Deck panels can be used for both new construction and rehabilitation projects. They have been used successfully on deck replacement projects and bridge widening projects. For bridge widening projects, the existing deck needs to be cut back in order to expose the existing deck reinforcing bars so that a simple closure pour can be made to connect the old deck with the new deck.
This depends on the details chosen. One common option is to use longitudinal post-tensioning to connect the panels and to provide net compression in the deck under all service load conditions (including negative moment regions of continuous spans). In this scenario, there are several distinct construction steps that need to be applied in sequence (see answer to sequence question under” Construction” below. Even with this sequence, it is possible to replace a single span deck in one weekend (including old deck removal). Note that this requires the use of high early strength materials for grouting and closure pours. For typical construction projects that are not under a critical time frame, it is safe to assume that even a multi-span bridge can be built in one to two weeks.
There are no national or FHWA standards for deck panels; however there are many states using them. States tend to use proven details from other states; therefore some level of standardization has emerged. Several organizations have developed state and regional standards. These are based on successful details that have been proven in the field. Research reports and the new PCI “State of the Art Report on Full-depth Precast Concrete Bridge Deck Panels” are also a good source of details.
This is a common question that has no easy answer. It depends on many factors including size of project, number of similar projects, details used and access at the site. We have seen some projects cost 50% higher than CIP (small projects with complex geometry), but others be less than CIP (large project with significant repetition). Another factor is the number of projects that are built in a region. As contractors and fabricators become familiar with the process, the level of risk decreases which normally coincides with a decrease in cost.
Many agencies specify overlays for deck panel projects. The overlays serve two purposes. First, they provide a final tolerance adjustment to the top surface of the panels. Even with the best fabrication and construction practices, the final riding surface can be rough. The installation of an overlay will provide a smooth riding surface. Overlays can also be used to provide an additional layer of protection for the deck. Two systems are in use in the US; bituminous concrete pavement with a high quality membrane waterproofing system and thin concrete overlays. Both systems can provide a significant measure of deck protection.
Another option is to use a thin overlay system. This is normally used in conjunction with profile grinding of the deck in order to get a smooth riding surface. Tighter fabrication tolerances are also recommended for this option since grinding is normally limited to approximately ¼”. Match casting or line casting of the panels is recommended to achieve this level of quality.
Roadway crowns complicate the fabrication process. Crowns can be cast into the panels, however this requires a special form that will be custom to each project (as opposed to a simple flat form that can be reused). This will tend to increase costs. If the panels are prestressed, the roadway crown complicates the matter even further.
The most cost effect way to accommodate crowns is to use a simple closure pour at the crown. The connection can be made with simple lapped bars. Hooked bars can be used for narrower gaps. Closure pours offer another advantage in that they allow for more adjustment of the panels in the field.
There is no standard panel size. Maximum panel dimensions are a function of shipping and handling. Eight to ten foot wide panels are common. A reasonable maximum length of panel is 40 feet. The maximum length of panels is somewhat controlled by the length of flat-bed trucks used to haul the panels. Longer panels may require the use of special cradles in order to prevent cracking during shipping. Longer panels will also require the use of special lifting hardware that may include spreader beams and multiple slings. If the bridge deck being constructed is very wide, a simple closure pour is suggested between adjacent panel groups.
There are no standard deck panel sizes and shapes; therefore different deck configurations can be accommodated with special pieces if necessary.
This is an important aspect of deck panel design. The design of supporting framing is normally based on the assumption that the concrete is a fluid load that places tributary weight on each beam. If a deck panel is placed with only support on a few beams, the amount of dead load will not be according to the beam design assumptions. To correct this, it is recommended that a support system be used at each beam that is under the panel. The most common system in use is leveling bolts. These bolts are used to set panel grades; however they can also be used to establish the proper dead load distribution in the bridge. It is common to specify that the torque in each leveling bolt to be adjusted to within 15% of each other. Minor variations in bolt load can be accommodates through the beam cross frames.
This aspect of deck panel design has been thoroughly studied by multiple universities and agencies. The connection is typically made by means of shear connectors placed within blockouts in the panels. The design of the shear connectors can be made using the same provisions that currently exist in the AASHTO LRFD Bridge Design Specifications. Pocket spacing is typically constant. Resistance to variable shear demand is accommodated by varying the number of shear connectors in each blockout.
The AASHTO LRFD Bridge Design Specifications limit the maximum spacing of shear connectors to 24 inches. Typically, designers use 24 inches as a maximum pocket spacing for this reason. Research has shown that precast deck panels can be built with pocket spacing up to 48 inches, with no detrimental effects to the performance of the composite connection. There have been opposite issues with short span bridges that require short connector spacing in order to accommodate the AASHTO shear spacing requirements. Designers have found that even with 24 inch pocket spacing, the number of studs per pocket can get quite large. The PCI Northeast Bridge Technical Committee has come up with larger reinforced pocket that can accommodate large number of shear connectors. There are discussions underway with the AASHTO Bridge Sub-committee on revisions to the current specification to reduce the number of shear connectors.
Other methods of connecting deck panels to girders are being studied including continuous blockouts on the underside of the panels.
