708 - Prestressed Concrete
GENERAL DESCRIPTION
Structural prestressed concrete members (also known as elements) are cast in forms at a Fabricator certified by the Prestressed Concrete Institute (PCI). A Fabricator supplying prestressed concrete bridge beams for MDOT use must also be an approved Supplier per MDOT’s Supplier Qualification Standard (SQS). A list of approved prestressed concrete beam Suppliers for MDOT projects can be found on the Structural Fabrication Unit (SFU) Website
Prestressing (pre-tensioning and post-tensioning) of precast concrete members allow for longer, more efficient spans than traditionally reinforced concrete members by introducing compressive forces into the concrete members. These compressive forces are achieved by using a high grade of steel or carbon fiber strands that are pre or post-tensioned to a predetermined load based on design requirements, temperature of steel and concrete, Contractor/Fabricator equipment, and other factors. Most prestressed concrete beams are longitudinally prestressed and some may also be transversely prestressed, such as side-by-side box beams that are post-tensioned.
Structural members that are prestressed include, but not limited to:
- Prestressed concrete beams
- Spun Poles
- Prefabricated Bridge Elements and Systems (PBES)
- Sound Wall Panels and Posts
Structural members that are not prestressed include, but not limited to:
- Culverts
- PBES
- Sound Wall Panels and Posts
MDOT requires on-site quality assurance (QA) shop inspection for the fabrication of prestressed concrete members. This inspection program is managed by MDOT’s Structural Fabrication Unit (SFU).
The SFU is responsible for implementing the Departments QA program for fabricated materials required to be accepted based on “Fabrication Inspection” per the Materials Quality Assurance Procedures (MQAP) manual or project special provision. The SFU also serves as the Departments structural fabrication expert and provides recommendation to design and construction staff regarding plan details, specifications, shop drawings, shop inspection, and field inspection.
In general, MDOT utilizes consultants to provide fabrication inspection and project management; however, the Structural Fabrication Unit will randomly perform fabrication inspection and project management on select projects for various reasons. The consultant provides weekly updates for active fabrication projects to the SFU and other important information such as program changes, overall quality, communication with Fabricator, and key performance indicators (KPIs) are discussed.
The SFU adheres to MDOT’s Structural Fabrication Quality Manual (SFQM). The purpose of the SFQM is to provide the SFU and their consultants with information on implementing MDOT’s QA program for fabricated materials required to be accepted based on “Fabrication Inspection”.
The SFQM can be found on the SFU Website.
MATERIALS
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EQUIPMENT
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PRECONSTRUCTION
Submittals and Shop Drawings
The Shop Drawing Review Process document summarizes the process for reviewing shop drawings for permanent structural elements, temporary bridges, and other temporary works. It is important for MDOT to respond quickly to shop drawing submittals in order to keep the project moving forward.
Shop Drawing Review Process can be found on the SFU website.
Shop detail drawings, commonly called shop drawings, are a type of working drawing that the Contractor (Supplier/Fabricator) develops prior to fabrication. The Contractor is responsible for providing drawings that accurately show the appropriate details, dimensions, material requirements, fabrication procedures, and other requirements necessary to fabricate and erect structural members in accordance with the contract drawings.
Note, some Contractor submittals may be labelled as a shop drawing in ProjectWise but don’t follow the shop detail drawing requirements listed above. These types of submittals are typically working drawings, construction submittals, manufacturing drawings, or product information sheets. These are not shop detail drawings and do not follow the Shop Drawing Review Process.
The Contractor must submit for review all shop drawings and calculations (when applicable) not furnished by MDOT for all parts of the work as required by the contract in Portable Document Format (PDF) into ProjectWise. The Contractor is responsible for the correctness of the shop drawings and calculations (when applicable) and ensuring their design (when applicable) complies with any permit and design requirements.
