A balsa-cored bridge deck installed in Louisiana

(Published on April 2010 – JEC Magazine #56)

On 30 September 2009, the first balsa-cored composite bridge deck installed in Louisiana (USA) was opened to traffic over the Pierre Part Bayou in Assumption Parish.

A fruitful partnership

The project was supported by an Investigational Bridge Research Deployment (IBRD) Grant from the Louisiana Transportation Research Center (LTRC). The development of the composite bridge deck was jointly undertaken by the Louisiana Department of Transportation and Development (LA DOTD), Louisiana State University (LSU), Crescent City Composites, and the Technical Services team of Alcan Baltek Corp., a part of Alcan Composites Core Materials.

This joint development project promotes composite construction as an alternative to steel or concrete bridge structures. The composite bridge deck design is expected to be the first of many in Louisiana to replace traditional steel grating structures over these many regional bayous.

Main characteristics

The balsa tree is the fastest growing tree in the world. It can grow from a tiny seedling to a 30 m-high adult in about 5-7 years. It grows straight and tall with a trunk of about 45 cm in diameter, topped by a lush crown of very large leaves. Each balsa tree produces hundreds of thousands of seeds. These are carefully culled and only the fittest are germinated in nurseries and planted on the farms. The other ingredients for rapid healthy growth are contributed by nature: the rich Ecuadorian soil, rainfall that averages 2,290 mm a year and hot tropical sunlight. Balsa is the only core material that comes from natural and renewable resources.

An ideal solution

Using balsa-cored composite panels has many advantages. These lightweight composite panels can be used for fabrication off site and quick installation, which results in less “down time” for the bridge and substantially less traffic disruption, as well as less constriction on important evacuation routes during the hurricane season. Moreover, these panels are immune to corrosion which shortens the life span of steel structures, especially over brackish waters.

Being replacement panels for existing steel construction means that the geometry, specifically the panel thickness, is dictated by the existing bridge structure. Therefore, to achieve the required stiffness within the restricted thickness, layers of Hardwire® high-tensile-strength steel reinforcements were used in conjunction with conventional biaxial glass fibre in the structural skins. The core chosen for this application was a specific density range of BALTEK® endgrain balsa. Only end-grain balsa could absorb both the high compression loads required of a bridge designed to withstand the continuous punishment of fully-loaded sugarcane trucks, and the high shear loads imposed by the restricted thickness and highly-loaded metal and fibreglass skins.

In situ monitoring

As a further long-term research programme, LSU installed fibre optic strain gauges in the bridge panels to monitor the performance of the laminate over several years. The Technical Services Department of Alcan Baltek Corp. installed conventional strain gauges on an adjacent deck plate to compare load responses for both methods of monitoring. Monitoring is essential because half of the composite panels on this bridge incorporate SWCNTs in the vinylester resin of the laminate, resulting in significantly increased flexural strength and fatigue properties.

All panels were vacuum-infused on site at the Crescent City lamination facilities in East Texas and trucked to the LA DOTD in Baton Rouge, where they were adhesively bonded to steel girders. Future production is expected to take place in Louisiana.