Just need a 2nd opinion on this slab on grade project


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Old 02-14-12, 04:37 PM
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Just need a 2nd opinion on this slab on grade project

Planning a 40’ x 30’ rebar reinforced thickened-edge slab on grade for an existing pole-type barn near Rolla, MO. Barn will become a storage area, carpentry/general workshop and tractor parking.

Slab will be 4” thick with #3 Grade 40 rebar on 4’ centers. Concrete will be a 5-6 bag (4000 psi) low water fiber mix. 65:35 gravel to sand ratio. Planning a 3-4 slump. Slab will be isolated from building exterior and poles with asphalt-impregnated fiber board. Finished surface will be 6” above grade and level (no slope). Control (expansion) joints will be Zip-Strips on 10’ centers.

Don’t plan on using a vapor barrier on top of 2” sand base (plate compacted) and 4” crusher-run rock sub-base (plate compacted since it is an unheated slab. Also, slab will be isolated from surrounding soil by crusher-run rock to prevent frost heaving. Nor do I plan on using any insulation (slab or wing) due to isolation of slab.

I will do all excavation and structural work and hire a crew to pour and broom-finish the concrete.

Questions:
1.Do I need a Ό” per linear foot slope (most recommended) on slab?
2.Should I reinforce using wire mesh on top of rebar (as recommended by a local pro)?
3.Should rebar/wire at contraction joints be cut? I would rather not have to cut the rebar if it is not really necessary but I have read that this should be done.
a.All rebar?
b.Only every other rebar/ wire as recommended by some pros?
4.See anything else that raises an eyebrow?
 
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Old 02-15-12, 05:41 AM
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uestions:
1.Do I need a Ό” per linear foot slope (most recommended) on slab?
2.Should I reinforce using wire mesh on top of rebar (as recommended by a local pro)?
3.Should rebar/wire at contraction joints be cut? I would rather not have to cut the rebar if it is not really necessary but I have read that this should be done.
a.All rebar?
b.Only every other rebar/ wire as recommended by some pros?
4.See anything else that raises an eyebrow?

(MissouriFLW)

============

If this is an inside concrete floor and not an outside paved area, I would suggest to you that you not broom finish the floor. Brooming for outside concrete is fine to prevent slipping when wet and snow covered. It's a bad idea to broom finish any inside floor. Just trowel it with a power trowel a couple of times and call it good so you can broom clean the floor. It won't be getting rain and snow on it if it is to be indoors. You will be sweeping the floor with a broom someday and will be glad it is not brushed finish but smooth.

I would saw cut the floor perhaps every 15-20 ft with a straight running diamond blade about 3/8" deep. The rerod will not be affected provided you keep the kerf depth controlled.

With the amount of steel rod you are putting in that floor you really don't need 6" X 6" mesh or even fibres. If you do tie 3/8 rod in a grid as you have indicated(OR mesh for that matter) make sure you assign one person and make all our helpers aware to keep that rod up in concrete not on bottom. If that happens the rod does little good. If you used the 6 X 6 mesh. Just splice a continuous length of 1/2" rod to mesh in perimeter thickened area and trust the mesh for the rest. It's going to crack anyway and your hope is it cracks on the 3/8" cut lines. Rod and mesh just keep the floor honest.

I don't think I would pitch the floor unless it's to a drain. That little bit of water that drips off the bottom of a truck or tractor does not magically run off a slab pitched 1/8 or 1/4" per foot a anyway.

compact the base mechanically and I would use visquine because the base will be pretty dry if you have had the roof over it for very long.

bs5
 
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Old 02-15-12, 06:52 AM
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Thanks for the reply.

Any particular reason you recommend cutting the joints over the Zip-Strips?

Why 15-20' for distance between cuts rather than the 10' I planned?

Why only 3/8" cut -vs- 1/4 slab depth (or 1" deep)?

I'm still gathering info (1st slab project I've planned) is the reason I'm asking -- not cutting you off at the knees -- I appreciate any and all input.
 
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Old 02-15-12, 03:34 PM
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I guess I did not pick up on the use of zip strips. No need to cut if you use those strips but are you going to plan on running them both directions? That might be a pain. 1" cut is fine as opposed to 3/8. I was thinking you were trying to stay away from your reinforcingbut 1" should work well. 10' zip spacing is fine also. That is just a matter of personal preference. Thats a cold joint, I don't know how that will work with your reinforcing.

bs5
 
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Old 02-16-12, 08:03 AM
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Thanks for the info...another question

All good poop, Bullshooter5 (no pun intended...OK, yeah there was!)
Anyway on to serious business..

