How high to put wire mesh

kmacht

I am getting ready to pour a concrete pad for my garage. I have done the pro's and con's of wire mesh vs rebar and elected to do wire mesh and also have fibers added to the concrete mix. Now my question is, how high do I put the wire mesh? The slab will be between 6 and 7" thick. I have read everthing from it should be 1" from the top, to the bottom of the top 1/3 of the pad, to in the middle, to just off the bottom. So what is the proper location for it and why?

Keith

RPH · Jun 23, 2010 #2 In the middle is what I have seen recommended.

In the middle is what I have seen recommended.

bimmer1980

I would highly recommend the rebar!!!!!!!!!!

Here's the theory behind the steel in the concrete. Concrete is great in compression--i.e. pushing the concrete molecules together. Concrete is very weak in tension--or when you are trying to pull it apart.

When you look at a slab cross-section, there are different loads throughout the slab. At the top of the slab, you have compression--both vertically, as well as horizontally. At the bottom of the slab, it is in tension. This is due to the "bending" of the slab. Since concrete is weak in tension, it will tend to crack once it is loaded beyond what it can handle. So the simple solution is to add structural members to the concrete to take the tension load. Hence the steel--rebar or mesh. On concrete floors in multi-story buildings, you will see the whole bottom of the floor a corrogated steel plate.

Therefore, you need the steel placed in the concrete that it can adequately "grip". I would recommend the lower 1/3 or place it about 1.5" off of the bottom of the slab.

I like to use plastic rebar chairs to support the grid of rebar.

If you insist on mesh, try pouring a 1.5" thick layer of concrete, roll the mesh out, and then finish pouring the remaining 4 to 5". This insures that the mesh is actually in the concrete, rather than laying on the bottom--which is practically pointless.

best of luck!

Dan in Pasadena

Personally, in 6-7" of concrete I'd use rebar but if you're going to use mesh, just have a hooked bar available to you to snag and pull the mesh up so it doesn't end up settling to the bottom of the slab. That or buy dobies and put them in at intervals to keep it a couple inches off the bottom. Good luck.

kmacht

I don't really want to get into the rebar/mesh discussion again. I had an earlier post on it and that wasn't what I was asking here.

I understand tension/compression in concrete but here is the problem. If for some reason one side of the slab starts to sag (errosion, bad compaction, etc) then it would seem that I would want the wire mesh towards the top since the top layer is now in tension and the bottom is in compression. If I want to park something really heave on it (say a tank) then the wire mesh needs to be in the bottom since the bottom is in tension now and the top is in compression. If I put the wire in the middle it would seem to me that it would provide no usefull benefit since it won't help in either situation. So what is the norm. Does it go towards the top, towards the bottom, or am I missing something with the middle?

Also, I will be using metal chairs to set the height. My local hardware store has them and they are adjustable.

Keith

kmacht

Oh, yeah. I forgot to mention, the 6" of concrete isn't because I need it for load. It is because the slab is an extension of another slab and I have to match its height off the driveway. I could probably get away from a load stand point of eliminating the re-enforcing metal all together if I wanted but it is so inexpensive I can't justify not putting it in. The area that will be done is only going to be used to house a kitplane I am building. It weighs less than 1000lbs.

Keith

bazzateer

Hi Keith,

If its so inexpensive and you could need the support both at the bottom and at the top how about putting one 'layer' of mesh 2" off the bottom and a second 'layer' 2" off the top?

ddawg16

Kmacht...do rebar in the middle.....bimmer explained it very well....mesh is NOT going to provide very much strength for tension...especially in the case you noted...even if it is 1" from the top.

When dealing with concrete, the cost difference between doing it right and doing it cheap is very little. The cost of the concrete is the same either way...so is the labor...the only difference is the cost of the material. I would estimate that rebar might be $100 more than mesh? And maybe take 1 hour more to lay than mesh?

But how much do you think it would cost if you had to do it over?

The big problem with mesh is that the wire is smooth...hence, it slides when in the concrete....in comparison, it provides very little strength to the slab.

One last thing....have you heard the phrase "We learn by our mistakes". Well....why do you think so many of us are so bloody smart........

tcianci

If it is welded wire mesh, how does it slide? Sometimes this site is like Ted Macks original amateur hour. There were several other not too bright posts in this thread.

