Damage to foam during slab pour

[ALinCarolina]

I'm in the process of planning a 36 X 60 shop (thread to follow). Specs are calling for foam board, 10 mill vapor barrier, mesh on chairs and the hydronic tubing wired to the mesh. The contractor doing the hydronic system thinks that here in NC that the bubble wrap is good enough and that the foam will be damaged by the concrete workers walking on it.
Maybe a stupid question, but how do the workers keep from damaging the foam or for that matter the wire? Will they lay boards temporarily over it?

 

[Leaflessshadetree]

The high density foam is pretty tough. As long as there aren't large holes underneath it it should be fine.

 

[myredracer]

I'm in the process of planning a 36 X 60 shop (thread to follow). Specs are calling for foam board, 10 mill vapor barrier, mesh on chairs and the hydronic tubing wired to the mesh. The contractor doing the hydronic system thinks that here in NC that the bubble wrap is good enough and that the foam will be damaged by the concrete workers walking on it.
Maybe a stupid question, but how do the workers keep from damaging the foam or for that matter the wire? Will they lay boards temporarily over it?

It is vital to have a good concrete contractor. When they are walking around with a hose spewing out heavy concrete, it's pretty hard for them to keep watching their feet to make sure they only step between the rebar & piping plus the hose will get dragged around. You need to make them know they can't walk on the piping and damage it and in fact I would write it into a contract. I would do a test on the piping immediately afterwards and if any has failed, it will be on the contractor. That will make them take notice and be careful. You obviously won't be able to repair a kinked pipe afterwards.

When you have rigid foam and rebar, the bar will be on supports and the problem is, if they stomp on a bar, they can push the bar & piping downwards and won't be where it should be. I tried the plastic chairs on a slab once that had foam under it. I had the vapor barrier over the foam. The chairs kept falling over and the concrete guys used hooks all over the place to pull the bar back up, but that punctured the vapor barrier too. It was a dismal failure. In our current house with foam and hydronic heating, I used pieces of broken concrete pavers that have more of a flat footprint and are more stable and worked great.

I would put the vapor barrier under the foam so it can't get punctured. That's what I did in our 2,000 sq. ft. of garage/shop and met our code like that.

Don't know what slab thickness you are planning, but I made ours (has hydronic piping) 5" thick. I used zip-strips for crack control joints. I also wet-doweled the rebar into the perimeter foundation walls.

 

[Rookie2]

You don't need chairs. you can lay the wire mesh with tubing on the foam board. High Density foam board (freezermate) is stronger and more costly than blue board. You can do away with the wire mesh and staple the tubing directly to the blue board and use fibermesh in the concrete. Many options ! The thicker the blue board ,the less earth you heat.

We used power wheelbarrows and I had to lay down a plywood racetrack to run on.

 

[brownbagg]

the foam is fine, its alot tougher than you think, remember thousand of people use it daily with no problems

 

[Autorotica]

Bubblewrap isnt as effective as the foam.

Pressurize your hydronic loop and make sure you have a large air reservoir on it too. If they poke a line, you want to know about it immediately so it can be repaired before the mud hardens.

Chris

 

[Perroflojo]

Pressurize the system before they pure the concrete and keep an eye on the pressure while they pure the concrete. If it holds pressure, you are good to go!

 

[pstnbly]

Bubble Wrap is virtually useless in underslab applications (google is your friend). You need 2.5 density XPS for underslab. 100s of thousands of square feet of concrete flatwork are poured every day over foam. I can't imagine why a professional concrete company would object to placing concrete over foam. Some people use any excuse not to do things properly.

P.S. use chairs, don't rely on the placement crew to lift the reinforcement, they won't.

 

[ALinCarolina]

A bunch of good advice, especially about pressurizing it before the pour. I was thinking the foam was fairly tough so good to know it is. I'm thinking the biggest worry then is the chairs and the wire. Just seems like it would be difficult to walk on the wire without bending it or dislodging the chairs or pushing them into the foam. Using pieces of pavers is a great idea.

