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Opinion concrete with fiberglass

Geo-TH,In

Geo-TH,In

Well-known Member
My pole contractor is a fan of fiberglass.
My concrete finisher is old school, instead of fiberglass he likes wire. The last job he did for me 10 ago has yet to crack. He did a 30x40x6 inch pole barn, continious pour, no forms. Used nails I put in posts 8 ft apart and shot grade in a few places in center. Finishing concrete is all he done for a living. He's now 65. May have retired. Doesn't answer phone. I may have to drive by and see if he's still around.
He's done a lot of work for me in the past. Always done a profession job. No cracks
 
Dont use fiberglass if you will be walking barefoot like around a pool or sidewalk. I use both everywhere else.
 
I put up a metal building 2 years ago to store my antique tractors in. We compacted the base-used rebar and wire mats-and fiberglass-poured it in cool weather-4000# mix with hot water 5 in thick. The finishers did an excellent job of finishing, came back the next day and sawed stress relief grooves and it cracked worse than any slab I have ever seen. I complained to the concrete company. They sent their rep out with the computer mix sheets on the loads. Said everything was good on their end. They pour certified concrete for the state and fed gov. Stuff happens. All I know is that if you don't want it to crack, don't pour it.
 
When I have used fiber glass it cracked. Of course rebar cracked to.

There is two kinds concrete, cracked and gonna crack.

Use fiber glass if you want but I would put in rebar and saw lines in for the cracks to follow. Rebar in my mind keeps the two sides of the crack even and together.

Being from WI I am always amazed how nice the concrete looks in the southern states.

Paul
 
I put concrete in my shop, don't keep it heated full time, I put fiber glass in had it sawed but some places it cracked on saw marks but some of it did shift, use rebar.

Pete
 
weldingman: Using the hot water in cool weather is what caused your concrete to crack. That made the concrete cure too fast for the weather conditions. If I pour in cooler weather I go rent concrete blankets and cover it for a week. I do not mean all day freezing either. If the highs are in the 30s I will cover the concrete.
 
Well it appears my e-mail has at least got you thinking, as I talked about in the e-mail I sent I have just had bad luck with the fiber reinforcement, cracks are of my least concern , it coming to the top and leaving the finish bad or undesirable are my real concerns. No larger than that building is going to be I think I would let him saw two reliefs at equal places for sure.
 
You are right on, the slower one can cure the concrete the stronger it will be in the long run. Drive way or flat work folks around here are brooming it off at about the third hour then covering it for three days or more.
 
George with the powder dry dirt your going to have under the slab I think your going to have cracks. The sad fact is it is going to move regardless of what you do to the pour. That being said I like fiber glass in floors IF your not wanting a really smooth finish. The fiber glass will stick up after it is finished and it will have wear off to be really smooth. The fiberglass will not stop deep cracks either. What it is very good for is shallow surface flaking and such. You also do not have to use as much rebar.

We just poured 3500 mix with 1/2 rebar on 3 foot centers, with fiber glass 5 1/2 inches deep, at my Grand daughter's. The rebar will keep the deep cracks from shifting levels and the fiberglass will keep the surface for flaking off. We just floated it off. We wanted a rough finish for livestock to walk on but not a broom finish.

The fiber glass is great for sidewalks. as the little rougher finish is not near as slick to walk on wet.
 
There are two rules concerning concrete, its going to get hard, and it is going to crack. If all goes as planned, control joints cut into your slab, will control WHERE the concrete cracks. The rebar or wire placed in the concrete keeps your slab in the same plane. You may have some random cracks but the rebar or wire keeps the crack from opening up or shifting up or down. I do not know what the upcharge is for fiberglass, I've talked to cement finishers who thought it was a little more difficult to get a trowelled out finish, but it does improve its strength. So if the budget will allow go with wire or rebar and fiberglass. The biggest culprit in cracking, scaleing in concrete is the addition of too much water. So remember, you are PLACING concrete, NOT POURING> keep wet enough to work, but not too wet. Remember the concrete slab rides on the subgrade below it so compact the ground or sand or stone below the slab. Good luck with you project. Hope your "old" finisher is still available. gobble
 
I wouldn't like fiberglass in concrete. I think wire is alright for a sidewalk or something like it that isn't structural. For a floor I only use rebar.
 
