Pole Barn Slab Questions

[PaulT]

OK,

I've finally gotten approval for the final stage of investment in the pole barn - the slab. I intend to do all of the prep work, and have a crew pour and float the slab. I have done some searches, but still have a few questions on the design for the TBN brain trust.

Background:
* The Barn is in upstate NY (cold winters, 42" frost line is code).
* I used a lot of aggregate fill (4" crushed rock with rock dust) to level the lot, but has had almost 3 years to settle, though it has not, and is quite compacted
* Plan to trench the edges down about a foot from the existing gravel level, about 18 inches wide, and use 2 inch insulation to the top of the 2x10 skirting boards.
* Slab will be 6" thick, with wire mesh appropriately supported
* will use foam insulation and vapor barrier under slab
* plan to put radiant heat pex tubing in at time of pour
* Intend to have approach "aprons" in front of main door and side door.

Questions:
1 - how thick should the insulation under the slab be?
2 - How thick should the approach aprons outside the barn doors be?
3 - It has been suggested that, in line with the door openings, I dig a trench below frost line, pour a footer, build a block wall (w/rebar and cement inside the block) and leave a lip on the block wall for the approach aprons to sit on. Is this overkill?
4 - What is the best way to tie the approach aprons into the main slab, if at all, so they float together with the main slab?

Thanks for your help.

 

[firewoodman]

Paul, We never tie an outside slab to an inside one due to frost-heave of the outside slab. I have seen many that were tied together and severely broken up near the joint due to the lift of the frost fighting the ties. We usually pour the outside slab 1" below the inside one for drainage and so they will be more level in the winter.

 

[linpat]

hi paul t
u will get alot of def ideas, i have a concrete crew and we pour about 200 floors a year for pole bldgs.
i would need to know what r u putting in the bldg before i would say it is over kill?

<font color="blue"> </font>
not sure what u mean here, but if u r talking about putting something like a footer around the bldg u r defeating y the pole bldg was designed for. the slab is a floating surface. with creating a surface thicker on the outside than the inside would not make it a floating slab.

if u r using the bldg for items less than 10,000 lbs yes i would say u r over killing your floor.

what PSI r u using

 

[linpat]

agree u should use a keyway 2 seperate the apron from the floor.

 

[jimmysisson]

We've built a few of these, in cold western Mass., 48" frost. We usually set the posts first (how ya' gonna' set the inside ones later?), then nail the 2x10 or -12 PT form boards between the posts, set to the finish slab height. They make the pour easy and give you something to push the outside backfill against. The main thing that makes these buildings, and their slabs, last is the 18-24" of porous crushed stone or bony bank-run under the slab. Your crushed stone sounds good, the stone dust not so great, as it packs tight. You just want all the water to drain away from under the slab. If the lot slopes toward the building, you'll have trouble. We don't trench the edges, and haven't had any heave yet. (20 yrs.) I usually wrap the posts in 30# felt, or Tyvek or something.
Anyway, I'd use 2" foam under the concrete, and if your site is dry, I'd forget the vapor barrier, as you're more likely to get a good finish on the slab without it, and the foam and gravel will keep the floor dry. The foam is for heat retention.
I agree with the other poster that any apron should be separate from the inside slab. If you can't live with a gravel apron, float a slab outside, don't try to pin it or you'll regret it. Make sure the slabs don't have any way to push up on the siding or trim or anything attached to the building. I don't think you need the block wall under anything. If you really, really want a poured apron tied in to the slab, then build the wall under all edges of the apron, so it'll stay put. Up to you whether that's worth the trouble.
Best approach here, IMO, is to build the shed first, then pour the floor when you have a roof on it. That way it's easier to control the effects of weather, keeps the sun off better while it's curing, and the gravel gives you a place to drive in stakes to brace the posts while you're framing. So: 1. prep the site, 2. set the posts, 3. frame and roof, 4. prep the slab, heat, etc., 5. pour and cure, 6. fill in walls, windows, doors, etc., 7. finish inside and out.

 

[PaulT]

I don't have any equipment that is over 10000 lbs.

I don't know what PSI I should use - also, should it be fiber reinforced or wire mesh reinforced?

The footer I was talking about would only be in the doorway. everywhere else it would just be a monolithic slab with footing (thicker slab) at the edges.

Since you pour a lot of floors for pole barns, and my barn is not going to be used in any special way, perhaps you could tell me how best to prep the floor if you were in charge. I assume the outside apron and sidewalk should be done after the inside slab.

