430-1 DESCRIPTION
Prestressed concrete members used for structures may be piles, girders, cored slab units, box beams, or deck panels. Girders are used to support the bridge decks. Cored slabs and box beams are positioned side by side to form a deck with shear keys on the vertical faces that are grouted after sections are placed in final position. A bituminous wearing surface or concrete overlay is placed over the completed cored slab or box beam deck. Prestressed concrete deck panels may be used on some bridges and are a part of the total deck thickness. Design and erection requirements for the deck panels are covered in the project Special Provisions.
430-2 MATERIALS
Prestressed concrete members are inspected at the casting plant by Materials & Tests Unit’s personnel. In the past, members were stamped with an “Approved” stamp prior to shipping to the project site. Members are no longer stamped; rather, RFID tags are now cast into all precast/prestressed members. Information about each member, including the Alternate ID and if it is accepted or not will be entered electronically by M&T personnel. The project inspector should use the RFID tag to receive the members as well as check that they have been accepted by the plant inspector. Further information and guidance on this process can be found on
M&T’s website.
430-3 HANDLING AND STORAGE
Members damaged while being handled or transported shall be rejected unless they can be repaired to the satisfaction of the Area Construction Engineer. Members are to be handled at the pick-up points shown on the plans unless other methods have been approved in writing. Any cracks or other damage should be called to the attention of the Area Construction Engineer. See
Article 1078-16 for alignment and dimensional tolerances for prestressed units. Materials and Tests personnel typically identify members that exceed the allowable tolerances at the casting plant. However, there are instances that problems are discovered after shipment that result in placement issues. Members exceeding allowable tolerances should not be rejected until specifics are discussed with the Area Construction Engineer.
430-5 BEARINGS AND ANCHORAGES
Steel sole plates shall be set in exact position and full and even bearing will be required on the bearing pad. Prior to setting the prestressed member, grind the galvanized surface of the portion of the embedded plate and sole plate that requires welding. Welding of the sole plate to the embedded plate should be performed as soon as possible along with painting of the welds. Technicians should utilize temperature indicating wax pens to ensure the temperature during welding of the sole plate does not exceed
300° F. Exceeding this temperature during the welding operation could damage the elastomeric bearing pad. In addition, Technicians should verify that the welds and painting of the welds are satisfactory on the interior bents while the Contractor still has easy access to the bent caps.
430-6 ERECTION AND INSTALLATION
Members shall be erected using only the pick-up points designated on the plans or other methods that have been approved in writing.
As soon as prestressed girders are set in position, levels should be run to determine elevations along the tops of girders. Elevations should be recorded in the Structure Workbook and used in determining the position of the deck slab form with respect to the top of girder. “Buildups” will need to be determined before the placement of the stay-in-place forms can be installed. (Refer to the
Engineering Control Section of the Construction Manual for further details and procedures for computing buildups.) In some cases buildups can result in a negative value and if such is the case, the top of girder will project into the floor slab. If this occurs, the Area Construction Engineer should be consulted.
When precast concrete deck panels are used to form the bottom of the deck slab, check to ensure that the minimum clearance under the deck panels can be obtained and for adequate cover over the top mat of reinforcing steel. If the minimum clearance under the deck panels, or the plan reinforcing steel cover cannot be obtained, consult with the Area Construction Engineer.
As cored slab and box beam units are set, error can grow due to irregularities in the sides of the units and unit sweep. the best practice is to begin setting individual units from the center, out to the edges. This way the error mentioned above is split between the two directions and does not all accumulate on one side of the structure. A video is available at the NCDOT YouTube channel regarding Setting Cored Slabs.
When prestressed cored slabs and box beams are utilized, they are required to be tensioned together with 0.6" diameter strands. when tensioning the strands, a double acting jack must be used and the strand shall be tensioned along the same axis as the strand. This type of jack allows the strand to be tensioned to the correct value, followed by a secondary mechanism that seats the wedges. The wedges grip the strand and allow the jack to be removed, leaving the tension in the strand. Offset or reaction frames are no longer required because the jack does not push against the wedges until after proper tension is achieved. During jacking, monitor the slabs for uplift at the bearings. If this occurs, release the tension and contact the Area Construction Engineer. Extreme caution should be utilized around the jack during tensioning. Never stand at the end of a strand due to the possibility of it accidentally releasing and shooting out like an arrow. A video is available at the NCDOT YouTube channel on the subject entitled Mono Strand Jack.
Box beams are similar to cored slab units but are deeper in section, therefore resulting in longer span lengths. Because of the depth, box beams have strands on the top and bottom half of the beam. During tensioning of the strands, a symmetrical tension force between each pair of strands must be maintained. This can be achieved with two jacks tensioning simultaneously, or incremental tensioning of the top and bottom strands with a single
jack.
After tensioning of the cored slabs or box beams is completed shear keys, dowel holes, and recesses at the ends of the transverse strands shall be filled with
Type 3 Grout. Water shall be added to the non-shrink grout in strict accordance with the manufacturer’s recommendations. The required compressive strength of the non-shrink grout is
5000 psi in
three (3) days. However, acceptance testing of the grout will be based on 14 day strengths. Three (3) day early breaks can be made to determine if the contractor can place construction equipment on the bridge prior to 14 days. The grout shall be satisfactorily water cured until the required strength is met. When filling the shear keys, the Technician should visually check under the cored slabs or box beams to ensure the grout does not leak between the slabs. If grout leakage occurs, the material should be removed immediately due to the difficulty removing when set has occurred. Prior to placing loads that exceed the legal limit on the tensioned and cured cored slab and box beam spans, the Contractor shall submit for approval a detailed description of the equipment along with his intended protection of the slabs while the equipment is on the structure. A video on the subject is available at the NCDOT YouTube channel entitled Grouting Cored Slabs and Box Beams.
430-7 PAINTING
All non-galvanized steel surfaces such as tie rod ends, field welds, and galvanized surfaces damaged by field welding or otherwise shall be cleaned and painted with two coats of an approved organic zinc rich repair paint meeting the requirements of
Article 1080-7 of the Specifications. This paint shall be applied only by brush to ensure a minimum dry thickness coat of paint of
1.5 mils per coat. The paint must be stirred continuously to avoid the settling of the zinc.
ADDITIONAL RESOURCES
