# Place DIY Basement Slab in Sections?



## MoreCowbell (Aug 11, 2011)

Hello.  This is another question that came up as I plan for my crawlspace-to-basement conversion project (details in this post: http://www.houserepairtalk.com/f39/new-amateur-project-11441/)

The basement slab I'll need to place will be approx 600 SqFt.  My understanding is that the pros charge roughly $5/SqFt which would cost me around $3k for the slab.  I'd really like to avoid that expense if I could. :hide:

Since the slab will need to have control joints cut every 10' anyway, I'm considering doing this myself in 10'x10' sections using my (admittedly small) 3.5 CuFt mixer.  I'm thinking that when each section cures, I can remove the forms and pour the next section right against it.  The cured sections would help in screeding new sections.

I'd probably embed a rebar grid to prevent any uneven vertical movement over time since separately poured slab sections would not have the mechanical interlocking properties that a crack along a control joint would have.

A 10'x10'x3.5" slab section would be 30 CuFt.  That would translate to about 10 batches in my mixer.  I think I can do that and get the slab screeded before it starts to set.

So ... am I really off the mark on this?  Thanks in advance for any replies.

-Rob


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## oldognewtrick (Aug 11, 2011)

Rob, if you are going to finish this yourself, why not just get a concrete truck to bring it out and get it all done at once? A LOT less trama from mixing all those batches.


Oh, and [ame]http://www.youtube.com/watch?v=fZOHY7Z5eaQ[/ame]


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## BridgeMan (Aug 11, 2011)

Working up some rough numbers, I came up with a delivered concrete material cost of about $1300, using a delivered quantity of 7 C.Y.  You're up to $1500 with rebar or mesh (more on that later).  A big disadvantage of dumping 7 yards at one time is that it will just about kill one person, doing the placement, consolidation, finishing and curing by himself.  Especially so in a basement, where there will be limited access for float handles and such during finishing.  I poured an 8 C.Y. garage floor by myself when I was young, healthy and maybe a bit stupid--it was one very long day, and just about killed me.

My vote would be to DIY with your small mixer.  I put many hundreds of hours on my 3.5 C.F. Sears mixer over the years, mostly runs of sidewalk and a few patios.  Doing your basement that way makes sense, in that you will set the pace for how hard you want to work, you will have finished sections to match grade with, and most importantly, places for access to make finishing a breeze.  You'll use about 40 sacks of Portland cement and 12 tons of plant mix (sand and rock, already mixed at your aggregate supplier's yard).  My best guess is about $800 total for cement and aggregate.  The other option would be pre-bagged, like Quikrete 5000.  That stuff is a tad pricey, going for close to $5.00 a 60-lb. bag around here, and you'll need about 400 bags.  That puts the price up and beyond the cost of delivered readimix, but you'd still have the option of doing the work at your leisure.

Getting back to the reinforcement issue--I'd strongly suggest using a roll of heavy gauge mesh instead of rebar.  Using rebar in a 3.5" slab is like using a howitzer to kill a mosquito--way too much for what you and your floor really need (unless you're into very heavy machinery, or Abrams battle tanks, etc.).  And the rebar cost will greatly exceed that of a roll or two of mesh.  You can wire the individual mesh runs together to develop continuity, and leave them an inch short of the forms.  Don't forget to place the mesh at the approximate center of the slab depth, either with small "chairs" or placing into the fresh mud and pushing it down with a shovel.  Then drill your forms at midpoint every 2' or so and insert 2' dowels of No. 4 rebar to tie the pours together.  

Feel free to PM me if you have any questions about the foregoing, or anything else regarding your project.  A co-worker of mine at the NM DOT Bridge Section (I'm a retired licensed civil engineer), when he was 70 years old, dug out the entire crawl space of his Santa Fe, NM house and built a basement under it, just like you're doing.  Had a neat conveyor system worked up to carry out the dirt and bring in the concrete.


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## MoreCowbell (Oct 2, 2011)

Hello.  Thanks for the replies.  Sorry for the being AWOL so long.

BridgeMan, what you said about mesh vs. rebar makes perfect sense.  The one thing that I'm concerned about though is how you avoid pushing it down when it is stepped on.  Do people try to step through the holes, or maybe just use a lot of supports so that it stays in the slab rather than underneath it?

Thanks again!

-Rob


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## BridgeMan (Oct 2, 2011)

Using standard rolls of welded wire mesh, you can properly support the stuff by placing chairs about every 18" in both directions.  Either buy dobies (or sand chairs) at your local masons or concrete supply place, or economically make your by breaking up a few hollow-core concrete blocks with a hammer, such that you wind up with small chunks about 1-1/2" square.  I like dobies myself, as the attached tie wires hold the mesh in place, such that it won't jump off or displace when stepped on.  Dobies come in variable thicknesses, so you'd probably want 1-1/2".  And no, you don't want to try to step in the mesh openings, because you will either trip or pull the mesh out of place.   

If you're just doing the 10' x 10' pours, you shouldn't really have to step in the wet mud very much.  After shovel-spading and striking off a few batches, use a resin or wood float to knock things down, then repeat the process with a few more batches.  You could probably wait until the entire square is filled and hand-floated before hitting it with a large (42") mag float or darby to get rid of the inevitable lumps/depressions.  Once the bleed water evaporates off, use a steel trowel to bring up the fines for a final finish, making the stuff "sing" to you in the process.  Do the trowel work from the middle out, using plywood "kneelers" if you can't reach the center from outside the forms (they won't leave much of a mark when pulled, and whatever they do can be easily troweled smooth).


