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Old 04-24-2008, 11:35 PM   #1
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


Hi everyone! This is my first post.

I'll be pouring a 3x3 concrete pad, 5" thick. The pad is thick because part of it will be slightly above ground. The pad will be a landing for a ramp. The ramp will "land" or rest about 8" from the edge of the pad.

I have viewed lots of stuff via Google on pouring a concrete pad. Of the pages that suggest rebar for reinforcement (which I plan to use), there seems to be no consensus.

Some say rebar in the middle. Others say rebar in the middle is a waste.

Ace Hardware seems to give the most explicit direction, although I'm still confused:
  • In some cases, concrete needs reinforcement with steel mesh (see image). You can use regular fencing material with 2' x 4' or 2' x 6' mesh.
  • If the pressure on the concrete is to come from the top of the slab, the reinforcement should be laid deep near the bottom of the slab.
  • If the strong point of the slab is at the center and the pressure will come on either end, the reinforcement should be laid as close to the top of the slab as possible.
So what does that mean? I've got the rebar, not mesh. I'm pouring tomorrow. I was planning two pieces going one way (1' apart each: i.e. at 1' and at 2') and two going the other, similarly. Should I make all the pieces exactly 3 feet, or less? Some say rebar should not reach concrete edges, others say nothing on this topic.

I'd guess that in my case the pressure is technically coming in at the end? Does that mean I lay as close to the top of the slab as possible? Again, the thickness is 5 inches. So would that mean I put the rebar an inch below? Less?

BTW the rebar I have is half-inch thick.

Thanks for your help guys. I'll be pouring tomorrow, so if you see this tomorrow morning (Friday, U.S.) and you're waiting until later to post please don't! I only have a limited time to work on this project, and I'll be done by tomorrow, late afternoon, eastern time.

EDIT: I have not had a chance to pour yet. Mon, 4/28/2008.


Last edited by Deck; 04-27-2008 at 11:34 PM. Reason: not pouring until tuesday probably
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Old 04-25-2008, 06:04 AM   #2
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


I'm not a pro but I've sei some concrete. I'd place the rebar about 1/3 from the bottom. If you're in a freezing climate you'll need something to stabilize the concrete to below frost or it'll heave. Assume you'll have a gravel base under the concrete. Good luck

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Old 04-25-2008, 09:11 AM   #3
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


For a non-structural application, the height of the rebar in the pad won't make or break your project. If this were bearing a steel column inside your house or something, I'd say that it definately needs to be no higher than the middle.

For a walking surface, just put it in the middle and it will work just fine.
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Old 04-25-2008, 12:11 PM   #4
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


Thank you guys! The rebar will be between 1 1/2" to 2 1/2" from the bottom. The rebar is half-inch thick so crossed that's about 1" thick.

I have several inches 3/4" gravel, compacted, for drainage but I'm not close to the frost line. I'm in NY and the frost line is like 42" deep.

I am also putting gravel around the edges once I remove the wood form. Will this prevent frozen dirt from essentially "grabbing" the concrete pad in winter and heaving it? None of the documents I have read have footing-like requirements for concrete pads. This includes NYS building code.

Pouring this afternoon. I've got bags of sakrete waiting to break my back =/
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Old 04-25-2008, 01:26 PM   #5
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


Since you are shallow and not down to the frost depth, you can expect the slab to heave, but it will move like anything else around.

Your only problem will be if it heaves and the other end of the ramp does not. For a ramp it should not be a big problem.
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Old 04-25-2008, 11:48 PM   #6
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


I didn't get a chance to pour today and I'm busy this Saturday. Saturday night or Sunday I will post some pictures so everyone can see how the form looks with the rebar in it.

EDIT: I'll post pics on Monday.

Last edited by Deck; 04-27-2008 at 11:34 PM. Reason: update
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Old 04-26-2008, 04:14 AM   #7
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


A ramp for what?!?!?!?!?
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Old 04-26-2008, 06:46 AM   #8
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


Have you considered putting a piece of rigid insulation between the gravel and the concrete? Should be cheap to do and I believe would help to prevent damage from frost heave. The insulation should function to absorb the maybe 1/4" movement from repetitive movement which could loosen your deck fasteners over the years. Good luck with the pour
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Old 04-27-2008, 09:02 AM   #9
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


Placement is due to the Neutral Axis. This is the line where the forces change from tension to compression. Concrete is very stong in compression and very weak in tension. The tension strength of concrete is not even accounted for in concrete design. The placement of rebar should be in the tension zone which in a slab is almost always going to be in the bottom half. Make sure you have 1.5" of concrete cover below where you place the rebar and you should be ok. Shrinkage in the top of the concrete can cause cracks as well. You may want to put some WWF in the top portion again observing a 1.5" cover
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Old 04-27-2008, 11:05 AM   #10
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


Putting insulation under the 3x3 slab itself is a folly and waste of time and money. Frost will occur under the small slab since heat flows in all directions and not just vertically. Frost heaving occurs under the slab and not just at the bottom.

