pouring a cement slab in sections
drptop70ss
| Posted in Construction Techniques on
I am ready to pour the slab in my 40×60 pole barn, but I cannot get the cement trucks all there in one day. So I am putting boards in to break the slab into 10 foot wide 60 foot long sections and will be doing a couple sections one day and the other sections a few days later. Has anyone ever done a slab in sections like this before? Just wondering if the seams will end up being a cracking point. Any suggestions to end up with this looking like a one piece floor? Since its a barn of course it doesnt have to be perfect but it will be my shop so I want it to be decent looking.
Dave
Replies
Time for the cement folk to answer this one. That floor is going to have cracks. The idea is to provide relief lines in the concrete via saw cuts or a break. The more experienced pros should be along shortly to define.
Is that a chevelle ss you're talking about?
Half of good living is staying out of bad situations.
Forget the primal scream, just Roar!
I doubt it ever can look like a seamless slab. As it cures, any contraction will be in from the edges, and that works against the seamless look. If you want to create a strong connection, there are metal keyways that act as an expansion joint and keyway to keep one chunk from lifting or settling. We also had to do one pour with a cold joint (truck dropped a driveshaft and blocked the driveway and then the pump blew a cylinder). Engineer recommended leaving the edge as ragged as possible and damp. The next pour bonded fine to it, but the finish across that joint wasn't as slick or even as the rest of the slab. I vote for the metal keyway and jjust pick its placement in the least obvious spot you can find.
we do this everyday. you want it to crack and at ten foot section , will be the perfect width. But you also need to cut at 15 feet long too. Also the slab will crack at all sharp corners so you might need to block out the poles.
Brownbagg,
do you mean after I pour a section I should go back and score the piece every 15'? All the way through or just partially? Also what do you mean block out the poles? When I pour a new section against a previous section should I also make a score line along the joint?
Rez, yep its a chevelle SS..cars are my disease and this barn is a new shop for me to do my resto work in.
thanks for the info guys, I wish I could do the slab all at once but not this time.
Dave
If you're going to do it in sections and you don't want uncontrolled cracks all over the place, there's a lot more to consider than the placement of sections and control joints.
40 ft. wide and 60 ft. long, any posts and where? What kind of base have you and did you compact it? How thick is this concrete slab going to be and what re-enforcing have you considered? What mix and expected loading ar you considering?
Gabe
Just a little side post here since the concrete guys are around. I've an old cottage that had a front porch room poured in the 1940s. Prior to my reno on the place no gutters on the roof of the porch ell so all water would cascade down and eventually removed the dirt under the porch corners aside the house by about a foot square. My question is what did the pourers do to make that 4 inch slab so durable? I removed old linoleum on it and on close inspection over the whole pad I cannot find any hairline cracks. No vapor barrier over sandy soil? Was it just the small size of the slab?Half of good living is staying out of bad situations.
Forget the primal scream, just Roar!
Rez,
Yes, the small size may have something to do with it but remember concrete has been understood for a long time, we're the ones who forget how to place and finish it properly.
I've demolished concrete structures that were older than I and structurally, the concrete was perfect. They were demolished to make room for bigger buildings.
Gabe
Guys,
I have done a few slabs, biggest was my 25x25 two car garage. I also helped do a 44x28 basement but had help with that one. The building is 40x60 clear span, the only poles are around the perimiter. Base is basically shale and clay, compacted by running my 15000lb backhoe over it by putting the loader bucket down, lifiting up the machine, and then running backwards. The clay packs down so hard its like concrete in itself. Over that I put down a 6mil plastic vapor barrier. Mix will be 3000psi, straight mix with no wire mesh (I have used it before, but also done slabs without and didnt get cracks)...slab will be 6" thick, I know its gonna be tough to get this done myself or with one helper, but that is part of the reason I made the sections 10' wide..I can handle a 12' screed board myself and then bull float it. I figure by the time a section is bull floated I should be able to get out on knee boards not long after and hand trowel it. No way would I try and do the whole slab this size in one shot, I know it would be too much without a lot of help. It will get done, just not in one day. I have never covered a slab with plastic after finishing it, is the extra cure time and strength it gives much more than if I let it cure normally? The building has a roof so isnt exposed to the sun.
A good point was brought up about the poles still being able to move a little..I will put some 1/4" foam sill seal around the bases of the poles, that should allow some movement and maybe spare the slab.
Dave
Edited 9/15/2002 9:12:41 PM ET by drptop70ss
Edited 9/15/2002 9:21:43 PM ET by drptop70ss
I wouldn't include the poles in the pour nor would I box around them. I would exclude them by installing a bottom plate made of 2x6 on edge to make a straight line. To lay out a pattern to control cracking in not as simple as "just" boxing them in.
