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Old 07-01-2010, 12:32 PM   #1
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Some questions about a concrete slab


Well we need to pour a small concrete pad (prob around 8x10) to put a shed on. I just have a couple questions since I have never done this before. Do I need rebar or would some sreen type stuff do the job? Is 4'' a good thickness for the slab? Any my last question is what should I look for when buying the bags of mix? Or would I be better off having a small concrete truck deliver some ready to go stuff? Thanks!

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Old 07-01-2010, 01:12 PM   #2
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Some questions about a concrete slab


for an 8x10 shed you'd be fine with just mesh, it all depends though on the type of shed or weight youll be putting on it. i would use rebar if it was me. and as far as concrete 3 1/2 is typical for a concrete pad
with your dimentions your looking at about a yard or concrete 4" (.33)x 8x10=26.4 divided by 27= .9777 which is equal to a yard. a truck would be more expensive than you mixing bags yourself, waay more expensive, but it would save you alot of time and backbreaking labor. hope this helps!

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Old 07-01-2010, 01:31 PM   #3
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Some questions about a concrete slab


Ok thanks I will probably just mix up the bag stuff. Can you give me some tips for a simple rebar layout? I dont want to under/over do it. What size and what kind of spacing should I do? Thanks again.
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Old 07-01-2010, 02:35 PM   #4
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Some questions about a concrete slab


number 4 rebar equal to 1/2 inch diameter, you can go 16 iches on center or 2 ft on center (both directions) would be sufficient. good luck !
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Old 07-01-2010, 10:48 PM   #5
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Some questions about a concrete slab


FYI, they drive trucks on concrete w/ wire mesh in it. Just be sure to either pull the mesh up a tad after the pour, or put LOTS of little rocks at wire intersections to keep the mesh approx 1.5" off the bottom. For a shed, don't lose a lot of sleep over that, though. Rebar is fine if "Overkill" is in your blood, of course. Rebar should also be lifted and wire tied at intersections. GL. j
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Old 07-01-2010, 10:59 PM   #6
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Some questions about a concrete slab


The topic of rebar in thin (i.e. 4") slabs has been extensively discussed in this forum. Do a search, you will find dozens of threads.

I have stated several times that placing steel in a 4 inch or thinner slab is a waste of time and money. The reason is very simple, you need at least 2 inches of cover to protect the bars against rust, thus in a 4 inch slab the bars go in the middle. The middle is the neutral axis, meaning the bars perform no work, i.e. they have ZERO structural function.

At best, the bars help to keep the slab together if it begins to come apart, which could occur if the mix is bad, the placement is bad, the foundation preparation is inadequate, or the curing is improper. It definitely will not happen if you purchase a good book on placing concrete, read it, and follow the directions.

The most important thing you can do to maximize success if proper preparation of the soil. Remove all organic soil, and dig down to naturally occurring firm soil. Mix the concrete thoroughly, follow mix instructions on the bag, do not use too much water. Place the concrete in the forms, vibrate it (rent a vibrator for the job), and finish it in accordance with instructions you will find in your concrete placement book.

Curing is important, you need to cover the concrete with wet burlap, let it cure for several days. You either need to install control joints when you place the concrete, or tool them in after it has cured a bit. I like to set them when I place, others prefer to sawcut the joints later.

For a one yard job, I would rent a drum mixer for the day, easier than mixing concrete one bag at a time, but you can go that way if you want to save the rental fee on the mixer.

In your case, there is nothing to be gained by placing steel, it will only cost you time and money. Since this is a shed, you probably can get away with no footers, but you really should check with your local code enforcement official about foundation requirements in your area. If you are in an area of expansive soils, or you are concerned about frost heave, footers would be a good idea.
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Old 07-01-2010, 11:00 PM   #7
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Some questions about a concrete slab


Here's a few of my thoughts. Note that these thoughts aren't based on theory, but on everyday practice, and will vary depending on the situation.

- Installing 3-4 #4 rebar (1/2") each way will cost you a few bucks at the most, and will not be overkill IMO. Actually, you'll probably find it easier & cheaper than wiremesh.

