Concrete slab questions.. fiberglass mix or no mix with wire mesh

bans25 · Oct 24, 2013

Hi All,

I have a question about slab in a new 32x32 garage addition to my current 24x24 garage.

Slab will be ~30x30. The contractor has in contract:

Supply concrete and labor to pour 4" slab with 4000 LB Mix with Wire Mesh and Poly, Flat smooth finish, Pin new slab to existing slab with rebar.

He then said to me today: "I think we will be going with fiberglass mixed concrete and there will be no need for the wire mesh"

I don't know about fiberglass mix?? I started reading online and reading about fiber coming out of concrete etc?

This will be a auto hobby garage, I will be on the ground all the time, I will also have a 2 post lift installed as well. The lift company requires 4" of 3000lbs concrete minimum.

Should I allow him to make this change? How bad are these fibers that show on the surface of the concrete? Some day I want to epoxy coat the whole slab but it will not be a year or two so I can save up for it.. Will the fibers cause a problem?

Also I asked about cutting expansion joints and he said they are not needed? I cant make heads or tails online if they are needed or not..

Any insight is welcome here.. I am the customer and want to make sure I get what will work for me, Why would he deviate from the contract? is this better or worse for me?

any insight is helpful

-Bill

readhead · Oct 24, 2013

Cut control joints the next day for sure. I would stay with the mesh and in fact I would go with rebar but mesh is ok. How much money was he returning for not using the mesh? Depending on the finisher you could have a fuzzy slab. For what you are doing I wouldn't use fibermesh.

bans25 · Oct 24, 2013

readhead said:
Cut control joints the next day for sure. I would stay with the mesh and in fact I would go with rebar but mesh is ok. How much money was he returning for not using the mesh? Depending on the finisher you could have a fuzzy slab. For what you are doing I wouldn't use fibermesh.

How do I determine what the cuts should be? (width/depth and locations?)

I was thinking the same, make him pour to contract.. but there is no mention of control joints in the contract.. this is something I could do myself correct if he really pushes back?

readhead · Oct 24, 2013

I sent you a PM. Give me a call.

tomroblee · Oct 24, 2013

I had a 36' x 48' barn built a few years ago. The builder preferred to use the steel mesh. The additional cost of fiber mesh was less than $100, so I used it as well as the steel mesh. The wee ends of the fiber mesh do stick out a tad, but haven't created any problems for me.

I wouldn't go without either rebar or steel mesh, but the fiber mesh is so cheap that it seems worth a try. Definitely go with control joints. Mine were cut the same day that the concrete was poured. After several years I haven't noticed a single crack or surface blemish.

Keithinsc · Oct 24, 2013

Had fiber in my old shop. We used a power trowel and after it was finish troweled, we let it sit for about 2 hrs.
We then came badck with the power trowel set as steep as the blades would go and 'shaved' the fibers off the surface.
Any that remained, I hit with a torch about 2 weeks later.
My current shop has "Buckeye" brand reinforcement fibers. Tiny, about 1/8 long but the diameter of a hair. Supposed to be just as strong. The masons love it because they don't even know it is in the mix. Finished floor is glass smooth.

Just googled up this facts page;
http://www.ultrafiber500.com/faq_index.html

edit; when the truck with the Buckeye fiber showed up, you could not tell it was in the mix. Got in an argument with the truck driver that I was getting ripped off! I had to take a sample of the concrete and mix it with water in a bucket, then I was able to see the tiny fibers.

gpflepsen · Oct 24, 2013

I used to pump concrete for 10 years. We would accumulate the concrete from cleaning the pump out after some jobs. These concrete "discs" would be about 6' diameter and a few inches thick. After a few years, we'd pick them up with a loader, drop them or otherwise abuse them to break them up and haul them away. Keep in mind, the concrete in a pump washout has been diluted with so much water that they have lost most of their compressive strength. The piles with fibermesh in them were a complete ***** to breakup.

