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9 2.1 Introduction The goals of this project were as follows: 1. Develop a full-depth precast concrete deck panel system with simplified connection details. The connection details include panel-to-panel and panel-to-girder connections that, specifically, â Provide satisfactory composite action with the supporting girders; â Meet the FHWA goals of ABC. Minimal construction steps, duration of each step, and safety risk to construction workers are of utmost importance; â Can be assembled from the top of the bridge deck; â Require simplified or no grouting between the precast deck panels and the supporting girders; â Keep the fabrication of the precast deck panels as simple as possible by minimizing shear pockets, minimizing cold top-surface construction joints, and relaxing tight tolerances in achieving satisfactory interface shear behavior; â Can be inspected for quality assurance during various stages of construction; and â Require minimal long-term maintenance. 2. Establish analytical procedures to investigate various concepts, such as constructability and structural behavior of the total system under various stages of the bridge life, including overloading. 3. Develop an experimental program to examine the various concepts to validate the analytical program and to select the system (or systems) to be retained for finalization. 4. Conduct more detailed analysis and testing of the developed system to establish design and construction guidelines. 5. Develop sample guidelinesâspecifications for the developed system that are validated by the analytical and experimental programs. 6. Develop a draft of proposed revisions to the AASHTO LRFD Bridge Design Specifications provisions to encourage widespread use of the research results. As a result of the research conducted in this project, it has been possible to develop a full- depth precast concrete deck panel system with shear connection spacing of up to 6 ft. Therefore, the shear pockets exist only in the transverse joints between the panels if the panels are made 6-ft long in the direction of traffic. However, the panels can be made as long as 12 ft, which generally is considered the maximum allowed dimension for shipping without special permit in most of the United States. In this case, an intermediate pocket would be required at a spacing not exceeding 6 ft. Panel width can be as much as the total bridge width or a partial bridge width for relatively wide bridge decks. The deck slab can be solid, or it can have a variable depthâthat is, ribbedâto reduce deck weight and, thus, increase allowance for additional loads in situations where such an upgrade is C H A P T E R 2 Research Goals and Approach
10 Simplified Full-Depth Precast Concrete Deck Panel Systems desirable. The deck system is pretensioned transversely and post-tensionedâor conventionally reinforcedâlongitudinally. The research was conducted in such a way that only the joints (pockets)âwhere the interface shear connectors are locatedâare required to be grouted. Such a system is referred to as a system with âdiscrete jointsâ between the deck and the girders. The haunch between girder and the deck may be left unfilled in the gaps between shear pocket joints. This option was used in the research to allow for both choices, regardless of whether the haunch was filled. The discrete joint system would reduce the labor and materials needed to fill the haunch. In addition, blind grouting of the haunch and associated questionable quality would be eliminated. The discrete joint system is similar to the system used in the Suehiro Viaduct at Japanâs Kansai International Airport Line (Matsui et al. 1994, Manabe and Matsui 2004). However, unlike the Japanese project, the discrete joint system developed in this project provides for composite action between the deck and the girders. However, there is some benefit to grouting the haunch. This research team is not opposed to doing that. Grouted haunches protect the space between the girders and the deck from being filled with foreign and unwanted material. After numerous trials, the research team reached a conclusion that the best material for grouting the shear pockets is ultra-high-performance concrete (UHPC) (Graybeal 2013, Graybeal 2014). Further, if the owner prefers not to introduce longitudinal post-tensioning in the field, the transverse joints should be filled with UHPC and reinforcing bars should project into the joints, as has been recently employed by New York and other states. If the deck is longitudinally post-tensioned, the transverse joint can be made much narrower, as it would not require space for splicing reinforcing bars, and it would be filled with nonshrink grout that has a compressive strength similar to the precast deck concrete. This would be much less expensive than UHPC. The innovative duct-in-duct post-tensioning method is proposed. The larger duct may be simply made of polyvinyl chloride (PVC) tube. As described in Chapter 3, the exterior tube need not extend beyond the formed edge of the panel. Once all the transverse and shear pocket joints are filled, preinstalled full bridge-length greased strands in rubber tubes are then tensioned to the required prestress. Thus, all grouting is done in one stage to simplify construction, and no duct couple or large coupling pockets are required. Analysis has shown that the loss of post-tensioningâtransferred from the deck to the girder because of the full connection before post-tensioningâis insignificant. The goals of the project were achieved by executing the following steps: 1. Develop a full-depth precast concrete ribbed slab deck panel system. 2. Analytically investigate performance of the system. 3. Experimentally investigate the behavior of the beamâdeck connection using push-off specimens. 4. Investigate the behavior of the full beamâdeck system using full-scale composite beamâdeck specimens. 5. Develop design examples and guidelines to facilitate implementation of the proposed system. 2.2 Precast Deck Panel System with Longitudinal Post-Tensioning An 8.5-in.-thick ribbed panelâas shown in Figure 2.1 and Figure 2.2âis proposed. The ribbed panel has longitudinal ribs between adjacent girder lines, spaced at 3 ft or less. In addi- tion, there are longitudinal ribs directly over the girder lines. If necessary, the panel overhangs
Figure 2.1. Plan view of the 6-ft- and 12-ft-long panel.