 

[wagspe208]

I did this in my shop.
High density foam. It can generally be found used off craigslist.. it is used under industrial roofing. MUCH CHEAPER. I used 2", ALSO be sure to use foam around the perimeter of the concrete. There is a HUGE heat loss if concrete is poured directly against a wood base board that is exposed to outside. (depending on your type of build)
I put down the mesh.
I put down the pex.
We wheel barrowed over all the pex. No issues. We did put plywood "runners" on top of pex so full weight was not on one tube.
My concrete guys were concerned a little also. As stated, pressurize the system. let it sit a couple days to verify no leaks. Leave pressurized for the pour. Monitor gauge during pour to verify no leaks.
No "chairs" necessary.
BUBBLE WRAP is useless.
Wags

IGNORE dog. he got stung by a wasp... HAHA... but you can see tubing, wire mesh, black plastic, and yellow foam in that order.
AND perimeter insulating blue foam.


The pex is much more durable than you would think. You can easily drive a tractor over it and not damage it. Walking on it won't hurt... even if it is an elephant. . (I was nervous also) Sharp edges will be the only thing that will f it up.

 

[myredracer]

I did this in my shop.
High density foam. It can generally be found used off craigslist.. it is used under industrial roofing. MUCH CHEAPER. I used 2", ALSO be sure to use foam around the perimeter of the concrete.

Good point. New foam is not cheap. I bought used 3" from a building material recycle/salvage place for a fraction of new cost. They get it off the topside of insulated roofs. I used foam sill gasket as a thermal break around the perimeter, glued in place with a few dabs of PL400 and trimmed it after the pour.

 

[rburke65]

You need to find another contractor.

 

[purplezr2]

A bunch of good advice, especially about pressurizing it before the pour. I was thinking the foam was fairly tough so good to know it is. I'm thinking the biggest worry then is the chairs and the wire. Just seems like it would be difficult to walk on the wire without bending it or dislodging the chairs or pushing them into the foam. Using pieces of pavers is a great idea.

The rebar in my was set on 2x2 squares so it was pretty easy to walk on. The foam is extremely tough considering it is foam.

 

[pstnbly]

The rebar in my was set on 2x2 squares so it was pretty easy to walk on. The foam is extremely tough considering it is foam.

They make plastic chairs that screw right into the foam that we used on my last job, those things are the hot setup, they went quick and really worked well.

 

[ALinCarolina]

Thanks guys. I haven't talked to a concrete guy yet, the contractor I mentioned was a hydronic contractor. Here is the tentative plan for the slab and wall. It will have one 4 post lift so won't need too heavy duty a slab.

 

[jevoy]

The rebar chairs I used had a circular base about 4" across. They were alot less shaky and held up well during the concrete pour. I bought them at HD.

 

[matt_i]

If you spend the time to wire tie about half the intersections (get pretwisted wires and a hook tool) the rebar grid will be so rock solid it would take a lot more than people to move it. Then you can use almost any chair. I used the Grip Rite ProLok chairs which has a snap-in 90 degree intersection built into it. Basically chairs under half of the intersections and tied the others. While I was at it I would take somewhere between a 6 foot to 8 foot piece, bend at 90 degrees and use to reinforce every internal or external corner.

Laying the rebar directly on the foam is a waste of good steel in my opinion. Might as well put it in the bottom third to midpoint there it can actually do some good.

I probably spent about 2 hours total tieing rebar in a 25x40 grid 16" OC. Goes very fast once you get the hang of it.

The pic doesn't open for me and is very small to make out any of the details.

 

[jevoy]

I agree with Matt i' s last comment. Wiring intersections does solidify the entire rebar grid.

 

[Gerald O]

Besides just being a bad idea, bubble wrap is not going to meet building code for NC.
If your contractor doesn't know this he is unqualified.

