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241 Chapter 8 Flange/Deck Connection: Concept Development of Joint Details for Accelerated Bridge Construction 8.0 Introduction Speed of construction, particularly for bridge replacement and repair projects, has become a critical issue to minimize disruption of traffic and commerce. Promising systems for rapid construction include precast bridge systems fabricated using decked bulb-T (DBT) concrete girders or precast deck panels on girders. This project focused on the development of cast-in-place (CIP) longitudinal and transverse joints between the flanges of the DBTs or between the precast panels. Because of the similarity in these systems, the discussion herein focuses primarily on DBTs which generally have greater constraints on deck thickness than precast panel systems. The bridge deck in DBTs consists of the girder flange, which is precast and prestressed with the girder. DBTs are manufactured in the precast plant under closely monitored conditions, transported to the construction site, and erected such that the flanges of adjacent units abut. Load transfer between adjacent units is provided by longitudinal joints (parallel to traffic direction). Figure 8.0.1 shows a DBT bridge being constructed. Figure 8.0.1: A DBT concrete bridge being constructed Depending on the specific site conditions, the use of other prefabricated bridge systems can also minimize traffic disruption, improve work-zone safety, minimize impact to the environment, improve constructability, increase quality, and lower life-cycle costs. This technology is applicable and needed for both existing and new bridge construction. Over the past 50 years, thousands of short to medium span bridges have been built using precast concrete elements. There are a large number of papers on the use of precast concrete
242 elements in bridge systems including research on the use of precast elements such as deck panels for rapid deck replacement. There are both longitudinal joints and transverse joints in prefabricated bridge systems. The DBT bridge system eliminates the time necessary to form, place, and cure a concrete deck at the bridge site. In addition, the wide top flange provided by the deck; improves construction safety due to ease of installations, enhances durability because the deck is fabricated with the girder in a controlled environment, and enhances structural performance with a more efficient contribution of the deck in stress distribution. Despite the major benefits of this type of bridge, use has been limited to isolated regions of the U.S. because of concerns about certain design and construction issues. One of the hurdles that must be overcome to enable a wider use of this technology is the development of design guidelines and standard details for the joints used in these systems, which must produce full strength joints, but still allow for accelerated construction. Figure 8.0.2 shows a typical DBT bridge consisting of five DBTs connected by four longitudinal joints with welded steel connectors and grouted shear keys (Stanton and Mattock 1986, Ma et al 2007). In order to reduce the total DBT weight, the thickness of the deck is typically limited to 6 in. Welded steel connectors are typically spaced at 4 ft. To make the connection, as shown in Figure 8.0.2, two steel angles are anchored into the top flange of the DBT and a steel plate is welded to steel angles in the field. Between two connectors, a shear key is provided at the vertical edge of the top flange. Grout is filled into the pocket of the connector and in the voids of the shear key to tie the adjacent girders together. A joint backer bar is placed at the bottom of the shear key to prevent leakage when grouting. Figure 8.0.2: A typical DBT bridge connected by longitudinal joints with welded steel connectors The typical longitudinal joint shown in Figure 8.0.2 has the strength needed to transfer shear and limited moment from one girder to adjacent girders. The width of the joint zone is small so that it facilitates accelerated construction. However, because the welded steel plates are located 4 ft from each other and at mid-depth of the flange, they cannot help to control flexural cracks along the longitudinal joint. Although the performance of this type of joint was reported as good to excellent in a survey of current users, problems with joint cracking in these systems have been reported in the literature (Stanton and Mattock 1986; Martin and Osborn 1983). This joint cracking along with joint leakage is perceived to be an issue Longitudinal Joint Zone (Typ.) DPPCG (Typ.) Bridge Rail (Typ.) DPPCG (Typ.) Bridge Rail (Typ.) Detail A Cross-Section View Plan View Longitudinal Joint Zone (Typ.) Welded Steel Connector Grouted Shear Key 4 feet Top Flange Detail A B B C C Grout Steel Angle Steel Plate Anchor bar (Typ.) B-B Grout Joint Backer Bar C-C 6 in .
