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-   -   wall to wall basement pour questions (http://www.diychatroom.com/f105/wall-wall-basement-pour-questions-135284/)

lee12 02-27-2012 07:21 PM

wall to wall basement pour questions
 
Planning to have concrete poured into a new section of basement (part of home addition). The area is roughly 16' x 20', concrete will be poured through a basement window which is about 16" x 24". Right now I have about 4" of stone down, planning to put plastic over the stone and then wire mesh elevated to sit at the center of a 4" pour. I've done smaller concrete projects before, but wanted to make sure I was clear on a few things before deciding if this was something I should try to tackle myself.

1) Do I need expansion joint material at the edges of the floor? Walls are block.
2) Do I need a relief cut anywhere for a floor this size?
3) How do I go about screeding when dealing with a wall-to-wall pour? When I've done concrete before there were always wooden forms in place so leveling was easy. I have some ideas how to do this but interested to see what people suggest
4) If I have the wire mesh elevated to sit in the center of the pour, will I be able to walk on it to do the screeding, etc. Without bending it all up? Should the sheets of wire mesh be tied together or just make sure they overlap?

A few other things to mention - this will basically be just a storage area so I'm not really concerned with how 'pretty' it turns out. Also not sure how much I need to worry about it being perfectly level and flat, I've never had water issues in this basement.

Might also think about putting in some piping for radon, just so it's there in the future in case I ever have a system added. I may post that as a separate question though.

Thanks in advance for any help

joecaption 02-27-2012 07:31 PM

Strong suggestion, hire this one out. You say you only want it for storage now but look over the dozens of other post of people trying to deal with poorly laid basement floors.
Far easyer to to have it right the first time.

Daniel Holzman 02-28-2012 06:41 AM

My experience with wire mesh has been uniformly bad news. It is very difficult to keep it in position, usually it ends up at the bottom of the slab, where it does nothing but rust. Note that the ACI (American Concrete Institute) does not even recommend use of steel reinforcing for slabs 4 inches thick, due to the fact that you cannot get appropriate cover (2 inches from soil) unless you put the steel in the center, which is the neutral axis of the slab, where it does nothing except provide a little temperature steel to minimize cracking. Crack control can better be accommodated by proper installation of control joints, which you definitely need on a pour this large.

If you are hell bent on having steel in the slab, use number 3 rebar, tied together, supported on chairs.

As for the actual placement, this is about four yards of concrete, which is not a huge pour, but will keep three experienced people very busy for several hours. If you lack experience, this is going to be a very challenging project, make sure your helpers have done this before, once the truck arrives is a poor time for training. Personally I would never tackle a job this big, too much work, too hard to get the floor level, too hard to get a good finish, but maybe you have the skill set to get it done. Best of luck, I admire your fortitude.

lee12 02-28-2012 08:08 AM

Thanks for the input. I was thinking that the wire mesh would be a bit tricky so I'm okay with leaving it out if it's not common practice.

Regarding tackling this myself, I am confident that it can be done. I have previous professional construction experience and this is the second addition I've put on my home, both of which I've done myself from footer to roof. The inspectors have been pleased with every stage.

I plan to have 4 people on this, myself included, all of us with construction experience to some degree. What's new about this for me is that it's wall -to-wall and it's larger than the other concrete work I've done. That being said, I am looking for advice from those with more concrete experience regarding:

1) methods for screeding a wall-to-wall pour
2) recommended expansion control for a floor this size

Again, thanks for taking the time to read and help

framer52 02-28-2012 09:06 AM

As you now have it figured out, you can't screed from wall to wall. I divide the pour in half to screed.

as far as expansion joints, why do you think you need them?

By the way, basement pours are not for beginners as you have been told.

concretemasonry 02-28-2012 09:21 AM

I assume this is a 4" slab sitting on top of the strip footings to provide the initial bearing against the base of the wall. This is method used commonly here for decades and required in some codes. Compact the soil well, even though the loads are minimal.

Snap chalk lines on the wall to use a guides. If you have a floor drain (good idea), slope the floor slightly for control over where water (if any) has a place to go and not pool elsewhere. You never know when there could be a plumbing or equipment failure. Placement without forms does take more skill and experience than when you can just dump from all side and just drag something around the edges.

You do not use expansion joints in a slab like this. I rarely have seen control joint either formed or sawed in a basement. The shrinkage of the slab will pull it away from the wall slightly since the slab is not connected to the wall.