For anyone who is interested...Rebar in a residential type garage slab is a total waste of time and money. Rebar is for STRUCTURAL concrete. WW mesh will not stop a slab from cracking is it is not properly supported but it will keep the 2 pieces aligned if it does crack.

Now, let the flames begin. And don't forget to comment how you like to "overbuild" things because you know how cheap and underhanded us contractors are. Please site more theory too. I'm sure lots of us have had a long day and could use a laugh.

Torque1st

In my experience the wire mesh always ends up on the bottom because everyone walks all over it during the pour and no amount of "hooking" will do much good. I have seen a few crews with a very experienced man on the hook that did ~OK but that is exceptional. They did not use any chairs at all. All of the nice little chairs or supports just get mashed down with the mesh.

ddawg16

tcianci said:
If it is welded wire mesh, how does it slide? Sometimes this site is like Ted Macks original amateur hour. There were several other not too bright posts in this thread.

For anyone who is interested...Rebar in a residential type garage slab is a total waste of time and money. Rebar is for STRUCTURAL concrete. WW mesh will not stop a slab from cracking is it is not properly supported but it will keep the 2 pieces aligned if it does crack.

Now, let the flames begin. And don't forget to comment how you like to "overbuild" things because you know how cheap and underhanded us contractors are. Please site more theory too. I'm sure lots of us have had a long day and could use a laugh.

What makes you think we consider contractors to be cheap and underhanded....yea, some might....but there are a 'few' who are not....

First....the sliding....because the wire is smooth...the is more chance for it to slide....and because it's is a much smaller gauge than rebar...it can stretch...hence, the section between two welds can stretch...and because the wire is smooth...the concrete will slide on it....

I guess I should have prefaced this with "Because it is a garage and will have a car or two on it...".

Contractors like mesh because it's quick....and if installed right it does in fact do a decent job....

In this case....because it's a garage with cars on it....I think rebar is a better choice.....if it was a patio...mesh is fine....assuming that it is positioned properly.

FYI....I just talked to a buddy of mine who has extensive experience with concrete....he says mesh is fine for a garage 'assuming' that it's installed properly...but he says that the biggest problem is that most contractors walk all over it pushing it down to the bottom while pouring and do a poor job of pulling it back up afterwards. Where as with rebar...you can walk between the rods and keep it at the right level during the pour.

I also did some searching on the net....it's pretty much 50/50 for the reasons noted above.

Side note....fibers in the concrete can end up causing the surface to be 'fuzzy'.

A_Pmech

If you want to make an informed decision on the subject of slab reinforcement I can recommend two publications:

1) To understand the relationship between floor loading, sub-grade condition and reinforcement, along with a good treatise on jointing, I recommend U.S. Army Technical Manual TM 5-809-12 "Concrete Floor Slabs On Grade Subjected to Heavy Loads"

2) Should you decide reinforcement is a good idea, it should be sized and installed per best practices. For a study on best practices, I recommend, in addition to the TM referenced above the book "Design of Concrete Structures" by Arthur H. Nilson. Chapter 5, "Bond, Anchorage and Development Length" covers the subject well.

PassnThru

Well - don't buy the tank and the problem is solved. They won't tell you this at the lot - you can't drive it and shoot it at the same time. It's a real bummer to find that out after the fact
Interesting discussion. I think this type of question comes from the fact that you can't get anyone to do concrete without basically telling you that it is going to crack. WTF? Why can't we pour a slab after all of these years using concrete and guarantee it not to crack? I don't understand that - I'm not sure anyone does. That's why we keep searching for just the right combination because we think the concrete contractor is holding out on us and for some reason won't give us that perfect mix of concrete and reinforcement that will give us decades of service with no cracks.

walrus

I've got no theory, just experience in Maine where the frost runs deep. I've either poured, help pour or been on many many slabs poured over gas tanks and around gas islands. #4 Rebar and 4000# concrete works. I wouldn't use #10 mesh for anything here, might work in other places

tcianci

They won't tell you it wont crack because there are just too many variables. Poor preparation, shrinkage, a bad mix, etc. etc. The variables are even more of an issue when you get into a DIY situation. If I get called to place and finish a slab or floor for someone where I have not done the grading, compaction and underfill, all bets are off. The concrete is only as good as what you're putting it on and even if you rebar the daylights out of it you still can't be sure it wont just settle off level in one big chunk.