You recommend "visqueen" as a vapor retarder.

Backstory: the goobers who had this place before me graded the dirt interior of the existing shed to the same level as the exterior grade. A LOT of water FLOWS under the metal siding on the high side and wets the soil (no idea the depth yet as I haven't excavated). I assume (and you KNOW what THAT does) the soil is damp pretty deep, accumulated over time.

Part of my current plan is to excavate the existing (llama-poo heavy) soil down to required depth, cover subgrade with 4" crusher-run rock and 2" sand to
1) isolate the slab from frost-heaving and
2) allow for sub-slab drainage if required.
Step 2 of the project is perimeter drainage around the new slab.

My understanding (and correct me if I'm wrong) is that if the new slab is to be in an unconditioned space, ACI recommends no vapor retarder/barrier.

Any thoughts?
 
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Old 02-16-12, 02:46 PM
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I understand and can appreciate the No visquine rule. As long as you have a good crew who can get that floor poured, placed and bullfloated and troweled while the getting is good.Visquine gives a crew pouring a big slab an extra layer of retarding the workable stage. It also get the concrete to hydrate slower which makes for a stronger floor. Provided you can get the sand layer of rock hosed down with water i am thumbs up on skipping the visquine.

If your ground is as wet as it sounds I think your idea of a french drain around the perimeter (if that is what you meant) is a good one. I don't think a 4" layer of wash gravel + 2" of sand will do much to prevent the frost from happening for a slab in Indiana however. I would suggest you do as you say and that is to start at the highest corner and get that floor base compacted and a good -4-6"above grade(at the very least). Frost will run much deeper than that given a good old fashion Winter with a few days of sub zero temperature and an unheated building.

The compacted base should be higher than the outside and the 4" of concrete insures a good pitch out of the building. I like to see a tounge and groove ground contact 2X 6 around the perimeter with groove side up and carefully cleaned of concrete at floor level and then perlins and siding left off sides. This applies to a newly or in the process of constructing building only. I think yours is up. The crew can than bullfloat the floor and finish perimeter by hand. If you do this you should probably do an additional course of 2X6 wolmanized and tongue and grooved together with thee one used as a screed for floor. That's why yoiu need to clean that first course up after the floor gets poured and finished.

Don't forget some kind of rat wall around perimeter in hopes of keeping that wood chuck from digging under that nice new floor.

bs5
 
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Old 02-18-12, 11:01 PM
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Can't help but think that 6 x 6 heavy mesh would be better than No. 3 bars at 4' centers. Less likely to get random cracks, and more control over differential settlement if it does crack. If you insist on rebar, at least use dobies or chairs to hold it off the grade. And don't forget to specify air entrained concrete, since it will be subject to freeze-thaw conditions.

I would never do a flat floor if vehicles will be brought in with snow and ice on them. You will eventually have supplemental heat in your work areas, and the resultant puddles will make you talk to yourself after a while. Might even consider going whole hog, and putting in an embedded sewer and water supply line for the future bathroom.
 
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Old 02-20-12, 02:28 PM
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Response to Bridgeman45

Thanks for your response.

Re: the mesh. The local pro suggested the rebar on 2" chairs since the mesh would be pushed down during pouring/finishing and, according to him, rarely gets properly raised to slab center. I really would like to use mesh since it's definitely cheaper here in central MO. As a novice, however, I'm inclined to listen to the pros, especially if it fits their mode of operation. Willing to listen to all opinions though.

Re: Air entrainment. Part of my concrete specs I will give the supplier is for 6-8% air entrainment.

Re: flat slab. What do you recommend for slope on the slab? I keep reading 1/4" slope per linear foot but that would mean a 7" difference over the 30' width of the slab...seems a little excessive. especially when you consider levelling woodworking equipment (radial arm saws, etc). I originally thought about the central drain idea but nixxed it after deciding to raise the slab 6" above grade. Stage 2 of the project will be perimeter drain piping and rock at the low point of the thickened edge of the slab.

Thanks to all you guys for your great input. Really helps me think through my decision-making process.
 
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Old 02-20-12, 02:50 PM
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Response to bullshooter5

Excellent point on the vapor retarder. I will plan on placing a vapor retarder. Never considered the slab moisture and curing. Muchas Gracias!

Would love to post a CAD of a cross-section but can't figure out how to post a jpg from my hard drive. Might give you all some more insight.