Continuous span bridges often make use of longitudinal deck reinforcement to enhance the negative moment resistance of the girders. Integral connections to substructures in high seismic regions also place high negative moment demand on girder systems. Deck panels have transverse joints that make the use of mild reinforcement difficult. There are two approaches to a solution to this issue. If the deck panels are post tensioned, the post tensioning system can be used to provide the desired resistance. Additional post tensioning is normally used to keep the deck panel joints at the specified compression under all service limit state loading conditions. The tendons can also be used to provide ultimate moment resistance for the girders. If mild reinforcing bars are used in the deck panel connections, they can also be used for the ultimate moment resistance.
Most states have specifications for bridge deck concretes, however these are not readily always available in fabrication plants. It is recommended that agencies use a performance specification (similar to girders) and let the fabricator meet the requirements of the specification. Typical design strengths range from 4-5ksi. Owners may also want to include durability requirements in the mix as well.
Closure pours typically use normal deck concrete. The restraint of the adjacent concrete can cause shrinkage cracks. Some states are investigating the use of shrinkage compensating admixtures to these standard mixes to reduce these cracks.
Another approach for rapid concreting is to design the joint for lower strength concretes. The Massachusetts DOT Fast 14 Project included a closure pour joint that was designed for 2000 psi. The specified final strength of this concrete was 4000 psi; however traffic was allowed to be placed on the bridge when the concrete met the 2000 psi strength. This approach greatly reduces the risk of trying to come up with a mix that can gain strength very rapidly. It is relatively easy to design a mix that can attain a strength of 2000 psi in a few hours, 4000 psi is much harder to attain in short order.
Narrow joints use typical non-shrink grouts. The AASHTO LRFD Bridge Design Specifications recommend a 5000 psi non-shrink grout. This has proven to provide very durable joints.
Research is on-going with joints that make use of UHPC. This material is expensive, but very useful. At this time, it is proprietary. There is hope that multiple companies will emerge that can provide this material. The FHWA is looking into ways to incorporate this material into typical federal aid projects.
Bridges using the F Beam will have a monolithic deck.
The D Beam has grouted mechanical connections provided (studs) that are. This joint provides continuity as well as a seal. This connection has been researched and tested to provide resistance to long-term fatigue loading and leakage. The joint was tested through two million cycles of loading and then successfully subjected to a ponding test.
Tolerances for riding surfaces are more important for decks with thin overlays (or no overlays). Deck grinding is normally required in order to achieve the tolerances that are expected for cast in place concrete decks. Match casting will most likely be required in order to achieve the type of tolerances required to grind a deck to the final profile. This will lead to higher panel costs. Decks with overlays need not be built with tight tolerances on flatness, since the overlay can make up for uneven deck surfaces.
There are two approaches used to minimize deck cracking during shipping and handling. The first approach is to use transverse prestressing. The deck can be designed to remain in compression under all handling situations. The second approach is to design the handling system to keep the concrete tensile stresses during handling within an allowable limit. The PCI design manual has significant information on designing panels for minimal or no cracking during handling. This manual includes recommendations on impact values and the calculation of lifting stresses. The allowable tension stress in the panels can be designed with a factor of safety of 1.5 applied to the modulus of rupture to produce a near zero cracking handling procedure.
In either case, it is possible to get minor cracking in deck panels; however the cracking noted to date has been significantly less than comparable cast in place concrete decks. In many cases, no cracking is found.
It is recommended that the design of lifting locations, hardware and lifting stresses be left up to the contractor or fabricator. This allows the contractor to take advantage to the lifting hardware that they have available. Specifications should require the submission of lifting and handling stresses for review.
Most designers detail leveling bolt systems for the adjustment of deck grades. Typically two bolts per beam are specified. The bolts can easily raise or lower the panel to obtain the exact grades specified on the plans. This can normally be done without power tools. The bolts also provide uniform dead load distribution to the beams. This is done by torqueing all of the bolts to within 15% of each other.
The installation of deck panels designed with longitudinal post tensioning requires a specific construction procedure. The key is to stress the post tensioning prior to making the composite connection with the deck. Otherwise, the post tensioning force would create a positive moment on the girder that would be undesirable. The following sequence is typically used:
There is a performance history of over 20 years for deck panels. The decks that have been installed with longitudinal post tensioning have been performing very well. The joints between the panels do not leak and the panels themselves are virtually crack-free.
The only significant issues to date are with respect to the concrete closure pours. The restraint of the adjacent panels can lead to restraint cracking in the closure pour concrete brought on by shrinkage of the closure pour concrete during curing. The cracking is similar to the cracking that is found on most cast in place concrete decks. Several agencies are experimenting with the use of shrinkage compensating admixtures to significantly reduce the shrinkage of the concrete used in the closure pours. The early results from this work are promising.
A properly designed and detailed deck panel bridge should perform better than an equivalent cast in place concrete deck. The lack of deck cracking combined with the high quality plant cast concrete leads to a durable deck that should require less maintenance than a conventional deck. There is no special maintenance that is necessary for a deck panel bridge.
Future repairs such as patching can be accomplished using typical patching details. Even decks with post tensioning can be repaired. The post tensioning is normally used for distribution reinforcement across the joints and it is normally installed as bonded reinforcement. Therefore is there is minor loss of a PT duct, the repair can be made using normal reinforced concrete. This essentially the same approach that is used for segmental bridges.