7-91 Section 708. Structural Prestressed Concrete Construction
708.01. Description This work consists of fabrica ting, furnishing, delivering, and erecting prestressed structural precast concrete. Precast/Prestressed Concrete Institute (PCI) certification is required for the prestressed structural precast concrete fabrication work listed below:
A.Category B2 – Prestressed Miscellaneous Bridge Products (Non-Superstructure) . Any precast , prestressed elements except for superstructure beams.
B.Category B3 – Prestressed Straight -Strand Bridge Beams (Superstructure) . All precast , prestressed superstructure elements using straight pretensioning or post -tensioning strands such as box beams, I-girders, bulb-tee beams, solid slabs, segmental box beams, and all products covered in Category B2.
C.Category B4 – Prestressed Deflected-Strand Bridge Beams (Superstructure) . Precast concrete bridge members that are reinforced with deflected pretensioning or post -tensioning strand. Included are box beams, I -girders, bulb-tee beams, solid slabs, segmental superstructure, and all products in Categories B2 and B3. Immediately correct findings that do not conform to PCI plant certification. Provide a copy of the PCI certificate of conformance to the Engineer before beginning production. If requested, make all annual plant certification and special imm ediate audit findings and audit dates available to the Engineer for all fabrication plants performing work on the project. 708.02. Materials Provide materials in accordance with the following sections : Cement ................................................................................................. 901 Fly Ash .................................................................................................. 901 Coarse Aggregate 6AA, 17A ................................................................. 902 Fine Aggregate 2NS .............................................................................. 902 Admixtures and Curing Materials for Concrete ..................................... 903 Bar Chairs and Wire Ties ...................................................................... 905 Prestress Strand ................................................................................... 905 Post Tensioning Tendons ...................................................................... 905 Steel Reinforcement ............................................................................. 905 Steel Welded Wire Reinforcement ........................................................ 905 Structural Steel ..................................................................................... 906 Water ..................................................................................................... 911 MDOT Standard Specifications for Construction Section 708
7-92 Mortar and Grout ................................................................................. 1005
Provide 6AA or 17A natural coarse aggregate; however, physical properties must meet 6AA in accordance with section 902, except that aggregate must have a maximum freeze-thaw dilation of 0.010% per 100 cycles based on testing of 6AA gradation. Provide 2NS fine aggregate. Do not use s lag aggregate. Provide structural steel for sole plates, position dowels , and other bearing components meeting the requirements of ASTM A709, Grade 36 or Grade 50. Hot-dip galvanize all structural steel in accordance with ASTM A123. Provide rigid monolithic polypropylene, high- density polyethylene, or other polymer plastic closed tube debonding material for prestressing strand with an outside diameter of 0.725 inch and a wall thickness of 0.04 inch. The tubing must not be reactive with the concrete or steel as approved by the Engineer . The material must have sufficient strength and durability to resist damage and deterioration during fabrication, transport, storage, inst allation, concrete placement, and tensioning. Do not use flexible polymer plastic split-sheathing. 708.03. Construction
A.Shop Drawings . Prepare shop drawings of fabrication details in accordance with subsection 104.02. Do not use contract plans instead of shop drawings . Do not start fabrication before the Engineer approves the shop drawings . Provide the QAI with a hardcopy of the approved shop drawings .
B.Shop Inspection . The Department will provide a QAI to perform shop inspection for structural p restres sed concrete and similar materials required to be accepted based on “Fabrication Inspection” according to the contract . The fabricator must establish and maintain an effective quality control program in accordance with PCI standards and the contract . The D epartment ’s shop inspection is not a substitute for the fabricator ’s quality control program .
1.Fabrication Notification . Provide the Engineer with a fabrication start date at least 7 days before beginning fabrication. If the f abricator suspends work for a period in which the Department ’s QAI leaves the fabrication plant, p rovide the Engineer with a fabrication start date at least 7 days , or as approved by the Engineer , before restarting fabrication.
