# Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?



## Deck (Apr 25, 2008)

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. 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.


----------



## Hancockian (Nov 27, 2007)

*Rebar Placement*

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


----------



## Termite (Apr 13, 2008)

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.


----------



## Deck (Apr 25, 2008)

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 =/


----------



## concretemasonry (Oct 10, 2006)

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.


----------



## Deck (Apr 25, 2008)

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.


----------



## Wanttodoitright (Mar 17, 2008)

A ramp for what?!?!?!?!?


----------



## Hancockian (Nov 27, 2007)

*3X3 Pad*

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


----------



## ncgrogan (Dec 3, 2006)

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


----------



## concretemasonry (Oct 10, 2006)

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.


----------



## Deck (Apr 25, 2008)

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:


----------



## Deck (Apr 25, 2008)

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.


----------



## so-elitecrete (Oct 27, 2007)

i wouldn't waste time OR $$$ for any reinforcement on a slab this size


----------



## Deck (Apr 25, 2008)

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.


----------



## MacRoadie (Apr 25, 2008)

concretemasonry said:


> 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.


----------



## Deck (Apr 25, 2008)

Ok thanks for the advice. I'm going to pour in a few minutes. The weather cleared up almost right after my last post. I'll take some pics and post them today or tomorrow.


----------



## 47_47 (Sep 11, 2007)

Don't forget the form oil.:jester:


----------



## so-elitecrete (Oct 27, 2007)

*rebar adds flexural strength,,,*

that's why its used in bdge decks & hi-rise structural floors,,, for slabs on grade, it, like welded-wire-mesh, adds strength during initial set when conc's subject to tension stress,,, when cured, it holds the broken pieces together :yes: that's all,,, any metal reinforcement should be plac'd 1/2way vertically according to aci specs,,, nevertheless, this is probably the best designed & most carefully engineered project i've ever seen from a h/o-diy'er.


----------



## MacRoadie (Apr 25, 2008)

Basically, that rebar isn't doing much of anything in that slab anyway, but it sure looks cool...lol

The slab is fully supported, so the concrete will always be in compression anyway. It's way too small and the width is so short that even if poorly supported, you'd have to put one boatload of weight on it to put the bottom of the slab in any measureable tension.


----------



## MacRoadie (Apr 25, 2008)

By the way, I'm not trying to be critical of deck's project. It's just that it's about 1000% over-engineered. To put it in context, a 5" slab on grade with #3 bars at 18" o/c both ways (as opposed to deck's #4 bars) would be pretty typical of your standard commercial slab. What you'd see in a light commercial tilt-up or a mixed-use project for example. For a 3' square pad, that's a bit overboard.


----------



## Deck (Apr 25, 2008)

I poured the concrete yesterday. I used 80lb bags of Sakrete High Strength Concrete Mix. I had estimated 6 bags for the pad. I based this on two concrete calculators -- basically web forms on the internet where you type in the size of your concrete form and it tells you how much concrete you need. For a 3'x3'x5" form, one concrete calculator said 5.7 80lb bags req, the other said 6.3 -- so I figured 6.

I ended up using 6.5 bags. There are directions on the bag that yielded a similar number. I used a $10 mixing tub that I bought at Home Depot, in the building materials aisle, to mix the concrete.

I always put the water in the tub first. I would slowly lower the bag in, upright. I would cut the bag towards the bottom, and lay it cut side down. I would pull until the bag slowly emptied into the tub. I did it this way because I read on the internet it was a good way to keep dust down, and it was. Sakrete recommends dry stuff first, then water.

In the tub I put four quarts of water, as Sakrete recommends. About half of the time this worked alright. The other half I needed to add more water because the mix was too dry, even after thorough mixing. Here's a picture of some dry mix:











When I needed to add extra water, I would always add very little. Just a quick spray/mist from the garden hose, and then mix. Repeat if necessary.

