ITEM 426 PRESTRESSING Definitions. For this Item, the following definitions shall apply:

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1 ITEM 426 PRESTRESSING Description. This Item shall govern for the furnishing, fabricating, storing and handling of prestressing materials, for the prestressing of precast members, and for the prestressing of cast-in-place structural units, in accordance with the plans and with this Item Definitions. For this Item, the following definitions shall apply: (1) Prestressing. The introduction of internal stresses into a structural member by tensioning and anchoring strands, bars, or wires to counteract the stresses that will result from the applied load. (2) Pretensioning. The application of prestressing force to the tensioning devices prior to the casting of the concrete. (3) Post-tensioning. The application of prestressing force to the tensioning devices after the concrete has hardened. (4) Tendon. Any single unit used to apply prestressing forces to the member. For post-tensioned units, a tendon shall be each bar, each group of wires, or each group of strands having common end anchorage. (5) Post-tension System. A complete tendon with couplers, end anchorage, and all necessary hardware Materials. All materials shall conform to the pertinent requirements of the following Items except as otherwise required herein: Item 420, "Concrete Structures" Item 421, "Portland Cement Concrete" Item 433, "Joint Sealants and Fillers" Item 435, "Elastomeric Materials" Item 440, "Reinforcing Steel" Item 442, "Metal For Structures" (1) Prestressing Hardware. Prestressing hardware shall be in accordance with the manufacturer's specifications. (2) Concrete. Concrete shall be of the class shown on the plans. (3) Prestressing Steel. Prestressing steel shall conform to one of the following types: Stress relieved seven (7) wire strand conforming to Departmental Material Specification D ; alloy bars conforming to ASTM A722; or steel wire conforming to ASTM A421. When shown on the plans or permitted by approval of optional designs, low-relaxation strand, meeting the requirements of Departmental Material Specification D , shall be used. The Contractor shall furnish certified test reports to the Engineer that the low-relaxation strand to be used meets all of the above requirements. In addition, the Contractor shall furnish samples for further testing in accordance with Test Methods Tex-710-I and Tex-442-A.

2 Strands, bars, or wires not produced in accordance with Departmental Material Specification D , ASTM A722, or ASTM A421 which meet or exceed the physical properties of the material specified may be permitted provided the physical properties are shown on the shop drawings. Testing shall conform to the applicable ASTM Specifications for the prestressing material used. Samples from each size and each heat of prestressing bars, from each manufactured reel of prestressing steel strand, from each coil of prestressing wire and from each lot of anchorage assemblies and couplers to be used shall be furnished for testing. With each sample of prestressing steel wires, bars, or strands furnished for testing, there shall be submitted a certification stating the manufacturer's guaranteed ultimate tensile strength and modulus of elasticity of the sample furnished. All materials for testing shall be furnished by the Contractor at his expense. The Contractor shall have no claim for additional compensation in the event his work is delayed awaiting approval of the materials furnished for testing. All bars of each size from each mill heat, all wire from each coil, and all strand from each manufactured reel to be shipped to the site shall be assigned an individual lot number and shall be tagged in such a manner that each such lot can be accurately identified at the job site. Each lot of anchorage assemblies and couplers to be installed at the job site shall also be identified in a similar manner. All unidentified prestressing steel, anchorage assemblies or couplers received at the site will be rejected and loss of positive identification of these items at any time will be cause for rejection of their use as intended. Samples of materials and tendons, selected by the Engineer from the prestressing steel at the plant or job site, shall be furnished by the contractor to the Engineer well in advance of anticipated use. Sampling shall be done in accordance with Test Method Tex-710-I. (4) Post-tensioned Tendons. Unless otherwise shown on the plans, the Contractor has the option of furnishing any type of post-tensioning system meeting the requirements of the specifications. The system selected shall provide the magnitude and distribution of prestressing force and ultimate strength required by the plans without exceeding allowable temporary stresses. Post-tensioned tendons shall be grouted or ungrouted, as shown on the plans. Tendon couplers shall be used only at locations specifically shown on the plans or approved by the Engineer. (a) Grouted Tendons. For grouted tendons, end anchorages and tendon couplers (when permitted) shall develop a minimum of 95 percent of the required ultimate strength of the tendon with a minimum elongation of two (2) percent