520Post Tensioned Concrete Bridge Members

520.1 Description

This Work consists of manufac turing and erecting post-tensioned concrete Bridge members composed of Portland cement concrete. Post-tensioning is tensioning in which the Contractor installs the post tens ion reinforcement in voids or ducts in the concrete and stresses and anchors it against the concrete after the developm ent of the required concrete strength. As a final operation under this method, the Contract or shall pressure-grout the voids or ducts.

520.2 Materials

520.2.1 Portland Cement Concrete Members

520.2.1 1 Cast-in-place Members

The Contractor shall provide cast -in-place concrete members sub ject to post-tensioning in accordance with Section 511, “Concrete Structures.” Concret e shall be Class AA – Superstructure Concrete unless otherwise specified in the Contr act.

520.2.1 2 Precast Members

The Contractor shall provide prec ast concrete members subject t o post-tensioning in accordance with Section 517, “Pr ecast Concrete Structures.”

520.2.1 3 Prestressed Members

The Contractor shall provide pres tressed concrete members subje ct to post-tensioning in accordance with Section 518, “Pre-stressed Concrete Members.”

520.2.2 Post Tension Reinforcement

Post tension reinforcement consis ts of high strength steel wire , high strength seven-wire steel strands, or uncoated high strength steel bars, in accorda nce with the Contract. The Contractor shall provide post tension reinforcement that is cle an, and free of dirt, loose rust, or other Deleterious Material when placed in the member.

520.2.2 1 High-Strength Wire

The Contractor shall provide hi gh-strength steel wire in accord ance with AASHTO M 204.

520.2.2 2 High-Strength Multiple-Wire Steel Strand

The Contractor shall provide hi gh-strength multiple-wire steel strands in accordance with AASHTO M 203M, Supplement S1 (low -relaxation), unless otherwise specified.

520.2.2 3 Uncoated High-Strength Steel Bars

The Contractor shall provide uncoated high-strength steel bars in accordance with AASHTO M 275. If the Department allows bars with a greater min imum ultimate strength, the Contractor shall produce and test them in accordance with AASHT O M 275.

520.2.2 4 Identification of Post tension Reinforcement

The Contractor shall assign post tension reinforcement and anch orage assemblies with a lot number and tag for identification purposes. The Department will reject high-strength Materials lacking identification.

520.2.2 5 Sampling of Post tension Reinforcement

The Contractor shall submit load elongation curves and mechani cal properties representative of the strands or bars to the Project Manager. The Contractor shall submit certified test results in accordance with AASHTO 203.

520.2.2 6 Protection of Post tension Reinforcement

The Contractor shall protect pos t tension reinforcement against physical damage, rust or other results of corrosion, from manufacture to grouting. The Project Manager will reject post tension reinforcement that is physically damaged.

520.2.3 Ducts

The Contractor shall use duct enclosures for post tension reinf orcement that are mortar tight. Aluminum ducts will not be permitted.

520.2.4 Anchorage Devices

The Contractor shall secure post-tension reinforcement at the e nds by means of approved permanent anchoring devices. The Contractor shall des ign anchorage devices to hold the post tension reinforcem ent at a load producing a stres s of at least 95% of the post tension reinforcement’s guaranteed minimum tensile strength. T he Contractor shall distribute the load from the anchoring device to the concrete with approve d devices. The Contractor shall provide anchorage devices in accordance wi th the following requirements: 1. Do not allow the average bearing stresses on the concrete cr eated by the anchorage distribution plates to exceed the values specified in the current AASHTO LRFD Bridge Design Specification . 2. Do not allow bending stresses in the plates or assemblies in duced by the pull of the post tensioning to exceed the Mate rial yield point, or cause vi sible distortion in the anchorage plate when 100% of the ultimate load is applied. The Contractor may omit the steel distribution plates or assem blies if the compressive stresses to the concrete are effectively distributed with ancho ring devices in conjunction with a steel grillage embedded in the concrete.

520.2.5 Grout

The Contractor shall provide pre-packaged grout with minimum 28 -day compressive strength of 6000 psi. The Contractor shall use grout specifica lly manufactured for grouting of post-tensioning ducts, and approv ed by the Project Manager prio r to use.

520.3 Construction Requirements

520.3.1 General

The Contractor shall use members manufactured in an approved es tablished commercial precast facility, unless the Contract specifies that members ma y be cast-in-place. The Project Manager must approve the method of post-tensioning of concrete before incorporating it into the Work. The Contractor shall have a representative certified by the Pos t-Tensioning Institute (PTI) as a Level 1 Unbonded PT – Field Installer to provide direct ov ersite of the crew performing the post-tensioning Work. The C ontract will specify the requir ed jacking force, anchor set, and coefficient of initi al post tension.

