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Discharge and placement of concrete can easily displace improperly secured ducts. Ducts should be properly secured and caution exercised when placing concrete. Figure 3.23 shows a case where concrete was placed down the webs and allowed to flow across the bottom slab of a segment where ducts were not well tied. The concrete displaced the ducts sideways and lead to significant difficulties with tendon installation. More duct supports, in this case between the top and bottom rebar in the bottom slab, and a change to the sequence of discharge and placement will solve this problem.

Figure 3.23 - Placing concrete in box segments

It is common practice to use form vibrators for concrete consolidation for many precast components such as piles and I-girders. External form vibrators may be used on casting cells for precast segments. However for most cast-in-place, and some precast, construction internal vibrators are usually needed.

Vibrators can displace ducts when they are not properly secured. Also, over aggressive or improper use of internal vibrators may lead to local duct deformation or damage. Care must be exercised. Place concrete in relatively small lifts of only two to three feet and allow internal vibrators to only penetrate sufficient to consolidate the lifts. Use care not to get a vibrator permanently lodged in the rebar cage (Figure 3.24)!

Figure 3.24 - Use of internal vibrators for consolidation of concrete

To prevent unnecessary and unwanted contamination of ducts in the period from casting to installing tendons, it is strongly recommended that suitable protection measures be implemented. For example, cover or temporarily plug open ends of ducts or install temporary caps over anchors to prevent water and vermin entering them. Drain holes in the bottom of ducts should be left open with grout pipes pointing downward to freely drain any rain water or condensation. Grout vents at high points and anchors should be temporarily closed. Areas of faces of components such as precast segments with multiple duct openings may be covered with suitable heavy duty plastic sheet. All temporary measures should be periodically checked, particularly if work is partially finished or components are in storage for extended periods.

Relatively simple precautions are worthwhile compared to the inconvenience and potential costs of repairs as a consequence of blocked ducts. Also, it is far more preferable to keep water out of ducts than to have to remove it prior to grouting. Excess water dilutes grout and can lead to bleed and grout voids which, in turn, may facilitate corrosion. Filling grout voids after the grout has set, is difficult and may require special vacuum grouting - which is costly. A little prevention is worth far more than the cure!

3.5 Tendon Installation

Main longitudinal, internal or external tendons set to a curved or draped profile are usually made up of multiple 12 or 15mm (0.5 or 0.6 in) diameter, seven-wire strands. The number of strands per tendon depends upon the range of anchor and wedge plate hardware available for that system.

Similar strands are used for transverse tendons in the deck slabs of precast or cast-in-place segments - but only three or four strands are laid side-by-side in a flattened oval duct draped to a very shallow profile. Occasionally, straight bar tendons may be used transversely.

Post-tensioning bars are most often used for temporary applications; erection of precast segments, securing erection equipment such as gantries and form travelers. Bars are more expensive than strands for a given post-tensioning force, primarily because of the cost of the anchor plates, nuts and couplers. Re-use is appropriate and economical for temporary work, providing that the stress does not exceed more than about 50% GUTS and the number of re-uses is limited normally to about ten; or as otherwise recommended by the manufacturer. For installation, post-tensioning bars are usually placed through straight ducts of sufficient diameter to provide adequate tolerance for construction (2.3.1.2).

3.5.2 Proving of Internal Post-Tensioning Ducts

Prior to installing internal tendons, it is recommended that ducts be proven to be clear of damage or obstructions by passing a suitable sized torpedo through the ducts. The torpedo should have the same cross sectional shape as the duct but 6mm (¼in) smaller all around than the clear, inside dimensions of the duct and should have rounded ends. For straight ducts the torpedo should be about 0.6M (2ft) long. For sharply curved ducts the length should be such that when both ends touch the outermost wall the torpedo is at least 6mm (¼in) clear of the inside wall; but it need not be longer than 0.6M (2ft). A duct should be satisfactory if the torpedo can be pulled easily through by hand without excessive effort or mechanical assistance.

For guidance, it is recommended that this test be performed on each individual tendon in a precast girder or similar component before it is released from the precast yard. For all cast-in-place construction with internal tendons, this test would be done on site. For internal tendons in precast segments, this test would be done on site after erection. Proving ducts with a torpedo is recommended for all internal longitudinal tendons over approximately 15M (50 feet) long and may be used, as necessary, for shorter tendons or as otherwise required by specific project documents. This check is not necessary for transverse tendons in slabs of precast segments when tendons are installed in the casting yard. It is not necessary for the short lengths of internal longitudinal tendons in precast segments while in storage.

