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Fibre Mesh v Steel Reinforcement in Slabs
5

Fibre Mesh v Steel Reinforcement in Slabs

Fibre Mesh v Steel Reinforcement in Slabs

(OP)
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

RE: Fibre Mesh v Steel Reinforcement in Slabs

(OP)
Ah, I see this has been covered before.

Thanks cvg

murdydrum
chartered engineer
http://www.dennanyreidy.ie

RE: Fibre Mesh v Steel Reinforcement in Slabs

I think the fiber works decent to control plastic shrinkage cracks and after that it is useless.

RE: Fibre Mesh v Steel Reinforcement in Slabs

I think plastic fibre in concrete is useless, period. Plastic shrinkage cracking should be controlled by proper placement and finishing procedures.

RE: Fibre Mesh v Steel Reinforcement in Slabs

The primary methods to prevent early age cracking are concrete quality, finishing, and curing. The primary method to control longer term cracking is subgrade preparation. For heavily loaded slabs, increased thickness and discrete reinforcement are the best controls.

The use of fibers is limited to non-structural purposes, with few exceptions. This is primarily because the distribution and orientation of fibers is not assured. Distributed fibers help reduce crack width by encouraging development of multiple, small cracks, or in some cases, helping to improve the properties of concrete which reduce cracking. Once cracks start to grow, they are controlled by the modulus and area of fibers and reinforcement crossing the cracks. The modulus of plastic fibers is far below that of steel (1000-1500 ksi for plastic v 29,000 ksi for steel.) The use of discrete (conventional) reinforcement assures that crack width is controlled once they start to grow. In slabs, this is usually relatively less important, but we have been seeing quite a few large companies move back to conventionally reinforced slabs after finding that fiber and unreinforced slabs did not perform well enough. Some slabs where poor performance was seen were well prepared and cracking was the result of loading and/or restraint, and the lack of reinforcement allowed wider crack propagation and/or vertical faulting.

In reinforcing an indoor slab, small, closely-spaced bar or wire will generally control cracks better than larger, widely-spaced reinforcement. Having reinforcement closer to a surface will restrain cracks on that surface better.

State of the practice for design of slabs is provided in ACI 360:
http://www.concrete.org/BookstoreNet/ProductDetail...

Construction practices are covered by ACI Committee 302:
http://www.concrete.org/BookstoreNet/ProductDetail...
This is due for an update, and the committee is now doing that.

RE: Fibre Mesh v Steel Reinforcement in Slabs

hokie66....
TXstructural....

EXACTLY!!

Fiber is an enhancement to the concrete mix....it is not reinforcement
It increases both the flexural (good) and compressive strength (not so much) (steel fiber only...polypropylene is worthless...as noted by hokie66)

I spent about 10 years on the ACI Committee for fiber concrete. It was a constant effort trying to keep the BS out....fiber provides some help, but it does not replace reinforcement and its crack limiting capabilities are limited as well.

The most significant effect that I've seen with fibers (steel fibers, again), is a reasonable increase in the modulus of rupture.

RE: Fibre Mesh v Steel Reinforcement in Slabs

I've heard that snake oil works as a curing compound, too...

Benefit from polypropylene fibres is the retention/retardation of bleed water and benefits of this...

Dik

RE: Fibre Mesh v Steel Reinforcement in Slabs

Steel fibres have a big benefit in improving the toughness/abrasion resistance of slabs, and for this reason are used a lot in industrial applications. Any area where tracked vehicles will operate will benefit from steel fibres.

RE: Fibre Mesh v Steel Reinforcement in Slabs

(OP)
Thanks Folks,

Seems to be a hotly debated topic.

The application I am considering is for a sand & aggregate storage shed (quarry). The floor (on grade) will be subject to loads from plant and materials. My approach is to design a floor slab which is suitable for use and to simplify its construction as much as possible.

Therefore, I am considering a deep slab (200mm / 8") with 1 no. layer A252 mesh (bottom) with fibre mesh reinforcement within the concrete mix. This will allow me to increase control joint spacings and allow for larger pours. The slab will be laid on graded hardcore well compacted in 9" layers. The slab does not need to be a super flat floor nor is the application the interiro of a 5* hotel, with a tiled finish, therefore, it does not need to be over-designed in terms of crack control. Primary objective is functionality & durability.

The viewpoints above are all valid and informative, many thanks.

murdydrum
chartered engineer
http://www.dennanyreidy.ie

RE: Fibre Mesh v Steel Reinforcement in Slabs

The thing I do not llike about fibermesh is the little hairs that remain after the finishing. If there is a finish topping such as carpet or tile to hide them, no worries, but if unfinished, it is extra work to remove them.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Fibre Mesh v Steel Reinforcement in Slabs

(OP)
Mike,
I agree. For a high spec finish, i don't think fibre mesh is suitable. I would adopt small bars at close centres in the top of the slab in this situation together with control joints at suitable locations.

