My new (1950s) house has large -- e.g., one is 3.5 feet by 8.5 feet by 5 inches -- concrete slabs in our back yard. I'd like to remove them but don't know which tool(s) to use. Could this be done with a sledgehammer (in, say, an afternoon), or do I need a breaker hammer or the like?
What you have is a concrete slab not a block. Semantics. I would rent a jackhammer, breaking that thing up with a sledgehammer will be a arduous task. You may also need an angle grinder with a metal cut off wheel to cut the metal lath or rebar reinforcement’s that should be inside the slab. You will want to break it down into small enough pieces to lift and handle in order to make the disposal job easier.
There are a whole spectrum of tools that might be useful for breaking concrete. It's hard to guess which one will be "just enough" to get the job done because there are several things that can affect the strength of the concrete:
- how much cement was used in the mix
- how the cure was carried out
- size of aggregates
- whether there's any embedded reinforcing steel, and what size
That said, if you're determined to have it out one way or another, you can certainly start small and escalate if you're not satisfied with the rate of progress.
One thing that can really help particularly when using hand tools is to get the concrete lifted off the soil. Concrete cracks much more easily when it's not well supported. Remove soil under one edge of the concrete enough to insert a hydraulic jack (bottle jack, automotive floor jack, etc) under the slab a few inches from its edge. Raise the slab an inch or more off the soil, then start striking the surface of the slab. Use at least an 8 pound hammer. Eventually it'll crack.. or you'll decide to go to the tool rental center and either an electric demolition hammer or a pneumatic jackhammer.
If the slab can be raised several inches at one of the narrow sides and you can start the breaking at the opposite side then you may get away with jacking up the slab in just the one place. When the pieces are broken to where they weigh just a few hundred pounds the raising can be done with a shovel or other prying tool instead of the hydraulic jack.
I did something similar a few years ago with a 12lb sledgehammer. Best/worst workout I ever got. Was about 12x12 and about 4" thick with the rebar laying on the bottom of the cement and not in the middle like it was supposed to be (that was nice). To make it simpler I rented a cement saw for the day and cut 2" deep cuts into the pad up like a checkers board. Dug under the cement a bit with a pick and then wailed on the cement. Nice chunks would break off and then repeat. Took about a week to do. I'll never do it again with a sledgehammer. If I ever, hope I never, need to do it again I'll rent a jackhammer. I paid someone to come haul it off.
The slab is probably only that thick at the edge. What I have done in the past for slabs that did not have reinforcements like wire mesh or rebar, was to pry 1 edge up then a 20 lb sledge hammer or better yet a 35 lb lead cannon ball slammed down the concrete breaks up quite nicely even at 4+ inches. If there is wire my best advice would be to rent a saw and cut it but only 2 of the residential slabs I have removed have had wire the first one I tried to break up until finally giving up and renting a saw the second one as soon as I realised it had wire I went and got a saw. I find a 4x4 or 3" piece of conduit is usually strong enough to slightly lift a edge of a slab , you don't need much and it will make the demo much easier, yes you can bust up concrete with out lifting it but it will go 100 times faster with a slight air gap below. My son and I took out a 20x 40 slab in about 6 hours using this method.