of the gage length when tested in the unbonded condition. Rigid metal ducts may be fabricated with either welded or interlocked seams. Metal ducts shall be bent without crimping or flattening and shall have sufficient strength to maintain their correct alignment during placing of concrete. Joints between sections of ducts shall be positive metallic connections which do not result in angle changes at the joints. Polyethylene (P.E.) ducts shall be sufficiently rigid to withstand placement of concrete, grouting and construction loads without damage or excessive deformation, while remaining watertight and shall be in accordance with ASTM Designation D3350, and D2239, D2447, or D3035 with cell classification PE or ASTM Designation D1248, Type 3, Grade 34, Cat. 5. Plastic material used shall not react with concrete or enhance corrosion or prestressing steel and shall be free of water soluble chloride. Smooth plastic ducts shall be used in locations embedded in the concrete. Corrugations of the plastic ducts shall be continuous spirals. The pitch of the spirals shall not be less than one-tenth of the radius (R/10) of the duct. The Contractor shall provide certified test reports on static pull-out bonding strength of the corrugated Polyethylene ducts for approval of the Engineer. These reports shall show that a force equal to 40

3 percent of the ultimate tensile strength of the tendon can be transferred from the tendon through the duct to the surrounding concrete at a length of two (2) feet six (6) inches. Twelve (12) pull-out tests must be conducted of which ten must meet specified strength. The duct and its connections shall be capable of withstanding the specified pressure for flushing the ducts in the event of an aborted grouting operation. Standard steel pipe shall have the properties defined in the latest edition of the American Institute of Steel Construction Manual and shall meet the requirements of ASTM A53, Grade B. The inside diameter of the duct shall be at least 1/4 inch larger than the nominal diameter of a single strand, bar, or wire tendon; for multiple strand or wire tendons, the inside cross sectional area of the duct shall be at least two (2) times the net area of the prestressing steel. The duct shall be adaptable for use with transition fittings as required to accommodate the necessary anchoring system. The duct shall be equipped with fittings for injection of grout and ports for venting. Grout to be used for the grouting of tendons shall be proportioned as follows: Portland Cement - Type I or II 94 Lbs. Water 5 Gals. Max. Expanding Admixture - to provide 2 to 4% expansion (Expanding admixtures shall not contain any chlorides, fluorides, sulfites, nitrates, aluminum powder or any other corrosive elements.) 50 lbs. Max. of Type A fly ash conforming to Departmental Material Specification D may be added to the above mixture if desired. When fly ash is used, additional water may be required to provide workability. Other pozzolans will not be permitted. Other mixtures of equal or better strength, workability, and free of corrosive elements may be approved by the Engineer. A change of brand or type of cement shall require reevaluation of the grout mix. (b) Ungrouted Tendons. For ungrouted tendons, end anchorages and tendon couplers (when permitted) shall develop at least 95 percent of the required ultimate strength of the tendon, with a minimum elongation of two (2) percent of the gage length, and shall withstand 500,000 cycles in a stress range of 0.6 F's minimum stress to 0.7 F's maximum stress, where F's is the guaranteed ultimate tensile strength of the tendon, without failure or slippage. Ungrouted tendons shall be coated with a nonvolatile, low friction mineral oil base grease, with a rust preventing additive having a relatively uniform viscosity under temperature ranges of 20 F to 120 F. A protective sheathing shall be provided around the tendon consisting of a waterproof material capable of maintaining the tendon tightly bundled and containing the lubricant Prequalification. Post-tensioning systems shall be pre- qualified prior to use on the project. Prequalification shall consist of tests on the complete tendons for compliance with the requirements of Subarticle (4). These tests shall be performed by a testing laboratory approved by the Director of Materials and Tests. For post-tensioning systems previously tested and approved on Department projects, complete tendon samples need not be furnished, provided there is no change in material, design, or details previously approved. Shop drawings or prestressing details shall identify the project on which approval was obtained. Prequalification testing will be at the Contractor's expense and the results certified, in writing, to the Engineer Project Samples. For post-tensioning applications only and unless otherwise shown on the plans, the Contractor shall furnish, for ultimate strength tests, one (1) specimen for each size of strand, bar, or wire to be used in the prestressing tendons. These specimens shall be five (5) feet in clear length. If the results of the test indicate the necessity of check tests, additional specimens shall be furnished without cost to the Department.

