Time for a slab! a few questions...

[Innovate1]

Temps are getting above freezing at night and I have a roof so time for the slab! I have read the undergrounds guide to slabs. Still have a few questions...

30 x 40 building. Figuring 5" thick over 2" foam. 4500 PSI. I have run the foam up the foundation walls to about the mid point of the slab and angled the top. Would like to insulate the edge better at the top - perhaps expansion joint material? How is that held in place as the concrete is poured. A lot of people seem to want to pin the floor to the walls but that seems like a poor idea to me. I have about 8" between the wall and what was dug out for the footing so not much to span (at least that's how it seems to me but what do I know?). Footings were poured directly in the trench with the earth as a form. If the slab settles slightly, and I expect it will, then the rebar will be holding the slab up and just lead to cracking it seems to me.

I take it I should wrap conduits and pipe penetrations with some sill sealer or something similar to give a little "give" and cushion to them?

Planed to hold the slab back about half the wall thickness at the OHD and put the outside slab on the outside half. At the people door was thinking I would run the slab to the outside of the foundation wall and pour the outside slab without foundation support.

Wife is thinking of colored slab. Concrete contractor pointed me to Butterfield integral colors. Should I be thinking of epoxy instead?

Floor will be flat with a slight slope for the last few feet at the OHDs

Control joints - from the guide recommendations probably 15' spacing across the 30' and 13.3' for the 40'. Reasonable? Cutting them - guide says right after finishing. How is this done without leaving impressions in the surface? I do have some cracks in my current floors so can see how doing this quickly is important but don't see how this can be accomplished.

Reinforcement - seems like that is just to keep cracks from expanding. Since the slab is contained on all sides (with some "give" to the expansion joint material around the edge if used) I don't see much utility in adding reinforcement.

Finish - the guide says hard troweled or burnished finish is standard. Seems that would be pretty slick, especially when wet. Other alternatives?

 

[ConCretin]

I'll take a run at your questions;

"I have run the foam up the foundation walls to about the mid point of the slab and angled the top. Would like to insulate the edge better at the top - perhaps expansion joint material? How is that held in place as the concrete is poured"

I'm not sure if additional insulation is necessary but if you want it, I'd attached 5" expansion foam to the wall first with spray adhesive and then apply the insulation over it.

A lot of people seem to want to pin the floor to the walls but that seems like a poor idea to me"

I agree. Dowels will do more harm by restraining the slab from shrinking than they will help. Get the subgrade right and let it support the slab.

"I take it I should wrap conduits and pipe penetrations with some sill sealer or something similar to give a little "give" and cushion to them?"

Probably not absolutely necessary for a modest sized slab but it will prevent obstacles from restraining the slab from shrinking. Also helps to soften re-entrant corners

"Planed to hold the slab back about half the wall thickness at the OHD and put the outside slab on the outside half. At the people door was thinking I would run the slab to the outside of the foundation wall and pour the outside slab without foundation support"

If you are going to place an apron, determine where the OH door will land and place your construction joint there. Install 1/2" expansion material or 1" rigid between placements and seal with caulking. This will keep the heated slab on one side of the door and the unheated apron on the other. An apron is nice on a man door too but your proposed detail is fine.

"Wife is thinking of colored slab. Concrete contractor pointed me to Butterfield integral colors. Should I be thinking of epoxy instead?"

A colored slab can look nice but will need to be sealed to protect it. An epoxy floor does both

"Control joints - from the guide recommendations probably 15' spacing across the 30' and 13.3' for the 40'. Reasonable?"

Yes but make sure you keep the water content low. Ask for a mid range water reducer to get a workable slump without excess water, which will increase shrinkage and might cause random cracks between your control joints

"Cutting them - guide says right after finishing. How is this done without leaving impressions in the surface? I do have some cracks in my current floors so can see how doing this quickly is important but don't see how this can be accomplished"

Ask your finisher if he has an 'early entry' or 'soffcut' saw. These saws are designed to cut green concrete before early age shrinkage cracks can occur. If not, he'll have to do it the next morning but it could be too late by that point. Cracks could occur even if you don't see them for a week or two

"Reinforcement - seems like that is just to keep cracks from expanding. Since the slab is contained on all sides (with some "give" to the expansion joint material around the edge if used) I don't see much utility in adding reinforcement"

This is going to be controversial but you are correct. If you have a good base and control shrinkage, you don't need reinforcing. Think of it as insurance. Don't let anyone convince you to use fiber instead of rebar. There's nothing wrong with it but it performs an entirely different function from steel reinforcing.

