I stumbled onto this article while working on a photobash of a solarpunk scene. I think it does a good job of explaining the concept but there seems to be something wrong with its certificates, which might throw an error in your web browser. https://nwedible.com/urbanite-broken-concrete-retaining-wall-as-a-garden-feature/
Just in case you don't want to check the link I'm also going to plagiarize a few quotes and images from the article:
"The marketing term for “old chunks of broken up concrete” is urbanite. Urbanite has a lot going for it: it’s durable and heavy like natural stone, reusing this product in garden and landscape design takes it out of the waste stream, it’s often a uniform thickness which makes it easy to stack or lay as a permeable patio surface, it’s often available in most urban locations, and it’s frequently free for the hauling. Free is good.
Drawbacks to urbanite can include potential contamination – this is more of an concern if your urbanite comes from a torn out commercial parking lot where all manner of auto fluids may have seeped into it than from the neighbor’s pool deck tear-out. Concrete itself can contain additives that might pose a health or contamination risk, although my feeling is that old, weathered concrete has probably already leached the worst of itself out somewhere else.
I probably wouldn’t use urbanite to build edible garden beds, but I can see great potential for turning this waste product in retaining walls, steps, and patio areas."
And a few examples of recycled concrete patios:
This last one came from https://www.terranovalandscaping.com/90/, which has a few other examples, including raised beds, so perhaps they knew their source of concrete was clean, or weren't worried about the potential for contanimation?
The pics are a bit deceiving. They make it appear as if you get uniform thicknesses, but is that often the case? I doubt it. When a building is demolished, it’s disturbing how sloppy and chaotic they are. They just smash it to bits, producing all different sizes and shapes. I wish they would think about reuse. They could take a cutter and cut uniform blocks off the building which can then be used like building blocks. Instead you got a gnarly mess of blobs with rebar sticking out.
Anyway, I think it’s common to buy much less concrete than you need for a driveway, then mix urbanite with the wet mix so a large portion is reused. I’ve not done it myself but probably entails soaking the urbanite in polyvinylacetate (PVA aka wood glue). I once had to repair a broken concrete step as well as patch some existing stucco. If I had just put new concrete where needed, it would not stick to the old concrete well. So many bonding layers are needed. You water down PVA and paint that onto the old surface. Then when that’s ½ dry you do it again but with a little concrete in it. It’s like a sloppy slurry.. gets everywhere. Then again with a thicker layer. Then you also add PVA in the new concrete mix. That’s how to make it bond. So it’d be the same idea with urbanite. It would only trust that for non-structural projects though. Probably wouldn’t want a foundation relying on it.
Good points! I suspect the 'even thickness' thing came from broken up concrete pads/sidewalks/patios, where the result would be irregulary shaped on X and Y but somewhat consistent for Z depending on how well they prepped the site for the slab. In that case it might end up pretty similar to landscaping rock. In some of the photos you can see they have a much flatter top and much more irregular edges and undersides.
100% agreed on demolition practices. There's a lot of potential in deconstruction for reclaiming building materials rather than consigning them to the waste stream. The tradeoff is in time, person hours (my grandfather once claimed a truckload of bricks from a demolished mill to build the family fireplace - my mother and her siblings weren't allowed to come inside that summer unless they chipped cement off some bricks and brought them with them). And in materials/energy - blades for a concrete saw and power to run it, perhaps. I'm sure there are other ways to get more-or-less regular building blocks but they'll have some cost to balance against the good of saving the materials and reducing the need for new manufacturing. Either way, there's some cool potential. Treating rubble or urbanite like quarried stone I think fits the solarpunk ethos.
Thank you for the details on bonding concrete! I've used cement to patch some holes in custom-shaped, 45° concrete blocks I made once (didn't shake all the air pockets out of the first couple) so I knew you couldn't just stick concrete to concrete, but not how to actually go about it when you had to. I'll refer back here if I need to fix it someday, or if it comes up in any of my stories.
Thanks!