Fibermesh works. If you can handle or remove the wiskers from the surface, there is no reason not to use it. I wouldn't solely use it in a structural application, like a thickened edge mat foundation or bolted post lift, but it will keep a slab together.

T-Mac · Oct 24, 2013

i have fiber mesh in my slab-didnt get **** about the finish work-still cant see the fiber on the surface

wssix99 · Oct 25, 2013

Your contractor is putting the screws to you. Fiberglass is not a substitute for wire mesh. But it is less expensive for your contractor.

There is a lot of research on this: http://www.cement.org/tech/faq_fibers.asp

The wire mesh will keep the cracks that will form smaller and hopefully less numerous. (You'll still need control joints cut in the surface.) The fiberglass will eliminate a lot of the fine hairline cracks and will give you a better finish. If possible, go for both. They work nicely together and rebar can be used in lieu of wire mesh to give you even more resistance to cracking.

Since your contractor is either pulling a fast one on you or doesn't really know what he's talking about, I'd inspect the job before concrete is placed. As per the link above, the mesh should be elevated in the slab to work. Often, contractors lay it on the ground, where it won't do anything for you. (It's also important to make sure the mesh is elevated in a way that it won't sink when concrete is laid on it. You should also not accept any plan to "pull the mesh up" or lay it down during the pour.)

Shadowdog500 · Oct 25, 2013

Find a better concrete guy!!!

Fiberglass is not a substitute for wire mesh.

A slab that size should have expansion joints.

Is there any way you can tear out the old floor by the lift so the concrete is extra thick there?

Chris

Rookie2 · Oct 25, 2013

I have a 40x60 x 5" and used 6x6 wire on the bottom to fasten my pex to. I used 3500# with fiber then saw cut 14' wide x 15' long pads (roughly)
Saw cut the next day after you seal it. If you saw cut first you'll have to scrub the fines off to seal it.
The mesh isn't too bad because the sealer kinda glazed over it. I did get 1 two ft. crack at the 10' OH door into the floor drain area. I think because I overheated the slab one day.
I think i'm in my 13th year and it looks good.
I did roll the heck out of the fill before pouring concrete, if you don't it will settle.

kbs2244 · Oct 26, 2013

Just look at the physics of it.
The wire mesh reinforces multiple square yards.
The fiber reinforces, at most, square inches.

How big is your slab?

brownbagg · Oct 26, 2013

fiber mesh does replace wire mesh

Daniel Dudley · Oct 26, 2013

brownbagg said:
fiber mesh does replace wire mesh

If you ever took a jackhammer to wire mesh reinforced concrete, you would know that there is no comparison. If you wind up having cracks or heaving, wire holds the slab together. Fibers will not, as in a crack they are broken.

In a perfect world, fibers will do. In my world, slabs get rebar or wire. Sometimes both, if I have a specific need. You might never see the difference, or you might see a significant difference after 20 years.

In any case, you will need a well compacted base to pour on. Especially with fiber.

KnurledNut · Oct 26, 2013

bans25 said:
Slab will be ~30x30. The contractor has in contract:
Supply concrete and labor to pour 4" slab with 4000 LB Mix with Wire Mesh and Poly, Flat smooth finish, Pin new slab to existing slab with rebar.

I have concrete flatwork experience and have poured and finished both. If you use the fiber, you will likely pick up on the difference in texture (against your old slab) depending on how they finish. It is more difficult to pull/float. If you go this route, i would recommend a 6'x6' gridwork of #4 bar with the fiber.
And definitely have them cut control joints. I would do every 10', which on your 30x30 would look like a tic-tac-toe grid.

rburke65 · Oct 26, 2013

I believe that a well compacted base is very important to having a quality floor. I did wire mesh in mine and if you wanted to cover yourself you could do both! I would not wait till the following day to cut the slab. Apply the curing compound, snap the chalk lines, and cut that puppy as soon as you can walk on it. I think your contractor is "tricking" you....The saw cut depth should be 1/4 of the slab thickness, and you can do cuts approx. every 10'. That would give you two cuts in each direction.