Pouring concrete is like painting a tractor. The results are a direct result of how well the subgrade was prepared. The subgrade needs crushed stone/gravel to lock it together and dissipate moisture, and needs to be compacted with vibratory type compaction equipment in thin lifts to achieve a level and smooth surface to pour the concrete on. Poor compaction and unstable substrate lead to cracks.. I suggest 4000# mix with added stone, 6 mil vapor barrier, and wire mesh with #4 rebar 5' OC both ways because sheets of mesh are 5x10ft. Stay away from rolled mesh. I also like like to see a 1-ft deep haunch at the outer edge with rebar parimeter tied to the the cross grid.
In Georges situation I would recommend 3 reliefs cut 10-ft on center the length of the building and one down the center lengthwise. George's substrate is going to move with just dirt and no stone or gravel and non vibratory compaction after the frost sinks in.-----Loren
 
thats not true--many concrete products are steamed cured overnight to get 4000 psi strength or more so the forms can be used again the next morning--rate of heating and cooling down of the concrete is very important, also moist curing for 7 days at 40 degrees minimum is also important to prevent cracking and obtaining the 28 day strength
 
My company requirs two separate applications of sealer for concrete with fibers in it or they won't warranty it. Second thing that comes to mind is ask about the different kind of fibers the supplier has to offer. When we pour concrete in foundries and other factories it almost always has "forta fibers" in the mix, much different than the fiberglass fibers.
 
If George does not use some form of barrier over his dirt, he is going to have several problems. First the dirt will wick away all the moisture in the concrete too quickly. Secondly he is going to get a lot of contamination in the crete as the driver lays lays it down out of the mixer shoot. The dirt is lighter than crete and wants to float up, where stone/gravel substrate won't. and as I said above, dirt is unstable when ground moisture gets into it.-------------Loren
 
Fiberglass in concrete is the lazy mans way.
I only use wire with rebar in the footing.

It was interesting reading some of the other replies about rock and soil compaction.
I do not deal with frost heave so their way may be best up north.

But there are 2 reasons concrete gets hair line cracks.
To much water in the mix and the water leaving the concrete to fast.

If you have time put down your dirt; roll it; flood it with a sprinkler; then let it dry out and grade it to your final grade.

By the way I have seen house slabs jacked up off the ground and put on blocks and still no cracks.
 
I'm more of a fan of traditional design, compared to using a reinforcing admixture. I would go with with a bit stronger then needed compressive strength design mix, 4000 psi would seem to be a good fit, and with more than sufficient deformed metal bar reinforcing, all used in a suitable slab design for the conditions. Nothing smaller than a #4 bar, maybe #5, and a grid dimension each way that reinforces adequately.

Your soil concerns me, what kind of bearing capacity is it? How well drained is it ? There'd be no way I would construct a slab unless it was on a subgrade of gravel that with optimum moisture will compact to the required compaction say 95% or better. That would have to be on top of undisturbed soil that will bear the intended loads, maybe with fabric to further strengthen. Well drained is a must.

Concrete material quality assurance is a must, and the single most important aspect that gets tampered with is the water to cementitious material ratio to change the slump to make it more workable. You can't do that and expect performance. Select the best design slump for what the job needs, maybe you need more people to place and finish, better than watering it down. Want to know what PSI it actually is, take 4 test cylinders, fill in 3rds by rodding each lift 25X with an iron rod of a certain diameter, testing outfit can then do the breaks for you at 7, 14, 21 or all at 28n days. Earlier breaks tell you if it will make it's design strength by how much it is increasing. Sounds ridiculous for a barn slab, but if you want to know, easy enough to prove.

This material requires some knowledge of lots of little things, but easy enough to get good result based on same.


Placing, you are headed for cold weather, that "placing" needs to start at 6AM on the best weather day you can get, as the day gets warmer, the hydration process is taking place without freezing unless it's really very cold temperatures. You are finishing at a good time in the day with lots of natural light. Getting started late is a mistake, been there done that, do it very early in the day. Slow cure, cooler weather is better, bridge deck construction they always burlap and sprinkle water for several days, slows the hydration process, and they typically do this even when not hot out.
 
As I recall from 10 years ago, there's several kinds of fibers; fiberglass, polypropylene, and maybe others. You might check on which is the best.
 
What sort of slump, and I mean real sump not what the finisher said was the slump?Too much water ruins more concrete than any one thing else.Anything over a 4 1/2 inch slump and its out of spec.One of my jobs at the concrete plant was to go out to jobs like yours while the concrete was being poured and check out how everything was being done a lot of times I'd see concrete we sent the trucks out with a 4" slump the finisher had added enough water to get it up to 7" or worse.We documented everything so when we got a call like yours we show the customer exactly what happened.If you want a professional job you need professionals doing it and that means an inspector that its been made clear to the finisher from the start that the inspector is calling the shots.
 
Lots of crazy stuff about concrete (NOT CEMENT!) in this thread.I worked at a concrete and block plant for 25 years BTW state certified to handle state concrete.If you really want
a first rate job get a copy of the state DOT specs and pour your concrete like they do.Won't be cheap but it'll be the best concrete slab you ever put down.
 