Keep the concrete sections simple, square and consistent.
10 ft. wide pours would be cut 1" deep across at every 10 foot increments.
Hand trowelling this size of a slab in not recommended, even for the experienced. Rent a power trowel and you won't be caught with a serious problem with a quick set up that will catch you by surprise and ruin your slab finish.
Gabe
Did the first 60x10 section today, and actually had extra concrete, so we layed it in the next 60x10 section about 2" thick and rough (just raked it around)..next problem! The second truck didnt make it and I cant do the next section till friday. I am still putting a few inches of concrete over the rough stuff, do you see any problem with the slab being weaker from being done in two layers? Also now the edge of the first slab is going to harden for another couple days, will it still be green enough to bond with the next section of cement we will be putting up against it? It is smooth as it was poured up against form boards.
Dave
Edited 9/17/2002 7:02:20 PM ET by drptop70ss
For your slab to slab joint I would put in a pc. of expansion, tack a half inch pc of wood to the top of that 1/2" expansion. Break that edge on the new slab. After finishing, pull off that pc of wood, let the concrete cure, then come back and fill that space left with either self leveling urethane caulk(it'll leak out of any openings), or regular urethane caulk. This'll give you a pretty smooth floor. You could do the same to the cut control lines too (cut a small tip on the cartridge and make it neat). Had this detail on a service station pour.
Next time pound some forms together for patio blocks for that extra concrete. Gives the kids something to leave their name in. Best of luck.__________________________________________
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
Calvin,
thanks, that is what I will do with the next cold joint, as there is going to be another. Unfortunately with this one when I pulled the form boards they chipped up the edge (rushing and forgot to use the edger before hand) so I have some chips that I was hoping to fill when I poured the next slab section against this one. The caulk wouldnt fill some of the chips, just too big. Normally I use a batch truck, but got a good deal on a few redi mix trucks, and this stuff set up way faster than I had expected so I was scrambling. I was also very suprised to have extra, normally I run pretty close and I was told another truck was coming so I figured no big deal to rake out a few inches and then cover it within a few hours. The deal with the cement is I cant schedule the trucks as well as I would like so I end up hoping the truck will show. At least I left the raked out stuff rough, my only concern was bonding to the existing smooth edge of the first slab. I am starting to think the money savings on the cement isnt worth it! Thanks,
Dave
In simple terms, yes it weakens the slab. Is it quantifiable? Not always.
Gabe
>At least I left the raked out stuff rough, my only concern was bonding to the existing smooth edge of the first slab.
With this part he wrote, I read it as wondering about an edge-to-edge bond of the second 10' section to the first 10' section (in addition to what you answered). On the edge bonding, my guess is that there is very little bonding, and if he thinks the second pour will flow perfectly to fill the chips outta the edge of the first one (and not chip later), he'll be disappointed. I'm guessing that an edge bond isn't even desirable--it's a joint, after all. Yes? No?
Absolutely it's a joint and will not be or should be bonded.
Bonding would serve no purpose and would fail in any event. It would have been advisable to pass an edger instead for a final finish.
Gabe
Thanks guys,
my fault for not running the edger before the stuff setup, like I said I was scrambling as it set up much faster than when I use the mix on site trucks. I figure I will pour the next section up against the first, TRY to float it some to fill some of the chips, then run the groover along the break line. I checked the cement tonight and it still looked very green; probably helps that the slab is indoors and doesnt have direct sun on it. I just want to try and make the break line look decent for some reason that I will forget about years from now. The next slab I will run the edger before I pull the forms, hopefully friday morning.
Dave
Dave, I was taught along time ago to break the edge first off. We usually went back after the flatwork if we wanted that framed edge look. If not framed edge, we would only clean up the edge if necessary. Not saying this is professional concrete protocol, but it worked well. Best of luck.__________________________________________
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
Rez
you are wondering why a concrete slab poured in 1940s, is showing no cracks.
The following is just my supposition, but most concreters today do not know how to place concrete. I've heard of contractors who build slabs 2 to 3 times thicker than required 'cause they don't know how to do it properly. Concrete is not self levelling, self compacting, or self finishing despite what the additive companies say.
Without pulling your old slab apart and looking for reo, I'd guess that in 1940 the concreters knew what they were doing and did it "right". The concrete was probably
mixed in the right proportions - modern mixes are often selected because they are the cheapest or the easiest to place;
leveled - the traditional tool is a square mouth shovel;
compacted (to get the air out of the mix) - these days you should use a vibrator and it takes on average about 20 seconds per location and insertion points would normally be about 12 inches apart;
finished at the correct time and then moist cured
Most import - the crew was big enouugh for the job.
I really worry about the longevity of large slabs poured by only a few people.
