2405Prestressed Concrete Beams

2405.1DESCRIPTION

This Work consists of providing and installing prestressed concrete beams and double Tee -beams for use in Bridge Superstructures.

2405.2MATERIALS

Provide concrete produced in a central-mix plant in accordance with 2462, “Precast Concrete.” The Contractor may use Type I, Type II, or Type III Portland cement. Use Mix No. 1W82 or Mix No. 3W82 concrete for prestressed beams. Use Mix No. 3W82 concrete for double Tee-beams.

2405.3CONSTRUCTION REQUIREMENTS

Provide beams manufactured from a precast/prestressed concrete plant certified by the PCI or by another organization approved by the Materials Engineer and in accordance with the following:

A General The Materials Engineer is the Engineer with authority concerning all matters of plant fabrication and inspection prior to delivery of the Materials to the Project. The Engineer has authority concerning all matters of fabrication at the Project Site. Submit a written notification to the Engineer immediately after placing orders for prestressed concrete beams. Include the name and address of the supplier and the beam manufacturing location. Notify the Materials Engineer at least 7 Calendar Days before beginning manufacturing operations. If the Contractor casts the beams at the Bridge site, notify the Engineer at least 7 Calendar Days before casting to permit inspection of the forms and reinforcement. If casting the beams at a plant away from the Bridge site, provide an office in accordance with Table 2031.3-1, Field Office with air conditioning and access to sanitary facilities. The Department will not require laboratory space. If, on any day, the Department inspects beam casting at a plant away from the B ridge site and less than two beams are cast, the Department will deduct from any monies due or becoming due to the Contractor the total cost of inspection for that day. Ensure the fabricator's Quality Control office maintains documentation containing the data required by the Specifications and the Materials Engineer. This documentation shall contain test data and measurements taken at times and locations approved by the Engineer, assuring that monitoring, by personnel not directly involved in production, is sufficient to ensure compliance with approved procedures. If the Materials Engineer's review of fabrication Work discloses that approved procedures are not being followed, the Fabricator shall immediately correct the procedure. The Materials Engineer will determine what additional testing Work must be done by the fabricator or, if necessary, what part of the Work must be repaired or replaced if fabrication Work is not properly monitored and documented by the fabricator.

Any and all costs of required additional monitoring and testing shall be at the expense of the Contractor with no additional compensation. Provide a PCI Level II Certified Technician on site at the start of fabrication and throughout fabrication of the prestressed beams . Provide PCI Level I Certified Technicians to perform Quality Control functions. Provide a PCI Level II Certified Technician as a supervisor for the Quality Control staff. Take precautions to prevent contamination of prestressing steel with oil, dirt, or other deleterious substances and to prevent damage that may result in weakening the prestressing steel that may result in its failure under stress. The Materials Engineer may reject nicked or kinked prestressing steel. Do not allow sparks or pieces of molten metal from welding or burning E quipment to contact the prestressing steel. Do not use prestressing steel as a ground for welding Equipment. Galvanize all steel inserts or devices that will be within 1 inch of the exposed surface of the finished Structure. Galvanize or coat with zinc-rich primer all other steel inserts or devices included in the beam. B Forms Provide forms designed to withstand pressure from concrete, vibration, and impact without distortion. Set and maintain forms mortar tight, free of warp, and on a rigid foundation. Set the side form at right angles to the vertical axis of the beam and with the plane of bearing surfaces flat and true . Set side forms during casting as shown on the Plans. Maintain side forms during casting until the concrete sets. Provide a tight fit without offset for joints in sectional forms. Set forms for prestressed concrete beams so the dimensions of the beam after prestress transfer will conform to the Plan dimensions in accordance with 2405.3H, “Tolerances.” Treat the face of the forms in contact with the concrete with form coating Material in accordance with 3902, “Form Coating Material,” before setting the form in position. Clean the forms of accumulations of oil and other substances before use. Provide beam end blocks as shown on the Plans. The Contractor may increase the length of the end blocks as much as 12 inches to accommodate sectional forms. Provide end blocks of the same length for beams of the same length in any one span. C Steel Units Place, support, and tie reinforcement bars for prestressed concrete beams in accordance with 2472, “Metal Reinforcement.” Cover the reinforcement bars with concrete at least 1 inch thick. Set sole plates for prestressed beams so that after prestress transfer the sole plate locations match the Plan locations within the tolerance specified for the Plan length of pretensioned beams. Place the sole plates in contact with the bottom form. Maintain position of the sole plates during placement of concrete. Position floor drains as shown on the Plans. Fasten the floor drains to the forms to ensure that concrete placement does not alter the alignment or location. Remove loose rust, dirt, oil, and other foreign substances from prestressing tendons before erecting the beam side forms.

