Why Put gravel under concrete?


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 concrete. WHY?
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 winter.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On May 6, 3:43�am, mister snipped-for-privacy@the-newzgroups.com 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 the garage.
Hank
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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 . . . .
  Click to see the full signature.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On May 6, 12:43 am, mister snipped-for-privacy@the-newzgroups.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.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
--
Nonny
On most days,
  Click to see the full signature.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It also keeps the organics out of the "ground layer" of concrete. Organics from soil contact can reduce concrete strength.(somewhat).
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I appreciate everyone's answers. You lost me on the WWM. What's that?
Thanks
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On May 6, 10:38 pm, mister snipped-for-privacy@the-newzgroups.com wrote:

Welded Wire Mesh.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Fri, 07 May 2010 09:19:31 -0700, Smitty Two

Don't tell me. Let me guess. Is it "The Witch Within Me?"
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
mister snipped-for-privacy@the-newzgroups.com wrote:

Besides all the other good answers, the "medium" sized rocks you throw in can easily weaken the concrete if too large. Maybe up to about 1" would be ok but not larger.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
responding to http://www.homeownershub.com/maintenance/Why-Put-gravel-under-concrete-440616-.htm MasonJoshua wrote: 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 snipped-for-privacy@att.net mister_friendly@the-newzgroups wrote:

-------------------------------------
--
+------------------[ SERVER SIGNATURE ]------+
| Delivered via http://www.homeownershub.com |
  Click to see the full signature.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
responding to http://www.homeownershub.com/maintenance/Why-Put-gravel-under-concrete-440616-.htm KenCon wrote:
mister_friendly@the-newzgroups wrote:

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.
-------------------------------------
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 18 Aug 2011 22:53:56 +0000, KenCon

Depends.
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 experience problems.

Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@snyder.on.ca wrote:

(...)
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 it.
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 freeze-thaw dammage.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 8/18/2011 9:28 PM, Home Guy wrote:

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. Proper curing.
You've now done everything as best you can for slab and finish floor.
Add pictures here
✖
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.