for Highway Construction Page 358 of 71 5 SECTION 506 – PRESTRESSING CONCRETE
506.01 Descri ption.
Provide, place, and tension prestressing steel in precas t or cast -in-place concrete. Prestressing includes the providing and installation of appurtenant items necessary for the particular prestressing system to be used, including ducts , anchorage assemblies, and grout used for pressure grouting ducts. For cast -in-place prestressed concrete, the term “member” means the concrete that is to be prestressed. Perform prestress ing using pre- tensioning or post -tensioning methods, unless the plans show only pre- tensioning details. If the plans show only pre- tensioning details, the Contractor may use a post -tensioning system only if complete modification details are approved.
506.02 Materials.
Provide prestressing reinforcement that meets 708.05. The Engineer will reject members with failing strand or steel. Grout and Mortar .......................................................................................................................... 705
506.03 Construction Requirements.
A.General. Submit complete shop drawings detailing the method, materials, and equipment proposed for use in the prestressing operations, including additions or rearrangement of reinforcing steel or prestress rei nforcing strands. Outline the method and sequence of stressing and include complete specifications and details of the prestressing steel and anchoring devices, working stresses, anchoring stresses, types of ducts , and other data pertaining to the prestressing operation, including the proposed arrangement of the prestressing steel in the members, pressure grouting materials, and equipment. Submit elongation calculations based on the sequence of stressing. Do not cast members to be prestressed before shop detail drawings are approved. Submit as -built shop drawings. Precast, prestressed concrete manufacturing plants are required to be certified by the Prestressed Concrete Institute plant certification program, in category B2 (prestressed miscellaneous bridge products), B3 (prestressed straight strand bridge members), or B4 (prestressed draped strand bridge members), corresponding to the products specified. Submit proof of current certification to the E ngineer before beginning production. For the fabrication of precast and prestressed members, use the provisions in the current edition, at the time of bid opening, Manual for Quality Control for Plants and Production of P recast, Prestressed Concrete Products published by the Prestressed Concrete Institute with the following exceptions:
1.The contract specifications govern where there are conflicts.
2.If a wire in a pre- tensioning strand fails, the Engineer will reject.
3.If a wire in a post -tensioning tendon fails, the Engineer may reject. Retain a professional engineer, who is experienced in post -tensioning operations and registered in at least 1 state of the United States. The Contractor’s professional engineer is to be in charge of post -tensioning for Highway Construction Page 359 of 71 5 operations and be present during tensioning and grouting operations. Two weeks before the start of the post - tensioning operations, submit in w riting the name of the professional engineer, the state(s) in which the professional engineer is registered, including registration number(s), and the date and place of post - tensioning.
B.Sampling and Testing. Submit a calibr ation of the post -tensioning jacking system and provide the appropriate display settings for the Department’s electrohydraulic pressure cell system. Prepare a graph showing the gauge pressure in pounds per square inch, and force in thousands of poun ds plotted through the whole range of the post - tensioning calibration. The Engineer will require re- calibration of the jacking system at intervals not exceeding 3 months or when it appears the equipment is producing erratic results. For each calibration, s ubmit a new set of charts from the calibration. The specified regular interval for re- calibration of equipment may be extended at the Engineer’s discretion on conclusive evidence there is no substantial change in the performance. How ever, in no case will the intervals exceed 6 months. The approval of material does not prevent subsequent rejection if the material is damaged in transit or later damaged and found to be defective.
C.Shipment and Rust Protection. Protect prestressing steel against physical damage, rust, and other results of corrosion from manufacture to grouting or encasing in concrete. The Engineer will reject prestressing steel with sustained physical damage. The Engineer may reject prestressing steel, if visible rust or other results of corrosion develop. Package prestressing steel in containers or other shipping forms for the protection of the steel against physical damage and corrosion during shipping and storage. Place a corrosion inhibitor, that prevents rust or other results of corrosion in the package or form and apply directly to the steel. The Department requires the corrosion inhibitor to have no deleterious effect on the steel o r concrete or bond strength of steel to concrete. Immediately replace or restore damaged packaging or forms. The Contractor may elect to use a corrosion inhibitor carrier -type packaging mate rial meeting the United States Department of Defense specification MIL- PRF-3420H. Clearly mark the shipping package or form with a statement the package contains high- strength prestressing steel, the care to be used in handling, the type, kind, and quantit y of corrosion inhibitor used, including the date when placed, and safety orders and instructions for use. If directed, submit the following for the corrosion inhibitor:
1.A sample, a list of chemicals and their proportions, and instruct ions for use.
2.Evidence the prestressing steel will be protected from rust and other results of corrosion.