If revisions are necessary after the shop drawing set is approved, the Contractor is required to submit a complete shop drawing set (proposed new sheets, revised sheets, and remaining sheets that are already stamped approved) for review to ensure each submittal is a standalone set.
The MDOT Bridge Design PM (or their delegate) is responsible for coordinating the shop drawing review process for all shop drawings required for pay items on bridge plan sheets. The coordinator is responsible for distributing shop drawings to the appropriate MDOT technical review areas (structural fabrication, load rating, ancillary structures, etc.) for review and comment. Once all comments have been addressed, the Bridge Design PM (or their delegate) stamps the drawing according to the following three stamp designations:
- Approved – This stamp is used when the shop drawings appear to be in general conformance with the contract plans. All sheets are required to be stamped.
- Approved Subject to Correction – This stamp is used when the shop drawings appear to be in general conformance with the contract plans, but there are very minor corrections needed for historical records that will not affect fabrication of the elements. Only the first sheet is stamped.
- Revise and Resubmit – This stamp is used when the shop drawings are not in general conformance with the contract plans and approval could result in elements being incorrectly fabricated. Only the first sheet is stamped.
Shop Inspections
Fabrication Notification
The Contractor (or the Fabricator/Supplier) must provide the Engineer with a fabrication start date at least 7 days before beginning fabrication per 708.03.B.1 of the MDOT Standard Specifications for Construction (MDOT SSC).
Prefabrication Meeting
SFU (or the consultant PM on behalf of the SFU) will conduct a prefabrication meeting for all design-build and design-bid-build projects. Exceptions are listed below:
- The frequency of prefabrication meetings can be reduced to an average of one meeting per month for Suppliers that perform continuous work for MDOT.
- The SFU PM approves the deviation in writing.
Some prefabrication meetings may be combined with other projects provided both projects have the same suppler (Fabricator) and Contractor, each Engineer for the different projects is okay with the meetings being combined, and the projects will be fabricated back-to-back with no excessive downtime between production.
Prefabrication meeting minutes are recorded to document topics that were discussed and to follow up on action items including who is responsible for the follow up. Draft meeting minutes are typically sent out 3-5 business days before production, but this can vary depending on when the fabrication unit was notified of the work or project schedules. Prefabrication meeting minutes are recorded to document topics that were discussed and to follow up on action items including who is responsible for the follow up. Draft meeting minutes are typically sent out 3-5 business days before production, but this can vary depending on when the fabrication unit was notified of work or expedited project schedules. Final meeting minutes are typically sent out within 1 business day of the comment due date established in the prefabrication meeting.
The following individuals are typically invited to the prefabrication meeting:
- Contractor
- Supplier (Fabricator)
- Quality Control Inspector (QCI)
- SFU:
- Consultant PM
- Consultant Quality Assurance Inspector (QAI)
- MDOT Structural Fabrication Engineer
- MDOT Transportation Engineer
- MDOT Structural Precast Concrete Specialist
- Engineer
- Bridge Design PM
The Contractor, Supplier (Fabricator), QCI, consultant PM, QAI, and MDOT Structural Precast Concrete Specialist must attend prefabrication meetings whereas the MDOT Structural Fabrication Engineer, Transportation Engineer, Engineer, and Bridge Design PM may choose to attend prefabrication meetings. MDOT encourages all individuals listed above to attend prefabrication meetings. Note that some unique projects may require the invitation of Supplier (Fabricator) specialty subcontractors (NDT, galvanizing, metalizing, etc.) to prefabrication meetings.
Inspection Facilities
The SFU is responsible for implementing the Departments QA program for fabricated materials required to be accepted based on “Fabrication Inspection” (also known as shop or QA inspection) per the MQAP manual or project special provision. Most prestressed members require QA inspection, and this inspection is performed by a qualified Quality Assurance Inspector (QAI) representing the SFU. Prestressed concrete Suppliers (Fabricators) are located nationwide and the SFU coordinates inspection at these Suppliers when MDOT work is occurring.
