Design Guide for Precast UHPC Waffle Deck Panel System, including Connections
CHAPTER 4. DECK REPLACEMENT
Over the life span of a bridge, it becomes necessary to replace or otherwise modify the existing/original deck. Possible reasons for replacing an existing bridge deck include structural deficiency due to normal wear and tear, concrete deterioration due to freeze-thaw cycles, and damage resulting from penetration of de-icing chemicals (particularly in the midwest and northern States that experience seasonal freezing), as well as other environmental factors or other functional considerations such as bridge widening to accommodate increasing traffic demands.
Although the bridge deck may be in need of rehabilitation/replacement, often other elements of the existing superstructure remain robust and capable of performing adequately for the remaining service life of the bridge. Therefore, an interesting challenge is presented in which the bridge deck needs to be removed and replaced while the supporting girders and piers receive little to no modification or enhancement. This procedure is known as deck replacement, or redecking.
This chapter addresses deck replacement procedures when UHCP waffle deck panels are to be used to replace existing CIP concrete bridge decks.
Benefits of Precast System
Traditionally, US highway agencies have used CIP concrete to build new bridge decks or replace deteriorating decks. However, there are drawbacks with CIP construction associated with construction time, including the time needed to cure the concrete, costs resulting from false work requirements, traffic disruption, worker safety, and so forth. Full-depth, precast concrete bridge deck panels provide an efficient alternative to CIP deck replacement and offer some desirable characteristics to help alleviate the drawbacks associated with CIP construction.
For example, traffic disruption can be reduced because precast components make it possible to perform repair/replacement work only at night or on weekends, leaving the bridge in full service during peak traffic hours. This ability may also help improve work zone safety by reducing the number of construction personnel exposed to moving traffic and the amount of time that they are exposed to it.
In addition, when full lanes of traffic need to be closed for work, staged construction can be utilized, leaving the remaining lanes open. Furthermore, construction time and costs are decreased due to reduced formwork requirements and concrete setting time, and higher quality can be achieved due to panels being cast in controlled environments.
Because of these benefits, interest in full-depth, precast bridge deck panels has been increasing steadily in the past few decades. In fact, more than 15 States have successfully used full-depth precast bridge deck panels in deck replacement projects and new construction on both concrete- and steel-girder bridges.(55)
That notwithstanding, the use of precast deck panels does not come without its limitations. Unlike CIP construction, precast construction may introduce fabrication and transportation challenges. These challenges include not only cost, but also many logistical considerations. In addition, due to the modular nature of precast construction, many connections and joints are obviously present in the bridge-deck panel system. These connections present increased opportunity for leakage and water, salt, and chloride ingress into the deck panels, creating significant potential for corrosion and degradation of the bridge deck. The concrete deck panels can also crack under service loading, presenting even more durability concerns.
Due to its durability properties, the use of UHPC as a primary material in precast bridge deck panels eliminates several of the concerns associated with conventional precast concrete deck panel construction. The use of UHPC panels realizes all of the advantages associated with precast construction, in addition to a few others.
The superior structural characteristics of UHPC compared to those of normal concrete or HPC present an opportunity to decrease panel sizes considerably. This, along with the possibility of using a waffle configuration, results in lighter panels, which are handled more easily by construction equipment while decreasing the dead load on the structure, improving overall structural efficiency.
The higher strength panels may also undergo less cracking than conventional concrete panels, increasing durability performance. As mentioned in chapter 3, the use of UHPC as a fill in the joints can result in simplified, constructible connections with exceptional long-term durability.
Several procedures are typical for most bridge redecking operations. These common procedures follow and assume a CIP existing deck removal replaced with precast deck panels:
- Removal and disposal/transportation of existing CIP deck.
- Removal of existing horizontal shear connections to beams/girders.
- Preparation of existing girders for new horizontal shear connection details.
- Design and fabrication of deck panels.
- Transportation of deck panels to site.
- Placement of deck panels onto existing bridge structure.
- Grouting of transverse connections.
- Placement of shear connectors in panel blockouts.
