How necessary is gravel under concrete?

I'm wanting to put a 20 foot concrete pad in front of my garage. Right now, the garage floor is concrete, but the driveway in front is just dirt. However, under that dirt, there is a bed of 2" to 3" rock, which I put there about 6 years ago. What I want to do is just make forms and have a just have a few yards of concrete delivered, to trowel myself. The base dirt is well packed and solid from years of driving on it.
To add gravel, I will have to disturb this well packed dirt with the rock underneath. (or the drive will be higher than the garage floor). Not only will this disturb the well packed base, but will double the cost of this job, because of the cost of the gravel, and the cost to rent some sort of machinery to dig up this dirt and rock base, since I already tried a shovel and with that rock base, the shovel wont even go in the ground unless it's after a rain when it's muddy, and even then the rock base could not be dug up with just a shovel.
My plan was to simply make forms, remove the small wooden ramp I made at the front edge of the concrete garage floor (I left the dirt about 4 inches lower than the garage floor intending to pour concrete, but made that ramp out of stacked 2x8's to drive in and out of the garage). Add a few bricks with some rebar on top, And pour/finish the concrete. A fairly simple job. However, someone told me I must put dravel under it first. Is this gravel really needed? If it is, why? After all, I already have the rock base, which is very solid, and rock is like gravel. It's just that dirt has filled in around the rock so it just looks like dirt now.
One other thing. For a 20 foot long by 12 foot wide strip of concrete, how many joints should I cut in the concrete? I'm thinking just one, so each section is 10ft. I plan to place one of those black fiber strips where it meets the garage slab. I'll probably just trowel the joint in the middle, to avoid having to rent a concrete saw.
Finally, did I figure this correctly?
20 X 12 = 240 sq ft. At 4 inches think, I estimated that I need roughly 3 cubic yards of concrete. (Actually 2.96 cu yd). Is this right? (I'm not the best at math).
Thanks
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You say nothing about your climate or location. Do you have freeze thaw cycles? Gravel base helps keep the water away from the concrete. Do you have seasons? Dry, wet cycles? Soil expands when it gets wet and shrinks when it dries. Rock helps isolate the concrete from soil movement. When the soil is wet its load carrying capacity is diminished. Concrete corners may not have good support and break. Generally rock under concrete is essential. Especially under driveways where there is loading from vehicles.
A thin layer of sand under the concrete allows up to 10 feet between joints. The joints control where the concrete cracks. If the joints are too far apart then a crack forms before the joint is reached. The sand allows a little slippage so the joints can be stretched a little.
I figure 2.7 yards.
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Pat wrote:

And no rebar?
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I would recommend putting a 1'X1' footing adjacent to your garage slab and one at the end of your new slab, also. Put an expansion joint at the center of your pour by placing a physical divider there. Just cutting a kerf in the concrete (after it sets up well?) is not a proper expansion joint and may likely not work to prevent the slab from cracking somewhere else.
Order 3000psi concrete. Costs the same as 2500 psi.
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Rebar for footings, yes. It's also a good idea to use reinforcement wire on any large slab.
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wrote:

I like the idea of rebar (or even mesh) in residential driveways but I've yet to see it in the dozen or so driveways under construction I've come across on Long Island or in Houston. All were poured on just sand without any gravel. Personally I like the idea of sand, gravel and rebar but that's not what I've seen so far. Of course one reason is probably to keep costs down.
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Been told also that rebar and wire are an 'old wive's tale' and not necessary today. And, worse, a huge nuisance if you try to break it up and remove later.
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wrote:

Probably right but for residential work, probably overkill besides the expense. I still like mesh but so far I haven't seen it in residential driveways and except for once, never in sidewalks neither. As someone pointed out, reinforced concrete will be harder to remove.
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wrote:

When rebar is used, how do they keep it IN the concrete, so it's not laying right on the ground? I figured I'd lay it across bricks, or pieces of broken patio blocks because bricks are a little too thick.
As far as mesh, the seems like it would be much harder to keep off the ground.
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snipped-for-privacy@toyotamail.com used improper usenet message composition style by unnecessarily full-quoting:

Rebar Chairs:
http://www.quadlock.com/images/icf_components/ICF_Rebar_Chairs.png
http://www.concreteconstruction.net/Images/885967607_1211_WOC_MIP_PS-Primesourcebuilding_tcm45-984353.jpg
http://i00.i.aliimg.com/photo/v0/222088874/Rebar_Chair.jpg

Any of those re-bar chairs could also be used for wire mesh.
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snipped-for-privacy@toyotamail.com wrote:

-snip-
I haven't seen this yet. It is of the utmost importance.
-snip-

Google 'rebar chairs' or cradles. They are plastic so they don't wick moisture up to the rebar, helping it rust.

It isn't bad-- You hook it up as you go along and make it float at the halfway point.
OTOH-- I've done a few concrete jobs in the past 40 years. Only one was a driveway and I was the bottom man in a crew of 6.
I'll do footings, piers, columns, and maybe a 10x10 square by myself or with a helper. But no way would I commit to pouring several yards on a driveway that I'd have to look at for a long time.
I'd hire it out and buy a lawn chair.
Pavers in a driveway are DIY--- concrete is for the professionals.
Jim
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On 4/27/2012 5:40 PM, snipped-for-privacy@toyotamail.com wrote:

Sometimes on large jobs they put it on 'chairs' but mostly, they just lift it as they pour. It doesn't sink back down.

same procedure.

