béton précontrainte de type caisson

OBS: béton type caisson - probably telegraphic shorthand for ""béton type utilisé pour la fabrication des caissons"" = standard box-girder concrete (as opposed to the standard concrete mixes used for bridge piers, crash barriers, etc.). Of course you might have concrete caissons used for the construction of the bridge piers in the river bed. Undoubtedly other interpretations possible.