Also, I figured about 40-41 yds for the inside and a 5 foot walkway/apron on two sides. Other than the materials and the prep, how much would I expect to pay a crew to spread and float the slab and sidewalk?

 

[PaulT]

Thanks for the feedback, Jim. The barn is built, except for the doors, so now I am on step 4 of your list. I have used 2x10 PT skirt (form) boards all around as you suggested.

Do you make the perimeter of the slab thicker, or is the whole slab 6" right up to the skirt (form) boards?

You're right, the gravel with stone dust doesn't drain water like washed gravel would, should I be concerned? I could scrape out a few inches of it and replace with 2 inches of #2 washed gravel, but I'd rather avoid that work. What happens if I don't do that? (water does percolate through, just not real fast)

The land does slope down to the barn. I have a french drain in front that has been working well to shed the water to either side, but recently it has silted in where I drive on it. I was planning to pour the outside slab apron with a drain trench in it, connected to the french drain at either end, and cover with metal grates. How thikc should this slab be? The same as the inside slab - 6"?

 

[ericbx1500]

This is the fourth winter since I built mine. I have a 4” floor, no rat wall, no foam insulation and 20 yards of sand underneath my slab. Mine heaves ever so slightly at my entry door (only during a really cold snap). And the reason I notice it there is because I have a 2x4 doorframe that goes threw my slab. I may redesign my doorframe someday just like they use on log cabins where the frame is able to float. I do live in Michigan so I do get some pretty cold winters. But one thing I do have is good soil drainage around my barn and even on the most humid days I never have a wet floor /forums/images/graemlins/grin.gif

 

[gates]

PaulT,

The only reason, besides structural integrity, to dig along the edges of the pole barn & pour a turned down slab edge is to keep the frost from the exterior from getting under the existing slab. It is the same concept for the door ways. If you did the entire perimeter and did not do the door ways you would defeat the purpose of protecting the interior from frost. This is why you were told to dig it down below the frost line. If you do not do the perimeter of the shop there is no sense in doing the doorways.
On typical radiant systems foam is placed next to the foundation wall, vertically, and then under the slab horizontally, to insulate the slab. We also insulate the slab edges with foam as well. This cuts down on end losses from the slab. With a pole barn with no foundation these are critical steps for a radiant system to perform correctly.
As for the the foam thickness. It is like the law of diminishing returns. There is no correct answer. The more insulation the better, but will the cost of the insulation justify the return?
Vapor barrier under slab. This has been argued for ever. I personally do not believe slab quality is effected. In a house basement we use a vapor barrier to hold back moisture.
I personally use wire and fibermesh in all of my pours. Once again their have been arguments both ways. To me $4 dollars extra per yard is a good investment. Finishers dont like it, makes there job harder for a burned in finish.
PSI is pounds per square inch this is the breaking strength of your concrete under lab conditions. Once again it is the law of diminishing returns. More thickness vs. richer(higher PSI concrete). I use 4000PSI air entraned concrete for exterior pours, 4000PSI non air entraned for interior slabs. I do not pour anything under 5" thick.
I assume you have a radiant layout for the pex. If not get one. This is extremely important.
You need to keep the water away from the building, by using what ever means you can.
Just my .02 worth.

Ron

 

[rambler]

Rat walls - the 1-2 foot edge around the outside - are required code here, and keep out raccoons, ground squirrels, wood chucks, etc from digging a hole under the slab & making it home. The honeycomb of holes they make will crack up the slab. i would _not_ be without this in my location. Would be against code, but as well I have seen the damage of _not_ having this - horrid with our critters 'here'.

The foundation under the door frame to below frost line keeps your doors square & free swinging & working all year around. I would do this too, but i live in Minnesota where a floor can move 3-4 inches in my clay soils. This foundation for the doors should be it's own footer, not attached to anthing & I would slip in a bit of felt or thin wood to let all other cement float up & down bside it. You want the door (rollup?) to be right on this foundation. Walk in doors or other will need a raised threshold so the bottom of the swinging door has clearance to other (moving) surfaces.

I would not tie in the apron to the floor - I would make sure there was a slip surface for them to move independently. As well you want something a bit insulating between, to save your heat. As others mentioned.

