3-4). Figure 3-7 shows a steel mould with removable side forms and a form vibrator for a bridge girder.

The forms are removed after curing the concrete and before releasing the tendons. They should be loosened or stripped in such a way that they do not restrain any longitudinal movement or vertical deflection of the member which may take place when the tendons are released.

To maintain a daily production cycle, the concrete must develop sufficient strength to allow the tendons to be released within about 16 hours after casting. This high early strength can be obtained either by using high early strength cement, by curing the concrete at an elevated temperature, or by combining these two options (Refs. 3-1 and 3-5). Curing at an elevated temperature is often done by steam curing, which involves the application of wet heat in the form of live steam under a confining cover. Steam curing generally commences 2 to 3 hours after casting and continues for 12 to 14 hours (Ref. 3-4). Other processes which can also be used to apply heat during curing include electrical-resistance heating and heating by circulating hot fluids through pipes contained in the forms or in the stressing bed (Ref. 3-5).

The long-line method is also used for the production of hollow core slab units. In this method, low slump concrete is extruded around the pretensioned tendons by an extruding machine which travels along the stressing bed to form a long hollow core strip. The tendons are released once the concrete has developed sufficient strength, after which the long hollow core strip is sawed into the required lengths. References 3-2 and 3-5 can be consulted for further information on this procedure.

Transverse sleepers

Form underties

External vibrator sled and track

Figure 3-7: Steel mould for a bridge I-beam (Ref. 3-5).