Wire mesh is also worthless to limit cracks on a 3x3 slab. There is not enough horizontal dimensions to create any real tensile/shrickage stesses to cause cracks. Rebar will do fine.

You only have a 3'x3'x5" piece of concrete, so don't micro-manage it.
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Old 04-29-2008, 12:04 AM   #11
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


Here are the pics. I'm only allowed a certain number of pictures (4) per post, so I will make multiple posts.

-----


I decided a 3x3 pad would be appropriate, and I set out to build a wooden form using basic untreated 2x4's. I cut pieces 37.5" -- 3' 1.5" -- so that I could overlap each piece on one end. That means each side of the form measures 3' on the inside.

I was preparing to assemble the form but I found that a few of the pieces I cut were warped. So I cut some new pieces until all my pieces appeared straight. I used a 3' level I held next to multiple sides of each piece to determine this.

I used a small rafter square (7") to make sure when I attached one piece to another that the form would be square. Here's a picture:





Unfortunately, the only good location for the pad has quite a slope. There is also rare flooding in the planned location. I needed a pad that was not only below ground, but a bit above ground as well.

A 3.5" high form was not going to cut it. For those who don't know, a 2x4 is actually 1.5"x3.5", so the highest I could get with one "layer" was 3.5". I decided 5" inches would be a good thickness for the pad. This would allow for the slope. So I had to make a second form.

I then placed one form on top of the other. Here's a picture:





I attached the two forms together on two opposite sides using stub pieces of 2x4. Here's a picture:





I made a line around the inside of the form 5" down, so I would know where to stop pouring gravel. Here's a picture of the final product:

Attached Thumbnails
Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?-002.jpg   Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?-005.jpg   Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?-004.jpg   Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?-003.jpg  
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Old 04-29-2008, 12:08 AM   #12
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


I moved the form to the dig location and used 4 metal shish kabob bbq metal thingies to mark the outside edges of the form. I inserted the bbq stakes all the way into the ground at each corner. I removed the form and I dug down, level, 6-8". There is no one measurement because of the slope. The depth was necessary to allow for some inches of gravel below the form, besides the two inches I'd have up to the line in the form.

I used a digital level to level the bottom of the hole properly. It was a lifesaver because I didn't have to move my head all the way down to see a bubble.

At Home Depot I bought 10 bags of 3/4" crushed stone at $3.25 ea, and 8 bags of Sakrete at $4 ea. I'll only need 6 bags of Sakrete I think. The other two are spares.

Here's a picture of the dig and materials. In this picture the ground slopes down toward the fence.





I alternated between pouring an inch or two of gravel, compacting it, leveling, and then fitting the form in until it was level. When it was level, I would check to see if the form was inches above ground on the highest side. I didn't have a specific number, I just wanted to make sure it was a bit above ground on all sides. The first two times it wasn't, and I repeated the whole process after removing the form: adding more gravel, compacting, etc etc.

Once the form was partially above ground on the highest side, I inserted the form and poured gravel up to the line and compacted, adding a bit more gravel. Here's a picture after I finished with the gravel pouring and form fitting:





Tomorrow I'll post pics of the rebar fitting.
Attached Thumbnails
Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?-001.jpg   Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?-006.jpg  
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Old 04-29-2008, 12:59 PM   #13
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


i wouldn't waste time OR $$$ for any reinforcement on a slab this size
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Old 04-29-2008, 01:06 PM   #14
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


At this point in building, I posted here to ask about the rebar placement. Following the advice here, I set out to make sure the rebar would be in the bottom half of the form. I decided to make the rebar sit between 1.5" and 2.5" from the bottom.
Read the first few posts of this thread for more info.