3.A certificate of materials as specified in 106.04. If steam curing is used, do not install prestressing steel for post -tensioning until the steam curing is completed. Where prestressing steel for post -tensioning is approved for installation in the ducts after completion of concrete curing, and if stressing and grouting are completed within 10 calendar days after the installation of the prestressing steel, the Engineer will not reject steel because of rust that may form during said 10 calendar days. The Engineer will not require the use of a corrosion inhibitor in the duct following installation of the prestressing steel if it was installed, tensioned, and grouted in this manner within 10 calendar days. for Highway Construction Page 360 of 71 5 Prestressing steel installed this way but not grouted within 10 calendar days, is subject to the requirements in this section pertaining to corrosion protection and rejection due to rust. When prestressing steel for pre- tensioning is placed in the stressing bed and is exposed to the elements for more than 36 hours before encasement in concrete , take adequate measures to protect steel from contamination or corrosion. Do not weld or ground the welding equipment on the forms or on the steel in the member after the prestressing steel has been installed. Clean and paint the exposed ends of pre- tensio ned members that will not be encased in concrete. Cover the surfaces with 2 thick applications of zinc -rich paint that meets MIL-P- 21035. Recess the anchoring devices so the ends of the prestressing steel and parts of the anchoring devices will be at least 2 inches inside the end surface of the members. Abrasive blast clean the surfaces of concrete against which concrete encasement over anchorage assemblies is to be placed. Clean any aggregate exposed after grouting the ducts. Fill the recesses with concrete and finish flush. Provide concrete that is the same as that specified for superstructure concrete.
D.Allowable Stresses and Losses . The Department requires the average working stress in the prestressing steel not to exceed 80 percent of the yield point stress of the prestressing steel and the maximum temporary tensile stress (jacking stress) in prestressing steel to not exceed 75 percent of th e specified minimum ultimate tensile strength of the prestressing steel. Anchor the prestressing steel at stresses (initial stress) that will result in the ultimate retention of working forces specified. The Department requires the initial stress at the anchor to not exceed 70 percent of the specified minimum ultimate tensile strength of the prestressing steel. The working force and working stress are defined as the force and stress remaining in the prestressing steel after all l osses have taken place, including creep and shrinkage of concrete, elastic compression of concrete, creep of steel, losses in post -tensioned prestressing steel due to sequence of stressing, friction and take- up of anchorage, and other losses peculiar to the method or system of prestressing have been provided for or have taken place. Ensure the stress loss in post -tensioned prestressing steel due to elastic shortening, shrinkage a nd creep of concrete , and relaxation of prestressing steel matches the plan details. Use the following formula and friction coefficients to calculate friction losses in tendons: T 0 = T x ( e (Ua + Kl) ) Where: T0 = steel stress at jacking en d Tx = steel stress a t point x e = 2.7183 (base of Naperian logarithms) U = friction curvature coefficient (see Table 506.03-1 ) a = total angular change of prestressing steel profile in radians from jacking end to point x K = friction wobble coefficient (see Table 506.03-1 ) l = length of prestressing steel from jacking end to point x for Highway Construction Page 361 of 71 5 Table 506.03-1 – Steel Coefficient Type of Steel Type of Duct K U Wire or strand Galvanized- rigid 0.0002 0.25 Plain bars Galvanized 0.0002 0.15 Deformed bars Galvanized 0.0003 0.30
E.Equipment. Tension prestressing steel using hydraulic jacks to meet specifications. The Contractor may equip each jack to stress tendons with a pressure gauge or a load cell for determining the jacking stress. Provide a pressure gauge with an accurately reading dial at least 6 inches in diameter. Provide a load cell with an indicator that can determine the prestressing force in the tendon. Ensure the indicator’s range is not in the lower 10 percent of the manufacturer’s capacity when determining the jacking stress.