4 For prefabricated tendons, the Contractor shall notify the Director of Materials and Tests at least 10 days prior to the installation of end fittings or the heading of wires in order that sampling and testing may be arranged Packaging, Storing, and Handling. All prestressing steel shall be protected against physical damage and rust or other results of corrosion at all times from manufacture to grouting or encasing in concrete. Prestressing steel that has sustained physical damage at any time shall be rejected. Any reel that is found to contain broken wires shall be rejected and the reel replaced. The wire shall be bright and uniformly colored, having no foreign matter or pitting on its surface. Prestressing steel shall be packaged in containers or shipping forms for protection of the steel against physical damage and corrosion during shipping and storage. A corrosion inhibitor which prevents rust or the results of corrosion shall be placed in the package or form, or shall be incorporated in a corrosion inhibitor carrier type packaging material, or when permitted by the Engineer, may be applied directly to the steel. The corrosion inhibitor shall have no deleterious effect on the steel or concrete or bond strength of steel to concrete. Inhibitor carrier type packaging material shall conform to the provisions of Federal Specification MIL-P Packaging or forms damaged from any cause shall be immediately replaced or restored to original condition. The shipping package or form shall be clearly marked with a statement that the package contains highstrength prestressing steel, and the care to be used in handling, and the type, kind and amount of corrosion inhibitor used, including the date when placed, safety orders and instructions for use. Low Relaxation (Stabilized) strand shall be specifically designated per requirements of ASTM A416. All strands not so designated shall be rejected. When acceptable prestressing steel for post-tensioning is installed in the ducts after completion of concrete curing, and if stressing and grouting are completed within three (3) calendar days after the installation of the prestressing steel, loose rust which may form during said three (3) days will not be cause for rejection of the steel. Prestressing steel installed, tensioned and grouted in this manner, all within three (3) calendar days, will not require the use of corrosion inhibitor in the duct following installation of the prestressing steel. Post-tensioning steel installed as above but not grouted within three (3) calendar days shall have a corrosion inhibitor, approved by the Engineer, used in the ducts and shall be subject to all the requirements in this section pertaining to corrosion protection and rejection because of rust. Tight rust or pitting due to rust at any time will be cause for rejection of the strand. When prestressing steel for post-tensioning is installed in the ducts prior to concrete placement, a corrosion inhibitor, approved by the Engineer, shall be placed in the duct immediately after concrete placement. If the corrosion inhibitor is disturbed and stressing of the strand is not anticipated for three (3) days, new corrosion inhibitor shall be installed Equipment. The Contractor shall furnish suitable hydraulic jacks for stressing the steel. The jacks shall be equipped with fluid filled pressure gauges at least six (6) inches in diameter and graduated to read directly to one (1) percent of the maximum gauge capacity or two (2) percent of the maximum load applied, whichever is smaller. Each jack and its gauge shall be calibrated as a unit with the cylinder extension in the approximate position that it will be at final jacking force. The calibration of the jack and gauge shall be done while the jack is in the identical configuration as will be used on the site, e.g., same length hydraulic lines. Certified calibration charts shall be furnished by an independent laboratory with each jack and gauge used on the project. Certified calibration of each ram shall be made prior to the start of stressing operations on the project and every six (6) months thereafter, or as requested by the Engineer. Any repair of the rams, such as replacing the seals, changing the length of the hydraulic lines, changing the type of pump or using gauges which have not been calibrated with the ram, shall be cause for recalibration of the jack and gauge with a load cell. No extra compensation will be allowed for the initial or subsequent ram calibrations.

