ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/297006
36 | ASPIRE , Summer 2013 S TAT E C oncrete bridges are an integral part of the nearly 14,000 state and local bridges along Wisconsin's transportation system. Approximately 70% of these state and locally owned structures are concrete and consist of concrete slabs, prestressed I-girder or box girder structures, culverts, and arches. Through research and the evolution of concrete technology, concrete has proven to be a durable and economical material for initial construction, as well as providing low life-cycle costs. The Past Wisconsin has a long histor y of concrete use for transportation structures. Many of the prestressed concrete girder structures remaining in service today were originally constructed in the 1950s. Wisconsin was one of the first states to use the American Association of State Highway and Transportation Officials (A ASHTO) I-girder shapes in bridge construction. Precast, prestressed concrete AASHTO I-girders were used exclusively through the 1990s for new, prestressed concrete I-girder type structures. The shallower A ASHTO shapes, such as the 28- and 36-in.-deep versions (Types I and II), are still used for new structures in Wisconsin today. However, current policy states that the deeper, traditional A ASHTO prestressed concrete I-girder shapes are to be used only for rehabilitation projects. Precast, prestressed concrete box beams, called box girders in Wisconsin were used concurrently with the A ASHTO I-girders in the 1960s, 1970s, and 1980s on many state- and locally owned projects throughout the state. Due to performance and maintenance concerns, mainly related to reflective cracking of the bridge decks between adjacent box beams, use of this beam type on state-owned projects has been limited since that time. However, the relative ease of construction has proven to be a reason why prestressed concrete box beam structures are still being used for locally- owned structures today. The Present Beginning in the mid-1990s, the Wisconsin Department of Transportation (WisDOT) began investigating the possibility of introducing more- efficient, prestressed concrete, wide-flange I-girders to replace the deeper AASHTO I-girder shapes for use on new structures. WisDOT, along with the precast, prestressed concrete suppliers and national technical experts, conducted research to produce and adopt more-efficient girder sections that could be used for longer spans. One alternative studied was the use of higher strength concrete. WisDOT adopted the use of 8 ksi concrete with the wide-flange I-girders, whereas 6 ksi concrete had been used with the traditional A ASHTO I-girders. Concretes with strengths higher than 8 ksi were evaluated, but were not ultimately used due to the longer curing times associated with the higher strengths. Another design change increased the size of the bottom flange of the girders, which maximized girder efficiency through the number of strands able to be housed in the flange. The Wisconsin version of the prestressed concrete, wide-flange I-girders has been used extensively since the early 2000s. WisDOT has also investigated improving the design details and fabrication techniques used with the wide-flange I-girders. In the early 2000s, as implementation of the wide-flange I-girders got underway, WisDOT inspection and maintenance engineers identified the frequent occurrence of end cracking of the girders immediately after production. Three types of cracks were noted: horizontal web cracking, inclined or diagonal cracking near the top of the girder web, and Y-shaped cracks near the bottom flange. W h i le t he st r uc t u r a l ef f icienc y of t he w ide-f la nge g irder s wa s a n improvement from the A ASHTO shapes, the girder cracking introduced a significant maintenance concern. WisDOT's bridges are exposed to highly corrosive environments due to the widespread use of deicing chemicals on roadways in the winter months. Although it is WisDOT's policy to coat the exposed ends of girders, the development of these end cracks was a concern given the location with respect to the potential leaking expansion joints on the bridge deck. Through research conducted by the Wisconsin H ig hway Re s ea r ch Pr og r a m ( W H R P), i n conjunction with WisDOT and the University of Wisconsin, it was determined that the cracks were generated due to the high concentration of prestressing strands located in the girder ends and the large forces induced during fabrication. Researchers used finite element modeling in addition to measurements made at prestressing facilities to analyze the cracking issue and create recommendations for updates to the WisDOT girder designs. Cracking of the wide-flange girder shapes was especially prevalent in the WISconSIn Innovative research produces concrete results by Aaron Bonk, Wisconsin Department of Transportation Accelerated bridge construction demonstrated with a geosynthetic reinforced soil-integrated bridge systems structure. All photos: Wisconsin Department of Transportation. Book_Sum13.indb 36 7/1/13 7:01 AM