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Old 05-19-2012, 09:42 AM   #1
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Concrete Rebar


We have a contractor building a barn 48' X 72'. The sections of the barn are in 24' sections. The contractor has placed rebar down already and it is about 2' apart and we don't think this is enough. The concrete is going to be 4" thick. Is this correct?

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Old 05-19-2012, 11:00 AM   #2
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Concrete Rebar


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We have a contractor building a barn 48' X 72'. The sections of the barn are in 24' sections. The contractor has placed rebar down already and it is about 2' apart and we don't think this is enough. The concrete is going to be 4" thick. Is this correct?






  • There's no generally-accepted rule of thumb as to how to space rebar in a concrete slab. There are many variables that influence the decision of how much space to leave, including the maximum load the slab is likely to have to support, the bearing capacity of the soil, the diameter of the reinforcing rebar, the likelihood of frost heaving the slab, and whether or not the slab is on a level grade. The steeper the grade, the more heavy loads will stress the slab, and the more reinforcement will be required. The same variables influence the type of concrete to be poured for the slab and how thick to pour it.

But hey,it's your building and if you want more rebar in the slab tell
him,just be aware it will cost more.

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Old 05-19-2012, 11:57 PM   #3
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Concrete Rebar


rebar is cheap, but every two feet is ok from what Im hearing, are you using steel mesh as well?
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Old 05-21-2012, 11:20 AM   #4
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Concrete Rebar


Why dont you think 2' is enough?
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Old 05-21-2012, 03:50 PM   #5
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Concrete Rebar


The topic of rebar in thin slabs (4 inches is a thin slab) has been discussed repeatedly on this forum, with many conflicting opinions offered. I suggest you do a search for "rebar" or "slab reinforcing" to see some of the thoughts. I have written numerous posts on this topic, I will try to summarize here.

A 4 inch slab is too thin to use rebar for structural support. The reason is that the rebar must be essentially at the center in order to have the minimum required 2 inches of cover recommended by the American Concrete Institute (ACI), and the center of the slab is the point of zero stress. Hence the steel provides ZERO structural reinforcing. If this seems confusing, you may want look up steel reinforcing on the ACI website, or Google thin slab rebar for a discussion.

Since the steel provides ZERO structural reinforcing, you may wonder why have it at all. My answer, and the ACI answer as well, is you don't need it. If it is improperly placed too close to one face of the slab, which is very common, it is likely to rust due to water penetration, therefore expand, and will pop the concrete out. At best, the steel reinforcing provides minimal crack control, which is better provided through proper curing of the concrete, use of low water content concrete, installation of control joints, and proper preparation of the support pad (typically crushed stone) beneath the concrete.

So my answer to your question is that the reinforcing is a waste of money, better to spend the effort on proper placement of the concrete. But if you must have it, the spacing matters little in this case.
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Old 05-24-2012, 08:13 AM   #6
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Concrete Rebar


what about welded wire mesh for thin slabs? vs fiber reinforced?

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Originally Posted by Daniel Holzman View Post
The topic of rebar in thin slabs (4 inches is a thin slab) has been discussed repeatedly on this forum, with many conflicting opinions offered. I suggest you do a search for "rebar" or "slab reinforcing" to see some of the thoughts. I have written numerous posts on this topic, I will try to summarize here.

A 4 inch slab is too thin to use rebar for structural support. The reason is that the rebar must be essentially at the center in order to have the minimum required 2 inches of cover recommended by the American Concrete Institute (ACI), and the center of the slab is the point of zero stress. Hence the steel provides ZERO structural reinforcing. If this seems confusing, you may want look up steel reinforcing on the ACI website, or Google thin slab rebar for a discussion.

Since the steel provides ZERO structural reinforcing, you may wonder why have it at all. My answer, and the ACI answer as well, is you don't need it. If it is improperly placed too close to one face of the slab, which is very common, it is likely to rust due to water penetration, therefore expand, and will pop the concrete out. At best, the steel reinforcing provides minimal crack control, which is better provided through proper curing of the concrete, use of low water content concrete, installation of control joints, and proper preparation of the support pad (typically crushed stone) beneath the concrete.