"Finish - the guide says hard troweled or burnished finish is standard. Seems that would be pretty slick, especially when wet. Other alternatives?"

You could ask for a 'smooth troweled finish' that isn't 'burned out'. They need to finish enough to remove trowel marks. You definitely don't want a sort of non slip finish such as 'floated' or 'broomed'

Holy cow! That was almost as long as the Guide itself!

Hope that helps and Good Luck with your slab.

 

[Pressingonward]

Good questions and answers. I have no experience so I'm just following along and trying to learn before I do my slab.

 

[larry4406]

Many of our slabs in our larger homes are specified to use Zip Strips to create the control joints. These are pressed into the slab as it is poured and the top piece pulled off as the concrete sets up. We finish over the crevice which later separates to create a very straight clean line with no cutting.
http://superiorprofiles.com/zip-strip.html

 

[OccupantRJ]

Be there when the slab is poured.

 

[ScaldedDog]

If you are insulating the floor with hard foam, prep whats under the insulation perfectly. It must be flat, and compacted, very, very well. If the insulation sheets can move, or have any air pockets beneath them, the slab will crack in those spots, and keep cracking forever.

5" is the minimum, IMHO. I did 6", and would again.

I'd also have enough rebar in the slab to build a battleship. I have no idea why anyone wouldn't do this. The cost is mouse nuts when compared to the cost a building, and the slab is the one thing that can't be redone at less than exorbitant cost.

I'm not a pro, just a homeowner who's hard earned money has purchased both great, and awful, slabs and driveways.

Mark

Sent from my SM-G935V using Tapatalk

 

[Innovate1]

Be there when the slab is poured.

Thanks for all the replies.

That is a concern. The contractor said they wanted to "check the grade" of the rock under the foam so didn't want me to install it. I am a little short on rock and didn't get it leveled. They are ready to go and said they would level the rock and get any additional needed (added to bill). But that also makes it easy for them to build up the level with rock and short the concrete. Unfortunately it's going to be hard for me to get it done anyway with my work schedule. Probably can drop in mid day to see what's happening.

They said they would put down the foam without tape and they do that all the time. (but they also said they dowel everything and most here said either not needed or don't). Do I need to have plastic sheeting in addition to the foam? Seems like perhaps not if it is taped but yes if not taped but looking for input with some experience on this.

 

[Innovate1]

I'll take a run at your questions;

"I take it I should wrap conduits and pipe penetrations with some sill sealer or something similar to give a little "give" and cushion to them?"

Probably not absolutely necessary for a modest sized slab but it will prevent obstacles from restraining the slab from shrinking. Also helps to soften re-entrant corners

"Planed to hold the slab back about half the wall thickness at the OHD and put the outside slab on the outside half. At the people door was thinking I would run the slab to the outside of the foundation wall and pour the outside slab without foundation support"

If you are going to place an apron, determine where the OH door will land and place your construction joint there. Install 1/2" expansion material or 1" rigid between placements and seal with caulking. This will keep the heated slab on one side of the door and the unheated apron on the other. An apron is nice on a man door too but your proposed detail is fine.

Thanks! No re-entrant corners - just a simple rectangle.

I plan to have an apron outside the man door also. I suppose I could do the same as the OHD an hold the break in some. They said they would drill and pin for rebar at the OHD joint. I can see that helping to keep the slab from sliding away from the building creating a gap but it seems to me pinning it to the foundation beside the OHD would also be effective and particularly at the left and right edge where there is a perpendicular wall to help support any stress. That would allow an open break between slabs for the expansion joint material. How is that normally held in place?

