Concrete Basics for the DIYer
First of all, let me tell you what this thread is NOT. It is not intended to make you a concrete expert, since I'm not one. It is not going to give you a lot of techniques on finishing, since there are members on here that have spent years on the opposite end of the chute as I am. It will hopefully take some of the apprehension out of ordering and placing concrete yourself.
What is Concrete?
Concrete, in its most basic form, is a mixture of water, course aggregate (3/4" and 3/8" rock), fine aggregate (sand), and cement (Portland, Flyash, Slag, etc.). Modern concrete almost always contains chemical additives, such as mid-range and high-range water reducers, air entrainment, shrinkage reducers, accelerators, and retarders (I'll get to those in a moment).
The rocks and sand make up the bulk of the actual mass of the concrete, with the water and cement binding everything together. The different mixtures of these ingredients determine a lot of things, such as the strength of the different mixes.
Once the water hits the cement, the hydration process starts, and the clock starts ticking. You've got about two hours to get the concrete where you want it.
So, What You're Saying, Is That Its Not Just Flat And Gray?
Well, yes and no. It ends up that way, but ordering from your local concrete supplier will be easier (and you'll sound like a professional!) if you know what kind of concrete is best for what you're doing.
So Get To It: What Kind Should I Order?
Geez, be patient. We'll focus on two basic types, a 3/4" rock mix and a pea gravel mix.
First, we need to determine what you're doing. Pea gravel mixes won't have any 3/4" rock in them, so they are used for decorate concrete, such as stamped patios, exposed aggregate driveways and sidewalks, and garage floors with radiant heat. They're generally more expensive than 3/4" rock mixes (no, I don't know why), so they aren't used when not necessary.
For almost everything else, a 3/4" rock mix will work fine. Things like foundation footings and walls, basic driveways, sidewalks, garage floors, piers, all that jazz, a 3/4" mix works great. It finishes just as good as a pea gravel mix, without the extra cost.
Should I Get a Truck Or a Bunch of Bags?
That depends on how strong your back is. A concrete mixer loaded with 10 yards is equivalent to 500 (!) 80lb bags of Quikrete. At $3.65 per bag at Home Depot, that works out to $1825 (before sales tax), or about double what the truck would cost. That doesn't figure in the extra time it takes, since it would take you all day to mix that much in a portable mixer (which also costs money to rent), and labor ain't cheap. If you order a 10 yard truck, it could be poured out in 15 minutes, as opposed to 8-9 hours.
If your job is less than one yard, you may want to look at bags. Concrete companies will charge a short load fee (since that truck isn't making much money hauling one yard), and that brings costs up. Its still a lot easier, but less cost effective, to order a truck.
How Much Do I Need?
Concrete is sold by the cubic yard 3x3x3. It is almost never placed 3ft thick, so for a slab, you measure L (ft2) x W (ft2) x H (in2) / 324. Or, you can go here:
and let a computer do it for you. If you order a truck, get at least a half yard more than what you think you need. There are many variables (grade variances, concrete sticking to the inside of the drum, etc.) which make getting the exact amount difficult. If you have to use a concrete pump, get at least a yard more.
You Said You Would Explain The Additives, But You Haven't Yet
Good Lord, you are impatient. So, without further ado:
Water Reducers (mid-range and high-range) - These are chemicals that, as the name suggests, increase the plasticity of concrete without using excess water (they make it wetter). Since the ratio of water to cement is a major factor in determining the strength of your concrete, the less water used, the better. You won't need to know much about these when ordering, since the mix will already be designed with them in there. Usually, though, the truck will have extra high-range with him, so if you want it a lot wetter than he brought it, he can add extra. The general rule of thumb is a gallon of high-range will increase the slump of a full load 2". Of course, there are many other factors, but the cool part about it is you can have the mix more workable, and it won't effect the strength.
Air Entrainment - Concrete naturally is about 1-3% air, or little voids in the material. This is fine for concrete that isn't exposed to freeze/thaw cycles, such as interior slabs or underground footings. However, for outdoor concrete (patios, driveways, sidewalks, etc.), you need more air voids because it will expand and contract. For this, an air entrainment additive is used to bump up the percentage to about 5-6%. The DIYer only needs to know to make sure the mix "has air" when he or she orders the concrete. The mix will already be designed with the proper amount in mind.
Accelerators - If you want your concrete to set up faster, order accelerator. There are two types, calcium chloride and non-CC. Calcium is cheaper, but can't be used with steel (rebar or mesh) because it is corrosive. If you want accelerator and are using steel reinforcement, you must get non-CC. I would recommend the DIYer not use accelerator, since it cuts the margin of error down. It is more for the professional who knows what they're doing and would like to be done quicker.
Retarder - A retarding chemical gives you about an extra 45 minutes to work with your concrete, which gives you a bit more of a margin of error. It also helps if you live in a remote area that is far from a concrete supplier, or if you can only con one friend into help you.
My Supplier Asked Me What Slump I Want. What is Slump?
Slump is a measure of how wet the concrete is when still in its plastic (semi-solid) state. A concrete tester fills a 12" tall metal cone with concrete. When he pulls up on it, the concrete inside falls down because of gravity (not just a good idea, its the law), and the slump is a measure of how far it falls. The most common slump is 4", but this differs depending on application. A dryer slump would be used in footings (2-3") so the concrete doesn't ooze out the bottom of the forms, whereas a wetter slump (5-6") would be used when doing most flatwork (driveways, sidewalks, etc.) because it makes it easier to work with and finish nicer.
