716 - POST-TENSIONING (Haunched Slab Bridges)
700-86 b. Qualification of the Post-Tensioning System Manufacturer (S ystem Manufacturer). Select a
system manufacturer with experience (in the United States) in post-tensioning concrete haunched slab or concrete box girder bridges that were designed and constructed according to AASHTO LRFD Construction specifications. Before materials are provided and any post-tensioning operations begin, the system manufacturer must be approved by the State Bridge Office (SBO). If the system manufacturer has not been previously approved, provide the SBO with the necessary information to consid er their qualifications. Provide the SBO with: Certificate of compliance with OSHA and other applicable industry standards for safety; In-house capability to design end anchorage assemblies, local zone and general zone design according to AASHTO specifications, sealed by a Professi onal Engineer licensed in the state of Kansas; Certificate of complian ce with AASHTO LRFD Construction specifications for testing of the end anchorage assemblies, performed by an independent testing laboratory, and sealed by a Professional Engineer licensed in the state of Kansas; In-house QC/QA implementation for manufacturing, assembling, storage, delivery, installation, stressing and grouting supervision; Names, qualification and experience of the field personnel to be assigned to assist the Contractor to supervise installation, stressing and grouting; The technician that supervises all grouting operations must be a valid American Segmental Bridge Institute (ASBI) Certified Grouting T echnician. Provide the SBO with verification of the technician’s ASBI Certification; Proof of continuous post-tensioning operations. Firms with less than 10 years of experience may be approved if sufficient related proj ect experience is demonstrated, bu t in any event a minimum of 7 years of experience is required; List of post-tensioned haunched slab bridges completed within the past 5 years including owner and identifying bridge information; and List any unfavorable claims within the last 10 years. The SBO will approve (or disappro ve) the system manufacturer within 5 working days of receiving the required information. A system manufacturer may submit the necessary inform ation to be considered for qualification at any time. The SBO will maintain a list of approved system manufacturers. Any change in a system manufacturer’s system or evidence of poor performa nce will require re-approval.
c.Shop Drawings. Submit shop drawings from the system manuf acturer to the State Bridge Office (SBO) for all work related to post-tensioning according to SECTION 105 . The shop drawings must be sealed by a Professional Engineer licensed in the state of Kansas. The shop drawings must be approved by the SBO before beginning fabrication. As a minimum, include in the shop drawings: A Post-Tensioning System that meets the re quirements in the Contract Documents; Tendon geometry and layouts; Distance from the bottom of slab to bottom of duct; Duct support detail and spacing accord ing to the Contract Documents; The locations of grout ports and grout vents; Connection details such as duct coupler, anchorage to duct and grout ports/ vents to duct; Anchorage local-zone reinforcement; Permanent grout cap details, concrete reces s, pour backs and temporary protection; Jacking forces and initial forces; Anchor set; Stressing operation and equipment data; All material specifications (e.g. strands, ducts and grout); Grouting operation and equipment data; Safety procedures; Elongation calculations and tolerances; All required computations; 716 - POST-TENSIONING (Haunched Slab Bridges)
700-87 If duct sizes different from the sizes shown in the Contract Documents are ap proved, modify the spacer
frame details shown in the Contract Documents; and Computations and a typical tendon force diagram (for all types of tendons), after friction and anchor set losses, based on an expected actual fric tion coefficient for the system to be used.
d.Installation. Install the hardware including ducts, te ndons, end anchorage assemblies and special reinforcing according to the Contract Documents and the instructions of the system manufacturer. Provide a qualified on-site representative of the system manufacturer, who is skilled and thoroughly experienced in the use of the system to supervise or provide appropriate guidance of the work. The system manufacturer’s representative will provide the Engineer pertinent information as required. The system manufacturer’s representative must be available full-time during post-tensioning hardware installation for inspecting and approving all installation prior to concrete placement, stressing, anchoring all tendons and grouting operations. As a minimum, the following items require inspection and approval by the system manufacturer’s representative: Installation of all hardware; Instructions to the Contractor regarding concrete placement around the ducts, end-anchorage assemblies and other appurtenances; and Supervision of stressing procedures, record keeping, certification of stressing results and grouting operations The representative of the system manufacturer that supervises all grouting operations must be a valid American Segmental Bridge Institute (ASBI) Certified Grouting Technician. Do not place any concrete in the bridge abutments an d superstructure until the hardware installation is approved by the Engineer and the syst em manufacturer’s representative. Reject all unidentified strands or anchorage assemblies. Provide all ducts or anchorage assemblies with inlet/outlet pipes. Provide concrete test cylinders at both abutments. Do not begin stressing until testing of concrete cylinders verifies minimum bridge concrete strength for jacking has been obtained. Do not begin the stressing before the concrete strength has reached the f' ci shown in the Contract Documents and a minimum of 72 hours after completing the slab pour. Complete the stressing within 7 days after completing the slab pour. Vibrate the concrete slab, as re quired, to obtain proper consolidation and compaction of the concrete specified in the Contract Documents. Proper vibration at the abutments and around the end an chorage assemblies is especially critical and should be considered a “confined” area. Exer cise care to obtain concrete consolid ation around the end anchorages without disturbing the reinforcing or post-tensioning assemblies.
e.Stressing Tendons.
