Mud Creek Bridge, Iowa, USA - Overlay

We are pleased to announce that a new Ductal® Ultra-High Performance Concrete (UHPC) structural overlay formulation is now available in North America.
08.02.2016
 
A bridge deck overlay involves the surface application of a thin layer of material in order to prevent deterioration due to water penetration and/or deicing salts. There are numerous commercial products available which may be used to create impermeable layers and add texture to the rolling surface. A typical overlay does not improve the performance capacity of an existing deck, whereas the Ductal® structural overlay provides long-term durability, improved deck protection and increased deck strength. It can also be used to rehabilitate a deck instead of a full deck replacement.
 
LafargeHolcim's Research Center in France developed the new Ductal® UHPC thixotropic structural overlay solution (a thixotropic material remains fluid when agitated but stays in place when left alone). It has been successfully applied on various projects in Europe, with the largest to date being the famous Chillon Viaducts in Switzerland - a 2.2 km (1.3 mile) long, 4-lane structure.
 
The new overlay formulation is very different than our Ductal® Joint Fill solution, which is used to connect joints between precast bridge elements. The Joint Fill material must be flowable and self-leveling in order to fill all voids. Most bridges are built with a sloped surface, and a self-leveling product can easily slide out of place. With this in mind, our thixotropic formulation was developed to permit placement on bridge decks with up to 10% slope.
 
Last month, a 38 mm (1.5 in.) thick Ductal® structural overlay was applied on a 30 m (100 ft) long bridge in Brandon, Buchanan County, Iowa. For this pilot project, we enlisted two Ductal® colleagues from France, who provided valuable on-site expertise and guidance for batching. Collaborators included the Iowa Department of Transportation (Iowa DOT), the Federal Highway Administration (FHWA), Buchanan County, Iowa State University (ISU) and local contractors.
 
When installing a Ductal® structural overlay, the first step is to grind the existing surface to remove damaged areas and provide a textured, rough surface for bounding. The surface must remain saturated dry.
 
 
For the Iowa bridge, overlays on the two 8.5 m (28 ft) wide lanes were completed in separate phases. Ductal® was batched on site using two 0.5 m3 (0.65 yd3) high shear mixers. Motorized buggies were used to deposit the material which was manually spread by workers with hand tools. A vibrating screed was pulled across the surface to vibrate and consolidate the 38 mm (1.5 in) thick layer. A curing compound was added immediately after placing to maintain the moisture. Upon completion, the lane was covered with plastic for curing and a few days later, the process was repeated on the second lane. After curing, the whole surface was ground and rooved to provide the required skid resistance.
 
The Iowa DOT consistently promotes innovations and have implemented many unique, often "first-time” Ductal® UHPC applications in the past. They have also established an excellent, long-standing collaboration with ISU, which has tested Ductal® for more than a decade. After the first day of casting, Iowa DOT and ISU conducted a workshop near the site to talk about UHPC and the overlay project. Approximately 30 professionals and students attended the workshop which included presentations by LafargeHolcim.
 
Overall, this innovative project demonstrated that the new Ductal® structural overlay can be mixed in standard high shear mixers: a simple vibrating screed machine can be used to place it and regular diamond blade grinding/grooving equipment can be used to texture the surface. The response to this first trial application has been extremely positive. The deck will be further monitored in the years ahead and provide an excellent reference for future overlay projects.
 

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