If you want your concrete wetter, the driver will add 1 gallon per yard per inch of slump. So if you have about a 4" slump and want a 5, add 10 gallons to a 10 yard load. Easy as pie.
If you want it dryer, tough beans. All you can do is wait. Have the driver stop his drum for about ten minutes and then look again.
Oh, probably. Feel free to ask questions in this thread, but perhaps if you have questions about finishing or other placing issues, a professional finisher could start a thread to that effect.
Talk about timing....
I do have a few other questions and hope you don't mind me asking here.
What about Portland cement vs. bags of sakrete? Correct me if I'm wrong, but I understand the portland is the 'glue' and you have to mix the sand and gravel in at about a 1-2-3 ratio. Sakrete is already mixed.
The real reason I ask is that I have to pour a few small slabs and a few deck footings. A brick mason neighbor/friend told me I could borrow his trailer mixer and use his trailer to go get the sand/gravel. I'm debating whether to buy the portland and go get the sand/gravel or just buy bags. No doubt the mixer will be handy either way. (Haven't figured exact yardage out yet, but am thinking by the time all said and done I'll be doing upwards of 50-100 80lb bags if I buy bags, at a rate of about 10-15 bags at a time. The large variance is that I do have some other projects in the works as well and may get to them if I have all the equipment.)
What I don't know is will I save much $$$ mixing the portland/sand/gravel vs buying bags or not? He said I should just buy the fill sand with gavel in it and not to worry about getting both sand and gravel. He also said about 40 heaping shovel fulls of fill sand to one bag of portland....he seemed very confident that was correct and that he would show me. I will admit I question this, even though would trust him 100% if it were motar vs concrete. Just not sure how much concrete work he's done.
I'll also admit if I could get a lot ready at once, I've considered renting a trailer of cement from a rental ranch as it seems to come out to about the same cost per yard as mixing bags.
One last question maybe you could touch on is the additives. I'll admit I haven't done much concrete work other than the occasional small slab, unless it was a truck bringing it and those weren't ordered by me so not sure of the mix. I have been doing some reading to insure my deck footing are proper and something I came across said to use dish soap to aid in the plasticity of the concrete if you are doing a smaller job and don't want to buy the special additive. They said generic soaps were better. What do these help, just the workability of the concrete? Pretty much everything I'll be doing will be covered, so I'm not looking for a perfect surface!
*edit, just called local sand company and they said fill sand is fine sand, but said they sold a concrete sand which is a course sand. I'm guessing I'd still need gravel, correct? She couldn't answere what I would need, but said they sell a 3/4" gravel as well as many others. I do need some pea pebble for another project, is that too smooth? The also have road gravel, which has some rock in it, would that work?
Thanks and if you wish for me to move this to my own thread, just say so!
I've found that mixing it yourself with gravel and sand and cement is often more trouble than its worth. I'd just buy the Sackrete and go for it. All you have to do is add water (and a little tip: I've found that you need to add a bunch more water than it says on the bag to make it workable).
Adding dish soap would spike the air entrainment way high, and I wouldn't recommend it. Little microbubble are what air entrainment is, so you're adding a lot of them. Excess air has a very detrimental effect on strength. Just add more water if you don't want to pay for the additives. It'll decrease the strength too, but not as much as 15-20% air.
There are different grades of sand (who knew?), with play sand and fill sand being fine, building sand being more coarse. In 3/4" mixes, there are both the 3/4" rocks and the 3/8" pea gravel, in different quantities. The road gravel wouldn't work, since the rocks are jagged and there are too many fines in them.
Thanks for the info Mort. Keep it coming.
The Up-sells: My Concrete Supplier Wants To Sell Me What?
There are a few things your ready mix supplier will have that you can just have delivered on the truck, without having to leave your house or anything.
Expansion Joint - Long sections of fiber, usually in 10 foot lengths. Used if pouring a slab up against an existing slab, so when it expands (heat/cold will do that to concrete), it doesn't crack or heave.
Release Agent - Used for stamped slabs. Stamping isn't really for the DIYer, but if you're feeling brave, you can get this delivered on the mixer.
Sealer - Makes it more water resistant.
Zip Strips - Used on larger slabs when a smooth surface is desired, in place of thicker fiber expansion joints.
Molasses - Used for exposing aggregates on slabs. You mix it with water in a sprayer and spray a finished slab before it sets up. DON'T MIX IT INTO THE CONCRETE! Anything with sugar will not allow the concrete to set, and you'll have a sloppy mess forever. We had a guy a few weeks ago that asked a driver to mix it into the load. The driver was charitable, so he didn't laugh or anything (until later).
Lamp Black - Powdered color. A little inside baseball: Mixer drivers hate this stuff because it gets everywhere. If you can, get liquid color. It looks better, and doesn't get all over the truck.
Thanks for all of the information.
What is a good starting point for the proportions in concrete and grout if you have to mix your own?
Example: Strong concrete = 1 shovel cement + 2 shovels sand + 3 shovels gravel + 1 gallon water ?
Example: Sticky/creamy grout = 2 shovel cement + 4 shovels sand + 1 gallon water ?
Thanks. I can fine tune it once I have a starting point.
"*edit, just called local sand company and they said fill sand is fine sand, but said they sold a concrete sand which is a course sand."
If you want to buy sand to mix concrete yourself,you want #2 torpedo sand,and whatever coarse aggregate you decide on.
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