I had already went to Home Depot and bought 4 pieces of 1/2x4 rebar at 2.27 ea., and 4 of what's called a "double rebar chair," at like .50c? ea. Home Depot sells several types of rebar chair. I bought the double because it's bigger, and I knew I'd probably need to bend any chair. Normally their chairs will hold the rebar several inches high.

Rebar chairs should be in the building materials aisle, in one of those cubby holes next to the rebar. I've been to several Home Depot stores, and they were always in the same place. Sometimes you have to reach far back in the holes though, as they might not be visible.

I cut the rebar pieces to 32" from 48", using a reciprocating saw with a metal blade. I wasn't sure whether the rebar should touch the edge of the form but I guessed no, and planned for the rebar to be 2" in from every side of the form. 2" x 2 sides = 4", 36" - 4" = 32".

I bent the very middle of each rebar chair so that its lowest point was 1.5" from the bottom, and I placed them on the gravel, vertically. I spaced the rebar chairs at 1' and 2' on the left side, and the same on the right side. All chairs were several inches from their parallel side. There were no exact positions for each rebar chair in the form. I figured (correctly) the actual placing of the rebar in the chair would cause movement.

*"Vertically/horizontally" might not be correct terms here but whatever, I can always edit this, just pm me.

I then placed two pieces of rebar horizontally, each resting on two vertical chairs. I measured to make sure both pieces were 2" from each edge and 1' from the closest parallel edge.

Here's a picture showing how close any piece of rebar is to an edge:





I then placed two pieces of rebar vertically, each resting on the two horizontal pieces of rebar. I then adjusted the vertical pieces similarly.

Most of what I read on the internet suggests wire to tie the pieces together and keep them from moving. Home Depot does sell wire for this purpose (check the cubby holes). However, using wire to tie the pieces together seemed cumbersome, and also I thought while doing it I could cause the pieces to move away from the measurements I had made. I guess I could have used a marker or something, but that didn't occur to me until just now.

Anyway.. I decided to use electrical zip ties instead. The whole idea is to keep the rebar from moving while the concrete is poured. Perpendicular pieces do not have to be connected structurally, and so really you just need something to make sure everything is held in place.

Here's a picture of a vertical piece of rebar, zip-tied to a horizontal piece, resting on a vertical chair:





And here's a picture of the final product:




Note in the picture above that I still have to trim the excess of each zip tie.

Also, note that I have started to backfill with gravel around the form. This is because I read somewhere on the internet about a form that came apart. I used 3" wood screws to make the form, so I don't think it will be a problem. But still...

Also, note the form is 2-3" above ground on the highest side, and 3" to 3.5" above ground on the lowest side (the right side of the above picture). I didn't mention the specific measurements in a previous post because I hadn't measured. Thinking about it, it seems high. With an occasional rare flooding I wanted it high though. I really am not sure about that. It means at one point only 1.5" of the pad will be below ground. What do you guys think?

This afternoon I will pour if it stops raining. Also note I had elevated the bags of Sakrete off the ground and covered them with garbage bags when I heard the forecast.
Attached Thumbnails
Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?-009.jpg   Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?-008.jpg   Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?-007.jpg  

Last edited by Deck; 04-29-2008 at 02:21 PM. Reason: spelling isle/aisle
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Old 04-29-2008, 01:31 PM   #15
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


Quote:
Originally Posted by concretemasonry View Post
Putting insulation under the 3x3 slab itself is a folly and waste of time and money. Frost will occur under the small slab since heat flows in all directions and not just vertically. Frost heaving occurs under the slab and not just at the bottom.

Wire mesh is also worthless to limit cracks on a 3x3 slab. There is not enough horizontal dimensions to create any real tensile/shrickage stesses to cause cracks. Rebar will do fine.

You only have a 3'x3'x5" piece of concrete, so don't micro-manage it.
I would have to agree. This is a tiny piece of concrete. The pad is so light, it's like two fleas arguing over who owns the dog. If you get any kind of soil movement, either from frost heave, expansive soil, settlement (I noticed the slab is on native without any obvious compaction), etc. that little chunk of concrete is going along for the ride. You said it's a landing for a ramp so I assume there won't be a static load on the concrete to resist any upward movement.

Don't get me wrong. You certainly have displayed an excellent level of workmanship and attention to detail, I'm just not too sure you'll get much bang for all the extra bucks you're spending in time and materials. On the upside, it will probably be the sturdiest 9 Sf of concrete in New York.

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