Design Guide for Precast UHPC Waffle Deck Panel System, including Connections
As described in chapter 2, full-depth, precast, UHPC waffle deck panels, which exhibit significant advantages over cast-in-place (CIP) bridge deck construction, can be adequately designed to meet design loads stipulated by the AASHTO LRFD Bridge Design Specifications.(27) However, as shown in previous studies on precast deck systems, the short-term and long-term performance of bridges constructed using these deck panels will be influenced by the quality of the connections among the panels (i.e., panel-to-panel connections in both longitudinal and transverse directions) and with panels to girders.(5) Hence, for successful broader application of UHPC waffle deck panels in bridges, increased emphasis must be placed on field-installed connections between deck panels and supporting girders in both longitudinal (parallel to traffic) and transverse (perpendicular to traffic) directions.
In current construction practice, these field-installed connections between deck panels and girders require interlacing of connecting elements such as mild steel reinforcement or headed shear studs extending from the supporting girders and deck panel elements. These approaches often cause fit-up and tolerance issues in the field. The problems can be exacerbated when pre- or post-tensioning is used in both directions of the panels for design. In addition, some studies have reported durability challenges due to field casting of cementitious grout used conventionally in these connections.(5,16)
The combination of these challenges reduces the performance of the connections and, therefore, reduces the performance of the precast deck panel system as a whole. Hence, to make precast UHPC waffle deck panels an economical, attractive, and long-lasting solution for accelerated construction, appropriate connections for the UHPC waffle deck panels must be established. In this context, use of post-tensioning technology is avoided, as this adds additional material and field labor costs. In addition, due to its superior structural and durability characteristics, and to match the durability performance of the waffle deck panels, UHPC is generally recommended in this guide as a fill in the field-cast connection regions. In particular, UHPC shows exceptional bond when placed against previously cast concrete and reduces development length requirements significantly for conventional mild steel reinforcement (see chapter 1). This allows for the development of simplified, less-congested connection details, which are not only construction-friendly but also provide improved long-term performance.
The next sections present several connection details for panel-to-panel (transverse and longitudinal) and panel-to-girder connections that were developed in earlier research projects and used successfully in practice for precast deck panels. These connection details have been obtained from numerous sources, including research performed under the National Cooperative Highway Research Program (NCHRP), FHWA, and the Precast/Prestressed Concrete Institute (PCI), as well as from experimental testing programs completed at ISU.
It is important to note that, in most cases, these connections were developed primarily for full-depth solid precast panels and did not consider UHPC as the fill material in the connection. Hence, a few of these connections may require modifications to incorporate them in UHPC waffle deck panels so that desirable outcomes can be achieved. In addition, the dimensions and reinforcement details of the connection regions may be decreased by taking advantage of the excellent bond and flow characteristics of UHPC. Where appropriate, comments and suggestions for potential improvement of the connection details presented are provided to enhance the performance, constructability, and/or economy of the connection when used with UHPC for the infill material in the connection regions.
Panel-to-panel connection and deck-level connections are critical to transfer loads between the precast deck panels and to distribute the load to different elements of the superstructure. Panel-to-panel connections are subjected to bending moments and vertical shear forces under vehicular loading. Traditionally, precast deck panel edges are provided with details to form a shear key along the connection when two panels are placed together. This detail prevents the relative vertical movement between adjacent panels under traffic load.
In recent decades, a wide variety of deck level connection designs have been deployed in bridge projects involving full-depth precast panels with substantial variance in observed performance under traffic loads. The connections that perform well typically consist of match-cast shear keys with epoxy adhesive or grouted female-to-female joints with discrete reinforcement, combined with field-cast concrete or grouted together with quality construction. In the grouted connections, the interface bond between the precast elements and the grout can help with water tightness of the connections, thereby improving durability.
From recent research conducted by Aaleti et al. and Marcu et al., it was found that the interface bond between normal concrete and UHPC can be improved significantly by introducing roughness or texture at the connection interface. (44,45) According to these studies, sand blasting, shot blasting, or creating a rough surface texture using form liners are options, which may be given consideration in appropriate panel-to-panel connections, for increasing bond characteristics between normal concrete and UHPC. Based on discussions with representatives from the Iowa DOT and the FHWA, the researchers recommend using details that can utilize field-cast UHPC in connections. Detailed descriptions of a few connections that are appropriate for the waffle deck panel are presented below. In addition, it is recommended that appropriate surface treatment preparation methods should be used to improve the long-term performance of the connections.