Fabrication Inspection: SFU Website (see section 2.2 of SFQM)
QAI Authority
Throughout the fabrication process, the QAI will inspect the materials and fabricated elements for conformance to MDOT specifications and project special provisions (SPs) in accordance with the Structural Fabrication Quality Manual (SFQM) and supplemental Inspection Test Plans (ITPs). In addition, the QAI will collect all documentation regarding fabrication and verify Buy America (BA) provisions were met. If problems arise during the fabrication, the QAI will contact their PM for resolution and the PM may seek additional guidance from the SFU. Once fabrication is complete and the members inspected for conformance to project specifications, the members will be stored for shipping.
QAI Requirements: SFU Website (see section 2.2.4 of SFQM)
Acceptance
Acceptance of prestressed members consists of the following two-part process per the SFQM:
- QA (Fabrication) Inspection Acceptance: After the structural members are inspected to verify conformance to project specifications, the QAI will stamp each main member “Approved for Use”. The approval stamp indicates that the QAI has inspected the members and found that they appear to meet project requirements; however, it does not relieve the Contractor of their responsibility to meet contract requirements. Some secondary members and miscellaneous components will not be individually stamped.
- Visual Inspection (VI) Acceptance: the Engineer must collect one copy of the stamped Bill of Lading and use it to verify the delivered structural products. Additionally, the Engineer must verify that the products are stamped and VI them for signs of damage that may have occurred due to shipping and handling. This VI should be documented in the field inspector’s daily report.
Figure 708.10 – “APPROVED FOR USE” Stamp
Acceptance: SFU Website (See section 2.2.7B of SFQM)
Furnishing and Fabricating
Equipment
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Void Boxes, Inserts and Attachments
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Design and Proportioning of Concrete Mixtures
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Concrete Strength
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Steel Reinforcement and Prestressing Strand
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Miscellaneous Steel
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Welding
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Strand Debonding
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Placing Concrete
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Cold Weather Precautions
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Curing Elements
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Workmanship
The QAI will inspect prestressed members after QC has performed their inspection and note any deficiencies or nonconformances (NCRs) caught by QC or observed by QA. The Supplier (Fabricator) is required to notify the MDOT QAI of all observed defects, damage, and potential NCRs and submit a proposed repair procedure to SFU for approval. The Supplier (Fabricator) must receive written approval from MDOT prior to performing any repairs.
Approved repairs are typically completed at the Supplier’s (Fabricator’s) facility prior to the member shipping to the project site; however, there may be some circumstances where approved repairs are conducted at the project site due to project schedules. In both cases, the Supplier (Fabricator) typically performs the repairs. Repairs conducted at the Supplier’s (Fabricator’s) facility will be witnessed by the QAI and repairs conducted at the project site will be witnessed by the MDOT Construction Inspector.
- NCR Process: SFU Website (See section 8 of SFQM)
The SFU has developed standard repair procedures for common defects observed on fabricated prestressed members. These repair procedures have been reviewed by industry and should be used by the Supplier (Fabricator) when applicable to expedite the review process. The QAI or Construction Inspector should be familiar with these standard repair procedures for common deficiencies.
MDOT Standard Repair Procedures: SFU Website (See Appendix A4 of SFQM)
Tolerances
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Stress Transfer
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Handling, Storage, and Transporting
When loading and shipping occurs, the Supplier (Fabricator) will load the members on a truck and the QAI inspects all members again for conformance to specifications since handling damage can occur when members are transported and loaded for shipping.
The Supplier (Fabricator) is required to provide the QAI with a minimum of five (5) copies of the Bill of Lading for each proposed shipment per the SFQM. Each Bill of Lading will be stamped “Approved for Use” (see Acceptance process below). One copy of the Bill of Lading is retained by the QAI and the other four copies are distributed to the Contractor, Supplier (Fabricator), Engineer, and Trucking Company.
