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Old 09-26-2010, 01:31 PM   #1
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Teach me about concrete mixing


premix vs ready-mix vs mix your own

I will be tearing up a slab to do some plumbing, and will need to fill it back in. But I am also taking this opportunity to broaden my DIY knowledge and learn about concrete mixtures in general.

Premix bags from bigbox stores (like quikrete and sakrete) i've heard are low in quality and high in price. Meant for small jobs for DIYers.

Ready-mix delivered on trucks are good for big projects but require you to know how to order the correct mixture.

Mixing your own sand, gravel, cement, water in a portable mixer or by hand also requires you to know the correct ratio.

So my questions are:
What ratio's do you need of each ingredient?
In what applications would you mix in different ratios? (indoor slab, outdoor sidewalk/patio, mortar, floor leveling)
In slump testing, when would you want 6"/8"/4"? 3? 2?
At what point does it make sense to go with ready-mix?

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Old 09-26-2010, 03:08 PM   #2
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you heard wrong,,, the bagg'd mixes are VERY good - just look on the bag for the design strengths,,, you'll find they're stronger that what you probably have now,,, the problem arises from the mixerman adding too much water i don't consider them high $ at all &, often, we'll buy 100b to use in a bsmt rather'n picking up fine & coarse aggregate + portland cement OR calling a plant for redi-mix.

while you'reat an apron store, but the book on concrete - its much better'n asking here,,, if, when you've read AND understand it thoroughly, you have more specific ???s, we'll give them a try.

i don't understand bio-mechanics yet but damned if i'd post such a general inquiry on 1 of those boards even if i knew where to find 1

for us, general slump specs: 0" slump, hgwy barrier; 2", hgwy paving; 4", driveway/sidewalk
,,, you didn't ask this part but, along w/books, doing it w/experienced men is a good teacher, too


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Old 09-26-2010, 03:58 PM   #3
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Itsreallyconc touches on good points. I'd also say that it's infinitely easier and cleaner using bag mix. Particularly for such a tiny job like repairing a basement slab after laying pipe. Getting a delivery is the last thing I'd do for that. And when it comes to making your own mix, I stopped doing that 15 years ago. Nowadays you have a wide selection of premix concretes, mortars, grouts, epoxies, vertical patch mixes, etc. Can't get all of those at the big box, obviously, but most builder supply and mason supply outfits carry them.
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Old 09-26-2010, 04:54 PM   #4
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Originally Posted by itsreallyconc View Post
you didn't ask this part but, along w/books, doing it w/experienced men is a good teacher, too
you're right, i forgot to ask, but had intended to add that to the list of questions. Any recommendations for books and websites?

As for doing with experienced men, I would gladly offer my time in labor for some education in exchange. But so far, all the contractors I've talked to to bid on my project was not interested in that proposition.
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Old 09-26-2010, 05:21 PM   #5
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The ratios of sand/gravel/cement depend on what the concrete is for.
1/3/6 cement/sand/gravel is often used for strip foundations or a ground bearing concrete floor, whereas a 1/2/4 mix is used for structural lintels or suspended re-inforced floors etc.
The ratios may be different in your area due to conditions (such as earthquakes).
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Old 09-26-2010, 07:16 PM   #6
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Well how big is the slab you are pouring? I just finished pouring a 40 bag pour for the same thing you are doing and it was a *****!
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Old 09-26-2010, 11:52 PM   #7
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The ratios of sand/gravel/cement depend on what the concrete is for.
1/3/6 cement/sand/gravel is often used for strip foundations or a ground bearing concrete floor, whereas a 1/2/4 mix is used for structural lintels or suspended re-inforced floors etc.
The ratios may be different in your area due to conditions (such as earthquakes).
thanks. that's the type of info i'm looking for. This is stuff that's not available in books, or at least not the books i've been reading. Mostly just how to work with concrete (pouring, forms, masonry, decorative stone, etc), but not the mixture.

even if i use premixed bags, i'm the type of person that would still like to know how to do the what goes in it, or what the "old fashion" way was.
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Old 09-27-2010, 12:16 AM   #8
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Well how big is the slab you are pouring? I just finished pouring a 40 bag pour for the same thing you are doing and it was a *****!
the bathroom i'm doing is about 6x10. And then there's a laundry area that I will have to do, but don't know how much of the slab would need to be broken yet. Could be little if a drain runs right through that area and I can tap into it, or could be large if the connection point is far.
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Old 09-27-2010, 09:23 AM   #9
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or what the "old fashion" way was.
Thats high Tech for my area. We have only just finished using a mix of cow dung, straw and mud and that's only because half of the cattle have got foot and mouth.
There is a concrete mix that only uses gravel and cement known as No Fines Concrete. This was used for a lot of houses thrown up after WW2.
It was poured into formwork and the walls were 8-10 inches thick.
The theory was that the air gaps reduced the heat loss and made the ingress of water less likely, apart from being cheaper. As many suffer from damp I don't think it's that good.
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Old 09-27-2010, 01:07 PM   #10
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I usually look for a book review first: http://www.google.com/search?tbs=bks...9c50a61d5b9175

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Old 09-27-2010, 01:46 PM   #11
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If it's not too late try to avoid tearing up the whole 6x10 to do the plumbing. Just cut the paths necessary to route the dwv.
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Old 09-29-2010, 05:11 PM   #12
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If it's not too late try to avoid tearing up the whole 6x10 to do the plumbing. Just cut the paths necessary to route the dwv.
thats the plan. But the existing and new differ quite a bit, so the trenching make up a good portion of the space.

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