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Support and hold -down devices should be of sufficient rigidity and have adequate support so that the position of strand will remain substantially unchanged under the induced forces in this method.

Procedures should be established to minimize friction at both high and low control points of the strand. The use of pin and roller assemblies generally provides the best method of reducing friction at up and hold-down points. Extra caution should be observed in tensioning harped strands to avoid undue exposure to safety risks of the personnel involved.

DIVISION 5 REINFORCEMENT AND PRESTRESSING

Standard Commentary

Page 5.22 MNL -116 4th Edition

5.3.15 Equal Distribution of Force in Harped Strands

Distribution of force in strands throughout the length of a bed can be determined by measuring elongation of a predetermined length of strand in each member. This procedure shall be employed for the initial setup of a newly installed bed if the bed differs substantially from other beds at the plant or if harping hardware or procedures differ from those normally used.

C5.3.15 Equal Distribution of Force in Harped Strands

Friction at each of the deflecting devices resists some of the force exerted in tensioning harped strands. The force applied to the strand is therefore decreased at each successive point of deflection away from the source of jacking. For this reason, this procedure is to be used when a new casting facility is first installed if it differs substantially from those already in use. For example, a new bridge beam bed to make Type IV beams should be evaluated when first set-up if it is the first of its kind at the plant. If the plant has already been manufacturing Type III beams with the same or similar harping hardware and procedures, the new bed can be considered additional capacity of similar products and not substantially different.

Force distribution should be checked when tensioning procedures change. An example is when strands were previously tensioned straight and subsequently harped and the procedure changes to tensioning in the final fully harped position.

This procedure is not intended to apply to regular, routine changes in form configuration or product layout within the bed.

Measurements of strand elongation at locations within the element are difficult to perform. Form, reinforcing, and equipment restrict access. Evaluation of a measuring tape or bar cannot be considered reliable if read closer than to the nearest one eighth of an inch. The tolerance between measured and calculated elongation should be 5% rounded up to the nearest 1/8 in. For example, if the desired elongation of a 20-ft section of strand is 1-1/2 in., then the acceptable range of measurements should be from 1-3/8 in.

to 1-5/8 in. (±1/8 in.). This is because 5% of 1-1/2 in. is 0.075 in. Rounding up to the nearest eighth results in a value of 1/8 in. Measuring bars or tapes should be maintained at or near the temperature of the strand to eliminate thermal differences in measurements. It must be noted that this procedure has significant inherent safety risks due to the necessity of having to work in direct contact with tensioned and harped strand. Therefore, it should be accomp lished only when considered necessary as required above. Great care must be exercised and appropriate safeguards applied.

5.3.16 Strand Debonding

Methods used to debond strands shall eliminate bond over the entire specified length of debonding.

Substances that permanently alter the physical and/or chemical properties of the surrounding concrete beyond the debonding interface shall not be used.

C5.3.16 Strand Debonding

Pretensioned members may be manufactured by including effective bond breakers on stra nd to reduce or modify concrete stresses at critical portions of the member.

Leakage of paste into sleeving must be prevented. The most commonly used element for debonding is plastic sheathing. Other elements may be used, such as retarder if lightly applied. Items such as animal fat or reactive greases should not be used as they affect the concrete beyond the debonding interface. PVC pipe with free chloride ions, which would migrate into the concrete, should also not be utilized.

5.3.17 Detensioning

Force shall not be transferred to pretensioned members until concrete strength, as indicated by test cylinders or other properly calibrated nondestructive test techniques, is in accordance with the specified transfer strength.

C5.3.17 Detensioning

Tests have shown that the bond transfer length for wet mix concrete is not appreciably affected by concrete strengths in the range of 2,500 psi to 4,000 psi (17.2 to 27.6 MPa) at release. Concrete strength does influence camber and dimensional changes due to strains in the concrete. A minimum concrete transfer strength of 3000 psi (20.7 MPa) is recommended.

If concrete has been heat-cured, detensioning shall be performed immediately following the curing period while the concrete is still warm and moist.

If concrete is allowed to dry and cool after steam curing and prior to detensioning, dimensional changes may cause contraction cracking or undesirable stresses in the concrete. Strands should be detensioned immediately upon stripping off covers, or a way should be developed to detension strands before stripping the covers (or partially detensioned before stripping covers). The use of self-stressing forms reduces the effect of dimensional changes.

In single-strand detensioning, both ends of the bed shall be released simultaneously and symmetrically to minimize sliding of members. Forms, ties, inserts, or other devices that restrict longitudinal movement of the members along the bed shall be removed or loosened.

Alternately, detensioning shall be performed in such a manner and sequence that longitudinal movement is precluded.

In multiple-strand detensioning, strands are released simultaneously by hydraulic jacks. The total force is taken from the header by the jack, then released gradually. The over-pull required to loosen lock nuts or other anchoring devices at the header shall not exceed the force in the strand by more than 5%.

General procedures for detensioning shall be developed, documented, and implemented by

For tension to be released gradually, strands should not be cut quickly but should be heated until the metal gradually

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Standard Commentary

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appropriate personnel for the tensioning system used by the plant and the typical product line. Specific procedures for unusual product shapes and prestressing strand configuration shall be detailed on production documents. The sequence used for detensioning strands shall be according to a pattern and schedule that keeps the stresses nearly symmetrical about the vertical axis of the members.

