Condo improvements without everyone's approval?

[Michaela G.]

I own 2 duplexes in a cul-de-sac of a total of 8 duplexes. 

After the purchase I found out from the city, that whoever built them, never finished subdividing the property completely. At this time there are 7 different owners.

6 of these duplexes are facing the cul-de-sac and 2 of them are owner occupied, facing other streets.

3 of us owners (4 duplexes) want to improve the cul-de-sac, but can't get cooperation from the 2 remaining duplex owners. Both of them vacant and boarded up. 1 I have a ph#, but the other is sitting in Florida and won't respond at all. 

Can we do things, without their approval? Would love to install an electronic gate to keep the riffraff of the neighborhood out. Or do something with the common parking lot. 

Is there any way to force them to be part of this, without filing suit? There is no condo association.

If we eat the cost, can they later force us to remove the gate or other improvements?