The Contractor may construct hold-down devices for deflected strands so that the Contractor can remove the device for a distance of at least 1 inch from the face of the concrete and plug the hole with mortar. Use free-rolling devices (hold-down and hold-up) at all deflection points. The device may rest on the bottom form and remain in-place. If resting the device on the bottom form, galvanize the part in contact with the form for a distance of at least 1 inch from the form. D Placement of Concrete Cast the beams in an upright position. Place the concrete in each beam without interruption. Modify the casting procedure if the length of the beams and placement conditions cause a cold joint to form when continuing each Lift full length before placing a subsequent Lift. Vibrate the concrete in each beam internally, externally, or both to produce uniformly dense concrete. Do not displace enclosures or steel units when vibrating. Internally vibrate in accordance with 2401.3D, “Compaction of Concrete,” using a vibrator with a non-metallic vibrating head no greater than 1 1/4 inches in diameter operating at a frequency of at least 100 Hz. After striking off the top surface of the beams to the required level, Work and hand float the surface to seal open tears in the surface and depress coarse Aggregate. Use transverse brooming to roughen the surface. Provide smooth finish on edges of top flanges as shown in the P lans. Apply a bond breaker shown on the APL to the smooth area after curing. E Concrete Curing Cure according to the requirements of 3240, “Precast Concrete Manufacturing.” F Tensioning F.1 Equipment Tension prestressing tendons using hydraulic jacks or dynamometers and hydraulic jacks. Equip each jack pump with a hydraulic pressure gauge. Calibrate jacks, gauges, and pumps as a unit under conditions similar to operating conditions . Provide a dated, certified calibration curve for each combination used. Recalibrate Equipment presenting erratic results during tensioning operations. Ensure the hydraulic pressure gauges can accurately determine the actual stress on the jacks within a tolerance of 2 percent of the total indicated stress during final elongation of the prestressing tendons. Calibrate the dynamometer used to measure an initial tension. Ensure the dynamometer can accurately determine the initial tension within a tolerance of 5 percent. F.2 General Procedures Conduct the tensioning procedure so that it is possible to compare the indicated force in the strands based on gauge pressures and the indicated force based on the corresponding elongation of the strands at any time during the tensioning operation. If the two indicated forces, corrected for friction loss, differ by no greater than 5 percent, stress the strands so the lower of the two indicated forces equals the required tension in the strand. Do not tension any strands to an indicated force greater than 80 percent of its specified minimum breaking strength at any time. If the indicated forces differ by greater than 5 percent, stop tensioning operations. Determine the source of the discrepancy and correct it before resuming tensioning operations. When the tensioning operation includes more than two girders with all deflection points included, demonstrate proper tension at both ends. When tensioning more than four girders with all deflection points included, measure and ensure proper elongation on t he interior girders that are more than one girder from an end.