 

[ALinCarolina]

Thanks for the support. I did some research and educated myself on types of chairs for the mesh. The ones with a wide base should work well and distribute the load enough not to puncture the vapor barrier or foam. Like:
http://www.rebarplastics.com/product/SM000 or
http://www.homedepot.com/p/Hercules-1-1-2-in-Rebar-Chair-50-Pack-911/204220120

But not these:
http://rebarchairs.com/html/wire_chairs.html

 

[Jim Riseborough]

You dont need mesh/rebar if the subgrade is prepared correctly. What did the contractor say the reinforcing was for? Proper subgrade and saw cuts is essential for having a crack free floor, also along with mix design.

 

[ALinCarolina]

Jim, if nothing else I think having the mesh to attach the PEX to is worth the minimal cost.

 

[matt_i]

You dont need mesh/rebar if the subgrade is prepared correctly. What did the contractor say the reinforcing was for? Proper subgrade and saw cuts is essential for having a crack free floor, also along with mix design.

I'll take your advice if you will give me a written 25-year complete-tearout & replace warranty with the payout funds sitting in escrow.

This is why reinforcement is necessary. You won't find anyone wanting to financially back a $5000-15000+ concrete replacement (depending on size of course) when $500-1500 of rebar will be a better insurance package.

 

[Jim Riseborough]

Warranty from what? Do they offer 25 year complete tear out and replace with the mesh? I dont see that an valid arguement.

IF its crack control you are looking for, then thats what the saw joints are for, if its strength you need, then make sure the mesh is sized for the load, and placed correctly.

I have designed and spec'd more than likely over 2 million square feet of slab in warehouses, offices, etc. If you design the subgrade and ensure its installed right, the slab is just a smooth surface over that. If it moves, the slab is going with it. Cracks are result of bad subgrade, freeze/thaw, bad concrete mix, or a combination of all.

If the mix design is correct, and the concrete is placed, and cured right, then cracks can be minimized. I would me more worried about he contractor adding 5-10 gallons of water to the mix at the size because it sat in the truck an extra hour waiting for some reason, than be worried about the contractor stepping on my mesh.

Use equivalent rebar spaced greater if you are that worried, so they can step between the rebar.

 

[Jim Riseborough]

Jim, if nothing else I think having the mesh to attach the PEX to is worth the minimal cost.

That is a good argument for it.

 

[wssix99]

Thanks guys. I haven't talked to a concrete guy yet, the contractor I mentioned was a hydronic contractor.

OK - This makes more sense, because...

Specs are calling for foam board, 10 mill vapor barrier, mesh on chairs and the hydronic tubing wired to the mesh.

This is likely impossible.

The contractor doing the hydronic system thinks that here in NC that the bubble wrap is good enough

If you have a licensed HVAC company that really believes this, they should not touch your home. ... or step on your property. Keep them away. Don't let them ooze their "brilliance" on to your land. The ground is a massive heat sink and pretending that something that isn't rated insulation will actually act like insulation is ludicrous.

Most of the energy you dump in to your garage is going to escape through your floor and into the ground. (Assuming you have good wall and door insulation.) If you don't insulate the heck out of your slab, you'll just be throwing money away every time you turn on your heat.

 

[wssix99]

This is likely impossible.

You are right to be concerned about walking on the mesh. That won't work for your concrete contractor. Even if they try, the chairs will concentrate the forces on your foam (unlike when you walk on the foam with a larger footprint) and the chairs will poke through.

It probably looks good to someone who doesn't have to place the concrete or may work on a sidewalk, but it won't work out well for a large slab. If you want mesh in the middle, probably the easiest way is to do the initial pour with your tubes and chairs (with nothing on top) in place and then roll out the mesh on top of all that as you go through the pour.

^ This is what I did on one section of my house floor. (Also radiant.) It worked OK, but the mesh really doesn't sit on the chairs right, nor does it stay straight or in the middle. It's just too weak. If you want a good, reliable mat of reinforcement in the middle of the slab to control any cracking that happens, you'll need to go with rebar.

