Removing part of a concrete slab
March 28, 2017 7:46 AM   Subscribe

I have a yard of wall to wall poured ~50 y/o concrete. I'm interested in removing an 18-24" deep strip about 15' long along one edge (to plant some greenery in). Is there a way to remove it cleanly?

We'd like to remove a long rectangle 15'x~18". Is there a way to do this without it looking messy? Bonus question: Any suggestions for how to prepare dirt that has been covered by concrete for 50 years for planting?
posted by stewiethegreat to Home & Garden (12 answers total) 3 users marked this as a favorite
Yes, there are handheld, gas-powered saws that can cut through concrete. The better blades are edged in diamonds, like those used for cutting ceramic tile. You can attach a garden hose to a fitting on the saw, and the flow of water keeps the blade cool and prevents a huge cloud of dust. This is a pricey piece of equipment, but a tool rental shop can help you out. After cutting the section free, the stuff being removed will have to be broken up (sledge hammer, jack hammer, etc) and lifted out.

The slab is probably 4" thick or better, so you'll be adding some fresh topsoil to replace the missing material. It would probably be a good idea to loosen up and enrich the compacted soil first.
posted by jon1270 at 8:00 AM on March 28, 2017 [8 favorites]

jon1270 has it -- saw cutting is the way to get a clean edge. There are also smaller electric concrete saws, but they won't cut as deep -- the larger gasoline model is probably what you will want.

If that is more work than you are comfortable taking on, every city has concrete cutting contractors, and it might be worth checking their price for the cut compared to the rental fee and your time.
posted by Dip Flash at 8:07 AM on March 28, 2017 [5 favorites]

If you rent the saw and do it yourself, get some ear protection. Loud.
posted by AugustWest at 8:24 AM on March 28, 2017 [3 favorites]

You want to rent a walk behind concrete saw. The local tool rental place wants about $70 for a four hour rental, they may also charge for blade usage. Hook it up to a hose, you'll get a nice clean cut. If you have time, cut up the waste for easier smashing with a hammer.

Turn the soil with a fork, then dump in some topsoil. You'll need about 1/3 of a yard.

Seconding the hearing protection.

Check with a masonry contractor, if they only want $200-300, it might be worth it. Smashing and disposing of concrete is a pain.
posted by Marky at 8:41 AM on March 28, 2017 [5 favorites]

How to prepare dirt that has been covered for 50 years for planting:
Your concrete layer is probably about 2-3” thick. It was probably poured on a bed of gravel. You’ll need to scrape off the gravel layer, and then scrape off the next layer of mixed gravel and dirt. You can use a strainer basket (easily made with ½” wire mesh from the hardware store) to pull gravel out and keep the dirt, or you can just get rid of the whole mess (because you’ll be taking a load of broken concrete to the dump anyway, and don’t feel like dealing with it). You’ll probably end up with a dirt layer at least 4” if not 6” below the level of the concrete.
Next step: buy stuff to fill it in. What stuff and how much depends on the content and aesthetic goals of this planter.
If you’re planting non-edibles, just get dirt, fill up the empty space. You don’t really need compost; topsoil is fine if you’re planting trees and bushes, maybe garden soil if you’re doing flowers. All you need to worry about is soil nutrients (which are probably fine) and soil being loose enough that roots can dig through it, permeable enough that it doesn’t trap water and drains ok, and non-rocky enough that you can dig with a trowel without swearing too loud. The larger the object you’re planting, the deeper you should loosen things with the shovel; smaller plants won’t poke roots very deep, may not see the old soil at all.
If you’re planting edibles, take samples and send them to your state agriculture extension or someplace that tests for lead and arsenic. Make sure you won’t be poisoning yourself by growing things there. Alternately, assume it’s bad, and get rid of another few inches of old dirt, then fill in with even more new garden soil (plus compost, peat moss, etc). Alternately, instead of digging out, build up – use a rail to create a raised bed and add 6” of new clean dirt above ground. Even if you don’t raise anything I’d recommend reading raised bed resources to get ideas about how to mix a good nutritious growing soil to fill in the trough.
posted by aimedwander at 8:44 AM on March 28, 2017

If you're removing concrete next to your house, check that it won't affect drainage/vapour barrier/damp-proof course. If you're removing concrete next to any wall, check that the wall isn't relying on your concrete slab for support. It shouldn't be, but many houses have hidden shouldn't-bes.
posted by scruss at 9:09 AM on March 28, 2017 [2 favorites]

Unless you have a place in your yard that needs concrete rubble, another good reason to hire a contractor is that they will haul the broken concrete away.
posted by Midnight Skulker at 9:33 AM on March 28, 2017

Unless you have a place in your yard that needs concrete rubble, another good reason to hire a contractor is that they will haul the broken concrete away.

Not counting other materials such as gravel that you might be removing, 15' x 18" x 4" of concrete is going to be around 1,100 pounds -- more than you will want to put in your weekly trashcan, say. No big deal to take to the dump if you have a truck, but kind of a hassle otherwise.
posted by Dip Flash at 10:06 AM on March 28, 2017

Seconding marky, you want to rent a walk-behind saw. It will make very quick work of this job, the line will be straight, and the risk of injury very low. Handheld, gas powered concrete saws can get away from you and because the blade is smaller, it is pretty easy to end up with something other than a straight line.
posted by rockindata at 10:11 AM on March 28, 2017 [1 favorite]

I did pretty much exactly this to cut a trench to run the electrical service to my workshop, and for some of the spaces for the foundation. Walk-behind concrete saw from the rental place (I think they rented it with the trailer to move it, which is way nicer than loading it into a truck). Some cross cuts to get movable chunk sizes (go smaller than you think you need to), once I pried put the first piece the rest came easily.

My rental place had a fixed charge for the blades, and an hourly charge for the tool.

Hearing and eye protection, but you knew that.

Even with the walk-behind it takes some care to get a straight line, which is an argument for "spend a few bucks extra to hire a contractor", but it's doable.
posted by straw at 1:11 PM on March 28, 2017

Snap a chalk line, or use wash away temporary spray paint and follow that line. I'd hire a guy though.
posted by fixedgear at 4:11 PM on March 28, 2017

On chalk lines and temporary spray paint: I'm a little confused, because the area I cut out for my foundation is dead straight, but the area I cut for my trench wavers a bunch, so I don't remember the difference. However...

What you'll probably want to do is stake down a string along one side of the proposed cut, just outside where the machine will run. When the blade coolant water is flowing and all the dust is coming off the blade and turning to mud 'cause of the water spray, anything temporary is gone.
posted by straw at 1:52 PM on March 29, 2017

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