Pouring four 10x15 slab sections Advice ?

[bcarwell]

I am putting in a concrete slab under my 40 x 15 foot roof overhang extension on my barn for some extra outdoor work room- not alot of floor load. Because I don't have a big concrete mixer and crew I'm thinking of doing it one section at a time, e.g. in four separate pours of 10x15 foot sections 4 inches deep. I'm around Austin TX with clay soil bed (e.g. drainage not so great but it will be on a well prepared bed with gravel if that matters. A few questions:
1) Would you attempt to tie the sections together mechanically with rebar or remesh and if so how ? I suppose I could make multiple slots in the form board starting on the bottom edge and extending vertically upward for rebar to extend through the form a few feet into the next section and guess the form would be removable on curing. Don't know how I'd do it with remesh other than sandwich it between 2x2 forms so the remesh could extend into the next section. I wouldn't think there would be a lot of shifting between sections though and wonder if tying is even necessary.
2) I'm figuring 50 cubic feet for a section. With a 3 cubic foot concrete mixer that would be 15+ loads. I'm guessing 5 minutes mixing + 5 minutes loading the mixer for each load or 10 minutes per load nonstop- probably more like 15-20 minutes per load including dumping. That would be 3-4 hours pouring for a section. Is that even doable (I don't have much experience with setup times, etc. but assume the "crew" could keep up with the pours and necessary screeding, etc. and would assume by the time the last pour is made it wouldn't be too late to start back at the beginning of the slab section for bull floating, trowel/finish work. Yes ?
3) NOTE: You may be saying "Why not order concrete for the whole job ?" As noted I have a small crew and don't think a concrete truck would hang around that long. Also I can do one section and wait a month or so before getting around to the next section.
4) And what would you do about the space between the sections ? Just leave the form board in place ? Or pull it and fill the space with what ? I don't really care about winning any beauty contests, just function.

Thanks for any advice, including "You are nuts".

Bob

 

[aczlan]

1) Would you attempt to tie the sections together mechanically with rebar or remesh and if so how ? I suppose I could make multiple slots in the form board starting on the bottom edge and extending vertically upward for rebar to extend through the form a few feet into the next section and guess the form would be removable on curing. Don't know how I'd do it with remesh other than sandwich it between 2x2 forms so the remesh could extend into the next section. I wouldn't think there would be a lot of shifting between sections though and wonder if tying is even necessary.

I would drill the forms, then put rebar through so that it sticks out a foot or so, tie that into the mesh on the pour side, then once you pull the form boards, tie into the mesh on the other side.

Is that even doable (I don't have much experience with setup times, etc. but assume the "crew" could keep up with the pours and necessary screeding, etc. and would assume by the time the last pour is made it wouldn't be too late to start back at the beginning of the slab section for bull floating, trowel/finish work. Yes ?

I think that using a small mixer like you are thinking will have the concrete set at one end before you get to the other.

3) NOTE: You may be saying "Why not order concrete for the whole job ?" As noted I have a small crew and don't think a concrete truck would hang around that long. Also I can do one section and wait a month or so before getting around to the next section.

What if you did 2 sections at a time? That would give you about 4 yards which should be doable with a truck and to have a small crew float out.
We did a 30ishx75 pad with 4 guys, 2 teens and some rented equipment (bull float, vibrator and power trowel IIRC)

4) And what would you do about the space between the sections ? Just leave the form board in place ? Or pull it and fill the space with what ? I don't really care about winning any beauty contests, just function.

I would probably pull the form between sections after the first pour has set but before you start the next one

Thanks for any advice, including "You are nuts"

You are nuts

I would go with a truckload delivery even if it cost a little more. That way it will be a unified pour and you should have an easier time floating it out. Also, what if the mixer gives out halfway through? That could get nasty in a hurry.

Aaron Z

 

[CurlyDave]

Get the concrete already mixed in a truck. Hire a professional finisher and have your small crew work under his direction on the day of the pour.

