Possibly one could find concrete pre-stressed slabs that could be set on top of your bridge structure, but guess pouring a concrete slab which would have the strength to support itself on the bridge structure would be quite a job. Seems to me our bridges in the north have a pretty short lifespan (believe it is 7 - 10 years) because of road salt, etc. but that may not be a problem with this bridge. Again, strength to weight ratios of wood plank will be real hard to beat with any other product, and high cost of other products will have to be factored in.