- I think you're looking at about 50 - 80# bags of "Quickcrete." At about $5 per bag, I don't think you're going to save astronomical amounts of money mixing the concrete yourself. I'd make a few calls to look for cartage charges from your ready-mix suppliers, as well as look for a local short load supplier. Ready-mix concrete is superior in every way as well, and a heck of alot less laborious.

- 4" thick should be fine, just make sure that you pour it on a hard, suitable base.
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Old 07-02-2010, 02:01 AM   #8
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Some questions about a concrete slab


Quote:
Originally Posted by Daniel Holzman View Post
I have stated several times that placing steel in a 4 inch or thinner slab is a waste of time and money. ... in a 4 inch slab the bars go in the middle. The middle is the neutral axis, meaning the bars perform no work, i.e. they have ZERO structural function.
Dan: I hate it when this happens. One engineer says yes, one says no. So why did the engineer that designed my house foundation and slab list #4 rebar 16" OC in both directions in the 4" slab? BTW: He also recommended covering the slab w/ visqueen to slow the drying, similar to your burlap idea.
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Old 07-02-2010, 02:35 AM   #9
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Some questions about a concrete slab


tyrell... DO NOT LISTEN to the rebar is a waste of time,money, has absolutely no structural function post!
put rebar on it 16 inches on center, pour it, and call it good. no steel in any pad is just asking for problems down the road!

Last edited by arty007; 07-02-2010 at 02:37 AM.
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Old 07-02-2010, 07:20 AM   #10
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Some questions about a concrete slab


not that i'm the last word but we've got a similar job coming up - 12' x 12' x 4",,, either we'll recommend 6" w/reinforcement OR 4" w/o,,, to be any good, the mesh MUST be placed at the vert midpoint +/- 5%,,, that means .1" leeway,,, no one's EVER got it right therefore the strength gained during the ' green ' stage's nullified & it then only holds the broken pcs together

haven't seen ANY engineer's post saying ' yes ' & dan only posted his recommendations,,, steel can't hurt but i wouldn't expect you need flexural strength UNLESS you're storing kilos of gold in the center,,, mesh can hurt,,, aci doesn't recommend steel/mesh in 4" ( per dan's post ),,, pay more attn to proper mixing, placing, finishing, & curing,,, jo does this work every day, too, therefore his post has definite meri.

knowing your location would've helped - especially if you experience any appreciable frost levels during cold weather
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Old 07-02-2010, 10:05 AM   #11
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Some questions about a concrete slab


most concrete places here have half barres you put in your truck to haul small amounts OR a small dump trailer they charge 5 bucks for and you hook it behind your truck and back it right to your pour,,,hit the button, open the door and your half done. MOST places have some way for small jobs cause they dont want them either.

get 6 1/2 bag mix with fiber mesh mixed right in it and its as good as it gets,,,or dig spade deep and wide all around edge to keep varmints out and edge thicker and add 1/4 yard.
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Old 07-02-2010, 12:57 PM   #12
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Some questions about a concrete slab


Ya I live in Canada and it gets very very cold in the winter...So the freezing and thawing does cause some problems sometimes. Thanks for al the suggestions.
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Old 07-02-2010, 06:31 PM   #13
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Some questions about a concrete slab


ah, all the new terms and recommendations overwhelm one. i'll put together a shopping/rent/borrow list and a supply list if you want. it is not hard and for this size of a pad, the driver will most likely help you drag the cement level.

Two ten foot and two 12' 2x4's called "forms" and wooden form stakes to lay on ground for future pad (inside area is 8'x10') "pinwheeling" the boards so you don't have to cut them. nail the forms into place. using string and level, stake them so that the tops of the "forms" are level. you'll double check them to be level just before laying cement. you'll want them either level or sloping to shed water.

dig the pit however deep. i put footers on mine to keep water from freezing in the winter that will "heave" the concrete. a footer is a trench of 6" to 12" deep along the edge of the pit. you can also plan to dump gravel and sand in there (a local concrete company will tell you if most homeowners do this in the area. it deeps on the soil, temp and rainfall there). pack the heck out of the soil, gravel, or sand or the concrete will push down any loose stuff as it dries and you'll get a low spot. rent a packer or use a homemade tap from scrap wood ("T" a short piece of 4x4 onto a 2x4 handle and tap ground until it is very dense. hard virgin soil will need little. but gravel, sand and loosened soil will need it. if you virgin soil is dense. if not, pack it. i've a low spot on my first slab b/c the pit was not packed properly). Dan is correct when he says to remove all organic debris, etc.

