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Old 12-23-2011, 07:30 PM   #1
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How much used concrete can be use with new concrete


I have a bunch of used concrete blocks and motor that I'd like to reuse when I pour a foundation and driveway.

I'd be using bagged ready-mix from HD. How much the old concrete blocks can I add to the mix without ruining it by making it a lot weaker?

Also, ready mix is a combo of sand/rocks and concrete, If I add some non-ready-mixed concrete, will that help the strength?

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Old 12-23-2011, 07:36 PM   #2
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How much used concrete can be use with new concrete


Total waist of time and a very bad idea.
For the few pennys you think you may be saving it may mess up the mix and screw up the integraty of the whole job.


Last edited by joecaption; 12-23-2011 at 09:34 PM.
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Old 12-23-2011, 08:35 PM   #3
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How much used concrete can be use with new concrete


Use it for the base and not within the concrete.
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Old 12-23-2011, 11:29 PM   #4
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How much used concrete can be use with new concrete


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Total waist of time and a very bad idea.
How are concrete blocks (IF clean) any different than other aggregate, if in small pieces? Still too wimpy? People use chunks of busted up concrete blocks to hold up wire mesh in slabs all the time. Granted, that doesn't add much volume. I don't see how they can hurt, but have never added enough to even worry about.
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Old 12-23-2011, 11:47 PM   #5
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How much used concrete can be use with new concrete


You're gonna cast a driveway with bagged mix from HD?
That is just plain a bad idea.

Have you done an overall cost analysis compared to using Ready-Mix.

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I'd be using bagged ready-mix from HD. How much the old concrete blocks can I add to the mix without ruining it by making it a lot weaker?
Ready-Mix will typically contain 60 to 75% aggregate and the rest will be a combination of cement, water, and possibly fly-ash. Other additives are available to prevent freezing, or stall curing, or reinforce the mix, or several other possibilities having to do with mix-design. You can bet the bagged mix already contains that basic same recipe.

Adding weird stuff can cause cracks and premature failure of the installation. Standing rebar on broken blocks and bricks really isn't recommended and could cause damage to the final results.
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Old 12-24-2011, 12:43 AM   #6
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How much used concrete can be use with new concrete


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Originally Posted by KarlJay View Post
I have a bunch of used concrete blocks and motor that I'd like to reuse when I pour a foundation and driveway.

I'd be using bagged ready-mix from HD. How much the old concrete blocks can I add to the mix without ruining it by making it a lot weaker?

Also, ready mix is a combo of sand/rocks and concrete, If I add some non-ready-mixed concrete, will that help the strength?
Havn't bothered to read any posts yet, so I don't know if it's already been said, but that said, your forms, when made, if you're using bagged concrete (which I'd advise against, because you could just buy sand, gravel, and bags of cement, 2 shovels gravel, 2 shovels sand, 1 1/2 shovel cement, mix together with water, you have concrete for 33% of the cost, and then whatever additives you personally choose to add to it)

when you make your forms, they should be completely clear of debris. NO FILLER!!! Not if you're using bagged concrete. You should have nothing but smooth dirt/clay. The occasional rock protruding from the ground, not too big of a deal. It's be preferred if there was no debris at all though.

To set your rebar, get metal wiring, and some pliers, and stack your rebar horizontally inside your concrete form. hopefully you're doing 12ft wide slabs at the SMALLEST. So you should lay rebar out every 12'' apart. then vertically on top of those from one edge ot another, every 8''. tie each overlap together with the metal wire.

Slabs should be no bigger than 6ft wide by 14ft long. Preferably 6ft by 12ft and make slab after slab. You don't want a single, continues slab. If you want curves in your slab, you can just take LP Siding, and some wooden stakes, and you can make your bends and curves and all that fun stuff in it.

But you need to make your driveway in sections, or it will look like complete crap. And you do not want to use any debris for "filler" because you'll screw up the slabs integrity, and it'll crack more easily.
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Old 12-24-2011, 12:45 AM   #7
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How much used concrete can be use with new concrete


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Originally Posted by Bud Cline View Post
You're gonna cast a driveway with bagged mix from HD?
That is just plain a bad idea.

Have you done an overall cost analysis compared to using Ready-Mix.