The other thing you need to worry about is concrete cover. You need at least 2" of cover on top of your tubes, reinforcement, etc. Depending on the thickness of your slab and what you do for reinforcement, you may need to put the tubing on the bottom no matter what. (For example, 1/2" rebar takes up 1" of the middle as it crosses in a grid. By the time you put 1/2" tube on top of that, you'd need at least a 6" slab.)


For my lifts, I also ran the radiant tubing around and "exclusion zone" for the posts, so there wouldn't be a chance I'd hit them when I drill the slab for the posts. If you staple the tubes to the foam, they can move around - so you'll either want to secure them to a solid mesh/grid or make sure you have access to a FLIR camera afterwards so you can be sure that you know where your tubes are. (In the event they pop loose and move during the pour.)

Another thing people forget often are the protective sleeves that need to go over the radiant tubes where ever they will cross a saw cut joint. So, you will also need to plan these out ahead of time and make sure you have those laid out.

 

[Jess]

Bubble wrap is for packages. Use foam, at least 2" over a very well compacted surface, vapour barrier, rebar grid with chairs and mesh wired to the rebar. Your piping plan can make use of the mesh to zap strap the runs in place. On my floor, we put some 6x6" scrap metal roofing on top of the foam to support the chairs. Kept them from being pushed down. We poured (line pump) 5" and kept it wet with a garden hose spray for a week after. No cracking of any kind so far. Be sure to insulate the perimeter and consider getting a heat load calculation and a hpex layout design. If you are only using the foam and don't want reinforcing, you can use staples designed for the purpose to attach the hpex to the foam.
We bring in heavy stuff, so reinforcing was part of the design requirements.

 

[ALinCarolina]

Thanks. I was never considering the bubble wrap, I believe in the foam. My concern was what would happen when they walked on the mesh. Thanks to you guys I think asking for rebar instead of mesh would be worth it just for the space it affords the workers to step between.

 

[TLCObsession]

I could not see the detail, but the vertical foam detail at the slab edge is pretty important too. 2" thick XPS cut at a 45 with the tip being at the slab top is what I have done. I like it so that you have 3/8" thick foam perimeter.

 

[jevoy]

In most cases the building code insists on rebar or wire mesh.

 

[pstnbly]

You are right to be concerned about walking on the mesh. That won't work for your concrete contractor. Even if they try, the chairs will concentrate the forces on your foam (unlike when you walk on the foam with a larger footprint) and the chairs will poke through.

It probably looks good to someone who doesn't have to place the concrete or may work on a sidewalk, but it won't work out well for a large slab. If you want mesh in the middle, probably the easiest way is to do the initial pour with your tubes and chairs (with nothing on top) in place and then roll out the mesh on top of all that as you go through the pour.

^ This is what I did on one section of my house floor. (Also radiant.) It worked OK, but the mesh really doesn't sit on the chairs right, nor does it stay straight or in the middle. It's just too weak. If you want a good, reliable mat of reinforcement in the middle of the slab to control any cracking that happens, you'll need to go with rebar.

The other thing you need to worry about is concrete cover. You need at least 2" of cover on top of your tubes, reinforcement, etc. Depending on the thickness of your slab and what you do for reinforcement, you may need to put the tubing on the bottom no matter what. (For example, 1/2" rebar takes up 1" of the middle as it crosses in a grid. By the time you put 1/2" tube on top of that, you'd need at least a 6" slab.)


For my lifts, I also ran the radiant tubing around and "exclusion zone" for the posts, so there wouldn't be a chance I'd hit them when I drill the slab for the posts. If you staple the tubes to the foam, they can move around - so you'll either want to secure them to a solid mesh/grid or make sure you have access to a FLIR camera afterwards so you can be sure that you know where your tubes are. (In the event they pop loose and move during the pour.)

Another thing people forget often are the protective sleeves that need to go over the radiant tubes where ever they will cross a saw cut joint. So, you will also need to plan these out ahead of time and make sure you have those laid out.

