Replies

1. 
   DanH | Apr 13, 2014 02:13pm | #1

   There are a half-dozen different ways (and different philosophies) of pouring a slab.  The main distinction is whether you have a "floating" slab that rides on top of the soil vs a slab with a proper (more or less) foundation around the edges.  For a dog kennel the floating slab is sufficient and probably better.

   You do want the soil under the slab to be reasonably prepped, though.   If much soil is moved to level the area it must be reasonably well compacted so it doesn't subside too much later.  Sand/gravel is a little easier to get adequately compacted, vs normal soil.

1. 
   bry123 | Apr 13, 2014 05:20pm | #2

   thanks for taking time to help

2. 
   AndyEngel | Apr 14, 2014 09:26am | #3

   Another advantage of gravel

   Gravel provides a break between the slab and the ground which allows the slab to move with changes in temperature. This should reduce the likelihood of cracks.

1. 
   deadnuts | Apr 14, 2014 01:31pm | #4

   Really?

   The floating slab  will move with changes in temperature when poured directly on the earth.  Gravel need not be present. So my question to Andy is: What kind of "break" is the gravel providing...and why does this reduce the likelihood of cracks?

1. 
   AndyEngel | Apr 14, 2014 01:38pm | #5

   It depends on several factors. Will the earth be smooth enough to allow motion? Protruding rocks, for example, might anchor the slab. Or the concrete could freeze fast. Or, as a wholly practical matter, you might need to dig down through a significant thickness of topsoil to get to mineral soil that you might pour a slab on. In that case, gravel certainly makes sense as fill material.

   And never forget a Fine Homebuilding editor's innate ability to overthink things. Honestly, I've poured patio and garage slabs right on grade more than once with no problem. It really depends on the situation.

1. 
   deadnuts | Apr 14, 2014 08:11pm | #6

   Let's get real.

   Andy,

   With all due respect, I think you're out of your league when dispensing advice about pouring concrete. First you advocate for gravel subgrades, then you say it "depends".  The only thing it "depends" on is if your too cheap or lazy to do it.  Also, concrete doesn't freeze; liquid water does. Saying concrete has the potential to "freezes fast" is like saying that a rock has the potential to freeze fast. Lastly, it should go without saying (at least in a building forum) that you don't pour concrete over organics.

   Look, I've been designing, prepping, and pouring residential concrete for almost 35 years. I can't think of any slab on grade situation where pouring concrete directly on existing grade is more advantagous than pouring gravel over a gravel subgrade. What really "depends" here are the details of how that's done within the parameters of the specific job at hand. The OP contained very few specifics regarding a general question. Really, the most appropriate answer is a simple "Yes". To use this post to go off on tangents and provide generalizations (which amounted to misinformation) about why concrete cracks is, in my opinion, foolish.

   Cheers!

1. 
   AndyEngel | Apr 15, 2014 10:19am | #7

   You're welcome to your opinion...

   ACI best practice is a compacted gravel subgrade with positive drainage, and I agree there's no good reason from a construction standpoint to skip the step. Except, we're talking about a dog run, so how much does it really matter? Enough to spend the time and money on a load of 3/4 clean? Maybe, maybe not. I've seen a lot of slabs done that way that have held up. And a lot that haven't. But again, it's a dog run, not a garage. As to not placing concrete over organics, I think you may overestimate the knowledge of some of the folks who visit here. Not everyone knows that.

2. 
   deadnuts | Apr 15, 2014 12:56pm | #9

   where's the logic?

   AndyEngel wrote:

    Except, we're talking about a dog run, so how much does it really matter?

   What  kind of logic that? You might as well carry that nonsense further and admit that you don't need a concrete slab in the first place. Dogs have been running without the aid of concrete slabs since before the dawn of mankind. Man has introduced enough crappy impervious surface material upon our plant; we don't need more.

   Sorry Andy. We're commenting and advising in a Finehomebuilding forum on construction techniques. You, in particular, should know better. If it's worth doing (and I'm not saying it is in this case), it's worth doing right.