- Longitudinal post-tensioning and, if required, transverse post-tensioning of deck panels.
- Grouting of panel blockouts to engage horizontal shear connectors.
- Closure and expansion joint pours.
- Barrier and overlay pours if needed.
Most precast panels used in deck replacement scenarios are also prestressed panels. Often, the panels are pre-tensioned transversely during the precast fabrication process, transported to the site, and post-tensioned longitudinally once they are placed on girders.
The advantages of prestressing are well known and include increased member flexural and shear strength, as well as the potential to eliminate/reduce cracking and enhance durability performance. The post-tensioning procedure, in particular, creates continuity between adjacent deck panels along their transverse joint interfaces, enabling transfer of moments across the connections. However, post-tensioning is costly and labor intensive and, as before, left out here in favor of more economical and construction-friendly connections.
Design and analysis of the deck panels, as well as the longitudinal and transverse joint connections between adjacent panels, is an obvious step prior to undertaking the deck replacement procedure and is presented in accompanying sections of this document (see chapters 2 and 3).
A critical aspect of the deck replacement procedure is providing horizontal shear connections between the existing superstructure and the new prefabricated deck panels capable of ensuring composite behavior usually assumed in design. The remainder of this section discusses some of the standard procedures outlined above and how they can be used in installing prefabricated UHPC waffle deck panels. It also provides details and recommendations for ensuring composite behavior between girders and waffle deck panels through horizontal shear connections.
Existing Deck and Shear Connection Removal
Prior to placement of the new precast deck panels, the existing CIP concrete deck must be separated from the existing structure and removed. If the existing deck was made to be composite with the superstructure via shear connectors, these connectors must be located and the concrete around them removed. Due to the nature of CIP construction, the existing deck must be cut/sawn into sections of limited size to facilitate removal. This should be done with equipment capable of removing the existing deck without damaging the superstructure.
Sawing/cutting is recommended over jackhammering because of previous cases in which the top flanges of prestressed girders have been damaged during the deck removal process. (56) In addition, the saw-cutting procedure should be monitored closely to avoid cutting through steel girder flanges by using improper blade depths.
Next, the existing shear connectors must be removed, as necessary, to avoid interference with the new precast deck panels. In many cases, complete removal of all existing shear connectors is warranted. For steel-girder construction, horizontal shear connectors are present in the form of headed shear studs. These studs should be removed by torch cutting them close to the base along with subsequent grinding of any remaining connector material. In concrete-girder construction, typical horizontal shear connection is in the form of conventional reinforcement left projecting from the top of the girder and then cast into the CIP deck. This reinforcement should be torch cut as close as possible to the top of the girder.
Upon removal of existing shear connectors, the top of the girder should be cleaned thoroughly to remove any remains of the old deck, regardless of steel or concrete girders. Figures 67 and 68 show the removal of an existing CIP concrete deck. Figure 69 illustrates the substantial amount of reinforcement that must be removed in deck replacement operations.
Horizontal Shear Connection Details
The horizontal shear strength at the interface between two interconnected elements is of primary importance to provide composite action. Between bridge decks and the supporting structure, horizontal shear connectors are provided, enabling composite behavior between the girder and deck slab system and preventing the separation of the girder and the slab.
Typically in CIP concrete construction, the shear connectors are distributed more or less uniformly over the length of the girders and the concrete is then cast over top. However, due to the nature of precast bridge deck panel construction and installation, this procedure is not practical. Instead, in precast construction, the deck panels are fabricated with blockouts, or pockets, at prescribed intervals and clusters of shear reinforcement are provided within these pockets, which are subsequently grouted to provide connections between the panels and girder. These pockets are illustrated in figure 70, which shows a deck panel placed on steel girders and resembles the operation adopted for installing the UHPC waffle deck in the field (see figure 5).
Composite action is created after the grout in the pocket gains adequate strength. A few options are available for providing horizontal shear connectors in girder bridges and are determined mostly by whether steel or concrete superstructure girders are present. Some recommendations and discussion of each follows.
