Near the Nebraska border in Sioux City, Iowa, a new type of bridge form let sunlight pass through its clear plastic. A crew of contractors, inspectors, technicians and engineers from the Iowa Department of Transportation (DOT) stood around on the plastic awaiting the concrete deck pour. At 85-feet long, the span was the first of its kind: a fully transparent bridge deck form made from acrylic plastic. When the concrete poured, they watched from below. And kept watching.
“Still looks like concrete!” the contractor joked. His job almost a wrap, he was already happy. Unlike wood forms typically used across the state, the ClearCast Forms did not need to be removed after the concrete cured. Their transparency, a monumental new feature for a bridge deck, let inspectors check the condition of the concrete during construction and throughout the bridge deck’s life cycle.
“The ClearCast Forms were a complete success, getting the contractor what he wanted, and providing the DOT the visibility they required,” said David Brodowski, Vice President of Inventure Civil. “The pour was exciting and we were pleased with the result and a happy customer.”
Inventure Civil knew the acrylic had to offer around 80 percent transparency – i.e., visibility of concrete surface area around a steel frame. Plus, each modular plastic unit needed to be under 60 pounds, so that two people could pick it up easily. The structural system had to meet those criteria, in addition to satisfying the strength and stiffness requirements typical for stay-in-place formwork, which in this case was 8 inches of fresh concrete and construction live loads of 50 pounds per square foot.
“At Inventure Civil, we understand materials and supply, and knew that we could isolate the right cost and parameters to make the ClearCast concept work,” Brodowski said. “But our business was growing fast, we were hiring people, moving into a new space, and taking on more complicated projects. Leading a growing company is challenging. We couldn’t just stop our day-to-day activities and focus on R&D.”
Thus Inventure Civil engaged NBM Technologies to provide structural analysis for this innovative new product.
“NBM was critical for finding a cost-effective support system to hold the transparent plastic we envisioned in our patent,” Brodowski said. “NBM not only did that, but figured out all the connections and detail work, which is the hard part of innovation – getting to an actual solution that works, from just a concept.”
A resourceful, innovative solution
Off-the-shelf plastic and steel components come in very specific dimensions. Anything outside these shapes tend to be custom-ordered in order to meet cost differentials.
NBM Technologies produced a design that uses off-the-shelf materials while meeting the target price of the highly-competitive market of highway construction. The structural system draws inspiration from the walls in office buildings and makes use of cold-formed steel, a material not currently found in built civil infrastructure.
Office building walls are frequently made up of cold-formed steel (CFS) channels and tracks. In cold-formed steel production, machines cut strips of steel sheet to any length in continuous production facilities, and then roll the strips into shapes. NBM Technologies was able to leverage its expertise in CFS to identify this material as a viable option for the ClearCast Forms, as well as identify those mass-produced and cost-effective shapes that could successfully meet the design criteria for any desired span lengths for the acrylic modules. Essentially a wall turned on its side, the design lets the steel channels hold each acrylic module firmly in place while concrete is poured on top of it.
Most bridge formwork is designed according to highly-prescriptive formulas outlined in the AASHTO Bridge Design Specifications. However, there are no codes or specifications for using plastic in a structural design, nor for using cold-formed steel in civil infrastructure. NBM’s proficiency in designing CFS systems, combined with its skills in analyzing the pure physics of material behavior, allowed it to reliably design this novel structural system.
“The NBM team checked all the failure modes and put math to it, using various codes and specifications that were mostly outside of our industry,” Brodowski said. “They put it all together to make a cohesive product that proved the concept. Not only that, their calculations were considered thorough by the Iowa DOT and helped secure our pilot project.”
With the help of NBM Technologies, the heavy highway engineering company materialized its vision for an ingenious product that expedites concrete bridge construction. It happened to be that the product made use of materials commonly used elsewhere: a hurdle cost-effectively dealt with through NBM’s high-level engineering judgment and advanced analysis.
“The Sioux City project was a complete success for Inventure Civil, the Contractor, and the Iowa DOT,” said Tim Brereton, CEO of Inventure Civil. “More states are starting to accept the ClearCast Form product. We expect to see this innovative product on more bridges across the country in the very near future.”
- Installs 5x faster than temporary wood forms
- Eliminates the need to strip forms from below
- Eliminates labor to patch underside of deck (Iowa DOT surface finish requirements)
- Reduces labor costs
- Workable support system
- Cohesive product
- Allows for inspection over the service life of the bridge deck