5 For post-tensioning, the jacks shall have provisions for measuring tendon elongation directly on the strand or bar. The jacks shall be capable of slow release of force to properly seat the tendon anchors. The Contractor shall furnish a grout pump of sufficient capacity to properly place grout in the quantities, pressures, and within the time limits required. For pretensioning, jacks for multiple strand stressing shall have sufficient capacity to provide the required stressing force and to permit simultaneous release of all straight strands. Single strand stressing will be permitted, but release of strands individually with single strand jacks will not be permitted. Single strand jacks for pretensioning shall have provisions for measuring the elongation directly on the strand Working Drawings. For cast-in-place prestressed units, in addition to forming and falsework plans required by Item 420, "Concrete Structures," the Contractor shall submit prestressing details. These details shall contain all necessary information for construction including: (1) Design Calculations. Unless otherwise shown on the plans, all design procedures, coefficients, and allowable stresses as well as tendon spacing and clearances shall be in accordance with the Standard AASHTO Specifications for Highway Bridges. Sufficient calculations to support the system and method of prestressing proposed for use, including realistic friction loss diagrams, shall be submitted. When the required jacking force for a particular type of tendon and duct and configuration is shown on the plans, and that type of tendon and duct and configuration are furnished, design calculations need not be submitted except as necessary to adjust for any conditions different from those shown on the plans. (2) Post-tensioning Details. These drawings shall show details of type, size, and number of strand, bar, or wire per tendon, tendon location and identity mark, jacking forces, lubricated tendons, seating loss, end anchorage systems, tendon profile, grouting and venting ports, grouting procedure, total elongation, measurable elongation, and other information necessary to properly complete the work. Jack gauge pressures required to monitor the stressing operation shall be furnished to the Engineer prior to the stressing operation. If friction reduction lubricants are to be used, the details shall state that use and the calculations shall be based on appropriate friction factors. A numbered layout and a step-by-step stressing sequence for the tendons shall be submitted. Complete provisions shall be made for each stressing operation, beginning with strand installation and ending with removal of excess strand at the anchors. All data pertaining to the stressing of the tendons, for a structure, shall be furnished in tabular form. Post-tensioning details shall include the method of support for the duct and location of the duct so that the center of gravity of the enclosed steel will be at the proper position. The details shall show the offsets from the bottom of the duct relative to the position of the prestressing steel within the duct and the distance from the bottom form to the bottom of the duct. The post-tensioning details shall be submitted on standard 22 inch x 34 inch sheets. Design calculations may be on standard letter size sheets. Six (6) sets of details and three (3) sets of design calculations shall be submitted to the Engineer, or directly to the Director of Bridges and Structures, Texas Department of Transportation, 125 East 11th Street, Austin, Texas , when so directed. When a railroad underpass structure is involved, an additional set of details and calculations will be required.

6 Construction Methods. (1) General. The general requirements of Item 420, "Concrete Structures", shall govern for cast-inplace construction and Item 424, "Precast Concrete Structures (Fabrication)" shall govern for precast concrete units or members. Prior to stressing, the Contractor shall furnish the Engineer certified copies of load calibration curves on all jacks and gauge systems to be used in the work. The control of concrete shall be by compressive tests of cylinders. The making, curing, and testing of test cylinders for "Release Strength", "Handling Strength", "Partial Tensioning Strength", and/or "Tensioning Strength" shall be in accordance with Test Method Tex-704-I. (2) Post-tensioning. The post-tensioning details submitted by the Contractor shall reflect the following general tensioning procedure, modified for each particular installation: (a) The modulus of elasticity shall not vary by more than one (1) percent for any strand within a tendon. (b) The tendons shall be tensioned in the sequence designated in the approved post-tensioning details. (c) Initial tensioning to take the slack out of the tendons will be 10 to 20 percent of the maximum tensioning load. (d) After the initial tensioning, the tendons shall be reference marked to determine elongation. (e) After tensioning to the specified jacking force, elongations shall be measured to determine tendon acceptance. After the tendon is accepted, the tendon anchors may be seated. (f) After stressing and anchoring all tendons and upon the Engineer's approval, projecting tendons shall be trimmed as shown in the approved prestressing details. If end anchorages permit, tendons may be inserted in the member after placing of the concrete. Positive means of holding the duct and reinforcing steel in the correct position shall be provided in all cases. The duct shall be supported at intervals of not more than four (4) feet, or as shown on the plans. They shall be securely fastened to prevent movement in any direction during placement of concrete. Tolerance for vertical positioning of tendons will be ± 1/4 inch, or 1/8 inch per foot of depth of the member, whichever is greater. After the concrete has reached a compressive strength equal to the "Partial Tensioning Strength" shown on the plans, as indicated by compressive cylinder tests, the members may be partially post-tensioned for the purpose of form removal or for removal of members from the casting bed to a curing area, provided satisfactory provision is made to bring all stressing units (including units stressed for handling) up to full required stress at the time of final stressing. The units to be stressed and the amount and pattern of the stressing for the partial post-tensioning shall be submitted with the post-tensioning details and approved by the Engineer. The final post-tensioning force may be applied when the concrete has reached a compressive strength equal to the "Tensioning Strength". These forces may be applied to one or both ends of the tendon, as shown on the post-tensioning details. The sequence of post-tensioning shall prevent overstressing the member in vertical or lateral bending at any time, and shall be indicated on the post-tensioning details. Calculations for elongation shall be adjusted to utilize either the modulus of elasticity for the strands being stressed or, when a bench test is specified, the apparent modulus derived from the bench test.