I had a floor put in my barn a few years ago. One thing I made sure I got was a vapor barrier. It takes longer to cure, but I have zero cracks in a 40x40 barn. I used the fiber in the mix, no rebar. I have had a 5000 lb tractor in there, no cracks.
 
I'm an old school construction worker, I believe in rebar, placed properly! Mesh does some good if your not going to expose it to a lot of salt. I've seen fiberglass hold up well, but it had a good base, might of been fine with nothing!
 
(quoted from post at 20:01:40 10/01/19) What sort of slump, and I mean real sump not what the finisher said was the slump?Too much water ruins more concrete than any one thing else.Anything over a 4 1/2 inch slump and its out of spec.One of my jobs at the concrete plant was to go out to jobs like yours while the concrete was being poured and check out how everything was being done a lot of times I'd see concrete we sent the trucks out with a 4" slump the finisher had added enough water to get it up to 7" or worse.We documented everything so when we got a call like yours we show the customer exactly what happened.If you want a professional job you need professionals doing it and that means an inspector that its been made clear to the finisher from the start that the inspector is calling the shots.
Click to expand...

For sure! You gotta watch the finishers - if you let them have access to the water they will raise the slump just to make it easier to work the concrete. Never let the finisher be the final judge of the slump unless your finisher is your neighbor or cousin and is a volunteer. If you are paying the finisher, you control the amount of water in the mix.
 
I will say this if you react to fiberglass I would not put it in the mix. I react to fiberglass. Meaning if I have my arm laying on a fiberglass item for much time like more than about 5-10 minutes I will get what looks like a rash or little red spots on my skin. I put in a floor for the shed with fiberglass fibers in it. I can't lay on the floor even with a shirt on for very long or I get to itching and the reaction to the fiberglass. Even now over 20 years later. I have to put a cardboard or something down to work on the floor.
I will never ever cut concrete for cracking. I cut the approach to my shed and it has cracked every way but the cuts. All nonsense. Creeper catches on the cut lines also.
 
Just a few pennies worth. When I was a kid in the late 50s into the early 60s the Interstate system was going full tilt in my area. Getting any concrete was a real b....ch for several months not for love nor money because all of the plants were pouring nothing but hy-way. My dad and I were trying to pour a couple of slabs for the green chop feeding aprons. My dad had a couple of friends who worked at the batch plant up the road. One day he gets a call about a "missing" load of concrete and were we ready to go. Eleven yards of Hy-Way grade concrete shows up. You pour this stuff and in a few minutes it starts to turn White! The only thing my dad had were a bunch of those wire screens you toss into the concrete for reinforcing it. We did put in several expansion half inch thick strips that were like tar paper.
These platforms are almost 55 years old and there are only one or two tiny cracks. No rebar, just that wire mat stuff. Now most of them are almost 6 inches thick, but they are still there. Now today the way they do curbs with slip forms and zero slump concrete is amazing to watch. Stuff never moves and if you look about fifteen feet back you can see it is getting dry. The surface that is. I did see some concrete being poured with those metal needles mixed in. Looked very interesting.
 
Not so sure. Army Core of Engineers has stated that concrete adheres better to slightly rusted rebar rather than "new" rebar with machine oil on it. Also, never use epoxy coated rods for slab on grade. They are made to bounce and flex in elevated slabs like bridge decks. I got a bunch "free" at work, and used them in my driveway. Bad idea. Hairline cracks over every rod.
 
Obviously fiber is much easier than mesh. Mesh is a pain, because you have to keep lifting it up as you pour, otherwise it will end up at the bottom of the slab where it does no good. But I believe mesh is a much better choice than fiber for a shop floor, and rebar is better yet. I used mesh in my shop, and I have zero cracks after fifteen years. For the slab in front of our garage, the contractor convinced me to go with fiber and it quickly cracked in a couple of spots.

I don't think there's any way that fiber can add the tensile strength of steel. If it did, they'd use it in Michigan roads instead of rebar, which rusts at the expansion joints, requiring frequent replacement. On the other hand, I've never had any problems with fibers protruding from our slab; I frequently walk barefoot on it with no problems.

If you go with mesh, figure out in advance where you want your control joints and cut every other wire that will cross the joint. That will encourage cracking at the joint.
 
Loren,
I need to locate my old concrete finisher. From comments about fiberglass sticking up I've determined I don't want fiberglass.

The last job he did 10 years ago was my 30x40 pole barn in the south. 6 inches, wire, no gravel, no plastic, NO CRACKS, not even in the relief cuts. Dirt was hard as rock. It was rained on many times. The rain compacted it. Little chance of rain next 7 days. After I get done rolling it, I'm getting out the sprinklers.