1.Stresses. Tension all strands using hydraulic jacks. The minimum force of the strands is the value shown on the approved shop drawings. Do not allow the maximum temporary tensile stress (jacking stress) in the strands to exceed 80% of the specified minimum ultimate tensile strength. Anchor the strands at stresses (initial stresses) that shall result in the ultimate retention of permanent forces of not less than those shown in the Contract Documents. After seating, do not excee d 70% of the specified minimum ultimate tensile strength at the anchorages locations and 74% of the specified minimum ultimate tensile strength in the span. Consider permanent force and permanent stress as the force and stress remaining in the strands after all losses, including creep, shrinkage, elastic shortening of concrete, relaxation of steel, post-tensioning losses due to the sequence of stressing, friction, take-up of anchorages and any other losses due to the method or system of post- tensioning. Complete stressing of the strands to within -0 to +5% of the forces shown in the Contract Documents.
2.Stressing Jacks. Apply post -tensioning forces only after the co ncrete has attained the specified compressive strength as determined by the cylinder tests and within the time requirements in subsection 716.3d . Equip each jack used to stress tendons with a pressure gauge (a minimum of 6 inches in diameter) for determining the jacking pressure. Calibrate each jack and its pressure gauge as a unit with the cylinder extension in the approximate position that it will be at final jacking force. As a minimum, provide 2 ja cks at each site to guard against breakdowns. Provide certified calibration charts (b y an independent laboratory) with each jack, hydraulic system and pressure gauge used on the project. Perf orm the calibration while the jack is in the identical configuration as will be used on the site (e.g., the same length hydraulic lines). 716 - POST-TENSIONING (Haunched Slab Bridges)
700-88 Provide a calibrated master gauge at each job site. Supply the master gauge in a protective waterproof
container capable of protecting the calib ration of the master gauge during shipment to a laboratory. Provide a quick- attach coupler on the calibrated master gauge to verify the permanent gauge readings. Calibrate and provide the Engineer with the master gauge for the duration of the project. Any repair of the jacks, such as replacing the seals or changing the length of the hydraulic lines, will be cause fo r re-calibration of the jack with a load cell. Conduct hydraulic jack calibration a minimum of every 6 months.
3.Elongations. Conduct the tensioning process so that the tension being applied and the resulting elongation of the strands may be measured at all times . Keep a permanent record of gauge pressures and elongations, and submit it to the Engineer. Using only a rigid metal ruler, measure elongations to the nearest ⅛ inch. Preload tendons to 20% of their total jacking for ce. Inspect dead end anchors for adequacy before completing the post-tensioning of the tendon. If a tendon’s measurable elongation is greater by more than 7% of the calculated measurable elongation, the tendon will be evaluated by the Engineer in conjunction with the State Bridge Office (SBO) and subject to rejection. If a tendon’s measurable elongation is less than the calculated measurable elongation by more than 7%, overstress the tendon to 80% of its ultimate strength from eith er end. If this yields an elongation within 7% of the calculated measurable elongation, the tendon will be accepted; otherwise it will be evaluated and subject to rejection. Evaluation of out of tolerance elongations procedure: It is of primary importance that the evaluation be performed as soon as the potential failure happens, since completion of the entire process is time critical. In the event of a failure to meet the 7% requirements above, a tendon evaluation is required, and will consist of one or all of the following procedures as determined by the Engineer in conjunction with the SBO: Modification of the friction and wo bble coefficient: Perform in-place friction tests or modify the K- factor by using the data from the tendon in question. Verification of the tendon modulus of el asticity: Perform additional bench tests. Re-calibration of the stressing jack: Verify elongation through jack or extension if used. Perform lift-off at dead end. Submit to the Engineer and SBO for approval, a propos al of a tendon evaluation sealed by a Professional Engineer licensed in the State of Kansas, representing the system manufacturer. Submit to the Engineer and SBO the results of the tendon evaluation. Should the evaluation fail to justify the discrepa ncy between the actual and calculated measurable elongation, the tendons will be rejected.