Once the structural prestressed member has been visually inspected and accepted by the Engineer, the construction inspector should verify the products are stored properly prior to erection. The method of stockpiling and transporting members is covered in detail in subsection 708.03 of the Standard Specifications for Construction. Below is a list of items the inspector should check regarding the storage of structural prestressed members:
- Support beams off the ground to prevent moisture and deleterious material intrusion.
- Support beams across the full width on two battens, each greater than 4 inches wide.
- Support beams on two battens, no more than 3 feet from the ends of the beams.
- Support stacked beams, one above the other along the same vertical plane at each end of the beams.
- Support the beams on level, stable ground, avoid storing beams near side slopes, or areas near heavy construction traffic.
- Do not support the beam at more than two points.
- Prestressed I-beam shapes may need to be laterally braced on site.
- Members are to be stored in an upright position and should be thoroughly braced to avoid overturning, which may damage the member itself, adjacent members or material, or injure personnel in the immediate vicinity.
- Related items such as bearings and bridge railing should also be protected from damage, dirt, and corrosion.
Loading and Shipping
SFU Website (See section 2.2.7A of SFQM)
CONSTRUCTION
Erection of Prestressed Beams
Considerable care should be exercised when handling structural precast concrete elements. In many cases, there is no way to repair a damaged element short of re-fabrication. Below is a list of items the inspector should check regarding the erection of structural prestressed members.
- Prestressed beams should be always kept in an upright position, as the beam’s own dead weight counter acts the internal prestressing force to keep the beam stable.
- Verify lifting devices, usually loops of seven wire steel strand, are the only attachment used when moving a beam. The beam must not be lifted at the beams center in any way or undesirable stresses may develop and the beam may crack.
- Ensure the lifting devices are removed once the beam has been placed in its final position on the substructure.
- Per Section 105 of the Standard Specifications for Construction, the Engineer reserves the right to reject any shipped product that shows visual signs of damage or does not meet specification requirements.
Check beam markings and compare to the beam layout sheet in the plans. Make sure the beams are placed in the right location and direction. They may fit in more than one place but there is only one correct location and direction for each beam.
Box Beams
Below is a list of items the inspector should check regarding the erection of structural prestressed Box Beams:
- Verify box beam widths will fit properly on the substructure units.
- Verify box beam bearing pads are shimmed (when necessary) on each beam in an approved manner during erection to provide full bearing contact with the bottoms of the beams.
- Box beams may camber more than anticipated or may experience transverse rotation such that they do not sit flush on the sole plates, which are typically beveled to accommodate camber and vertical alignment angles.
- (Expansion Bearings Only) Verify the beams line up with the position dowel holes, and ensure the holes are filled with hot poured rubber sealant type filler at least 3 inches above the position dowels. Ensure the remainder of the hole is filled with Type H-1 grout.
- (Fixed Bearings Only) Verify the holes are filled with Type H-1 grout.
- (Side-by-Side Box Beams Only) Verify beams are set with spaces as specified on the contract plans or shop drawings and that seal washers are placed around the transverse post-tensioning conduit holes.
- Note in the Daily Work Report (DWR) when erection has been completed.
Below is a list of items the inspector should check regarding the post-tensioning of side-by-side box beams after erection:
- Verify the space between the box beams is completely filled, full depth, with R-2 grout mortar and cured for at least 48 hours.
- Ensure the contractor does not grout when temperatures are below 40° F.
- The deck width should be measured by the Contractor as soon as the beams are set to ensure the post tensioning rods or strand will be fabricated to the proper length.
- All hardware dimensions must be as shown on the plans or shop drawings.
- Tendons or rods for post-tensioning should be tensioned with calibrated jacks according to a Contractor-supplied calibration chart showing the corresponding jack pressures and elongations necessary to gradually build up to the required post-tensioning force.
- After the post-tensioning is complete, ensure the conduit is flushed with water, then compressed air.
- The tendon or rod holes will be grouted under pressure with Type E-1 grout for steel tendons or rods as specified in subsection 708.03B of the Standard Specifications for Construction.
