615.01 618 SECTION 615 – PRECAST/PRESTRESSED CONCRETE BRIDGE MEMBERS
615.01 Description ................................ ................................ ................... 618
615.02 Prestressing Methods ................................ ................................ ... 619
615.03 Materials ................................ ................................ ...................... 619
615.04 Equipment ................................ ................................ .................... 620
615.05 General ................................ ................................ ......................... 620
615.06 Forms ................................ ................................ ........................... 620
615.07 Stressing Requirements – General ................................ ............... 621
615.08 Pretensioning Procedure ................................ ............................... 622
615.09 Proportioning and Mixing of Concrete ................................ ........ 623
615.10 Handling, Placing, and Consolidating Concrete ........................... 625
615.11 Removing Forms, Finishing, and Curing ................................ ..... 626
615.12 Post -Tensioning Procedure ................................ .......................... 628
615.13 Combined Prestressing and Post -tensioning ................................ 629
615.14 Transfer of Stress ................................ ................................ ......... 629
615.15 Handling and Installation ................................ ............................. 630
615.16 Grouting ................................ ................................ ....................... 631
615.17 Tolerances ................................ ................................ .................... 631
615.18 Method of Measurement ................................ .............................. 632
615.19 Basis of Payment ................................ ................................ .......... 633
DESCRIPTION
615.01 Description
This work consists of manufacturing precast/prestressed structural concrete members and hauling, storing, and placing the precast/prestressed members on a prepared substructure. Fabricate these items in plants that have been certified by the Precast/Prestressed Concrete Institute (PCI). The fabricator shall be certified by PCI for the product that it will be manufacturing and supplying to the State or the Contractor. 615.02 619 615.02 Prestressing Methods Prestress the individual bridge members by pretensioning, post tensioning, or a combination of both methods. Plans for prestressed members will fully detail at least one of these methods. The Contractor may use a prestressing method other than that shown on the Plans if approved by the Engineer. Fabricate precast/prestressed concrete members in a PCI Category B3 certified plant. Have at ea ch fabrication site a technician skilled in the approved prestressing method. This technician shall give specialized aid and instruction in the use of the prestressing equipment and installation of materials as may be necessary to achieve the required res ults. The fabricator of precast/prestressed concrete members shall also have at each fabrication site a quality control technician capable of performing all necessary quality control inspection and testing to ensure that the precast/prestressed member is applicable for its intended use. The quality control technician shall have direct lines of communication to engineering, production, and management with responsibility only to management and shall not be subject to control by production (plant superintend ent). Quality Control personnel shall meet the training requirements as specified in the Departmental procedures. MATERIALS
615.03 Materials
Provide materials as specified in: Portland Cement ................................ ................................ .... 901.01 Fine Aggregate ................................ ................................ ....... 903.01 Coarse Aggregate ................................ ................................ ... 903.03 Joint Filler, Preformed Type ................................ .................. 905.01 Steel Bar Reinforcement ................................ ........................ 907.01 Prestressing Reinforcement Steel and Anchorages ................ 907.04 Structural Steel ................................ ................................ ....... 908.01 Elastomeric Bearing Pads ................................ ...................... 908.12 Paints ................................ ................................ .......................... 910 Cement Concrete C uring Materials ................................ ............ 913 Water ................................ ................................ ..................... 921.01 Air-Entraining Admixtures ................................ .................... 921.06 Chemical Admixtures and Additives ................................ ..... 921.06 Grout, Type 1 ................................ ................................ ......... 921.09 615.04 620 EQUIPMENT
615.04 Equipment
Provide equipme nt as specified in 604.04. Also provide a Type A field laboratory as specified in 106.06. CONSTRUCTION REQUIREMENTS
615.05 General
Construct the substructure for the precast/prestressed structural concrete members as shown on the Plans and as specified in these Specifications. Fabricate t he panels in a PCI Category B3 certified plant . Construct the precast/prestressed structural concrete members in an approved plant under plant control conditions and place them upon the substructure to the established lines and grades as shown on the Plans. Ensure that each plant has a quality control plan as specified in 604.03.B.
