7" too thick for walkway slab?

Hi Everyone,

First I want to thank everyone who contributes here! I've already learned a lot from lurking. I'd really appreciate your help and advice on this project:

I have a circular driveway separated from my entryway (slab) by about 12 feet. I want to pour an 8 foot wide walkway between the two. So total slab size will be 8 foot wide by 12 foot long.

Links to a couple photos:

http://i26.************/2zg4z1c.jpg
http://i27.************/1zd0lxk.jpg

The forms are 2x6. Right now they're mounted with angle iron stakes at each end, and the angles are screwed to the concrete with TapCons. I also have steel stakes I'm going to drive about every 4 feet but they're not in yet.

I had to pull out a big rock sticking up in the middle, which left a large depression. I had some material (sand, gravel) so I bought some portland, mixed some hand loads and filled in the hole. You can see the gray area in the photo where I did this. I'm going to put plastic over this concrete before I pour the slab so if the stuff underneath cracks, the crack won't transmit upward into the new slab.

The rest of the area is undisturbed soil; we live in a very rocky Central Texas area so it's actually mostly limestone not far below. I removed all the grass, what little was growing here that is.

I'm planning to put 3/8" rebar on a 12 inch grid. Planning to drill into the existing slabs to receive the ends of the rebar. Also have expansion joints to put between existing concrete and the new pour at each end.

As you can see I have two 2" drain pipes laid in, and I'm going to put in a third pipe in the obvious open space.

Now for my questions.

1) If I don't put any fill material in, the slab will average 7" thick, and the thickness varies somewhat (minimum 5 1/2", maximum 9"). I know the typical depth of a slab for this purpose is more like 4". OTHER than taking more concrete, is this a bad thing? Will a slab that thick be weaker, more prone to crack, or otherwise bad? I could put down gravel and compact it, but for this small a slab, and considering the time and cost to rent a compactor, get more gravel, etc. it's not cost-effective UNLESS there's a structural reason to do so. Plus, I figure having slab on undisturbed soil -- even a thick one -- is the best way to keep stuff under the slab from shifting. The pour will be right at 2 yards total.

2) How deep should the rebar be placed in the slab? I have heard 2 1/2" from the bottom -- and I have "chairs" that are 2 1/2" high -- but that's for a 4 inch slab. Is it important to have the rebar closer to the surface or closer to the bottom?

3) How close to the edge of the slab should I put the outermost piece of rebar?

4) I have read here on the forum that you should groove every 8 x 8 feet. But for aesthetic reasons I don't want a groove halfway up the slab -- I want it just one piece. Can I get away with going a little further than recommended, do you think?

5) What kind of concrete should I order? xxxx PSI? Fiber entrained? What slump?

6) Anyone notice anything else amiss that I haven't mentioned? Any other advice?

My biggest concern is cracking. Because of where it is, I DO NOT want it to crack. That's why I'm doing what I consider to be overkill on the rebar.

Thanks for your patience, I realize this is a long post but I wanted to mention everything I think might be important. Will be glad to supply more info if I'm leaving anything out.

I poured a driveway once in my younger days so I have some awareness of what's involved -- but it has been 25 years or so. I think this is a do-it-yourself project, but please correct me if I'm wrong.

Thanks a bunch!

Jeff

 

based on my small amount of experience i would say looks good to me. as for the rebar if I remember from the concrete institute closer to top would be better but in no case closer to the top than 2 in. all concrete cracks eventually, but the steel will keep them from growing. thickness will add to strength not make it weaker. the higher slump the stronger all other things being equal. be sure to wet the soil under the pour to help slow down drying, and in tx. you probably want to place burlap or something simular over the slab after finishing so you can wet it to help the surface not to dry to quickly. I would say you will need at least 2 other helpers to move concrete and help with the initial screeding. good luck.
Murphy was an optimist

 

Okay, here is my advice based on 22 years in the concrete biz, as well as observing cracking causes firsthand.
First, as to the pipes in the slab. If concrete is poured around these pipes, you will get cracking directly over the top of them. Sticking up in the bottom of the slab like that, they will act like a joint from the underside (like a groove in the top does). Ditto for the chunk of concrete if it sticks up in the slab. The top of the pipe needs to be under the bottom of the concrete, preferably with some fill stone or sand over the top of it. If it even bumps up an inch into the bottom of the slab, it can crack there.
2) rebar should be placed near the center of the slab.
3) The higher the slump, the wetter the concrete and hence the weaker it is. 5 inch slump concrete is plenty workable and not too runny. Higher than that, you'll get a lot more shrinkage.
4) Your plan (7 inches thick, 12 inch rebar grid) is fine for a concrete walkway, or in case a small plane ever needs to land on it. A typical walkway (or driveway) is 4 inches thick, and your rebar requirement could be cut in half (2x2 grid). I've torn out countless 20 + year old slabs where there was no reinforcement at all, and the concrete looked perfect. We only tore them out because the homeowner wanted a stamped driveway. One of these drives was only 2 inches thick with no cracks outside the joints! The rebar can be placed within an inch of the form boards, as long as the concrete is well-consolidated around it.
5) Even with a rebar grid, the concrete will probably crack. Ive seen 2 foot thick slabs loaded with rebar exhibit cracking. They just don't separate when they do crack. A lot of cracking has to do with shrinkage from pouring too wet. Rebar won't stop it if it is shrinkage related.
6) I'd pour a minimum of 4,000 psi, air-entrained, with stealth-type (very fine, not coarse) fiber reinforcement.
7) Why don't you want the joint(s) in it? There is no better way to control cracking, and it's a crap shoot without the joints. Remember though that to be effective, a joint needs to be at least 1/4 the depth of the slab. If you pour 7 inches thick, the joint will need to be 1.75 inches deep. You'd have to saw it in afterwards, as I've never seen a jointer (groover) that deep. If it were mine, I'd dress it up by jointing it lengthways down the center, then every 4 feet across it. The joints would be a nice decorative element and with so many, the cracking would be much less of a potential problem.
8) Sorry for the long post, and good luck with your project!