2.Prefabrication Meeting . A prefabrication meeting will be held between the Engineer and fabricator before fabrication begins . The fabricator must provide the following representatives: plant manager MDOT Standard Specifications for Construction Section 708
7-93 and quality control manager , engineering and production personnel as
appropriate, and if applicable, subcontractor representatives . The fabricator must review all aspects of the project during the prefabrication meeting to avert problems that may complicate or delay fabrication and provide the Engineer with the proposed schedule of fabrication and delivery of completed elements .
3.Inspection Facilities. Provide facilities for inspection of material and workmanship at no additional cost to the Department in accordance with subsection 809.03.A, except as follows:
a.Provide an office close to the work with at least 120 square feet of floor space and shared by no more than one other QAI . The Engineer may approve sharing larger offices with additional QAIs .
b.Provide a parking space for the QAI next to the office.
c.Include a desk, chair, locker, plan rack, secure storage space for testing equipment, and high-speed broadband internet service. Metal -covered board or steel plate and stove for heating and drying aggregate are not required.
4.QAI Authority . The QAI has the authority to inspect in accordance with subsection 104.01.E . If an issue arises that the QAI cannot resolve, the Engineer will arrange a meeting with the QAI, Contractor, and fabricator to resolve the issue.
5.Acceptance. The Engineer will base acceptance of prestressed structural pre cast concrete material on the two-part process described in MDOT’s Structural Fabrication Quality Manual . The Engineer may reject fabricated members at the project site that the QAI accepted at the shop because of damage or deficiencies . The Engineer will evaluate the damage or deficiencies on a case-by -case basis if the fabricator submits a n NCR requesting the Engineer ’s review.
C.Furnishing and Fabricating
1.Equipment
a.Forms . Use metal forms . Forms must meet the requirements of subsection 706.03.D . Wood forms may be used for bulkheads , voids, and keyways .
b.Strength -Test Specimen Molds. Use specimen molds for making test specimen in accordance with ASTM C470/C470M .
c.Curing Tank . Use a Department -approved curing assembly consisting of a water tank equipped with thermostatic c ontrols . MDOT Standard Specifications for Construction Section 708
7-94 Maintain lime -saturated water at 70°F ± 5°F. Provide a tank sized
to contain the required number of 28-day test specimens.
d.Compression Testing Machine . Use a compression testing machine in accordance with ASTM C39/C39M . Subm it a calibration certif icate no more than 12 months old at any time during fabrication.
e.Scales. Use a calibrated scale. Submit a calibration certificate no more than 6 months old for weight scales and 3 months old for volumetric scales at any time during fabrication and whenever there is reason to question accuracy.
f.Stressing Jacks. Submit a calibration certification no more than 12 months old at any time during fabrication and whenever there is reason to question accuracy.
2.Void Boxes, Inserts, and Attachments. Design and const ruct void boxes, inserts, and attachments to withstand forces imposed during fabrication without bulging, sagging, collapse, or other deformation. Fasten void boxes, inserts, and attachments to maintain the proper position during concrete placement and co nsolidation . Place weep holes to provide drainage for voids . Puncture weep holes immediately after removal from casting bed.
3.Design and Proportioning of Concrete Mixtures . Design a concrete mixture meeting the following requirements:
a.Air content of fresh concrete of 5.5 % to 8.5%;
b.Minimum specified 28- day compressive strength as shown on the plans;
c.Slump from ¾ to 2½ inches if not using water -reducing or retarding admixtures;
d.Slump no greater than 4 inches if using a Type A or Type D chemical admixture;
e.Slump no greater than 6 inches if using a Type MR chemical admixture;
f.Slump no greater than 8 inches if using a Type F or Type G chemical admixture; and
g.At least 564 pounds of cementitious material per cubic yard of concrete. Provide ceme ntitious material with fly ash content no greater than 25% of the total weight of the cementitious material . Provide slag MDOT Standard Specifications for Construction Section 708
7-95 cement content no greater than 40% of the total weight of the
cementitious material . If using fly ash and slag cement in the same mixture, do not exceed 15% fly ash and 25% slag cement measured as the total weight of the cementitious material .