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## itsreallyconc (Oct 3, 2011)

i don't see any reason for mesh OR fiber in a thin ( 4" ) floor neither does ACI ( mesh/steel ) since the placement must be so precise - +/- 5% on vertical axis = .2",,, you're absolutely right - when jabonies get thru stepping in the mud, the mesh will be out of spec anyway,,, yes, it does give some added strength while the conc is curing in the ''green stage' but, after that, all it does is hold the cracked pieces together


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## toddmanqa (Oct 26, 2011)

I had a few questions in regards to doing multiple pours for a basement floor (with rebar tying the sections together).

1. If you do multiple pours, do the 'joints' between the sections become natural crack lines?

2. When doing a new pour next to an existing cured section, what (if anything) do you need to place between the new and existing section?

3. When all the concrete pours/sections are done, can you pour a self-leveling concrete over the whole floor?

Thanks!


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## nealtw (Oct 26, 2011)

If I was to do this job, I would form it up in strips four feet wide to make screeting and finishing easier, if I wanted to tie new to old I would use maybe two foot wide mesh and I would form it with 2x2 screwed to 2x3 with the mesh trapped between.
White carpenters glue is good for attaching new to old and they sell products for this.


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## mudmixer (Oct 26, 2011)

MoreCowbell -

itsreallyconc is right about no reinforcement in a basement slab. It is not needed and is hindrance. The reason it is not needed is because in a basement, there is little temperature change or change in moisture. For that reason, control joints are not needed and are troublesome later.

Place (not pour) the concrete and finish it properly. It will take some paid or volunteer labor, but make sure the boss is experienced and IS the boss (not you). An enclosed basement area can be an ideal curing environment.

Virtually every home here has a basement and is done that way since the first floor platform is always completed before the basement floor and concrete comes in through a window. If you have truck access for other access points, the bull work is reduced.

Just order a low slump concrete (wet). You will have better concrete if it comes from a plant. A DIYer usually has multiple batches with different properties and too much water and cement for good concrete work.

Dick


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## itsreallyconc (Oct 26, 2011)

*1. yes but they're now called ' construction joints ';

2. in a bsmt, nothing;

3. you can do anything you want 'cause its YOUR bsmt    but why the need for self-leveling overlay if the work's been done correctly ?  self-leveling conc is very tricky stuff to do properly !*


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## toddmanqa (Oct 27, 2011)

nealtw said:


> If I was to do this job, I would form it up in strips four feet wide to make screeting and finishing easier, if I wanted to tie new to old I would use maybe two foot wide mesh and I would form it with 2x2 screwed to 2x3 with the mesh trapped between.
> White carpenters glue is good for attaching new to old and they sell products for this.



So, like this?

Then, remove the wood form, and you have the wire mesh hanging out, to be inserted into the next strip?


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## itsreallyconc (Oct 29, 2011)

*what wire mesh ?  why create problems & add something dick ( a prof eng ) AND aci [ concrete.org ] discourages ?  you want to place it in strips ?  fine but lose the wire  :agree:*


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## MoreCowbell (Oct 30, 2011)

Thanks for all the great advise.  It looks like both Toddmanqa and I have similar projects going.  I am definitely getting a lot out of the replies to this thread.

I just have a few more questions:

1) The 2x2 & 2x3 idea that Nealtw had for the forms... is that to provide some interlocking of the slab sections to keep them even in case of settling or just a convenient way to create a partial form when mesh extends past the section being placed at the time?

2) It looks like the majority say that rebar/mesh is overkill.  However, wouldn't reinforcement give the slab some crack resistance if there is some settling?

3) Is 3.5" good for a basement slab?  Is it a bad idea to go thinner?

4) If placing the slab in sections as we discussed (i.e. sections in direct contact) do I need expansion joints around the perimeter with the slab meets the foundation?

Thanks again for the great advise! :


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## mudmixer (Oct 30, 2011)

Cowbell -

You are trying to reinvent the wheel to find a unique DIY approach that has proven to usually not be acceptable and is more costly in the end.

It has been shown the reinforcement and mesh is not worth the problems and can be detrimental.

By the way, you do not need expansion joints since concrete shrinks and does not expand in a basement environment.

Regarding the thickness, 3.5" is a practical minimum and many codes require a 3.5" slab to be in contact with the wall for additional lateral restraint.

Dick


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## nealtw (Oct 30, 2011)

The idea of stacking 1.5" on top of 2.5 " was just to capture the wire. I would only do that if a customer was bent on having it there. I agree with Mudmixer, plant mixed concrete and placed by a pro would be the best. If you are worried about setling rent a compacter.
Now how about the other problems. If this area was not intended to be basement, is there drainage around the outside and is the out side of the foundation sealed.


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## MoreCowbell (Oct 30, 2011)

OK - It looks like I'll skip the wire mesh.  That will save a few $ which is nice.

Nealtw, you got me rethinking another part of my plan.  Moisture should not be a problem in the basement but I am planning on installing a perimeter drain under the slab just to be safe.  I was also planning on laying a few inches of gravel under the whole slab as well, followed by a vapor barrier, before I place the concrete.

I know its a good idea to have a perimeter drain, but is it really necessary to have gravel under the entire slab?  If not, I'll be real happy to save that expense as well.


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## nealtw (Oct 31, 2011)

We always compact and use sand becuase it's easy to work with, I don't know if there is a code requierment.


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