54.1
General
The stressing of post-tensioned members is by many of the considerations applicable to pretensioned concrete.
Commentary
Post-Tensioning of Plant-Produced
Products
General
Many architectural panels which do not lend themselves to being because of difficulties with long line ing, such as jacking bulkhead self-stressing require- ments, can be easily post-tensioned. Tbe process of post-ten- sioning incorporates the installation of either bonded or bonded tendons in preformed voids ducts throughout the length of the member, through a section of the member. After curing the member. strands are stressed and anchored against the hardened concrete.
Bonded tendons are installed in voids ducts and made monolithic with the member and protected from
by grouting after the stressing operation is completed. tendons protected against corrosion by a prop- erly applied coating of galvanizing. epoxy, grease, wax, plas- tic, bituminous or other approved material, and carefully cast in concrete in a sheathing of heavy paper or plastic. bonded tendons connected to the member only through anchorage hardware, which should be sired and designed in accordance with 318.
Stress in the tendons shall always be measured by gauge readings and verified by elongation. Due to frictional losses peculiar to post-tensioned mem- bers and generally due to their relatively short length (as compared to most pretensioning beds) the predetermination of jacking loads and elonga- tions and accuracy and reconciliation in measure- ment are particularly important. The elastic short- ening of the concrete member during tensioning shall be given due consideration in computing ap- parent elongations.
Records shall be maintained for plant post-tension- ing operations in a similar fashion to other plant op- erations.
Post-tensioning systems shall be installed in accor- dance with manufacturers’ directions and proven procedures. Manufacturers’ recommendations shall be observed regarding end block details and spe- cial reinforcement in anchorage zones applicable to their particular systems.
Plastic coated unbonded tendons with a low of angular friction looped within the panel, and anchorages installed at one end only or at both ends may be used. Curvature in the tendon profile shall preferably not be closer than 3 feet (0.9 m) from the stressing anchorage. Tendons shall be firmly supported at intervals not exceeding 4 feet (1.2 to prevent displacement during concrete placement.
The strand (tendon) most frequently used in architectural pre- cast post-tensioned concrete is called the monostrand. Al- though can be fabricated to be grouted, they are usually coated with and covered paper plastic. Thus, they typically used in the condition. If friction is exceptionally high due to length curvature of tendon, a strand coated with and encased in plastic tube is available. These have a coefficient of angular friction = 0.03 to 0.05). Anchorages and pocket for- ums should be rigidly attached to the to prevent
Standard
Commentary
of cement paste into the anchorage cavity. Ties between the sheathed tendon and support steel should not tight as to cause visible deformations (indentations) in the
Details and Positions for Duets
Materials commonly used for formed ducts are 22 to 28 gauge galvanized or bright spirally wound or longitudinally seamed steel strip with flexible semi-rigid seams.
Although most ducts are formed using metal tubing, occasion- ally collapsible or inflated rubber tubes that can be removed after the concrete has hardened used to form void in the member. This would not be a preferred method if grouting were to be utilized, due to the lack of composite action be- tween the cylinder left by the void and the grout placed in the void. For grouted tendons a corrugated HDPE or polypropy- lene duct may be used if the materials meet the appropriate Post-Tensioning Institute recommendations.
Short kinks wobbles in alignment will result in appreciable increases in friction during tensioning.
5.4.2 Details and Positions for Ducts
Ducts for post-tensioning tendons shall be con- structed of flexible or semi-rigid metal or corrugated HOPE/polypropylene tubing installed within the member. Tendons which are not to be bonded by grouting may be installed in ducts of plastic, or other material. Metal ducts shall be of a ferrous metal and may be galvanized. Aluminum or PVC shall not be used for ducts.
The alignment and position of ducts within the member shall be controlled. The trajectory of ducts shall not depart from the curved or straight lines shown on the design drawings by more than in. (13 mm) per ft (3 m). For curved members, the tendons, and consequently the ducts, shall be placed on or symmetrically about the axis of the member that is parallel to the direction of the curva- ture. The position of ducts with respect to the thick- ness of the member, especially at critical locations shall be maintained within a dimensional tolerance consistent with the size and usage of the members, with a maximum variation from specified position of in. mm) or in. per foot of depth, whichever is smaller.
The alignment of ducts shall be such that tendons are free to move within them and, if grouting is to be used, area shall be sufficient to permit free pas- sage of grout. The inside diameter shall be at least in. (6 mm) larger than the nominal diameter of single wire, bar or strand tendons; for multiple wire or strand tendons, the inside cross-sectional area of the duct shall be at least twice the net area of the prestressing steel.
Ducts installed in members prior to casting the con- crete shall be of such construction that they will not admit concrete or mortar during casting. Ducts or duct forms shall be so supported and fastened that they will maintain their positions during casting and compaction of concrete. Joints between duct sec- tions shall be adequately coupled and taped to maintain geometry and prevent concrete paste in- trusion during casting. After placing of ducts and re- inforcement and forming is complete, an inspection shall be made to locate possible duct damage. All holes, openings, or excessive dents shall be
Tendons may be installed in the ducts either prior to or subse- quent to placing concrete. In general, it is preferable to place the tendons subsequent to the concreting operation so that
and can be blown cleaned from the duct to avoid blockage of the duct.
Standard
paired prior to concrete placing.
All ducts shall have grout openings at both ends. Grout openings and vents shall be securely an- chored to the duct and to either the forms or to re- inforcing steel to prevent displacement during con- crete placing operations.