Ok, all that said, I've seen plenty of slabs that are decades old with no cracks in them. The one in my garage is nominal 4 inch 2500 psi mix on 6 inches of crushed stone, been there 31 years, one small hairline, no rebar, no welded wire. The only thing I know for sure about this one is that we puddled the crap out of it for days before we placed the stone and since the hairline exhibits no "lippage" from one side of the crack to the other, I'm gonna chalk it up to shrinkage. The stuff shrinks more than you would think. Take a look around the junction between your basement or crawl space walls and the floor... I't not uncommon to see a sixteenth of an inch space. Very interesting discussion and thank you to the guy who provided references to the technical publications about concrete reinforcement.

willymakeit

The mesh will work fine. It is there for a small amount of tension. Mesh is still a pita when it comes demo time. Some times worse than rebar.

kmacht

I give up. I asked a simple question. How high do I put wire mesh in a concrete slab. Then I get blasted with a bunch of you don't know what you are doing, you should use rebar, etc, etc. The decision has already been made. I am using wire mesh in my pour. You can provide all the posts you want about using rebar vs wire mesh but the fact is that I am still going to use the wire mesh. We have had this conversation here before and my decision has been made and it is final. I am not hiring a contractor to do the pour so if it gets mashed down into the bottom then its my own fault so. Who cares if some contractors don't care enough to do the job right. I am doing it, not a contractor. Yes, I know a slab can crack no matter what you do. If it does crack because I used wire mesh instead of rebar then I am ready to deal with those consequences. I am just trying to find out what the best practices when using wire mesh to help prevent it as much as that is possible. So I am going to ask once more.

How high do I put the wire mesh in a concrete slab.

Keith

nehog

kmacht said:
I understand tension/compression in concrete but here is the problem. If for some reason one side of the slab starts to sag (errosion, bad compaction, etc) then it would seem that I would want the wire mesh ...

IMHO wire mesh won't help in that situation...

But if you insist, I'd center it in the pour, either with dobies or some other way to ensure it stays put where you want it. So for a six inch slab, I'd space it at about three inches. Personally I doubt there is a major difference between higher, middle and lower, as long as it is not on the bottom (so doing a double pour can make sense), or right at the top.

willymakeit · Jun 24, 2010

As suggested earlier center it. You can use wire chairs to hold it off the floor and a hook tool to pull it up if it sags. This is done in comm. const. all the time.

Torque1st

kmacht said:
I give up. I asked a simple question.

There is the problem, you think it is a "simple" question. Unfortunately it is not. Try to put it in the middle.

WNYflyer

Generalizing here since I have no idea of your slab and foundation system.

But if your slab is not supporting the structure in any way then it is a slab on grade. For a typical slab on grade the concrete thickness is determined ignoring any structural load capacity contribution from the steel reinforcing. For a 6" slab on grade the mesh would be put down 2" from the top to help control shrinkage cracking and slab curling. Mesh is located in the upper third primarily because the slab wants to dry quicker on the exposed top thus more and wider possible cracking. I would cut every other wire at contraction joints also

Now if your slab supports your structure in any way then it is not a typical slab on grade, it is structural slab and all bets are off and there is no easy answer.

For those in construction ACI 360R-10 is the current bible for slabs on ground along with ACI 302 I believe

Good luck

Mattlt

I've always been told there are two kinds of concrete slabs:

1) Those that ARE cracked.
2) Those that WILL crack.

Mike83

Mattlt said:
I've always been told there are two kinds of concrete slabs:

1) Those that ARE cracked.
2) Those that WILL crack.

Yup - there are two guarantees with concrete: It will get hard. It will crack.

To the op: for a slab on grade I would bias the mesh toward the top to control the shrinkage cracks. If you get cracks in the bottom of the slab - who cares?! You cant see them!

Try to limit the strength of the slab since more cement = more heat of hydration = more shrinkage when it cures. After you float, keep the slab nice and cool with burlap and a soaker hose for several days. Space your contraction joints no more than 15ft I'd say.

Sorry if you knew all that...just trying to help out.

bimmer1980

to the OP: If you are not concerned with the load, why not fill in the void with 2 to 3" of rock or gravel, plate pack and then place 4" of concrete? It would certainly be a little cheaper. Or 2" of foam on the bottom to keep it from getting so cold.....

For what it's worth, I believe I answered your question in my first post. I had not seen your thread about debating the merits of mesh VS rebar. I know how I would do it, but if you want mesh, that is your deal.......

