Cement questions

A cubic yard of concrete needs at least 5 bags of cement. 4 inches thick would be the minimum. It will strengthen it if you put reinforcing wire in the floor. Using the premixed bags would be far to expensive for a project this size.

 

If you want this to be a two car garage you really will be happier going 24x24. You could put a load of pea gravel or fine crushed stone on the floor and get by until your ship comes in. Using poles would save putting in a foundation until you could put in a floor. Mixing concrete by hand is hard work. it's a real back breaker for anyone not toughened in to bending and lifting.

 

I heard on local talk radio today that the price of cement was becoming more expensive because China is doing a great deal of building and has basically cornered the market - making it much more expensive here in the U.S.A. - it all sounded reasonable the way it was presented but I'm curioius - doesn't China have it's own cement producers? It must be expensive to ship also.

 

Here is a military manual that should be helpful:

http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/5-428/toc.htm
FM 5-428 CONCRETE AND MASONRY

 

The premixed bags are only intended for post holes, not cement floors, it is a lower grade and has a horribly reduced lifespan compared to the real stuff. Are you a pay now or more later after the tear out, person.

 

an 80 pound bag of premix is 2/3 of a cubic foot.
a 60 pound bag is 1/2 cubic foot.
a 40 pound bag is 1/3 cubic foot.

a cubic yard equals 27 cubic feet

a 22 x 20 pad is 440 sq feet,
most pads are 4"
thats a little more than 146 cubic feet
or 5.4 cubic yards

Thats about 220 bags of 80 lb mix

Cement to sand always confuses me. You can stretch the sand but it weakens the cement. I hear pro's say things like "7 bag mix is pretty rich." I take this to mean that 7 bags of portland cement per cubic yard is light on sand and heavy on cement. Some Portland comes in 92.6 pound bags. A yard weighs 3240 pounds. That works out to about 1 part portland to 4 parts sand by weight. I don't know enough to know how much sand is too much.

If you buy portland and sand separately you will need at least 40 bags of portland, if you scrimp a little on the cement. That means you need 4-4.5 cubic yards of sand delivered. You can replace up to 35% of the portland with fly ash, but now days they charge the same. Its still a good idea because it makes the cement easier to work and much stronger when you're done. It also dramatically cuts carbon dioxide emission during the cure.

It may not be cheaper to mix your own. Consider the cost of getting 3000 pounds of cement home and unloaded, then the delivery and purchase cost for the sand. You might save a few hundred on the whole project, but you'll have to work hard. Of course if you have 5.4 yards delivered, you'll need a crew.

 

I wouldn't even consider mixing that much concrete, even with a portable mixer. My first concern would be being able to mix it quick enough for a more-or-less continuous pour. And just the thought of shoveling sand and cement and gravel into a mixer makes my back hurt!

Also, while a 4 in. slab calculates out to about 5 1/2 yards, order at least 6 yards(10% over) just in case. I've never been sorry I ordered extra. I try to have something else formed up (stepping stones, sidewalk, etc,) in case I have too much.

 

I am going to pour part of my barn 24 x 24 and have prices it both ways. You don't save much by doing it yourself. I figure only about $20, my time is spent better doing other things.

Bob.

 

You also need to add in the cost of gravel under the slab and a moisture barrier. I hate to be negative but there is a lot more to concrete work than just knowing the correct mix. If you do this right, and you will very likely be sorry if you don't, it will take a lot more than 6 yards. I wouldn't even consider pouring a slab that size without a footing at least twelve inches wide and eighteen inches deep, or below the frost line in your location, all the way around the perimeter. Unless you have a crew willing to do a lot of hard work, and put in a long day I would forget the self mixed idea. Unless you are willing do it in sections, which is not a good idea because of "cold joint" problems you need some with a good bit of experience in concrete work to help with this project. Concrete should be poured on a continuous basis, so it can be screeded (leveled) and then worked (troweled) at the proper time. To get maximum strength and finish concrete has to be troweled, do you have access to and experience in running a power trowel? On a self mix pour that size unless you have a very large mixer, and depending on the weather that day, the first part you pour could need to be worked before you are finished pouring. While this is not impossible for you to do, unless you have some real "forming to finish" experience or can get help from someone that does, this could turn into an expensive nightmare. You don't want to spend the amount of money it will take, even if you do it yourself, and possibly end up with a unlevel, weak, or poorly finished floor.

