How to make a concrete pad

[W4EWH]

As a sort of followup to the posts about KA1BWO's new building, I'll ask a semi-related question.

I'm going to pour a six-by-six concrete pad to mount a small structure, so I need an education about cement and small building mounts.

• Do I need to have reinforcing wire and/or rebar in the cement?
• What kind of cement is appropriate? Can I use "patching" cement?
• If you've done this, please tell me what you would do differently the second time.

Thanks for your help.

73,

Bill, W1AC

[wd9ive]

what type of a structure will sit on the pad, are we talking a lot of weight? how deep or thick are you planning on making the slab. At 6' X 6' and say 4" thick you may want to consider a Redi Mix truck unless you're going to rent a mixer, even then it's a lot of bags. You dont want to use patching cement you want concrete mix. Reinforcing wire is a good idea.

[k4kyv]

I would recommend re-bar, since it is relatively cheap.  Locally, they sell re-mesh (reinforcing wire) only in 50' and 100' rolls, and that would be more than you need for 6' X 6'.  Plus, re-bar is stronger.  In any case, make sure the  reinforcing steel is well contained inside the concrete, since even a small piece near the surface in contact with the soil will set up electrolysis causing the steel to rust, expand and eventually crack the concrete.

I wouldn't use patching cement.  That stuff is designed for use as a filler for patching holes and cracks, not for strength.  Your job is probably too small for a ready-mix delivery, so your best bet would be to use a wheelbarrow, hosepipe, shovel and garden hoe for mixing your own. You can rent small mixers, either powered by hand or a motor. You could order the sand, gravel and cement separately, but probably the easiest way would be to purchase bags of dry ready-mix like Quickrete, and simply add water, mix it in, and pour or shovel the concrete in place.  The sand, cement and gravel would be cheaper, but getting it delivered to the site could be a hassle and/or expensive. The instructions on the bags of ready-mix tell how to estimate the number of bags needed and how much water to use.  I usually purchase bags at a big-box merchant like Lowes or Home Cheapo, and order a couple of extra bags to make sure I  don't come up short.  The store will take back and refund unopened bags if you return them within 30 days or so.

That would still be a lot of bags, so you might call a ready-mix company or two and see if they will deliver that small amount. They WILL NOT haul left-over concrete back to the plant for disposal.  They will dump it on the property somewhere, and it is up to you to get rid of it before you end up with a huge boulder somewhere at the edge of your property.

Lay the re-bar near the mid-point of the depth of the wet concrete, and keep them at least 3 or 4 inches away from the edges of the slab. They will stay in place where you lay them, so don't worry about them sinking to the bottom or shifting position once you lay them.  Just make sure there is no oily substance on them before you lay them. No need to wire or weld them together.  I would lay them in a cross hatch, in both directions, perpendicular to each other as in a grid. Laying the ones parallel about a foot apart should be plenty.

[KA2DZT]

Thanks Don,  saved me all the typing to tell him what you just did.

Fred

[W2VW]

Fine a job site where they are pouring slabs. See if you can get enough plastic coated wire mesh for your job. Bring beer.

The mesh is a whole lot faster than tying steel. Probably less cutting too.

Usually you use a hook to adjust the mesh into place within the slab's thickness during the pour. It will move some.

[K6JEK]

There are several videos on the Internet about this. Here's one from Home Despot.

http://www.youtube.com/watch?v=VykeWQLYnsk&feature=related

What's the pad for? If it's just a patio, I think you can get away with no rebar but I bet someone on the forum could give you a definitive answer. If you're parking a tractor on it, that's another matter.

I remember jack hammering up a pad I'd poured with plenty of rebar. The stuff works. It was a lot of work getting this thing out. I resolved to not overbuild in the future.

I have mixed concrete and I've picked it up in a pour trailer from a local place. These are miniature dump truck trailers. That sure was a lot easier.

What would I do differently? Figure out something to do with the extra concrete. You'll probably will have some. My neighbor had some stepping stone forms. That worked out.

Besides that. Have help. Have the right tools. Don't do it on hot days if you can avoid it. The stuff is exothermic. One more thing I do differently: Don't pour concrete if I don't need to. I've grown fond of interlocking pavers which work for a lot of purposes, can be removed easily, drain, and look good.