Friction in Ducts C5d.3 Friction in Ducts
The jacking force necessary to provide the required stress and overcome frictional force shall be indi- cated. Production documents shall also show the techniques to be observed in jacking, which may consist of over-jacking and over-elongation fol- lowed by a reduction of load for seating the anchor- ages or of jacking from both ends.
Maximum jacking force shall not exceed the appli- cable limits in ACI 318.
Friction on the post-tensioning tendon is due to length and cur- vature of the ducts The length effect is the amount of friction that would be expected in a straight tendon due to minor mis- alignment (wobble of the duct). The curvature effect results from friction due the of the duct. Both components of this friction are to the respective coefficients of friction between the tendon and the side of the duct. Coefficients constants be used for computing fric- tional effect have been established by research for all duct and tendon combinations in common usage.
5.4.4 Tensioning
A schedule indicating the minimum jacking strength and a sequence of tensioning tendons to keep stresses within predetermined limits of symmetry about the axis of the member shall be established and shown on the production drawings. The con- crete compressive strength shall be determined from test cylinders.
A minimum initial force of 10 percent of the jacking force shall be applied to the tendon to take up slack and to provide a starting point for elongation mea- surement. The jacking force shall then be applied, including any overload and release that may be called for in the procedure. The rate of application of the force shall be in accordance with the post-tensioning manufacturers’ recommended pro- cedure.
Final force applied to tendon and actual elongation measured shall check the theoretical values within 7 percent and shall agree with each other within 7 percent. If stressing is not achieved within this tol- erance, then procedures shall be altered until toler- ance limits are observed. For post-tensioned ten- dons the stress at the end anchorages, immedi- ately after tendon anchorage lock-off shall not exceed 0.70
5.4.5 Anchorages
Anchorage devices for all post-tensioning systems shall be aligned with the direction of the axis of the
Tensioning
Post-tensioning in plant produced members is generally in short lengths so elongation is usually a small value which places added emphasis on carefully obtaining accurate readings. For post-tensioned tendons shorter 25 (7.6 special stressing methods and elongation measurement methods are required.
An gauge is a necessity for tendons since stressing the strands by calculating the elongation would be difficult because of the elaborate strand configurations (multi- tude of tendon curvatures) and losses which occur during the jacking process.
The actual elongation of the strand should be checked against the theoretical elongation that the strand is entirely stressed. The strand may become bonded kinked or the anchor may not be working preventing the strand being fully stressed.
Sample calculations for tensioning setups are shown in Ap- pendix
Anchorages
For systems, the anchorage provides the only point from tendon member; therefore, it is critical that the anchorages be capable of 95 percent of the ultimate 124 MNL-117 3rd Edition
tendon at the point of attachment; concrete sur- faces against which the anchorage devices bear shall be perpendicular to the tendon axis. Anchor- age losses, due to seating loss or other causes, shall be measured accurately and compared with the assumed losses shown in the post-tensioning schedule and shall be adjusted or corrected in the operation when necessary.
The connections attaching the anchorages to the form shall be sufficiently rigid to avoid accidental loosening during concrete placement. The anchor- age area shall be sealed immediately after the ten- dons or strand are post-tensioned. Minimum con- crete cover for the anchorage shall not be less than the minimum cover to the reinforcement at other locations.
Plastic pocket formers used as a void form at stressing anchorages shall prevent intrusion of con- crete or cement paste into the wedge cavity during concrete placement. Pocket formers shall be coated with grease prior to insertion to help prevent concrete leakage into the anchorage and to aid in their removal during form stripping.
5.4.6 Grouting
Ducts shall be blown free of water after curing of the concrete and provision made to keep water out of the ducts prior to grouting. To provide maximum protection to the tendons, grouting shall be per- formed within ten days after completion of the ten- sioning operation unless otherwise specified. If a delay is expected in grouting, a rust inhibitor can be applied to the tendon before placement in the duct. Grout shall always be applied by pumping toward open vents. Grout shall be applied continuously under moderate pressure at one point in the duct until all entrapped air is forced out the open vent or vents. Vents shall not be closed until they dis- charge a steady stream of grout. Once all vents are closed, pumping shall continue until a steady pres- sure of 100 psi is maintained for ten seconds. Thixotropic grouts shall be mixed with a shear-type mixer rather than a paddle mixer.
5.4.7 Sealing of Anchorages
Tendon anchorages shall receive a concrete or grout seal to provide the minimum cover required for the tendon material elsewhere in the structure. This seal shall be adequately covered for curing since shrink- age or contraction cracks will permit moisture
of the tendon.
Alignment of anchorages is critical for seating of tendons. Misalignment during casting can reduce effectiveness of an- chorages.
C5d.6 Grouting
Post-tensioning members which are to heavy fluctuating or dynamic loads or which subject to frequent wetting or or severe climatic exposure should have ducts con- taining the tendons pressure grouted following the completion of tensioning. Grouting is an important operation, serving to protect the tendons, relieve the anchorage of stress fluctuation, and increase the efficiency of the tendon in resisting ultimate moments.
C5.4.7 Sealing of Anchorages
Care should be exercised to protect end anchorages of tendons. Even with grouted tendons, the end anchorage is an integral part of post-tensioned system.
Lack of proper protection access point for moisture and a developing point for corrosion. If salts are present in
Standard
tration. Low-shrinkage, non-metallic grout shall be chosen for anchorage pocket sealing.
Commentary
moisture or water that accesses anchorage, then is enhanced and often accelerated, making adequate end tion mandatory.
If a concrete or grout seal cannot be provided, then the anchorage and tendon end shall be completely coated with a corrosion-resistant paint or other effec- tive sealer. The anchorage and tendon end shall then receive a cover which will provide fire resis- tance at least equal to that required for the structure.