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
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
RE: Fibre Mesh v Steel Reinforcement in Slabs
Thanks cvg
murdydrum
chartered engineer
http://www.dennanyreidy.ie
RE: Fibre Mesh v Steel Reinforcement in Slabs
RE: Fibre Mesh v Steel Reinforcement in Slabs
RE: Fibre Mesh v Steel Reinforcement in Slabs
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
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
Benefit from polypropylene fibres is the retention/retardation of bleed water and benefits of this...
Dik
RE: Fibre Mesh v Steel Reinforcement in Slabs
RE: Fibre Mesh v Steel Reinforcement in Slabs
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
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Fibre Mesh v Steel Reinforcement in Slabs
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
RE: Fibre Mesh v Steel Reinforcement in Slabs
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
Dik
RE: Fibre Mesh v Steel Reinforcement in Slabs
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
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
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
RE: Fibre Mesh v Steel Reinforcement in Slabs
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
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
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
"We shape our buildings, thereafter they shape us." -WSC
RE: Fibre Mesh v Steel Reinforcement in Slabs
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
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
Dik
RE: Fibre Mesh v Steel Reinforcement in Slabs
Also, spats you are correct...I did not correctly read your post. My apologies.
RE: Fibre Mesh v Steel Reinforcement in Slabs
RE: Fibre Mesh v Steel Reinforcement in Slabs
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
http://www.helixfiber.com/
I wonder if these fibers align with magnetic fields :P
RE: Fibre Mesh v Steel Reinforcement in Slabs
"We shape our buildings, thereafter they shape us." -WSC