Again, for the loads associated with it, I recommend the mesh 1" to 2" above the bottom of the slab. THe key is for the mesh to be actually IN the concrete, not merely laying on the ground.

Regardless of mesh or rebar, the key is to get the sub base well compacted.

If you already knew all of things mentioned above, then why ask? We were just trying to be helpful and keep you from making mistakes that we have already seen happen.

Best of luck on your endeavor.

Daniel Dudley · Jun 24, 2010 #25 +1 compact the snot out of it.

+1 compact the snot out of it.

Phil S. · Jun 24, 2010

What Tcianci (post #9) said. Your right on Tciani. Oh, and one more point, Concrete does two things "it gets hard and cracks!!!!!" period

tcianci

To the OP:

HOW DARE YOU CONTINUE TO ASK THE SAME QUESTION OVER AGAIN JUST BECAUSE WE'RE TOO DUMB TO ANSWER IT THE FIRST TIME?

kmacht

tcianci said:
To the OP:

HOW DARE YOU CONTINUE TO ASK THE SAME QUESTION OVER AGAIN JUST BECAUSE WE'RE TOO DUMB TO ANSWER IT THE FIRST TIME?

HA! Now that's funny.

I have done my research before asking here and basicly got the same response that was posted here.

"I recommend the mesh 1" to 2" above the bottom of the slab."

"I would bias the mesh toward the top to control the shrinkage cracks"

"I'd center it in the pour"

Everyone has an opinion and nobodys opinion matches. I'm going to go with the bottom of the top 3rd in the slab. As someone else mentioned, who cares if I get shrinkage cracks on the bottom. No one will see it. To answer a few questions:

I already built it up with 6" of crushed stone. 6" of concrete was a comprimise in how much to build it up with stone vs how thick to make the concrete. Yes, it was compacted very well. I rented a jumping jack compactor because I have clay soil. I compacted the stone as we spread it. I stopped every 2" of stone and ran the compactor. I also made sure I had plenty of good drainage all around the slab so that I would minimize the chance of frost heave in the winter.

Keith

Torque1st

The point is that no matter where you try to put mesh, -IT WON'T END UP THERE. You can draw all the pretty pictures you want. Study all the pretty illustrations. Install all the chairs in the world. But in the real world those pretty pictures and spacers are absolutely meaningless.

Ch3No2

Middle if you choose wire mesh...and to the guy that said to put it at the bottom 1/3, you don't think there is lateral tension in the top when you drive on it?......and to the guy that said it's for structural only, bullsh*t!..#3's 24 on center in a slab without expansion joints sure helps!

Justanoldguy

tcianci said:
If it is welded wire mesh, how does it slide?

HaHa.
That was my exact thoughts as I read that statement..

Personally, I would compact hell out of the base course and lay concrete with the mesh in the center. eg between 2 and 3 inches from the bottom.

Bruce4310TX

It should be 2/3 of the way of the thickness of the slab, so near the top on a 4" then roughly 2.5-3" 6" then about 4 to 4.5 but with mesh it will never be where you want it to be, it will end up looking like a roller coaster up and down all the way thru yeh its better than nothing, As for the fiber it only works during the time its setting up its used to keep surface curing cracks from forming. Does not provide any structural strength when its cured zero. Do a little research you find this fact out but a lot of un-informed contractors try to sell it as the crack cure all . Spend you money on higher PSI crete. Good luck.

kmacht

Still not sure why you guys think that the wire mesh will move when I pour the pad. The addition I am pouring is L shaped and 11' at the widest part. I can reach the center of the pour from both sides so there should be no reason to have to walk on it. With enough chairs it shoud sit just where I put it.

Yes, I am aware that the fibers only provide minimal strength after the cure. I am doing it specificaly for surface cracks during the drying process. At $4 a yard extra it is cheap insurance.

Keith

2chipped

Put the wire 2" from the top, and here is why. Concrete is heavy crap (an example is cut lattice out of a sheet of paper set it on spacers and pour water on it which will cause it to compress unless you want to install chairs every 16in)so you start out 2 in from the top some will go to 3 and 4 in due to the weight. which means nothing if the prep isn't perfect the batch isn't or the finished work isn't. I'm not saying it wasn't prepped right or that you won't do a good finish job. But the mesh will only hold it together around the cracks. myo2 Good luck

How high to put wire mesh

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