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seth 05-04-2009 12:58 PM

Question on Monolithic slab - Pour all at once?
 
The contractor quoted to pour all at once - the 15" X 24" footing, 3 feet retaining wall on two sides and the 20 X 32 feet slab. Is this feasible? Quotes received from other contractors split this into three days – one day each to pour the footings, wait for some time, and than pour the retaining wall, back fill, compact and finally pour the slab on a separate day.

Is this acceptable practice? Pouring all (footing, retaining wall and slab) in one pump?. I am concerned that it might develop cracks since this would be a one huge piece of structure. Also I thought that the better idea is to pour the footing first and let it get cured and than pour the rest since they all sit on top of footing. Now the guy who quoted monolithic slab is the lowest quote, also have excellent reputation, so I would like to know if this is a correct way.

Please enlighten me on this. Thanks

Willie T 05-04-2009 02:09 PM

Quote:

Originally Posted by seth (Post 269271)
The contractor quoted to pour all at once - the 15" X 24" footing, 3 feet retaining wall on two sides and the 20 X 32 feet slab. Is this feasible? Quotes received from other contractors split this into three days – one day each to pour the footings, wait for some time, and than pour the retaining wall, back fill, compact and finally pour the slab on a separate day.

Is this acceptable practice? Pouring all (footing, retaining wall and slab) in one pump?. I am concerned that it might develop cracks since this would be a one huge piece of structure. Also I thought that the better idea is to pour the footing first and let it get cured and than pour the rest since they all sit on top of footing. Now the guy who quoted monolithic slab is the lowest quote, also have excellent reputation, so I would like to know if this is a correct way.

Please enlighten me on this. Thanks

Assuming that by "retaining wall", you are referring to the stem walls beginning at the footing, and extending vertically to the surface of the slab... the walls to which the edges of the slab will be poured, no, this is not acceptable.

And here is the reason:

You suggested you have at least part of three feet of fill within the perimeter of those walls that will require some compacting. If, indeed, you anticipate filling all of that three feet, the fill needs to be compacted in progressively prepared layers. ie. fill eight inches... compact it all... add eight more inches... compact it all again... add eight more inches.... do the total compacting thing again. And on and on till you fill the required depth.

This cannot be accomplished without the stem walls first being in place to backfill your fill dirt against.

If, on the other hand, the intention is to simply dig down for the footers, you still have to compact the relatively small area of fill all around the backside of the form that would be used for the inside of the stem walls.

And you couldn't get the form boards out... Not good construction practice to leave form boards in the ground.

Remember, even pockets and edges left uncompacted are not good, and are subject to excessive settling and eventual cracking.

A monolithic slab, one poured with slab and "haunch" footing done all at the same time is fine...... IF done upon undisturbed or thoroughly compacted soil. No way this is going to happen with three foot walls.

seth 05-04-2009 02:24 PM

Thanks Wille. Yes you are right; right now all the three foot of the wall is below the slab grade. So It needs to be filled completely with dirt/57 gravel and compacted.

Can I pour the retaining wall and the footing in one pump and the slab separately?

Willie T 05-04-2009 03:59 PM

Quote:

Originally Posted by seth (Post 269312)
Thanks Wille. Yes you are right; right now all the three foot of the wall is below the slab grade. So It needs to be filled completely with dirt/57 gravel and compacted.

Can I pour the retaining wall and the footing in one pump and the slab separately?

Yes, you can. However, this entails a rather complex and expensive process of formwork, and is a bit impractical if the walls are not now in place. But you say your walls are there at this time? Well then, the slab and cavities of the block walls can be poured at one time with no problem. In fact, that is the preferred method. It is only the pressures of compacting close to unfilled walls that will give you concern..... the walls can be bowed outward from the compacting.

The most economical, and at the same time, effective way is to dig and pour the footing first. And as really SHOULD be done with all poured concrete (but rarely is anymore) after a 28 day wait, go ahead and either lay up block stem walls, or form stem walls and pour them.

The full 28 day wait is not necessary after pouring the stem walls before you pour the slab. But the walls should have at least 10 to 14 days to develop enough strength to help resist the lateral forces which will be applied from the compacting process.

In reality, it should go like this:
  • dig footing........... one day
  • compact footing... same day
  • prep footing......... two days
  • pour footing......... one day
  • wait for curing...... twenty eight days
  • lay up block walls. one to two days
  • or................ form walls... three to four days
  • pour walls........... one day
  • wait for cure....... two weeks
  • fill and compact.. one day
  • prepare slab....... two days (not counting time for electric and plumbing prep)
  • pour slab
  • wait for cure...... 28 days if laying block... (Wood frame can begin in as little as a week)
Now, does anyone in these days of greedy "hurry-up and make the money" ever allow these lengthy curing times? Of course not. But neither did the old foundations and slabs have the cracking we have today.

BTW, Your slab should be kept cool with water during this curing time. (Curing concrete creates immense internal heat that hurts the strength-developing process if left unchecked.) Often hay or burlap sacks are spread on the concrete to retain the moisture from occasional (every few hours) sprinkling. Concrete should never be allowed to dry out during the curing cycle. As crazy as it sounds, wetting concrete, and letting the water evaporate can also cause cracking. Once you begin wetting it down, keep it wet for the whole curing cycle. Doesn't hurt to hold the moisture in with plastic sheets.

