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The photograph shows a test being performed on the waffle slab panels. The photograph shows the underside of a waffle slab panel.

Full scale laboratory tests of the waffle slab system were performed.             

Ultra-High Performance Concrete Waffle Slab Bridge Deck for Wapello County, Iowa
Dean Bierwagen, Ping Lu, and Ahmad Abu-Hawash, Iowa Department of Transportation; Brian Moore, Wapello County, Iowa; and Terry Wipf and Sri Sritharan, Iowa State University, Ames, Iowa.
Since 2003, Iowa has been helping expand the knowledge base for the design, casting, and construction of highway bridge components using ultra-high performance concrete (UHPC). With funding support from the Federal Highway Administration's (FHWA) Innovative Bridge Research and Deployment (IBRD) program and the Iowa Highway Research Board (IHRB), two bridge projects have been completed using UHPC. The first was the Mars Hill Bridge, a single-span, 110-ft (33.5-m) long, prestressed I-girder bridge, in Wapello County, Iowa. The three girders of the cross-section were cast with UHPC. The second bridge was the Jakway Park Bridge, a three-span 110-ft (33.5-m) long bridge, in Buchanan County, Iowa. The center 50-ft (15.2-m) long span was constructed with UHPC using the pi-girder cross section developed by the FHWA and the Massachusetts Institute of Technology. More details about these two projects are given in HPC Bridge Views, Issue No 57.

In keeping with the trend of trying to develop cost-effective bridge sections using UHPC, the FHWA had investigated the potential use of a UHPC waffle deck precast system for bridges and had developed a design procedure for a full-depth deck system. The Iowa Department of Transportation (Iowa DOT), in cooperation with Wapello County, Iowa, and Iowa State University (ISU), sponsored a workshop in Iowa with attendance by representatives of industry, academia, contractors, and the FHWA to discuss the potential for using the UHPC waffle slab in a demonstration bridge. A grant secured by Coreslab Structures of Omaha, Nebraska, from the FHWA's Highways for LIFE program along with research funding from the IHRB, provided the funds to pursue this project. The Iowa DOT, Wapello County, and ISU are now working on a bridge project where a precast deck will use UHPC in a waffle configuration. Field cast UHPC will be used in the connections between the panels and the girders and between adjacent panels. This article summarizes the project activities through November 2010.

Project Details
The project was divided into two phases. In Phase I, Coreslab Structures completed casting of two test panels at its plant in Bellevue, Nebraska, and ISU performed laboratory testing and evaluation. Following the approval by FHWA of the Phase I work, Phase II of the project began with casting the bridge deck panels in summer and fall 2010. Construction of the test bridge in Wapello County is planned for spring 2011.

Phase I—Design
The Iowa DOT with assistance from ISU designed the waffle deck panels for the bridge. The required compressive strength for the UHPC was 24,000 psi (165 MPa). The waffle deck design procedure was based on a modified version of the empirical design method of the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications. The design was validated through laboratory testing that included a study of the expected performance of the jointed waffle panel system. The demonstration bridge will be 60 ft (18.2 m) long and 33 ft (10 m) wide. The superstructure will consist of five standard Iowa Type B precast, prestressed concrete girders spaced at 7 ft 4 in. (2.23 m). The 8-in. (200-mm) deep waffle slab is designed to be composite with the girders. Steel reinforcement in the panels consists of No. 6 bars. The deck cross section is composed of two precast UHPC waffle deck panels that are connected at the centerline of the bridge. Each panel is 8 ft (2.4 m) long (measured in the bridge's longitudinal direction) and 16 ft 2½ in. (4.94 m) wide.

Phase I—Testing
Two laboratory specimens representing a full-scale portion of the Wapello County Bridge were tested in the ISU Structural Engineering Laboratories. The laboratory test specimens were designed to simulate service, fatigue, and ultimate loading of the deck panels. Several connection details between the two adjacent decks panels and between the deck panels and the girders were included. The test panels were 8 ft (2.4 m) long (the same lengths as the proposed bridge panels) and 9 ft 9 in (2.97 m) wide. The two panels were placed end-to-end and spanned 7 ft 4 in. (2.23 m) across two 24-ft (7.32-m) long prestressed concrete girders as shown in the drawing at the end of this article.

A single point load was applied to the laboratory specimen at various locations to represent a wheel load. The loading locations were selected based upon finite element analysis of the test specimen to identify critical locations. The laboratory testing of the UHPC waffle deck system indicated that the waffle deck including the UHPC joint systems performed satisfactorily. No fatigue damage was noted after 1 million cycles of loading at each loading location. Displacements of the bridge deck were considerably smaller than the allowable limits of the AASHTO LRFD Bridge Design Specifications. Further testing will be performed in the field once the bridge is constructed as part of Phase II of the FHWA grant.

Phase II
After successful validation of construction and performance of the waffle deck system in Phase I, the deck panels for the prototype bridge were cast in the summer and fall of 2010. The measured compressive strength of the UHPC in the production panels was 33,700 psi (232 MPa). Because of flooding during the summer in Wapello County, bridge construction was delayed until spring 2011. Through the conduct of this study, multiple organizations gained significant knowledge about the design, construction, handling, fabrication, and structural performance of this innovative deck system. Once the Wapello County Bridge is constructed, field testing and evaluation will be performed to provide further information about this new precast deck system. The field data, combined with the laboratory test data, will provide important information for potential use of these deck types in the future and allow design, construction, and fabrication modifications if needed. In addition to the durability benefits of UHPC, the waffle slab concept offers several potential benefits including a viable precast deck alternative for use in accelerated bridge construction of new bridges, and a weight-saving precast deck alternative for use in replacing deteriorated decks on existing bridges.

References
Garcia, H. M., "Analysis of an Ultra-High Performance Concrete Two-Way Ribbed Bridge Deck Slab." Federal Highway Administration, Report No. FHWA-HRT-07-056, 2007, 64 pp.

More Information
More information about this Iowa project and UHPC is available in the above document and at www.fhwa.dot.gov/hfl/partnerships/coreslab/phase1/app_index.cfm or contact the lead author at dean.bierwagen@dot.iowa.gov.
The drawing shows a cross section of the test specimen.

HPC Bridge Views, Issue 65, Jan/Feb 2011