[W4EWH]

what type of a structure will sit on the pad, are we talking a lot of weight? how deep or thick are you planning on making the slab. At 6' X 6' and say 4" thick you may want to consider a Redi Mix truck unless you're going to rent a mixer, even then it's a lot of bags. You dont want to use patching cement you want concrete mix. Reinforcing wire is a good idea.

It's going to be an "Enclosure" instead of a "Structure", because my brother Tom, W3TDH, who is an electrician, says that I don't need to put in ground rods if it's not large enough for human habitation. I don't know if there's a standard size where an "enclosure" turns into a "structure", but it's going to be 1mm smaller that that. 

It will be made of whatever wood I have lying around, and I really don't know how thick the cement should be. I don't think I'll need too much room, and I'd like to have a tilt-open top or similar arrangement that will make it easier to get at everything.

The first thing that will go in is the filter and pump for my swimming pool, since the pool lines had to be dug up to fit my new septic system. That part is to get SWMBO (http://en.wiktionary.org/wiki/SWMBO) to approve the expense, but I'm also going to use it for a "phasor" shed to mount the matching network for a vertical I'm planning.

The filter, even without water in it, requires two men to lift, so I'd guess we're talking about a couple of hundred pounds all told when the pump and electrical boxes are included.

Thanks for your help. I appreciate your time.

Bill, W1AC

[W2VW]

The local concrete supplier may be able to provide you with a short load. That is left over concrete in a truck not used on another job. Sometimes it is cheap. Beats mixing.

[W4EWH]

I would recommend re-bar, since it is relatively cheap.  Locally, they sell re-mesh (reinforcing wire) only in 50' and 100' rolls, and that would be more than you need for 6' X 6'.  Plus, re-bar is stronger.  In any case, make sure the  reinforcing steel is well contained inside the concrete, since even a small piece near the surface in contact with the soil will set up electrolysis causing the steel to rust, expand and eventually crack the concrete.

I hadn't known that: are there plastic supports for holding up the ReBar or mesh while I pour the concrete? Would a plastic barrier under the concrete prevent electrolysis?

I wouldn't use patching cement.  That stuff is designed for use as a filler for patching holes and cracks, not for strength.  Your job is probably too small for a ready-mix delivery, so your best bet would be to use a wheelbarrow, hosepipe, shovel and garden hoe for mixing your own.

Just like the good old days!

I asked about the patching cement because I know a guy who offered me a bag for free, so that's good to know.

That would still be a lot of bags, so you might call a ready-mix company or two and see if they will deliver that small amount. They WILL NOT haul left-over concrete back to the plant for disposal.  They will dump it on the property somewhere, and it is up to you to get rid of it before you end up with a huge boulder somewhere at the edge of your property.

I'll have to ask what a truckload costs. Come to think of it, I'll have to ask what a truckload contains: I know what a "yard" of concrete is, but not the minimum delivery amount. Come to think of it again, how do I calculate how much I'll need?

Lay the re-bar near the mid-point of the depth of the wet concrete, and keep them at least 3 or 4 inches away from the edges of the slab. They will stay in place where you lay them, so don't worry about them sinking to the bottom or shifting position once you lay them.  Just make sure there is no oily substance on them before you lay them. No need to wire or weld them together.  I would lay them in a cross hatch, in both directions, perpendicular to each other as in a grid. Laying the ones parallel about a foot apart should be plenty.

I'll "test for transfer" here: do you mean that I can put the re-bar into the wet concrete and push it down to the appropriate depth, or that I must support it somehow while the cement is being poured?

Thanks for your post. I owe you one!

Bill, W1AC

[WA1GFZ]

use at least 3500 pound concrete. Any ground rods through slab put in PVC with a layer of sand around rod. If you put the rod against concrete it will explode the slab if hit by lightning. Do not mix by hand life is too short.
5 yards mininum around here but a local guy might cut you some slack if you are flexable about delivery time.

[W4EWH]

The local concrete supplier may be able to provide you with a short load. That is left over concrete in a truck not used on another job. Sometimes it is cheap. Beats mixing.

That is really nice to know!

(Scratches head, thinks about it)

Questions:

• Will they deliver part of a partial load? In other words, can I get a truck to pour "just enough" for my slab, or will they want to dump all the cement they have left?
• How long is the "usual" wait? Days? Weeks? Will they call me and ask before coming?
  
• How long does concrete last while it's in the truck? I don't want to take a chance of getting something that should have been dumped, but I don't know how much margin I have or what the trade-offs are.

