Precast Bridge with UHPC Joints Shortens Vital Route for Tall Vehicles.
By Kirk Stelsel
When Henry Ford rolled the first Model T off his revolutionary assembly line in 1908, he forever changed the world. The Model T was the first affordable, mass-produced automobile and it ushered in a new era for transportation.
To imagine a world without motor vehicles today is almost unfathomable, but the automobile could never have made an impact without a proper network. Streets, highways, interstates and the focus of this issue, bridges, connect us and make vehicle transportation possible.
Today’s bridges create efficiencies for drivers by making new routes possible through advancements in engineering and building materials. Such is the case with the Hawk Lake Bridge in Ontario, Canada, about 28 mi. (45 km) east of Kenora.
It’s all in the planning
President Dwight Eisenhower, known for his military prowess, once said, “In preparing for battle I have always found that plans are useless, but planning is indispensable.” His wartime theory also applies well to infrastructure work. To ensure projects meet budget, timeline, structural and performance requirements, extensive planning is required.
When planning began for the Hawk Lake Bridge project, a number of criteria were taken into consideration. For years, drivers of tall vehicles had been forced to take detours around the existing railway underpass on King’s Highway 17, a main artery that plays a critical role in the flow of traffic between eastern and western Canada. The project consisted of replacing the aging structure with a new alignment to span over the railway, rather than under, eliminating all vertical restrictions previously created by the underpass.
Given the importance of the highway, the removal of height restrictions was the No. 1 priority. However, the bridge designers also had to account for heavy traffic flow – it’s estimated the bridge carries approximately 3,600 vehicles per day, one quarter of that from heavy trucks – and reduce the construction timeline as much as possible. Achieving these goals required high-strength and durable materials than could stand up to traffic loads and be installed quickly.
Lastly, the remote location of the new overpass and Canadian weather played a role in planning. The weather limits what crews are able to do on site, particularly in the colder months when temperatures consistently remain below freezing. The distance between the overpass and access to construction materials also had to be taken into consideration.
The bridge owner, the Ministry of Transportation of Ontario (MTO)/Northwest Region, and consultants TBT Engineering of Thunder Bay and Philips Engineering, considered all the factors and designed an 89.2 ft. (27.2 m) overpass consisting of 12 side-by-side precast concrete box girders and precast concrete approach slabs. The precast components were produced by Lafarge Construction Materials Precast, and construction was completed by Carillion Canada. In addition to the precast concrete, the project used Lafarge’s Ductal ultra high-performance concrete (UHPC) as joint fill between the box girders.
According to Paul Quinless, executive vice president of the Civil Infrastructure Division for Carillion Canada, the use of precast box girders for the Hawk Lake Bridge project solved a number of issues. He said the company was well versed in precast concrete installation and aware of its many benefits, thanks to previous projects such as the installation of precast on Highway 416 near Ottawa.
First and foremost is the dual benefit of time savings and quality assurance precast concrete offers. “A conventional bridge with a poured deck would take substantially longer and have more potential quality issues,” Quinless said. “The precast option ensures that the deck is manufactured under controlled conditions in a precast facility.”
The MTO/Northwest Region has also successfully completed a number of projects utilizing precast bridge components, so the Hawk Lake Bridge project was a continuation of the concepts and methodologies it has developed. For the precast elements, MTO specified a concrete with a high compressive strength of 7,000 psi (50 MPa), because it had found that precasters in the area were exceeding those strengths on previous projects. The use of a side-by-side precast box girder design, without a cast-in-place top slab, also helped to shorten on-site construction, eliminating the time it would take for forming, casting and curing. Another unique feature of the structure is that the concrete reinforcing consists of glass fiber reinforced polymer (GFRP), as opposed to the more traditional steel rebar design.
Using precast box girders and approach slabs, the bridge exceeded structural requirements, and installation minimized impact on traffic. “Precast components for bridge projects have demonstrated improved quality and have shortened the on-site construction time,” said Gary Weiss, MTO’s senior structural engineer. “While other materials and concepts are always evaluated, due to the remoteness, long travel distances and quality concerns in our region, precast concrete has been adopted as the standard construction method to be used.”
Peter Seibert, technical director-Ductal, Lafarge North America, echoed those thoughts. “This project was in a very remote area in northwestern Ontario, far from ready-mix plants, and the highway is one of the main arteries between east and west Canada – so you cannot be shut down too long, and timing was important.”