Maximum eccentricity about the vertical axis of the member in the casting bed shall be limited to one strand or 10% of the strand group. Vertical axis eccentricity shall be limited at initial cutting of the ends of the bed and as strands are cut between members in setup. Detensioning shall be performed in a manner that will minimize sudden shock or loading.

loses its strength. This becomes much more significant as the ratio of prestressing force to the area of member increases.

Cracking of the concrete in the end region of the product may be the result of the detensioning method and sequence used. Implementation of a well designed strand cutting procedure and sequence has been shown to reduce or eliminate cracking.

5.3.18 Detensioning of Harped Strand

Hold-down forces shall always be computed and compared with the weight of the member if hold-downs are to be released prior to release of the strands from the anchorage.

C5.3.18 Detensioning of Harped Strand

For members having harped strands it is particularly important that no longitudinal movement be allowed along the beds until the hold -down devices are removed, as any such movement may cause serious cracking of concrete or destruction of the hold -down devices or both. It is therefore advisable to release hold-downs and remove bolts prior to releasing the forces at the anchorages; however, release of hold-downs without release of anchorage force may result in dangerous concentrated vertical loads that can crack the top of the member. In general, if the sum of hold-down forces is not more than half the weight of the member, it is safe to release hold-downs prior to release of the anchored force. If the hold-down forces are less than three-quarters of the weight of the member and nominal reinforcing steel is carried in the top flange or slab, cracking is unlikely. If the hold-down forces approach the weight of the member and if hold-downs are released prior to the anchorages, some cracking is inevitable in the upper surfaces immediately above the hold-downs. Sizing of reinforcing steel for this is addressed in ACI 318.

The effect of vertical forces resulting from initial release of hold-downs can be minimized by weights or vertical restraint applied over the hold-down points.

Cracks resulting from initial release of hold-downs will generally close upon release of the anchorage force. No rules regarding the sequence of hold -down and anchorage forces can be given as the incidence of upper surface cracking will vary with the size and shape of the member, concrete strength, and the amount of reinforcin g steel in the upper region of the member.

Detensioning of heavily stressed sections should be done at all points between members simultaneously to avoid damage caused by product movement or shock. Heavily stressed sections with harped strand may require a partial release of harped strand at the end of the member before releasing hold-downs to avoid top fiber cracking.

5.3.19 Detensioning of Dry-Mix, Machine -Cast Products

Each plant shall have a program to determine and evaluate strand slippage.

C5.3.19 Detensioning of Dry-Mix, Machine-Cast Products

In dry-mix, machine-cast products, achieving good bond between the concrete and the prestressing steel is primarily a function of the degree of consolidation of the concrete.

Inconsistencies in the concre te mix or mechanical malfunction in the equipment may also lead to bond failure.

When this occurs, strand slippage may be evident and the product shall either be rejected or its performance evaluated on the basis of some loss of bond. Detensioning slippage must be monitored beginning at a solid section of concrete.

After concrete placement is finished, it is necessary to clean the last trailoff or beginnings of the concrete placement away from the strand to allow the mark for measurement of slippage to be made on the strand directly adjacent to a solid section of the product. The mark on the strand should preferably be placed approximately one foot from the solid section of slab. A standard grillage should be used to make the mark on the strand during the operation each day and set beside the bed. This grillage can then be used to evaluate the slippage as well as making the marks.

If the mark is made excessively far from the end of the product, then the mark will move due to the movement of the strand as it is detensioned between the point of the mark and the end of the product. For this reason, placing the mark close to the end is preferable.

Evaluation of slippage is imperative. The loss of prestress is particularly hazardous for dry-mix products since shear capacity is chiefly provided by the prestress force. A lack of shear capacity creates a potentially dangerous situation of member failure, possibly with no prior warning.

Loss of prestress due to slippage also reduces load-carrying capacity since the mo ment capacity reduces as the slippage increases. The affected length of slipped strand is determined by considering an overlap of the zone of flexural bond with the zone of strand stress development bond.

5.3.20 Protection of Strand Ends and Anchorages

C5.3.20 Protection of Strand Ends and Anchorages

DIVISION 5 REINFORCEMENT AND PRESTRESSING

Standard Commentary

Page 5.26 MNL -116 4th Edition

Special attention shall be directed towards finishing the ends of members in the area of strand ends and anchorages, as specified on the shop drawings.

Unless such areas are maintained in a permanently dry condition after erection, strand ends and anchorages shall be protected against moisture penetration.

Lack of proper protection of end anchorages allows an access point for moisture which may lead to corrosion. This is especially true in environments where chlorides or other deleterious substances may be present in the water. Under such circumstances, adequate protection may be mandatory.

When exposed to view, anchorages (stressing pockets) should be recessed and packed with a minimum of a 1-in.

(25 mm) thickness of non-metallic, non-shrink mortar and receive a sack finish. Prior to installing the pocket mortar, the inside concrete surfaces of the pocket should be coated or sprayed with a bonding agent. This mortar seal should be adequately covered for curing as shrinkage or contraction cracks will permit moisture penetration. When not exposed to view, strand ends should be coated with a rust inhibitor, such as bitumastic, zinc-rich or epoxy paint to avoid corrosion and possible rust spots.

5.4 Post-Tensioning of