7 Suitable means shall be provided for measuring the elongation of the steel to the nearest 1/16 inch. Elongations for determining tendon acceptance shall be made before anchor seating and shall compensate for dead end anchor loss, anchor set, and elongation of strand in the jack. In the event of discrepancies of more than five (5) percent between calculated and measured elongations, the entire stressing operation shall be checked carefully and the source of variation determined and corrected before proceeding further. Loss of elongation due to anchor set shall be checked for agreement with the anticipated value used in the stress calculations. Adjustments to the jacking force may be required on future tendons to compensate for anchor set greater than anticipated. Failure of individual wires of a seven (7) wire strand or of wires in a parallel wire tendon is acceptable provided the total number of wire failures is not more than two (2) percent of the total number of wires in the tendon group. Failure of an entire strand will be cause for rejection of the tendon. Slippage of anchor wedges will be reason to reject the tendon pending the results of a structural review. For grouted tendons, the ducts shall be grouted within 48 hours after the completion of the tensioning operation. A sufficient number of two (2) inch grout cubes shall be made and job cured per grouting period. The temperature of the concrete shall be between 35 F and 90 F, from the time of grouting until the average compressive strength of three (3) consecutive cubes exceeds 800 psi. No individual cube of the three (3) may be less than 90 percent of the specified compressive strength. Grout must be pumped toward an open vent. Grout shall be pumped continuously under moderate pressure at one end of the duct until all entrapped air is forced out the open vents downstream from the grout pump. The open vents shall be closed as soon as grout flows in a steady stream. The grouting procedure shall continue as long as the established grouting time limit (if applicable) is not exceeded. After all grout ports have been closed, the pressure shall be increased to a minimum of 75 psi and held at this pressure for approximately 15 seconds. The grouting entrance port shall then be closed. Grout ports will be required at the far end of the tendon, and at the high points of the tendon profile when there is more than six (6) inch variation in the vertical position of the duct. Tendons with less than six (6) inch vertical variation shall have grout ports at the far end and at intervals between the far end and injection end not to exceed 100 feet. Grout ports shall consist of 1/2 inch minimum diameter pipe, with caps, valves, or other positive means of closing the pipe and having a minimum port diameter of 1/2 inch, as approved by the Engineer. The tops of the grout ports shall be set approximately 1/2 inch below the finished surface of the concrete. Upon completion of grouting, recesses caused by the grout ports in the concrete surface shall be filled with mortar and finished to the satisfaction of the Engineer. (3) Pretensioning. All strands to be pretensioned as a group shall be brought to a uniform initial tension of 1000 pounds (plus or minus 50 pounds) per tendon prior to being given their full pretensioning. Initial tension greater than that specified herein may be used when designated and approved on the shop drawings. The tension shall be measured by a dynamometer, or other suitable means to check the computed and measured elongation. The modulus of elasticity shall not vary by more than one (1) percent for any strands that are being gang tensioned. After initial tensioning, the straight strands shall be tensioned to a total stress as required by the plans by means of single strand or multiple strand hydraulic jacks that are equipped with calibrated gauges. The induced stress shall be measured by elongation of the strands and checked by gauge pressure. The results shall agree within five (5) percent. Means shall be provided for measuring the elongation of the straight strands to an accuracy of one (1) percent of the theoretical elongation or 1/8 inch, whichever is smaller. In the event of discrepancies of more than five (5) percent between stresses indicated by gauge pressure and elongation, the entire operation shall be checked carefully and the source of variation determined and corrected before proceeding further.