Some places insulate down to below frost level, at the wall edge vertically. Then they use the earth inside your building for a heat sink, no insulation. The ground evenually gets to be 55 degrees, so you are not ever losing much heat, _if_ you keep you building a constant heat all winter & you don't make it 75+ degrees all winter. This saves on insulation. It requires a bit more heat, but is a huge thermal heat sink & buffers your shed for a few days on heat gain or loss.

If not the vertical ban of insulation, I hear of 2 inch insulation close to the walls, and an inch - some use 2 - for the center of the floor. If you heat all the time, that middle will not freeze anyhow, so it will be diminishing returns to build up insulation in the middle of the building. You need to carefully plan to insulate the edges & outer surfaces of the slab tho, as that is where all your heat loss will be.

Many folks believe 6" is a wonderful floor, & end up building a 4 inch when $$$$ reality sets in. With the heat tubes, I'd not want less than 5. 6 will be great, if you have it priced & can, go with it!

Generally rebar is better than wire mesh. The floor will crack someday, & the rebar will be strong enough to keep it from shifting. Wire mesh catches all the salt we get from road salting in winter, & will rust away in short order at crackpoints, leaving nothing to keep the concrete from shifting.

Some hate fibers for a floor, say they stick up & scratch if you kneel/ lay on the floor, end up allergic to their building. Others say it helps prevent cracks, wouldn't be without the fibers. I donno.

You should be sure the poles are insulated from the concrete, so the slab can lift without lifting the poles, and to keep the concrete from rotting the wood.

Water is your enemy, you need to keep the water away. slope away from the building, drainage to someplace else even in heavy melts or rains.

If you keep the water away, your floor prep sounds pretty good. Be sure it's tamped _hard_. If it was dumped in thick & not packed, be sure it has settled well & is _hard_. Keeps the floor from cracking. Should repack any digging you do in 6 inch or less lifts, pack _hard_.

With minus 30 degree temps, lots of varments, lots of wind, lots of salt on the roads all winter, and clay soils..... We gotta do a lot to make these things work. /forums/images/graemlins/smile.gif

--->Paul

 

[QRTRHRS]

I don't know that this would be redundant with the foam insulation or not. There is a bubble wrap type of insulation for use under slabs that has properties that allow for movement of concrete.

I can tell you that when I built my last barn, I personally rolled the ground with a 13 ton vibratory roller while my contractor did the bulldozing. When the builder did the center aisle slab (16x120'), he dug out 4" and put in 2b stone and tamped it. I forget now if it was 3000 or 5000 psi but it is 6" thick with wire, poured in sections then cut every 12' for relief. No insulation or heat. Danged if everyone of those slabs did not crack in the center (parallel with the length of the aisle). It is a horse barn but I had gone with 6" just in case I wanted to run through with heavier tractors and such.

 

[erict]

When I built my pole barn I did not need a rat wall(was not code) nor did I need a permit. Since I dont have a rat wall I installed 3' galv field fence around the whole outside of the pole barn to stop any digging of critters. So far so good. The slab looks the same as the day I had it installed. This is my workshop. Code has change since then.

 

[EddieWalker]

This is turnign into a very informative post! Reading all that goes into a slab up north compared to what we do down here is just amazing!!!

From personal experience and what I've read, I'd use rebar over wire mesh every time. Wire mesh will give you more strengh for allot less money if it's installed correctly, but it's just about impossilbe to install correctly. The wire mesh needs to be in the middle of the slab to function. The way to get it there is to pull it up by hand while pour the concrete. If you get half of it where it's supposed to be, your lucky. But there's no way to know where the bad spots are until it cracks.

Rebar can be held in position with chairs or blocks and will stay where you need it during the pour. For me, this is a huge benifit and enough of a reason to spend more for rebar.

Good luck,
Eddie

 

[gates]

Edddie,
You are 100% correct about the wire placement. Most people will not spend the money to put the wire up on chairs. They depend on picking it up as they go. The chairs make it a no-brainer and assure the wire is where it needs to be.
We use rebar only when there are design loads on the slab that are not obtainable with wire. We use wire mesh in multiple gauge sizes 90% of the time.

Ron

 

[PaulT]

So let me summarize what I think the consensus seems to be so far for a floating slab with radiant heat.