Do not stand behind the jacking machine or rod holes during jacking procedures. If a tendon or rod snaps, it can cause serious injury
I-Beams, 1800 Beams, and Bulb-Tee Beams
Below is a list of items the inspector should check regarding the erection of structural prestressed beams and girders:
- After the first and second member (beam or girder) are erected, the second member must be attached to first through the diaphragm connection(s). In addition, each subsequent member must be attached to the previously erected member through the diaphragm connection(s).
- Prestressed beams and girders, especially taller and longer members, can be more prone to sweep, transverse deflection, or even rotation. For this reason, they should be rigidly blocked in place via diaphragm connections to ensure proper spacing between members and to avoid beam rotation during construction.
- Verify member (beam or girder) bearing pads are shimmed (when necessary) on each member in an approved manner during erection to provide full bearing contact with the bottoms of the beam/girder.
- (1800 Girder only) Verify use of beveled sole plates on each girder and note in the Daily Work Report (DWR).
- Note in the Daily Work Report (DWR) when erection has been completed.
Spun Pole
Below is a list of items the inspector should check regarding the erection of prestressed spun poles:
- During installation, the poles must only be lifted and stored using the locations and methods shown on the approved shop drawings or manufacturer’s instructions.
- Visually inspect spun pole for any damage, cracks, or spalls and notify the Engineer if present.
- Note, the concrete clear cover for spun poles is typically less than a prestressed beam so cracks or spalls may increase the members susceptibility to steel corrosion.
- Spun concrete poles must be safely secured after being placed in the drilled shaft installation before the concrete foundation is poured and cured
Sound Wall
Below is a list of items the inspector should check regarding the erection of sound walls:
- In most cases the sound wall panels are attached to the posts using tongue and groove joints. It is important during erection that the panels are lowered slowly into position and do not crack or spall the posts.
- If the sound wall posts contain grout pockets (prestressed sound walls only), then the grout pockets at the ends of the posts must be filled with non-shrink grout and should be free of cracks.
- Note, a ladder should be used to visually inspect the tops of soundwall posts.
- Visually inspect sound wall posts for damage, shrinkage, cracks, or spalls and notify the Engineer if present.
- Note, cracks and other deficiencies will be subject to moisture and chloride intrusion which can lead to rapid deterioration of the posts.
INSPECTION & TESTING
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MEASUREMENT, DOCMENTATION & PAYMENT
Basis of Payment
There are three methods of payment for structural precast concrete members: Stockpile payment, partial shipment payment, and final shipment payment. No method of payment relieves the Contractor (or Fabricator) from damage due to transporting, storing on the project site, or erecting the elements. Additional information on payment can be found in Section 109 of the MDOT Standard Specifications for Construction.
The Engineer will make stockpile payments for fabricated structural steel products in accordance with section 109 of the MDOT SSC. See subsection 2.2.6 of the SFQM for SFUs responsibilities with stockpile payment. These materials can be stored at the fabrication facility or at the construction site. If stored at the construction site, the materials must be inspected by the Engineer as stockpiling occurs since the approval stamp ink could wash away over time. If the approval stamp(s) appear to be fading away, the Engineer must mark the accepted structural steel products in a more permanent method.
After the Supplier (Fabricator) has shipped a portion of the structural concrete elements to the project site, visually inspect the members and verify they are stamped “Approved For Use” according to the sections above. If all the documentation is provided and there are no deficiencies, then partial shipment payment may be made. The Engineer must follow section 109.04 of the MDOT Standard Specifications for Construction.
After the Supplier (Fabricator) has shipped all of the structural concrete elements to the project site, visually inspect the members and verify they are stamped “Approved For Use” according to the sections above. If all the documentation is provided and there are no deficiencies, then final shipment payment may be made. The Engineer must follow section 109.04 of the MDOT Standard Specifications for Construction.