615.06 Forms
Unless othe rwise allowed, for external forms, provide metal forms that are mortar-tight and sufficiently rigid to prevent distortion due to pressure of the concrete and other stresses incident to the construction operations. The forms shall be substantial and unyiel ding, and designed, set, and maintained so that the finished concrete will conform to the proper dimensions and contours. Provide forms that are filleted at sharp corners and that have a bevel or draft in the case of projections to allow for easy removal. Wooden bulkheads may be used. Treat forms with oil immediately before placing the concrete. Do not use any material that will adhere to or discolor the concrete. Internal forms may be of cellular polystyrene, meeting the requirements specified in Table 615.06-1, or reinforced cardboard. 615.07 621 Table 615.06 -1: Cellular Polystyrene Property Value Density (ASTM D1622) 0.90 pounds per cubic foot Compressive Strength @ 10% Deformation (ASTM D1621) 10 psi minimum Absorption (ASTM C272) 3% (vol.) maximum Each individual form shall have a cross -sectional area, equal to the cross - sectional area of the beam void, and a minimum length of 5 feet. When individual forms are constructed by gluing pieces together, use an approved glue. The Engineer will reject fo rms that show signs of glued joint separations.
615.07 Stressing Requirements – General
Calibrate all jacks, together with their gauges before using in the manufacture of prestressed members. Wh en required by the Department, perform calibrations using proving rings or other acceptable methods performed by an approved testing laboratory at the Contractor’s expense. Have a calibration chart for each device readily available on site and furnish suc h charts to the Department. Recalibrate jacks and gauges at least once every 6 months. Provide means for measuring the elongation of reinforcement to at least the nearest 1/16 -inch. For all methods of tensioning, determine the force in the tendons by monitoring either the applied force or the elongation, and then independently checking by measuring the other. At the completion of tensioning operations, the two control measurements, force and elongation, shall agree within 5% of the computed theoretical v alues. If discrepancies exceed 5%, suspend the tensioning operation and use qualified personnel to determine and evaluate the source of error before proceeding. If the source of the discrepancy cannot be determined, recalibrate the devices. Additionally , ensure that the control measurements of force and elongation algebraically agree with each other within 5%. If the measurements do not agree within 5%, the Contractor may add a load cell at the “dead end,” and if force measurements agree within 5% between the gauge at the live end and the load cell at the dead end, the Department will waive the requirement for elongation agreement. When taking elongation and gauge readings, include appropriate allowances for chuck seating, bed shortening under load, abut ment rotation, thermal 615.08 622 effects, gauge correction based on calibration data, friction, and any other compensation for the setup. To measure the tensioning force, use one or more of the following methods:
1.Pressure gauges to measure force from the pressure applied to hydraulic jacks.
2.Dynamometers connected in tension into the tensioning system.
3.Load cells connected into the tensioning system, so the action of the tensioning operation imparts a compressive force to the sensing element.
4.Digital readouts connected to a pressure transducer to measure force from the pressure applied to the hydraulic jack.
5.Force computed from the actual elongation of the strand based on its physical properties and compensation adjustments. For pretensioned members, establish independent references adjacent to each anchorage to indicate any yielding or slippage that may occur between the time of initial stressing and final release of the cables. Stressing of strands for pretensioned members or tendons for po st-tensioned members may be from one end, unless otherwise indicated. However, if there is a discrepancy of more than 5% between stresses in the strand or tendon, as computed from the elongation measurement and those indicated by gauge readings, perform j acking from both ends.