Thanks for the thought on the woodchucks/groundhogs. Haven't seen any around my place but we do have a plethora in the area. Considering my options as we speak. Might be easier to install when I do Phase II (perimeter drainage)
 
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Old 02-20-12, 03:33 PM
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If you're not going to be snuggling your trucks and tractors up amongst the radial arm saw and other power tools, why not just pitch the floor over half the length, closest to the equipment door?
The work shop area can be totally level, and the vehicle parking area sloped downward towards that door, or even an embedded grate draining to a drywell beneath the slab, just before the door. Moot point if the vehicles will never be used when it's raining or snowing--make everything level.
 
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Old 02-20-12, 04:49 PM
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sloping the whole floor

We never encouraged the concept of sloping a floor. For one thing it's not worth the effort. It's not so simple as just sloping a floor for a big building like a pole barn whether you pitch it 1/4 or 1/8 " per foot over 35 or 70 feet. The reason is if the only thing you are trying to control is a little bit of snow or road water that drips off a vehicle coming in out of the weather you might as well forget it. Park that dripping tractor in the middle and that water coming off the tractor aint gonna make the door opening. It will dry up before it makes the door. Now if you were going to run a steady stream such as when washing a car some water would get pushed out with that slope.

That brings up another point. You have to pitch the slab out the door only because you are not going to like the way a skirt board looks on a pole barn when it is level around the structure and the floor drops off 3 or 4" from the middle to the end. So, you have to just pitch a deepening trough out the door. If you started at the center and pitched the floor 1/8" per foot of your building you have over 4" of difference between the level perimeter and right at the edge of the door you have a 4" difference in elevation at door edge enough to trip or at least stub your toe assuming you have poured the edge level as we have talked about earlier.

If you want to pitch a section of your floor pitch only 10-15' either side of a floor sump and grill. and reserve parking at that location for your inside and out equipment. What you are doing there is keeping the water away from the perimeter because it will at least head to the center where the sump is and not the outside of the floor

If pitching the whole floor works for you that is fine I am only giving an example of what has worked best for us over the years. I have tried to do that for people before and I don't think it ever worked out. If it was my floor it gets a sump or drain with a limited amount that is pitched. No offense to those that swear by any other method.

Good luck with your project.

bs5
 
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Old 02-20-12, 08:30 PM
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True Story--Last place we lived in was an upscale subdivision in SW Colorado. A custom home builder (from Texas) built a new $500,000 spec home 2 doors down from ours. He obviously learned his skills from the same place bs5 did, as the huge garage floor was poured perfectly level. Nada pitch. Interestingly enough, no curbs were cast under the exterior walls, either.

A few months went by before someone bought the place. The new owners invited some of us neighbors over one winter night for a coffee-dessert potluck. Nice people. So when I asked if they were happy with their new place, the husband said, yes, except for a problem out in the garage. He then invited all of the men out for a look, but I already knew what he was going to show us (having regularly monitored the place's construction months before). There were puddles of standing melt water covering close to 2/3 of the entire floor, under and around both parked cars (we had just had another foot of heavy, wet snow).

The dirty water was close to 1/2" deep in the space at the house entry door, and the pile of dirty towels on the landing to get some of the wet, salty water off their shoes was a silent testimony to the advantage of pitching floor(s) to drain. As was the line of dark mold growing along the saturated wall sill plates, just below the bottom of the sheetrock. The owner wasn't very happy when I told him what a proper repair would require.

And contrary to what someone has said, a slope of 1/4" per foot will move plenty of water. At least that's what half a million (+) highway bridge decks in this country seem to show, with their 2% normal crown.
 
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Old 02-20-12, 10:10 PM
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My apologies to Bridgeman, I don't think I had even read your post when I responded to the original posters comment about pitching the whole floor. In your post I see that you provided an accepted practise and that was pitching the water out the door. I was certainly not trying to demean anything you said because I happen to prefer pitching to a drain if you feel the need in the case of a pole barn floor.

In your example given you say bridge decks are pitched and I'm glad they are and agree with you that they should be. I believe practically any exterior concrete surface should be pitched I'm sure you do too. After all they are subject to constant rain and snow.

You are obviously a person with experience at concrete flatwork. I probably tend to do things my own way and I am not always right and I'm not very good at describing my methods if somebody does things differently. If I have offended any contributor's for that please accept my apologies.

bs5
 
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Old 02-21-12, 12:58 AM
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No offense taken, no apology needed. We're all entitled to practice what serves us best.

And it's still great to live in a country where we have such freedoms.
 
 

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