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

 

[cvg]

http://www.eng-tips.com/search.cfm?...+mesh&t=&h=&searchSite=3&searchArea=1&d=1&s=1

 

[murdydrum]

Ah, I see this has been covered before.

Thanks cvg


murdydrum
chartered engineer

http://www.dennanyreidy.ie

 

[dcarr82775]

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

 

[hokie66]

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

 

[TXStructural]

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.aspx?ItemID=36010

Construction practices are covered by ACI Committee 302:

http://www.concrete.org/BookstoreNet/ProductDetail.aspx?ItemID=302104

This is due for an update, and the committee is now doing that.

 

[Ron]

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.

 

[dik]

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

 

[hokie66]

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.

 

[murdydrum]

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

 

[msquared48]

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

 

[murdydrum]

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

 

[spats]

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

 

[msquared48]

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

 

[dik]

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

 

[spats]

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.

 

[Ron]

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.

 

[TXStructural]

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.

 

[spats]

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

 

[dik]

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