How to make the unbreakable easily breakable?

May 1, 2008 3:18 PM

How can I make concrete somewhat easily breakable?

I'm hoping to get a motor-scooter very soon. I have an assigned spot behind my apartment that I'd like to park it in, unfortunately there's nothing to chain it to. My hope is to get a big ass planter that I can fill with concrete and set an anchor in.

I don't want to leave a heavy planter filled with cement when I move out. So my thought was "How can I make this easily breakable with a sledge-hammer?"

Waxedpaper balls?
Additives?
Animal Sacrifice?

Also, how long will concrete take to set when it's poured that thick?

posted by piedmont to home & garden (26 comments total)

concrete takes about twenty eight days to fully cure.

anything you do to make it more breakable, also defeats the purpose of what you are trying to acheive

i say simply fill the pot with concrete mix, stick a plastic tree in it while the concrete is still wet, along with your eyebolt, so when you move out you are leaving something decrative behind
posted by Mr_Chips at 3:31 PM on May 1

Are you allowed to mount an anchor in the ground? Something like these?

I searched for motorcycle floor anchor, there are probably lots of other sites, this was just the first I found.

As for making weak concrete I think you just add too much water, but then the bolt or whatever you put in there will be easy to pull out.
posted by JulianDay at 3:34 PM on May 1

I'm thinking back to concrete design class and there's not an additive that weakens concrete. The ultimate strength is determined from the mix of cement, lime, and aggregate. I'm guessing you're not formulating your own concrete but rather buying it in the 'just add water' bags, so you can't really change the composition. Overwatering the mix could weaken it, but it'll also take longer to set.

Creating voids in the planter is a good idea, but as heavy as concrete is, anything deformable would just get flattened. Anything rigid would bond to the mix (or sink to the bottom), and then would be a structural part of the finished product.

How deep is your planter? What if you built levels in it, so that you pour 1/3 of the bottom, build a wooden frame that fills another 1/3 after the first pour sets, then fill the top 1/3 again. You'd only have 2 smaller slabs to break up then and it would still be plenty heavy.
posted by hwyengr at 3:36 PM on May 1

While the concrete would take around 28 days to fully cure, it will set (firm up) in less than a day. I also cannot think of a process for weakening it.

Could you fill the planter with heavy gravel or something, maybe lay a concrete circle on top of that to give the weight and impression of a full planter?
posted by chudmonkey at 3:40 PM on May 1

hwyengr is thinking along the lines I am. Not that that should stop anyone from presenting other ideas.

If I were to layer with something non-concrete, when could I pour the second layer? Is it set enough after a day to make this doable?
posted by piedmont at 3:46 PM on May 1

just fill the bottom half with sand

Lay thicK sheet of plastic over sand, fill upper half with concrete mix and eye bolt.

when you are ready to move, pound a hole into bottom of planter so sand can pour out, then the concrete will break easily since there is a void below it.

problem solved
posted by Mr_Chips at 3:53 PM on May 1 [1 favorite]

hywengr,
So adding more of which ingredient weakens it? Can't peidmont just take the ready mix and add more aggregate or lime, since it seem that those could be purchased at hardware store?
posted by bdc34 at 3:57 PM on May 1

concrete is susceptible to Spalling, due to water getting into small crevices and freezing. if you could make your anchor with cracks/ holes already in it, then when you move, all you need to do is soak it and then chuck it in the freezer.

of course, this will only be a workable solution if you live in an area where it doesn't get below freezing normally.
posted by ArgentCorvid at 4:00 PM on May 1

you'd also need a pretty big freezer!!
posted by Mr_Chips at 4:02 PM on May 1

It somewhat defeats the purpose of trying to create a heavy anchor for your scooter, but you might look into making it of Light Weight Concrete (LWC). This is sort of the commercial version of the crushable concrete pioneered by Engineered Arresting Systems Corporation for runway overrun arresting applications.

Also, since the mid-1930s, most concrete intended for outdoor use in areas with freezing weather has been air entrained, for greatly increased resistance to freezing and cracking issues.
posted by paulsc at 4:04 PM on May 1

Mr_Chips' idea seems brilliant to me.

Just build the shell out of concrete, fill with rocks, lead shot, sand, whatever.
posted by Netzapper at 4:13 PM on May 1

Absolutely, adding more sand and stone while keeping the cement content the same, will weaken the concrete. But a solid mass of any composition is going to be tough to break up. Layering will be a lot cheaper, too.
posted by hwyengr at 4:16 PM on May 1

Use Quikcrete Mortar Mix (#1102). This is a type N mortar that has a compressive strength of only 750 pounds per square inch compared to a typical concrete of 3000 psi or more. Don't use Mason Mix which is stronger.