UHPC and precast: better together
The use of precast elements for bridge structures provides a durable solution that can handle high traffic loads, but in some cases the poured-in-place joints between the pieces are not able to stand up to the wear and tear as well as the precast. To ensure the joints would not fail, a UHPC material named Ductal Joint Fill (JS 1000) was chosen. “This region really prefers precast elements and now, by joining the precast elements together with Ductal, it provides an excellent solution,” Seibert said.
UHPC is a relatively new family of concrete products that provides high levels of compressive and flexural/tensile strengths, as well as durability and ductility – resistance to tensile loads even after initial cracking. The unique mix design consists of portland cement, silica fume, quartz flour, fine silica sand, high-range water reducer, water and fiber reinforcements.
In testing, UHPC has reached compressive strengths up to 33,000 psi (230 MPa) and flexural/tensile strengths up to 7,000 psi (50 MPa), depending on the use of fibers and heat treatments. The fibers, either steel or organic, provide the reinforcement and eliminate the need for steel reinforcing bar cages. With no bulky reinforcement and its inherent high strength, UHPC allows for thinner, more lightweight sections.
The first UHPC Joint Fill project was completed in 2006 for another MTO project; a bridge over a rail track in Northwest Ontario. Since then, its use has only continued to grow. Last year alone, the Lafarge Ductal group worked on six Joint Fill projects and it has 10 projects currently planned for 2011.
The Ductal Joint Fill for the Hawk Lake project was batched on site and used to join the precast elements together. “Whenever we have bridge structures, particularly box girders, precast concrete is a very efficient and economical cross section,” Seibert said. “In the past, the joints within the box girders were typically the weakest link due to inherent problems with the available joint fill products. What happens is there’s no load transfer within the box so the joints crumble, fail and corrode. Now the UHPC joint is the strongest link in the system.”
The Ductal solution creates a long-lasting joint with exceptional strength and a high level of bond development. Most importantly, the Ductal joints effectively distribute the wheel load from one girder to the next, reducing the stress on each joint. Also, they perform well when exposed to road salts, which are commonly used in the region.
Using the precast elements together with the Ductal Joint Fill ensured the rapid construction of the bridge and also meant that the structure could provide a long service life to the client. Seibert was pleased with how the project went and what his team was able to learn from it. “We are happy with the results,” he said. “Every bridge has new challenges and we learn from these experiences. This project in particular was quite a challenge because it had a 4% slope. We had to develop new operational procedures for optimal placement of the joint fill and keep it in there, which really helps for future, similar projects.”
UHPC applications for precast
While the only use of UHPC on the Hawk Lake Bridge was for joint fill, the product also holds many possibilities for precast bridge applications such as beams, decks and pilings. According to Seibert, a UHPC, full-depth, waffle deck panel for a bridge in Iowa will be completed this year using the Ductal joint fill, making it the first 100% UHPC deck in North America.
Since UHPC can be cast thinner while maintaining high-strength requirements, it also holds possibilities for additional applications such as superstructures, substructures, sound barrier walls, crash and blast protection systems, precast pavements and hybrid systems. According to Seibert, UHPC can also be used for architectural applications such as ultra-thin cladding systems with complex shapes and textures for building facades. The use of UHPC can even offer potential LEED benefits. Due to the elimination of up to 40% in material (compared to conventional concrete construction), CO2 emissions are reduced, and it provides a long-term service life and lower maintenance costs, which reduces life-cycle costs.
To date, the wide-spread adoption of UHPC in North America has been limited for a variety of reasons. The most important limitations, and hardest to overcome for precasters, include a lack a familiarity with the product, how to train plant employees on casting procedures, and initial cost, including licensing fees.
By using a precast concrete deck system, in conjunction with UHPC joint fill, the MTO simplified the installation process while ensuring improved tolerances, increased speed of construction, high durability, structural strength and overall cost savings.
The bridge has now begun its service life, one that both the designer and owner expect to be a long one given the strength and durability of the materials used, as well as the unique design elements chosen. And, the top priorities of removing the impediment to large vehicles and ensuring a fast construction timeline were achieved. In fact, Weiss expects to see more streamlined timelines and budgets on similar projects in the future.
“We have utilized this concept on a number of structures in our region and have been very pleased with the results,” Weiss said. “As contractors become more familiar with this construction methodology, we are seeing a reduction in on-site construction time and a reduction in overall project costs.”
Kirk Stelsel is communication manager for the National Precast Concrete Association.