Is all this wet curing a little bit of a hassle? Yes, but it pays off in the long run. This is one of those "Do the job right, and it will last." things.

Of course it goes without saying that concrete should be poured as much on the dry side as is practical. Many contractors soup it up to facilitate faster and easier placement. This is hurting you. Don't let them do it. Soupy concrete has way more potential for cracking than dryer stuff.

seth 05-05-2009 09:04 AM

Thank you again for your help; No Currently I do not have a retaining wall. I need to pour the footing, retaining wall and the slab

Willie T 05-05-2009 09:19 AM

2 Attachment(s)
I was quite serious about the likelihood of the retaining walls (stem walls) being pushed outward from compacting forces. I've seen it many times, and even lost a wall once, myself, before I wised up and began doing it right.

Leave your concrete pour low by about 2/3 of the header blocks (about an inch to an inch and a half of empty space for the slab pour to lock [key] into). It will create a better lateral lock all around the perimeter.

The bottom drawing shows somewhat how it would look with a slab poured.

seth 05-05-2009 09:29 AM

Hi Wille

Could you please explain the second para. When you say Header, what exactly you mean? Also when you say Leave your concrete pour low you mean the slab?

Willie T 05-05-2009 09:45 AM

Quote:

Originally Posted by seth (Post 269726)
Hi Wille

Could you please explain the second para. When you say Header, what exactly you mean? Also when you say Leave your concrete pour low you mean the slab?

A clarification drawing has been added above. I meant on the wall pour. Leave it a little low, and the slab pour will lock into the wall better. Just makes things a little more solid.

The header row is the final top row. The one with part of the side of the block cut out so you can pour your slab right up to the outside edge.

seth 05-05-2009 10:01 AM

Thanks again

I am not planing the retaining wall as a block wall. Don;t want to use precast blocks. I am asking the contractor to POUR the retaining wall which will be 3 foot tall at one side and gradually tapers down to a foot

walkman 05-05-2009 10:20 AM

WillieT: if the slab was at the bottom of the stem wall, is it OK to pour the footing and slab in one pour and then build the block stem wall? Or should the footer be poured, then stem wall, then slab?

Willie T 05-05-2009 10:39 AM

Quote:

Originally Posted by walkman (Post 269747)
WillieT: if the slab was at the bottom of the stem wall, is it OK to pour the footing and slab in one pour and then build the block stem wall? Or should the footer be poured, then stem wall, then slab?

I'm thinking you're talking about pouring a basement slab in with the footer? Then building a stem wall up to the height of the house floor?

It generally isn't done. I can't give you an engineering explanation of why, but the basement slab is usually poured as a free-floating separate component after the footer is poured, and the walls built.

Perhaps someone with a better engineering background than I have could tell you why...... and maybe there's really no reason? Could be one of those "We've always done it that way." things.

Truthfully, we don't do a heck of a lot of basements here in Florida, so I really am not that well acquainted with the whys and wherefores of basements. The few we do are just done as the drawings call for them.

Willie T 05-05-2009 10:46 AM

Quote:

Originally Posted by seth (Post 269741)
Thanks again

I am not planing the retaining wall as a block wall. Don;t want to use precast blocks. I am asking the contractor to POUR the retaining wall which will be 3 foot tall at one side and gradually tapers down to a foot

Then you don't even have to be concerned with things like header blocks.

Mop in Hand 05-05-2009 11:30 PM

Willie T, (speaking of we've always done it that way) I have never poured the footings and then the walls, we always did a monolithic pour when it came to the footings and walls. In my opinion a monolithic pour is just easier, well because, we just have always done it that way. However, never did a footing, wall and slab at the same time, except for a few small freestanding garages. I will have to agree about waiting a few weeks to backfill, compact and then the slab.

Willie T 05-06-2009 08:21 AM

Quote:

Originally Posted by Mop in Hand (Post 270142)
Willie T, (speaking of we've always done it that way) I have never poured the footings and then the walls, we always did a monolithic pour when it came to the footings and walls. In my opinion a monolithic pour is just easier, well because, we just have always done it that way. However, never did a footing, wall and slab at the same time, except for a few small freestanding garages. I will have to agree about waiting a few weeks to backfill, compact and then the slab.

I can see it if you are already all excavated, and have everything open. Up north, you're going to form basement walls anyway, so a small forming bump out for the footing isn't too much of a bother.

But here in FL, we almost never dig down more than a couple of feet for house footings, and formed stem walls are a rarity. It would be an expensive hassle for us to form and pour a combination footer and stem wall for residential work. Our houses only sell for about half of what they do up north, but materials seem to cost as much or more, so it would be financial suicide for the average builder to go to such unnecessary extremes on a foundation.

walkman 05-06-2009 03:27 PM

Quote:

And as really SHOULD be done with all poured concrete (but rarely is anymore) after a 28 day wait, go ahead and either lay up block stem walls
WillieT: if 28 days is the ideal time before laying block, what would you consider the minimum time since many people don't have that much time?


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