When you mix it, add extra water to make is very thin and soupy. Excess water makes weaker cement. If you want it even weaker, add extra sand to the mix.

You should have no problem breaking it into chunks with a sledge hammer
posted by JackFlash at 4:20 PM on May 1

what about putting fruit, like apples in your concrete? as the apple decays, it creates a void, so when you attack with a sledge hammer, concrete breaks easily
posted by Mr_Chips at 4:24 PM on May 1

Not sure how to make it weaker to begin with, but when it's time to get rid of it, I'd suggest building a fire on top of the planter to help weaken the concrete before busting it up. As I recall, my dad did something similar when it was time to demolish a big concrete chunk in our front yard when I was a kid, lighting a big fire underneath to help weaken it.
posted by limeonaire at 5:07 PM on May 1

When it's time to go, drill a couple holes in it and use stone splitting wedges to break it into pieces.

Alternately, get like a dozen cinder blocks and some chain. Once you've chained them together, they will be pretty much immobile. Chain your scooter to this. When you move, un chain them and you can walk away with them.
posted by gjc at 5:33 PM on May 1

Why not drill an anchor into the pavement and attach your scooter to that? Something like this.
posted by Mach5 at 5:56 PM on May 1

I did the same thing to secure my bicycle outside my apartment. I used an eyebolt, 80 lb bag of regular Quikrete concrete mix and a 10 gallon bucket. If I were to do it over again, I would have just dug a hole close to my patio, poured concrete into the hole and sunk in a piece of rebar bent into an upside down U or V. Once it dries, you can pass the chain through the rebar loop. You can cover up the concrete with dirt or mulch.
posted by Andy's Gross Wart at 6:22 PM on May 1

I would take the opposite approach -- get a nice-looking planter, put concrete in the bottom 1/3, plus an eyebolt coming out the side, and plant a tree in the top 2/3. When you move out, simply rotate the planter (if it's not too heavy to rotate) to hide the eyebolt, and leave the planter/tree as part of the landscaping. No need to destroy it or move it or anything else.
posted by Forktine at 7:33 PM on May 1

I'm thinking that normal concrete, un-reinforced, will be relatively easy to break up with a sledgehammer and that making it weaker will just cause crumbling problems.
posted by bz at 11:31 PM on May 1

If I was going to create "pre-broken" concrete, I'd pour a few inches, lay some clingfilm or other non-permeable plastic on top, then pour a few more inches of concrete. Repeat until planter is full. But I prefer the security anchor, or failing that, the gravel-topped-with-concrete approach.
posted by Leon at 4:30 AM on May 2

JulianDay's suggestion seems much better than all this cement chemistry to me, since you could sink something crazy, like feet of a u shaped steel rod into the ground (right into concrete if you like) leaving only a two-inch U poking out the top. Lock your bike to that.

That's innocuous enough to leave as-is when you move out.
posted by rokusan at 5:18 AM on May 2

...like SIX feet of u shaped steel rod....
posted by rokusan at 6:17 AM on May 2

Easily breakable makes your scooter easily stealable. As others have said, epoxy a ring in the pavement to chain your scooter to.
posted by electroboy at 6:36 AM on May 2

If you add extra sand to the concrete mix, it will be weaker. However, if it is contained within a regular-strength concrete container, the weakness won't really matter; it will still be a lot of work to remove. If you want a planter (assuming the embedded eyebolt mentioned above won't work for some reason), try putting something in it that is still heavy, but removable, like gravel, sand, or I don't know, a plant? A decent sized planter is going to weigh hundreds and hundreds of pounds when it is filled with dirt and a shrub/tree, which is more than enough to deter anyone from trying to move it. Planters are frequently used as a security measure outside of government buildings, and more often than not, they are filled with plants. Possibly they have additional stuff in there (concrete, gravel, pop-up machine gun emplacements), but why? That would be more work, and heavy is heavy...
posted by schwap23 at 7:25 AM on May 2

Just a note: I don't know what you ended up doing, but if you sunk anything into the ground near the parking area, esp. if it's in a grassy spot, you'll want to spray-paint that bright flaming orange or something similar so that when they come to mow the lawn, they don't roll over that and break a blade. I promise your landlord (and/or his lawn-care contractors) would be very pissed if that were to occur.
posted by limeonaire at 7:31 AM on May 21