   By the way, the type of "break" you had a loose grasp of is called a capillary break. Capillary breaks can keep the concrete from wicking up unneccary moisture which can lead (amongst other things) to premature slab failure.

   Cheers!

3. 
   DanH | Apr 15, 2014 07:32pm | #10

   You're right -- it should have a full foundation down to below the frostline.  And, since it will not be heated, the slab must be poured on a hollow form, so that expanding soil below will not affect it.

4. 
   deadnuts | Apr 15, 2014 08:03pm | #11

   Dan,

   Two questions regarding your last post here:

   1. What do you mean by  "full foundation"?

   2. What sort of "hollow form" for the concrete pour must be used here?

5. 
   DanH | Apr 15, 2014 10:20pm | #12

   1) What it sounds like -- a poured or filled-CMU wall running down to footings of the appropriate width.

   2) What it sounds like -- a "disposable" form under the concete, generally made of light plywood, that supports the slab off the soil several inches.  (Sometimes you can get away with cardboard boxes.)

   (Oh, and I forgot the rebar -- or maybe post-tensioned cables.)

   (Note to the OP:  I am being facetious.  Not sure what Deadnuts is being.)

6. 
   deadnuts | Apr 16, 2014 07:41am | #13

   Sounds just like this...

   Cardboard box forms. What a great idea Dan! You're so smart.

   But don't you think the bleed water from the concrete pour will cause them to collapse?

7. 
   AndyEngel | Apr 16, 2014 09:14am | #14

   Respectfuly, I disagree.

   It's a dog run. Building some things to the highest standard is a waste of time and money. In my life at least, those are finite resources.

3. 
   Mike_Mahan | Apr 15, 2014 10:59am | #8

   Not that it applies here, but anytime concrete is poured over a poly vapor barrier, at least 2" of sand should be placed  on top of the poly. Pouring concrete directly over poly can result in improper curing.

   Most of the soil in my area is decomposed granite, an ideal base for concrete.

1. 
   florida | Apr 19, 2014 04:29pm | #16

   In my 40 years in construction including building shopping centers, office buildings, houses, patio's, you name it, I've never seen sand placed over poly. I've never seen it done on adjacent jobs or jobs I've driven past. Never.

   Here's a Building Science article refering specifically to sand over poly in California. The first sentence, "Perhaps it was the drug culture of the 60’s that turned brains into coleslaw but it is hard to understand the lunatic practice of placing a layer of sand over the top of a plastic ground cover under a concrete slab in California."

   http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems

1. 
   DanH | Apr 19, 2014 04:58pm | #17

   Maybe the sand was to keep the poly from blowing away.

2. 
   Mike_Mahan | Apr 20, 2014 01:07pm | #18

   I appreciate that information.

   I guess that sometimes we get stuck in "the way things have always been done" mode. My information has come from finishers with years of experience. The only time I omitted sand I had the worst cracking I've ever experienced in a small slab. I just assumed that the finisher was right. And I've never experienced the problems that the article lists in slabs with sand over the vapor barrier. But I guess it may be time to rethink things.

   Thanks,

4. 
   Mongo | Apr 16, 2014 12:06pm | #15

   I'd want...

   I'd want gravel for drainage.

   My biggest concern would be heaving and the slab possibly cracking. A compacted gravel bed would provide for that.

   If you do go right over earth, then just cut some control joints in the slab to control whatever cracking might occur. If you want to go fancy and keep the joints clean and free of debris, then get a tube of vulkem or some other self-leveling sealant usually used on pool deck joints, and fill the control joints with that. It is self-leveling, so tape the joints at the edges of the slab so the vulkem doesn't flow out.

5. 
   sapwood | Apr 20, 2014 01:34pm | #19

   Forget the concrete and install a layer of good free draining gravel. The dog will appreciate it. 

1. 
   DanH | Apr 20, 2014 02:16pm | #20

   Yeah, but what sort of base should you put under the gravel?

1. 
   Mike_Mahan | Apr 20, 2014 02:42pm | #21

   Concrete, what else.

