To Rebar or Not?

[JasonF]

I'm having a 32x48x12 pole building put up this summer and all plans are set except for how I want the concrete done. Plans are to have the floor of the barn 4" thick with two pads 8" thick for a future lift. Outside there will then be a 30x24 4" thick approach to the overhead doors as well as a small walkway off the side off the approach leading to the entry door. I had planned on just doing wire like I have done in previous builds but lots of reading on here go me thinking about using rebar throughout? So pricing it out, 1/2 bar spaced 18" gridded OC would add approx $2200 to the build, what are the opinions on whether this is needed or not? All concrete will be 4000psi placed on a compacted sand base. I also priced the materials out with me buying and laying the bar down and the price was only a few hundred less, so it appears to be a fair quote just don't know if I need the added expense?
Just for reference I don't have any real heavy equipment or anything, mostly vehicles, <8000gvw trailers, quads and sleds. My thought is that I only get one chance to do it right, but then again I like $2200 in my pocket too!

 

[LEVE]

Install rebar, otherwise you'll regret it.

 

[raceman17]

Also don't put rebar directly where the posts of your lift will eventually sit. Hitting rebar when you go to drill and bolt down the lift posts would be bad. When I did my slab I had it formed to 4" thick and then I dug out two areas about 6'x6'x 8" deep in the area where I thought my lift would go. Then I layed rebar around those areas but not where I knew I would be drilling. It didn't have any issues.

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[GYPSY400]

If your in an area that freezes, rebar and mesh would be a must.. Along with thickened perimeter.. My local building code requires engineered pad if it's larger than 560sqft.

Here is a pic of my thickened perimeter with rebar stirrups



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[RVDan]

It's really just a question of how stable your ground is. If it's all been dug up and put back down no amount of compacting will make it solid enough to skip the rebar. If it's virgin earth that's been settling for thirty years, it's fine without.

 

[kbs2244]

There is no reason to not use re-bar.
If you want an example to follow, check out how they build an Inter State highyway.
Cost is inportant, but performance is more so.
They use a whole lot of iron.

 

[jhasafety1]

I used cattle panels in my shop floor 5 years ago and not one crack. Wasn't sure about it when I did it but it was suggested by an old concrete man. At the time it was definitely better than wire and cheaper than wire/rebar.

 

[3/4 drive]

Rebar makes for a better job , even with the rebar does not make it foolproof finished one for a buddy last year rebar and wire still heaved .

 

[Angelfire]

Rebar is certainly the best way to go. That being said, there are millions of pads poured with mesh that work just fine. Weather certainly should play a part in your decision. And regarding a compacted base, you most definitely get a solid base through compaction of disturbed soil. My compacted base was miles harder than the earh that had been sitting for 30 years. I had a proctor done and compaction tests done when I built an addition and my detached garage...no issues with my pads stability.
Cheers

 

[elsensei]

Definitely use rebar. And it's worth renting a concrete cutter and cutting joints the next day and then filling them with sikaflex afterwards...looks way cleaner than forming joints in wet concrete.

 

[ConCretin]

There's a lot of mis-information on this subject. First off, it's important to understand that rebar or wire mesh won't prevent cracks resulting from shrinkage or an inadequate sub base. What they will do is hold the concrete together across cracks if they occur and maintain the integrity of the slab.

If you are 100% confident that your base material is sound, you don't really need reinforcing for a typical residential slab but if there's any question (and there often is), then steel reinforcing is cheap insurance.

The major difference between mesh and rebar is that rebar is more rigid and easier to support, which is why I prefer it.

The other alternative, fibermesh is intended to prevent early age shrinkage cracks and buy you time to cut control joints. It won't help once cracks occur including those that occur under those control joints. Folks get away with fiber all the time but truth be told, they probably wouldn't have noticed anything different without it.

 

[Frost0824]

Definitely use rebar. And it's worth renting a concrete cutter and cutting joints the next day and then filling them with sikaflex afterwards...looks way cleaner than forming joints in wet concrete.

Yup they are all right. I used 3000 PSI concrete, with Fiberglas fibres throughout... this winter two long cracks. I know it's a pain and more expensive but it will pay off.

