Steel Fiber STRUCTURAL Slab 1

[LPPE]

I have a 2-way structural slab, columns at 20'-0" on center, light storage loading. The contractor wants to eliminate the rebar in the slab and use steel fibers. He keeps mistakingly calling the 2-way slab a "slab-on-grade". I think this may be where he is mislead.

I do not know much about steel fiber reinforced slabs, except I thought they were used for slabs on grade for industrial applications (for high wheel loads, impact resistance). I did not think it could be used as a replacement for rebar in a structural slab.

Does anyone have any experience in fibers for structural slab? I don't think I would take the rebar out of a conc. beam and replace it with steel fibers, know what I mean?

And if anyone has done this, what is the design procedure for analyzing a steel fiber concrete slab/beam?

 

[Qshake]

I've seen this query before and found it by searching for "fiber reinforced slabs" within this site. The thread is thread507-3127. RON gives some good insight in replying and I would hope he would turn up here to expound further.

 

[TA]

I have had limited past experience of the use of fiber reinforcement. The advice I received from the manufacturers is that it may safely be used as an anti-crack reinforcement, with the added bonus that it increases surface durabiity of the slab to impact and abrasion. I do not believe however that it should be used as a substitute for tension steel.

 

[dlew]

I agree with TA. I have seen steel fibers used only on slab-on-grade to control cracking. I do not believe that their use as reinforcing steel is covered in the ACI Code

AEF

 

[Ron]

I consider fiber reinforcement an enhancement to the mechanical properties of concrete, not a substitute for continuous, deformed bar reinforcement. Fiber, particularly steel fiber, will provide an increase in the compressive strength of the concrete, increase its tensile strength, and help in tensile stress distribution to mitigate some cracking problems.

While the increase in tensile strength (and corresponding flexural strength) from fiber addition is substantial by proportion, it is not adequate to accommodate the flexural stresses typically imposed in unsupported slab applications.

 

[LPPE]

Thank you all for your input. I spoke with a steel fiber manufacturer, and he basically reiterated what Ron said. You may use the fibers for slab-on-grade, slab on metal deck, etc. but not as a structural slab (meaning as a substitute for tension reinforcing). He also said that ACI-318 does not have any provisions for using steel fibers in place of continuous deformed bar reinforcement.

 

[redhead]

Be careful about the use of fibremesh concrete on metal deck. Remember, it is OK for a composite deck, but not if the deck is only a permanent steel form.

 

[JAE]

We recently completed the design of a replacement slab at a loading dock where there was a very high incidence of cracking in the one-way (non-composite) slab. No metal deck was used and the slab (5 1/2" thick) spanned 11 feet across steel beams which were tied to the concrete slab with headed anchors creating a composite beam system.

The first slab system (designed and built in the 1960's) lasted until the mid-80's when it was replaced with essentially the same slab with reinforcing bars. This replacement slab cracked almost immediately due to a change in the loading criteria - much heavier forklifts and a higher loading rate.

We determined that the composite beams created a very high flexibility in the floor such that, when combined with the wheel loadings, initiated cracks. The owner was adamant that the new slab not crack.

Our design included new support columns and beams to reduce both the spans of the existing steel beams as well as the slab itself. This, by itself, eliminated the cracking stresses but we went a step further by specifiying fiber reinforcing in the slab (not to replace the reinforcing bars but to supplement them).

The fiber was a combination of steel and poly material. This was intended to increase the durability of the slab and further reduce crack sizes throughout.

So far, the deck has performed well.

 

[LPPE]

Sounds like a pretty solid design. Steel and poly fibers with rebar. Nice. Another thing I have found in my readings is, as JAE mentioned, fibers are often used in conjunction with flexural steel. Steel fibers may take the place of compression steel. Just another footnote.

 

[bill30206]

At a local ACI chapter meeting, I heard a speaker for a steel mesh company located in Ohio. He showed pictures of slender, tilt-up walls for buildings at his company's manufacturing site. No rebar was put in the walls, and it was simply amazing. The downside is that this practice is not accepted by building authorities, and the project was only permitted as an experiment/demonstration. I can't remember the name of the company. It seems like the contractor you're dealing with is misguided.

 

[austim]

Hi, pylko

I would suggest that there is quite a good analogy between fibre reinforced concrete and fruitcake (which Ron likes, as he has told us before).

In any fruitcake the amount of visible fruit on the cut surface of any slice is quite variable (even though there is usually sufficient fruit to provide the required overall flavour).

Similarly, the proportion of fibres in the tension zone of a suspended slab is basically uncontrolled and will be quite variable. At some sections it may be dangerously low and pose a structural risk (even though the general 'flavour' of crack-resistance is quite OK).

I support all those who say 'don't even think of using it as replacement for properly design structural reinforcement'.

 

[LPPE]

How about a fruitcake reinforced slab?
This is great. From steel fibers to flavor of fruitcake. Excellent.
A followup - After reading all your posts and doing some research, I called the contractor and left a voicemail explaining what conditions we have, what the slab is expected to do, and why steel fiber reinforcement wouldnt work in this situation. Haven't heard a peep from the guy since.