6. 
   Mike_Mahan | Apr 20, 2014 02:48pm | #22

   Stabilized earth.

   Stabilized earth is an alternative. This is not spam:  it's  the brand I'm familiar with. http://www.stabilizersolutions.com/

1. 
   DanH | Apr 20, 2014 02:56pm | #23

   I recall, probably 40 years ago, reading an article about making a patio using screened topsoil mixed with portland cement.  IIRC (which is doubtful, given my memory), the guy first screened the soil by throwing it through 1/2" hardware cloth, then mixed dry with a relatively small amount of cement.  Packed the surface with a tamper, then wet it down.

1. 
   MarkH | Apr 20, 2014 08:05pm | #24

   I remember reading that article.  Sounded dumb to my teenage mind then, sounds even more stupid now.  But wudda I no?

1. 
   AndyEngel | Apr 22, 2014 09:53am | #25

   I don't know about with topsoil, but mixing Portland into mineral soil is a legitmate way to increase its bearing capacity for roads and driveways. Engineering is required, though.

1. 
   MarkH | Apr 25, 2014 07:50pm | #27

   I imagine mineral soil would improve in strength with added concrete, but average clay bearing or loom top soil, probably not so much.  It could possibly work for a patio, but I doubt it here where I am.  (SW Ohio)  Too much freezing, frost, and clay.

7. 
   deadnuts | Apr 25, 2014 03:37pm | #26

   Before you jump to any conclusions....

   I believe the article referred to in an eariler post about a sand base prep'd over poly is by Joe Lstiburek. While an accomplished and well respected (at least by me) building science engineer, I believe Mr. Lstiburek makes an erroneous assumption in this article that all concrete placed is of equal value. It isn't.

   Concrete strength and durability is influenced by many factors including:

   1.initial mix (incl. modifications during placement) ambinet weather conditions,

   2.duration of placement (including drive time),

   3. finishing techniques.

   If all the afore mentioned variable are perfect, then I would agree that a sand base should not be needed to ensure a well performing concrete slab pour.  Usually this means concrete that is mixed with only the amount of water needed to ensure  full hydration of cement and allowing for a low slump concrete placement.. Rarely in residential work is this achievable.

   In my experience, even seasoned concrete subcontractors have very little concept of concrete chemistry.  To the contrary, they have a much better concept of getting the work done with as little effort as possible and getting paid. This lack of concrete chemistry understanding and care can ruin a perfect batch of  ready-mix concrete. The tradegy usually begins to unfold where the delivery is asked modify the ready mix (aka "loosening")  by adding post mix water (under full responsiblity of the contractor mind you) thereby tturning a 2-3" slump into a 6-8" slump that is much easier to place, screed, and level. If the contractor is short handed on labor, he also knows a high slump pour will allow a pour to be more easily placed and worked over a longer time period before getting away from him/her.  An irresponsible finisher will also get on a slab too early ( while excessive bled water is  present) and perahps be more prone to over-troweling whereby the initail finish draws up too much moisture needed for core hydration. Since slump and finishing procedures are not normally specified and recorded on residential job sites, these poor placement factors are rarely perceived as the genesis of future failures. Thus, it is my contention that high slump pours are all too common and lead to excessive bleed water resulting in:

   1. weak concrete

   2. increased finishing times

   3. cracking, surface crazing, and spalling (premature failures)

   I believe the use of a dry sand base can help mitigate the disadvantages of high slump pours by absorbing excess bleed water and allowing for better placement and finishing.  Further, the use of a sand base can also help mitigate curling and/or plastic shrinkage binding which can be caused by poor slab preperations (i.e. lack of expansion/control joints and decoupling measures). Rarely would I find a sand base proving to be disadvantagous to any concrete slab pour.  In many cases it's cheap insurance against less than optimal pouring and curing conditions. I'm guessing that is why California has chosen to err on the side of caution and written it into code. We don't have that requirement where I operate, but if I know the conditions for pouring concrete slabs are not going to be optimal, I regularly prep with  1-2" of sand over polyethelyne.