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Old 04-29-2008, 03:24 PM   #16
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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.

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Old 04-29-2008, 03:55 PM   #17
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


Don't forget the form oil.
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Old 04-29-2008, 06:03 PM   #18
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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 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.
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Old 04-29-2008, 06:28 PM   #19
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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.
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Old 04-29-2008, 06:38 PM   #20
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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.
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Old 05-01-2008, 01:17 AM   #21
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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:

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Old 05-01-2008, 01:23 AM   #22
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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
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Last edited by Deck; 05-01-2008 at 01:32 AM. Reason: clarification on edging
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Old 05-01-2008, 02:22 AM   #23
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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.

Last edited by MacRoadie; 05-01-2008 at 02:24 AM. Reason: spelling
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Old 05-01-2008, 12:45 PM   #24
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


Ok thanks. I have a mallet and a hammer so I will try using both this afternoon.
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Old 05-02-2008, 11:01 PM   #25
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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
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Old 05-03-2008, 06:03 AM   #26
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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 ?
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Old 05-03-2008, 10:46 AM   #27
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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...

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Old 05-04-2008, 01:50 AM   #28
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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.
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Old 05-04-2008, 02:25 PM   #29
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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?
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Old 05-10-2008, 11:12 AM   #30
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Rebar Placement in a 3x3 concrete pad. Top, bottom or middle?


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.....

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Last edited by Taylor; 05-10-2008 at 11:16 AM. Reason: comment on wetting concrete
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