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MoreCowbell 08-11-2011 04:13 PM

Place DIY Basement Slab in Sections?
Hello. This is another question that came up as I plan for my crawlspace-to-basement conversion project (details in this post:

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.:D

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


oldognewtrick 08-11-2011 04:23 PM

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

BridgeMan 08-11-2011 09:27 PM

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.

MoreCowbell 10-01-2011 10:12 PM

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!


BridgeMan 10-02-2011 04:24 PM

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).

itsreallyconc 10-03-2011 12:15 PM

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 :rolleyes:

toddmanqa 10-26-2011 12:55 PM

Multiple pours and self-leveling concrete
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?


nealtw 10-26-2011 03:21 PM

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.

mudmixer 10-26-2011 04:30 PM

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.


itsreallyconc 10-26-2011 06:14 PM

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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