4.Record of the Post-Tensioning Operation. Keep a record of the following post-tensioning operations for each tendon installed: Date strands installed; Date strands stressed; Date grouted; Supervisor’s and Inspector’s names; Coil number for strands installed or heat number for bars installed; Jacking ends; The theoretical and actual elongation; The theoretical and actual anchor set; Actual tendon lengths; Gauge pressure and forces (initial, intermed iate intervals, and before anchor set); Accepted or number of items rejected; and Comments as to problems. Upon completion of the post-tensioning, submit to the Engineer for approval a record of gauge pressures and tendon elongations. The records submitted must be seal ed by a Professional Engineer (licensed in the state of Kansas) representing the system manufacturer. Do not cut off stressing tails of tendons until the stressing records have been approved by the Engineer. Do not torch cut strands or bars at any time. 716 - POST-TENSIONING (Haunched Slab Bridges)
700-89 f. Grout. Provide the Engineer with a minimum of 3 days advance notice of the beginning of the grouting
operations to allow adequate time for ASBI-certifie d KDOT inspection staff to get to the job site. Grout the annular space between the duct and the te ndons after the tensioning of all tendons has been completed and the strands have been anchored. Weather permitting, begin grouting immediately or within 72 hours after stressing all tendons within an independent unit.
1.Equipment. Provide a grouting mixer capable of continuous mechanical mixing which shall produce grout free of lumps and undispersed cement. Batch all ma terials using batchi ng equipment which provides accurate solid and liquid measures. Provide grout equipment capable of continuously grouting the longest tendon on the project within a maximum of 20 minutes. Use grouting equipment utilizing gravity feed to the pump inlet from a hopper attached to and directly over it. Keep the grout hopper at least partially full of grout at all times during the pumping operation to prevent air from being drawn into the post-tensioning duct. Use positive displacement type pump s, capable of producing the outle t pressure required by the grout manufacturer. Use a pump with seals, adequate to prevent introduction of oil, air or other foreign substance into the grout, and to prevent loss of grout or water. Place a pressure gauge at some point in the grouting line between the pumping outlet and the duct inlet, and having a full scale reading of a maximum of 300 psi. Use grouting equipment containing a screen having clear openings of ⅛ inch maximum size to screen the grout prior to its introduction into the grout pump.
2.Mixing. Mix the grout accordin g to the manufacturer’s directions. The pumpability of the grout may be determined by the Engineer accordin g to ASTM C 939. When this method is used, do not exceed 11 seconds for the efflux time of the grout sample immediately after mixing. Do not use the flow cone to test grout which incorporates a thixotropic additive.
3.Grout Operations. Keep all grout inlets and high point outlets open when the grouting operation begins. Allow grout to flow from the first outlet past the inlet unt il any residual flushing water or entrapped air has been removed, at which time close the outlet. Close remaining outlets in the same manner and in the order shown on the plans. Perform normal grout pumping operations at 75 psi. Do not exceed 250 psi for pumping pressure at the inlet. Pump the grout through the duct and continuously waste at the outlets until there is no evidence of water or air being ejected. If the actual grouting pressure exceeds the maximum recommended pumping pressure , inject grout at any outlet that has been, or is ready to be closed as long as a one-way flow of grout is maintained. If this procedure is used, fit the outlet that is to be used for injection with a positive shutoff. When one-way flow of grout can not be maintained as outlined above, flush the grout immediately out of the duct with water. To keep the tendon filled with grout under pressure, close the outlets and inlets in the order shown on the plans, when the tendon duct section at the outlet and inlet is completely filled with grout. Do not remove or open positive shutoffs required at the inlets and outlets until the grout has set. In temperatures below 32°F, keep ducts free of water to avoid damage due to freezing. Maintain the temperature of the concrete slab between 35 and 85°F fr om the time grouting begins until jobsite cured 4 inch diameter cylinders of grout reach a minimum compressive st rength of 800 psi, when te sted according to AASHTO T22. The maximum grout temperature is 90°F, during mixing or pumping. When required, cool the mixing water. Remove ends of inlets and outlets a minimum of 1 ½ inches below the concrete surface after the grout has set and fill the recess in the concre te with an approved epoxy grout. Grout all anchorages, before the winter shut down.
g.Protection of End Anchorages. Clean exposed end anchorages, st rands and other metal accessories of rust, misplaced mortar, grout and othe r such materials as soon as possibl e after tensioning and grouting is completed. Immediately following the cleaning operation, apply a coat of zinc-rich epoxy paint, minimum thickness of 4 mils.
h.Recess Pocket Filling. Fill all longitudinal and transverse end an chorage recess pockets as shown in the Contract Documents. Apply an approved epoxy resin bonding agent according to the manufacturer’ prior to placing an approved non-shrink, non-metallic grou t. Apply grout according to the grout manufacturer’ s instructions. Finish the outside exposed surfaces of the recess pockets smooth an d flush with the surrounding concrete surface. Select grout to match the color of the surrounding concrete slab.
Source: Kansas Standard Specifications for State Road and Bridge Construction, 2015 Edition. Pages 417–420 of 978.