Fiber or steel reinforcement for concrete

Looking at having a floating slab poured for a new building (shop) and am planning on it being 6" thick. I have two shops and two aprons that were poured 6" thick with fiber reinforcement and no steel. Never had a problem.

I was calling around for quotes and the first guy I talked to said he has had better luck with good old steel reinforcement and recommended it over fiber. He would pour it either way and it would cost the same either way, but he recommended steel and no fiber. He said the only good thing about fiber is it's easier to rip out

What do you guys recommend and why?

i'm a don't fix what aint broke kind of guy.You have had success with fiber so stick with it fiber is rated same as steel wire.If I were going to pour (shop)floor I would add 3/8 rebar in floor as well as fiber if there is going to be heavy eqipment on it.4'grids

I had a concrete slab 30' X 55' poured in my Morton bldg using fiberglass and no steel over twenty years ago. It was poured 6" thick with a monolithic beam down the center lengthwise about two feet wide and about 10" thick. They never put expansion joints in and to date, there are no cracks. I vote for fiberglass. LOL

Fiber.

Mesh is only as good as fiber when it's positioned perfectly in the slab.

When's the last time that happened?

In my opinion fiber wins every argument, but quite a few of the old-schoolers always hang on to the old ways.

But then again, I like Wal Mart, too.

Most cracks I ve seen in concret run off of solid hard objects , Post , etc . Rap post or anything else that wont give with 1/4 or 1/2 in. styrofom and you wont have any cracks from the presure .

I have did it both ways as a General contractor and for most Contractors, Fibreglass is a no brainer but if doing for my Customers I used wire mesh with 1/2" rebar around the perimeter and maybe on a 36" grid across the site, plus 5 bag mesh mix, Overkill? Maybe ,but it is my concrete and my customers. For myself I have used 5 & 6 bag mix out of the truck and can not honestly see the difference.Pouring pads for grain bins or buildings on a contract, 5 bag wins hands down for economics.Its going to crack in ILL.where I lived and it is spelled out in the contract,I tried but not my fault.Nature of the beast.Stopping movement is the whole purpose. Any one who says they have poured large expanses of crackless concrete in the midwest and most of the winter prone USA is BSing .

I vote with John. Be sure to specify proper amounts of Fiber, there is skimping on some jobs that give it a bad rep for strength when it is the amount that makes it work. JimN

Will have to agree with John. Most contractors here in IL. will guarantee one thing, that it will crack. I've also seen as much cracking with fiberglass as wire and or steel. The most important thing is to watch what you are pouring over. I poured a 40 X 48 solid slab, no expansion joints 30 years ago over virgin clay with 3" - 4" of sand and wire and has not cracked to this day. Was in a heated shop.

Control joints are for cracks.

Neither mesh nor fiber have anything to do with preventing cracks.

a GOOD sub base will help more than any thing else.

Rusted nuts your right I totally agree with that

Having a good sub base is 90% of how well your concrete slab is going to hold up to heavy loads and abuse. My test of a good sub base is-- It has to support the weight of the concrete truck and not leave tire tracks over 2 inches deep.

The comments about good sub-base are dead on. Even if you put down an aggregate base, it should be rolled or hand tamped. Aggregate bases can be compacted up to 25% of their initial thickness. Concrete is usually specified as 6 bags portland per cu. yd. with no more than 6 gallons of water per bag (36 gallons per yard). The water is the key to cracks. The slab as it cures will develop tremendous shrinkage forces internally. The lower the water/cement ratio the less shrinkage and the fewer the cracks. Normally slabs are poured in 20 ft squares to control cracking but cracks do occur anyway, usually a foot away from the control joint. The fiber mesh folks made huge claims of strength when the product was introduced in the late 6o's. Most of the claims have proven to be false. The only accepted benefit of fiber mesh is that it absorbs or disapates the internal shrinkage forces thus helping to prevent cracks. Flexural strength, the ability to carry loads is usually attained by the addition of steel reinforcing bars. The usual spacing for a 6" thick slab on a good sub base would be #3 bars at about 12" centers each way. If the slab is to carry unusually heavy loads, the bars need to be #4 or thicken the slab another 2 inches. The bars to be effective must be held at a point at least 2" clear of the base not to exceed 3". This usually means driving bars to support the mat or buying bar chairs to hold the mat at the proper height in the slab. The bars must be tied together at at least every other intersection to prevent them from being displaced by pouring the concrete. In summary the best way to prevent cracks is to isolate immoveable surfaces, i.e. columns and walls and limit the water/cement ratio. Pour it as dry as can be worked. Wet self leveling concrete will in a month look like a spider web of cracks. Good luck with your project.......