3.5.3 Installation Methods

Post-tensioning strands may be pushed or pulled through ducts to make up a tendon. Pushing should be done with care using a protective plastic cap provided by the PT system supplier so that it does not get caught or damage the duct. Pushing single strands into a duct already containing many strands may become difficult as the duct is filled with more strands.

Sometimes it may be easier to pull the entire bundle through together using a special steel wire sock or other device securely attached to the end of the bundle (Figure 3.25). Welding strands together with a pulling eye is not recommended because the heat of welding alters the steel properties and reduces its strength even when a few feet are wasted.

For transverse post-tensioning in deck slabs, sometimes a Contractor may wish to place strands in ducts before concreting to provide extra rigidity. If this is done, the transverse strands should be checked to see that they can move in the ducts after casting in order to ensure that they are indeed free before they are stressed.

In any event, if strands are placed in ducts before casting concrete, the time for completion of stressing and grouting commences from the moment the tendons are placed in the duct.

Figure 3.25 - Steel wire sock for installing multi-strand tendon

For aggressive environments, when ducts may be contaminated with chlorides, they may require flushing before installing tendons. Only clean water should be used for flushing. The ducts should be well drained. All water should be removed before grouting. If necessary, flushing water should be blown out of the ducts before installing tendons, using dry, oil-free compressed air.

3.5.5 Time to Grouting and Temporary Tendon Protection

The time between the first installation of the prestressing steel in the duct and the completion of the stressing and grouting operations should not exceed the recommendations of the AASHTO LRFD Construction Specifications.

Any light surface corrosion forming during this period of time should not be sufficient to reject the prestressing steel. However, unless approved by the Engineer, failure to grout tendons within the time limit might be sufficient reason to stop work until the concerns are resolved.

The use of water soluble oil to reduce friction for installation and stressing or for temporary corrosion protection of an installed tendon is not recommended as it has been shown to reduce bond. Furthermore, it can never be satisfactorily removed from the strands and ducts and any residual water in the ducts spoils the grout, leading to excessive bleed, grout voids and possible corrosion.

Ends of tendons should be protected by coverings until approval for cutting off the stressed strand tails after satisfactory stressing has been obtained from the CEI.

3.6 Jacks and Other Stressing Equipment

Jacks for stressing single (mono) strands generally have two cylinders, one each side the strand, with a wedge device for gripping and pulling the strand (Figure 3.26).

Figure 3.26 - Mono-Strand Jacks (Courtesy VSL Corporation)

Occasionally, longitudinal multi-strand tendons may be stressed one strand at a time, although this is usually only practical where each strand is clearly identifiable at each end and there is no risk of trapping an underlying strand in the process.

3.6.1.2 Multi-Strand Jacks

Multi-strand post-tensioning tendons are usually stressed as an entire group, using very large custom made jacks. This ensures that all strands are tensioned together and avoids the risk of trapping an individual strand. Stressing jacks are generally of the center-hole type - i.e. tendons pass through a hole in the middle and are attached at the rear of the jack (Figure 3.27).

Prestressing jacks must be very accurate - which is difficult to achieve. Stressing jacks have more wearing surface and packing than a conventional jack of the same capacity. This, and the necessity of a long jack stroke, increases the potential for variations in the accuracy of the applied force. Other factors that affect the accuracy and efficiency of stressing jacks are: use of dirty oil, exposure of the system to dust or grit, eccentric loading, type of packing, ram position, oil temperature, hydraulic valves, ram and packing maintenance, and readout equipment.

Figure 3.27 - Typical multi-strand, center hole, stressing jack

The jacking system should be fitted with a pressure gauge which registers the pressure of the hydraulic jacking fluid. The pressure gauge and jack must be calibrated together and remain together as a unit throughout all stressing operations. Pressure gauges and jacks should not be interchanged. If they are, then the new system must be recalibrated before use in production stressing.

Pumps for hydraulic fluid delivery must be kept in good working order. Breakdowns in the middle of the stressing operation are undesirable.

