Specifying Steel Fibers in Slab

[RPMG]

In a past project, we specified a jointless slab by providing a preliminary spec and dosage requirement, then stating that the final requirements would be per the delegated designer, assuming that would be the steel fiber vendor. We eventually got what we intended but the contractor tried to buy the cheapest fibers and pour without submitting anything. So, my question is how do you specify steel fibers and do you use a specific vendor?

 

[BridgeSmith]

As I understand it, it's not only the dosage that's important, but also the mixing when it's batched has to be done in a specific way, so that the fibers get evenly distributed in the concrete matrix. Otherwise, the strength increase cannot be relied upon, because it may not be present throughout concrete section.

 

[Seppe]

Really good question that I’d like to expand upon.

Is it really feasible to delegate the design of the entire slab to a third party? I can see it more clearly with discrete components like prefabricated wood trusses or cold-formed steel “infill” framing, but it’s not as obvious to me how the interaction would work with something like a SOG (or, say, an elevated PT slab).

If there are in fact SOG delegated designers in the marketplace, are they standalone engineering consultants? Or do the fiber producers themselves actually operate as delegated designers as well, going so far as to furnish PE-sealed documents that are specific to the project? It just seems like it could be a breeding ground for conflicting interests if the manufacturer who produces the fiber is also the entity tasked with formally deciding just how much of the material is needed in the design.

In the end, the EOR obviously still owns the overall responsibility of the “integrated” structure, fiber-reinforced SOG included. But it’s the execution and integration of the fiber-reinforced design itself along the way that has always created a pause.

 

[RPMG]

Most slabs on grade are not part of the building structure, not covered by ACI 318, and do not require a stamp. For steel fiber slabs, the vendors submit their product and calculations similar to a concrete mix design. As far as installation, the vendor provides the instructions similar to post-installed anchors.

Just as a public service announcement, any sort of dike, basin, pool, or containment structure, where the concrete provides the containment, the design is supposed to be per ACI 350, which requires additional reinforcement for crack control, and not in the arrangement that you would put for strength. I've encountered this error 4 times in the last decade. It gets overlooked, but it's the engineer's mistake.

 

[Seppe]

So if a SOG doesn’t perform properly, say in the form of joint displacement or excessive cracking that ends up being detrimental to the operations / function of the building space, who is responsible for that? The fiber manufacturer? The architect? Do they know that they are expected to take formal responsibility for the entirety of the concrete slab design? Seal or no seal, where does the lawsuit land?

I am keenly aware that not all SOGs are intended to be structural elements and as such don’t fall under the ACI 318 umbrella. I’d venture to guess that most lawsuits involving SOGs are related to those slabs that DO NOT qualify as ACI 318 structural elements. Still, the buck for the design of the non-structural SOG has to stop somewhere, and I’d be surprised if it’s with the fiber manufacturer.

 

[Heaviside1925]

Speaking from my landscape so may not be applicable but we typically will require the contractor to select a vendor with a performance guarantee, including language that the guarantee must contain certain conditions. Not saying this is always a perfect solution and a contractor may try to substitute anyway but it at least makes the contractor aware when they bid the job, that not just the lowest cost vendor can be selected without further consideration.

 

[BAretired]

The performance of a slab on grade is dependent on more than just the amount of steel fibers in the slab. It is also dependent on the preparation of the subsoil. Steel fiber reinforcement cannot be relied upon to develop a particular moment resistance. If it could, steel fiber reinforcement could be used for suspended slabs, not just SOGs.

 

[rapt]

It also depends on the type of steel fiber. Not all fibers are equal.