How to Dry Stack Broken Concrete

Old concrete can be "stones" in dry-stacked walls and raised beds.

Dry stacking broken concrete is a skill almost anyone can master. Chunks of concrete, a building material sometimes known as “urbanite,” are handled just like stacking stone. Urbanite can be stacked dry – without using mortar – to build raised garden beds, landscaping planters and low garden walls. Walls of dry-stacked urbanite are composed of two parallel lengths of layered stone, one on each side of the wall. Each layer of these vertical stacks is connected every few feet by a “bond stone” that spans the wall’s entire width. Also, build strength by following the stone-stacking tradition of “one over two” and “two over one.”

Things You'll Need

• Measuring tape
• Coarse construction sand
• Stone-sized broken concrete pieces
• Construction stakes
• Twine or string
• Work gloves
• Garden shovel
• Garden spade
• Wheelbarrow
• Safety goggles
• Sledgehammer
• Stone chisel
• Hammer
• Carpenter’s level
• Mortar (optional)

Instructions

1. Measure out and outline the shape of the wall or other broken-concrete structure using stakes and string. Adjust the proportions as needed, once you see dimensions on the ground.

2. Assess each piece of broken concrete. Set aside larger rectangular pieces as bond stones – to span the wall’s width and hold both sides together – and also to “top off” your finished work as heavy capstones, to add stability. If you’ll need them, save the squarest pieces for corners.

3. Dig a trench 8 inches deep for the foundation. Make the trench the same width as the wall base, typically 24 inches. Remove the stakes and twine. Lay in 2 inches of coarse construction sand, using the level to make sure the base is even.

4. Lay the foundation course with parallel exterior lines – “wythes” – of urbanite. Orient urbanite pieces so outer edges are an inch or so higher than inner edges to shift weight inward, for stability. Fill in the center space between the wythes with smaller pieces. Place a bond stone every few feet. Sift coarse sand into every seam, to lock the foundation in place.

5. Follow the same process for every course or layer, without the sand. Strengthen and interlock the wall structure by always placing “one over two” wherever two pieces butt together, in the next course lay one piece over the top of that seam. Similarly, lay “two over one.”

6. Fit concrete pieces together like a puzzle, as tightly as you can. Cut and shape pieces with the hammer and chisel as needed. Be sure to wear safety glasses. Shim smaller pieces into place between stones that don’t fit firmly, making sure these wedges support inward tilting.

7. Stop after every few courses to fill in the wall faces with small concrete chunks – outside face only, for raised beds – tapping them into place with the hammer. This process, called chinking, locks together and further strengthens the wall.

8. Level the final layer by placing slightly thicker concrete chunks between the wythes. Finish by placing recycled concrete capstones along the wall’s entire length. You may mortar these in place, if desired, to make sure they stay put.

Tips & Warnings

• For walls up to 3 feet tall, the wall base needs to be at least 2 feet wide. For higher walls, add 8 inches in base width for every extra foot in height.

• Some municipalities don’t allow you to build walls higher than 3 or 4 feet without mortar, so check local regulations before starting tall walls.

References

• The Mail Tribune: Broken Concrete -- Handsome Is As Handsome Does
• Concrete Materials: DIY Garden Wall
• Creative Homeowner: Building a Dry-Stacked Stone Wall