Thanks for your suggestions!

Bill, W1AC

[WBear2GCR]

I think you need to come clean with what the project really is?

- wait I just read that part - pool filter and a phasor network -
That makes all the difference in how to do it and what you need.

Depending on what it is, you may or may not need footings...

EDIT: so you want a guy with a post hole digger, some cardboard tubes (sonotubes), go below
the frost line , concrete to fill the tubes, and a way to get them all at the same finished height, and an attachment point. Then build a frame with treated lumber, lay a floor down, ur done.

That you can do solo.  The slab requires a form, then laying in the reybar or mesh, and then when pouring, one of those agitators that gets the bubbles out, then someone to work the surface properly... not a job for a novice.

If you go for a slab, max the size... you do not have to build on the entire surface... but you can later.

Also you have to look at slope to keep the water running off...

The reybar or mesh goes down in the form before the pour.

Concrete comes in different "pound test" strengths.

I'd do some online research to get the basics under ur belt.

You might want to just hire someone to do the job and be done with it.
Unless you want a workout. In that case concrete work beats a gym, especially
in the summer sun.

[WA1GFZ]

most concrete trucks carry 10 yards. I got a guy to deliver 12 in a load once but he told me he was pushing the machine. A concrete guy plans 1/2 hour per site visit and plans for you to get good quality by keeping the mixer turning. He will charge you extra if he has to stay longer. Bear is right you want to support it below the frost line or it will heave. You could do it all in one pour if the sono tubes top is at the bottom of the slab. The more metal you put in it the better it will last. If you do it in one pour get a couple buddies and some beer and you can finish a 6 by 6 slab. Just spray some water on the surface to show you the low spots. It is an art so do your home work. A dumb guy like me can mix a yard by hand in a day. That is over 20 bags of home cheapo shitcrete but I added portland.

[W2VW]

The trick is to get leftovers from a large job up the road. Best to talk with the local concrete people.

[W4EWH]

... you want a guy with a post hole digger, some cardboard tubes (sonotubes), go below the frost line , concrete to fill the tubes, and a way to get them all at the same finished height, and an attachment point. Then build a frame with treated lumber, lay a floor down, ur done.

That you can do solo.  The slab requires a form, then laying in the reybar or mesh, and then when pouring, one of those agitators that gets the bubbles out, then someone to work the surface properly... not a job for a novice.

I just watched the video that K6JEK pointed to, and a couple of other videos on Utube, and you're right: it's not for a novice.  More important, I realized that I'd have to have a cement truck drive over my brand-new leaching field, and I don't even want to think of what all that weight could do.

If you go for a slab, max the size... you do not have to build on the entire surface... but you can later.

That's good advice: it's just a little more work when you add it all up, and there's always going to be another project someday.

You might want to just hire someone to do the job and be done with it. Unless you want a workout. In that case concrete work beats a gym, especially in the summer sun.

Thanks for the advice. I can tell you've been "in the trenches"!

73,

Bill, W1AC

[k4kyv]

No need to tie the rebar together or use supports for pouring a simple slab, unless you are pressed for time.  Just fill the space about half full and then lay the re-bars on top of what's poured, then pour the rest.  If you pour all the concrete first, you will be surprised at how hard it is to push the re-bars down into the liquid concrete.  That will give the best assurance that the re-bars are well covered.  Of course, if you are having it delivered by a ready-mix truck, you might have to use supports and tie it together so they can simply dump the concrete and go on their way; they might charge extra for the time they have to spend waiting for you to lay down the re-bars, but for a 6' X 6' slab, with the  re-bars pre-cut, you should be able to lay them down in just a few minutes, so the concrete guy might wait for that long. All you have to do is put them in place.  They will stay put in the liquid concrete.  If you do tie and support them, make sure they are rigidly enough in place that the falling concrete doesn't knock them out of position.

If the hole is deep and the slab thick, you need to have a wooden stick on hand and punch the liquid concrete  as it poured, to get out as many air bubbles as possible.  With my tower base, I waited until they had finished pouring to do that. There was extra concrete left over from my guy anchors; I just told them to top off the holes where the anchors were already poured.  Since the slab of concrete was to be entirely buried below grade, that didn't hurt anything and gave some unanticipated overkill. Of course, that is not an option with a slab.
 
