Should you want to remove an existing fence or just replace one, you may need to remove posts that were set in concrete. While not a difficult job, this process can be labour intensive.
Determine how the post is set into the concrete. It likely will be set in one of three ways: the post is bolted to the surface (got to step 2), the concrete was poured around the post as an anchoring weight below ground (go to step 3), or the post is metal and set into a concrete structure such as a slab or stairs (go to step 4).
Loosen the nuts on the anchor bolts coming up from the concrete and through the base plate of the fence post using the socket set. Lift the post straight up and off the bolts. The bolts will remain sticking out of the concrete. If necessary, cut the bolts flush to the surface of the concrete using a grinder.
Dig down the entire length of the concrete fence post base with a shovel. Don't dig directly against the post. Start about 25 cm (10 inches) away from the post. Dig around the post and base until you have exposed all of the concrete. Create a 5 to 7.5 cm (2 to 3 inch) gap between the concrete and the surrounding dirt. Rock the post back and forth to loosen it from the ground, then pull it straight up and out of the hole.
Cut the post with a bandsaw so that only 5 or 7.5 cm (2 or 3 inches) remain above the top of the concrete. Clean out any debris that has fallen into the hollow centre of the post. Pack the post with dry ice, which will create an extreme cold condition inside the pipe that causes the metal to become brittle. Wait a half hour, then strike the remaining piece of post with a hammer several times. The metal of the post will shatter on impact.
If the fence posts are extremely tall or heavy, rig a sling around the post and lift each post out of the concrete using a lifting machine (such as a forklift, excavator or crane). This will be faster and safer than doing it by hand.
Do not use dry ice in an area that lacks ventilation. Dry ice is a hard form of carbon monoxide and as it "melts" the gas will force oxygen out of the area and you could asphyxiate easily if there is no source of fresh air.