I have poured many small sidewalks and shed floors and never put
gravel under the concrete. I want to pour a driveway section in front
of the garage and was told by a guy at the concrete company (who came
to measure the amount needed), that I need to put gravel under the
I should mention that I always put lots of small to medium sized rocks
under concrete to save on the amount of concrete needed. I save all
rocks I get just for that use, and leave them get rained on to clean
them. Any rock that will not exceed the height of the forms, goes in
there. Because this will be driven on, I intend to put in some rebar,
which I never do on sidewalks and shed floors. I have never had any
problems with these sidewalks cracking or substantially lifting in
On May 6, 3:43ï¿½am, mister firstname.lastname@example.org wrote:
I agree with the others, to drain any water that may accumulate under
the pad and possibly freeze in the winter. If you live in a warm
climate, not so much of a problem.
Since I live where it can get to 25 below, I usually will drop the pad
down about an inch from the garage floor for 2 reasons. 1. The
concrete will rise in cold temps. 2. To keep any water from entering
I wish you had asked him for a better clarification.
Coarse gravel, like 57 stone, under a slab is used as a capillary
break to prevent ground moisture wicking up through the concrete.
All good concrete needs the bottom almost as flat as the top,
creating a uniform thickness to maximize strength. The concrete
is prone to cracking wherever the slab gets thinner no matter how
thick it is. This is usually accomplished by using a select
subgrade fill which varies around the country depending on what is
available and inexpensive. The material should have a low plastic
index, have some self-compacting characteristics, and be easy to
work. Perhaps in your area this material is a compactible gravel.
Keep the whole world singing . . . .
On May 6, 12:43 am, mister email@example.com wrote:
Gravel doesn't exactly make the underlying soil more water permeable.
What it does is allow airspace (at least for a while) between rocks
which permits water to flow through the gravel layer and get access to
a larger surface area of soil.
Without gravel, the soil near the center of the slab stays dry, while
the soil near the periphery gets soaked. With the gravel layer, there
is a slightly more even distribution of soaking.
All the answers you got were good ones. The gravel under a slab
permits water to even itself out and hopefully exit from under the
slab. It makes it easier to level the underside to prevent
thin/thick areas in the slab. The side benefit of water and
thickness control is that the slab will be more stable in a frost
situation and will not crack as readily under loading.
By the way, don't forget to use WWM. I don't think it contributes
much strength to most slabs, but tends to hold the crack tight and
level when the inevitable happens.
replying to Nonny, Kendall Concrete wrote:
I disagree with the WWM. I have replaced and poured concrete for over 30 years
and what we see is that concrete is going to crack no matter what you do. When
cracks occur in concrete containing WWM, moisture gets in and rusts away the WWM
within a year or so. I have torn out nicer looking concrete without WWM than
most with it. Another thing is that when we cut control joints in the concrete,
that causes controlled cracking which then rusts out the WWM. As far as gravel
under a slab draining water- What happens when there is nowhere for the water to
go? The gravel stays saturated. And again, I have torn out some very nice
sidewalks that had no gravel under them and some were 30 years old and older.
Gravel allows the moisture to drain into the dirt below. This will prevent
cracking in your concrete. If you extend run of or drainage pipes runs of
gravel are very worth the time. Hope I was able to help. If further help
Is needed feel free to email me at firstname.lastname@example.org
I have poured concrete for 30 years now and can show you many places where
we never used gravel under the slabs and they look just as good as those
with gravel under them. Too many people watch Bob Villa.
If you have good drainage and no frost, gravel is not required. If you
have good drainage and frost, you might get away without gravel. If
you have no frost and iffy drainage you might get away without gravel.
If you have poor drainage and get heavy frost, you likely will
Gravel is what you use when you want to move water away from a structure
and you want the water to flow passively and freely - aided by gravity.
Having a concrete pad sitting on gravel on a low spot of ground is not
doing the pad any good.
A concrete pad sitting on a high spot of ground will have a natural
tendency to dry out and gravel isin't needed.
In fact, I can't think of any reason why a pad would need gravel under
In the case of foundations, you want any water that reaches the
foundation to flow easily down to it's base where (hopefully) it's
collected by a tile, pipe, etc. That's where the gravel helps.
If you pour a pad on top of gravel, and you don't put down a membrane to
separate the pad and the gravel, then what you end up with is a pad with
a very jagged, permeable bottom surface where repeated cycles of
freeze/thaw will eventually break up the concrete if any water reaches
the underside. What you want is a smooth bottom surface, which is
naturally impermeable to water infiltration and therefore resistant to
Gravel is the best capillary break possible under a slab floor. It is
even better if the gravel is capped with Perminator or other heavy mil
vapor barrier, but the gravel by itself will usually insure freedom from
vapor transmission problems. The gravel has little or no value for the
concrete itself. Each geographic area has a compactible select fill of
some type that is used under commercial slabs. Here it is red select
which is decomposed sandstone. It is fairly easy to generate Proctor
densities/ modified Proctors at 95% or higher.
Here's a good recipe for a strong well supported slab with best attempt
at preventing moisture problems:
Removal of loams and other organic soils and, in some cases, high
plastic index clays.
Proper compaction in six inch lifts of compactible fill to subgrade.
A six inch lift of 57 stone as a capillary break.
Heavy mill vapor barrier.
Concrete with Water/Cement ratio below 50, using plasticiser if necessary.
You've now done everything as best you can for slab and finish floor.
replying to mister_friendly, clint wood wrote:
I'm surprised that not a single person here actually knows what aggregate
beneath a concrete slab is actually used for. It has nothing whatsoever to do
with moisture drainage. I think we all know that concrete is not water
permeable. Any water that makes its way beneath concrete has wicked through the
dirt from adjacent areas and will continue to wick its way out from beneath the
concrete. Aggregate beneath a concrete slab is used entirely to minimize
settling and to more evenly distribute the load the concrete imparts onto the
soil beneath. Soil compacts easily, and the weight of a vehicle will cause your
driveway to eventually compress the soil beneath and ultimately lead to concrete
failure. Aggregate spreads a concentrated load over a larger area, thereby
minimizing soil compaction. Generally speaking aggregate is not needed beneath
a sidewalk because a sidewalk is typically exposed to loads that are
insufficient to produce soil compaction.
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