There were triumphal arches built in the mid-late republic. I saw a picture of one built to celebrate the conquest of coastal Gaul(Province) by Ahenobarbus in 121?bc. It was only in the imperial age that large scale construction of arches and other great buildings were done mainly to glorify the current emperor and they were built at state expense as opposed to republican buildings which were paid for by the individual, usually out of loot gained from the particular conquest(Pompey's theater etc..).