Q: I have a concrete slab (about 2 to 3 inches thick with small rocks embedded on the surface) in front of my house. The area is about 3 feet by 20 feet. I would like to remove it.
Can I break up the slab, then cut it into smaller blocks and use them to build a raised garden bed? If so, do you have any suggestions on how to do the job? If not, do you have any other suggestion for reusing this concrete?
A: We applaud your effort to go green. You’re in for a lot of work, but if you persevere, you will end up with a handsome raised bed for your veggies or flowers. Here are a couple of suggestions:
Your first job is to decide on the finished look you want. Do you want to go rustic, like the old rock walls seen in the Gold Country, or do you want to feature the exposed aggregate for a more finished look?
Both jobs call for power. For the rustic look, go after the slab with an electric jackhammer. For the more finished look, a water-cooled gasoline-powered concrete saw is the ticket. Either way, rent the tool. You’ll be able to turn the big slab into smaller building blocks with a helper and half a day’s work.
To make rustic building blocks, draw a pattern on the slab freehand with some chalk. Draw the pieces more or less 8 inches wide and 12 inches long. Start at one corner of the slab and score the lines you’ve drawn with the jackhammer. Start at an edge and work around the pattern.
Cut 3/4 inch deep — any more and you risk breakage. Then go to about a foot away from the edge and cut deeper, gradually moving away from the edge. The slab should crack along the line you’ve scored. Pry your newly made rustic building block out with a crowbar. You’ll probably have to buy this tool, but if you do any yard work, it will be invaluable.
Repeat the process until you’ve got all of your blocks cut, removed and stacked. Then wheel the pieces off to the planting bed.
We suggest you dry stack the pieces to form the bed. Make sure to stagger the joints. The irregular edges will look good, and the joints will act to keep the soil drained.
For the uniform look, lay out a grid on the slab using a chalk box and red chalk. Each segment should be about 4 inches wide and 10 inches long. Basically you’re making bricks.
Cut the slab on the lines with the concrete saw. If the slab is indeed 3 inches thick, one pass will do it. With all the cuts made, pry the "bricks" out with a crowbar, stack them and move the bricks to the planting bed.
Face the exposed aggregate to the outside and use mortar to build the bed walls. We like to enrich the packaged mortar mix with additional Portland cement. Embed No. 3 steel reinforcing bar (rebar) into the mortar joint on the first course of brick and every third course thereafter.
Do not mortar the vertical joints on the bottom course to allow for drainage. Finally, we think a nice look would be to give the aggregate a coat of polyurethane to bring out the texture in the stone.