Do not tension prestressing strands in a bundled position with direct contact between adjacent strands. Maintain a clear space of at least 1/4 inch between adjacent strands during tensioning. Depress tensioned strands into a bundled position after the completion of tensioning. Record the gauge pressures, indicated forces, and elongations, and submit the record to the Materials Engineer. The Contractor may tension strands as a group if the strands in the group are from the same manufacturer and the strands receive the same initial tension. When tensioning, consider initial strand tension no greater than 150 pounds per strand to be zero tension. If the contract requires an initial tension greater than 150 pounds per strand, use a dynamometer to measure the tension. Add the elongation due to the initial tension to the final elongation measurement. Tension the deflected strands so that final tension is uniform in all parts of the strand. Provide freely turning rollers to reduce frictional forces at the deflection points. Correct tension and elongation measurements for losses due to slippage of grips or anchorages, and friction to ensure the prestress force shown on the plans is met. When tensioning strands, make temperature corrections using the temperature of the strand at stressing and 95°F for the temperature when concrete bonds to the strand for the tensioning correction. If a temperature differential in the strands at the time of tensioning and 95°F exceeds 14°F, consider the change in the final elongation measurements to obtain the required prestress force at the time of casting. Base the change in elongation due to temperature on 1/8 inch per 100 feet of strand length for each 15°F variation in temperature. Tension prestressing strands when the ambient air temperature and prestressing strand temperature are greater than 32°F and will not fall below 32°F at any point. G Prestress Transfer Perform the prestress transfer after the control cylinders indicate that the concrete has reached a compressive strength of at least 4,500 psi unless otherwise shown on the Plans. When breaking the controls cylinders, use a testing machine that prints all test data, including time and date, directly to a printer, and initial the printout. Make the prestress transfer when the concrete is still warm and moist. During the prestress transfer sequence keep the lateral eccentricity of the prestress to a minimum and prevent cracking in the top flange of the beams. The Contractor may perform the prestress transfer by the gradual release of hydraulic jacks, by heating exposed portions of individual strands to failure, or by a combination of these two methods. If heating the individual strands, heat in accordance with the following:

along the casting bed. Use a sequence of heating each strand along the bed and a sequence of heating individual strands so that no detrimental damage results

Control the heat to ensure that failure of the first wire in the strand does not occur for at least 5 seconds after the application of heat followed by gradual elongation and failure of the remaining wires Completely strip the forms from the beams before beginning prestress transfer.

H Tolerances After prestress transfer, check the dimensions of the prestressed concrete beam to verify that the dimensions match what the Plans show within the tolerances in the PCI, Manual for Quality Control for Plants and Production of Structural Precast Concrete Products, MNL-116. Inform the plant Inspector of any measurements not within the tolerance limits. The Engineer will measure differential camber between adjacent members of the same design with the beams erected in the final position. The Engineer or the Materials Engineer may reject members that do not meet the dimensions shown on the Plans or the specified tolerances. I Rejection The Materials Engineer may reject beams failing to meet the requirements of this section or beams showing any of the following after the removal of forms:

The Materials Engineer will not reject beams showing minor surface cavities or irregularities that do not impair the service value if repaired as approved by the Materials Engineer . Make repairs after the Materials Engineer inspects the irregularities. Use repair Materials and methods approved by the Materials Engineer. J Marking, Handling, Storage, and Transportation The Department will allow beams to be moved from the casting bed to a nearby storage area after attaining the minimum compressive strength for prestress transfer . Do not transport or install beams until the beam attains a compressive strength of at least 5,000 psi, as evidenced by control cylinders. When casting off-site, do not ship the beam until it has reached the full design strength shown in the Plans. Mark each beam with the casting date and piece mark. If casting beams away from the Bridge site, mark each beam with the name or trademark of the manufacturer and the B ridge number. The Materials Engineer will stamp each approved beam with the official mark of the Department before shipment. Complete repairs before the Department stamps the unit. Do not ship beams without the Department stamp of approval. After completing the repair Work, notify the Materials Engineer at least 1 full Business Day before intent to ship. The Engineer will perform a final inspection of units upon delivery. Ensure markings remain in evidence after erection, but not readily visible in the completed Structure. Keep prestressed beams in an upright position at all times. Support prestressed beams during storage, lifting, and transportation at two points only. Considering beam stresses and stability, determine the support point locations in accordance with standard PCI methods. Obtain permits as required by Road authorities. K Installation Erect prestressed concrete beams in a manner that will provide safety to the workers, Inspectors, and the public always, as well as reasonable assurance against damage to the prestressed members. Prior to the placement of diaphragms, temporarily anchor, brace, and stabilize the prestressed beams as they are erected to preclude sliding, tipping, buckling, or other movement that may otherwise occur.