3.6.1.3 Bar Jacks

Bar jacks have a central hole through which the bar passes and is secured by a nut at the rear of the jack (Figure 3.28). Most jacks have an enlarged nose to accommodate a bar-coupler.

Figure 3.28 - Prestressing Bar Jack

3.6.2 Calibration

Jacks should be calibrated every six months as a minimum.

3.6.2.1 Jack and Gauge

Calibration is most important. This is a process where the load delivered by the jack to a tendon is measured by a precise load cell or other equipment. The readings of the jack's pressure gauge are noted against the readings of the load cell through the entire jacking range to create a chart of pressure gauge reading versus actual load recorded by the load cell. The chart only applies to this particular jack and gauge combination - it does not apply to any other.

When used for stressing, the actual force in the tendon is easily found from the pressure gauge and calibration chart. In general stressing jacks are about ninety-five percent efficient; but actual efficiency will vary depending on the age and condition of the jack. Any calibration chart which shows jacking forces much greater than ninety-five percent of pressure multiplied by piston area should be questioned.

With use, a jack and gauge system can drift out of calibration. So, on large projects, a calibration load cell is normally kept on site and the jack and gauge are periodically checked. On small projects, the jack and gauge system should be calibrated immediately prior to use. This is often done by the supplier of the system or by a local, approved laboratory.

Figure 3.29 - Jack Calibration

3.6.2.2 Master Gauge

The master gauge measures hydraulic pressures accurately. The load cell operates on the principle that changing pressure results in a corresponding change in electrical resistance. The readouts are made with a so-called Transducer Strain Indicator.

Gauge readings should not be taken while the ram is retracting or in a static condition as hysteresis will likely result in erroneous values. The calibration curves and master gauge readings are only valid when the ram is extending.

If there is any indication of damage to the gauge, the stressing system should be checked with the master gauge. For this reason, the master gauge should be kept locked away in a safe place so that it is always in good working order. If there is more than 2% difference between the master gauge and the calibration chart, the jack and gauge should be recalibrated. Usually the stressing Contractor has the jacks calibrated with the master gauge and at least one other gauge (B) as a back-up.

3.6.2.3 Calibration Curve

A calibration curve relates the pressure recorded by the jack's own gauge to the actual force delivered by the jack (Figure 3.30). The curve is established by the above calibration process. It can be found for the jack's gauge and the master gauge. The jack and gauge must remain together as a unit at all times while in use in order to avoid mix-ups and incorrect results. Periodically during use, the jack and gauge should be checked by inserting the master gauge. Significant variation from the calibration curve would be reason to examine the jack system.

Figure 3.30 - Calibration Chart for Pressure Gauge and Jack Force

If a jack needs repair, then the repaired jack and gauge should be re-calibrated. (Repair to pressure hoses alone would not be reason for recalibration.)

3.7 Jacking Methods


3.7.1 Single (Mono) Strand Stressing

Single strand stressing using a monostrand jack is normal practice for transverse tendons in deck slabs (Appendix C) where each strand lies side by side in a flat-oval duct where it cannot interfere with or trap another strand. Similar applications might include relatively short longitudinal strand tendons in precast planks or solid or voided slabs.

Single strand stressing can be used on multi-strand longitudinal tendons only if they are straight or curve only in one direction so that the strands on the inside of the curve can be stressed before those on the outside to avoid trapping. For this reason, single strand stressing is not suitable for multi-strand tendons of reverse curvature.

When single strand stressing is used for a small section girder, allowance should be made for the elastic shortening loss induced in the earlier stressed strands by the stressing of subsequent ones. This should be taken into account in the design or construction engineering of the component.

Mono-strand stressing techniques are available for greased and sheathed strands for cable-stays and similar, external tendon, applications for repair or rehabilitation.

The sequence in which tendons are stressed and the ends from which they are stressed should be clearly shown on the Contract Plans or approved Shop Drawings, and must be followed.

3.7.1.1 Single Strand, Single End and Alternate End Stressing

When single mono strand stressing involves short tendons, it is usually only necessary to stress from one end because friction loss is small (although care is needed to make sure wedge set loss is not excessive on a short tendon).