Walking and working on mesh properly supported by chairs is how it's done every day everywhere in the county to the tune of thousands of square feet a day. This is nothing new and not rocket science. These products are designed and made for this industry. I place my mesh, tie it, attach tubing to it, lift the mesh, support it with dobies or chairs on 2 to 3 foot centers, and place concrete. It's far easier and produces a better more consistent finished floor then trying to lift the reinforcing mat into the concrete when it is unsupported.

 

[brownbagg]

wire mesh and fiber are really only good for the first 48 hours to control shrinkage cracking. after that it does nothing. Rebar is for load factors, it does nothing till a load is applied. Regular concrete without rebar can hold any normal load with no problem, except for maybe a peterbilt or a dozer. But reqular car or truck no problem. If you want to feel safe by added rebar then go for it, easlier to put in now before the pour.

 

[jevoy]

Rebar helps against frost heaving. I have witnessed this first hand. I had a slab with no rebar and a slab with rebar. Both with the same base and compaction. The slab without the bar heaved worse by far. Hands down.

 

[jevoy]

If you can afford the slab you can afford the rebar. Concrete is expensive and you would be foolish imo to not use it.

 

[wssix99]

I place my mesh, tie it, attach tubing to it, lift the mesh, support it with dobies or chairs on 2 to 3 foot centers, and place concrete.

Where I live, the workers who place concrete are not skilled enough to do this properly and will F' up the entire thing attempting this method. (Anyone who can accomplish such a feat quickly becomes a supervisor or foreman.) The cost of the extra labor to get the mesh flat (after walking around on it) is also more than putting down rebar.)

Given what is at stake in a radiant floor, I would not chance trying to put the tubes and mesh both in the middle of the slab. By the time any leaks might be discovered, the concrete company will be looooong gone.

 

[ALinCarolina]

WS, sounds like your local workers are similar to some of ours. We can both speak from experience apparently. I'm going to get the price difference between rebar and mesh and will report back.

 

[myredracer]

I had a structural engineer friend design our 2,000 sq. ft. garage floor (3 sep. floors with hydronic heating) - 5" thick, #3 bar 18" o.c., bars wet-doweled into perimeter walls on top of 18-24" of plate compacted sand (and 3" foam). After 10 years, the only cracks are where the zip strips are. Installing rebar is easy. I can't see mesh and piping working very well and would rather know my rebar and piping is where it ought to be.

Zip strips work like a charm. Place during the pour and it's done.

 

[Jess]

A consideration with design of the reinforcing in the floor, is where it will sit and where the pex will be. My rebar and mesh is in the 1 1/2 " position, 1/2 pex, then 3" of concrete placed on top for a total depth of 5". The mesh was used to tie the pex to, and not really for anything else. We used over 400 ties to secure the pex. If the pex is deeper, or stapled to the foam, the response time to heat up the floor is longer. In colder climates where you heat all season, stapled to the foam will work fine as once it warms up the slab will stay at the set temperature. Lots of reference material available for this and the biggest thing is proper insulation to avoid loss to the perimeter. Good discussion on here.

 

[joes169]

I've poured thousands of yards of concrete over heated floors in my career, and I can't see any good reason to suspend the pex, other than waht Jess just listed above, although I doubt the response time is very noticeable.

25 years ago, we laid mesh for the plumbers to tie to. Now, 99% of pex installs we pour over are stapled to the foam. As the guy who has to cut hundreds of feet of control joints over every floor, and pay for repair if I hit a line, I much prefer the staple method, as long as the denser, 25 psi foam is used. It allows you to keep the stell slightly above the pex, where it should be, and as a bonus, as you're sawing, if you see a few sparks, you know that you're treading lightly.

I think it might have been mentioned earlier, but make sure the lines all have pressure on them when th efloor is poured. They need to be tested before pour anyways, there's no reason not to leave them pressurized, and it gives a peace of mind when pouring & sawing.