4.Concrete Strength . The Engineer will base acceptability of concrete strength on the results of compressive strength-test specimens using standard 6- by 12-inch or 4- by 8-inch cylinders . Mold, store, and test compressive strength-test specimens at no additional cost to the Department.
a.Molding and Curing . Mold and cure compressive strength -test specimens in accordance with ASTM C31/C31M , except as modified by this subsection. Make at least six strength-test specimens from concrete representing each sublot according to the contract . Make one-third of the strength-test specime ns from the beginning one-third of the sublot pour, one-third of the strength-test specimens from the middle one-third of the sublot pour, and one-third of the strength-test specimens from the final one-third of the sublot pour. Leave strength -test specime ns with the element in the same curing enclosure until stripping the element . At that time, r emove the 28-day strength-test specimens from molds and place in a water curing tank until testing. Store remaining work progress specimens with the element or in an alternat ive location exposed to the same environmental conditions as the element until testing. The fabricator may mold and cure alternate strength-test specimens to determine concrete strengt h at less than 28 days . Alternate strength-test specimens must be molded and cured with the element or in an alternat ive location exposed to the same environmental conditions as the element until testing.
b.Testing and Acceptance. Conduct compressive strength tests in the Engineer ’s presence . Test specimens in accordance with ASTM C39 /C39M , except test compressive strength-test specimens in a moist condition resulting from the required curing conditions. Consecutively test a set of three strength-test specimens obtained by taking one strength-t est specimen from each of the three sublot sample locations to determine concrete strength. Two of the three specimens must meet the specified concrete strength at release requirement, and the third specimen must meet or exceed 95% of the specified concrete strength at release requirement . If strength-test specimens do not meet the specified requirement , MDOT Standard Specifications for Construction Section 708
7-96 retest . Retesting consists of testing a set of three strength-test
specimens obtained by taking one strength-test specimen from each of the three sublot sample locations. Alternate strength-test specimens must be tested before the end of the 28-day curing period. Consecutively test a set of three strength-test specimens obtained by taking one strength-test specimen from each of the t hree sublot sample locations to determine concrete strength. Alternate strength-test specimen results will be accepted in place of the 28-day strength test results if alternate strength-test specimen results equal or exceed the specified 28-day concrete compressive strength. If alternate strength-test specimen results d o not meet or exceed the 28-day concrete compressive strength requirements , continue curing remaining 28- day strength-test specimens for the full 28-day period. Consecutively test a set of three strength-test specimens obtained by taking one strength-test specimen from each of the three sublot sample locations . Do not ship the prestressed element before strength-test specimen results equal or exceed the specified 28 -day concrete compressive strength. The 28-day concrete compressive strength must meet the following conditions: The average of the compressive strength of the three strength-test specimens in which a test specimen is obtained from each of the three separate sublot sample locations equals at least the required minimum 28-day concrete compressive strength; and At least two of three strength-test specimens meet the required minimum 28 -day concrete compressive strength and the third specimen exhibits at leas t 60% of the required minimum 28-day concrete compressive strength. If the 28-day concrete compressive strength test does not meet the criteria stated above, the Engineer may reject the prestressed element represented by the tests or determine whether the concrete has sufficient structural strength. If the Engineer determines that the concrete has sufficient structural strength, the unit price for the pay item and quantity represented will be prorated based on Formula 708-1: MDOT Standard Specifications for Construction Section 708
7-97 𝐴𝐴= 𝑆𝑆𝑡𝑡
𝑆𝑆𝑟𝑟×𝑈𝑈 Formula 708-1 Where: 𝐴𝐴 = The adjusted unit price, 𝑆𝑆𝑡𝑡 = Tested strength, 𝑆𝑆𝑛𝑛 = Required strength, and 𝑈𝑈 = Unit price.
5.Steel Reinforcement and Prestressing Strand . Tie uncoated steel reinforcement with wire ties . Tie epoxy -coated steel reinforcement with epoxy -coated wire ties . Do not weld steel reinforcement. Prestressing strand must be free of oil, dirt, paint, corrosion, and deleterious material that prevents bonding between strands and concrete. Do not use a reel or pack of strand when one or more wires in the strand show a coating of adherent rust that light rubbing cannot remove . Do not use strand that contains adherent rust, pits that are visible to the naked eye, or kinks, bends, nicks, or other defects . Cover prestress ing strand stored outside with waterproof tarps and block above the ground to prevent contact with soil and water . Do not use strand without identification.