8 Where a combination of straight and draped strands are used, the measured elongation of the draped strand shall not vary more than five (5) percent from the calculated elongation. Measurements on individual deflected strands to establish differential stresses at selected points on the member shall be averaged at a cross section of the member, and the averages shall be within five (5) percent of the calculated elongation. The measured elongation of any individual strand shall not vary from the calculated elongation more than ten (10) percent at any measured cross section. Independent references shall be established adjacent to each anchorage to indicate any yielding or slippage that may occur between the time of initial stressing and final release of the tendons. One (1) splice per pretensioned strand will be permitted, subject to the following: Splices shall be located outside the members; strands which are spliced shall have the lay or twist in the same direction to avoid unravelling. When splices are used in a multiple strand tensioning operation, all the strands shall be spliced so that an adjustment can be made for the average seating loss. When shown on the plans, strands with bond breakage (debonding) shall be encased in plastic tubing along the entire debonded length and ends of tubing sealed with waterproof tape. Split plastic tubing may be used provided the seam of the tubing is sufficiently sealed with waterproof tape to prohibit grout infiltration. Wrapping of strands with tape to provide debonding will not be permitted. Full-length debonding of straight strands shall be permitted only as approved by the Engineer and shall be done on an individual basis. Full-length debonding, when permitted, shall be symmetrical about the vertical centerline of the beam and shall be limited to ten (10) percent of the total number of straight strands or a maximum of six (6) strands, whichever is less. Full-length debonding of draped strands will not be permitted. With the strands fully stressed as prescribed above and all other reinforcing in place, the concrete shall be cast to lengths necessary to provide the lengths required by the plans, after shrinkage and elastic shortening has occurred. After concrete strength requirements are attained, the tension in the strands shall be gradually and simultaneously released and the strands cut off as required, using a sequence to minimize shock and reduce premature tendon breakage. Release of strands individually with single strand jacks will not be permitted. Flame release of deflected strands in beams and boxes, and straight strands in piling, panels, and tee beams will be permitted provided the method and sequence is submitted to and approved by the Engineer. When draped strands are used, positive external hold downs may be required to offset the vertical forces in the beam at the time of stress release. (4) Combined Pretensioning and Post-tensioning. Where the plans call for a combination of pretensioning and post-tensioning, all of the requirements of both the pretensioning and post-tensioning shall apply Measurement. (1) All precast concrete members (except piling) either preten- sioned, post-tensioned, or combined pretensioned and post-tensioned, will be measured and paid for as specified in Item 425, "Prestressed Concrete Structural Members". Prestressed piling will be measured and paid for as specified in Item 409, "Prestressed Concrete Piling". (2) Cast-in-place prestressed concrete structures and units will be measured as follows:

9 (a) Grout and ducts for post-tensioning will not be measured but will be considered subsidiary to this Item. (b) The prestressing system required and the work involved in prestressing of cast-in-place structures will be measured by the product of the required final prestress force and the horizontal length over which the prestressing is applied, expressed in thousands of kip-feet (MKF). Unless otherwise shown on the plans, the required final prestress force used will be the maximum value required within a unit, and the length of prestressing will be taken as the overall dimensions of the unit, no deductions being made for the clearance distance between the ends of tendons and the ends of the unit. This is a plans quantity measurement Item and the quantity to be paid for will be that quantity shown in the proposal and on the "Estimate and Quantity" sheet of the contract plans, except as may be modified by Article 9.8. If no adjustment of quantities is required, additional measurements or calculations will not be required. The maximum percent variance from the plans quantity for which no adjustment will be made will be as follows: Percent Variance Plans Quantity Percent Over 5000 MKF 1/2% 1000 MKF through 5000 MKF 1% Less than 1000 MKF 1-1/2% Payment. The work performed and materials furnished in accordance with this Item and measured as provided under "Measurement" will be paid for at the unit price bid for "Prestressing (Grouted)" or "Prestressing (Ungrouted)". This price shall be full compensation for furnishing all prestressing steel; for fabrication, transportation, erection and post-tensioning; for furnishing all encasing ducts, grout fittings, grout, end anchorages, and bearing plates; and for all materials, labor, tools, equipment and incidentals necessary to complete the work.