Concrege aggregate - round vs crushed?

[zmoz]

I have a few projects that I want to finish, and I plan on buying a concrete mixer to do it. I have noticed that generally under concrete they use compacted crushed gravel...because it packs down better, but in the concrete mix itself, usually they use round river rock for aggregate.

Is there a specific reason for this? I happen to have about 10 extra yards of crushed gravel and sand from another project, I'd like to use this stuff as my aggregate. Is there any specific reason I shouldn't?

 

[Egon]

in the concrete mix itself, usually they use round river rock for aggregate.

Cost. Don't have to crush the rock.


Shouldn't be a problem using your crushed rock for concrete mix.

 

[zmoz]

Huh, I thought the crushed was cheaper...I only paid $11/yard. That's why I bought too much. I guess it depends largely on where you are, where I live there are hills of basalt being mined for gravel all over the place.

While we're talking about it, does the size of the aggregate have any impact on the strength of the finished product?

 

[Egon]

Running a gravel crusher costs money.

On size its a yes and a no. It will depend on the design and rebar placement and clearance from edges and rebar all come into play. The ultimate strength of concrete is determined by the aggregate.

Note: 40 years out of date.

 

[oldnslo]

On crushed Vs rock, this depends on where you live. In central MN where I was raised it was 99% gravel straight from the ground because it was plentiful. Here in northern IL it is 95% crushed since this is limestone country.

On aggregate size Egon gave a good answer. The only other variable might be thickness of the concrete pad. 12" thick pad Vs 4" may allow a little larger rock but it still comes down to rebar placement.

Roy

 

[zmoz]

Thanks for the info guys. THEORETICALLY speaking, if I poured a 4" slab (like a sidewalk) - would it be stronger with 1" aggregate vs 1/2" aggregate?

Another thought - crushed rock should have more surface area than round, right? Would that also mean more contact area for the cement, and better strength?

Not that I'm doing anything too important - a few posts, a few bricks, and a few square feet of slab. God knows what I'll use the excess for...

 

[EddieWalker]

Concrete has been refined into a science where each little modification can have significant results. Depending on what you are doing, it might make a difference, or it might not. For a walkway that will only see human traffic, then you can get away with a lot of things that you wouldn't want to do for a foundation.

There is an ideal size for your rock, and then what will get the job done. For a walkway, you won't notice any difference. What you really want to address is how will you keep it from cracking?

Rebar works great for adding strength to the pour and fiberglass helps cut down of cracking. Concrete that is exposed to the elements, like walkways, expands and contracts. You have to provide for this or you will have more cracking then normal. A solid base is one of your most important considerations. For a walkway, dirt is fine if it's compacted dirt and not loose fil. Be sure that water drains away from it and not wanting to go under it.

Use the rock you have, it will work fine for a walkway.

Eddie

 

[Egon]

Another thought - crushed rock should have more surface area than round, right? Would that also mean more contact area for the cement, and better strength?

Same strength.

Check out the Portland Cement Association website and spend some evenings reading. :thumbsup:

 

[/pine]

the basic formula for (average) 2000# concrete is "1-2-3"

one part portland, two parts sand (fine aggregate) , three parts rock (course aggregate)...The water to portland ratio will also effect the compression strength...

If you can get "crusher run" it is usually about the perfect mix of fine and course aggregate...perfect for making concrete...just add portland..."air entraining" agents that will protect it against freeze-thaw cycles can be found at the big box stores...likewise for fiberglass re-inforcing...

 

[MikeD74T]

zmoz, Portland cement & sand is the glue that holds the rocks together. Fill a bucket with round rocks & another with crushed rocks. Which can you push a stake into easier? The better the rocks resist moving the stronger the concrete. BTW rebar and fiber are used to increase tensile strength, aggrigate determines compressive strength. A few years ago I poured some warehouse floors for a Watts Fluid Valve foundry. It was a 6" floor with 1-1/2" crushed stone with no reinforcing rod ,wire, or fiber. They were running forklifts with bins of castings on it in 3 weeks with crush tests coming back at 3800 lbs. Good concrete is about good ground compaction & stability and a good mix. Reinforced concrete is for vertical work or flatwork on unstable ground. MikeD74T