I see you point on a thermal break under the door but the door will be inside the foundation wall or very close to the edge. Some support on the edge seems like a good thing mechanically (but not thermally) to keep the top aligned with the outside pad. A minor amount of settling seems inevitable but I suppose the inside could be made just slightly higher than the outside pad to allow for that.

If both inside and outside slabs both rest on the foundation wall that is a thermal path regardless of the break between slabs (although not quite as direct) unless I am missing something.

Not sure what you mean by "apron". I always just took that to mean a slab outside and either way I will have that.

 

[ConCretin]

They said they would put down the foam without tape and they do that all the time. (but they also said they dowel everything and most here said either not needed or don't). Do I need to have plastic sheeting in addition to the foam? Seems like perhaps not if it is taped but yes if not taped but looking for input with some experience on this.

Poly or rigid insulation (if taped) acts as a vapor retarder, which will limit moisture intrusion into the structure, which is a good thing. If you anticipate any kind of adhered floor covering such as epoxy, you might want to consider an actual vapor barrier such as Stego which is a 15 mil product.

By apron, I just mean an outside approach slab. As I mentioned, I like to place the slab/apron joint under the OH door, which is typically a few inches in from the inside face of the foundation depending on wall thicknesses. Done this way The apron bears on the top of the foundation wall where it drops down across the door opening and is held in place with dowels coming out of the wall. The slab doesn't bear on anything other then the sub grade just the way it does everywhere else. You can place the apron 1" lower to create a step to further limit water intrusion under the door.

The apron should pitch away from the structure 1/4" per foot. Use non-air entrained concrete for the slab and air entrained concrete with a broom finish for the apron. You can pin them if you want to but I'd use smooth dowels to allow a little lateral movement while keeping them aligned. Finally, I like to place rigid insulation under the aprons and extending out 4' to keep the frost from getting under them.

 

[Innovate1]

Should the vapor barrier go under or on top of the foam? I mentioned taping but they don't do it. Thinking I should at least ask them to do the normal poly. Thinking I will probably get better sealing around pipes and things with poly that with taped insulation board.

They also mentioned something about a slump of 6. I didn't have any point of reference at the time but did a bit of reading and isn't that a little high for a slab?

 

[matt_i]

The steel-troweled finish should approximate a bare basement floor. Imo plenty of grip even when wet.

I had the control joints cut the next morning after the pour, it was however colder weather and so curing was probably slowed. No issues in the ~3.5 years since.

I would think its tricky to place foam on a crushed stone base, I think you'd have to use a laser level to get it "flat" so there's good bearing all over. I got to the point where I was messing with "1 stone" of 3/4" nominal which is probably a little overkill. I didn't use foam but wanted a well-compacted base. I sprayed the stone base lightly with water before plate-compacting it in 2 directions.

I'd urge 10mil plastic, its thick and more expensive but won't get shredded like 6 mil would.

 

[ConCretin]

Should the vapor barrier go under or on top of the foam? I mentioned taping but they don't do it. Thinking I should at least ask them to do the normal poly. Thinking I will probably get better sealing around pipes and things with poly that with taped insulation board.

They also mentioned something about a slump of 6. I didn't have any point of reference at the time but did a bit of reading and isn't that a little high for a slab?

The location of the vapor barrier is a but subjective. I prefer to put it under the rigid insulation to better protect it. There is another school of thought that wants it on top because they fear bleed water will pool in the spaces between the board causing the slab to take longer to 'dry'. This isn't generally a concern in a residential situation and I want the bleed water to have some place to go other than up to the surface.

A 6" slump is WAY too high if they are achieving it with water alone. It's OK if they are using a water reducer. Concrete's advertised compressive strength is usually based on the amount of water required to produces a 4" slump. In addition to virtually guaranteeing you won't get the strength you paid for, the excess water will create all kinds of surface problems if the resultant bleed water isn't handled properly.

 

[Innovate1]

Thank you for all the information. I haven't worked much with concrete. Was just reading that water reducers shouldn't be used in cold weather but not sure how cold is cold. I will ask if they are using reducers. They plan to pour Tuesday and the temps from that day on are:

59/39
57/37
64/36
52/29
54/39
then around 60/45 for the next few days.

