Design Guide for Precast UHPC Waffle Deck Panel System, including Connections

CHAPTER 6. SUMMARY AND CONCLUSIONS

Summary

Full-depth, precast, UHPC waffle deck panels with longitudinal and transverse ribs and a viable set of connections suitable for field implementation were developed for this HfL project. Following a successful laboratory validation of the full-scale unit, which consisted of two panels and recommended connections, the waffle deck was installed successfully on a replacement bridge in Wapello County, Iowa. The subsequent load testing confirmed the desirable performance of the UHPC waffle deck bridge. The project described in this report was undertaken as a follow-up study to broaden the applications of UHPC waffle deck panels to new bridges as well as deck replacement projects.

To make the broader applications of UHPC waffle deck panels feasible, this report was formulated as a guide to provide technical and practical information necessary to allow bridge owners to consider the use of UHPC waffle slabs for a wide variety of bridges. Using a series of simple and detailed finite element analysis models, every effort was taken to improve efficiency of design while ensuring safety, integrated versatility, and simplified design calculations.

Conclusions

This study serves as a guide for complete design, fabrication, and installation of UHPC waffle deck panels with mild steel reinforcement. The conclusions drawn from the development of this design guide, as well as construction and performance of the waffle deck panels in the laboratory and field, are summarized in this section.

Design

The waffle deck panel should be designed to act compositely with the supporting bridge girders under traffic loads. For broader application of the precast deck panel in different bridge types with variable girder spacing, several deck panel configurations with variable transverse and longitudinal rib spacing were developed and evaluated under traffic loads. A maximum girder spacing of 10 ft was considered while arriving at panel and reinforcement details. AASHTO HL 93 truck loading was used to estimate the demand on the waffle deck panel.

• The deck panel should be at least 8 inches thick to satisfy AASHTO requirements and to resist the effects due truck loading.
  
  
• The maximum allowable rib spacing in transverse and longitudinal directions is limited to 36 inches.
  
  
• The minimum thickness for the slab over the ribs should be taken as 2.5 inches to prevent punching failure under truck loads and ensure adequate cover for reinforcement.
  
  
• Two different configurations for transverse rib reinforcement, which are applicable for different girder spacing, are proposed and shown in figure 85. The first configuration (UWD6T6B) consists of #6 bars at the top and bottom of the transverse rib. The alternate configuration (UWD6T7B) consists of a #7 bar at the bottom and a #6 bar at the top of the transverse rib.
  
  
• The cracking moment capacity of the deck panel according to the strip method in the positive and negative bending directions was found to be between 1.86-4.14 k-ft/ft and 5.76-8.17k-ft/ft, respectively, depending on the transverse and longitudinal rib spacing. The cracking moment capacities are presented in tables 18 and 19 in chapter 2.
  
  
• The nominal moment capacity of the UWD6T6B deck panel using the equivalent strip method in positive and negative bending directions was found to between 10.19-18.59 k-ft/ft and 23.39-32.98 k-ft/ft, respectively, depending on the transverse and longitudinal rib spacing. The details of the nominal moment capacities are presented in tables 18 and 19 in chapter 2.
  
  
• The UWD6T6B configuration can be used for waffle deck panels with any rib configuration (rib spacing < 36 inches) in bridges with a maximum girder spacing of 8.25 ft. This configuration can be used for bridges with a girder spacing of 8.5 to 10 ft if the transverse rib spacing is limited to 21 inches.
  
  
• The UWD6T7B configuration can be used for waffle deck panels with variable transverse and longitudinal rib spacing in bridges with a maximum girder spacing of 9.25 ft. For girder spacing of 9.5 to 10 ft, the transverse rib spacing is limited to 30 inches.
  
  
• The overhang region in the waffle deck panels should be designed as a solid section rather than a waffle configuration to resist the effects of barrier rail crash loads adequately. Although this region may be tapered without satisfying current AASHTO requirements, economic benefits of this modification are expected to be small.
  
  

Connections

A set of connections suitable for UHPC waffle panels were tested as part of this HfL project and proved satisfactory. While these connections can be used readily, additional connections established especially for full-depth precast panels have been identified as acceptable, as discussed in chapter 3.

While using these connections, it is recommended that all voids in the field be filled with in situ UHPC. With this modification, a designer may take advantage of increased compressive and bond strengths associated with UHPC and simplify the reinforcement details within the connections.

If a connection is expected to experience negative moments (e.g., top of a pier), it is recommended that the connection design include some precompression, such as that due to localized post-tensioning, to keep the connection from experiencing any tensile stresses.

Redecking

Using traditional procedures combined with those recommended in this report, full-depth, precast UHPC waffle deck panels can be used effectively for replacing deteriorated bridge decks. Regardless of the use of steel or concrete girders in existing bridges, old decks should be removed using locally approved methods along with the appropriate guidance provided in chapter 4. The selection of the waffle panels and connections can follow the recommendations in chapters 2 and 3.

Constructions Issues

It is important make sure that the precaster has appropriate training from the UHPC producer for fabrication of the UHPC waffle deck panels and the ability to mix large volumes of UHPC and thermally cure the panel decks. In addition, it is advisable to have the UHPC manufacturer on-site with the contractor when casting the UHPC fill connections in the field, to ensure the expected product quality is achieved.

It is important to engage the supplier of the UHPC in the early stages of project design so that quality control with mixing and placing of UHPC can be satisfied adequately in the precast plant and the field. While an overlay can be used on a UHPC waffle deck, it can also be omitted by integrating a suitable riding surface on the top surface of the panel during construction.