Steel Fibers in Concrete Slab 4

[ludvik]

I have a concrete slab on grade in an electrical substation I am designing. The slab is 9" thick, reinforced with two layers of rebar, and 4000psi grade.

The substation will have heavy equipment rolling across it once in a while - the equipment is heavy, (~5kips) and has small steel rollers. I recently met with my client who expressed a concern that the concrete would spall in time because of the rolling action, and also the small impact loads that apparently occur when these draws of circuit breakers are pulled out. My client said that I should investigate the use of steel fibers to enhance the concrete's durability. He further said that in India this is a common practice.

I have heard of steel fibers being used in roadway pavements, but never in a building. Has anyone heard of this being done in the US? Does anyone have any info on the topic?

 

[Bannu]

Refer to the thread on this topic published earlier.

thread592-23598

Sree

 

[flamby]

ludvik,
Use of fibre just to prevent the surface of concrete from minor damages is, I believe, an overkill. The main purpose of fibres is to provide flexural strength to the concrete, not hardness.
I have used fibre reinforced concrete in lining works of tunnels, but never in buildings. Not even seen anybody doing that, here in India. Perhaps you can use a dense & rich mix with silica fumes, if wear is an issue. Or you can use granite tiles over the slab. I dont know, what is the availability of granite at your location. In our parts, it it very common to have granite floorings and they are wear-proof to quite some extent. If you dont want to use tiles, you can use granite or similarly hard aggregates in the concrete mix.
Consider silica fume concrete and do some testing. I feel this will suffice your needs.

 

[Ron]

ludvik...using steel fibers will achieve the results your client desires, but as "flame" stated, it might be a bit of overkill. There are other ways to achieve hard, durable surfaces such as through mix controls, or by using an integral metallic aggregate topping ("dry shake process&quot to densify and "toughen" the surface, that will be less expensive. Be careful with the use of silica fume, as this can create more "curling" in the slab unless great care is taken.

Check products from Master Builders. They have several that would work.

 

[flamby]

Ron,
Your each advice is invaluable. Can you please elaborate on silica-induced-curling of slabs?
Thanks and regards,
flame

 

[ludvik]

Many thanks Ron, Flame, and Bannu for your responses. It struck me also that steel fibers might be overkill, and I will look into Masterbuilder's products. Flame: A granite surface would probably be very expensive in this location because of both high material and installation costs.

Does anyone have an idea of the cost of steel fiber? I note from reading the other thread that workability will be substantially reduced with the addition of fibers. Will this be a problem given that it is only a slab, and that my reinforcement is on a 12" grid?

 

[Ron]

flame...
Silica fume is a very finely divided particle matrix, acting both as a filler and reactant in the hydration process. The addition of fine particles in a concrete mix usually results in a greater tendency toward shrinkage. When coupled with a normally higher shrinkage potential at the surface of concrete, this exacerbates the tendency of a slab section to "curl" or shrink more on the top than on the bottom.

 

[kieran1]

STEEL FIBRES ARE VERY EXPENSIVE I HAVE USED POLYPROPYLENE FIBRES FOR FLOOR SLABS IN HEAVY INDUSTRIAL UNITS THESE ARE A LOT LESS COSTLY. ALSO HAVE YOU CONSIDERED LOCALLY THICKENING THE SLAB UNDER THE ROLLER POINTS Kieran

 

[ludvik]

kieran: Thanks for the tip about polypropylene fibers. Are these a kind of a standard for slabs with heavy industrial loads? Can you give me more details such as manufacture or a web link?

I would prefer to avoid local thickening if possible because by the time the building contractor is working, the electrical designers will not have finished - they are indecisive and have a difficult job, so i would prefer to give them as much positioning flexibility as possible.

 

[jeg]

Ludvik

I have built several sub stations with moving circuit breaker assemblies and all of them had a steel strip approx 100mm wide (or angle) with fixing tangs, set into the concrete for the rollers to run on. This ensures a hard and level surface which is critical to enable the heavy circuit breakers to be moved by hand.
I appreciate that the electricals have not yet defined the position of their equipment, but I believe you should apply some pressure and tell them to be decisive and fix the locations so that you can design your part of the works.
If you must wait for a final electrical design and have to proceed with the civils, then you could concrete the main floor slab 100mm low and finish later with a light mesh reinforced screed incorporating the runner plates.
Good luck

 

[sven]

Ludvik,
Steel fibre slabs are used in Australia for heavy machinery slabs both in buildings and in the open. Usually, this is done because of the significantly improved tensile strength of the mix, with resistance to wear a bonus. Steel fibres are expensive, here they increase cost by 50% to 100% over plain concrete.
One option is to use it as a "topping" slab, say 4" thick, but this requires steel loop ties between your 2 pours.
Either a topping "shake" or cast-in rails also sound like good ideas.

 

[djbohn]

I agree with Sven completely; we don't consider increased durability from the use of steel fibers. Also, the slab you mention is a very strong slab. An embedded plate or channel, or a shake or topping of emory or mettallic is ideal. Incidently, I don't consider 5 kips that a heavy of a load.
By the way, the majority of damage you will see in a floor slab from moving loads occurs at the joints. We are going to more and more un-jointed slabs with steel fibers.
Lastly, I consider polypropelen fibers to be next to worthless

 

[ludvik]

Thank you jeg and sven for your replies.

Jeg: I would like to have the level of certainty about the electrical equipment that you speak of such that I could specify rails or a slab thickening, however, as I'm sure you can guess, this is easier said than done. We may even have to force the issue for other reasons also, but currently we are attempting to provide the more flexible solution for the slab. Secondly, at what equipment size do you normally need to start putting in rails? We have 13kV switchgear/circuit breakers, and the heaviest possible piece is about 8 kip (but probably less).

The reinforcing steel in my structure is electrically grounded because of the high electric flux surrounding the electrical equipment. It is obviously impossible to ground every steel fiber, so I am intersted now in polypropylene fiber as a substitute. It works in pretty much the same way as steel fiber, and I understand that it is easier and cheaper to install because of its higher workability and more straight forward finishing. Has anyone got any experience with polypropylene fiber that they could share?

Sven: The dry shake products I have come across include a metallic aggregate which again cause a grounding problem. Do you know of a shake that is non-metallic?

 

[LPPE]

Dramix is a steel fiber supplier. Give them a call with your loads and soil information and they are more than happy to help you out.

http://www.bekaert.com/building/dramix1.htm

 

[jeg]

Ludvik

Assuming 8 kips = 8000 pounds (we dont use kips in the uk)= 35600 newtons applied through 4 steel roller type wheels = approx 9000 Newtons per wheel.
There you have a load that will cause local crushing of the concrete surface no matter how well it has been reinforced.

I agree with Sven and DJBohn about fibre reinforcement. Fibre will not improve the hardness of the concrete surface. A good steel power float finish will be more effective.
In this case I would certainly use steel plates.

 

[jeg]

Ludvik

Just a thought on the "dry shake" and other similar products.
I believe these products protect against abrasive wear of the concrete surface and NOT against local crushing. Do any other forum members have better experiences of these products?

Your problem is local crushing due to a heavy load applied by a steel roller to a small area of concrete.
The only certain way of resisting this is a steel runner plate.