520.3.1 1 Post-Tensioning Plan and Grouting Plan Submittal

The Contractor shall provide a post-tensioning plan that shall include complete details and calculations prepared by a licensed New Mexico Professional Engineer supporting the methods, sequence of stressing, Materials, and Equipment to use in the post tensioning operations. The Contractor shall submit this to the Project Ma nager at least 30 Days before incorporating it into the Work. The Contractor shall ensure the details completely describe the proposed post tensioning system, including the following: 1. Type and arrangement of post tension reinforcement; 2. Complete Specifications and details of post tension reinforc ement; 3. Anchorage devices; 4. Anticipated elongations and bas e calculations for the nomina l area; 5. Forms for cored holes and voids; 6. Ducts for the enclosure of post tension reinforcement; 7. Any additions or rearrangemen t of reinforcing steel from tha t shown in the Contract; 8. Sequence of post tensioning; 9. Certification of Equipment calibration; 10. For the operation of grouting post tension reinforcement - the Materials and proportions for grout, details of equipment for mixing and plac ing grout, and methods of mixing and placing grout (also, locations and detail s of inlets and outlets for grouting and the direction of grouting); and 11. Other data pertaining to the post tensioning operations.

520.3.1 2 Approval of Post-Tensioning Equipment

The Project Manager must approve Equipment for post-tensioning concrete members before the Contractor starts the Post-tensioned post tensioned concrete Work. The Project Manager may reject unsatisfactory machinery, Equipment, or appu rtenances. The Equipment shall be calibrated with a load cell and provided with an indicator to determine the force in the post tension reinforcement. The Con tractor shall use a load cell range that does not include the lower ten percent (10%) of the Fabricator's rated capacity to determine the jacking stress. The Contractor shall use an independent testing Laboratory appr oved by the Project Manager to calibrate the Equipment. The Contractor shall ensur e that the Equipment has been calibrated within a year of the Work. The Project Manager shall check certified calibration charts for the hydraulic jacks, pressure gages, or load cells used for tensioning post tension reinforcement before and during post tensioning operati ons.

520.3.1 3 Placement of Reinforcing Bars, Post tensioning Ducts, and Appurtenances

The Contractor shall place and secure reinforcing bars, ducts f or enclosing post tension reinforcement, forms for cored holes, and cellular spaces with methods approved by the Project Manager. The Contractor shall pay particular attention to concrete cover requirements over reinforcing bars near and at ends of beams.

520.3.2 Ducts

520.3.2 1 Formed Ducts

The Contractor shall provide duc ts that allow the entrances of grout. The Contractor shall use a duct that will transfer bond stresses as required and wil l retain its shape under the weight of the concrete. 520.3.2.2 Grout Opening or Vents The Contractor shall provide ducts with grout openings in locat ions to ensure air is not trapped in the ducts. The Contractor shall provide grout openi ngs or vents with provisions for preventing grout leakage.

520.3.2 3 Placement of Ducts

After placing the ducts and comp leting reinforcement and formin g, the Contractor shall inspect the Work for possible duct damage. The Contractor shal l securely fasten ducts at close intervals to avoid displac ement during concrete placement . The Contractor shall repair unintentional holes or openings in the duct before concrete pla cement as approved by the Project Manager. The Contractor shall securely anchor grout openings and vents t o the duct and to the forms or reinforcing steel to pre vent displacement during concr ete placement. After installation in the forms, the Contractor shall cover the ends of ducts as necessary to prevent water or debris from entering. The Contractor shall ensure the vents are mortar tight and provide means for injecting grout through the vents a nd sealing the vents. The Contractor shall remove the ends of vents two (2) inches below the Roadway surface after completing grouting.

520.3.3 Tensioning

The Contractor shall apply tension to post tension reinforcemen t with hydraulic jacks to produce forces in accordance wi th the approved post tensioning procedure (reference Section

520.3.1 1, “Post-Tensioning Plan and Grouting Plan Submittal”).

During strand stressing, the Project Manager may accept individ ual wire failures, if no more than one (1) wire in any strand is broken and the area of broken wires does not exceed two percent (2%) of the total area of the post tensioning steel in the member. The Contractor shall stress the members in a sequence that prod uces minimal eccentric forces.

520.3.3 1 Tensioning Equipment

The Contractor shall apply stress to tendons using hydraulic ja cks that can provide and sustain the necessary forces and have either a pressure gauge o r a load cell for determining jacking stresses. The Contractor shall use a pressure gauge with at least a six ( 6) inch diameter dial accompanied with a certified calibration chart. The Contractor shall calibrate each jack and its gauge as a unit with the cylinder extension in the approximate position that it will be at final jacking force.