615.08 Pretensioning Procedure
Apply to each cable the amount of stress shown on the Plans. Perform pretensioning by either the single strand or multi -strand jacking method. When prestressing i s performed by the multi -strand jacking method, bring the cables to a uniform initial tension of approximately 5,000 pounds before applying the full pretensioning stress. Measure the initial tension of each cable by a dynamometer or other approved means. 615.09 623 After the initial tensioning, stress the cables until the required elongation and jacking pressure are attained and reconciled within the limits specified in 615.07. Tension the deflected pretensioned strands by either partially jacking at the end of the bed followed by raising or lowering the strands to their final position, or entirely by jacking, with the strands being held in their final position during the jacking operation. Tension the deflected strands so that the final ten sion in all parts of the strand is uniform and provide means to reduce frictional forces at the bend points to a minimum. When strands are deflected after partial tensioning, raise or depress the strands simultaneously at all points or in an approved speci fic sequence. Obtain the Engineer’s approval of strand splicing methods and devices. When using single strand jacking, provide only one splice per strand. When using multi -strand jacking, either splice all strands or no more than 10% of the strands. Spl iced strands shall be similar in physical properties, from the same source, and shall have the same “twist” or lay. Locate all splices outside of the prestressed units. The Engineer may accept wire failures, provided not more than one wire in any strand i s broken and the area of broken wires does not exceed 2% of the total area of the strands. After final stressing, all strands shall be positioned as shown on the Plans, and the stress in the strands shall be uniformly distributed throughout the bed length. With the cables stressed in accordance with the Plan requirements and the foregoing Specifications, and with all other reinforcing in place, cast the members to the lengths specified. Maintain cable stress between anchorages until the concrete reaches th e compressive strength specified in 615.09.
615.09 Proportioning and Mixing of Concrete
Proportion Class P concrete as specified in Table 615.09 -1. 615.09 624 Table 615.09-1: Class P Concrete Class of Concrete Minimum 28-Day Compressive Strength (psi) Minimum Cement Content (pound per cubic yard) Maximum Water/Cement Ratio (pound/pound) Air Content % (Design ± production tolerance) Slump or Slump Flow (inches) P 5,000 (1) 658 0.45 0-8 (2) 2 + 1 (3) P-SCC(4) 5,000 (1) 658 0.45 0-6(2) 26 ± 5
1.Or as shown on the Plans or approved shop drawings.
2.Air entraining is optional with the Contractor, unless otherwise shown on the Plans or shop drawings.
3.Not to exceed 3 inches before the addition of high range admixtures, and not to exceed 10 inches after the addition of high range admixtures. If water -cement ratio is equal to or less than 0.35 then the maximum slump is 10 inches. If the water-cement ratio is 0 .36 – 0.45, the maximum slump is 8 inches.
4.Maximum coarse aggregate size of a No. 67 stone. Comply with all applicable provisions of 604.03 except as modified herein. Submit a concrete mix design template in accordance with Departmental procedures for review and approval. Do not use calcium chloride. Handle, measure, and batch materials; mix concrete; and comply with the limitations of mixing as specified in 604. Make concrete test specimens for Class P and Class P -SCC, in accordance with AASHTO T 23 and ASTM C1758 respectively, to determine the adequacy of the concrete design and the minimum time at which the stress may be applied to the concrete. Cure the test spec imens used to determine the time at which stress may be applied in the same manner and under the same conditions as the bridge members. The initial curing of specimens to determine the design strength of the concrete shall be as specified above with additional curing water, as provided in AASHTO T 23. The compressive strength of the concrete will be determined from the average strength of at least two representative test specimens made and cured as specified above and tested in accordance with AASHTO T 22 . The frequency of sampling and testing will be in accordance with Departmental procedures. 615.10 625 615.10 Handling, Placing, and Consolidating Concrete
A.Handling and Placement Handle and place concrete for prestressed bridge members as follows:
1.Before placi ng any concrete, clean the forms of construction debris and other extraneous matter. Place and secure the reinforcing bars, of the size and type specified, as shown on the Plans.
2.Place concrete in the forms immediately after mixing. Do not use any con crete that does not reach its final position in the forms within 30 minutes after adding the cement to the mix. Place the concrete to prevent segregation of the materials and displacement of the reinforcement.
3.Use metal or metal -lined open troughs and chutes and keep them clean and free from hardened concrete. Discharge water used in flushing clear of the forms.
4.Keep the temperature of forms, headers, cables, reinforcing bars, or other steel that comes in contact with freshly placed concrete to belo w 110 F during casting operation, except keep forms for deck panels below 90 F.
5.Do not begin to place concrete if the ambient temperature is below 26 F; and if the ambient temperature falls below 26 F during the placement of concrete, discontinue th e placement as soon as practicable.
6.Do not begin placing concrete during precipitating weather; and if precipitating weather occurs during the pouring operation, only leave the area where the concrete is being placed uncovered. Limit this uncovered are a to no more than 10 feet of bed length.