6.Miscellaneous Steel . Galvanize carbon steel items cast into the structural concrete element s to facilitate br idge construction including, but not limited to, forming, finishing, fals e decking, safety tie- offs, or any other temporary wor ks in accordance with AASHTO M111, AASHTO M232 , ASTM B633 Service Condition 1, ASTM B695 , or epoxy coat in accordance with ASTM A775/A775M .
7.Welding. Welding must be in accordance with PCI certification requirements .
8.Strand Debonding . If strand debonding is required, furnish and install rigid polymer material around prestressing strands . Place the debonding material at the specified locations and to the lengths shown on the plans . Seal the ends of the debonding material with duct tape or other materials approved by the Engineer to prevent the intrusion of cement paste or water within the debonded regions of the strand.
9.Placing Concret e. Place concrete in accordance with subsection 706.03.H and as modified as follows:
a.External vibrators may be used;
b.Remove all water , snow, and ice from forms before placing concrete; MDOT Standard Specifications for Construction Section 708
7-98 c. Protect fresh concrete and forms from rain, snow, and ice during
any interruptions of casting operations ;
d.Maintain concrete temperature between 45°F and 90°F but as close to 70°F as practical during placement ; and
e.Do not hold vibrators against forms or reinforcing steel, and do not use them for flowing or spreading concrete. Do not disturb partially hardened concrete.
10.Cold Weather Precautions . Protect concrete to prevent damage from cold weather . Replace the prestressed element at no additional cost to the D epartment if concrete becomes frozen. Before placing concrete in forms, construct an enclosure around the forms to assist with maintain ing the specified temperatures . Arrange heating and the enclosure to allow removal of a portion of the enclosure during placement and finishing of concrete without interrupting heating. Provide uniform forced air , radiant heat , or steam heat in the enclosure. Vent the heating system to prevent exposure to carbon dioxide and carbon monoxide exhaust gases during concrete placement and curing. Before placing concrete, preheat reinforcing steel and form surfaces to temperatures between 40°F and 75°F. During placement of concrete and up to the concrete initial set, operate the heating system to maintain an air temperature in the enclosure between 40°F and 75°F.
11.Curing Elements . Provide curing enclosures to allow air or steam circulation around exposed portions of the concrete. Cure concrete at temperatures between 70°F and 150°F until concrete attains the release strength shown on the approved shop drawings. Maintain the required temperature during the curing period with steam or radiant heat . Apply steam or radiant heat after concrete reaches initial set in accordance with ASTM C403/C403M without damaging the concrete. Do not direct steam or radiant heat at the concrete or the forms because this caus es localized high temperatures . During initial application of steam or radiant heat, increase the concrete temperature in the curi ng enclosure by no greater than 36°F per hour until reaching the curing temperature. Maintain the maximum curing temperature within the enclosure until concrete reaches the desired temperature and strength. Do not exceed a maximum concrete temperature of 150°F during the curing cycle . Upon curing completion, MDOT Standard Specifications for Construction Section 708
7-99 reduce the concrete temperature in the enclosure at a rate no greater
than 50°F per hour . The cooling rate must continue until the concrete temperature is 40°F or less above outside ambient temperature. Provide recording thermometers for steam or radiant heat curing that are capable of showing the time-temperature relationship in the curing enclosure from the time of concrete covering to transfer of prestress . Use at least two recording thermometers per product line at locations approved by the Engineer to monitor the concrete and curing rate. Graph time-temperature documentation and provide a copy to the Engineer for evaluation.
