Tendons may be stressed either by pre-tensioning or by post-tensioning, according to the particular needs of the form of construction. In each system, different procedures and types of equipment are used and these govern the method of tensioning, the form of anchorage and, in post-tensioning, the protection of the tendons.
All wires or strands stressed in one operation should be taken, where possible, from the same parcel. Each cable should be tagged with its number and the coil number or numbers of the steel used. Cables should not be kinked or twisted, and individual wires and strands should be readily identifiable at each end of the member. No strand that has become unravelled should be used.
8.7.2 Safety precautions
A tendon when tensioned contains a considerable amount of stored energy, which, in the event of any failure, anchorage or jack, may be released violently. All possible precautions should be taken during and after tensioning to safeguard persons from injury and equipment from damage which may be caused by the sudden release of this energy.
8.7.3 Tensioning apparatus
Hydraulic jacks are the normal means for tensioning tendons, although weights or levers may sometimes be used. The tensioning apparatus should be in accordance with the following.
a) The means of attachment of the tendon to the jack or tensioning device should be safe and secure. b) Where two or more wires or strands are stressed simultaneously, care should be taken that they are of approximately equal length between anchorage points at the datum of load and extension
measurement.
c) The tensioning apparatus should be such that a controlled total force is imposed gradually and no dangerous secondary stresses are induced in the tendons, anchorage or concrete.
d) The force in the tendons during tensioning should be measured by direct-reading load cells or obtained indirectly from gauges fitted in the hydraulic system to determine the pressure in the jacks. Facilities should be provided for the measurement of the extension of the tendon and of any movement of the tendon in the gripping devices. The load-measuring device should be calibrated to an accuracy within ±2 % and checked at frequent intervals. Elongation of the tendon should be measured to an accuracy within 2 % or 2 mm, whichever is the more accurate.
8.7.4 Pre-tensioning 8.7.4.1 General
Where pre-tensioning methods are used, the tension should be fully maintained by some positive means during the period between tensioning and transfer. The transfer of stress should take place slowly to minimize shock, which would adversely affect the transmission length.
8.7.4.2 Straight tendons
In the long-line method of pre-tensioning, sufficient locator plates should be distributed throughout the length of the bed to ensure that the wires or strands are maintained in their proper position during concreting. Where a number of units are made in line, they should be free to slide in the direction of their length and thus permit transfer of the prestressing force to the concrete along the whole line.
In the individual mould system, the moulds should be sufficiently rigid to provide the reaction to the prestressing force without distortion.
8.7.4.3 Deflected tendons
Where possible, the mechanisms for holding-down or holding-up tendons should ensure that the part in contact with the tendon is free to move in the line of the tendon so that frictional losses are nullified. If, however, a system is used which develops a frictional force, this force should be determined by test and due allowance made.
For single tendons, the deflector in contact with the tendon should have a radius of not less than five times the tendon diameter for wire or 10 times the tendon diameter for a strand and the total angle of deflection should not exceed 15°.
The transfer of the prestressing force to the concrete should be effected in conjunction with the release of hold-down and hold-up forces, so that any tensile stresses in the concrete resulting during the process do not exceed permissible limits.
8.7.5 Post-tensioning
8.7.5.1 Arrangement of tendons
Where wires or strands in a cable are not stressed simultaneously, spacing members should be sufficiently rigid not to be displaced during the successive tensioning operations.
Tendons, whether in anchorages or elsewhere, should be so arranged that they do not pass round sharp bends or corners likely to provoke rupture when the tendons are under stress.
8.7.5.2 Anchorages
All anchorages should conform to BS EN 13391. The anchorage system in general comprises the anchorage itself and the arrangement of tendons and reinforcement designed to act with the anchorage. The form of anchorage system should facilitate the even distribution of stress in the concrete at the end of the member and should be capable of maintaining the prestressing force under sustained and fluctuating load and under the effect of shock.
Split-wedge and barrel-type anchors should be of such material and construction that, under the loads imposed during the tensioning operation, the strain in the barrel does not allow such movement of the wedges that the wedges reach the limit of their travel before causing sufficient lateral force to grip the tendon, or at or before the limit of travel the wedges cause an excessive force in the tendon.
If proprietary forms of anchorage are used, the anchoring procedure should be strictly in accordance with the manufacturer’s instructions and recommendations.
All bearing surfaces of the anchorages, of whatever form, should be clean prior to the tensioning operation. Any allowance for draw-in of the tendon during anchoring should be in accordance with the engineer’s instructions, and the actual slip occurring should be recorded for each individual anchorage.
8.7.5.3 Deflected tendons
The deflector in contact with the tendon should, where possible, have a radius of not less than 50 times the diameter of the tendon, and the total angle of deflection should not exceed 15°.
Where the radius is less than 50 times the diameter of the tendon or the angle of deflection exceeds 15°, the loss of strength of the tendon should be determined by test and due allowance made.
8.7.5.4 Tensioning procedure
Before tensioning, it should be demonstrated that all tendons are free to move in the ducts. Tensioning should be carried out under competent supervision in such a manner that the stress in the tendons increases at a gradual and steady rate. Tensioning should not be carried out at temperatures below 0 °C without the approval of the engineer.
The supervisor in charge of stressing should be provided with particulars of the required tendon loads and extensions. Allowance should be made during stressing for the friction in the jack and in the anchorage although the former is not necessary when using load cells.
Stressing should continue until the required extension and/or tendon load is reached. The extension should allow for any draw-in of the tendon occurring at the non-jacking end, but measurement should not commence until any slack in the tendon has been taken up. A comparison between the measured tendon force and that calculated from the extension provides a check on the accuracy of the assumption made for the frictional losses at the design stage; if the difference is greater than 6 %, corrective action should be taken but only with the approval of the engineer. Full records should be kept of all tensioning operations, including the measured extensions, pressure-gauge or load-cell readings and the amount of pull-in at each anchorage.
Where a large number of tendons or tendon elements is being tensioned and the full force cannot be achieved in an element because of breakage, slip or blockage of duct, if the replacement of the element is not practicable, the engineer should consider whether a modification in the stress levels can still conform to the relevant limit state.
In the case of curved tendons, or tendons made up of a number of constituent elements, or tendons loaded in stages, the engineer should specify the order of loading and the magnitude of the load for each component of the tendon.
Tensioned tendons, anchorages and duct forms should be effectively protected against corrosion during the period between stressing and covering with grout, concrete or other permanent protection. Ducts should be plugged at their ends and vents.