Seems just about perfect to me for concrete - not too hot or too cold but have no idea about additives.

The walls and roof are up/on so protected a little although the OHDs aren't on so some BIG holes.

 

[ConCretin]

You are right about the temps. They are pretty ideal for concrete.

I'm not sure where you got the information about water reducers. We regularly place in below freezing temps and a mid range water reducer is standard in all our mixes. We use high range water reducers (superplasticizer) less frequently but I have never heard of an issue with those either.

I don't know if you are buying the concrete or your contractor is but either way tell them you want a mid range water reducer and are willing to pay for it. It should only cost a few dollars per yard. It will give the finishers the 6" slump they want while maintaining the compressive strength of the concrete and avoiding a whole bunch of surface problem caused by excessive bleed water.

 

[Innovate1]

Very interesting and helpful. I saw the note about temps on a web site. A lot of misinformation on everything on the web...

Was just looking at the order sheets for the foundation walls done by another contractor(two companies work together with one doing foundations and the other slabs). It states slump of 6 and a charge for "winter service" and on the wall a charge for 1/2% calcium chloride due to cold temps. But nothing about a water reducer or anything like that. I saw them placing it and the consistency looked fairly thick without a lot of water. They did the door opening cutouts with just end blocks and leveled out the bottom by hand after the concrete was placed - I am thinking that the calcium chloride might have been partly to speed the cure so they didn't have to stick around as long for that step.

I should be able to see the order sheets for the slab but of course by that time it is too late to change anything.

 

[Bigblockyeti]

I'm prepping for my own 16' x 22' slab and I've had the supplier quote 3500psi with a 4" slump and nothing more. I'm wondering if I need to give them any more information beyond that. The finishers are coming to look at what I've got sometime this week so there's no surprises when they show up and the mixer is on it's way. I have seen these same guys really crank up the water while still in the truck and that has me slightly concerned but they're doing that for a house slab. It's much larger and I don't know if they really give a **** about long term quality or not when doing that.

 

[ConCretin]

Was just looking at the order sheets for the foundation walls done by another contractor(two companies work together with one doing foundations and the other slabs). It states slump of 6 and a charge for "winter service" and on the wall a charge for 1/2% calcium chloride due to cold temps. But nothing about a water reducer or anything like that...

Water reducer will show up on the slip by the name of the admixture, which obviously varies from supplier to supplier. MasterGlenium 7500 is common around here. Many of our ready mix producers include it in their mix design. They know the concrete is likely to be tested and it gives them some margin for error.

Winter service generally means that the mix water is heated and you are correct about the calcium, its accelerates the set, increasing the heat of hydration thereby protecting the concrete from freezing.

I'm prepping for my own 16' x 22' slab and I've had the supplier quote 3500psi with a 4" slump and nothing more. I'm wondering if I need to give them any more information beyond that. The finishers are coming to look at what I've got sometime this week so there's no surprises when they show up and the mixer is on it's way. I have seen these same guys really crank up the water while still in the truck and that has me slightly concerned but they're doing that for a house slab. It's much larger and I don't know if they really give a **** about long term quality or not when doing that.

It's wise to get out in front of this. If the truck shows up with a 4" slump, you can bet the crew is going to start screaming for water. Call the supplier and ask if a mid range is included In the mix design and if it's not, ask for it. The crew will do a better job and you'll end up with better concrete.

Most the issues we read about on here; dusting, crazing, shrinkage cracks, blisters, de-lamination, etc can be traced back to excessive mix water.

 

[Innovate1]

No additives other than what I mentioned before. I told them I wanted water reducer, said I would pay for it and they added it to the paperwork. THey also said I would get a copy of the concrete order/receipt which makes me feel a lot better that they aren't going to overfill with rock and cut the concrete and also on the additives. When I first brought up the issue that they had said 6" slump they said it would be " 4 maybe 5" which sounded like they would be adding water.

Thought they might complain that I was telling them how to do their job but they didn't but probably thought it....
I did see listed 1% calcium on the list and wondering why when the temps will be above freezing except for a couple degrees below in 3 days. Only saw 1/2% on the foundation order.