Get it all prepped, gravel under slab compacted (very important), and form the outside edges. Use rebar to reinforce the concrete, support it on dobes or chairs before you pour. If the finisher says there is no need for that, and he can "pull it up" while finishing, get a different finisher. I have demolished many slabs and I have never seen one where the rebar ended up anywhere but on the very bottom using the "pull it up" technique. The outside edges need to be ~12" deep and 12" wide, tapering back to the slab thickness.

In my opinion, the 6" x 6" x 10 gauge wire sold for slabs is worthless, It always end up at the bottom and doesn't contribute anything to the slab.

I would suggest a black polyethylene vapor barrier under the slab, on top of the gravel, to prevent water vapor transmission through the slab, but you don't have to have this on an outside slab. I have always used a total of ~20 mils thickness, which is two layers of 10 mil plastic.

Talk with your finisher about whether to cut expansion slots in the finished slab or to form them into the concrete. I have always had them formed in during the pour, but have lately read a few excellent reasons to cut them in with a diamond saw after some curing.

Be certain your finisher has grade stakes and knows how to use them. I had a contractor hire a finishing crew once that did not understand grade stakes--the slab ended up being dished about 1.5" lower in the center than at the edges and turned into a great big shallow pond every time it rained. Those guy never worked on any job I had again.

I would never even consider doing this is small sections with a small mixer. You will pay more, break your back doing it, and come out with an inferior result. It is extremely difficult to get adjacent slabs all level and at the same elevation, and if you are doing it for the first time it may not work as nicely as you planned.

Slope the entire slab about 1/4" per foot to the outside, away from the building. You do not want puddles on the slab.

If you have a professional finisher, he will keep up with the truck, if he doesn't I would far rather pay for an hour of waiting time on the truck than do this in small sections.

 

[Egon]

How big is your crew and what experience do they have? Can a concrete truck drive along the forty foot length?

 

[Western]

x's 2 what Aaron said.

If you do it in sections, get some expansion material (felt, neoprene, ect tra) at your local supply and install at each joint before the next pour. (easy)

If it where me though, I would save up to do the whole pour and hire a crew for the finish, or as Aaron said rent the float. Bull float can be had for less the $20 a day most of the time.
Allot of concrete contractors will do the pour and the finish for $1-$2.50 a sq ft. Worth calling around. All you do is the prep, forms and steel.

With no footer, your looking at around 7.5 yards of concrete, I just paid $79 a yard up here in DFW, so probably close to the same in your area.

Using a "mixer" would sure be a labor of love

 

[Ken45101]

Check with your ready mix operator (or a couple of them). Around here, there is a 2 yard minimum delivery. If so, you might not save anything mixing it yourself. And the truck operators wait a reasonable amount of time, assuming you know anything about pouring and finishing concrete.

As for mesh/rebar, I don't know. The original 1500' driveway here was poured 40 years ago without any rebar or mesh, but they poured it 6" thick. It has held up very well. When we added/widened the driveway, we did the same: 6", no steel. They did use a 3000# mix with fiber added, that's supposed to give it additional strength.

Ken

 

[rambler]

Base preperation is _the_ deal on concrete, not 'if it matters or not...' It is the _whole_ deal on getting good long lasting concrete.

Rebar holds up, mesh often is installed wrong (ends up laying at the bottom of the concrete), and is weak and will rust off at the cracks that form, doing nothing for you really. Rebare is the real deal for holding concrete together.

To do sections, you run rebar through holes in the form board, when that section sets up for a day, you pull the form away, and then attach the next section of rebar to the protruding rebar.

I think you will save the most money by hiring one pro to set up and finish off and supervise your project, and you get a group of willing friends who can listen to instructions to help do the grunt work. do it all at once. This will give you the best work, for the cheapest money.

Concrete work takes a lot of prep, and a lot of experience to do it right; the day of the pour it takes a lot dumb muscle. Don't try to confuse one for the other.

--->Paul

 

[Red Horse]

My two cents.

I think Curly Dave has a lot of good points.

