Fibre Mesh v Steel Reinforcement in Slabs 5

[murdydrum]

Hi folks,
I'm new to this forum.

What are peoples thoughts (both specifiers & contractors) on the use of fibre mesh or steel reinforcement in concrete slabs.
The applications I am considering is for warehouse / large building floor slabs. Obviously ground bearing (floating) floor slabs.

I find the application of fibre mesh useful for large pours & to minimise the extent of control joints. I would like to hear other peoples opinions on choosing a method of crack control in large slabs.

Thanks


murdydrum
chartered structural engineer

http://www.dennanyreidy.ie

 

[concretemasonry]

When using fibers, the key is the proper mixing time after adding to distribute the fibers uniformly without "balling up"(over mixing).

Steel fibers are a little more difficult to mix if deformed than poly fibers.

We even had to make heavy duty concrete pavers (4x4x8 nominal) with steel fibers for heavy equipment repair and service shops for tacked equipment. Since we needed a zero slump mix, dispersion was difficult and adding water was necessary. The good with adding water was the dispersion was better and the strength increased to 12,000 psi (but not needed), but the dimensional tolerance for proper installation suffered. - A totally different application but the owners specified the products.

For poured in place concrete, mixing time is critical for optimum performance for any fiber application. Canned specs do not always serve the purpose because variable aggregate shape affects the mixing and distribution.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[MJB315]

With steel fibers- what kind of slab-on-grade joint spacing are we talking about?

Say you have a lightly-loaded, 4" slab on grade, that is 100'x100' and you are worried about crack-control and curling. Say there are no penetrations or obstructions on the slab, the sub-grade is well prepared, and you're sitting directly on a vapor retarder. Can you use steel fibers (omitting WWF), pour the entire area at once, AND eliminate any saw-cutting control joints?

We would typically use WWF and put saw cuts in the slab about 15' on-center. The risk usually is curing of the panels at the saw cuts and messing up any movement sensitive floor finishes above. Are steel fibers are cure-all?


"We shape our buildings, thereafter they shape us." -WSC

 

[MJB315]

...The risk is usually curling of the panels...

"We shape our buildings, thereafter they shape us." -WSC

 

[Ron]

Steel fibers are not a "cure all". For a 4" slab on grade, the maximum joint spacing should be 12 feet. If you enhance the mix with steel fiber, you can go to 15 or 16 feet for the spacing. Use the same controls you would for no enhancement....

1. Make sure the subgrade does not vary more than 1/2" in 10 feet and not abrupt changes in flatness...this causes restraint and cracking.
2. Make sure the concrete mix is well controlled and that indiscriminant water addition is not allowed at the site. If any water addition is made, make sure it is done by the supplier, not the concrete finisher.
3. Keep the water-cement ratio as low as practicable...preferably below 0.50.
4. Saw-cut the joints on the same day as placement. Do not wait until the next day....initial cracking will have already occurred.
5. Make sure the saw cuts are deep enough...typically 1/4 of slab thickness is sufficient.

 

[hokie66]

No, steel fibres are not appropriate for that type floor, and are not a cure-all. Stick with your current methods. Steel fibres are appropriate in heavy storage, warehousing, and industrial applications where deterioration of joints is often the main maintenance problem. Steel fibres also improve tremendously the abrasion resistance of slabs in areas like industrial workshops.

For a commercial floor of that size where you really don't want joints, consider a bonded post-tensioned floor.

 

[dik]

Ron... and make sure it's properly cured... Done like a turkey...

Dik

 

[Ron]

Yes, dik....

Also, spats you are correct...I did not correctly read your post. My apologies.

 

[ajk1]

Steel fibres at the appopriate dosage (and no rebar and no mesh) is the way many exposed concrete (no tile or other finish on them) industrial or commercial floors have been built in the last few years. For example, the Home Depot floors in my area are generally of this type (in Home Depot's case they are then ground/polished). Other companies have also gone the steel fibre route. We recently completed a storage warehouse steel fibre reinforced 9" thick slab on grade with shelf stacking, for a large supplier of cement and grout products. The main advantage of the steel fibres is that they provide the slab with post-cracking ductility which allows signifcant reduction in slab thickness. Some suppliers of steel fibres (such as Dramix) will design the floor thickness and steel fibre doasge for you and provide you with a sealed drawing. Of course you should study the design method and satisfy yourself that you understand it. All the other good practice comments by others in this string of responses of course apply, about concreting procedures etc. Just one comment that I would add, is to use the largest size of course aggregate possible because the larger the coarse aggregate the smaller the shrinkage of the mix will be if they design it right. This means that you should use at least a 4.5" thick slab for a mix with 1.5" course aggregate (3x1.5 = 4.5). If the floor is subject to frequent wetting though, I am not so sure about steel fibres because you may have a rusty appearing floor, although stainless steel fibres are available at added cost. I suggest you call one of the steel fibre supplier's engineering departtment and discuss it with them.

 

[hokie66]

ajk1,
You skirted by the question of joints. What joint spacing is used in the Home Depot slabs? In the warehouse which you did? What type joints? Thanks.

 

[jwilki]

These guys go as far as saying it may substitute reinforcement....

http://www.helixfiber.com/

I wonder if these fibers align with magnetic fields

 

[MJB315]

Is there any justification for pouring such a slab (100'x100') without any control joints? I have read / heard feedback from steel fiber reps saying that its possible and that they have had success. What gives?



"We shape our buildings, thereafter they shape us." -WSC