Concrete slab question.

[Boxman]

I'm getting ready to have a new building put up for my woodworking shop. The building will be a 24'x48' pole barn type of construction. One of the cement contractors that I've talked to suggested that the slab be reinforced with fibermesh instead of wire of rebar. Does anyone here have any experience or advice on fibermesh vs the other options?

Boxman

 

[beenthere]

I assume you mean fiber in the concrete, vs steel mesh and/or reinforcing rod in the concrete.

I would do both, have the fiber added as well as have the steel 6" mesh wire (or re-rod per the following suggestion). Each contribute their own strength to the concrete floor. And be sure to have plastic vapor barrier under the concrete (IMO).

 

[gray]

Boxman.

I like the fiber alot. I also believe in rerod at 4 to 6 ft intervals. Cut or form in control joints at 16 to 20 ft. cross centers. The remesh is good but if you are using fiber and rod you will not need the mesh. Also the fiber will produce a harder more scare resistant surface finish.

I have a wood and consruction shop of 40 by 80 in a near monolithic slab (64 cy) that has a few small surface cracks that may not have shown up if I had sut control joints into it. I put mesh in it 12years ago when it was built but no fiber at that time.

Fiber with some rerod is my choice now. Very little mesh use for me. Place poly under the slab as well. Hope you are doing at least a 10 ft ceiling. 12 would be better.

BTW- I teach woodworking and cabinet building in HS and have been a licensed contractor for 25yrs. This is not to say I know what i am talking about, it just means I have experience with this stuff.

Good luck.

 

[Fungus]

Fibermesh does not replace rebar although it can be used instead of wire mesh. Both fibermesh and wire are used primarily to control shrinkage cracking at the surface while the concrete is curing. This type of cracking results from uneven curing of the concrete where the top dries out faster than the bottom.

Besides shrinkage cracks (a surface condition) concrete can crack completely through due to settlement or improper base preparation. In this case something is needed to keep the concrete together to prevent the crack from widening. Fibermesh provides no help here. If you don’t have any steel installed the crack can grow and/or shift.

A lot of slabs have been poured using only wire. You must get it positioned in the top 1/3 to help control shrinkage cracking. If the concrete cracks it helps keep it together but has limited ability here. Due to the small wire size it easily rusts once exposed and breaks. IMHO most wire ends up at the bottom and does no good.

Rebar is used in a slab to keep it together when it cracks. Due to its size it has the strength and durability to keep the crack from widening and/or shifting. Control joints are often cut into the slab to force or “control” where the cracks occur.

My preference is a rebar grid installed on chairs (<font color="blue">see here</font>). Shrinkage cracking can be controlled through proper curing – wet cure or curing membrane. If you are concerned about shrinkage cracking or want the tougher finish add fibermesh.

 

[jmc]

<font color="blue"> "My preference is a rebar grid installed on chairs"

<font color="black"> That job looks too nice to bury in concrete!

 

[Boxman]

Thank you guys for your responses. I'm always impressed with the depth of knowledge and experience available here.

beenthere: You interpreted my question correctly and since I live in the Pacific Northwest a vapor barrier is a definite must.


gray: I'll have 2 control joints across the width of the building. The ceiling will be 13' above the slab...I work with a lot of 10' rough lumber and I wanted to be able to spin it and still clear my dust collection ductwork and my lighting. I don't think that any of us ever become "experts" at woodworking, we just gain experience.... HS woodshop class is where I got the bug for woodworking many years ago, I'm sure that you've inspired many former students in the same way....

Fungus: Your explaination makes everything crystal clear (the photos helped a lot too) I'll take your suggestion of fibermesh and a rebar grid on chairs.

jmc: I agree with you. It sure is nice to see such attention to detail in an area that you'll hopefully never see again....

Boxman

 

[slowzuki]

I have some comments too, have been planning a slab for a few months now, dredging up info.

Concrete always shrinks when it cures, about 1/4" over 16 ft if I can remember right. (or is it over 32'? oh well you get the idea)

Concrete contracts when cold, expands when warm. From below freezing to hot summer day this is about a 1/8" over 16 ft change.