A 10 ft x 60 ft slab, 6 inches thick would normally require 2 shovelers, at least 1 vibrator (preferably 2) and two to pull the screed (which ideally is a vibrating one). Absolute minimum crew size is 3, but a crew of 4 would allow you to start screeding before the truck had finished discharging.
A few years ago I managed the reconstruction and widening of an urban road which had to stay open to traffic whilst we did the work. The pavement design called for concrete and the way the job was designed meant that the typical work area was about 700 ft long by 2 lanes wide, spaces too small for any of the local concrete pavers, so the concrete was all placed by hand. The crew size was 9 and typically they could pour, finish and apply a curing membrane on one 8 inch thick, 11 ft wide x 300 ft long slab per 7 hour shift, using 2 inch slump, 4500 psi concrete - getting the concrete trucks through the traffic was a continuous problem and most concrete was placed from one side of the slab rather than between the forms. These slabs were unreinforced and scoured at 15 ft intervals for contraction joints. Jobs on the crew were:
shovellers - 2 to 4,
immersion vibrators - 2 shared between shovelers and screed pullers
screed pulling - 2, assisted by 2 of the shovellers
hand finishing behind the screed - 2 (one each side of the slab)
bull floating - 1
When there were 11 or 12 on the crew, all processes could occur similtaneously and on days when the batch plant could keep the trucks coming, output would approach 600 lineal feet per shift. However, on days when there were only 7 on the crew, output dropped to less than 200 lineal feet.
I have gone on a bit. The point I wanted to make is if you want a quality slab that will last, you need the right concrete and enough people to place, compact and finish the concrete.
Dave, have you ever poured a slab this size before? No offense, but your question kinda makes me wonder...........How much help have you got lined up for the pour? People you absolutely KNOW will show up and know what to do when the first truck gets there?
When that stuff starts down the chute you better know what to do with it. You are talking about doing "a couple" of these 10' x 60' sections in one day, you better have a lot of experienced help on hand.
Good luck, Joe H
PS: Gabe Martel is the guy you need to ask about this, he wrote an article for JLC last year about this. You might find him at Joe Fusco's place or Quitting Time if he doesn't show up here.
Edited 9/15/2002 4:53:53 PM ET by JoeH
What the (*&^??? - neither Gabe's or rez post was there when I first saw this, now they are both up before mine? Is this a new Pisspro feature? Joe H
to answer your question, Yes. about scoring, cut about 1/4 the thickness. On 60 feet cut it 15 feet section.
as the wind blows and freeze and thaw the poles will move a little maybe 1/4 inch, this will crack the slab. Blockout around poles about 18 inches. That way if it does crack it will be in the small square instead of the big square. compact the hell out of subgrade, put down a vapor barrier, pour kinda dry not soupy and cover with plastic for seven days after placement. do not let slab dry out for seven days. Cut scoreline within 12 hours, as soon as you can walk on it without leaving marks.
Is this where we register our Poorspero complaints?
Dan, I checked with them and they said to send them all to Larry. Or post them on the portapot wall for faster service. Joe H
Complaints about what?
Good golly,
Real time.
I was reading the thread, saw your toung-in-cheek remark and thought of a smart a** remark as well. I replied to your post but by the time mine finally posted I was at the end of the thread. I stopped to stuff my two year old son's mouth full of quesa-dias and corn on the cob.
I returned to see my message chronologically discombobulated. I started trying to delete it so as not to disrupt the flow of technical conversation, when I changed my mind and decided to edit it with a "case in point..." comment about poorspero when I was greeted by your response.
Yeesh.
Thanks for the information though...
Dan
Dan,
missed my "tonge in cheek" remark? All I give is my info, no need to screw with someone online..what came across as smart a**?
Dave
drptop70ss,
I was attempting to reply to Joe H, and I was being the smart a**. Apparently I was being a dumb a**.
His comment was
"What the (*&^??? - neither Gabe's or rez post was there when I first saw this, now they are both up before mine? Is this a
new Pisspro feature? Joe H"
So I responded with "...is this where we... etc"
Not trying to raise a big stink about a free service, just attempting to make a one line joke in the middle of an otherwise serious conversation.
Hence, my previous "yeesh".
I'll try to refrain from feeble attempts at humor in the future.
Did you guys hear the one about the Montanan, North Dakotan and the New Yorker?
Dan
I have a possible solution - one I have used in the past(and even has been pictured in prints):
Where you end your pour for the day, pre-drill your interior form boards - where you will be butting up another section of slab - and place 4' dowels of rebar every 2 feet or so so that half of the dowel will be in the section you are ready to pour and half will end up in the subsequent pour. That way, with each section you pour, you can pull your forms and reuse them, and you have structurally connected sections of slab that won't sink or heave, but can flex slightly.
What do you think?