 

[JasonF]

Alright you guys are pretty much telling me what I expected. The build sight was filled around 25 years ago which is why I was leaning towards the rebar, just a little added assurance for when it does crack. Like was said by some above the reason I questioned the use was mainly due to my last barn of the same size did not have it and I had zero cracking.
What are peoples thoughts on putting rebar in a 4" slab? Is this routinely done as most of the time I see guys doing it in at least 6" slabs? Don't know if I can stomach the bill for 50% more concrete too. lol

 

[ConCretin]

JasonF, a 6" slab isn't all that structurally different than a 4" slab. The section just isn't thick enough to allow the concrete and rebar to act as a beam and span a significant distance. It's the soils under the slab that end up supporting the loads placed on it.

Rebar in a typical slab on grade isn't going to enhance it's load bearing capacity - it just holds everything together as it floats on whatever it was placed upon. Build yourself a well compacted granular base that drains and you'll avoid most problems.

An unreinforced slab on a good base will perform better than a heavily reinforced slab on a crappy base.

 

[rustyjames]

Alright you guys are pretty much telling me what I expected. The build sight was filled around 25 years ago which is why I was leaning towards the rebar, just a little added assurance for when it does crack. Like was said by some above the reason I questioned the use was mainly due to my last barn of the same size did not have it and I had zero cracking.
What are peoples thoughts on putting rebar in a 4" slab? Is this routinely done as most of the time I see guys doing it in at least 6" slabs? Don't know if I can stomach the bill for 50% more concrete too. lol

From what I've researched rebar in a 4" slab in not best practice. I think you'd be better off with WWF instead.

 

[JasonF]

This is were I question if the rebar does much more than wire mesh in my case. On a 4" slab the rebar put on 2" ladder puts the bar right in the middle of the pour which as stated really doesn't add to the load capacity, it only helps to keep everything together. In this case isn't WWF doing the same thing or does the wire just rust out once there is a crack and break? From what I've read throughout this forum is seems like rebar on a thicker slab positioned lower in the pour actually does provide some gains in load rating due to the bar not being able to flex like when its in the middle. My build sight is located at the highest point of my back property and does drain pretty well in that area. The area leading up to the barn is whats wet now, probably won't be able to start contruction for at least a month.

JasonF, a 6" slab isn't all that structurally different than a 4" slab. The section just isn't thick enough to allow the concrete and rebar to act as a beam and span a significant distance. It's the soils under the slab that end up supporting the loads placed on it.

Rebar in a typical slab on grade isn't going to enhance it's load bearing capacity - it just holds everything together as it floats on whatever it was placed upon. Build yourself a well compacted granular base that drains and you'll avoid most problems.

An unreinforced slab on a good base will perform better than a heavily reinforced slab on a crappy base.

From what I've researched rebar in a 4" slab in not best practice. I think you'd be better off with WWF instead.

 

[ConCretin]

You really don't pick up much load bearing capacity by moving the rebar lower in the slab until you get into much thicker sections.

Rebar is overkill with regard to the amount of steel needed to hold a slab together under most conditions - it's just easier to support. Depending on the gauge, you might need to support mesh every 2' to keep if off the ground. Rebar supports can be 2 or 3 times farther apart.

Good luck with your project

 

[dlhindiana]

Maybe a dumb question but if rebar just holds together concrete if there are cracks and you are putting control joints in, couldn't you run rebar just in the area of the control joints only? Maybe like 3' pieces perpendicular and then tie them together parallel to the control joints?

 

[Angelfire]

Just a note. If you do go with mesh, make sure it's installed properly. Too often it's there but when the guys pour, they walk all over it and it ends up under the concrete where it does no good.

 

[chrislxf]

@LLWillysfan you first paragraph is not correct. I suggest you to read 7.12.1.1 from ACI 318-08. This is the reason why you first need to have the slab dsigned by a licensed civil engineer. Any repair or changes of fdn and paving will be expensive after the concrete is cured. Again, concrete will not crack if designed correctly.

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[The mean fish]

Again, concrete will not crack if designed correctly.

My grandfather did concrete work for years and he always told me "All concrete cracks". I'm 37 years old and I've never seen a piece of concrete without some sort of cracks in it, whether it's some homeowner poured driveway, floors in high rise buildings or the face of a billion dollar dam. I tend to believe my grandpa, all concrete cracks at some point.

 

[Exit 61]

Stable Base = good slab............ rebar or mesh ? what ever is easier for you, it all needs to be set on dobers or stands..... 4000psi mix no cc or very little. Wife and I and a few helpers are going to do 36+ yards as soon as the freezing nights are over. We are using rebar just because I got a good deal on it. should have less than $5000 in it including the bbq for the helpers............ remember its just concrete no need for a 6 year degree to do this......... And as stated before it will crack, its heavy,and no one will steal it.