For this application though, the finish is not "aesthetically critical" for want of a better phrase.

Thanks

murdydrum
chartered engineer
http://www.dennanyreidy.ie

RE: Fibre Mesh v Steel Reinforcement in Slabs

Forget fibermesh. To accomplish what you're look for, use STEEL fibers.

RE: Fibre Mesh v Steel Reinforcement in Slabs

Spats:

Oh, after six months I can see the rust on the slab now, let alone all the love sent your way from the local janitor! :)

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Fibre Mesh v Steel Reinforcement in Slabs

Depending on the soil type, an 8" slab for moderate loading should be OK... you can check the design. A good high strength concrete, low slump, sawcut into proper sizes at the right time and curing should all be used. I'd probably use regular reinforcing steel properly chaired and not WWM/WWF. I'd skip the fibres completely, steel and/or polypropylene. The former should be used if there is heavy equipment moving on the slab. In addition to the rust noted by MM, they are really difficult to work with.

Dik

RE: Fibre Mesh v Steel Reinforcement in Slabs

2
I stick by my recommendation to use steel fibers, based on the desire to minimize the need for crack control joints in an industrial slab where they're storing sand and aggregate. The slab will be subject to heavy abuse, both from the product itself, and it's likely that front end loaders will be used... they'll tear up the joints! The use of steel fibers allows larger slab areas to be poured without joints.

For normal industrial floors, I'm all for reinforcing bars (mesh is a joke), but you generally cannot stretch the joint spacing with reinforcing, and it does nothing for durability and abrasion resistance. Steel reinforcing is normally placed in the upper 1/3 of the slab to control cracking, rather than being used for bending strength, so it generally doesn't help you there. Murdy stated his "primary objective is functionality and durability". Steel fibers give you that... improved bending strength, crack control, fewer joints, abrasion resistance, lower maintenance costs, etc., etc..

Msquared... you seem real hung up on the fuzzy/rusty floor thing. Have you ever seen a properly constructed steel fiber floor? The fibers to not stick out. Sure you'll get a little surface rust, but who cares? This is an industrial building that somebody is dumping rock and sand on, and running over with heavy equipment.

Another suggestion to increase strength and limit joints, Is the post-tension the slab. Post-tensioning can also be used to reduce the thickness of the slab.

RE: Fibre Mesh v Steel Reinforcement in Slabs

While I agree with spats's basic premise, I do not agree that the use of fiber will allow no joints. It will allow different spacing of the joints (distance between joints can be a bit more), but it will not preclude the use of joints.

An hokie66 noted, the toughness of the slab will be increased by the use of fibers.

As dik alluded, don't let the fiber addition substitute for sound concrete mix design and placement practices.

RE: Fibre Mesh v Steel Reinforcement in Slabs

As an aside, the next editions of ACI 360 and 302 will have better guidance for slabs, and both should have good coverage on steel fibers.

Ideally, slabs are designed for their particular use, and if structural fibers impart needed properties to the concrete, they should be used. Similarly, fibers are not a replacement for discrete reinforcement which is capable of restraining cracks sufficiently to prevent structural failure, usually faulting or displacement of areas of concrete, or the failure of a slab in transmitting gravity and lateral loads properly to soils or substructures.

RE: Fibre Mesh v Steel Reinforcement in Slabs

Ron, I never said NO JOINTS... read again.

RE: Fibre Mesh v Steel Reinforcement in Slabs

I've only ever used them to provide an 'armoured' surface and partial substitution for reinforcing. Generally the same construction is used for both steel reinforced or regular SOG construction.

Regarding fibres, either polypropylene or steel... try to stick a spade into the mix... I've found them to be really tough to work with...

Dik

RE: Fibre Mesh v Steel Reinforcement in Slabs

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.

RE: Fibre Mesh v Steel Reinforcement in Slabs

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

RE: Fibre Mesh v Steel Reinforcement in Slabs

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

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

RE: Fibre Mesh v Steel Reinforcement in Slabs

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.

RE: Fibre Mesh v Steel Reinforcement in Slabs

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.

RE: Fibre Mesh v Steel Reinforcement in Slabs

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

Dik

RE: Fibre Mesh v Steel Reinforcement in Slabs

Yes, dik....

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

RE: Fibre Mesh v Steel Reinforcement in Slabs

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.

RE: Fibre Mesh v Steel Reinforcement in Slabs

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.

RE: Fibre Mesh v Steel Reinforcement in Slabs

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

http://www.helixfiber.com/

I wonder if these fibers align with magnetic fields :P

RE: Fibre Mesh v Steel Reinforcement in Slabs

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

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