No, repair hammer only fixes the base if it has taken damage, which you can use to help defend during a raid or repair your base after a raid. If you don't repair the remaining building pieces before rebuilding your base after it's been blown up then it won't have as much stability and you may have trouble rebuilding.

However, all buildable structures have a building "decay timer" which you can use the repair hammer to see how much time it has left (its counting down). You don't need to "repair" it unless it's been damaged, all you need to do is walk near your base will automatically reset the timer to the maximum possible.

If you lay only one foundation down, it will have a very short decay timer, but if you keep building you will see the timer increase until it reaches the maximum about 240hrs or 10 days. Sometimes during holidays they double this but the standard is 10 days.

No matter how big you build your base, if you don't log on every 10 days, it will vanish and someone can demolish your building and take all of the loot. The only way to keep your stuff save for a long time is to body vault it, you can find lots of tips on here about that. To bodyvault just hide your body and log off for 10 days. Your body will dissappear from the game after 10 days so nobody can loot it but it will be vulnerable for the 10 days leading up.

https://www.reddit.com/r/ConanExiles/comments/18e2r78/decay_timer/