7.Do not deposit any concrete until the Engineer has inspected and approved the placement of the reinforcement, conduits, anchorages, and prestressing steel. Clean all exposed steel of all concrete, other than lig ht deposits of cement paste, immediately after placing and consolidation is completed. 615.11 626 B. Consolidation During and immediately after the placing operation, consolidate the concrete with vibrators and suitable spading tools. Apply vibration at the point of de posit and in the area of freshly deposited concrete. The vibrators used may be internal or external or a combination of both. Vibrate for a sufficient duration and intensity to compact the concrete thoroughly, but so as not to cause segregation. Place c oncrete in the precast units in one continuous operation for each unit, except for the minimum delay required for installing voids. Deposit and consolidate the concrete so that the concrete will be smooth and dense and free from honeycomb and pockets of segregated aggregates.
615.11 Removing Forms , Finishing, and Curing
A.Removing Forms The Contractor may remove side forms after 6 hours, where the removal of forms will not cause distortion of the hardened concrete. Do not remove the members from the bottom forms until they have been s tressed sufficiently to sustain all forces and bending moments that may be applied during handling.
B.Finishing Finish all formed surfaces of the bridge members as specified in 604.22 or other methods when approved by the Engineer . Finish roadway surfaces of the members as specified in 604.21.C. Transversely score the top surface of members that will not become a part of the roadway surface with a stiff wire brush, or by other approved methods. After removing hold down devices from the bottom of the beams, coat the resulting holes with an approved bonding compound and plug with mortar.
C.Curing Cure the bridge members by the water method or by steam curing as specified below. The Contractor may use oth er methods, provided the details of the proposed methods are submitted to the Engineer and approved. 615.11 627 1. Water Curing. As soon as the concrete has hardened to a degree that the finish will not be harmed, perform water curing as follows:
a.Cover the member wit h a pre-dampened material suitable for use with the water cure.
b.Keep continuously and thoroughly wet until the member has attained the strength required for stress transfer with a minimum curing time of 24 hours. Do not cure using the water method when t he ambient temperature is expected to drop below 45 °F.
2.Steam Curing. Perform steam curing as follows:
a.After placing and vibrating the concrete, wait until the member attains initial set, 2 to 4 hours, or 4 to 6 hours if a set retarder has been used, befo re applying steam. If the ambient air temperature is below 50 °F, apply enough steam during the delay period to hold the air surrounding the member at a minimum temperature of 50 °F and a maximum of 10 F greater than the temperature of the concrete at pl acement.
b.To prevent moisture loss on exposed surfaces during the delay period, cover members as soon as possible after casting or keep the exposed surfaces wet by fog spray or wet blankets.
c.Perform steam curing beneath suitable enclosures that will allow free circulation of steam about the sides, ends, and tops of members, and that will contain the live steam with a minimum moisture loss. The Contractor may use tarpaulins or similar flexible covers, provided they are kept in good repair and secured in a ma nner that will prevent the loss of significant steam and moisture.
d.Steam at the jets shall be low pressure and in a saturated condition. Steam jets shall not impinge directly on the concrete, test cylinders, or forms. During application of the steam, the temperature rise within the enclosure shall not exceed 50 F per hour. The curing temperature throughout the enclosure shall be not less than 90 F nor more than 160 F, and the relative humidity shall be not less than 95%. The difference in the 615.12 628 temperature adjacent to the concrete at different locations within the enclosure shall not exceed 20 F at any time. Continue steam curing until the concrete has reached the required transfer strength. In discontinuing the steam, do not reduce the temperature w ithin the enclosure more than 50 F per hour until the temperature inside is within 10 F of the temperature outside. The minimum time from the end of placement and finishing operations to the removal of required covers shall not be less than 12 hours.
e.When curing has been done at elevated temperatures, begin to transfer the prestressing load after releasing the forms and discontinuing the steam and while the concrete is still hot in order to prevent cooling shrinking and cracking. If directed by the Engineer, cover members or otherwise protect them to cool the concrete slowly after release in order to prevent thermal shock and the evaporation of moisture in the members.