A basement slab is not prone to shrinkage cracks because of the uniform temperatures, contrary to an exterior slab, so cut/formed joints can become a problem. Use a low slump to maximize the strength and minimize curing shrinkage. Cure slowly and the poly under the slab will maintain much of the moisture to aid in curing. Avoid excessive amounts of artificial heat and mist the floor lightly for the first week or so if it appears to dry out. Avoid the temptation to use a wet mix to make the movement of the concrete around easier and consider another window (if possible) to shorten the length to move it since a truck has wheels for a reason. I have even seen pumps used with super plasticizers used to avoid placement problems and reduce the chance of paying waiting time on the truck.

Mesh or rebar are not common, but can be used if desired.

Dick

lee12 02-28-2012 09:30 AM

Dick - thank you for the input, much appreciated.

jomama45 02-28-2012 10:59 AM

Dick did a good job of clearing most things up, but here's a little more:


Quote:

Originally Posted by lee12 (Post 865145)

1) Do I need expansion joint material at the edges of the floor? Walls are block.

No, the concrete will shrink away from the walls. If you have a few odd corners close together, like thru a doorway, it doesn't hurt to wrap those corners with expansion material to allow the slab to move freely.

2) Do I need a relief cut anywhere for a floor this size?

As a professional I would w/o a doubt, BUT, I have to answer to a homeowner/builder/customer. I've found it's much easier to justify cutting control joints than trying to explain random cracking. But again, it's completely up to you in this case.

3) How do I go about screeding when dealing with a wall-to-wall pour? When I've done concrete before there were always wooden forms in place so leveling was easy. I have some ideas how to do this but interested to see what people suggest

As Dick said, you need to snap chalk lines around the floor at the top of concrete finished floor level. You need to be careful not to cover up these lines with concrete when placing the floor, as it will not be visible when you scrape the concrete off. The process for striking it off is called "wet-screeding", and is extremely common practice, although it does take some practice to get good at. You need to hold the strike-off flat on the concrete you mag out, sometimes easier said than done.


4) If I have the wire mesh elevated to sit in the center of the pour, will I be able to walk on it to do the screeding, etc. Without bending it all up? Should the sheets of wire mesh be tied together or just make sure they overlap?

I would skip any re-enforcement as the floor will likely see no substantial load.

lee12 02-28-2012 11:45 AM

jomama45 - thank you! exactly the type of information I was looking for.

I had also been thinking about driving rebar straight down in a few places toward the center, leaving the top of the rebar at the height of the finished floor. Between this and snapping lines along the edge I figured I'd be able to get the floor level enough. see any problems with that plan?

jomama45 02-28-2012 11:54 AM

Quote:

Originally Posted by lee12 (Post 865718)
jomama45 - thank you! exactly the type of information I was looking for.

I had also been thinking about driving rebar straight down in a few places toward the center, leaving the top of the rebar at the height of the finished floor. Between this and snapping lines along the edge I figured I'd be able to get the floor level enough. see any problems with that plan?

The rebar will work fine, just be careful no one falls on it or tries to drive a wheelbarrow over it........

I would recommend keeping a hammer handy as well to drive the rebar below the surface at least an inch after you hand mag the height around it.

lee12 02-28-2012 12:00 PM

good points - thanks again

Gary in WA 02-28-2012 01:16 PM

My 2 cents:http://www.buildingscience.com/docum...ts?full_view=1

Snap a second line 3-1/2" above the grade chalk line for your screed board when you cover parts of it by accident..... been there, done that, LOL.

Gary

cleveman 02-28-2012 10:18 PM

I put a 1x4 on the footing around the perimeter and screed off this. Hopefully, you can pour two pours.

concretemasonry 02-29-2012 05:46 AM

cleveman -

If you pouring a slab between walls and on the strip footing, which I assume is also common in your area, how did you remove the 1x4 and fill in the gap between the new concrete and the walls? A 1" gap there is not a good situation. It is best to have floor continuous.

Dick

cleveman 02-29-2012 08:07 AM

Dick-I didn't remove the 1x4 and haven't.

I think your concern is justified, that the wall will shift in on top of the footing. Typically, the top of the footing has a bit of a "channel" or "key" tooled into it, plus the rebar every 4' or so.

So it hasn't been a problem, but I get your point.

I have seen a few block basements where the bottom course is held in place by the slab, and the second course is shifted over. These were on foundations with no perimeter drain, bad grading outside, poor soffit coverage (gable end) and obviously no cores filled. So all the planets were in alignment.


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