Concrete Expansion Joint

Need some collective wisdom here. I'm getting ready to pour a slab basically 30 ft by 40 ft. There are an existing row of pole barn poles down the middle, so I plan to pour 15x40, then another 15x40. The entire slab will be inside after the building is finished, and it seems like I need an expansion joint between the two pours. But I don't really know. This will be a maintenance shop, I've though about just putting 1/4"masonite between the slabs, or nothing. I don't want something wide which would catch casters rolling across the seam.

8" slabs, and I plan to order 4,000# concrete, and expect to put 3/4" smooth dowels between the pours.

And what if anything would you put around the poles? (5x5 wood)

What say ye?

Masonite will be just fine, we have cut quarter inch plywood for expansion and it works fine.

Soil type is the determining factor. I'm on black clay that expand's and contracts with moisture fluxation. I've had no slab shift when I poured 18"deepx12"wide beams around perimeter and 20' oc. When I expect to extend a slab with a secound pour,I dig an 18"wx18"d trench then extend form 4"below grade so that a 6"wide x 14"deep shelf is formed 4"below grade. That seem's to tie the pours togeather well enough no dowels are required but they are good insurance.

4 in. x 50 ft. Expansion Joint for Concrete
Ref

First off Congratulations on your new bldg. It wont be big enough.PERIOD.
Next, take this with a grain of salt and talk to a contractor IN YOUR AREA
so you can match your soil structure with your bldg. if you don't
you will get the advice that you paid for.

I would suggest wrapping the poles with something like 3/8 Styrofoam to keep the concrete from contacting the wood. Unless you're in an exceptionally dry area make sure there is gravel and good drainage under the slab.

If you poured the whole floor at the same time, would you put in a expansion joint? Not saying I'm right , but I have seen many floors bigger
than yours that don't have an expansion joint down the middle. If you are going to do two pours I would cut a joint from each post to the wall,
and skip the expansion joint in the middle. If you have footings on your outside walls put the expansion there if you want to.

Depends on your soil or how you have prepared the site. If you pour the slab on a good layer of sand you can get by without an expansion joint. Where I'm at we have a sugar sand for soil and I poured a slab 32'x42' without an expansion joint in 1991 around the posts of a pole building and the slab is in as good condition as when it was first poured.

First off, concrete does not expand. It contracts as it cures, but it will never grow back to the original dimensions of the pour. "Expansion joint" allows movement between two slabs, but I'm not convinced it's needed in a typical concrete floor. I DON'T want my slabs to move.

Second, typical expansion joint material is 3-1/2 inches wide. If your pour is 6 inches thick, how is the expansion joint allowing movement in the remaining 2-1/2 inches of the slab?

Before you make up your mind, take a look at the floor of a new Home Depot. HD must be very demanding in their floor specifications, and it shows. I've never seen an expansion joint in one of their floors. There are plenty of control joints cut into the floor, though.

If you really think you need expansion joint, don't use the fiber crap at the big box stores. That stuff will rot out, leaving a big gap in the floor. Most redi-mix plants have a rubber joint that won't rot; that's what you want to use. As for masonite, it's not very compressible; why not just cut a control joint?

Here's what I did for the floor of my 30x40 shop: I did three pours: The first two were 7-1/2 feet on each side, then the remaining 15 feet was poured later. The slab is 4 inches thick along the walls and 6 inches thick in the middle. I used wire mesh, and cut every other wire where I planned to cut my control joints. Before pouring the middle slab, I drilled the edges of the first two pours and inserted rebar dowels to prevent movement between the slabs. I cut one control joint across the middle of the floor, that is to say halfway between the two ends. I figure the "cold" joints between my pour serve as control joints also. Ten years later there are no cracks in my shop floor.

We also did a 25x35 foot slab in front our garage. I had originally planned to do it myself in one pour, but my wife insisted we contract it out and the contractor did it in two pours. He insisted on putting an expansion joint between the two pours, but I made sure he used rubber joint, not fiber. Since he did it in two pours, he ended up wasting a LOT of concrete. I cut control joints in the slab every six feet, and there are only a couple of cracks from inside corners where I should have cut but didn't.

If I ever pour another shop floor, I think I would try to do it in one pour. That does mean having a big crew of guys who know what they're doing, but you get it over and done with. And you don't have to screw around with dowels, since you can run rebar all the way across the slab.

Hello Andy Martin,

How about wrapping roof shingles around each pole? I am going to use my left overs for my walk. I think
you still can by the 4" asphalt material for that purpouse,

Guido.

Only 30X40 do it in one pour ! Nothing needed around the poles or if you do run a seam down the middle nothing needed there either. The concrete will dry and contract away the posts and itself. Expansion might be ok if it was outside exposed to the weather where it would freeze and raise up. You would want the fiber kind that will not rot away.
8" slab ? How much weight are you gong to be running on it ? That's really thick.

Hello Andy Martin,

My walk as this kind of expansion strips. Some kind of plastic. The man said it was industrial strips?
10 + years, they are level with the concrete,

Guido.

A couple "quick & dirty" and probably "not recommended" tricks that we used to do.

To keep the rebar or mesh centered in the slab- plastic "chairs" are available for this, place as needed to hold the
rebar/mesh. No need to try pulling it up later. Old bricks are usually lying around someplace, usually free, just the
right thickness for a 4 inch slab.

For a clean joint line when making 2 pours- extend the rebar/mesh a foot or 2 past the joint. Place a rough cut 2x4 flat
on top of rebar/mesh. Stake it down to prevent slipping out of position. Let the mix flow/slobber under the 2x4 on the
first pour. After finishing off the first pour, remove the 2x4. This leaves a nice step to support the edge of the second
pour, & the extended rebar/mesh elminates the need for dowels.
Even if the slab moves a bit from frost action, the step & rebar make it all move at the same amount.

Willie

Well thank you very much guys. I can always count on this forum. A few points:

I lied, 6-inch slabs (typo).

I've got a small experienced crew which no longer does much concrete, and the price is reasonable. I just have to work around hay hauling. I would rather have a cold joint than pay for a pumper truck. Pretty expensive out in the country. I know lots of places they'll bring in a pumper for any pour, but not here, and they can drive alongside each 15 ft pour and shoot the mud to the far wall.

You guys have convinced my I will do nothing, cold joint with dowels through the formwork, no drilling.

Thanks again!

That's pretty good advice. I've been doing it 30 years, and most people have never caught on to the realities of your first sentence, even many engineers. Expansion joint is pretty much worthless. I would only use it exterior where pouring against a brick wall or something like that.
Nobody wants to hear it, but wire mesh is pretty much worthless too. At least the light gauge stuff. It does not actually reinforce, it only keeps a cracked slab locked together. I believe fibermesh is well worth the money to do the same thing and it's foolproof.

An expansion joint is used to isolate independently moving slabs or objects. Use it around your columns so that as the columns or slabs settle, move under load, or expand/contract at different rates during temperature fluxuations they can do so independently. If you pour right up against the column and the column settles by 1/4", or a strong wind load causes to to deflect by 1 or 2 degrees, it can cause your slab to crack around the column. If one end of your slab is exposed to sunlight, it will expand in the heat, if this slab directly abuts a slab poured independently that is in the shade, you can get some issues at the joint. Also when you join the slabs using slick dowels either drill them in to one side, or use a dowel pocket to ensure that one end is free to move in and out, while the other end is locked into the adjacent slab.

As others have said, contact a local Geotechnical Engineer about soil related issues. 9 times out of 10 a contractor knows what is normally done, but does not know why. Contact an engineer to give you the why, and to determine your exact needs.