So my answer to your question is that the reinforcing is a waste of money, better to spend the effort on proper placement of the concrete. But if you must have it, the spacing matters little in this case.
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Old 05-24-2012, 11:19 AM   #7
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Concrete Rebar


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Originally Posted by Daniel Holzman View Post
The topic of rebar in thin slabs (4 inches is a thin slab) has been discussed repeatedly on this forum, with many conflicting opinions offered. I suggest you do a search for "rebar" or "slab reinforcing" to see some of the thoughts. I have written numerous posts on this topic, I will try to summarize here.

A 4 inch slab is too thin to use rebar for structural support. The reason is that the rebar must be essentially at the center in order to have the minimum required 2 inches of cover recommended by the American Concrete Institute (ACI), and the center of the slab is the point of zero stress. Hence the steel provides ZERO structural reinforcing. If this seems confusing, you may want look up steel reinforcing on the ACI website, or Google thin slab rebar for a discussion.

Since the steel provides ZERO structural reinforcing, you may wonder why have it at all. My answer, and the ACI answer as well, is you don't need it. If it is improperly placed too close to one face of the slab, which is very common, it is likely to rust due to water penetration, therefore expand, and will pop the concrete out. At best, the steel reinforcing provides minimal crack control, which is better provided through proper curing of the concrete, use of low water content concrete, installation of control joints, and proper preparation of the support pad (typically crushed stone) beneath the concrete.

So my answer to your question is that the reinforcing is a waste of money, better to spend the effort on proper placement of the concrete. But if you must have it, the spacing matters little in this case.

Would you agree that wire remesh in a 4" pour would also be redundant?
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Old 05-24-2012, 02:17 PM   #8
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Concrete Rebar


Wycoda -

Wire mesh is not really reinforcement for structural purposes, but does a good job on crack control especially with properly sawed (same or early the next day) control joints.

The question on whether rebar is required depends on the quality and type of base. I had my personal driveway poured without rebar because the base was so solid and rigid. - I found this out when the contractor with all the equipment and tools could not remove enough base material (40 year old limestone, well used and compacted) for a 6" slab without blasting. I went with a 4" slab with only wire mesh and fibers (since fibers are cheap, but do not really control cracks) about 20 years ago and there is still not a crack despite the trucks the current owner put on it. When this concrete contractor does use rebar, he spaces lighter rebar at 16" (same amount of steel per foot) because he discovered he did not have an employee with shoes over 16" long, so they could walk without pushing the steel down. That is a minimum cost increase and closer spaced is more efficient than heavier bars spaced wider. There was rebar or wire allowed in the driveway crossing the city sidewalk and the apron to the street since the city owned the land and the utilities run there and the depth to rock is less that the frost depth. My contractor friend says the city policy provides a lot work for him because city trucks frequently pull in to turn or maneuver on unreinforced slabs on poorly compacted base and he ends up doing the removal and replacement. He never had job from a 4" slab with 16" o.c. rebar even when the utility trucks went of the standard driveway. This is not specific to your building, but give some guidelines.

Depending on your location, climate and projected future use is, you might want to consider air entrained concrete and higher strength (4000-4500 psi) is a good investment. In your building, there could be salt dripping from vehicles or animal urine, etc. that can attack concrete. Make sure he places the concrete and just does not dump wet soup in the forms without some consolidation and delays for proper finishing followed by controlled curing.

Dick
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Old 05-24-2012, 03:03 PM   #9
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"Depending on your location, climate and projected future use is, you might want to consider air entrained concrete and higher strength (4000-4500 psi) is a good investment. In your building, there could be salt dripping from vehicles or animal urine, etc. that can attack concrete. Make sure he places the concrete and just does not dump wet soup in the forms without some consolidation and delays for proper finishing followed by controlled curing."



Air entrained concrete is only neccessary in concrete exposed to the elements,and 4000-4500psi for this application is totally unneccessary and a waste of money, 3000-3500 psi is more than enough for this application, if you really want to assure a good job,have the contractor use a curing compound as soon as finishing is complete,and a sealing compound after curing is complete.
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Old 05-24-2012, 04:03 PM   #10
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Canarywood -

The higher strength concrete can give a better finish and is more durable from factors such as temperature, chemicals and abrasion. Depending on the OP's standards it may be a good investment considering it is just minor material price difference and the labor placement and and delivery is not different.

It is based on the location of the OP as I mentioned in my post. In some areas, it may seem like an unnecessary minor cost increase, but in other areas, it could be a good investment. Depending on the use, in some areas, trying to find a supplier to deliver low strength 3000 psi non-entrained concrete than can limit you to the low end suppliers with limited capacities. There is nothing magical about high strength concrete and common admixtures since the delivery cost of concrete is usually one of the highest costs because of the cost of fuel, trucks and labor. Some producers describe concrete as the most expensive way of selling aggregate.