Just saw the write up of details on the concrete and I figured 17.1 yds (30 x 40 building outside with 8" foundation walls so actual slab is 28.7' x 38.7' x 5" + a bit more at the door openings). They show 21 yards. Unless I figured wrong that would be a slab about 6.5" thick. Wondering if that is to insure the floor is at least 5"... Seems like a lot of overage but if the floor is a little thicker I wouldn't complain.

 

[Innovate1]

Got the inside slab done. Looks pretty good although the corners are a little low - seems like a common problem but shouldn't be. Now on to the outside slab. Talking to them about pinning they think the inside slab should be pinned to the walls but don't want to pin the outside because of float caused by freezing and the chance for cracking. I can kind of see how the outside might be worse than settling inside but it also seems like the same basic issue. Anyway they suggested NOT pinning to the foundation. The slope of the outside slab is minor - about 1/4" to the foot. I have about have the foundation wall (4") at the OHDs for the slab to rest on. When I asked about it they suggested they could dig out along the foundation, insert some lower rebar and pour separately a wider ledge but not tied to the slab to allow lift. They also said they could dig and install some sort of piers near the outer edge of the slab to resist but again not tied to the slab to allow lift - I really didn't understand the details of this.

I have had some movement of a sidewalk that goes down a hill but considerably steeper - at least several times more slope.

Should I be worried about the outside slab pulling away? Is this typically done with no restraint? The current apron on the attach garage is not pinned and on a slight slop with no issues of movement so perhaps I am overthinking this...

Have seen some comments about using smooth pins to allow them to slide but once they get a bit of rust on them movement seems difficult.

They are saying they can do it at about 4.5" slump and don't need a water reducer. I suppose using one will reduce shrinkage but how important is that? Broom finish.

 

[Bigblockyeti]

Well, I have my shed slab poured (most of it) and I ended up specifying 6 yards of 4Ksi. I had it formed and staked, 6 mil vapor barrier under #4 rebar on chairs all tied together and steel mesh from edge to edge minus an inch. The difficult thing for me in addition to working around the weather was coordinating the power buggy, flatwork guy and readymix. I finally got it nailed down at 8am on 5/22 with cool, overcast and humid weather and a buggy I had picked up the afternoon before. The buggy made a bigger mess than I had anticipated in the muddy parts of the yard and on the driveway but couldn't have done without. When shooting grade on the gravel I noticed a few spots 1/2" - 3/4" low and I scooted things around to make it "good enough" which as it turns out actually made for a bit more slab than calculated. I should have needed 5.5 yards based on the thickened slab edge dimensions but it wasn't enough. I had to make an emergency box to leave a void for the extra 1/2 yard I should have ordered, luckily my flatwork guy has seen it happen many times and immediately knew what to do. I'll finish the last bit with twenty 80# bags and my boys as labor when the weather allows. Later reviewing my invoice for the truck, it states only 3000psi mix with fiber was sent so I'm hoping I don't have a problem, everything was ordered verbally over the phone so no record indicates I didn't get what I asked for.

I chose the flatwork guy as he was doing nice work on slabs for a builder a few streets over. It only took a little over 1.5 hours to place and screed everything but quite a while for it to set very slowly. Bleed water was still very present 4 hours after we started but started to go away quickly when the sun broke through. He went over the surface 8 times total waiting between troweling sessions and slightly increasing the blade angle with each subsequent pass. We finally wrapped up a little before 4pm, I carted his power trowel back to his truck on my trailer behind the lawnmower and we started tarping. He left and 90 seconds later the sky opened up with a monsoon instantly dumping buckets and blowing the tarps off. I had no assistance from my boys other than them running around terrified screaming "tornado" while I was getting saturated. My wife headed out too, worried all the work and money I had into the project so far had just been destroyed. I got the tarps staked tight and for good measure screwed through the grommets to the forms so they would go nowhere. The slab was fine and the tarps will come off tomorrow. I'm looking forward to knocking out the last void and assembling a lumber package to start framing.

PSA, if you think you know how much concrete you need, order more!