Figure 2.2. Precast deck panel system: Section A-A, B-B, and C-C.
Research Goals and Approach 13 are expected to be made of full 8.5-in. thickness to resist impact loading on the bridge railing (Precast/Prestressed Concrete Institute 2011A). The longitudinal ribs and the solid overhang help eliminate the potential for torsional stresses caused by moving truckloads. The precast concrete panel was designed with 6-ksi-specified concrete strength at service. The panel can be made 6-ft long to avoid in-panel shear pockets. Alternately, the panels can be 12-ft long with shear pocket at mid-length (6-ft) spacing, as shown in Figure 2.1. Depending on the bridge design, other panel lengths are possible. This project gives evidence that connection spacing can be as large as 6 ft, but it does not dictate the length of each panel or whether each panel is a ribbed slab or a solid slab. Figure 2.2 and Figure 2.3 show the cross sections of the panel, which has a shear key at the transverse joint. At the top, the shear key has a 2.5-inch-wide gap between panels to fill the transverse joint with grout. If the deck is not post-tensioned and a UHPC conventionally reinforced joint is desired, the shear key is intentionally roughened by applying a retarder to expose the aggregates and enhance the bond between the panel and the grout, according to Iowa DOT (Wipf et al. 2011). Feedback from the precaster of the panels used on this project indicated that a simplification could be achieved by reducing the projection of the bottom lip of the shear key because that length is relatively thin and could crack during handling and shipping of the panels. The research team agrees with this improvement and recommends a modified shear key geometry, as given in Figure 2.2. The precast panel is pretensioned in the transverse direction. Experience has shown that about 250-psi average effective prestress produces adequate capacity in this direction for handling and service loads. Additional reinforcement is needed to help distribute wheel loads to the transverse ribs. The longitudinal post-tensioning system is made of sheathed 0.6-in.-diameter post-tensioned tendons placed in larger PVC tubes. The post-tensioned tendons are designed to introduce a minimum average compressive prestress of 250 psi at the transverse joints. The net stress after allowance for the effects of superimposed loads in continuous span bridge decks should be designed to be less than the limit permitted by the AASHTO LRFD Bridge Design Specifica- tions. For continuous spans expected to have negative moments at the piers, the post-tensioned compressive stress may be as large as 600 psi to 700 psi. The tendons are unbonded seven-wire strands coated with corrosion-inhibiting grease, regardless of whether transverse joint grout flows into the outer PVC ducts. The strands are continuously encased in polyethylene sheathing, according to ASTM A416 and as shown in Figure 2.4. Thus, post-tensioning is guaranteed to extend for the full length of the tendons and across the transverse joints, even after all joints are filled with grout. The strands would have an excellent multilayer corrosion-protection system. Traditional metal or plastic corrugated duct materials have traditionally been used and can still be used if the designer desires. However, a simple, locally available PVC tube can be used, as its function is only to create space for the fully encapsulated groups of strands to go through the deck end-to-end. This system of unbonded tendons was successfully used on the Arbor Rail Line Bridge replacement in Nebraska City, Nebraska (Hennessey and Bexten 2002), and later on bridges in Oregon and Michigan. UHPC is recommended to fill the shear pockets. In addition, it can be used to fill the transverse joints if no longitudinal post-tensioning is employed. The proposed minimum compressive strength at the time of grinding the joint and opening the bridge to traffic is 12 ksi. The UHPC mix used for this project was supplied by Lafarge North America. While able to use custom-designed mixes with local materials, the research team opted to use the bagged mix from this experienced supplier to minimize variability in the project. Future users of the system may opt to develop their own mixes, as long as the material meets the UHPC criteria
Figure 2.3. Precast deck panel system: Section D-D on concrete and steel girders.