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On Fri, 27 Apr 2012 17:40:25 -0500, snipped-for-privacy@toyotamail.com wrote:

If you can, go by a highway under construction. You will see plastic chairs used to keep the rebar off the ground before they pour.
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On 4/26/2012 10:55 PM, snipped-for-privacy@toyotamail.com wrote:

Your math is correct as far as it goes. It is VERY difficult to maintain a uniform thickness. The main way to accomplish is to carefully and accurately grade a granular subgrade material, here it would be "fill sand". Unless you have this type of tolerance, be very prepared for additional yardage. There is nothing worse than running out and waiting for delivery of a 1/2 yard to finish. Much cheaper to send back a 1/2. A truly uniform 4" slab is much stronger than one that varies from 4 to 6 (think about tire ruts from the concrete truck if you are backing into your forms). How are planning to pour?
Gravel capillary break is a best practice method under a floor slab, not under exterior pavement. It does no harm, but will add to the expense that is not necessary. What is important is proper compaction of the subgrade. If is gets goey when it is wet, this sounds like clay - wet clay expands. A typical commercial spec would require removal and replacement with a select material (red select here) or lime stabilization of the clay that is compacted to 90-95 Proctor density. I am fairly sure you are fine to go ahead with what you have.
Wire mesh is NOT reinforcement. It keeps cracked concrete from getting bigger cracks. In practice, it is very difficult to keep at the proper location. Artificial or steel fiber is considered an equivalent. Cost depends on dosage per cubic yard.
4" concrete does not gain appreciably by using #4 bar. If the bar is run both directions there is not sufficient depth of concrete for minimum concrete cover. Reinforcement is usually placed in the tension dimension of the concrete which is at the bottom 1/3 point with 2" of concrete cover from earth forming (bottom).
Your tar joint at the existing is good practice. It is an expansion joint. If the concrete is not trapped, there is no need for more expansion. Tooled or sawn joints are contraction joints. These help the concrete break in a straight line when the shrinkage forces form. Make sure that contraction joints are T/4 - at least 1/4 of the slab thickness. Concrete should always have contraction joints at least every 12' both directions.
YOu need to cure the concrete at least the first 3 days. You need to keep vehicles off the concrete at least 7 days. Make sure you use air entrainment for any exterior concrete. I would order 3500# with air. Let the bleed water come out before any trowel work of any type. If you don't know concrete, see about getting an informed buddy or assistant.
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On Apr 26, 11:55pm, snipped-for-privacy@toyotamail.com wrote:

Many times gravel is used to bring the grade up to a 4 inch pour. Gravel is cheaper than concrete. Sand may wash away from under the concrete. I advise against using sand unless you are sure it won't wash away.
Rule of thumb is figure 1 yard will do 80 sq.ft. at 4 inches. So, your math is correct.
One more thing to think about is to drop the concrete approx. 1 inch below the entrance of the garage floor. This will serve 2 purposes. One is that water won't enter under the garage door if you get a sudden wind driven down-pour of rain. The other is if there is a cold snap and the outside concrete heaves up a little.
Since the pour is only 20x12, I wouldn't worry about the gravel for drainage too much if you have a good slope to the pour ( 1 inch in 10 ft ).
I wouldn't drive on it for at least a week, 3 weeks is recommneded. Do not let anything heavy, like a car, sit in one spot very long. Even after a year of curing, a heavy object will cause the concrete to develop low spots ( puddles).
Good luck.
Hank
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wrote:

Wash away??? Yes it can but not common.

1 inch to me sounds like a lot but I guess a judgement call. I think you have a good suggestion at least for water tho I still would try to slope the driveway away from garage if possible. I've seen driveways in NY and I haven't seen heave problems but I suppose it's possible, just haven't seen it. Good suggestion tho to ?? inches below entrance to garage.

That's what I would have thought (about the one week) but I've seen industrial setting where light traffic was allowed after just 3 to 4 days. I think I heard it's ok because concrete is about at half strength then. I know I was very nervous when I first learned of this. Regardless, three weeks is overkill for most residential driveways unless you are going to have a lot of turning or heavy vehicles other than cars.
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I agree that sand washing away is uncommon. But if the underside of the pour is exposed, it happens. Keeping dirt, sod and etc. next to the concrete does away with the washing out of sand.
Since neither of us can see the site, shoot it for grade, or know the climate, I am just offering suggestions. When my garages were built, I had the men drop the concrete about a half inch, just inside the door ( like a little pocket the door sat in) and it went out to the skirt. Then the skirt dropped another half inch below that. Believe me it helps in the pour rains and heaving here in Ohio.
Also, I didn't put my e450 van on my new concrete for over a month. I parked it in a spot and didn't drive it for a couple months later. Now I have 4 little puddles where the concrete sagged. A year later a friend parked his 6X12 enclosed trailer in another spot for a few months and the concrete still sagged and created small puddles. Drivng over it probably won't do that as much as just leaving something sit on it. Also, I have seen bus stops and exits where there is 8-12 inches of concrete pads with rebar and it still sags over time becaues of the weight.
Hank <~~~~ learned from experience :-(
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wrote:

Earlier when you said an 1 inch drop I was thinking 1/2 inch myself but it is a judgement call of course.
Yep, I won't argue with experience ...
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replying to tangerine3 , Sid in Sacramento wrote:

From my understanding of concrete, you have to make it thicker that two inches minimum, to keep it from cracking easily. With that said, I would buy some wire mesh to keep it together, in the event it does crack. The job seems pretty large for one person, as the saying goes," concrete waits for nobody ". I would enlist the help from a concrete worker from Craigs list ? The wire mesh will run you about seven dollars a piece at Home Depot. Here is a website that u can use to calculate how much concrete you will need. http://www.concretenetwork.com/concrete/howmuch/calculator.htm
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