I can haul 3 ton at a time. What size P-gravel? I realize you can't put down concrete in my flower like powder.
 
There's two different major types of cracking a concrete slab can have.

1. Flexural cracking due to shifting of the soil below, and/or loading from above.
This is handled by proper preparation and compaction of the sub base and adding rebar (although some of the manufactures claim the fiber will handle this too...)

2. Cracking caused by shrinkage and creep. As the concrete cures, it shrinks slightly (shrinkage) and also can settle (creep; mostly in elevated slabs)
For this you need surface reinforcement; Either wire mesh or fiberglass will work. The fiberglass should be added in the proper proportions per the manufacture and mixed in.

When I was working on my masters, one of the classes we added fiberglass to the mix to see how it would affect the blast performance of a slab. However, after pouring the first slabs without, and adding the fiber to the truck, the driver thought the mix was "too thick" and took the liberty of adding water to thin it back out again (without consulting us first). This appears to have affected the strength as the test cylinder for the fiber were much lower than the non fiber ones. So if you are going to use fiber make sure you add it properly.

They also make a metal "fiber" admixture that can be used as well.
 
When they poured my 24 X 30 garage floor the used both and I have not got any cracks nor fibers sticking up but you can see them in the concrete.
 
Nah , many years back like 1977 my buddy and i poured the floor in his new 40x72 , just he and i from the wall to the edge of the big door it tapered fro 4 inches to 8 inches and down the center it was eight inches thick with wire all the way across from the walls to the door we started off mixen ourself and pouring a little here and there and had about half hand poured before we said enough was enough . He and i cut a deal with the local cement place that we would haul them two big loads of sand a day in the evening when we came home for concrete as he and i ran coal buckets and came by the sand and gravel plant each night on the way home . So the deal was struck and we had a 6 1/2 sack mix bought out . we formed up half way down the middle and he and i poured over 32 yds the one day and Hand finished it and that evening pulled the forms and the next day it was a 30 yd pour . four days later myself my buddy and his brother backed in our three coal buckets loaded with his brother being the lightest at a 119 and change and Mike and i being 126 128 and change . and to this day there are NO cracks in that floor , it was put down ontop of 6 inches of 57 slag and wire and LOTS OF BEER CANS , Beer cans are the secret . Many 120000 lb plus loads were placed on that floor.
 
I put up a 40 X 60 building 4 years back. I had a local contractor come get the pad read this was in September. They used screening from a rock crusher they watered it as they put it down and had a roller/packer there.
The pad sat until the following April when the cane to put the building up. That base was just like concrete. After they got the building up they put #5 rebars on one foot centers both ways and poured 5" of concrete. have been very happy with it and no cracks.
 
That kind of point loading on "green concrete" 7 day compressive strength must have been high, let alone at 4 days. 6 1/2 bag of Portland per cubic yard, wonder what PSI mix that would make. Amazing it did not fracture running on it that soon. I have put in structural slabs with 5000 PSI mix design and a nice grid of #4 bar at entrances to buildings where I knew we would be running across with heavy loads, often times part of a sidewalk, but just a section so we did not fracture the sidewalk where we crossed. Never fails on a site, sidewalk is put in too early or they forget and always has to be fixed, few extra yards and bar, good to go.
Only difference I'd wait at least 2 weeks or the full month.
 
Mark
I looked at contractors floor in his garage. A long crack and surface looked like elephant skin, tiny fracture cracks.

NO thanks to fiberglass.
 
Up here in winter wonderland, that epoxy rebar is used so salt intrusion does not rust away the rebar. Epoxy is just a protective coating, it neither adds or takes away from the strength of the rebar. gobble
 
> Mark I looked at contractors floor in his garage. A long crack and surface looked like elephant skin, tiny fracture cracks.

Well George, that sounds like too much water in the concrete. Whether you use fiber, steel mesh, rebar or nothing at all, if it's too wet it's going to shrink and crack.

That said, if the contractor cuts corners on his own work, it's a fair assumption he'll cut them on yours.

Lessee, you don't like the way he does concrete, so you're going to hire your own concrete guy. And you don't like the way he roofs, so you're going to hire your own roofer. What's left for the barn builder to do? At this point you might as well contract out the posts, girts and siding and be your own general contractor.
 
I worked in a large manufacturing plant where we had a lot of outside storage on concrete. We tried fiber. Personally I would not have it. Rebar is best, wire will do. 5000lb concrete does not cost that much more. I think we worked with a 5 inch slump.
 
My brother, who has poured hundreds of slabs for housing, driveways, etc. says there are only two kinds of concrete: concrete with cracks and concrete that hasn't cracked yet.
 
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