jhochst · Oct 26, 2013

I had a 30x48 slab poured 1 year ago. My floor is 5" with rebar. still perfect. Great base and control joints were cut 10x8

Bruce4310TX · Oct 27, 2013

wssix99 said:
Your contractor is putting the screws to you. Fiberglass is not a substitute for wire mesh. But it is less expensive for your contractor. There is a lot of research on this: http://www.cement.org/tech/faq_fibers.asp

The wire mesh will keep the cracks that will form smaller and hopefully less numerous. (You'll still need control joints cut in the surface.) The fiberglass will eliminate a lot of the fine hairline cracks and will give you a better finish. If possible, go for both. They work nicely together and rebar can be used in lieu of wire mesh to give you even more resistance to cracking.

Since your contractor is either pulling a fast one on you or doesn't really know what he's talking about, I'd inspect the job before concrete is placed. As per the link above, the mesh should be elevated in the slab to work. Often, contractors lay it on the ground, where it won't do anything for you. (It's also important to make sure the mesh is elevated in a way that it won't sink when concrete is laid on it. You should also not accept any plan to "pull the mesh up" or lay it down during the pour.)

Correct glass gives absolutley Zero strength to your slab complete waste of money use mesh at the minimum but save the money on glass and spend it on re-bar you wont regret it. It only saves your contractor time and puts more $$$ in his pocket.........its not intended to add structural strength to your slab.

Bruce4310TX · Oct 27, 2013

brownbagg said:
fiber mesh does replace wire mesh

Only for the uninformed consumer and a contractor who you should not use if he pushes fiber instead of wire or preferably re-bar.

Dick in Wisconsin · Oct 27, 2013

Keithinsc said:
Had fiber in my old shop. We used a power trowel and after it was finish troweled, we let it sit for about 2 hrs.
We then came badck with the power trowel set as steep as the blades would go and 'shaved' the fibers off the surface.
Any that remained, I hit with a torch about 2 weeks later.
My current shop has "Buckeye" brand reinforcement fibers. Tiny, about 1/8 long but the diameter of a hair. Supposed to be just as strong. The masons love it because they don't even know it is in the mix. Finished floor is glass smooth.

Just googled up this facts page;
http://www.ultrafiber500.com/faq_index.html

edit; when the truck with the Buckeye fiber showed up, you could not tell it was in the mix. Got in an argument with the truck driver that I was getting ripped off! I had to take a sample of the concrete and mix it with water in a bucket, then I was able to see the tiny fibers.

How much as the upcharge for the Buckeye fiber? how big was the slab? Trying to get an approx. cost per yard of concrete.

ZRX61 · Oct 27, 2013

Well I'm glad we cleared that up.......

bcradio · Oct 27, 2013

I would not use rebar on a slab less than 6" thick as that will be the spot a crack is likely to occur. I would use mesh on this as mentioned above or go thicker on the concrete (5"+ slab) and use fiber. My personal preference is to use the thicker slab approach and possible mesh as well if you like. My rule of thumb is 4" if no cars will be on it otherwise 5" - 6" minimum.

Also, I need to mention that crack control joints and expansion joints are a must. Do not cut them 1/4 thickness of the whole slab though unless you will be filling them with the expansion joint paper/fiber pieces. For crack control joints, 1/4" to 1/2" depth will do fine.

Keithinsc · Oct 27, 2013

The Buckeye was $5 a yard extra. I think it was $100 a yard, 40+ yds, a 30x60 with footers. About 3 yrs ago.

RbrtAWhyt · Oct 27, 2013

The garage in my first house had fiberglass fibers in it. It always looked there was cat or dog hair on floor in someplaces. You could lead down to sweep it away and see that it was embedded in the concrete. May be an issue if you are wanting a fine finish on the floor such as epoxy or such. If you dont plan on doing any finish work to it, and you're not an OCD clean freak it will be fine.

wssix99 · Oct 28, 2013

ZRX61 said:
Well I'm glad we cleared that up.......

That Buckeye Fiber link is misleading.