F.Preparation and Stressing . For pre -tensioning, accurately hold in position the prestressing elements during stressing. The Contractor may cast in 1 continuous line several units and stress at 1 time. Do not transfer bond stress to the concrete until the concrete has attained the compressive strength specified. Cut and release the elements in an order that minimizes lateral eccentricity of stress. Before placing forms for closing slabs of box girder cells, the Engineer must verify that ducts are unobstructed. For post -tensioning, install the prestressing steel after the concrete has been placed. The Engineer must verify the ducts are without water and debris immediately before installation of the prestressing steel. During installation, ensure the prestressing steel is laid out in an orderly manner corresponding with its final positioning in the ducts, without overlaps and kinks. After installation and before post -tensioning a member, the Engineer must verify the prestressing steel is free and un- bonded in the duct. Do not prestress cast -in-place concrete until at least 10 calendar days after the last concrete has been placed in the member to be prestressed. Additionally, ensure the compressive strength of the last placed concrete has reached the strength specified for the concrete at the time of stres sing. Conduct the tensioning process so tension being applied and the elongation of the prestressing steel may be measured. Keep a record of gauge pressure and elongations and submit for approval. For prestressing tendons in continuous post -tensioned members, tension by jacking at each end of the tendon. The Contractor does not need to jack both ends simultaneously. Re-tension the unit before grout is placed if slippage of prestressing steel after tensioning is greater than 1/8 inch of the amount assumed in elongation calculations in a unit. for Highway Construction Page 362 of 715 For post -tensioning, do not stress more than ½ of the prestressing force in a girder be fore an equal force is stressed in the adjacent girders. During the stressing operations, do not apply more than 1/6 of the total prestressing force eccentrically about the centerline of the structure. Distribute the prestressing force with an approximately equal value in each girder and place symmetrically about the centerline of the structure. In slabs, uniformly distribute the prestressing force across the slab.
G.Ducts.
1.Size.
a.Provide ducts for tendons made up of multiple wires, bars, or strands that require an area of at least twice the net area of the prestressing steel.
b.Provide ducts for prestressing bars with a minimum inside diameter 3/8 inch larger than the diameter of the bars to be used.
2.Placement . Accurately place ducts as specified or approved and securely fasten at close enough intervals to avoid displacement during concreting. After installation in the forms , cover the ends of ducts as necessary to prevent the entry of water or debris.
3.Grout Openings. Vent ducts for continuous structures over each intermediate support. Use a ½ inch minimum diameter standard pipe and connect to ducts. Seal vents mortar -tight and tape as necessary. Remove the ends of vents 1 inch below the roadway surface after grouting has been completed. Install mechanical shutoff valves capable of withstanding grout pumping pressure at vents and the inlet and outlet of the ducts .
4.Forming. Use rigid galvanized ferrous metal ducts . The Contractor does not need to galvanize duct transition couplings connecting to anchoring devices. The Contractor may fabricate rigid ducts with welded or interlocked seams . Galvanizing the welded seam is not required. Provide rigid ducts that have sufficient strength to maintain their correct alignment dur ing placing of concrete . Provide joints between sections of rigid duct that are positive metallic connections that do not result in angle changes at the joints. Use waterproof tape at the connections.
H.Anchorage Devices. Secure post -tensioned prestressing steel at the ends by means of approved permanent type anchoring devices. Do not deviate from the equivalent anchor set more than plus 1/8 inch. Use anchorage devices for post -tensioning that hold the prestressing steel at a load producing a stress of at least 95 percent of the specified ultimate tensile strength of the prestressing steel. Distribute the load from the anchoring device to the concr ete by means of approved devices that will effectively distribute the load to the concrete. Use devices that meet AASHTO Standard Specification for Highway Bridges, Division I -Design Article 9.21 and Division II -Construction section 10.
I.Grouting.
1.Equipment. Provide grouting equipment capable of continuous mechanical mixing that will produce a grout without lumps and undispersed cement and be able to pump the mixed grout that complies for Highway Construction Page 363 of 715 with provisions of this recommended practice. Provide accessory equipment (e.g., scales, liquid measures) to accurately batch materials and a pump able to produce an outlet gauge pressure of at least 100 psi with seals adequate to prevent introduction of oil, air, or other foreign substances into the grout and to prevent loss of grout or water. Place a pressure gauge readable in 20 psi increments having a full -scale reading of not more than 300 psi at some point in the grout line between the pump outlet and the duct inlet. Provide the system with an effective manual or automatic control to limit the buildup of excessive pressure. Provide grouting equipment c ontaining a screen having clear openings of 0.07 inch maximum size to screen the grout before its introduction into the grout pump. Make the screen easily accessible for inspection and cleaning. Use gravity feed to the grouting equipment pump inlet. Provide grouting equipment capable of continuously grouting the largest tendon in 20 minutes or less under normal conditions. Provide standby water flushing equipment, separate from the grouting equipment, that uses a different power source than the grouting equipment, is capable of developing a gauge pressure of at least 250 psi, and has sufficient capacity to flush out partially grouted enclosures due to blockage, breakdown of grouting equipment, or inability to maintain one- way grout flow.
2.Preparation of Ducts. Clean ducts and ensure the ducts are without water and deleterious materials that would impair bon ding of the grout or interfere with grouting procedures. Use water for flushing ducts that contains quick lime (calcium oxide) or slaked lime (calcium hydroxide) at 1.5 pounds per gallon. Ensure compressed air used to blow out ducts is oil free.