Submit an erection Plan if active vehicular or railroad traffic is permitted to travel beneath beams prior to complete erection of all the beams and diaphragms in a span. Address comments made by the Engineer prior to performing any work. Ensure the plans and specifications are prepared by an engineer, thoroughly checked by a second engineer for completeness and accuracy and certified by one of the aforementioned professional engineers licensed in the State of Minnesota. Include sufficient details so that construction of the proposed system can be completed solely by reference to the plans and specifications. Show the design criteria on the first sheet of the Plans. Ensure struts, bracing, tie cables, and other devices used for temporary restraint are of a size and strength that will ensure their adequacy. Arrange the Work schedule so that each beam will be connected to an adjacent beam. Before operations are suspended for the day, ensure at least two adjacent beams are erected (including diaphragms and bolts fully tightened), braced, and stabilized in any one open span. Install and permanently fasten the prestressed concrete beams as shown on the Plans. Provide intermediate diaphragms for prestressed concrete beams as shown on the Plans. Provide structural steel shapes and plates for the steel intermediate diaphragm option in accordance with 3306, “Low-Carbon Structural Steel,” or 3309, “High-Strength Low-Alloy Structural Steel.” Galvanize structural steel plates and shapes in accordance with 3394, “Galvanized Structural Shapes.” Provide fasteners, including washers, for the intermediate steel option in accordance with 3391, “Fasteners.” Provide fasteners meeting the requirements of ASTM F3125 Grade A325, Type 1. When used with galvanized structural steel, use ASTM F3125 Grade A325, Type 3. Galvanize fasteners for use with galvanized structural steel in accordance with 3392, “Galvanized Hardware.” Use cast-in-place anchorages to connect the steel intermediate diaphragms to the fascia beams . Provide anchorages capable of providing an ultimate pull-out strength of at least 15 kips per anchorage. Provide approved plastic or galvanized steel Material to form holes in beam webs to connect bolts for steel intermediate diaphragms. Ensure threaded rods used to attach prestressed concrete beams to cast -in-place concrete diaphragms are either galvanized per 3392, “Galvanized Hardware” or electroplated in accordance with ASTM B633, service condition SC4. Completely remove or cut off flush within 1/2 inch of concrete surface any uncoated items, such as used for static safety lines, anchors, lifting loops, etc., prior to casting the deck. Provide an ordinary surface finish to the exterior faces of the precast elements in accordance with 2401, “Concrete Bridge Construction.” Provide a surface finish as per the Special Provisions with the beams in place and in conjunction with the final finish of the remainder of the S tructure in accordance with 2401, “Concrete Bridge Construction.”

2405.4METHOD OF MEASUREMENT

The Engineer will separately measure Prestressed concrete beams Type ___ as individual units regardless of minor variations in Plan details between beams of the same type. The Engineer will measure Prestressed concrete beams ___ by summation of the individual lengths, out to out, along the centerlines of beams. The Engineer will measure intermediate diaphragms for prestressed concrete I -beams by length based on the horizontal distance from centerline to centerline of beam along the axis of the diaphragms.

2405.5BASIS OF PAYMENT

The Contract Unit Price for Prestressed concrete beams includes the costs of manufacturing, transporting, and erecting the beams in the final position, and the cost of temporary bracing in accordance with 2405.3K, “Installation.” The Contract Unit Price for Prestressed concrete beams includes the cost of constructing the intermediate diaphragms complete-in-place and structural steel or concrete and reinforcement bars as shown on the Plans. The Department will pay for Prestressed concrete beams on the basis of the following schedule: Item No. Item Unit

2405.502 Prestressed Concrete Beams Type ___ each

2 Prestressed Concrete Double Tee-Beam Type ___ each

3 Diaphragms for Type ___ Prestressed Beams linear foot

3 Prestressed Concrete Beams ___ linear foot