It's hard to describe what you want from a mix, but "just moistened," where the concrete is "pasty" worked for me. When you have it mixed properly, you can push a rake along the bottom (tines up) from one end to the other. The concrete "moves" but is not soupy. Here's a picture of some good mix:











Here's a picture of an 80lb bag of mix in the tub, right before I finished mixing it:











I used a rake for mixing almost exclusively (occasionally a shovel). I broke the first rake I used, pictured below on the right, after the first two bags. This is because I either pushed or pulled too hard in a dry mix. I don't know when the rake actually broke. Luckily I had a spare of a different type, pictured below on the left:


----------



## Deck (Apr 25, 2008)

Here's a picture after I shoveled 3.5 bags in the form:











I needed about 6.5 bags to fill the form. Each bag took about 15 minutes to mix and shovel in. Here's a picture after I filled the form:











I used a long 2x4 (6 feet?) to screed the concrete. I did this multiple times. Occasionally I had to move some of the excess from the high areas to a low area, screed some more, etc ,etc.

After I was done screeding I pushed in two 1/2" x 6" galvanized anchor bolts (.89c ea, building materials aisle, Home Depot). I then packed concrete around any indentation. This was a last second thing, and I did it just in case I would have a need for them when securing the ramp.

If you ever have a need to buy those anchor bolts from Home Depot, check the threads. A lot of them have threads that are totally unusable due to the galvanization. I checked before I bought by fitting a nut on as a test. I used painter's tape to cover 2" of thread before I inserted in the concrete. So about 4" of bolt is actually in the concrete. Also, any bolt is about 4" from any side.

I used a 15"? magnesium float (about $14, building materials aisle, Home Depot) for the finish work. I then used a 6"x3" concrete edger ($5, building materials aisle, Home Depot) to edge the concrete. I ended up continuously refinishing the pad. I was way too meticulous. Finally I decided to go over the pad with a shop broom, which left some really slight ridges that I read might be good for traction.

I finished about 4 hours after I started. About half that time was mixing, the other half was prepping, finish work and cleanup I guess. Here's a picture of the pad after I finished, but before I edged or ran the broom over it and re-edged:











I covered the concrete with some left over crushed stone bags, so it wouldn't be exposed to direct sunlight. I used 2x4s on opposite sides on top of the form, and I used a staple gun to secure the crushed stone bags to the 2x4s. Then I used another 2x4 perpendicular and on top of those 2x4s to stabilize it. I wanted the cover elevated so that it wouldn't affect the concrete. Really I don't know if it's necessary though. Here's a picture:











I'm trying to spray the pad every 12 hours, so that it cures properly. I only mist lightly and high up. The first time I sprayed I left a mark in the concrete and had to retrowel that area.

I tried to remove the wood form today, and it just won't come off. I did not use form oil (whoops). I had figured (incorrectly) that since the form was soaked, it would be easy to remove after a day.

Any ideas how I can get this form off? I'm going to remove all screws from the form. What then? Can I bang on it? Chisel? What do you guys suggest?

Thanks


----------



## MacRoadie (Apr 25, 2008)

A good whack should (hopefully) do it. The forms should actually have been pulled a bit sooner. The longer they sit, the tougher it's going to be to get them off (without spalling the concrete). You might just use a rubber mallet, or a block of wood but a well-placed whack with a hammer will likely do the trick.


----------



## Deck (Apr 25, 2008)

Ok thanks. I have a mallet and a hammer so I will try using both this afternoon.


----------



## Deck (Apr 25, 2008)

I took the form off yesterday. I had to unscrew the wood pieces in place. This was time consuming because part of the form was underground. I dug down at the corners. Here's a picture:











I used one of those flex extension things because I couldn't get in with the drill. I'd really like a right angle thing for my drill. Not like one of those expensive ones with the right-angle chuck, I want something low-low-profile that I can just change hex bits and (un)screw stuff.

Anyway.. the flex extension was ok but you really need to use force to keep it focused on the screw. I took my hand away from the end only for the picture. And here it is:











The top half of the form came off easily once the screws were removed. The bottom half was below ground, and for that I needed a pry bar to pry up the wood pieces. Here's a picture of the pad after I removed the form:










As you can see in the picture the concrete appears to have some small gaps... Air pockets? Did I not screed well enough? Or is that how it's supposed to look?

Also:
I'm at odds for how to re-grade near the concrete. I filled up to where the pad starts with gravel. What now? Should I put in more gravel up to the soil line or just pack in soil over the gravel? I have a tamp to compress the soil.

I have been spraying the concrete regularly, but the top is flaky. Why is that?