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Old 05-24-2012, 04:58 PM   #11
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I basically agree with concretemasonry on his posts. Higher strength concrete really only means lower water/cement ratio, there is no increase in material cost. It does make the concrete stiffer, hence harder to finish by inexperienced personnel, but the ultimate product is more resistant to attack by various chemicals, as he noted. Air entrained concrete is standard where I live, and in most of the country, you would need to specify non-air entrained concrete if for some reason you needed that.

As for concrete mesh, as noted it has no structural value in thin slabs, however it does offer some crack control. It is even harder than rebar to position correctly during the pour, I would not bother with it for a floor surface, but to each their own. Fibers offer limited control over shrinkage cracking, they don't hurt anything, but are not really necessary for a slab application.

All of this changes when you place structural concrete, so bear in mind that all of my comments are specific to thin slab non-structural slabs.
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Old 05-24-2012, 06:44 PM   #12
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Originally Posted by Daniel Holzman View Post
I basically agree with concretemasonry on his posts. Higher strength concrete really only means lower water/cement ratio, there is no increase in material cost. It does make the concrete stiffer, hence harder to finish by inexperienced personnel, but the ultimate product is more resistant to attack by various chemicals, as he noted. Air entrained concrete is standard where I live, and in most of the country, you would need to specify non-air entrained concrete if for some reason you needed that.

As for concrete mesh, as noted it has no structural value in thin slabs, however it does offer some crack control. It is even harder than rebar to position correctly during the pour, I would not bother with it for a floor surface, but to each their own. Fibers offer limited control over shrinkage cracking, they don't hurt anything, but are not really necessary for a slab application.

All of this changes when you place structural concrete, so bear in mind that all of my comments are specific to thin slab non-structural slabs.


Dick and Daniel,with all due respect we're talking about the floor of a barn here,why in the world would you use 4000-4500 psi for this application,i worked in the field for a redi-mix supplier for 35 years,so i'm talking from a pracital view point, we routinely supplied concrete for 30 story high rise floors that were 3750 psi,and columns were 4000 psi quoting from the books is fine,working in the field is another story.
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Old 05-24-2012, 07:01 PM   #13
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I never said you needed 4000 psi concrete for strength, the floor slab performance has nothing to do with strength and everything to do with finish and durability. Where I live, 4000 psi concrete is standard, effectively no extra cost. If it costs extra where the OPS lives, nothing wrong with 3000 psi concrete if it is placed, cured and finished correctly.
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Old 05-24-2012, 07:16 PM   #14
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[quote=Daniel Holzman;928053]I never said you needed 4000 psi concrete for strength, the floor slab performance has nothing to do with strength and everything to do with finish and durability. Where I live, 4000 psi concrete is standard, effectively no extra cost. If it costs extra where the OPS lives, nothing wrong with 3000 psi concrete if it is placed, cured and finished correctly.[/quote


Excatly what i said, glad we could agree.
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Old 05-24-2012, 10:53 PM   #15
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Quote:
Originally Posted by Canarywood1 View Post
"Depending on your location, climate and projected future use is, you might want to consider air entrained concrete and higher strength (4000-4500 psi) is a good investment. In your building, there could be salt dripping from vehicles or animal urine, etc. that can attack concrete. Make sure he places the concrete and just does not dump wet soup in the forms without some consolidation and delays for proper finishing followed by controlled curing."



Air entrained concrete is only neccessary in concrete exposed to the elements,and 4000-4500psi for this application is totally unneccessary and a waste of money, 3000-3500 psi is more than enough for this application, if you really want to assure a good job,have the contractor use a curing compound as soon as finishing is complete,and a sealing compound after curing is complete.
The "elements" may be too vague of a term. In reality, anywhere there is a chance that the slab may be subjected to moisture & freeze/thaw cycles, air entrainment is necessary. The OP stated this was a "barn", which could certainly be subjected to both of these scenarios............

As a contractor I'll always opt for a 4000 psi+ mix for hard troweled finishes. Everything else aside, it's makes fiscal sense to spend a few dollars more per yard to get a floor that finishes better & faster. IMO, pouring a 3000 psi to save a few dollars on material will cost far more in extra labor waiting for adequate concrete to finish..........

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