708 - Prestressed Concrete
-This section currently undergoing substantial revision-
Structural precast concrete members are cast in forms at a facility certified by the Prestressed Concrete Institute (PCI) that is typically a location other than their final position in the structure. Precast concrete beams are also longitudinally prestressed (bonded pre-tensioned strands) and in the case of side-by-side box beams, are transversely prestressed (post-tensioned strands) as well. Prestressing and post-tensioning of precast concrete elements allows for longer, more efficient spans than traditionally reinforced concrete members by introducing pre-compression into the concrete members. The pre-compression force is typically designed to be larger than the effect of applied loads on the element, and therefore, stresses are well controlled.
Other structural elements that are prestressed include concrete spun poles and in some cases, Prefabricated Bridge Elements and Systems (PBES) other than beams, and sound wall posts and/or panels.
Structural precast concrete elements that are not prestressed include, but may not be limited to, sound wall posts and panels, culverts, and some PBES. Although structural precast concrete elements are typically fabricated off-site, they require on-site inspection to ensure successful forming, casting, transport, handling, and placement into their final position in the structure.
Shop drawings with complete detailed dimensions are submitted to MDOT for review and approval. Approved shop drawings are required prior to the start of fabrication, but MDOT does not check every dimension for accuracy. The submission is reviewed to ensure it is complete and in general conformance with the contract plans. The Contractor is solely responsible for the correctness of shop drawings. Errors found during the review of the shop drawings will be pointed out by MDOT but undiscovered errors are the Contractor's responsibility.
It is the responsibility of the MDOT Design Project Manager (PM) to ensure their project’s shop drawings are being reviewed by all applicable technical reviewers. This process is outlined in Chapter 10 of the MDOT Bridge Design Manual. Figure 708.1-1 and Figure 708.1-2 below summarize MDOT’s electronic shop drawing review process and provides a listing of shop drawings to be reviewed with annotations indicating which MDOT Review Areas need to be involved in their review. Note that this shop drawing listing is general in nature and is applicable for most projects; however, exceptions may apply on a case by case basis and it is the responsibility of the PM to ensure the shop drawings are being reviewed by all applicable parties.
Upon receipt of five sets of approved shop drawings from the Design Support Area, the Engineer shall forward two sets of these copies to the Contractor.
Prestressing, refers to the process of pulling steel tendons in the beam into tension before the concrete is placed in the forms. After the concrete has hardened, the stressed tendons are released and transmit a compressive stress to the concrete. This offsets tension forces on the concrete and increases the load-carrying capacity of the beam. The result is similar to pressing on the ends of a set of books in order to pick up the whole set of books at once.
There are three general kinds of precast prestressed concrete beams in use:
- the box beam
- the prestressed I-beam
- the prestressed 1800 beam
The precast prestressed concrete beams vary in width, length, and depth. The deeper types will span longer distances for the same loading conditions. Also, for the same span length, the deeper types will carry a larger load.
When the beams arrive on the job site, inspect the beams before they are unloaded. Check one end of each beam for a stamp or tag stating “approved for use.” If the beams are approved for use, a memorandum will be sent to the TSC from Construction and Technology stating the approval of each beam and verification of all certification and test results. Look for damage to the concrete surfaces in either the form of cracks, particularly in the area near the bearing plate (diagonal or radiating), and fine tension cracks near mid-span, or spalled off pieces. Bring any damage to the attention of the Engineer and make a record of the damage in the daily report. Also note in the daily report when the beams arrive in good condition.
Considerable care should be exercised when handling prestressed members. Usually there is no way to repair a damaged member, short of refabrication. Beams should be kept in an upright position at all times. The method of stockpiling and transporting members is covered in detail in subsection 708.03 of the Standard Specifications for Construction. Beams shall be supported at two points no more than 3 feet (1 m) from the ends of the beams.
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.
Lifting devices, usually loops of cable, are cast into the member and these should be the only attachment used when moving a beam. The lifting device must have adequate capacity to lift the beams and set them in place. Never lift the beam near the center. If the beam has to be set down before being placed, never allow it to be supported at the center. Crib it under the ends. The prestressing forces pulling in on the ends of the beam hold up the dead weight of the beam and there is no top reinforcement. If the beam is supported or lifted at the center, the prestressing forces will cause the beam to crack.
The beams in a structure may also be different widths. When setting the beams, be sure to have the right size neoprene bearing pad is used and is centered under the bearing plate of the beam.
To ensure that box beams will fit properly on the substructure units, the beam widths should be checked. Beam bearing pads must be shimmed in an approved manner during erection to provide full bearing contact with the bottoms of the beams. This needs to be checked on every box beam. Note in the daily report when the beam erection has been completed.
The beams should be set with spaces between the beams as specified on the plans or shop drawings and with the seal washers placed in the duct holes. The space between the beams should be completely filled, full depth, with R-2 mortar and cured for at least 48 hours. The deck width should be measured by the Contractor as soon as the beams are set to assure that post tensioning rods or tendons will be fabricated to the proper length.
All hardware dimensions must be as shown on the plans or shop drawings. Tendons or rods should be tensioned with calibrated jacks according to a Contractor-supplied calibration chart showing the corresponding jack pressures and rod elongations necessary to gradually build up to the required stressing force. After the tensioning is complete, the tendon or rod holes will be grouted as described in the standard specifications.
Do not stand behind the jacking machine or rod holes during jacking procedures. If a tendon or rod snaps, it will cause serious injury.