Thanks


----------



## so-elitecrete (Oct 27, 2007)

miss'd a little consolidation is all,,, hitting it w/shovel, trowel, or broomstick vertically would probably've solved that,,, great looking lil slab for a 1st-timer - congrats.

flaky = too much wtr on top of slab, not removing bleedwtr prior to broom finish, too much wtr in mix - just 4 that come to mind,,, again - great job,,, do you travel ? :thumbup:


----------



## BuiltByMAC (Jan 31, 2008)

This is an informative thread, guy. Clear set of blueprints for other HO/DIYs to follow...

A slightly wetter mix and working the concrete in the form more would've gotten rid of those air pockets. On your next project, after you pour the concrete in the form, use the rake to "settle" the concrete. Stir the concrete mix, vibrate the rake through it. This will "soup up" the mix so the liquid concrete fills in the gaps around the stone.

Something to keep in mind for your next concrete form project - on the nail aisle, they have something called duplex nails. These are nails with two heads. They're used a lot for concrete forms because they're easy to remove when you break your form down. The first head secures the two pieces of wood tightly, the second head stands proud about 3/8" so it's easy to hook your hammer's claws under it and pull the nail out.
8d for 1x and smaller material, 12d & 16d for 2x material...

Mac


----------



## MacRoadie (Apr 25, 2008)

I agree. Could have used a bit more water. Was going to say something earlier when you posted the mix picks, that your mix looked pretty dry. For a small project like this, that you're hand-mixing, a little more water is better than a little less. The concrete will consolidate better, you'll eliminate some of those pockets, and you won't run the risk of lowering the water to cement ratio too far. You'll have extra bleed to deal with, but that's managable. Too little water and you don't have enough moisture to properly hydrate the cement paste through the whole matrix (the chemical process that causes the concrete to set up). Keeping a slab moistened aids curing by maintaining a layer of moisture at the surface of the slab, preventing the moisture in the slab from evaporating before that hydration can take place.

It's all good though. Great job on your first ever slab.


----------



## Deck (Apr 25, 2008)

Thank you for your advice guys. And thank you for the compliments, I appreciate it 

I scrubbed most of the flakes off using a thick bristle nylon brush, and now it doesn't look flaky anymore.

What do you guys think about re-grading the soil near the concrete? Should I compact gravel or soil to the soil line?


----------



## Taylor (May 10, 2008)

*comment on rebar*

I happened on this thread while looking for information on installing footings in my basement....for anyone else that reads this thread, I would just note that the structural engineer advising me on my project has recommended 5 #4 rebar in either direction, in a 30"x30"x12" footing....this is for a support column for a load-bearing wall on the ground floor that has nothing underneath it, unlike this thread.....a lot of good information for reluctant house restorers in this thread BTW.

...adding, I would be careful about adding more water than prescribed to the concrete, my understanding is that the concrete will lose strength and crack as this moisture evaporates....it will also corrode the rebar faster, due to producing carbonic acid sooner....perhaps it might be worth renting or investing in a small concrete vibrator.....


----------



## MacRoadie (Apr 25, 2008)

The original poster stated that he liked his mud on the dry side rather than the wet side (less rather than more slump). We are talking about sack mix here which is typically Type 2 concrete with a higher specified water to cement ratio to begin with (it's the wettest and least strong by design). Reducing water in this instance will definitely hamper hydration of the cement paste, and the photos bore out the additional problem of poor consolidation.

As far as the concerns re: excessive cracking, yes excessive water can and usually will lead to plastic shrinkage cracking, but we're talking about 9 sf of mud here, not 9,000. WRT the rusting steel and carbonic acid issues, again this is a tiny slab and evaporation is going to occur very rapidly (and pretty evenly given it's size). You'd need ALOT of water before those issues are a concern.

You are wise to follow your structural's specifications to the letter as you are building an engineered footing designed to distribute load over the entire surface area of the pad (hence the #4 bars each direction). I wouldn't confuse the structural engineer's requirements for the engineered spread footing with the little project outlined here in this thread.


----------



## Taylor (May 10, 2008)

MacRoadie said:


> TI wouldn't confuse the structural engineer's requirements for the engineered spread footing with the little project outlined here in this thread.


Absolutely. I only commented in case someone with a structural case such as mine relied on the internet rather than a structural engineer for their planning. In my case the footings will be supporting lally columns and a steel i-beam to support a load-bearing wall above.

Thanks for your clarifying comments.


----------



## yesitsconcrete (May 11, 2008)

usually 2 mats of #4 tied rebar's spec'd here


----------