In steel-girder bridges, the most commonly used option is to provide connections between the girders and deck system using headed shear studs welded to the top flange of the girder. These studs are embedded in the deck slab for an adequate height to provide for full anchorage. The Utah DOT recommends using 3/4-inch-diameter studs, while a diameter of 7/8 inches is the largest common size that has been used in highway bridges for the past three decades.(58)
Utah DOT recommends that the length of the stud be determined by the following criteria: the bottom of the head of the connector is at or above the mid-height of the panel while maintaining 3 inches of clear cover or at least four times the diameter. Another study suggests that the studs should be developed at least 5 inches into the slab to prevent pry-out failures.(59)
Given the shear connectors provided for precast decks systems must be located in discrete locations that coincide with the shear pocket blockouts fabricated into the panels, their spacing is considerably larger than in CIP systems, where they are distributed uniformly along the length of the girders. Therefore, to still provide adequate shear resistance at the interface between the deck and girders, the shear studs are clustered within the shear pockets, rather than provided individually. Figure 71 illustrates clustered headed shear stud arrangements in deck panel shear pockets.
The current AASHTO LRFD Bridge Design Specifications (Article 18.104.22.168.2) state that the spacing between shear connectors shall not exceed 24 inches.(27) The specifications do not distinguish between spacing requirements of individual studs and those of clusters of studs. Designers have interpreted this provision as applicable typically to both situations. However, this specification is based on research conducted more than 30 years ago and has been a point of contention recently.(58)
Under NCHRP Project 12-65, researchers conducted tests concluding that the maximum spacing of clusters of studs can be extended to 48 inches for stud diameters of 7/8 and 1 1/4 inches when confinement of the grout surrounding the stud clusters is provided. This spacing has also been implemented successfully in practice during a redecking procedure of the Interstate 90 Door Creek Bridge in Madison, Wisconsin.(53)
It is advantageous to be able to increase spacing of shear connector pockets for a number of reasons: simplify and speed up the panel fabrication process, reduce grout usage decreasing cost and construction time, reduce possibility of water leakage at deck/grout interface, and increase flexibility in layout of required transverse reinforcement. In addition, tests have shown that using 1 1/4-inch-diameter studs in favor of 7/8-inch-diameter studs can reduce the number of studs by about 50 percent and the shear pocket size by 40 percent, further increasing economy. This approach has been used successfully in Nebraska.(58)
For concrete-girder bridges, horizontal shear connection is provided typically by conventional mild steel reinforcement that projects from the top of the girder and into the slab. However, similar to steel bridge shear connections, headed studs can be welded to a steel plate embedded into the concrete beam.
For deck replacement projects, Utah DOT suggests providing T-headed mild steel reinforcement placed in drilled and grouted holes.(61) The recommended bar size is a #6 or equivalent with a head size able to fully develop the bar. Embedment is recommended to be 6 5/8 inches to provide a minimum of 26.4 kip in tension, which is based on fully developing the yield strength of the bars in tension.
The shear connectors need to be embedded far enough into the drilled/grouted holes as well as the precast deck panel to develop the full tensile strength of the reinforcement based on the shear friction theory of horizontal load transfer. A Virginia Tech study found that single-leg, post-installed rebar was a very convenient type of connector when using epoxy to fill the drilled holes.(59) In this study, #5 bars were embedded 5 inches into the drilled/epoxied hole and 5.5 inches into a 5,000 psi precast concrete deck; #6 bars were embedded 6.5 inches into the drilled hole and 5.25 inches into the precast deck. Figure 72 shows a connection configuration that can be used when placing precast bridge deck panels on concrete-girder bridges.
Use of UHPC Waffle Deck
Following surface preparation and installation of connection elements such as shear studs or reinforcement to the existing girders, installation of UHPC waffle deck panels as part of the redecking process can proceed. The process will not be any different than what is recommended for new bridge construction (see chapters 2 and 3). The dimensions of the waffle deck panels and connections need to be designed adequately using the procedures recommended in the previous chapters. During this process, the recommended length for reinforcement and shear studs can be used rather than those recommended for typical full-depth precast concrete panels.