Concrete contractor is setting up and pouring my floor in the morning, I have not had an opportunity to discuss specifics about reinforcement and mix. I want this done right, these guys are good but what you don't ask for you don't get. 4" pour, rebar size and spacing?? Im leaning toward 5000 psi mix. They are going to tell me 4000 is fine and 5000 is overkill. The garage is detached, on footers, stem walls with ledge to carry the floor. Stem walls are insulated inside with 2" foam down to footers. Frost depth in my area (MN) is 42". No in floor heat. Planning to pour over a 6 mil poly vapor barrier. Soil has been compacted. Building will be heated intermittently with a unit heater. Suggestions welcome.
Pouring garage floor tomorrow
- Thread starter sdo
- Start date
kaiser715
Well-known member
Mine was 5-6", 6x6 wmm in field, perimeter (monoslab) with rebar. Each column base (8) was dug out about 4'x4' and 2 feet deeper, had a 'cage' of rebar bend up and tied into footing rebar. 4000psi. This was a 42xx60.
Depending on your uses, building size, and if you are going to put in a lift...you might want more than 4". And figure out now if you need to thicken for lift footers, and avoid rebar where lift columns will be (iirc, lift mfgrs have spec on how close, like 12-18", someone here will know for sure.)
Don't forget the saw cut. They did mine late afternoon same day, 10x10 pattern. Cut 25% depth of slab.
Depending on your uses, building size, and if you are going to put in a lift...you might want more than 4". And figure out now if you need to thicken for lift footers, and avoid rebar where lift columns will be (iirc, lift mfgrs have spec on how close, like 12-18", someone here will know for sure.)
Don't forget the saw cut. They did mine late afternoon same day, 10x10 pattern. Cut 25% depth of slab.
wssix99
Well-known member
It's a little late in the game to be asking...
4000 is fine. 3000 is fine, too. The higher you go, the greater the cement content and the greater the the slab will shrink/contract. The more this happens, the more rebar you need to counteract it.
4000 is way overkill structurally, but folks like it because the high cement content gives a nicer finish. 5000 is super way overkill. <- Stay away - you will just get a greater chance for cracking.
The need for steel is a function of the thickness of the slab, the size of the rebar, the properties of the concrete, and the spacing of your contraction (saw cut) joints. Technically, if you space your control joints 10-12 feet, you shouldn't need any steel, at all.
^ The more steel you have, the greater you can move those contraction joints apart (a lot of steel will allow you to move the joints just a bit) and the more insurance you buy against cracking happening outside of your control joints.
matt_i
Well-known member
Personally I like the crushed stone underneath to break surface tension with water in the subgrade, but I don't think you are going to get that by tomorrow.
Its a good idea to have plans in place to cure the slab to help achieve the design strength. Either wet-curing or else can use a curing sealer as soon as you can walk on it without damage.
My vote is for the 4ksi mix.
Either spacing sounds OK for rebar, although the 24" would be preferred. But I would make sure they set it on chairs or dobies. Having it lay on the ground and then "hooked" into position is mostly just wasting your money imo.
Its a good idea to have plans in place to cure the slab to help achieve the design strength. Either wet-curing or else can use a curing sealer as soon as you can walk on it without damage.
My vote is for the 4ksi mix.
Either spacing sounds OK for rebar, although the 24" would be preferred. But I would make sure they set it on chairs or dobies. Having it lay on the ground and then "hooked" into position is mostly just wasting your money imo.
Update, pour is done, I met with the contractor when they arrived and confirmed the plan, decreased rebar spacing down to a 2'x 2' grid, confirmed 4000 psi mix with air, they did some final prep and hit it again 2x with a plate compactor before laying down the poly and rebar. When I left they were just starting to power trowel and it was looking pretty good.