12.Workmanship
a.Concrete Defects. Remove concrete fins and irregular projections from surfaces immediately after removing forms . Patch air holes larger than ¾ inch in diameter and ⅜ inch in depth with Type R -2 mortar . Keep surfaces saturated with water until pointed and trued with mortar . For fascia beams , use a mortar mixture composed of two-thirds Type 1 cement and one-third white cement or as approved by the Engineer to closely match the beam . Restore consistency by reworking but not by re-tempering. Honeycombing, voids, and cracks are cause for rejection. The Engineer wi ll evaluate them on a case-by -case basis if the fabricator submits a n NCR for the Department ’s review .
b.Finishing I-Beams. Smooth finish the outer 1 inch of the top surface of the I -beam . Rough finish remaining I -beam top surfaces to provide a ¼ -inch surface texture.
c.Finishing 1800 and Bulb -Tee Beams. Smooth finish the outer 6 inches of beam top surfaces . Rough finish remaining beam top surfaces to provide a ¼ -inch surface texture. Clean the outer 6 inches of the top surface and apply a debonding compound in accordance with the manufacturer ’s recommendations . Prevent compound from spreading over beam flanges or toward the center of the beam . Remove compound that exceeds the 6-inch boundary before it cures . Use solvents approved by the debonding compound manufacturer.
d.Finishing Box Beams. Smooth finish the outer 1 inch of the box beam top surface. Rough finish remaining box beam top surfaces to provide a ¼ -inch surface texture unless otherwise required. If hot mix asphalt (HMA) overlay is required, provid e a wood float finish on the top surface. MDOT Standard Specifications for Construction Section 708 7-100 e. Bearing Surfaces. Fabricate bearing surfaces to meet a flatness tolerance of ⅛ inch over 12 inches.
13.Tolerances. The Engineer will evaluate beams that do not conform to the dimensional tolerances specified in Table 708-1. Table 708- 1: Dimensional Tolerances for Beams Beam Type Tolerance Length of I -beams , 1800 beams, and bulb- tee beams ±¼ in ch/25 feet, ±1 inch max Length of box beams ±¾ in ch Width of I -beams , 1800 beams, and bulb- tee b eams +½ in ch, –⅛ inch Width of box beams ±½ in ch Height of I -beams, 1800 beams, b ulb-tee beams, and box beams +¼ in ch, –⅛ inch Camber deviation from design v alue ( measured within 24 hours of strand r elease) ±⅛ inch /10 feet Thickness of top s lab of box beam +½ in ch, –¼ in ch Length of I -beam end blocks +2 ft, –0 inch Sweep of I -beams , 1800 beams , and bulb- tee beams ( horizontal deviation of c enterline from a straight l ine between ends m easured at both top and bottom) ¼ inch/10 feet Sweep of box beams ( horizontal d eviation of centerline from a s traight l ine between ends measured at both top and bottom) ⅜ inch up to 60 f eet, ½ inch over 60 fee t Vertical deviation of s ide forms between top and bottom of beam ≤¼ in ch from plan location Prestress strand ≤¼ in ch from plan location Location of conduit for transverse post- tensioning ≤½ in ch from plan location Location of holes for position dowels (I- beams , bulb- tee beams, and 1800 beams) ≤½ in ch from plan location Location of holes for position dowels box beams ≤1 inch from plan location
14.Stress Transfer . Do not transfer prestress force to the concrete element until the concrete strength, as indicated by strength- test specimens, meets the specified prestress concrete compressive strength at release. Cut or release prestressing strand to minimize lateral eccentricity of prestress. Flame cut the prestressing strand at least 6 inches away from the end of the element . Cut strands flush wit h the concrete surface using mechanical means and cover ends and depressions around MDOT Standard Specifications for Construction Section 708 7-101 prestressing strand ends with asphaltic material approved by the Engineer.
15.Handling, Storage, and Transporting . Handle and store prestressed elements to prevent damage. The following special requirements for beams apply :
a.Keep beams upright.
b.Use lifting loop devices shown on the plans unless the Engineer approves alternat ive lifting devices and procedures . Apply equal loads to each pair of lifting devices.
c.Support stockpil ed beams across the full width on two battens that are at least 4 inches wide. Do not s upport beams at more than two points.
d.Use battens to hold beams off the ground over the full length of the beams . Place battens in from the beam ends no greater than 1.5 times the depth of the beams, or 3 feet, whichever is less . For skew beams, measure the distance along the centerline of the beam . Place battens to support stacked beams, one above the other, along the same vertical plane, at each end of the beams.
e.Suppor t beams during transport the same as stockpiled beams, except that trucks with two rear bolsters may be used. If truck bolsters are worn, use wood shingles to give bearing. Place wood blocks under chains to hold beams in place on the trucks.