In order to maintain relatively even dispersal of post-tensioning, transverse tendons in deck slabs should be stressed from alternate ends - i.e. stress all the strands of one tendon from one side of the bridge and switch to the opposite side for the next tendon - and so on. This may be referred to as "Alternate End Stressing". It should only be necessary in special cases (as determined by the Designer) to stress the strands of one tendon from alternating ends.

3.7.1.2 Single Strand, Two End Stressing

Two end stressing means stressing the same strand from both ends. This may be done sequentially, from one end at a time or simultaneously using two jacks. However, stressing from both ends would normally only be needed for long tendons where friction loss is significant. Stressing from the second end should not be done if the calculated elongation is less that the length of the wedge grip. Re-gripping in a portion of the old grip length should be avoided.

3.7.2. Multi-Strand

Multi-strand tendons are the most frequent choice for main longitudinal tendons in bridges. All the strands of one tendon are tensioned together using a multi-strand jack. The sequence in which tendons are stressed and the ends from which they are stressed should be clearly shown on the Contract Plans or approved Shop Drawings and must be followed.

3.7.2.1 Multi-Strand, Single End and Alternate End Stressing

When a multi-strand tendon is stressed from one end it is often referred to as "single or one end stressing" to distinguish it from tendons stressed from both ends. However, with a number of similar and often symmetrical tendons in a superstructure, that need only be stressed from one end, it is desirable to keep the overall post-tensioning effect as even as possible by stressing similar tendons from alternate ends of the structure. When this is done it is often referred to as "alternate end stressing" and it means that tendons are stressed from one end only, but from opposite, alternate, ends of the bridge.

The location of the jack is switched from one end of the structure to the other in such a way that an equal number of tendons are stressed at each end (Figure 3.31). If stressing starts with T1 on the east side of the structure, tendons T2 and T3 are stressed from the west side and T4 again from the east side.

Figure 3.31 - Alternate end stressing

By alternating the stressing ends the overall effect is more or less symmetrical. Since the design of the structure is usually based on a relatively even distribution per the alternate end stressing sequence, it is very important to adhere to the correct, specified sequence.

3.7.2.2 Multi-Strand, Two-End Stressing

When the tendons are very long, losses over the length of the tendon due to friction and wobble become large. Stressing the tendon from the second end results in a higher force in the tendon than if only stressed from one end. Also, for symmetrical tendons two-end stressing becomes effective when the effect of anchor set at the jacking end affects less than half of the tendon (Figure 3.32). Stressing from the second end should not be done if the calculated elongation is less that the length of the wedge grip. Re-gripping in a portion of the old grip length should be avoided.

Figure 3.32 - Stresses along tendon for different modes of stressing

When the tendon has been stressed to the final force at the first end the wedges are seated and the stressing operation moves to the other end. At this second end the tendon will already have a considerable force (there is no slack to be pulled out of the tendon and elongation measurements can start immediately). Elongations at this end will be relatively small and re-gripping of the jack is not normally necessary. The total elongation for the tendon will be the summation of the elongations measured at each end. Re-gripping is a source of error in the measurement of elongations. Care should be taken that no reference marks are lost during the re-gripping.

The second method involves a simultaneous jacking operation at both ends using two jacks. Each jack pulls approximately half of the total elongation. There is no reason why these elongations should be exactly the same. The advantage for the Contractor should be that the stressing operation and movement of equipment from one location to another can proceed somewhat faster since this method involved less individual re-gripping. Two sets of equipment are required and some reliable means of communication to synchronize operations.

3.7.3 Bar Tendons

Bar tendons have either a coarse or fine thread and are anchored by a nut bearing against an anchor plate. Bars are stressed individually using a special jack (Figure 3.28).

Curved bar tendons are rarely used nowadays; the vast majority of bar tendons are straight. With good clearance around the bars, there is no friction loss. Also, when the nut is gradually tightened using the ratchet on the jack as load increases, there is little or no seating loss. Consequently, the force is the same throughout and there is no need to stress from both or alternating ends.

The sequence in which PT bars are stressed should be clearly shown on the Contract Plans or approved Shop Drawings. For example, many PT bars are used for erecting and closing epoxy joints in precast segmental construction. It is important to maintain as uniform pressure as possible in order to evenly compress the soft epoxy. This is achieved by stressing the bars in a certain specified sequence. Similar situations apply to vertical PT bars in pier columns. Consequently, the sequence should be followed.

3.8 Stressing Operations