Advice on concrete slab

[BigMike]

I am getting ready to build a 16 x24 pole barn. I was planning on setting the poles with concrete collars and was also going to pour a 4" concrete slab in conjuction with the collars. I heard someplace that the slab should be independent of the collars and not be in contact with the poles because the expansion of the slab could move them. Can anybody confirm or refute that statement. Sure hate to have the thing shifting on me.

 

[Bird]

</font><font color="blue" class="small">( Can anybody confirm or refute that statement )</font>

Mike, I'm sure we have members who know a lot more about this than I, but yes, I think you've been told right.

 

[SPIKER]

I agree wi9th what you were told too

I've heard that the poles should not be attached to the slab. and no concrete should be poured onto the poles... the poles should only contact dirt and set on a underground pad. dirt holds them firmly. if the concrete is poured onto the poles then they can easly snap off rather than move a bit in high winds or if they get bumped with tractor or somethign... also hear that the concrete can cause rott as it traps too much moisture aginst the poles if they are encapsulated in concrete.

Mark M /forums/images/graemlins/ooo.gif

 

[CTyler]

You will be pouring a floating slab so you don't want it tied to your poles. I put down 6 mil plastic to keep moisture from coming thru the slab. The plastic extended up to cover the poles and skirt boards as well. I insulated the perimeter of the slab with a bubble type reflective insulation. Stapled it to the skirtboards and poles. This also allows a little room expansion as well as some insulating value.

The plastic covers the insulation otherwise the concrete would attack the foil.

 

[jimgerken]

The advice is right, but I'd add this: the reasoning behind not allowing the slab to bond to the poles or collars is because the slab WILL move over time, and the poles, if set properly, will not move. If you have freezing weather, frost may move the slab up and down seasonally. My slab inside my shed moved about an inch up this winter (and went back down this Spring), an extreme case due to very cold weather here in MN. Even without frost, the slab will probably settle slightly over time if placed on fill and packed sand. It happens. If the moving part and the stationary parts are kept from bonding together, both will survive without damage. The slab would crack of held by the poles.
On the subject of collars around the poles, after reading a couple books and listening to some of the best advice on this TBN and other sources, my method was this: Drill hole, tamp bottom dirt flat with 4 inch diameter wooden fence post, then add three or four shovels full of crushed rock (or sand), retamp with fence post, drill a 1/4" hole into each surface of the pole, about a foot up from the bottom end, drive a landscape spike (about 10 inches long, maybe 5/16" diameter) into the holes so maybe three inches of the spike is in the wood and most of it is sticking out, drop the pole in the hole, locate and brace for plumb, now pour in the concrete collar, concrete will bond to the pole and especially to the landscape spikes. I use about half the hole depth for concrete. The bottom of the pole can drain since it is setting on compacted crushed stone or sand. The concrete collar adds a lot of bearing area and makes withdrawl much more difficult. After concrete cures a day or so, I backfill the hole with more crushed stone, and a layer of dirt on top.

 

[espn]

Now are these poles pressure treated or otherwise rot and termite resistant?

 

[BotaGuy]

I had a 24X54 Pole garage built and It was built on top of a floating slab. The company used steel plates 90 degrees to bolt the poles to the concrete. It worked perfectly and there were no issues. I have also seen plenty of concrete poured as floors....my current shed had a 1 inch thick perimeter insulating styrofoam between the concret and the poles....that has worked good too.

 

[BigMike]

Jim, you described exactly they way I was going to set the poles. They are pressure treated. I will now set it up so that the floor slab will float independent of the poles. Like to thank everybody for the help. Always good to have a few other people chime in when you have some doubts. Please feel free to offer any advice you think will help on this project. I will do the picture thing and post when I get started on this project...sometime near the middle of June (I hope).

 

[keeney]

I have a large porch on my house with a slab-on-grade patio under it. The contractor set it up like this:

Footings for porch posts are 24" x 54" deep with rebar.

Post sets on footing and is kept from sliding off by being dowled to the footing with a hole hole drilled in the end of the post. These posts provide vertical support only - the porch roof horizontal and racking stability comes from being attached to the house.

Posts are pressure treated 6x6 "Glue-Lams". These are basically 4 2x6's glued together with opposing grain radius directions. Will warp a lot less than a solid 6x6.

Patio slab is poured over footings such that it can heave upwards with frost, but cannot settle lower than the footing.

Post is surrounded with expansion joint material to prevent any contact that might crack the slab or move the post up and down.

Slab has its control joints aligned with the post holes since that is where it will likely crack anyway.

- Rick

 

[Poppa]

</font><font color="blue" class="small">( Post sets on footing and is kept from sliding off by being dowled to the footing with a hole hole drilled in the end of the post. These posts provide vertical support only - the porch roof horizontal and racking stability comes from being attached to the house.)</font>

Rick, Were the dowels, in the footing itself, placed at the time of pouring the footing or were they drilled in afterward? I am assuming they were placed during the pour.

</font><font color="blue" class="small">( Posts are pressure treated 6x6 "Glue-Lams". These are basically 4 2x6's glued together with opposing grain radius directions. Will warp a lot less than a solid 6x6.
- Rick
)</font>

And, were these true gluelams or were they something the contractor built on site?

I ask these questions because you have just about described what I have been thinking of doing to build my own porch roof.

Thanks, Mike

 

[neilly2]

Don't forget a couple things when pouring your slab:

1. Welded wire fabric - contrary to popular belief this is not reinforcing. But, it limits shrinkage cracks (the small spider cracks). If you want a re-inforced slab (I doubt it), you need to go to 5" thick with #3 rebar mats.

2. Control joints. These are 1/2" deep grooves that are saw cut in the concrete the day after the pour. They provide a place for the concrete to crack. They are usually sawn from column to column like a checkerboard. ALL concrete will crack in a floating slab, but you can prevent that one large diagonal crack through your barn the spreads apart slowly for years. The control joints can be caulked with a commercial grade caulk.

Of course, you're in Texas so frost heave isn't much of a concern.

I can answer all construction related questions.

 

[neilly2]

DON'T SET WOOD POSTS DIRECTLY ON CONCRETE.

I don't care if its glu-lam or pressure treated, eventually mother nature wins. Especially, with the new requirements in pressure treated wood---its not what you think for longevity.

Go to the Simpson Strong Tie website, or to Home Depot and get a post bracket that sets in the concrete and keeps your wood post 1" off the face of the concrete.

Sawn edges of pressure treated should be covered with tar.