Seeing an Ex in person again

My Ex and I have been separated since July. He broke up with me saying I "deserve someone better than him". I really had tried to be a good girlfriend and support him and we never had any fights. The break up was traumatic and I've been doing a lot of work on myself. I was recently transferred at work and now he and I are in the same building. I knew we would eventually run into eachother and I tried to mentally prepare. We ended up in the elevator yesterday. I tried to be cordial and say "Hey. How have you been?" He wouldn't really look at me and kinda stared at the floor and said "Can't complain." I am worried that this will be an ongoing thing. The interaction dredged up some feelings of hurt from the break up. I don't know what to do other than to just continue to be polite.