Mesh does not stop cracking. It restrains them and makes them smaller. Control joints are still required with wire mesh or rebar, where these cracks will occur. Cracks can occur anywhere, but the tighter the spacing and the more cross sectional steel in the slab, (wire mesh or rebar) the higher the likelihood that they will occur in the control joints.

Fiber mesh doesn't stop or control this cracking either. Control joints are still needed and wire mesh can still do good. The Buckeye FAQ is consistent with this, but it doesn't give all the information and its clear they are selling something...

bans25 · Oct 28, 2013

I had a talk with the contractor, I think I am back on track to contract. He will use wire mesh on chairs. I decided not to go with the fiber mix in the concrete.

Thanks much for all your help!

Now I need to figure out why email notification is not working when I get replies to the posts!

ozyborn · Oct 28, 2013

Poured my slab inside my garage, 5 " with 4500 psi mix, rebar and the fiberglass added. Heated shop. Since this was entirely indoors no control cuts needed. 10 years later not a single issue. I say always use wire mesh or rebar. No fly ash in the mix either.

houseostyle · Oct 28, 2013

ozyborn said:
Poured my slab inside my garage, 5 " with 4500 psi mix, rebar and the fiberglass added. Heated shop. Since this was entirely indoors no control cuts needed. 10 years later not a single issue. I say always use wire mesh or rebar. No fly ash in the mix either.

Did you pour over an existing slab?

I have a sunken room in my house that I want to bring all to one level. Its on an external wall and measures 15' x 12' x 6" I need to pour concrete OVER an existing concrete slab.

I have a few quotes and a few differences in opinions on how to get it done.

Contractor 1: Wants to lay down Styrofoam blocks and pour in lightweight concrete. No rebar - no wire mesh.

Contractor 2: Wants to pour in fiberglass mesh concrete only. No rebar - no wire mesh.

Any advice? I am laying tile down over in a few weeks after it is poured.

wssix99 · Oct 29, 2013

ozyborn said:
Since this was entirely indoors no control cuts needed.

Control cuts have no bearing on if the slab is indoors, or not. All concrete shrinks and cracks - no matter where it is. If you have enough rebar, however - that may keep you from needing the cuts. (PCA calls out a 0.5% Steel Area Ratio as the point where control cuts can be skipped.)

houseostyle said:
I have a sunken room in my house that I want to bring all to one level. Its on an external wall and measures 15' x 12' x 6" I need to pour concrete OVER an existing concrete slab.

The general rule is that a slab is only as good as the foundation. If you are sinking, you have an issue underneath. The only way to "fix" this is to fix the foundational problem.

houseostyle said:
Contractor 1: Wants to lay down Styrofoam blocks and pour in lightweight concrete. No rebar - no wire mesh.

If you don't want to tear up the slab and re-do the foundation, this is probably a good way to go. Keep the topping/leveling course light as to not put more weight on the old slab and trigger more sinking.

houseostyle said:
Contractor 2: Wants to pour in fiberglass mesh concrete only. No rebar - no wire mesh.

This is probably not a good idea due to the weight.

Trey T · Oct 29, 2013

You can never have fiber by itself because you can never guaranteed that the fibers are overlapped to create a continuous flexure strength in any direction. The fibers are there to minimize the cracking but it WILL NEVER BE A REPLACEMENT FOR REINFORCEMENT BARS.

As long as you are using portland cement and wanting flexure, you will need steel rebars. fibers are strong but brittle and it will not be able to replicate the flexure strength model of a reinforced concrete.

houseostyle · Oct 29, 2013

The room was designed to be a sunken room and is currently raised up with plywood flooring - no foundation issues at all ( its low like a garage floor - sorry I don't know the proper terminology for this ) see pictures of what I mean:

wssix99 · Oct 30, 2013

^ probably doesn't matter then. I'd definitely put in wire mesh to make sure wide cracks do not develop. (They will go right through and will either crack the tile or grout joints.)

Concrete slab questions.. fiberglass mix or no mix with wire mesh

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