Very little problem as the concrete would neutralize it to non hazardous calcium formate. Also the formic acid is biodegradable, and non flammable. Easily neutralized with backing soda. If they use sodium formate for the fuel (sodium salt of formic acid) there is no safety issue at all.
Want to see a real disaster try that same truck accident scenario with ammonia (I narrowly escapeed this one 40 years ago) or hydrogen (PLUG 's fuel) or gasoline (Detroit's fuel) and view a bleve.