No way would I do this with a little mixer- that is a lot of work and I think you would end up with a lousy job just because of the time factor.

this is not a big slab. You are talking about 7.3 yards of concrete-or roughly 10 yards if you pour a 12" x 12" "haunch" all around the perimeter.

Key point has been made as to base. Use a big plate compactor to get the gravel solid.

I would set 2 x 4's at each 10' interval. You will then pour each 10 x 15 "box" and screed off of these 2 x 4's. Obviously the better the job you do setting those, the flatter the floor.

Fill a wheelbarrow or two with concrete when you are done with the pour.

Once the floor starts to set so it will bear some weight, get on it and pull the three 2 x 4 "screed boards". Then fill the void space with the material you saved in the wheelbarrows.

As for the wire, no doubt I guess the best job is to set the wire on chairs but I think with some care you can hook the wire with a rake and pull it up-you are not building an interstate! Just make sure you are pouring the concrete so it is stiff-if not, curly is right, the minute you release the wire, its down to the bottom again. Stiff concrete is a bit tougher to work but its stronger too.

this opinion and a buck eighty will get you a medium coffee in Dunkin donuts!

 

[Stimw]

I poured a 40' X 65' slab. I graded and formed it myself. I ask at a local tool rental if they could recommend someone to do the drop and finish. They gave me a guys phone number who did a lot of tool rentals that they felt was competent. His price was reasonable and he and his crew started at 7am and were done at 5pm with 34 yards finished and power troweled.
Sometimes you will just have to go with a pro, concrete is one of those times. I have worked my share of concrete and even driven the trucks. I have seen too many inexperienced jobs that didn't turn out well.

 

[charlz]

Sometimes you will just have to go with a pro, concrete is one of those times. I have worked my share of concrete and even driven the trucks. I have seen too many inexperienced jobs that didn't turn out well.

^^This, I did my fair share of concrete projects as a teenager and it is one of those things where it is worth it to pay the money. Just have to make sure you get a guy who will do it right.

 

[Egon]

I am putting in a concrete slab under my 40 x 15 foot roof overhang extension on my barn for some extra outdoor work room- not alot of floor load./QUOTE]

Have you considered useing precast blocks. It may be a simpler easier way to go and much easier on your back.:thumbsup:

 

[2manyrocks]

CurlyDave's post covered it well.

It's probably not economical to mix this volume of cement yourself compared to the cost of concrete truck delivered ready mix. You get a better job and save if you do the form work and correct placement of your vapor barrier and reinforcement. The savings on the prep work allows you to budget for a good finisher.

The finishing part is extremely important unless you want to end up with an otherwise good concrete job that spalls.

 

[lovemytoys]

x's 2 what Aaron said.

If you do it in sections, get some expansion material (felt, neoprene, ect tra) at your local supply and install at each joint before the next pour. (easy)

If it where me though, I would save up to do the whole pour and hire a crew for the finish, or as Aaron said rent the float. Bull float can be had for less the $20 a day most of the time.
Allot of concrete contractors will do the pour and the finish for $1-$2.50 a sq ft. Worth calling around. All you do is the prep, forms and steel.

With no footer, your looking at around 7.5 yards of concrete, I just paid $79 a yard up here in DFW, so probably close to the same in your area.

Using a "mixer" would sure be a labor of love

i didnt come close to the yards you did here what i got with a 12x12 footing it 2.25 yards with no footing i got 1.85 yards that with 4inch slab

 

[lovemytoys]

how much stone would he need ??

 

[Mike476]

I poured the 20X30 slab for my shop, I didn't use mesh. We rebarred around the perimeter and used fiber in the concrete, no stress reliefs. Slab is 6 inches thick around the perimeter and through the middle, 4 inches through the rest of the pad.

That was 11 years ago, no signs of cracking so far. 4 inches of gravel around perimeter, 3 inches under the rest of the slab with 20mil plastic on top of the gravel.