Fiber is a secondary reinforcment, and not intended to replacde steel. We use fiber when price is a big deal.

Uniform well drained fill is utmost important if cracking is a concern.

Some wil argue with me, but if anyone does not care of the cost we place 1/2" rebar 12" OC, wired together, using chairs to keep it where it belongs. We consider it best placed in the lower 1/3 of the slab. After years of practice, I can tell folks pretty much where it is going to crack.

I have not poured less than a 4,000# mix for the last fourty years, even footings.

I lost alot of bids due to price, but they never bat a eye when they ask me to tear it out and do it my way, and perhaps it is a overkill.

Often a person has the comnt, we must have gotten some bad concrete when we poured this project. Lets just say, I have been lucky and never got any of this so called bad concrete ever. I can't manage to drive in the yard and then call for the trucks like alot of the so called --FAST GUYS do.

Concrete work is a art, and often there are bum jobs, usualy because someone thinks they can chew off as much as they bite off. You get to see it all. A friend of mine borrowed my compactor, for a hr, and compacted his entire project with one tank of gas. He had sixteen foot of fill, and run my compactor across the top--it is rated for 6" lifts, not 16 ft lifts.

That was about four years ago, and you should see it now. A very expensive rock pile looking slab.

Mesh in my opinion is a waste of money, because even if you pull it up while pouring you simply can not walk over it otherwise it goes to the bottom. In our area 10 guage wire will snap when it gets rusted in a slab.

Rebar controls caracks, which will happen.

Mesh is too weak to really help, it rusts out shortly after a crack appears.

Fiber helps hold small pieces in place, it is not for major control of the whole slab.

If that's unclear - use rebar. The fiber is optional, but use the rebar for sure.

--->Paul

Rebar controls cracks, which will happen.

Mesh is too weak to really help, it rusts out shortly after a crack appears.

Fiber helps hold small pieces in place, it is not for major control of the whole slab.

If that's unclear - use rebar. The fiber is optional, but use the rebar for sure.

--->Paul

I have to go along with NE IA. The best way to go is with 3/8 to 1/2 inch epoxy coated re-rod. Another thing to consider is the aggregate in the mix. Aggregate and fines[sand] make up 85% of concrete. I had a slab poured with fiber and bad rock. It's cracked in a few places but not to bad. But the rock is popping[spauling] pretty bad.If you live up north make sure you pay for good rock. Don't try to go cheap on concrete, I learned the hard way.

Phil in Pa. I agree with both of your statements and I am a certified concrete batchman. If the fiber is added correctly it will blend in at all directions. By nature there is 2 types of concrete (cracked) and (gonna crack)expansion joints will help control the line it follows when it does. If you don't want the joints have them put in what is called a keyway joint.As a side note we had some concrete spill out of a truck at the plant and figured it would come up easy with the jack hammer and the loader... wrong the fiber made us have to completely break it into almost gravel just to remove 1 yard

When i replaced my cracked drive way turn around the people who did the work talked me out of steel reinforcement saying fiber worked just as well.Big mistake.A year later my cement is cracked in the same place as what they tore out.From now on it's my way or the highway,the heck with the experts.

I put up a new Morton deluxe machine shed summer of 2008 60' X 88' with a five inch thick 6 bag mix concrete floor poured in Oct of 2008. Sub base was firmly packed fine wet sand. Wetted down to keep concrete from drying too fast. Re-rod on 36" centers with more at doors. No one mentioned scoring although joints were mentioned. Concrete will crack. Scoring controls where it will crack. Contractor said: "Cracks will start at side and end posts so we'll score in from posts" which were on 8' centers on the sides. So, I have scores 8' apart across and scores lengthwise between the end posts. Only visible cracks are from the posts to where the scoring starts. About one inch. The cracks are there but you can't see them because they're at the bottom of the scores. Beautiful white concrete too, because I kept it fairly wet for about a month after pouring.

