as someone who writes specifications for (much smaller) construction projects it is pretty common to include language akin to "once you start working on it you've bought it". if the contractor felt that the decking was insufficient for the project they should have either: a) not engaged in the work; or b) change the contract to place all the liability on the County. they obviously didn't choose option a, so did they get some concessions?

that certainly won't answer the question of whether or not they did shitty work, but it would point to who gets to eat the surfacing material and/or the repairs required to make the bridge operate safely for the long term.