* Compacted soil/gravel
* Vertical insulation at the perimeter
* Horizontal insulation under the slab
* 6" thickness
* Wire mesh properly placed with chairs
* exterior aprons/walkways floating independently of interior slab - air entrained, poured at a later date from the interior slab
* 4000-5000 psi
* Fiber reinforced
* No need for a frost wall/footer in doorways
* No need for a vapor barrier if horizontal insulation is used

What I am still not sure of:
* Do I need a thicker edge, and is the purpose to strengthen the slab or to keep the frost out?
* If it is to keep the frost out, will it really do that if its only 18" thick around the perimeter, with a frost line at 42"?
* How thick should the exterior aprons and walkways be?
* if the barn is 28 ft deep, with 3 bays (12 ft wide, 16 ft wide, 12 ft wide), how should the slab be poured and separated (monolithic, monolithic then cut with a saw, poured in sections)

 

[gates]

Pault,
You do not need a thicker edge for concrete strength.
18" will not keep the frost out in a 42" frost line area.
5" is adequate if poured correctly and you have a good base.
The slab can be poured either continuos or individual pours.
It depends on your man power, you do not want it to get away from you.
Something that size can be easily poured continuos.
For the control joint placement in feet go 2 1/2 times the thickness of the slab in inches times 12. An example would be a 6" slab would have control joints at 15' on center. I do not exceed 20' spacing no matter the thickness. If you have an inside corner, ie. corner protruding into the slab, pick up a set of control joints off of the corner. If you saw cut the joints be very careful of the radiant tubing. The joints should be planed and considered when you install the tubing. Some tubing manufactures want sleeves on the tubing where it bridges a joint. Others just set the tubing deeper in the area of the joint. The floor should be cut as soon as you can walk on it, the sooner the better. If you own a soffcut saw, cut it as soon as you can send the saw accross it. If you pour the floor in sections you will need to put in a construction joint. This can be either a keyway or dowels. This will lock the slabs together and keep them in the same plane. Your concrete placement guys should know this info. If they do not get someone who does!

Ron

 

[PaulT]

Thanks Ron,

I was really looking for a no-nonsense response like yours to help me understand just what I needed to do to prep the pour and reduce total cost, assuming I would hire experienced guys to do the placement.

 

[gates]

Pault,
This is one part of the job, concrete work, that you truly get what you pay for. There is a lot more to concrete placement then meets the eye. All of the following come into play. Site conditions, concrete additives ie. plasticizers, admixes, water reducers, water content, air content, PSI, concrete temperature, air temperature, humidity and even if the wind is blowing at pour time. Concrete placement is definetly not rocket science but it does require experience. Remember this, you will look at that slab for a long time. It is the finished product. I would not start cutting corners now.

Ron

 

[rambler]

</font><font color="blue" class="small">(
* Do I need a thicker edge, and is the purpose to strengthen the slab or to keep the frost out?
* If it is to keep the frost out, will it really do that if its only 18" thick around the perimeter, with a frost line at 42"?
)</font>

The thicker edge is to keep varmits from tunneling under your floor & making it weak (air voids).

It will still be a floating slab, and it will still rise & fall with the frost. It is not a foundation, it does nothing for the frost line.

The thicker part will make it stronger, but not the primary reason for the rat wall. Keeping rats & etc. away is.

--->Paul

 

[bugstruck]

You are getting excellent information here but I want to throw this out there for consideration. At risk of starting a concrete additive debate (don't want that), some of us old-timers don't have much good for them. Retarders in very specific situtations, yes. Hardners, rarely. Air, virtually never. You can't make that "no additive" sale in the modern world, but the Romans had it right IMO. Yes, they had some geographical materials availability (binders) we don't even have in the US today and we have some they didn't. But their product longevity is indisputable and they did it with nearly zero reinforcement. The few failures I've experinced in Maryland supplied concrete were always with additives. Many newer bridge structures with the most advanced concrete mix designs were failing permaturely in the 80's and early 90's. Perhaps they have improved some, but I'm still seeing the same old list of additives. I'm just throwing it out to the younger crowd for consideration. If you haven't lived long enough to see the results, you'd do admixes to the 9's. They sure do sound good and I'm sure someone has done structural, weathering, .... freeze tests, but they just don't improve the base product in many situtations, assuming you handle and place the straight product properly. That's the most important quality conrol measure you can take IMO. Keep the water at mimimums for workability of course.

Other than my little digression on concrete, I know the recommendations presented for incorporation into this job are right on target. You can skin specific construction issues a few different ways, but virtually all of the methods discussed are very workable IMO.

HTH