You can control where the cracks will form by combinations of sawcuts(or zipstrips), welded wire mesh, fiber reinforcing or rebar.

Only welded wire mesh and rebar can keep the concrete locked together across crack.

A W1.4 6" square wire mesh can keep cracking small up to about 16' square sections.

Saw cuts (or zip strips) at this point can relieve stress and force it to crack there.

Our slab is designed to use welded mesh and sawcuts to control the cracking and movement. I could use fiber too but I have been told it can take a long time to wear off the surface in low-traffic areas. I will be laying on the floor working and it will be a bare floor surface in our apartment so I can't be having splinters.

The engineer involved in our project suggested using rebar instead of mesh but the mesh gives a nice grid to tie our PEX heat tubing to.

 

[gray]

In the past few years the only time I have used remesh #10-6 has been to tie off heating tubing to it. It will help with cracks but is not all that structual.

The fiber in the mix has never been a surface finish problem. I always use cement sealer as soon as I can walk on the pour without leaving marks. It helps the cure and protects the surface from water and dusting.

I use fiber/ rerod #4 or 5/ and cut the slab at 16 to 24 ft squares. Also, if it is a heated area I go to 24 ft squares but if it is unheated I use 16ft squares. The remesh is only used for heating tubing. Always use a vapor barrier under the slab and in most areas it is code.

I have 12 local contractors who compete agianst me that are former students and hundreds that are in the construction field. I am happy for them all. We still talk often and will share any info I have with them. I may lose some jobs but at least I feel they may be done well.

Not enough cabinet needs locally for full time shop work. Not enough money for custom cabinets either. I have taught many students basic cabinet making that they then went out and found employment in. Happy about that as well.

Lots of big decissions to make in such a business. Good luck and any info I can share- ask. It is in my blood to teach even if I am going to retiire soon.

 

[slowzuki]

Gray,
It is nice to hear someone with obvious good intentions working in your fields. I can tell you care about the quality of work you do.

In our area it can be hard to find a reputable contractor to perform work. Even a reputable one can be poor if he has a fresh crew and is very busy.

/forums/images/graemlins/cool.gif

 

[SPIKER]

Hi All:

I'm planning on doing my slab some day and since there seems to be soem good experiance I like ot post whet I'm planning.
38x50 pole barn with poles down ctr of the 38 (so 2 sides 19x50 with poles in there) Everything is 6x6 poles with 2x6T&G treated side skirt boards 3+ high. Planning on in-floor radiant heat, already have PEX-AL-PEX (2000 feet of 1/2" and 500' of 3/4") planning on using 5" fiber reinforced crete ~5000 psi mix. and steel reinforcing mesh. (this is where I'm not sure. I don't know the best way to go I want the mesh just for tieing up the PEX to but what is best meterial to use. (Also I can get my hands on some used chain link fence in the 8'x3~400 feet lenght range.) would that work better than the 6x6mesh? I know puilling up the mesh or the link would be best but puling th elnik may be near impossable.? also the barn has been up for 3 yrs now and Ive been backfilling using sand/gravel creek wash mix. (I have still a bit of back filling to do. I also have already bought soem "Bubble foil bubble/poly" for under the crete as a vapor barrier and raidaint heat reflector.

should blue board still be used under the poly-bubble-bubble-foil or over the foil? the manufacture claims that this replaces 2" of blue board but some how I'm thinking more is beter... the space is going to be down stairs and upstairs will be liveable space for now, later this will be all shop space and upstairs will be more space or woodworking shop.

downstairs I'm still contemplaiting a in-floor pit for vehical service with a possable underground room for storm shelter & pump room.

also there is a 10x50 overhand/poarch which will be creted in same way with tubing inthere but probably not used. I may someday inclose this and JUST IN CASE I was planning the tube in there. anyhow this currently needs lots more back fill (one corner/end is down 24~30" so slow backfill & time setteling with tractors & rollers going in-out & over it all the time. base it getting very hard. outside base has not even started back filling as I'm storing extra wood & impliments there for now.

Well anyhow thoughts or ideas would be good and thanked.

MarkM