Concrege aggregate - round vs crushed?

[Egon]

As a professional cement finisher for several years I can say that finishing does not change water/cement ratio

Only if it is done properly.

[QUOTENow you're letting that 'Super Star member of the month' thing go to your head. ][/quote]

Nope!

But I can recall a time when I was involved with miles of concrete roadway, curb and gutter or sidewalk's being poured.:thumbsup:

 

[CurlyDave]

I think the answer might be that crushed rock has more surface area than round rock and will require more cement in the mix to cover all of the surface area.

For the greatest strength crushed aggregate is necessary, but this is strength well beyond the ordinary.

I am only an amateur concrete finisher, but once worked for a company that searched far and wide for the aggregate that made the strongest possible concrete in order to make nuclear reactors from it. We did a lot of research on this and wound up with crushed from a particular quarry.

 

[zonta223]

Hi
Well I have never poured big foundations.
Never done highways or footpaths etc.
But what I do everyday is make concrete in a test lab .
A test mix consists of (for 30 MPA concrete).
5.6kg of cement.
15 kg of sand.
11.okg of 10mil aggregate.
8.0Kg of 14 mil aggregate.
4.0kg of water. (added is 19mil of GWA to reduce water usage)
That is for cement with no flayash or slag added.

To increase the strength to 40mpa.
Cement 7.47kg
sand 13.5kg
aggregate the same as above.
Water slightly less. (the exact amount determined by a slump test)

We cast concrete cylinders and break them at 1,3,7,28,56 day intervals.
The mix is measured to 1gram tolerance.
The sand , aggregate etc is dried to set moisture levels.

The only variable is in the cement.

However if you mess with the above water cement ratio by as little as 100mls is can have a dramatic effect on breaking strengths

Just something to keep in mind when mixing your concrete.

regards

 

[Egon]

That's nice information to have Zonta.

 

[zmoz]

Hi
Well I have never poured big foundations.
Never done highways or footpaths etc.
But what I do everyday is make concrete in a test lab .
A test mix consists of (for 30 MPA concrete).
5.6kg of cement.
15 kg of sand.
11.okg of 10mil aggregate.
8.0Kg of 14 mil aggregate.
4.0kg of water. (added is 19mil of GWA to reduce water usage)
That is for cement with no flayash or slag added.

To increase the strength to 40mpa.
Cement 7.47kg
sand 13.5kg
aggregate the same as above.
Water slightly less. (the exact amount determined by a slump test)

We cast concrete cylinders and break them at 1,3,7,28,56 day intervals.
The mix is measured to 1gram tolerance.
The sand , aggregate etc is dried to set moisture levels.

The only variable is in the cement.

However if you mess with the above water cement ratio by as little as 100mls is can have a dramatic effect on breaking strengths

Just something to keep in mind when mixing your concrete.

regards

That's very interesting! Have you done any research specifically about the differences between round and crushed aggregate?

 

[zonta223]

That's very interesting! Have you done any research specifically about the differences between round and crushed aggregate?

Hi
No all the aggregate in used in South Autralia is what we call crushed "blue metal"
What we try to do is make concrete that is universal through out Australia so results are relative to other cement works.
We also do tests using cement and a special sand called Norm Sand.
This is a sand (from Germany) used by labs all over the world mixing it with specific (450 grams) of cement and 225 grams of water. So labs worlwide can compare results.
Its interesting testing cement from around the world to see how many shades of "grey" cement the world produces.

regards

 

[zmoz]

Wow, I could really impress my friends if I told them I made my concrete out of imported German sand and Australian aggregate! Most people think I'm nuts for wanting to mix my own concrete in the first place...

 

[wroughtn_harv]

If you do a search on my name you will often see an old nine cubic foot concrete mixer with an old man stuffing it full. I mix my own concrete for just about everything I do.

The nine cubic foot formula that I picked up from my dad who learned it long before he passed it on to me is:

ten gallons of water if the sand and gravel is dry, less water if it's wet.

one ninety four pound bag of portland cement (if it's for fence posts or where finish isn't a concern, paper bag and all).

Add rock and sand (we usually order re-mix which is concrete sand and three quarter rock re-mixed before it's loaded into a truck) until you get the mix you want. This is for three thousand psi mix. I add portland to bring it up if I need stronger concrete.

 

[zonta223]

Wow, I could really impress my friends if I told them I made my concrete out of imported German sand and Australian aggregate! Most people think I'm nuts for wanting to mix my own concrete in the first place...

Well as I said before don't mix a lot of concrete but do make a lot of adobe mix for water tank bases etc. I buy cement at $25 a ton so cheaper to make adobe than concrete and for what I need to use it for does a great job.
Now just need a box blade so I can make a cement drive:laughing:

 

[Dargo]

I can tell you that ripping out concrete that was made with 'pea gravel' vs. crushed limestone, you can tell a HUGE difference!! The pea gravel (mix you'd use for exposed aggregate) is 10X a hard to tear out and break up. Adding fiber to the mix is a joke. I can't tell at all if someone has put 10 bags of the stuff in their mix; it doesn't make it any more difficult to bust up and tear out.

A good example is to take a piece of crushed limestone aggregate, put it on a slab of thick steel and smack it with a small 3 lb. sledge hammer. You'll get dust. Try the same with a pea gravel river rock and your metal will ring and you'll maybe split the rock at best. It's a much harder rock by a large margin. If you use a 5000 psi mix with river rock along with heavy wire matting and rebar and it's an all day job to tear out a 12'X12' section which will require extensive use of a demolition saw. Use regular 3500 psi mix with all the fiber you want to add and it's maybe a half hour (if that much) to tear it out. There is positively no comparison between the two.