520.3.3 2 Measurement of Stress

The Contractor shall provide a record of gauge pressures and te ndon elongations for each tendon, for review and approval by the Project Manager. T he Contractor shall measure elongations to an accuracy of within 1/16 inch. The Contractor shall not cut off stressing tails of post-tensioned tendons until the Project Manager approves th e stressing records. The Contractor shall use the gauge or load cell readings to det ermine the stress in the tendons during tensioning and verify with the measured elongati ons. The Contractor shall use the modul us of elasticity to calculate anticipated elongations and base calculations for the nom inal area (provided by the Fab ricator per lot of tensioned steel), or as determined by a bench test of strands used in the Work. Before starting elongation readings, the Contractor shall tensi on the tendons to a preliminary force to eliminate take-up in the tensioning system . The Contractor shall ensure this preliminary force is between five percent (5%) and 25% of the final jacking force. The Contractor shall measure the initial force with a dynamometer o r with other approved methods. The Contractor shall mark each strand before the final stressin g to permit measurement of elongation and to ensure that anchor wedges are set properly. The Contractor shall address a discrepancy in indicated stress between jack gauge pressure and elongation. If this occurs, the load used, as ind icated by the gauge pressure, will produce a slight overstress rather than understress. The Contr actor shall check the entire operation to determine the source of errors, if a discrepancy o ccurs between gauge pressure and elongation; more than five percent (5%) for tendons longer than 50 ft, or seven percent (7%) for tendons shorter than 50 ft. The Contractor shall corr ect errors before proceeding.

520.3.3 3 Tensioning Post-Tensioned Members

The Contractor shall not post tension cast-in-place concrete un til at least ten (10) Days after placing the last concrete in the member and until the com pressive strength of the concrete has reached the 75% of the specified strength. The Co ntractor shall use the methods described in Section 510.3.5.2, “In-Place Concrete Strength Measurements” to determine the actual in-place co mpressive strength of the concr ete. The Contractor shall perform the tensioning of post tension rei nforcement in the members so that the Project Manager may continuously observe the tensio n applied and the elongation of the post tension reinforcement. The Contractor shall apply tensi oning forces to post tension re inforcement in the sequence and amount approved by t he Project Manager to avoid ex cessive eccentric loads about the vertical axis of the member. The Department will not allow the maximum temporary stress (jac king stress) and the stress in the steel before losses due to creep and shrinkage to exceed the values in accordance with Section 5 of the AASHTO’s LRFD Bridge Design Specification .

520.3.4 Grouting Post tension Reinforcement in Post-Tensioned M embers

The Contractor shall bond the post tension reinforcement to the ducts 24 hours after completing tensioning and anchori ng of the post tension reinfor cement. If the end of a post-tensioned assembly will not be covered by concrete, the Contractor shall recess the anchoring devices so that the ends of the post tension reinforcement and the anchoring devices are at least tw o (2) inches inside of the end surface of the members, unless the Contract requires a greater embedment. Patch Material shal l be in accordance with Section 533, “Concrete Structure Repair.”

520.3.5 Grouting Keyways in Precast Members Subject to Post-Ten sioning

The Contractor shall use fibrous Material that retains grout bu t does not intrude into the keyway openings. The Contractor shall inspect keyw ay joints in precast box girde rs immediately before placing grout. The Contractor shall keep the keyways clear of Material, and keep interior surfaces clean and moist. The C ontractor shall place and conso lidate the non-shrink grout into the joint so that it fills the keyway to the deck surface. The Contractor shall strike off the grout at the deck surface. The Contractor shall cure in accordance with Section 511, “Conc rete Structures.”

520.4 Method of Measurement

The Department will measure concre te for cast-in-place post-ten sioned members in accordance with Section 511, “Concrete Structures,” or Section 512, “Superstructure Concrete.” The Department will measure reinforcing steel for c ast-in-place post-tensioned members in accordance with Section 540, “Steel Reinforcement.” The Department will measure quantities for precast members subj ect to post-tensioning in accordance with Section 517, “Pr ecast Concrete Structures” o r Section 518, “Pre-stressed Concrete Members.”

520.5 Basis of Payment

Pay Item P a y U n i t Post Tensioning Lump Sum

520.5.1 Work Included In Payment

The following items will be considered as included in the paym ent for the main item(s) will and not be measured or paid for separately: 1. Post tension reinforcement i n post-tensioned members; and 2. Ducts, anchorage devices, dist ribution plates or assemblies, grouting recesses and ducts, and all other incidental p arts and processes required fo r post-tensioned members. Section 521: Non-Shrink Grout Page 432