243 limiting wider use of this type of bridge. The State of Washington limited the use of DBTs for roads with high ADT and for continuous bridges. As part of a research project to address issues that influence the performance of DBT bridges, a specific objective was defined to develop improved joint details which allow DBT bridge systems to be more accepted as a viable system for accelerated bridge construction. As mentioned in Chapter 1, the focus of NCHRP 10-71 was to develop specifications, guidelines, and examples for the design and construction of durable cast-in-place (CIP) reinforced concrete connections for precast deck systems that emulate monolithic construction, considering issues including speed of construction, durability, and fatigue. This project focused on systems that eliminate the need to place and remove formwork thus accelerating on-site construction and improving safety. The three systems considered in NCHRP 10-71 to accomplish these objectives were identified during the 2004 Prefabricated Bridge Elements and Systems International Scanning tour (International Scanning Study Team, 2005). These systems included: (1) precast composite slab-span systems (PCSSS) for short- to moderate-span structures, (2) full-depth prefabricated concrete decks, and (3) deck joint closure details (e.g., DBT flange connections) for precast prestressed concrete girder systems for long-span structures. Each system uses precast elements that are brought to the construction site ready to be set in place and quickly joined together. Depending on the system, the connections are either transverse (across the width of the bridge) or longitudinal (along the length of the bridge); however, practices differ in detailing the transverse and longitudinal connections. Chapters 2 through 7 summarized the research effort conducted by the University of Minnesota research team, which was associated with the development of design recommendations for PCSSS bridges. These remaining chapters, Chapters 8 through 14, summarize the research effort conducted by the University of Tennessee at Knoxville (UTK) research team, which was associated with the development of longitudinal and transverse connection concepts between full-depth deck panels and decked bulb-T flanges including the development of durable closure pour materials for accelerated bridge construction. As noted in Chapter 1, Appendix A contains the recommended design recommendations including suggested changes to the AASHTO LRFD Bridge Design code and commentary that were developed during the study, and Appendix B contains detailed design examples developed by Roy Eriksson and his team from Eriksson Technologies, Inc. The first two design examples are associated with the PCSSS bridge concepts, and the latter three examples provide guidance on the detailing of longitudinal and transverse joints between precast panels and decked bulb-T flanges. 8.1. Longitudinal and Transverse Connection Concepts between Precast Panels and Decked Bulb-T (DBT) Flanges This section summarizes the conceptual designs proposed and evaluated for the longitudinal and/or transverse connections between full-depth deck panels or deck flanges. Five different connection details were originally presented to phone survey respondents for comment (see Appendix C), the results of which are briefly summarized in Section 8.1.1. Issues considered in finalizing the connection details and description of connection details further investigated in the NCHRP 10.71 study are summarized in Section 8.1.2.
244 8.1.1. Summary of Phone Survey in Association with Longitudinal and Transverse Connection Concepts between Precast Panels and Bulb-T Flanges Survey respondents provided feedback and insight on the longitudinal and transverse connection concepts in general, as well as regarding the five specific joint connection details outlined in a handout that was distributed prior to the phone survey conducted by the research team (see Appendix C). The five connection concepts that were addressed were: loop bar (U-bar) detail, straight bar detail with spiral to reduce lap length, headed bar detail, welded wire reinforcement (WWR) detail, and structural tube detail. In general many respondents considered all five of the connection concepts to be potentially useful in bridge construction, and especially when rapid construction was critical. A common concern regarding the connection of each of the precast elements was that of differential camber, and many respondents who voiced this concern suggested that the use of a steel plate or haunch should be included to assist with the leveling of adjacent precast panels. Many of the respondents preferred the U-bar detail over the other options, and noted that a loop bar detail has been successful in Japan and Korea. Some expressed that although the U-bar detail was the most promising, it could require a thicker deck to accommodate the bend radius of the loop detail, which would add weight to the structure. It was suggested by another, that the key to the U-bar detail would be to obtain a waiver on the minimum bend radius of the looped bar. Experience of a similar detail used in Nebraska four years indicated exceptional performance of the system; it was emphasized that the connection must be either nonshrink or expansive to prevent cracking. Some respondents