Wssix, they will have plenty of rebar on hand, in other words if they say "we always just space 36" oc" I have the opportunity to say, we'll I want 24" instead. They also call in the order for the mix truck just as they are finishing up prep, so again today I can tell them what I want. 4000 psi in a detached unheated building I believe is min allowed by code because of the potential for freezing under the slab. Luckily I'm in sand so that helps, thanks for all the suggestions
Wssix, they will have plenty of rebar on hand, in other words if they say "we always just space 36" oc" I have the opportunity to say, we'll I want 24" instead. They also call in the order for the mix truck just as they are finishing up prep, so again today I can tell them what I want. 4000 psi in a detached unheated building I believe is min allowed by code because of the potential for freezing under the slab. Luckily I'm in sand so that helps, thanks for all the suggestions
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wssix99
Well-known member
They always do have rebar and they always say stuff. I hope you were able to see the rebar before the pour to make sure they actually did what they said they would do. (They don't always do that. It's expensive!)
... My favorite was when my contractor showed up to my job, which spec'd #5 rebar, with a truck full of #4 and I had to re-calculate new spacing on the spot.
When are they going to saw cut your joints? Will they be back tomorrow?
matt_i
Well-known member
My thoughts exactly...wet cure or curing sealer, get those joints cut tomorrow morning asap.
oilslick
Well-known member
I thought power trowels and air entrained concrete don't mix? Educate me.looks like a nice sized workshop.
Why did they spec air in a slab that was power troweled? You might have issues with the surface Spaulding.
Air is for exterior cement so water can expand and contract without blowing it apart. To do this the surface must be open not sealed over with a trowel.
Air is for exterior cement so water can expand and contract without blowing it apart. To do this the surface must be open not sealed over with a trowel.
They came back this morning and saw cut the control joints, everything looks pretty good, and yeah I saw the rebar, I watched until they had all the mud down. I don't know about the air entrainment, when I asked yesterday the contractor told me because they were pouring over a vapor barrier, no more than 3% air entertainment. Not sure if he was blowing smoke or what, but I have the slip from ready mix plant and it doesn't call out air. It did call out 1% calcium chloride because they were pouring early and mornings in MN are chilly this time of year.
As I watched yesterday when they started bringing mud down the chute ,the contractor got it in the wheel barrel and poured it. He came back and told the ready mix operator to mix the batch more, so they sat for a bit while the drum mixed longer. Not sure what he was after, and I didn't ask.
This crew has poured 3 large building floors for my dad over the last few years and they all turned out well and no issues to date, so I trust they know what they are doing.
Communication prior to them showing up was just difficult because they are busy as ever, building is booming in this area right now and everyone is in a rush to get things done before it gets too cold.
As I watched yesterday when they started bringing mud down the chute ,the contractor got it in the wheel barrel and poured it. He came back and told the ready mix operator to mix the batch more, so they sat for a bit while the drum mixed longer. Not sure what he was after, and I didn't ask.
This crew has poured 3 large building floors for my dad over the last few years and they all turned out well and no issues to date, so I trust they know what they are doing.
Communication prior to them showing up was just difficult because they are busy as ever, building is booming in this area right now and everyone is in a rush to get things done before it gets too cold.
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That's actually not true, natural air is typically around 2%. When we order "half air" (which is perfectly acceptable to pour in an exterior building AND power trowel) the actual air content is between 3.5 - 4% when tested. Full air shouldn't exceed 6%, and 5.7-5.8% is an ideal goal in our climate.
OP, you have nothing to worry about from what I can tell, the contractor seems to know what he's doing, and this is far from his first rodeo. If you read here long enough, you'll realize that most "internet experts" here know just enough to be dangerous........
Some of us know what we know from experience not from a book. While you call half air 3.5 to 4 that is not the case around here. When it’s speced with no air added 3.5 to 4 is acceptable as natural air. Maybe it’s not the scientific term you’re referring to but my advice equated to the same thing you said.
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My Dad's been pouring concrete as his career for 54 years, I've been in the business myself for 18+ years, I'm not regurgitating what I've read. I warranty every single yard of concrete I pour, which is thousands of yards per year. It's in my best interest to educate myself on best practice, as well as pay close attention to failures and investigate "why" they happen.
I can tell you from my experience, the OP has nothing to worry about using partial/some air entrainment on an exterior slab (that very well could freeze in it's lifetime many times) with a power trowel, it's done on a very regular basis. To be honest, it's far more likely to spall if there was NO AIR added to the concrete.
Not sure what's different with Canadian ready-mix, but either they have more "natural air" for some reason, or you may be confused slightly.
James-W
Well-known member
Looks good as far as I can tell.