Also, if you get a lot of uncured Portland cement on your skin, don't surprised if by the next day you have some big, raw, ugly sores where the top layer of skin just seems to dissolve, and not necessarily in the exact spots where you were in contact with the wet cement.  I discovered that the hard way years ago. I got it all over my hands, but the raw spots appeared on my arms all the way up to the elbows.  Fortunately, the lesions dried up and healed within a few days, but I should have kept my gloves on more.

Avoid running ground rods through the concrete.  Drive them just off the perimeter of the slab.

[Opcom]

First I'm not a concrete expert. What I wrote is from watching 3 heavy duty slabs being built (30x40, 12x12, and 2x2) and some from a couple of engineering and do it yourself books because it got me interested.

Concrete mix also is rated in "lbs" (maybe I missed where that was said). The higher the lbs, the stronger it will be.

I won't argue with anyone about not tying it and not using chairs, but I did each time, and on a 4 FT wide, 4" thick sidewalk as well.

I always use  rebar even on a 2x2 slab I did, and I space it closely, 8" on a little job and tie it together or see that it's done., and It is cheap so I use the thickest the engineer says is useful. For a 6x6 maybe 3/8" or 1/2" and use plenty, as was said it is cheap. Cut with torch or sabre saw, or chop saw if available.

The little plastic supports are called chairs and should be at the height about 1/2 the thickness of the slab. Don't fear to make the slab thick. Nothing wrong with a 6" slab, but 4" is OK.

A moisture barrier is always a good idea, cover the foundation with it then apply chairs and rebar. Don't want water getting up in the concrete and freezing, it will eventually damage it. Also you can drape it over the edge down into the beams, which are the part of the slab that goes into the ground, which could depend on ordinance, but for a 6x6 slab, 1FT deep x 10" wide might be OK.

The dig for the beam is like a 'moat' all the way around, will become the real support that is under the edge of the slab. Rebar goes in there too and ties to your slab rebar. We stuck two rows of pieces vertically into the ground 2 FT apart along the length of the beam and 6" from each other, pair wise, and then tied off the long horizontal rebars to them, every 6" down from the top of the beam. It's to help keep the slab in place and give the edges strength.


This is for Texas and our ground moves and does bad things, your miles may vary. Look at it this way, you will never have to fix it if it does not break, and it is far cheaper to overdo it now than fix it later.

[KA2DZT]

Your slab is outside with no heat inside the structure.  Don't dig footings down below the frost line if the footings and the slab are going to be the same pour of concrete.  Frost will just lift the slab and the footings will be pulled up with the slab.

Most of the time 3-4" of gravel is put in the hole first.  Pour the concrete over it.  Put the rebar in the hole before you pour the concrete.  Use a hook of sorts to lift the rebar up a bit into the middle of the mix.  That's how it is usually done.  Use some wire to tie the rebar so the concrete doesn't move it around too much.

Use 4000Lb mix, 6" will support tons.

Fred

[KB5MD]

Pouring and finishing cement....you guys have my back aching already...

[K6JEK]

In 2010 I had some steps poured by pros. They showed up with a mix-on-site rig and a nice pump. "You know what my last job was? White concrete." "Really? I didn't know there was such a thing"

Sure enough when my steps cured they had white spots. The contractor tried to fix it with a concrete stain, a nice rusty orange color that would have looked nice. But it took unevenly leaving red spots. I liked it but my wife said it looked like a murder scene. They ended up covering the whole job with a light coat of the gray stuff.

You can get an idea of the rebar we put in for just this little job. Heaviest anticipated load? A couple of guys and an AR-88. I can't remember when or how Rex got the forms off and stakes holding the forms out. All I did on this job was kibitz.

[WA1GFZ]

Use 6 mil plastic under the slab. This will hold water in the concrete so you have time to play before it sets up. Water will actually help it cure stronger.

[W3GMS]

Use 6 mil plastic under the slab. This will hold water in the concrete so you have time to play before it sets up. Water will actually help it cure stronger.

Plus, it keep moisture coming up through the concrete.  That really helps if you ever want to put any kind of surface treatment on the concrete.  All my cement projects around here have vapor barriers under them.

Joe, W3GMS

[KF1Z]

would take 30,  80lb bags of Sakrete for 6' x 6' x 6"  pad.

That's 2/3 of a yard

[WA1GFZ]

I would add portland to shitcrete for slab applications.

[KF1Z]

Sakrete 5000 IS portland