Research Goals and Approach 15 set by FHWA (Graybeal 2013, Graybeal 2014). If a flowable fill is used to fill the haunches over the girders, holes in the panel would be needed to place the grout and vent the air. The two cases for the interface shear pockets considered in this project were 1. Panel-to-concrete girder connection (Figure 2.5): An innovative shear connector assembly was developed for this project. Based on numerous trials and iterations, the new CDR con- nection hardware was developed in this research. It consists of two 1Â½-in.-diameter (ASTM A193 B7: minimum specified yield strength = 105 ksi, minimum tensile strength = 125 ksi) threaded rods that are connected with a collar and a Â½-in.-thick horizontal top plate assem- bly. The collar is made of two tubes (outer diameter = 2 in., inner diameter = 15â8 in.) welded to a Â½-in.-thick vertical plate. The collar assembly is 7-in. tall and is embedded 3â4 in. into the top flange of the concrete girder, as shown in Figure 2.5. A 3-D print of the shear connector assembly was made to help understand and communicate its features, as shown in Figure 2.6. Note that the CDR is placed in the precast girder before girder concrete is placed. The two threaded rods are made 12-in. long to allow for field cutting to proper length and for haunch variability. Additional No. 4 rebars were added at the base of the collar in the girder top flange to pro- tect the concrete from crushing because of the high bearing stress produced by the threaded rods and the collar assembly, as shown in Figure 2.5. Nonlinear finite element models were developed to examine the behavior of this unique collar assembly, and the results are shown in Figure 2.7. As can be seen, the collar helps reduce the bending of threaded rods and the stress concentration in the grout. 2. Panel to steel girder connection (Figure 2.8): The shear connector assembly consists of nine 1.0-in.-diameter studs (ASTM A108, Fy,min = 51 ksi, Fu,min = 65 ksi). The studs are welded to the top flange of the steel girder. Transverse and longitudinal spacing between the studs satisfy the minimum spacing requirement given by the AASHTO LRFD Bridge Design Specifications of four times the stud diameter. Although the testing was performed on 1-in.-diameter studs, it can be shown analytically that Â¾-in.-, 7â8-in.-, and 1Â¼-in.-diameter studs can be used, as long as the stud cluster does not exceed nine studs. If a larger number of studs in a cluster is necessary, it is recommended that further experiments be undertaken to determine the group effect. The precast panel is provided with additional reinforcement around the pocketâas shown in View C-C of Figure 2.5 and Figure 2.8âto help confine the concrete in this zone of high-concentrated shearing force. Figure 2.4. Unbonded seven-wire strand coated with corrosion-inhibiting grease and encased in polyethylene sheathing.
16 Simplified Full-Depth Precast Concrete Deck Panel Systems Figure 2.5. Details of the panel-to-concrete girder connection.
Research Goals and Approach 17 (a) Components (b) Full assembly Figure 2.6. A 3-D print of the shear connector assembly for concrete girders.
18 Simplified Full-Depth Precast Concrete Deck Panel Systems Assembly with collar Assembly without collar (a) Von Mises stresses in the grout around the shear connector assembly (with collar, smaller red area is in high stress). (b) Von Mises stresses in the shear connector assembly (with collar, smaller red area of the stud is in high stress). (c) Von Mises stresses in the shear connector assembly at the girderâhaunch interface (with collar, stresses in the rods are smaller). Figure 2.7. Von Mises stresses in the grouted shear pocket and the shear connector assembly.
Research Goals and Approach 19 Figure 2.8. Details of the panel-to-steel girder connection.
20 Simplified Full-Depth Precast Concrete Deck Panel Systems Figure 2.9. 3-D rendering of the complete deck. Since the width of the transverse joints is narrow and the shear pockets are covered, the complete deck has a uniform width joint, as shown in Figure 2.9. 2.3 Precast Deck Panel System with Conventional Longitudinal Reinforcement The proposed system can be used without longitudinal post-tensioning. Figure 2.10 shows the plan view of the panel. The longitudinal reinforcement of the panels would be coupled rebars across the transverse joints. The top layer of rebar is made of No. 5 at 10 in. in the 5-in.-thick slab. The bottom layer is made of two No. 6 rebars at each of the longitudinal ribs, which are spaced at 3 ft or less, as shown in Figure 2.11 and Figure 2.12. Top and bottom layers are provided with two 3â8-in. and 2-in. concrete covers, respectively. Additional longitudinal reinforcement over the piers in continuous span bridges may be required by design. The longitudinal reinforcement extends into the transverse grouted joint, as shown in Figure 2.13. Because of the use of UHPC, the length of the joint to produce full continuity in the bars is greatly reduced compared with conventional concrete or nonfiber reinforced grouts. To avoid interference between the spliced bars, the bars may be bent horizontally at 15 degrees or staggered in production. A similar splicing concept has been found to provide full development of the spliced bars by FHWA (Graybeal 2014). Figure 2.13 shows a plan view of the spliced longitudinal reinforcement bars in the panel-to-panel joint.
Research Goals and Approach 21 B Figure 2.10. Plan view of the precast panel with conventional reinforcement (for clarity, reinforcement is not shown). Figure 2.11. Precast panel with conventional reinforcement: Section A-A.
Figure 2.12. Precast panel with conventional reinforcement: Section B-B.
Research Goals and Approach 23 Figure 2.13. Splice of longitudinal rebars at the panel-to-panel joint.