Divided the slab into two 20X15 sections, screed board was a 2X4 that I milled with a slight crown in the middle, drove 6 pieces of rebar into the ground that I drilled corresponding holes in the 2X4 for it to sit on. That left two inches beneath the screed board for continuous pour. Once I had finished with the screeding and before I started with floating and finishing I pulled the screed and filled the cavity with concrete. Turned out perfect and I liked the idea of a continuous wet pour.

 

[sandman2234]

Just keep in mind that water added to concrete at the time of pouring weakens the concrete. Some minor amounts of water added to the finishing operation aren't what I am talking about, more like the guys that grab a hose and start adding it to make it "easier" for them to pour. I prefer to keep the hose put up and give the finisher a 5 gallon bucket to use...
Preset on concrete typically takes around four hours, depending on the temp, etc. Working the concrete at, after or near this point is extremely detremintal to the strenght of the concrete.
David from jax

 

[mattbatson]

just to chime in, we had a 4 inch addition to our driveway that was 10 ft by 35 ft done about 10 yrs ago.
no rebar.
fiber mesh was used in the concrete instead.

it has no cracks and has held up wonderfully.

also, for the base, isnt gravel self compacting?

 

[ironnut]

My wife and I have mixed and poured a number of slabs. We did a 40x15 slab in 5 foot wide sections. We used 1/2 inch rebar as re-enforcement in the slabs. I had holes in the forms to allow the rebar to overlap into the next section.

The real secret to concrete that doesn't crack and settle is the base preparation. 3/4" minus beat into submission with a plate compactor makes a great sub base and supports the concrete so it doesn't crack and settle. Concrete just spreads a point load out so that the dirt underneath can better support it. If it isn't supported adequately it will fail.

As to finishing, get some books to start with. A decent darby is a major key tool to an acceptable finish, that plus a bit of knowledge on how to use it. Place the concrete, tamp it, screed it, float it, then wait for the bleed water to disappear and get your darby out. Back and forth once and then wait for the sheen to disappear, then repeat. The key is to not over work it.

We have also done slabs with a crew of our friends and the redi-mix folks. The first time we did that it worked out okay, but the slab was not a thing of beauty, functional yes, but it could have been better. But then I didn't know the secret of the darby at that point. Be prepared, with your forms and site preparation. Also be prepared to instruct and direct your crew. It really helps to have someone running the chute who is experienced or at least has a good eye on keeping the pour the correct thickness. The chute person can make or break the pour.

A 5x15 section 4" thick is around a cubic yard. That was enough for my wife and I to do in a day. A couple of hours for the pour and then a few minutes every so often for the next couple of hours longer to finish the slab. If you are not in a big hurry i.e. you can spread the pouring of the slab over a number of weekends, it is quite doable. I went a bit upscale on my cement mixer and it can mix a 1/5th of yard in each batch. It makes doing 5 sack mix easy as one bag of cement plus the aggregate make a batch. I have also poured pads using the little mixers that are so common.

I have not had any failures in our pours of concrete. I park a 14,000 lbs backhoe on our first pour. It is a 2x4 thick slab with mesh in it and it has not cracked and settled. Pouring your slab is definitely doable.

gordon

 

[bcarwell]

Many thanks to all for your very valuable advice and sharing your experience and knowledge. Because I have some other things going on (like building a funicula down a cliff) I've opted to job out the slab not because I don't think I can do it but just have a higher priority now. Actually several of your appends encouraged me and I will be doing one myself not quite as big when the funicula is completed. Again many, many thanks- very valuable resource I am now printing off.

 

[charlz]

I went a bit upscale on my cement mixer and it can mix a 1/5th of yard in each batch. It makes doing 5 sack mix easy as one bag of cement plus the aggregate make a batch. I have also poured pads using the little mixers that are so common.

You built that? Looks awesome! You must have really been expecting to pour a lot of concrete to go through the effort to build that.

I just checked out your dozer thread, not sure how I missed that. Do you have pics of this concrete buggy build to put up a thread with? That would be cool. Welcome aboard neighbor!