Ready-Mix will typically contain 60 to 75% aggregate and the rest will be a combination of cement, water, and possibly fly-ash. Other additives are available to prevent freezing, or stall curing, or reinforce the mix, or several other possibilities having to do with mix-design. You can bet the bagged mix already contains that basic same recipe.

Adding weird stuff can cause cracks and premature failure of the installation. Standing rebar on broken blocks and bricks really isn't recommended and could cause damage to the final results.
Bad idea? Understatement......

The biggest project I did using bagged concrete was the small space behind my garage to the rear back wall. Very small space...2'x20'.....took about 18 bags of concrete....Easy part was picking up the bags....as in, back the truck up to HD...they load a pallet on the back......yea baby...you find out how much those bags really weigh when you start to unload them. After about the 10th bag I'm thinking to myself...."This is really a stupid way to do this."

Never again.....

Adding the old concrete? An even worse idea. Sounds good until you end up with a chunk sticking out of the surface.....

Concrete is one of those things where a 'do over' does not apply....
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Old 12-24-2011, 01:03 AM   #8
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How much used concrete can be use with new concrete


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Adding the old concrete? An even worse idea. Sounds good until you end up with a chunk sticking out of the surface.....
Now THAT answers my question. I did not think about a chunk (approx same density, likely a bit less) floating to the top. Ugly. Thanks, dawg.
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Old 12-24-2011, 03:06 AM   #9
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How much used concrete can be use with new concrete


Actually the driveway is already there, I'm adding 22" side strips to it, so they really aren't that big of an area. The blocks would be busted up and mixed just like gravel.

Looks like it might be a bad idea either way, unless I use if for base under the concrete.

I was under the impression that concrete was 100% recycleable. What do people use old concrete for?

Edit: looks like Wikipedia has a site for this, I guess if I mix in concrete rather than readymixed I'd be ok.
http://en.wikipedia.org/wiki/Concrete_recycling

From Wiki:
"Crushed recycled concrete can also be used as the dry aggregate for brand new concrete if it is free of contaminants"

Last edited by KarlJay; 12-24-2011 at 03:12 AM.
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Old 12-24-2011, 03:23 AM   #10
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How much used concrete can be use with new concrete


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Originally Posted by KarlJay View Post
From Wiki: "Crushed recycled concrete can also be used as the dry aggregate for brand new concrete if it is free of contaminants"
"Crushed", in this case, would be very similar to "pulverized" if you want to avoid the aesthetic mess ddawg found out about. If you have a machine to do this for you then mixing with the concrete is a viable option. If you're planning to break up the old conc./cmu with a sledge hammer, then I suggest planning on using it for the base beneath the cast slabs.
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Old 12-24-2011, 08:35 AM   #11
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How much used concrete can be use with new concrete


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Originally Posted by KarlJay View Post

I was under the impression that concrete was 100% recycleable. What do people use old concrete for?

Typically it's only used for base under the pavement.

From Wiki:
"Crushed recycled concrete can also be used as the dry aggregate for brand new concrete if it is free of contaminants"
It's a little more complicated than Wiki leads on. The aggregates need to be washed, and really should be saturated before hand. I've never seen crushed concrete re-used in ready-mix concrete around here, mostly because aggregate is far cheaper than everything that's involved in re-using crushed concrete and certifying that it's a good replacement.
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Old 12-24-2011, 09:24 AM   #12
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How much used concrete can be use with new concrete


I second JoMama's comment. In 35 years of engineering, I have never seen old concrete reused in fresh concrete. This is different than asphalt, where old asphalt is routinely ground up and added to the mix. Broken concrete is routinely used as structural fill material, where it functions quite well.
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Old 12-24-2011, 09:59 AM   #13
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How much used concrete can be use with new concrete


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From Wiki:
"Crushed recycled concrete can also be used as the dry aggregate for brand new concrete if it is free of contaminants"
True, however your bagged mix is already properly proportioned with the proper amount of aggregate, sand and cement. If you start adding more aggregate you are changing the mix. If you want to start with a pile of sand, a pile of aggregate and bags of cement then you could possibly use some broken up concrete. I would not use the mortar as it is much softer than cement.
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Old 12-24-2011, 06:02 PM   #14
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How much used concrete can be use with new concrete


Ok, so I think I'll just truck the old stuff to the recycle yard and go with the fresh stuff. The pours really aren't that big so it won't be that much concrete.

Thanks for the input!

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