 

I would just move the pipes so they are equally spaced, mark the locations on the form and joint over them.

 

Tscar hit the nail on the head. Ifyou're adding another drain pipe, they should be spaced at 3', 6', and 9' with a joint over each. As I said before, it would be more decorative (in my opinion) if you also included the joint up the length as well. Good luck.

Pecos

 

Speedwrench, Pecos, TScarborough,

Thanks a million to all of you for the advice! I really appreciate it.

TScarborough, I'm leaning toward your suggestion of equally spacing the pipes and scoring above them. Pecos, thanks for sharing your expertise, it's plain you know your stuff (you're funny too, I particularly liked the "land a small plane" thing). And Speedwrench, your tips were useful too.

ONE MORE QUESTION:

The delivery company I have lined up is one of those that mixes on-site (in other words, the square truck, not the rotating round one). He says he can do 4000 PSI fiber-entrained. In y'alls opinion, is mixed on-site just as good as the "rotating" cement loaded at the plant and mixed in transit?

I'll post pics when I'm done.

Thanks again,
Jeff

 

It is usually better, and you don't really need air-entrained or fiber, so if Custom is charging exra for that, decline the option.

 

The fiber suggestion was for added insurance, it costs about $7 extra per yard. We use it in everything we pour in addition to rebar. They claim it reduces cracking, and since it reinforces the entire matrix instead of in one plane like rebar or wire, I figure it can't hurt anything.
The air entrainment helps add protection against delamination in freeze/thaw areas. I know Texas isn't one, but another benefit of air is less bleed water. Newbies or people who don't know better often overwork the surface, trowelling the bleedwater back into the top of the slab. This makes for a weak paste with a high water/cement ratio and therefore a weaker top. If your surface is bleeding a lot, wait to work it until the bleed water is gone. Personally, I'd leave them both (fiber and air) in, but it's your call.

Pecos

 

Pecos, I've decided to go with fiber -- mostly 'cuz I'm the belt-and-suspenders type. It'll add $20 to the total cost.

I didn't mention air entrained and the cement contractor didn't either -- I don't think I'll need air. It's a cold day in you know where when it freezes around here.

I'll be ready to pour by noon tomorrow. The weather is nice (not too hot) but the 20% - 30% "chance of thunderstorms" thing every day this week has me a little worried. How long after pouring should I wait before I let it rain (ha- as if)?

Can I cover up with a tarp after the pour? Will that protect the slab if it should rain? It's small enough that it's practical to do that.

Thanks,
Jeff

 

If you cover the slab, don't use plastic or anything with a plastic facing which will touch the concrete. The wrinkled or bubbled areas where the tarp is not in actual contact with the concrete will cure a different color than the areas that are in contact with the surface. It will end up looking like hell, and the mottled stains will never bleach out, at least not for several years. I see this all the time, especially during the winter where contractors use curing blankets to cover their work, or whenever a slab is covered to protect it from rain. There is actually a post about it right now on another concrete forum I frequent. The best bet is if you could build a framework to tent over the slab without touching it. Make sure it is strong enough that the wind won't knock it down onto the surface.
It's hard to say when it's safe from rain without being on site evaluating it. Concrete sets up at such different rates that it just depends on how yours goes. Good luck on your pour.

Pecos

 

Pecos: "The best bet is if you could build a framework to tent over the slab without touching it. Make sure it is strong enough that the wind won't knock it down onto the surface."

Can do! That's what I was thinking too. I have porch posts at one end of the slab to tie to.

Thanks.

 

Be sure to post a picture of your results, and a description of any problems you may have run into. It could be invaluable to the next guy. Good luck!

Pecos

 

I've been slow to post pictures of the results, but here they are... three years later, NO CRACKS! The biggest problem I encountered was that the plastic I used to protect the finished slab from the elements did sag onto the moist slab during a thunderstorm and left a distinct mark... it has pretty much faded over the three year period however.

I really appreciate everyone's help and advice -- most of which I took. I'll let the results speak for themselves. -- Jeff

https://picasaweb.google.com/1113837...eat=directlink

 

That looks great - thanks for the update

 

I had the same problem with plastic sheeting leaving marks on a walkway I poured last year. I put the sheeting on supports but we got a gullywasher during the night and it sagged down ointo the concrete. I'm glad to hear that the marks fade. Mine haven't yet but it's only been 8 months.

 

Nice job. I'm glad it came out as you hoped!