D.Erection of Prestressed Beams
1.Box Beams . Shim beam bearing pads during erection to provide full bearing contact with the bottom of the beam . Place seal washers or other devices meeting the Engineer ’s approval between the beams at the transverse conduit holes . After setting beams, drill position dowel holes into bridge seats through holes provided in each beam end. Insert dowels. At expansion bearings, fill position dowel holes with hot -poured rubber -asphalt type filler to at least 3 inches above the position dowels . Fill the remainder of the hole with Type H -1 grout . Fill holes at fixed bearings with Type H -1 grout. After setting beams in their final position, clean the beams with water, and mortar longitudinal joints and the surfaces between beams . Use Type R-2 mortar with a slump of 5 inches and place when the air temperature rises above 40°F . Fill spaces between beams full depth. Rod the mortar into the space to form a t ight, solid joint . Cure mortar for 48 hours . After mortar cures, post -tension the deck transversely . MDOT Standard Specifications for Construction Section 708 7-102 Tension tendons to the required force but do not exceed the yield stress of the material. After post-tensioning, clean the annular space between the tendon and hole by flushing with water . Remove water with compressed air . With the grouting vent open at one end of the hole, inject Type E -1 grout under pressure at the other end. Continue injecting grout until grout comes out through open vents . Close open vents while maintaining grout pressure. Gradually increase pressure to at least 50 psi and hold for 15 seconds . Close the inlet valve. Remove lifting devices.
2.I-Beams, 1800 Beams, and Bulb -Tee Beams. Place beam bearing pads over the position dowel and shim to provide full bearing contact with the bottom of the beam . Position beams on the substructure and rigidly block them in place before erecting steel diaphragms or beginning deck and concrete diaphragm forming. Remove lifting devices. 708.04. Measurement and Payment Pay Item Pay Unit Prest Conc Deck, __ inch .............................................................. Square Foot Post Tensioning (Structure Identification) ........................................ Lump Sum Prest Conc Box Beam, Furn, __ inch ......................................................... Foot Prest Conc Box Beam, Erect, __ inch ........................................................ Foot Prest Conc I Beam, Furn, __ inch .............................................................. Foot Prest Conc I Beam, Erect, __ inch ............................................................. Foot Prest Conc 1800 Beam, Furn .................................................................... Foot Prest Conc 1800 Beam, Erect ................................................................... Foot Prest Conc Bulb -Tee Beam, Furn, __ inch by __ inch ............................... Foot Prest Conc Bulb -Tee Beam, Erect, __ inch by __ inch .............................. Foot The unit prices for Prest Conc, Erect pay items include the cost of position dowels, shimming to provide full bearing contact, bracing, and blocking. The Department will pay for prestressed concrete box beams as Prest Conc Deck when the space between adjacent box beams is filled with grout . The Engineer will measure Prest Conc Deck based on the nominal overall length of the beams , multiplied by the overall plan width. Plan width is the sum of the widths of the beams plus the sum of the spaces between beams. The Engineer will measure spread prestressed concrete box beams as Prest Conc Box Beam, Furn, __ inch , and Prest Conc Box Beam, Erect, __ inch. MDOT Standard Specifications for Construction Section 708 7-103 The Engineer will measure Prest Conc, Erect and Prest Conc, Furn pay items based on the nominal length of the beams . The unit prices for Prest Conc I Beam, Furn, __ inch , Prest Conc 1800 Beam, Furn , and Prest Conc Bulb -Tee Beam, Furn, __ inch by __ inch include the cost of debonding the beam flange. The Department will not allow additional compensation for costs incurred in the certification of prestr essed concrete plants or claims by the Contractor for delays or costs associated with prestressed concrete fabrication plant certification.
Source: Michigan Standard Specifications for Construction, 2020 Edition. Pages 455–467 of 1,146.