f.Provide temperature recording devices that will provide an accurate continuous perman ent record of the curing temperature. Provide a minimum of one recording device per 200 feet of continuous bed length with a minimum of two devices per bed.
g.Leave side forms in place a minimum of 6 hours after the concrete is cast or until the concrete ha s set sufficiently to withstand damage when the forms are removed. When the side forms are removed during the cu ring cycle, only remove the minimum area of the curing enclosure that is necessary to remove each individual form section area in the enclosure . Immediately close the open area in the enclosure as each form section is removed.
h.When the temperature is not expected to rise above 32 F, protect the beams from freezing temperatures until the design strength is reached.
615.12 Post-Tensioning Proced ure
When post -tensioning, set the anchor plates normal in all directions to the axis of the steel. In all stressing operations, keep the stressing forces symmetrical about the vertical axis of the member. Do not begin tensioning 615.13 629 until the concrete has reache d a compressive strength of at least 3,500 pounds per square inch, unless otherwise shown on the Plans. The amount of tensioning to be retained in each post -tensioned unit after anchorage shall be as shown on the Plans. Apply a slight overstress, as determined in the field, to overcome friction between steel and enclosure and to allow for relaxation of the anchorage. Tension the units until the required elongations and jacking pressures are attained and reconciled within the limits spec ified in 615.07, with such overstress as approved by the Engineer for anchorage relaxation. Then allow the male anchorage element to be driven home by the jack action. Conduct the tensioning process so that the tension being app lied, and the elongation of the prestressing elements may be measured at all times. Determine the friction loss in the element, i.e., the difference between the tension at the jack and the minimum tension, in accordance with the current edition of AASHTO Standard Specification for Highway Bridges.
615.13 Combined Prestressing and Post -tensioning
If the members are manufactured with part of the reinforcement pre -tensioned and part post -tensioned, comply with the applicable portions of the requirements spe cified above for each part.
615.14 Transfer of Stress
Do not transfer the stress to the bridge members until the compressive strength of the test specimens is 4,000 psi or greater, unless otherwise shown on the Plans. Before transferring any stress to the bridge members, obtain the Engineer’s approval of the pattern and schedule for releasing the strands. Strip or loosen forms that tend to restrict the horizontal or vertical movement of the member prior to stress transfer. Transfer stress by either the multiple strand release method or the single strand release method. When the multiple strand method of release is used, gradually and simultaneously release either a symmetrical group of strands or all the strands. Remove the load on the strands from the anchorage and place on the 615.15 630 jacking system. Gradually release the jack or jacks until the strands are relaxed. When the single strand release method is used, heat strands with a low oxygen flame played along t he strand for a minimum of 3 inches until the metal gradually loses its strength and allows the strand to slowly pull itself apart. Apply heat at such a rate that failure of the first wire in each strand will not occur until at least 5 seconds after heat is first applied. Release the strands in the sequence of the approved pattern and schedule of release.
615.15 Handling and Installation
Prepare an erection plan in accordance with 602.42 and with the requirements of this Subse ction. The Contractor may handle all members immediately after completion of stressing. If the stressing is not done in a continuous operation, do not handle members before they are sufficiently stressed, as determined by the Engineer, to sustain all forc es and bending moments due to handling. When handling members, maintain them in an upright position at all times and pick them up from the points designated on the Plans. The Contractor may incorporate the members into the bridge structure at any time after completion of stressing and grouting, provided representative test specimens indicate that the concrete in the members has attained the design strength specified in 615.09, unless a minimum beam age is shown on the Plans. Do not ship any beams from the fabrication plant for incorporation into a structure until the Department has formally accepted the beams in accordance with Departmental procedures. Erect all prestressed beam members and establish the grade of the roadway on th e bridge before starting to form for the bridge deck, unless otherwise shown on the Plans or approved by the Engineer. If the Contractor wishes to start forming for the bridge deck before all beams have been erected , and the Plans do not preclude this prac tice, first obtain written approval from the Structures Division before constructing any forms . 615.16 631 615.16 Grouting Equip conduit, used for installing the bars or cables through post -tensioned members, with approve d grouting vents. After completion of stressing, grout the space between sides of bar or cable and sides of conduit as specified in 616.09. Clean recesses in girders at the ends of diaphragm bars, holes left by form ties, or all other surface irregularities, and patch with an approved epoxy grout.