3.Injection of the Grout . Fill the entire void space between the duct and the tendon with grout. Pump the grout through the ducts by pumping continuously from one end of the duct. Ensure valves are op en when pumping starts. After a continuous stream of grout emerges, close the valve at the first vent first. Close the remaining vents, in sequence, in the same manner. Pump grout through the duct and continuously waste at the outlet until no visible slugs or other evidence of water or air are ejected and the efflux time of ejected grout is at least 11 seconds. Close the outlet pipe, hold the p umping pressure momentarily, and then close the valve at the inlet while maintaining this pressure. Do not remove or open valves until the grout has set.
4.Cold Weather. Keep ducts free of water to avoid damage due to freezing. Concrete Temperature: Do not grout , unless the temperature of the concrete is 45°F or higher and maintained at this temperature, until job- cured 4 inch cubes reach a minimum compressive strength of 800 psi. Grout Temperature: Ensure grout temperature is less than 90°F during mixing or pumping.
J.Transportation and Storage . Transport precast, prestressed stringers in an upright position and ensure points of support and directions of the reactions with respect to the stringer are approximately the same during transportation and storage as when the stringer is in its final position. for Highway Construction Page 364 of 71 5 K. Camber . Check and record the vertical deflection (camber) of each girder. Girder camber is the measured variation between the girder center elevation and a straight line between the girder end elevations. Check and record the camber within 72 hours after transfer of the prestressing force, and within a 2- week period before shipment, but no less than 3 calendar days before shipment. Perform and record each camber check when the alignment of the girder is not influenced by temporary differences in surface temperature. Compare the actual camber measured with the required camber values and the permissible tolerances. The following are allowable tolerances from the design camber :
1.Plus or minus 1/2 inch for up to 80 feet in length.
2.Plus or minus 1 inch for over 80 feet in length. Meet the specified girder camber values in the contract documents within the allowable tolerances when girders are erected. Make provisions to control the camber growth. Control the vertical deflection of prestressed concrete girders by scheduli ng fabrication or other means. Use a method to control the vertical deflection that does not cause damage to the girders or over stress when checked in accordance with AASHTO specifications. Show the method of controlling the vertical deflection on the shop drawings . The Department may reject girders not meeting these minimum requirements as specified in 105.03. Make data from intermittent camber checks the Contractor performs available to the Engineer on request. Ensure the recorded measurements of the girder camber accompany the girder to the project site to document the girder complies with the specifications. Control girder deflection at no additional cost to the Department.
506.04 Method of Measurement.
The Engineer will measure acceptably completed prestressing of cast -in-place concrete by the lump sum.
506.05 Basis of Payment .
The Department will pay for accepted quantities at the contract unit prices as follows: Pay Item Pay Unit Prestressing Cast -in-Place Concrete ...................................................... LS Providing and placing additional deformed bar reinforcing steel required by the particular system used, ducts , anchoring devices, distribution plates or assemblies, incidental parts, for providing samples for tes ting, working drawings, and for pressure grouting ducts, is incidental and the cost included in the contract unit price. When there are no contract pay items for prestressing precast concr ete, prestressing concrete members is incidental and the costs included in the contract unit price for providing precast members as specified in 502. for Highway Construction Page 365 of 71 5 SECTION 507 – BRIDGE BEARINGS
507.01 Description.
Provide and place bearings including plain unreinforced elastomeric pads, reinforced elastomeric pads with steel laminates, or polytetrafluoroethylene (PTFE) pads with stainless steel mating surface that meet AASHTO Specifications for Highway Bridges at girder supports as specified. Provide bearings with the dimensions, material properties, elastomer grade, and type of laminates specified. Show the design load specified and testing requirements. If filled PTFE sheet is used, only glass -fiber filler will be approved.
507.02 Materials.
Provide bearings as specified in: Elastomeric Bearings ............................................................................................................... 720.02 Polytetrafluoroethylene (PTFE) Bearings ................................................................................ 720.03 Provide manufacturer certificates of compliance for materials used in t he bearings.
507.03 Construction Requirements.
Fabrication. Fabricate bearings as specified in 720.02. Testing. Test materials for elastomeric bearings and finished bearings as specified in 720.02. Installation. Install bearings as specified in 720.02.
507.04 Method of Measurement.
The Engineer will measure acceptably completed work by the each.
507.05 Basis of Payment .
The Department will pay for acceptable quantities as follows: Pay Item Pay Unit Elastomeric Bearings – Plain .................................................................. Each Elastomeric Bearings – Laminated ......................................................... Each PTFE Bearings ....................................................................................... Each
Source: Idaho Standard Specifications for Highway Construction, 2023 Edition. Pages 394–400 of 768.