How necessary is gravel under concrete?

[tange...@toyotamail.com]

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

[Pat]

> 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.
>

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.

[Tony Hwang]

And no rebar?

[Sonny]

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.

[Sonny]

> And no rebar?- Hide quoted text -
>
> - Show quoted text -

[Sonny]

On Apr 27, 1:05 am, Tony Hwang <drago...@shaw.ca> wrote:
>

> And no rebar?-

Rebar for footings, yes. It's also a good idea to use reinforcement
wire on any large slab.

[Doug]

On Fri, 27 Apr 2012 00:05:55 -0600, Tony Hwang <drag...@shaw.ca>
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.

[Jim Elbrecht]

"Doug" <noemail...@msn.com> wrote:

>>
>>And no rebar?
>
>
>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.

I like some iron in there, too-- but I keep hearing folks who know a
lot more about it than I do say that the fiberglass right in the mix
is a better binder. To a casual observer it looks like there is
*no* reinforcement, but it is in the mix.

Jim

[Steve Barker]

There'll be 1/2" rebar on two foot centers in any slab i own. No steel
in concrete is just stupid.

--
Steve Barker
remove the "not" from my address to email

[Robert Macy]

On Apr 27, 6:22 am, Jim Elbrecht <elbre...@email.com> wrote:

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.

[Doug]

On Fri, 27 Apr 2012 09:22:15 -0400, Jim Elbrecht <elbr...@email.com>
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.

[tange...@toyotamail.com]

On Fri, 27 Apr 2012 07:39:06 -0500, "Doug" <noemail...@msn.com>
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.


---
* Message Posted by computer - Cell phones kill from radiation *

[Home Guy]

tange...@toyotamail.com used improper usenet message composition style
by unnecessarily full-quoting:

> When rebar is used, how do they keep it IN the concrete, so it's
> not laying right on the ground?

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

> As far as mesh, the seems like it would be much harder to keep
> off the ground.

Any of those re-bar chairs could also be used for wire mesh.

[Jim Elbrecht]

tange...@toyotamail.com wrote:

>On Fri, 27 Apr 2012 07:39:06 -0500, "Doug" <noemail...@msn.com>
>wrote:
>
>>On Fri, 27 Apr 2012 00:05:55 -0600, Tony Hwang <drag...@shaw.ca>
>>wrote:
>>
>>>
>>>
>>>Pat wrote:

-snip-

>>>> You say nothing about your climate or location. Do you have freeze thaw
>>>> cycles?

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

-snip-

>
>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.

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

>
>As far as mesh, the seems like it would be much harder to keep off the
>ground.

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

[Steve Barker]

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

>
> As far as mesh, the seems like it would be much harder to keep off the
> ground.

same procedure.

>
>
> ---
> * Message Posted by computer - Cell phones kill from radiation *

[DanG]

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.

--


___________________________________

Keep the whole world singing . . .
Dan G
remove the seven

[Doug]

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.

[Hank]

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

[Doug]

On Sat, 28 Apr 2012 03:38:47 -0700 (PDT), Hank <nineb...@aol.com>
wrote:

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

>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.
>

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.

>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).
>

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.

[Hank]

On Apr 28, 8:48 am, "Doug" <noemailaddr...@msn.com> 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.

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 :-(

[Doug]

On Sat, 28 Apr 2012 07:34:20 -0700 (PDT), Hank <nineb...@aol.com>
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 ...

[tange...@toyotamail.com]

On Sat, 28 Apr 2012 03:38:47 -0700 (PDT), Hank <nineb...@aol.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

Thanks for all the replies

I'll look up those plastic rebar chairs.

I plan to order 3.5 yards to make sure I have enough. I have a barn
with a small feed room, and want to put concrete in there too. What ever
excess concrete is left over will go in there. The room is only 6 ft
wide, so I'll just have a 6 foot 2x4 that I can move to accomodate
whatever is left over. Whatever part of that room is not covered, I can
do later with bagged concrete, since I have a concrete mixer. But 3
yards is too much to mix by hand.

I'm assuming that when I use rebar, in both directions, that I skip the
place where there is a joint. In a 20ft run, that should only be one
joint.

Thanks

[Sid in Sacramento]

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

--

[Cynthia O.]

replying to tangerine3, Cynthia O. wrote:
Rebar sits in chairs. The newer chairs do not require wire ties around the
rebar, and are optional. At least 2 rows of rebar, placed parallel, and equal
distance in the concrete footer, are required. WWF is used in slabs, and also
sit in chairs to keep it off the ground and at the correct height.

--
for full context, visit https://www.homeownershub.com/maintenance/how-necessary-is-gravel-under-concrete-694985-.htm

[Cynthia O.]

replying to Robert Macy, Cynthia O. wrote:
I am only replying because others may read it....even though it is a 5-year
old question. Using rebar OR WWF (you don't need both) in concrete is not
only the right thing to do, but is the smart thing to do. Those who say it is
"old" do not know concrete and its ability to crack. Concrete has excellent
compressive strength, but poor tensile strength. Code requires certain
concrete applications to have either rebar or WWF (Welded Wire Fabric AKA
WWMesh). You put rebar rods in footers (on chairs), and you put WWF in slabs
(on chairs). This places the rebar / WWF off the ground, and into the
concrete where it is doing what it is supposed to do.