 

[jhochst]

i had a 32x48' slab poured. 5" thick with rebar grid. will be 2 years this fall, not a single flaw. I have been very pleased. was well worth the extra.

 

[GYPSY400]

This is were I question if the rebar does much more than wire mesh in my case. On a 4" slab the rebar put on 2" ladder puts the bar right in the middle of the pour which as stated really doesn't add to the load capacity, it only helps to keep everything together. In this case isn't WWF doing the same thing or does the wire just rust out once there is a crack and break? From what I've read throughout this forum is seems like rebar on a thicker slab positioned lower in the pour actually does provide some gains in load rating due to the bar not being able to flex like when its in the middle. My build sight is located at the highest point of my back property and does drain pretty well in that area. The area leading up to the barn is whats wet now, probably won't be able to start contruction for at least a month.

what are you doing for footings?? Are you going to have just a flat slab, or thickened perimeter? As you can see in my previous post, the rebar is just in the perimeter, with mesh across the middle. This is an engineered design, which in my area, is required by code.

 

[wssix99]

There's a lot of mis-information on this subject.

Amen. And in this thread, too!!!

ACI 318-08. This is the reason why you first need to have the slab dsigned by a licensed civil engineer. Any repair or changes of fdn and paving will be expensive after the concrete is cured. Again, concrete will not crack if designed correctly.

I know many licensed civil engineers who are not qualified and can't properly figure this out. ACI does have a guide for the design of these slabs and - BTW - ACI spells out very clearly that all concrete cracks and all you can do is control the width of them.

For this type of application, hiring an engineer to design the slab is overkill. (It's only light duty.) However; ACI does provide the best practices and guidelines for the design.

if there are cracks and you are putting control joints in, couldn't you run rebar just in the area of the control joints only? Maybe like 3' pieces perpendicular and then tie them together parallel to the control joints?

There are applications where you might put "pins" across the joints to hold the slab sections together, but in this application, the reinforcement (WWF or rebar) is for crack width control. For crack width control, you want a consistent cross sectional area ratio of concrete to steel, otherwise you'd get a differential effect in different areas of the slab.

Plans are to have the floor of the barn 4" thick with two pads 8" thick for a future lift.

If you do this, you'll want to separate the slab sections (with differing thicknesses) with an expansion joint. The thicker section will build up greater forces related to shrinkage, so you'll need more reinforcement there if you want consistent crack control across the various slabs. Adding in the expansion joint (full section) also allows you to design the reinforcement for the different sections independently.

In the house I'm building, I isolated my garage slab from my house slab and also divided the house slab in to two sections with an expansion joint. I then had a different reinforcing plan for each of the three sections. My foyer got over-reinforced with lots of rebar so I didn't need any control joints cut at all, my shop space got WWF and closely spaced control joints, and my garage got a more typical rebar reinforcement with more distantly spaced control joints. It worked out really well and the combination of strategies paid off. I was able to invest in rebar where I needed it and forgo the expense where I didn't need it - an no cracks outside of the control joints.

So pricing it out, 1/2 bar spaced 18" gridded OC would add approx $2200 to the build, what are the opinions on whether this is needed or not? All concrete will be 4000psi placed on a compacted sand base.

If cost is a factor and you are going to keep one part of the slab thin, then I'd isolate the different sections with an expansion joint and only put rebar or WWF where you need it.

To figure out how much reinforcement (WWF or rebar) you really "need" for this type of application, you have to decide how you want to do crack width control. How many control joints do you want and how closely spaced do you want them to be on each of the slab sections? Once you know that, you can figure out how much reinforcement you need. IMO - 8" on the lift slab is overkill. I'd just go with 5 or 6" to make sure you get the minimum 4" required by the lift manufacturer. That will save you some money also. As LLWillysfan points out above, the strength is in the base.

BTW - There is also a game of probabilities here. It's impossible to calculate how much cracking you will encounter or where they will occur. Adding reinforcement or placing control joints closer together will only increase the probability that your cracks will occur within the control joints. Even a properly designed floor can develop cracks in unwanted places. So, you can also add reinforcement as an insurance policy, etc.

 

[Aquamoose]

Mine's a 36' x 48' slab. I used mesh and fibers. The base was carefully prepared by carefully taking off any layers and only filling in with sand and compacted to form a level surface with 1/4" tolerance. 2" insulation boards were installed & taped. Mesh came next and tubing tied to it.