615.17 Tolerances
Unless otherwise specified, standard sections shall meet the manufacturing tolerances specified in Table 615.17 -1. Any variation beyond these tolerances will be subject to the Engineer’s approval. 615.18 632 Table 615.17 -1: Manufacturing Tolerances in Standard Sections Description Tolerance I-Sections Box Sections Nominal Depth ± ½-inch ± ½-inch Nominal Width ± ½-inch ± ½-inch Nominal Length Computed Elastic Shortening ±1/2 -inch Computed Elastic Shortening ±1/2 -inch Variation in Straightness , inches ¼-inch x (Total Length in feet)/10 ¼-inch x (Total Length in feet)/10 Variation in Camber, inches Beams in any 1 span not more than : 1/8-inch x (Total Length in feet )/10 Beams in any 1 span not more than : 1/8-inch x (Total Length in feet )/10 Location of Voids --------- Length ± ½-in Wall Thickness ± ½-in Bearing Full Bearing – Full Width of Beam Full Bearing on at Least 2/3 of Width of Beam Tendon Placement ± ½-inch ± ½-inch Reinforcing Steel Placement ± ½-inch ± ½-inch Reinforcing Steel Concrete Cover ± ½-inch ± ½-inch Reinforcing Steel Splice Lengths Minus 1-1/2-inches Minus 1-1/2-inches COMPENSATION
615.18 Method of Measurement
The Department will measure structural members by the linear foot of precast/prestressed concrete structural members of the several types and sizes installed, complete in place. The Department will determine the linear measurement from the nominal lengths shown on the Plans. 615.19 633 615.19 Basis of Payment The Department will pay for accepted quantities at the contract prices as follows: Item Pay Unit Prestressed Concrete I -Beam (Type) Linear Feet Prestressed Concrete Bulb Tee Beam (Size) Linear Feet Precast Concrete Beam (Description) Linear Feet Prestressed Concrete Box Beam (Size) Linear Feet Prestressed Concrete Beam (Size, Description) Linear Feet Concrete Channel (Size) Linear Feet If the Engi neer allows concrete that does not meet the specified strength to remain in the permanent construction, the Department will adjust the bid price in accordance with 604.31. 616.01 634 SECTION 616 – POST-TENSIONED PRESTRESSED CONCRETE
616.01 Description ................................ ................................ ................... 634
616.02 Prestressing Methods ................................ ................................ ... 634
616.03 Materials ................................ ................................ ...................... 635
616.04 Reserved ................................ ................................ ....................... 635
616.05 Protection and Installation of Prestressing Steel .......................... 635
616.06 Anchorages and Distribution ................................ ........................ 637
616.07 Ducts ................................ ................................ ............................ 638
616.08 Prestressing ................................ ................................ .................. 639
616.09 Bonding and Grouting ................................ ................................ .. 641
616.10 Form Work ................................ ................................ ................... 644
616.11 Sampling and Testing ................................ ................................ ... 644
616.12 Method of Measurement ................................ .............................. 644
616.13 Basis of Payment ................................ ................................ .......... 644
DESCRIPTION
616.01 Description
This work consists of prestressing cast -in-place concrete by furnishing, placing, and tensioning prestressing steel in accordance with the Plans and these Specifications, and furnishing and installing all appurtenant items necessary for the particular prestressing system to be used, including ducts, anchorage assemb lies, prestressing steel, and grout.
616.02 Prestressing Methods
Perform prestressing using post -tensioning methods. Submit to the Engineer for review and approval complete details of the proposed method, materials, and equipment to use in the prestressin g operations, including any additions or rearrangement of reinforcing steel from that shown on the Plans. Such details shall outline the method and sequence of stressing and shall include complete specifications and details of the prestressing steel and a nchoring devices, working stresses, bursting stresses, anchoring stresses, type of ducts, and all other data pertaining to the
Source: Tennessee Standard Specifications for Road and Bridge Construction, 2021 Edition. Pages 627–642 of 1,072.