Concrege aggregate - round vs crushed?

[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