This connection detail was developed as part of the FHWA HfL program supporting the development of UHPC waffle deck panel systems for accelerated bridge construction. The details of the connection, which are shown in figure 48, use 1-inch-diameter dowels bars (epoxy-coated or stainless steel), extending from the panels, spliced together with additional transverse reinforcement and filled with UHPC in the gap.
The structural performance of this connection was evaluated under simulated traffic loading as part of an experimental study conducted at ISU. A summary of experimental testing is included in chapter 1. More details about the laboratory and field performance of this connection can be found in Aaleti et al. (39)
In a recent study, Graybeal investigated the structural performance of field-cast UHPC connections representing the transverse and longitudinal connections between precast deck panels and the connections between the top flanges of deck-bulb-tee girders, respectively.(36) The connection details are shown in figure 49.
These connection details consist of headed reinforcement or straight bars or hairpins protruding from the panels into the connection region. The connection region is filled with UHPC mixed on site. The experimental testing of these connections under repeated loading (up to 5 million cycles) has shown the excellent performance and water tightness of the details. These connection details are already implemented by the New York State DOT (NYSDOT) in a few bridges.
In today's practice of using precast deck panels, the connection regions are typically filled with conventional grouts and utilize mechanical connectors (such as couplers) and confinement using steel tubes or hoops to develop the reinforcement in the connection regions. The details of such connections are presented below as connections 3, 4, and 5.
These details were developed primarily for conventional cementitious grouts and HPC precast deck panels. Some of these connection details may be relatively difficult to construct and, therefore, they may not be cost-effective. However, by taking advantage of the excellent bond properties of UHPC (as it is used in the connections instead of grout), these connection details can be modified appropriately to find a use in bridges using UHPC waffle deck panels.
Connection 3, recommended for panel-to-panel connections herein, was developed as part of NCHRP Project 10-71 to develop design and construction specifications for durable CIP-reinforced concrete connections for precast deck systems.(46) The typical details of the connection, which consist of a grouted shear key interlaced with lapped U-bar reinforcement along the length of the panels, are shown in figure 50.
Experimental studies completed on this connection indicated that the proposed longitudinal joint had sufficient strength, fatigue characteristics, and crack control for the maximum service loads determined. Although NSC was used in the connection, the use of UHPC is suggested here. Because of the use of U-shaped bars and UHPC, the length of the connection can be reduced appropriately to a value comparable to connection 1.
A similar connection was developed for the second Strategic Highway Research Program (SHRP 2) Project R04, which aimed to develop standards and codes for accelerated bridge construction by constructing a demonstration bridge. A UHPC transverse deck joint over a pier was the primary focus of a laboratory experiment conducted at ISU due to its location in the tension zone resulting from negative moments. This connection was then employed in a demonstration bridge in Pottawattamie County, Iowa (see figure 51).
Testing this connection revealed that the interface between the UHPC joint fill and HPC deck panels experienced debonding and significant opening under service level negative moments, raising concerns over the durability of the connection. However, similar design details have been used successfully at transverse connections in single-span bridges. A post-tensioned retrofit over a short distance was developed and implemented successfully with adequate prestressing force to eliminate opening at the interface from negative moment experienced over the bridge piers.(47) Instead of the joint, if the waffle panel is positioned with the center of the panel over the piers, the inherent tensile resistance of the panel may be taken advantage of in ensuring adequate performance of this connection without any prestressing.
- If this detail is desired to be used in areas of high negative moment over interior supports of multiple-span bridges, further investigation regarding its applicability in any tension zone should be carried out. One such option would be to use prestressing, and the other is to move the connection away from the top of the pier.
- In typical transverse connection details, the width of the void at the top of the panel can be limited to ˜8db, or 6 inches for #6 bars. This would reduce the separation of the panels to 9 inches at mid-depth.
- The U-bar reinforcement may be replaced with conventional, straight reinforcement due to superior bond strength between UHPC and steel reinforcement and the corresponding reduction in development length.