Both. Put the steel in 2" from the bottom. Use a good compacted granular subbase. I would use #53s.

I see everybody specifying rebar sizes and placement, but nobody is mentioning how much fiber per yard of concrete. I don't know the cost of fiber, but the plant could put a thimble full in a load and it would contain fiber. Some folks would do that, and for that reason, I think you should specify how much fiber per yard.

I worked on an experimental project for the state of Illinois. If I remember right, the contract specified 3 lbs of fiber per cu. yard. I don't know how they proportion the fiber in the private mixes.

The finishers grumbled a little about fiber buildup on their tools, but it was a burlap finish and caused no problem. Might be a problem with a trowel finish - I don't know.

I'd expect it to crack, no matter what you use.

Paul

First off there make sure you have a solid base nothing will work right if you don't.Then put in rebar tying it in place.Fiber is better than wire mesh for 2 reason 1)Its all thru the concrete 2)Finishers usually stomp the wire mesh to the bottom of the slab where it does little if any good.And this is the MOST important thing make sure the concrete is put down properly.Most finishers want to put way too much water in the concrete which makes the gravel go to the bottom and delutes the cement which can then turn 3500lb concrete into 1000lb concrete.Make it plain to the finisher that the concrete will be poured with no more than a
4 1/2" slump 5" at the very most by a SLUMP TEST,
NOT the Finisher's estimation which most finishers think a 7" slump is a 4". Most Non Commerical concrete is poured out of spec.If I were doing a big slab I'd pay an Independent Testing Agency a couple hundred bucks to come out and supervise the job and take test cylinders.Also get all involved together and make it plain to them before the pour that a tester will be there and his is the final word.

Concrete is a material that is subjective to versatility and value engineering, admixtures and reinforcing designs can and will reflect that.

The simple thing to remember when constructing anything is what you have designed, been done so thoroughly, and does it meet the intended purpose ?

What will you be using this concrete slab for, heavy point loading, trucks, tractors, heavy machinery etc. ? Whatever it may be you want the design of it to match the loading or exceed it for future or unintended uses that may require it to be stronger. Will it be exposed to harsh substances, like salt is on a highway bridge for example.

Most times it pays to build a slab according to a design that meets the use and then some, woven wire mesh is fine to use, though concrete is porous and has capillaries, you can address issues with corrosion of reinforcing like woven wire mesh reinforcing slab design, which will most likely provide a higher compressive strength than a fiber reinforce mix design, same is true with using deformed metal bars (rebar) just that it will cost more. Woven wire mesh will be fine for reinforcing, for an interior slab, sidewalk maybe not the best choice for a wash bay for example, that is where you spend the money on epoxy coated rebar or similar.

6" slab will provide you with a nice structural loading value, especially with reinforcing. Maybe you call for 5000 psi and place rebar accordingly to achieve an acceptable structural loading value or vice versa, less psi, more reinforcing etc.

Sub-base compaction is critical to the stability of the slab, that has to be done right. I can't say what the differences in compressive strength will be between woven wire mesh, fiber and or rebar, but there will be cost differentials of same. If were me, I'd want a proven mix design from the supplier, one they use all the time like for D.O.T. or similar, and do some research on structural values, like compressive strength, for the different slab designs, woven wire mesh, fiber, and rebar, see what matches what you need for loading the closest and what you can afford. I think we'd all prefer a 12" x 12" grid of #4 or larger deformed bars, but doing that would be cost prohibitive, would make a very strong slab though.

I always suggest taking 4 test cylinders, and make sure the supplier, contractor is aware of that, it would be very helpful if there was a claim or the material failed, very simple thing to do when the material arrives.

Do not forget all stub ups, for the buildings needs, now and the future, easy to put in a sleeve, extra conduit and similar for future use and make sure the layout and or structural details are coordinated with the new building if has not already been done, good to plan ahead here, once the material is placed, too late to do any of that.