Last October a century-old, one-lane concrete bridge in Bex, Switzerland, was replaced by a two-lane bridge with a composite deck. Aside from being lighter and corrosion-resistant, the prefabricated bridge deck offered another advantage – quick installation.

“The bridge is located in a mountainous area, on the only road providing access to the valley,” says Sébastien Lavanchy, project leader/engineer for 3A Composites. “Therefore, the time of installation was critical.” 3A Composites in Switzerland provided the COLEVO composite bridge deck, which was installed in less than three hours.

The new Bex bridge over the Avançon River in southwestern Switzerland has another distinction: It is the first application of a balsa sandwich road bridge in the country. The core of the composite deck is made of a structurally-bonded balsa-based material called BANOVA, developed by 3A Composites. According to Lavanchy, BANOVA has two to three times lower density than traditional construction wood products and offers optimal performance through special orientation of the wood fibers. “It’s ideally suited as core material for lightweight and highly-loaded sandwich elements used in construction applications and composite bridges,” says Lavanchy.

The sandwich deck elements were produced on a flat mold by vacuum infusion. The 285-millimeter sandwich plate is composed of a 240-millimeter BANOVA core wrapped on all sides and fully sealed into glass fiber and vinyl ester face sheets. “The total thickness is similar to that of a concrete deck for the same application, however with a significantly lower weight of only 160 kg/m2 against 700 kg/m2 for the concrete solution,” says Lavanchy.

Material and structural testing were key to the success of this novel project. 3A Composites worked in close collaboration with many partners, including Suisse Technology Partners in Neuhausen, Switzerland, and the Composite Construction Lab (CCLab) at Ecole Polytechnique Fédérale de Lausanne (EPFL). For instance, Suisse Technology Partners tested the complete set of material properties, with special attention on the balsa wood’s thick core dimension. The CCLab performed fatigue and ultimate strength tests on full-scale samples. In addition, construction details such as panel joints and the integration of guardrail posts were tested at full scale prior to implementing them on the bridge.

“We may not have 100 years of experience working with this material,” says Lavanchy, “but drawing on 50 years of use in maritime construction as well as extensive accelerated aging tests, we are confident that these structures perform just as well as concrete ones.”

The composite deck was prefabricated in three 40-square-meter pieces by 3A Composites in Altenrhein, Switzerland, then transported across country by truck. Next, the pieces were pre-assembled on the side of the construction site by adhesively bonding the sandwich deck on the upper flanges of two galvanized steel girders. Then the whole bridge was installed in one single piece with a crane. “This would not have been possible with standard construction materials at this location as access of a suitable crane was not possible,” says Lavanchy.

Overall, the road was closed for 10 days: It took two days to remove the old bridge and only a few hours for installation. The remaining road closure was necessary for casting the transition slabs and standard road reconstruction on both sides of the bridge. By comparison, a cast-in-place concrete solution would have required closing the road for six to eight weeks, says Lavanchy.

While critics argue that sandwich composite materials cost more than traditional construction materials, composites may be the most economical solution where lightweight, durability and longevity are critical. “For the road bridge project in Bex, taking into account the full project cost, the sandwich composite deck is a technically and economically competitive solution,” says Lavanchy.