lay a grid of 1/4" rebar in pit spacing about 16" - 18". don't let the rebar touch the forms (so as to not stick out of the future concrete slab). use "rebar ties" bought at Home depot to tie the pieces together. prop-up the rebar grid up with the little place rebar holders (home depot).

the day before or morning of, call a concrete company and say you want 8'x10' pit at 4" deep with a footer (1 yard concrete but i'd ask for 1.5 yards concrete if you put in a footer and so you don't have to pay premium if you use a little more). 3,000lb strength, slump of no more than 5.5, etc. if you need help, ask a neighbor if he/she'd stand by to help drag the concrete level if the driver can't. you'll pay the same or more in renting a mixer and hauling the bags, etc. order and pay for an extra 1/4 or 1/2 yard.

Pour Day - Check the forms to be level. either have concrete mix pumped into the pit or haul it by wheel barrel (the concrete should be thick enough to pour into a road cone (missing the top 1/4 of the cone) /turned upside down/ and not "slump" down very far (if the concrete mix slumps down 5.5" the concrete is a ~5.5" slump". there are better indicators of strength than slump but it is the most convenient and fastest). pour into pit and make sure it gets back into the corners of the pit. take a 9+ foot straight 2x4 and drag it over the concrete to level it out. the driver will most likely be cool and help you. it only takes a few minutes for this size pit (you could ask if the driver could help drag it level when you order the concrete if you want). here concrete mix is ~$45 delivery and $80/yd.

three finishing tools - using the wide flat thing rented at Home depot, smooth out concrete. you'll also use a flat hand concrete smoother and another one with a "fin" down the middle to put in the control joints. the driver can tell you when to do this (probably three times). you can rent/buy these things at home depot. job is done.

i left out the burlap as it would have to be pretty hot/dry/windy to need that (if it is cool at night, ask for pour late in the day. you could "finish" it for a few hours and then let it cure over night). i'd see if you see this being done at construction sites around town. i'd say nothing more than a very light mist to cool it off during the first few hours is at most all that is needed. you don't want to provide a means for chemical to leach out of the concrete as it dries. but you also don't want the surface to dry out before it sets-up or it will crack on the surface. don't pour it if it will rain in the next 24hrs or if it is too hot or when it is windy.

one huge recommendations and i'm sure i'll catch some grief for it. do not let anyone add water. if they need it, tell them to walk. you'll be the one with the pad in thirty years and they won't know your name. i saw grown men almost in tears screaming that water must be added. i had one contractor walk off the job. just call a certifier and have them show up during the pour if it becomes an issue. or just say a certifier will be showing up and they'll shut up about it.

Last edited by rosco; 07-03-2010 at 01:04 AM.
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Old 07-02-2010, 07:04 PM   #14
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Some questions about a concrete slab


Wow, I learn something new here everyday. That’s why I love this place.

I’m confused on one thing though, what 2x4’s from HD should I use for forms, the plastic or cardboard?
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Old 07-02-2010, 08:05 PM   #15
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Some questions about a concrete slab


OK, back to the rebar. The claim that rebar can't hurt is nonsense. If you use a #4 bar, that is 1/2 inch diameter. This means the MAXIMUM possible cover is 1-3/4 inch, assuming you get the bars EXACTLY in the middle, which is not going to happen. 1-3/4 inch is less than required minimum cover per ACI.

The reason this is a problem is that with less than minimum cover, the bars are prone to rusting. When they rust, they expand. When they expand, they crack the slab. This is serious trouble, easily avoided by NOT USING bars.

I challenge anyone who thinks bars are useful in a 4 inch slab to find some independent, knowlegeable organization that supports the practice. Or at least offer a well thought out explanation as to why you think steel bars IN THIS application are useful.

Bars are used in structural applications because they provide tensile strength, which concrete lacks, but ONLY when they are placed in the tension side of the slab. When placed in the middle, they are in the neutral axis, which by definition carries no stress or strain. So all they do is sit there and rust, and in the long run they crack your slab.

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