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Note: 40 years out of date.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

 

[Egon]

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.

What does the Portland Cement Association say?:thumbsup:

The cement is the glue. Sand is part of the aggregate of a well graded cement mix.:thumbsup:


Shape and Size Matter

Particle shape and surface texture influence the properties of freshly mixed concrete more than the properties of hardened concrete. Rough-textured, angular, and elongated particles require more water to produce workable concrete than smooth, rounded compact aggregate. Consequently, the cement content must also be increased to maintain the water-cement ratio. Generally, flat and elongated particles are avoided or are limited to about 15 percent by weight of the total aggregate. Unit-weight measures the volume that graded aggregate and the voids between them will occupy in concrete. The void content between particles affects the amount of cement paste required for the mix. Angular aggregate increase the void content. Larger sizes of well-graded aggregate and improved grading decrease the void content. Absorption and surface moisture of aggregate are measured when selecting aggregate because the internal structure of aggregate is made up of solid material and voids that may or may not contain water. The amount of water in the concrete mixture must be adjusted to include the moisture conditions of the aggregate. Abrasion and skid resistance of an aggregate are essential when the aggregate is to be used in concrete constantly subject to abrasion as in heavy-duty floors or pavements. Different minerals in the aggregate wear and polish at different rates. Harder aggregate can be selected in highly abrasive conditions to minimize wear.

 

[zmoz]

The void content between particles affects the amount of cement paste required for the mix. Angular aggregate increase the void content.

Seriously...? I thought the reason crushed rock packed down better is because it fills the voids better?


For what I'm doing it's not going to be all that important, I could probably use kitty litter and broken beer bottles.

 

[Egon]

Seriously...? I thought the reason crushed rock packed down better is because it fills the voids better?

Yes!:thumbsup:

The Aggregate is being used for very different purposes. For base material a well graded crushed aggregate properly packed is the way to go go.

Concrete is not packed. You want the entire surface of the aggregate particles to covered in cement paste which is the glue that binds.

You will have no problem using your crush in a gravel mix.:thumbsup::thumbsup:

It's all about economics and service conditions.

Just a little Google research will give all sorts of answerers.

 

[MikeD74T]

Egon, I think you may be misunderstanding some of your researched data. I've poured many acres of concrete floors. This does not make me an expert on mixing concrete but I've never got concrete from a company that uses screened rock, which would be more round than angular. Everything I've ever poured contained crushed stone. Your selected text says that round aggregate has more effect on cements ability to flow than it's strength. If the strength was the same & round stone/sand flows better, why do concrete companies use crushed stone? I also know that beach sand, very round, makes terrible concrete.
And for what it's worth, cement finishing is in fact a packing/compacting process. MikeD74T

 

[zmoz]

Everything I've ever poured contained crushed stone.

It's interesting you say that, because (although I am no concrete professional) all the concrete I've ever seen has the round rock. That's why I asked this question. I just chiseled a hole in the concrete floor in my shop, 12" thick. (concrete must have been cheap that year?) It was all round stone in the concrete, and then crushed underneath, from about 15 years ago. And I know every time I've bought an 80# bag of "premix" it comes with round rocks.

I think for 'piddley' little projects like mine it's not going to matter much, although it does make sense that crushed rock could need more water in some circumstances. I'd bet it is mostly a matter of location, and which type is cheaper...

 

[Egon]

, cement finishing is in fact a packing/compacting process.

The least work possible should be done when finishing cement to retain the proper water cement ratio. The finishing is really a smoothing operation.

Actually the data posted was for others reference. I didn't have to research for the comments I made.

Not having seen uncrushed aggregate in concrete in your area is very likely a result of the type of aggregate available and the ability to grade properly.

I'd bet it is mostly a matter of location, and which type is cheaper...

:thumbsup::thumbsup::thumbsup:

 

[lostcause]

it's been a while since i got an education (and even longer since i was paying attention) but if i remember correctly, you'll find that there are multiple sizes of aggregate required for concrete. that way when two larger and oddly shaped pieces do not fit together closely, the smaller aggregate fills the voids instead of the cement, which does not have any real strength.

think of it like a stone fireplace. generally, people pick or cut the stones so that the shapes fit together with a minimum of mortar between them. take that image and extrude it out in a third dimension and you have a working concrete mix.

also, round (smooth) aggregate is good if you are going to do an exposed aggregate patio or walkway.

 

[Egon]

instead of the cement, which does not have any real strength.

The cement has great strength. It holds the concrete together. Ultimate strength depends on the strength of the aggregate.

Beach sand usually has a lot of organic material include which reacts with water to form liquids that are very acidic or basic.

[is all that white in the sand white rock or the calcium remains of crustacean/ mollusk and such shells?]

 

[MikeD74T]

It's interesting you say that, because (although I am no concrete professional) all the concrete I've ever seen has the round rock. That's why I asked this question. I just chiseled a hole in the concrete floor in my shop, 12" thick. (concrete must have been cheap that year?) It was all round stone in the concrete, and then crushed underneath, from about 15 years ago. And I know every time I've bought an 80# bag of "premix" it comes with round rocks.

I think for 'piddley' little projects like mine it's not going to matter much, although it does make sense that crushed rock could need more water in some circumstances. I'd bet it is mostly a matter of location, and which type is cheaper...

It may be a regional thing. Had friends go to Virginia for a project a few years ago & they could not buy crushed stone, only grades of "river rock". They were doing drainage & I don't know what was in concrete there. MikeD74t

 

[MikeD74T]

The least work possible should be done when finishing cement to retain the proper water cement ratio. The finishing is really a smoothing operation.

Now you're letting that 'Super Star member of the month' thing go to your head. As a professional cement finisher for several years I can say that finishing does not change water/cement ratio & the "smoothing' operation is in fact micro compaction. MikeD74T