- If U-bar reinforcement is used in conjunction with UHPC, the length of connection may be reduced appropriately, provided it does not compromise the workability of the connection in the field.
FHWA published Connection Details for Prefabricated Bridge Elements and Systems to promote the advantages of using prefabricated elements and systems in bridges.(48) The document represents the state of the practice with respect to accelerated bridge construction and focuses on presenting quality details for use by bridge engineers in the structure-type study phase of a design project. The remaining details in this section are selected details obtained from this document.
The connection 4 detail is a grouted shear key typical of a transverse joint between adjacent full-depth deck panels from the Iowa DOT. This connection, shown in figure 54, has been classified as Level 2, meaning it had been used on only one project but was found to be practical to build and had performed adequately. (48)
Similar details have been used typically with longitudinal post-tensioning applied across the joint to ensure that the joint remains in compression under the negative live load moment. Although some States have used details without post-tensioning, there have been reports of concerns specific to leakage.(48)
The connection shown in figure 55 is another transverse shear key joint, but includes the post-tensioning details, as illustrated, which were proposed by the NYSDOT.
This connection is classified as a Level 1 detail-one that has been used on multiple projects and has become standard practice for at least one owner/agency. (48) A suggested alternative would be to fill the pocket with UHPC to improve durability of the connection. With this detail, the connection can provide dependable negative moment resistance.
- Numerous shear key geometries have been used successfully in practice. It may be desirable to simplify the shear key geometry from that shown in figures 54 and 55 to reduce fabrication difficulty (see detail in figure 56). This decision should be made on an individual project basis after consideration of the appropriate factors.
- Post-tensioning is often costly and time-consuming and necessitates the contracting of specialized labor, complicating the construction process undesirably. Thus, alternative details based on combining UHPC and mild steel reinforcement should be evaluated prior to selecting such a detail. However, if post-tensioning is deemed necessary, it has been shown to provide excellent crack control and water tightness in panel-to-panel connections, reducing many durability concerns. If post-tensioning is used, a minimum prestress level of 250 psi should be maintained across the joint after losses.(49)
In the connection 5 detail, on one side of the panel, #6 reinforcement is embedded 6 inches into a galvanized, bulged hollow structural section (HSS 4 in. x 12 in. x 3/8 in.). On the other side of the panel, the reinforcement extends 7 1/2 inches beyond the panel. A 1-inch-diameter plastic pipe is attached to the top surface of the HSS tube and is used to fill the tube with grout after installation. The HSS tube has an oversized hole (1.75 inches in diameter) on the free side of the panel to help in installing adjacent panels.
The HSS panels are installed such that the first panel is laid with the HSS exposed and ready to receive the protruding reinforcement from the adjacent panel. After the adjacent panel is laid, the HSS is filled with grout to complete the connection between panel elements. Note that installation of this connection requires tilting of the panels to avoid interference with shear connectors attached to the superstructure. An individual panel and the connection interface between adjacent panels are shown in figure 57 and figure 58.
Use of HSS tubes in the panel was motivated by a tube's ability to confine the grout used conventionally in similar details. Confinement allows a bar to develop its yield strength in a shorter distance than when unconfined. Tests showed that conventional grout was able to fully develop the yield strength of #6 bars in these details. Thus, in light of UHPC's enhanced bond characteristics, bar development should not be a concern in the detail presented.
This connection is described in NCHRP Report 584, Full-Depth Precast Concrete Bridge Deck Panel Systems, which documents a research project motivated by the potential to develop a totally prefabricated bridge construction system.(50) One of the project objectives was to develop connection details for new deck panel systems. The requirements were that the details satisfy the following conditions: high durability, quick construction time, good ride quality, and high structural performance. The research produced two panel-to-panel connection details, one of which is the one presented here as connection 5. (The research also developed one panel-to-girder connection detail each for concrete and steel girder bridges, and these details are presented in the next section of this chapter.)
Although these details were not designed explicitly for use with UHPC, they can be expected to perform as well or better when UHPC is used. Thus, they are presented here as viable options in precast deck construction. For reference, the deck panels for which this connection was developed are 8 ft long and cover the entire width of a 44-ft bridge; the panels have an 8-inch structural depth with an additional 1/4-inch sacrificial layer at the top that allows for texturing of the top surface of the slab (for rideability treatment). These panels are reinforced longitudinally with #6 bars at 13.3-inch spacing.