 

I have done garage pads in an unorhtodox method, with good results. The last one was 24' x 25' x 6" deep (11.1 yards).
I had a dump truck deliver "road mix" that we spread out evenly with the tractor and scraper blade. I framed in the pad space with 2x8's prior to putting the gravel down. For a five bag mix, I purchased 56 bags of Portland Cement. I drew divisions in the gravel for the 56 bags and spread out the cement evenly. Then we brought in a rototiller and dry mixed the gravel and cement.
Next we had a garden hose on each side of the tiller putting water where it was mixing. The tiller would not get tight at the edges so we shoveled that mix into the wet mix we were making. This method uses incrediblily larger amounts of water that standard cement mixing. After is was all mixed we ran a shaker across it to drive the rocks down. That was followed up by a bull float to bring it all realitvely flat. (My handle was a bit short, so I have a dip in the back 2' of the garage). After it was set up, I troweled it out twice.

There were five of us doing the work and the actual mixing and floating phase took about 1-1/2 to 2 hours. The total cost for it was about 20 percent of what the local concrete company would have charged.

We did this seciton in '98 and park a car and pick-up on it and have no cracks.
Bret

 

Ready mix is the way to go IMHO! I have a "back to the land" friend who learned the hard way building his house. The time is what kills you as much as the workload. It is difficult to pour a slab with some curing while you are still adding fresh, and not have it crack later.

Best way to save is to be sure you get only what you need (the concrete plant can calculate it based on dimensions and depth), get a couple quotes if you can, and don't make the floor any deeper than you need to. 4 inches is plenty deep for most garages where heavy equipment won't be parked.

You can also investigate the fibered concrete, which has fiberglass mixed in it. That allows you in many cases to skip rebar and wire reinforcements, saving you some cash right there. It is very strong.

 

Although it wouldn't be cheap either, can you get by with a wood floor? Perhaps have the foundation walls and base prepared as if concrete were going to be poured. Then put down a vapor barrier, spaced apart 3" x 6" treated timbers (called bridge timbers) and then treated 2" x 4"s side-by-side across them, screwed into the timbers. If you get around to a concrete pad in the future you would just need to take up the wood.

On the fiberglass strands. When I had the creek crossing pad poured I ordered superstrength concrete as fully loaded trucks of riprap would be crossing it. On one load it had to sit and wait for the fiberglass to be brought from another plant. When it arrived the driver carried up a bag of about 3" x 14" x 24" for the entire load. However, when it was being floated you could see the strands were thoroughly mixed with the concrete.

Ken S. in WC TN

 

please bear in mind that those bags of ready mix are onlyabout a half a cubi foot;I have a half a pallet of them on my truck as I type this. I live so far out that the minuinmum the ready mix company will deliver is three yards. I only need 21 cubic fee. The cost doing it this way is 105 dollars per yard,a lot of back brakikng labor,and concrete that hasn't the strength of redimix with fiber mesh. Get the truck in there and swallow the price. Make absolutely sure that you have rebar and that they include that fibermesh. You will be happier!!!

 

While the mentioned 4 inches is adequate for the main part of the slab, the edge where the vehicles enter should be dug out at least 6 inches for about 1 foot inward; most cracks start where there is load stress. Secondly, concrete has a 'compaction factor', 5 cubic yards of concrete takes about 5 1/2 (by volume) dry materials, the air space between the sand and gravel is filled by the wet cement mix to cause this.

 

i work for a testing firm that verifies construction materials , while i can tell some horror stories of bad concrete from the ready mix companies the majority and i mean 99.9% is better than you will probably mix yourself usually the ready mix calculates out cheeper than the bag mixes and you would be handling tons! of materials and not getting it uniform by hand mixing mabey cost effective iff you do like the old timers and have lots of large rocks to get rid of and dropp them in a thin concrete to make up volume ,still handling tons and sacrificing quality , concrete should be poured as dry as possible for max streanth ,free water should not seperate from the concrete after pouring . the fibers are over rated if the slab will be exposed to the weather you will want an air additive . another consideration is if the sand and gravel you intend to use in hand mixing is sutable for concrete there could be high levels of clay ,organics or othe deleteriouse material in it resulting in soft concrete or surface spalling the ready mix companies have sand and gravel sources tested to make shure they are usable . a lot of what you do depends on local soil conditions when considering how to prepare under the slab , we have areas of expansive clays here and even the best builders have trouble.most concrete disasters are caused by attemping to pour in extream weather and ether cooking or freezing the concrete before it can cure .china was exporting cement a few years ago dropping the prices now of course they are using it in realation to the 3 gorges dam and other developement

 

Interesting Bret F!
I enjoy seeing new ways to do things.