commented that the loop detail would require perforations in the formwork, and therefore the bar spacing should be standardized as much as possible. The straight bar with spiral reinforcement to reduce the lap length detail was also favored by many of the respondents. A common concern regarding the connection was that it was expected to be more expensive, and would also require additional field labor to complete the connection. In addition, it was suggested that the spiral reinforcement may create alignment issues during construction, which could add to the amount of time required for construction. The headed bar detail was often praised for the fact that it would come to the field site nearly completed, which would reduce construction labor as well as the time required to construct the system. Many respondents conceded little experience with the headed bar details and suggested that testing would be required, though many said that they expected that the detail would work adequately. Some respondents also suggested that the detail may be difficult to fabricate, and that the alignment and placement of the longitudinal steel could be complicated. The welded wire reinforcement detail was generally liked by most of the respondents, especially because the wire reinforcement detail was expected to promote rapid construction in the field. A few respondents voiced concern regarding the ability for the welded wire reinforcement to be adequately developed in the space available. In California, it was noted that welded wire reinforcement was not permitted, and that it may be due to a fatigue concern, though manufacturers are promoting its use. Many of the respondents viewed the structural tube detail as being exceptionally robust, with one respondent describing it as being âbomb proof.â A common concern regarding the structural tube detail was the potential for alignment and other constructability issues. Also of concern was the potential for sloppy field work to degrade the connection, especially if the tube were not correctly and completely filled with grout.
245 8.1.2. Criteria Considered and Finalization of Longitudinal and Transverse Connection Concepts Investigated in NCHRP 10-71 In further developing the connection concepts, the following criteria were considered: ⢠The connection detail should not only be able to transfer shear but also provide moment continuity across the joint. Where possible, two layers of steel should be used in the joint. ⢠The connection detail must allow the precast units to be joined together quickly to minimize disruption to traffic. For the joint connections, it is desirable to minimize or eliminate forming of the joint to expedite construction and reduce cost. Field placement of reinforcement within the longitudinal joint area after erection should be minimized. In joints where forming is required, provide sufficient room to facilitate connection completion and use CIP rather than special grout mixes. ⢠The closure pour (CP) material to precast unit interface is an area of concern for durability. The focus in this area must be on minimizing cracking in this location to reduce intrusion of water that may result in corrosion. Place the reinforcement as close as practicable to the top and bottom surfaces to help control cracking. ⢠Cumulative fabrication and erection tolerances, particularly differential camber in deck flanges, will result in some degree of vertical flange mismatching. A temporary welded connector detail should be considered for leveling flange mismatching before the permanent connection is placed. When selecting CP materials, performance-based specifications for durability need to be developed to proportion concrete mixtures or other grouting materials that are capable of protecting structures against a given degradation for a specified service life in given environmental conditions. To improve the current joint detail (Figure 8.0.2), the proposed new details should control joint cracking better, and maintain the accelerated construction features. One concept was to replace the current welded steel connectors with distributed reinforcement to provide moment transfer as well as shear transfer across the joint. Well-distributed reinforcement can control cracks much better than widely spaced welded steel connectors. However, straight lap-spliced reinforcement requires a much wider joint to develop its strength. It is very important for the proposed joint width to be as narrow as possible. Joint width minimization will reduce the required expensive grout which results in a reduction of cost and faster construction time. As a result, options to reduce the joint width were explored. Such options included bars with hooks (U-bar), bars with headed terminations, and bars with spirals. To allow for accelerated construction, the details were also developed to minimize deck thickness which would reduce the weight of DBT girders. As a result, the U-bar detail and the headed bar detail were selected as the most viable candidates for this research. U-bar details are oriented vertically in the joint to provide two layers of reinforcement fabricated with a single rebar. The U-bars provide continuity of the deck reinforcement across the joint by lapping with the U- bars from the adjacent flanges. The 180o bend of the U-bar, embedded in the joint, provides mechanical anchorage to the detail necessary to minimize the required lap length. The extended reinforcement of the U-bar details is staggered (i.e., out of phase) with the adjacent lapped U-bar to facilitate constructability in