- Connection 5 would not be suitable for typical waffle deck construction because the specified size of the HSS tubes is larger than typical rib widths in waffle deck panels. However, connection 4 is presented as an option for connecting internal waffle panels to overhang sections that may be solid panels. In this case, the HSS tube would be embedded into a solid overhang panel and would receive reinforcement extending from the adjacent waffle panel. The HSS tube embedment can be achieved with short shear studs anchored into surrounding UHPC in the overhang panel, requiring no reinforcement extending from this panel.
- In addition to filling the tubes with UHPC, the void between panels should be filled to provide protection against corrosion of reinforcement crossing the joint interface.
Connections 6a and 6b
Connections 6a and 6b were designed for the ISU test mentioned previously. In this study, two deck panels spanned the width of the bridge and were supported by five prestressed concrete girders. The two deck panels met at the center girder, requiring a longitudinal panel-to-panel-to-girder connection. This connection can be established using dowel bars extending from the panels and shear hooks protruding from the girder, which are tied together with additional reinforcement placed longitudinally (along the girder length). The void between the panels is subsequently filled with UHPC. This connection detail is shown in figure 59. (Note that the shear hooks protruding from the girder are not shown, for clarity.)
The deck panels are also supported between the edges by prestressed concrete girders. To provide a connection between the panel and girder, shear pockets are provided at intervals along the deck panel and clusters of horizontal shear connectors are left protruding from the supporting girder at these locations. In this connection, shown in figure 60, shear hooks extending from the girder are embedded into UHPC that is poured to fill the shear pocket. (Additional information about shear pockets and horizontal shear connections is included in chapter 4.)
These two connections were designed expressly for use with UHPC fill in the connections and were shown to exhibit excellent performance.
The FHWA developed a novel composite connection detail for joining precast concrete bridge decks to supporting superstructure elements.(51) The effort was focused on creating a simple detail with little to no interference or field fit-up issues, increased long-term durability performance, and reduced aesthetic and rideability concerns. The FHWA designed this connection to engage the sustained tensile capacity of the field-cast UHPC.
The connection at the supporting girder resembles that of traditional CIP deck construction, where the headed studs/reinforcing steel protrude from the girder into the haunch continuously along the member. One difference, however, is that the extension of the height of the connectors is reduced to avoid interference with the precast deck. A continuous channel/void is cast into the underside of the deck panel. The bottom mat of transverse reinforcement crosses this void. This void under the precast deck eliminates the need for full-depth shear pockets; thus, clustering of connectors can be avoided, and the shear connectors can be placed continuously along the girder.
After the precast components are assembled into position, the haunch is formed and UHPC is cast into the void to fill the haunch connection completely. Vent holes can be provided in the deck panels to ensure the UHPC fills the void completely. The research concluded that the connection developed is capable of meeting the requirements associated with horizontal shear connections and ensuring composite behavior, and that it surpassed the performance displayed by a companion conventional test specimen. Figures 61 and 62 show, conceptually, the FHWA-developed connection detail for steel girder and concrete girder superstructures, respectively.
- Roughening the interfaces and surfaces between the precast components enhances the interface resistance to horizontal shear stresses.
- The detail presented above is only conceptual and has not been deployed in actual bridge construction projects. Thus, relevant dimensions, bar sizes, etc. must be determined and evaluated before the connection can be specified confidently in practice.
Connections 8a and 8b
As mentioned in the last section, two panel-to-girder connections (one each for steel- and concrete-girder bridges) were developed as part of the NCHRP 584 research.(50) The first connection detail, shown in figure 63, includes headed shear studs welded to the top flange of supporting steel girders at prescribed intervals, with 1 1/4-inch-diameter studs placed in clusters of eight and spaced at 48 inches.
Partial-depth shear pockets are fabricated in the deck panels at locations corresponding to the locations of stud clusters along the girder, which are recommended to be filled subsequently with in situ UHPC (via a grouting pipe extending through to the top surface of the deck) to complete the connection. These pockets were designed with either an HSS tube or three #6 closed ties to provide confinement and increase the compressive strength of the grout used; because of its superior compressive strength, this may not be necessary when UHPC fill is used.