In the area where the main door went, I went down to 5" and lined the front apron with rebar. The edge went down to 8" on plenty of gravel. It slopes slightly starting about 2 ft inside the shop.

My RV pad is 6" and lined with rebar with two separate layers of compacted gravel. Serious amount of drainage! 3 years and not a single crack anywhere.

Preparation is key.

 

[Eriehunter]

Just get heavier mesh.

 

[MG44]

In October I had a 60 x 105ft parking lot poured at one of my businesses. No rebar, just wire mesh. We had one of the worst winters on record. I walked the parking lot a few days ago and not one crack on it. The lot did heave by the garage bay doors over the winter, it went up about 3/4 of an inch but has went back down since the ground thawed. Still no cracks.

Contractor told me rebar wasn't necessary, I took his word on it and so far so good. We will see.

 

[ConCretin]

nevermind

 

[JasonF]

Thanks for all the replies fellas, I appreciate all the input no matter how varying some of the opinions may be. If for nothing more than piece of mind I'm going to spend the money and put in #4 on 18" centers, for what the total cost of the build is this is not that much of an added cost.
BTW, not going to consult a Civil engineer either for my basic pole barn and floating slab. LOL I love this forum but no where else do I see "get an engineer to look at it" more than here, maybe I'm just biased because I work with so many different types of engineers day in and day out. For me an engineer is consulted when trying to complete a task that is outside the norm or safety is involved, in this case I'm just looking for best practice.

Not going to do any footings, probably will just thicken the perimeter some and call it good.

what are you doing for footings?? Are you going to have just a flat slab, or thickened perimeter? As you can see in my previous post, the rebar is just in the perimeter, with mesh across the middle. This is an engineered design, which in my area, is required by code.

 

[Diesel Dan]

10+ yrs ago we poured my 32x48 slab near Midland, MI. It was a 4" slab, no rat wall, no mesh, no rebar, just fiber. Top soil was removed and 12" of sand brought in and packed with multiple passes of the backhoe while watering the sand. It cracked at the ZIP strip locations and no other places for the time I lived there and stopped back years later and still doing good.

Barn floor after that was 6", no fiber, with 1/2" rebar 24" OC and welded at every joint. Current barn is 5-6", no fiber but wire mesh. Personally I wouldn't use the fiber again since I'm fond of smooth sealed floors now.

 

[Shocker]

My floor is 6" with 8" perimeter footers and 4" stem wall. They used rebar throughout and fibermesh mix and it is solid and crack free for the last 6 years or so.

 

[GYPSY400]

Thanks for all the replies fellas, I appreciate all the input no matter how varying some of the opinions may be. If for nothing more than piece of mind I'm going to spend the money and put in #4 on 18" centers, for what the total cost of the build is this is not that much of an added cost.

BTW, not going to consult a Civil engineer either for my basic pole barn and floating slab. LOL I love this forum but no where else do I see "get an engineer to look at it" more than here, maybe I'm just biased because I work with so many different types of engineers day in and day out. For me an engineer is consulted when trying to complete a task that is outside the norm or safety is involved, in this case I'm just looking for best practice.



Not going to do any footings, probably will just thicken the perimeter some and call it good.

If you don't need an engineer , then by all means.. Like I said, in my area it's required by code.
My drawing cost me $900 for something he most likely had saved on his computer.
I believe the dimensions of my pad is 14" thick at the outside edge, for 12", then tapers up to 5" in the centre ( code minimum is 4").. Like many have said, prep work is key.


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[Aquamoose]

I should add,

You should contact whoever makes & supplies cement to area contractors and ask for references through the sales department. They certainly want the best guys handling their products so they likely have the top three on a post-it note right by the phone for people who called. I used two of three that were provided to me in the past and they both did great work.

It never gets old to see prior work and asking for references.

 

[sophijo]

....no mention of curing protocol here, and that has a significant impact on the final product. After it's finished keep it wet and covered or sealed. 28 days for the initial cure as I understand it, and it never stops.

 

[BigGMC]

........ And as stated before it will crack, its heavy,and no one will steal it.

I was worried at first that my new 50yd pad would walk off in the night, luckily we live in a low crime area.

JasonF - I went with fiber mix..... wish I didn't as I work on the floor alot. I suppose eventually all those protruding fibers will wear away but untill then......

 

[03HD-F150]

Just my experience, but I had a very well compacted base on my 30x48, and did no rebar or wire, but had a 24" rat wall included on the perimeter. After 3 years, I have fine cracks in only the control joints, and I've notice no heaving/shifting whatsoever. MI weather.