246 the field. The stagger cannot be too large, or the transfer of forces across the joint would be difficult to achieve. To minimize deck thickness, the U-bar detail was designed to utilize an extremely tight bend. The inside bend diameter that was used was three times the diameter of the bar (3db), thus with No. 5 bars used, the inside diameter of the bend was 1-7/8 in. ACI Committee 318-08 (2008) set minimum bend diameters for different rebar sizes and materials. For a No.5 bar made of conventional steel, the minimum bend diameter, per ACI 318-08 (2008), was six times the diameter of the bar (6db), and for D31 deformed wire reinforcement (DWR) the minimum bend diameter was four times the diameter of the bar (4db) when used as stirrups or ties. Clearly the U-bar bend diameter that was used (3db) violated the minimum allowable bend diameters established by ACI 318-08 (2008). The minimum bend diameters were established primarily for two reasons: feasibility of bending the reinforcement without breaking it and possible crushing of the concrete within the tight bend. To ensure that the reinforcement would not be broken while bending, two ductile reinforcing materials were used: deformed wire reinforcement and stainless steel reinforcement. Concrete crushing in the tight bend was closely observed in the experimental investigation to determine if it would occur. As an alternative to the U-bar details, two layers of headed bars were considered to provide continuity of the top and bottom deck steel through the joint. The previous NCHRP 12-69 project explored the use of single large-headed bars to provide continuity across the joint (Li et al. 2010, Li et al. 2010a). In that project, Headed Reinforcement Corporation (HRC) provided the headed reinforcement, which consisted of a No. 5 bar with a standard 2 in. diameter circular friction welded head with a head thickness of 0.5 in. Large- headed bars such as these with the bearing area (Abrg) exceeding nine times the area of the bar (Ab), are assumed to be able to develop the bar force through bearing at the head. Bars with smaller heads, (e.g., Abrg/Ab⥠4) are assumed to be able to develop the force in the bar through a combination of mechanical anchorage and bond, where the development length for these bars is less than that required to develop a hooked bar (ACI 318-08). In the current study, the headed reinforcement used was No. 5 bar with Lenton Terminator® bearing heads. The diameter of the head was 1.5 in., and the thickness of the head was 7/8 in., which gave Abrg/Ab of 4.76. The smaller head dimension was necessary in order to fit the two layers of reinforcement within the deck while minimizing the deck thickness. The large-headed bars in two layers would have resulted in a much thicker, uneconomical deck system. Both longitudinal joints (parallel to the traffic direction) and transverse joints (perpendicular to the traffic direction) were designed and tested in the NCHRP 10-71 study utilizing each joint detail. Both joint directions were investigated so that the results of this experimental program could apply to several precast deck systems (e.g., DBT systems and full-depth precast deck systems). Figure 8.1.1 shows the two joint directions tested and the specimen orientations used to represent the joints.
247 Figure 8.1.1: Orientation of joints and corresponding test specimens 8.2. Organization of Report Regarding Investigation of Longitudinal and Transverse Connection Concepts between Precast Panels and Bulb-T Flanges Chapter 9 describes the process of selection of trial longitudinal and transverse joint systems and the laboratory testing of trial selected joints. The performance of different joint details was compared in Phase I experiments. In Phase II experiments, additional tests were conducted on the most promising connection detail from Phase I to investigate parameters including overlap lengths, rebar spacings, and concrete strengths. In all of these tests conducted to select the most viable joint details, the details were cast in monolithic concrete specimens. Chapter 10 presents the parametric studies conducted to determine live load forces to be applied in the connection tests. Parameters investigated included girder depth, girder span, girder spacing, single- and multi-lane loading, skew, diaphragm spacing and stiffness, and location of wheels relative to the joints in the precast deck for the panel-to-panel and/or flange-to-flange connections for fully continuous transverse or longitudinal deck behavior. Chapter 11 describes the selection of two closure pour materials (overnight and 7-day cures) based on the specified performance criteria for freeze thaw, shrinkage, bond strength and permeability. Chapters 12 and 13 summarize the slab tests to determine serviceability (including crack control), static load strengths, and fatigue characteristics for the selected longitudinal and transverse joints, respectively, fabricated with durable grout material in the connection region, selected through research described in Chapter 11. A brief summary and conclusions are provided in Chapter 14.