The 1 1/4-inch studs are used in favor of 7/8-inch-diameter studs to decrease the number of studs per pocket, thus decreasing pocket dimensions. The 48-inch spacing dimension violates the AASHTO LRFD Bridge Design Specifications for stud spacing. However, as part of the research, NCHRP conducted tests that concluded that the maximum spacing of clusters of studs can be extended to 48 inches for 7/8-inch and 1 1/4-inch stud diameters when confinement of the grout surrounding the stud clusters is provided.
This design has been implemented successfully in practice by the Wisconsin DOT during a deck replacement procedure of the Interstate 90 Door Creek Bridge in Madison.(52,53) The connection is shown utilizing partial-depth shear pockets; consequently, additional considerations arise that must be accounted for when evaluating the suitability of this type of shear pocket.
- Confinement of the shear pockets via closed tie reinforcement or an HSS tube can be eliminated when UHPC is used as the fill material due to its increased compressive strength.
- The size and number of horizontal shear studs and their spacing need not be as described above. Individual project requirements dictate the size, number, and spacing needed.
The second detail developed in the NCHRP 584 report calls for clusters of three 1 1/4-inch-diameter double-headed studs spaced at 48 inches, as shown in figure 64. These studs are embedded into the top flange of the concrete girder. The reason cited for using three double-headed studs rather than extending the vertical web shear reinforcement of the girder is to minimize the dimensions of the shear pocket. Similar to the connection to the steel girder, the partial-depth shear pockets are provided in the panel at locations corresponding to the shear studs, and the shear pockets can be filled with UHPC to finalize the connection.
- Confinement of the shear pockets via closed-tie reinforcement or an HSS tube can be eliminated when UHPC is used as the fill material, due to its increased compressive strength.
- Again, the size, number, and spacing of the shear connectors can be modified from those presented, to meet specific project demands.
Figure 65 shows a typical panel-to-girder connection that facilitates horizontal shear transfer and the development of composite action of the deck/girder system. The deck panel is fabricated with tapered shear pockets, or blockouts, at prescribed intervals along the length of the beam. At locations corresponding to the blockouts, headed shear studs are welded to the top flange of the steel beam. After the deck panel is transported to the site and properly set into place and any required post-tensioning procedures have been performed, the blockout is then filled to complete the connection.
Due to its superior characteristics compared to typical grout used to fill the shear pockets, UHPC could presumably result in smaller pocket sizes and/or smaller shear connectors provided per pocket. Additional information on horizontal shear connections between steel girders and precast concrete deck panels is included in chapter 4.
- The wearing course shown in the detail is optional. It may be eliminated based on individual project requirements, as UHPC exhibits unique durability characteristics, including excellent abrasion resistance.
- It may be desirable to provide a tapered blockout as shown to obtain a mechanical bond between UHPC and deck panels in addition to the interface friction between the two separately cast materials.
- In addition, oval-shaped blockouts (in plan) may minimize cracking and could therefore be preferred over typical rectangular blockouts, provided the cost difference is justified. (The additional cost of oval blockouts is usually insignificant.)
Another form of horizontal shear connectors that can be provided in concrete girders, protruding into the precast deck panel to ensure composite behavior, is hooked reinforcement. This reinforcement is usually provided by extending the shear stirrups in the girder out of the top flange, as shown at the top of figure 66. This protruding hooked reinforcement is provided at regular intervals, rather than continuously, at locations corresponding to shear pockets fabricated in the deck panel, similar to the procedure mentioned for the previous two connections.
However, another option is to provide field-installed hooked reinforcing bars in holes drilled into the top flange of the concrete girder and filled with epoxy. This is particularly applicable during deck replacement projects, when an existing CIP concrete deck is removed. When a CIP deck is to be replaced with precast deck panels, it is required to remove all existing horizontal shear reinforcement from the girders (to avoid interference of existing reinforcement with placing the precast deck) and replace it with new shear connectors at the prescribed shear pocket locations during the deck replacement operation. Figure 66 illustrates both of these concepts. The top schematic shows extended shear stirrups, and the bottom one shows field-installed reinforcement.