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macro sythetic fiber SOG

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structSU10

Structural
Mar 3, 2011
1,062
I am looking into a proposed design using macro synthetic fibers for a slab on grade - it uses the eurocode TR34 to determine flexural strength with the fibers. ACI 360 appears to use a similar procedure based on yield line and such, so the general approach seems acceptable to ACI.

The one issue I run into is ICC-ES reports for the various macro fibers I research; many of them are noted that the macro synthetics cannot replace required structural reinforcement.

I am confused by this as the ACI 360 and TR34 procedures treat it as strengthed plain concrete with an improved modulus of rupture to help reduce slab thickness, but per the ICC ES report it appears the fiber is to be added as more of a fail safe than to be relied upon structurally. Anyone else run into this that may have some insight?
 
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Is this something that goes beneath the slab, over the granular base? I've use geotech fabric for aircraft runways, but not for SOG construction. You may find reference to their use by looking at some specialised pavements.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
these are the fibers embedded in the concrete to create fiber reinforced concrete. there is micro and macro fiber - micro is just for crack control, but ACI and the TR34 allows macro fibers to be considered to improve the modulus of rupture / strength as well.
 
thanks... not a fabric below... know nothing of them as long as they don't introduce a plane of weakness...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
ACI permits the following methods for the design of FRC slabs. The inclusion of the fabric reinforcement for strength is discouraged for using the "thickness design methods". See ACI 360 section 3.3 for the use of other methods.

- PCA, WRI, and COE thickness design methods;
- Elastic method;
- Yield line method;
- Nonlinear finite modeling;
- Combined steel FRC and bar reinforcement.

 
My issue is the fibers themselves - the ICC reports I run across have this statement or similar:

"fiber must not be used to replace any structural reinforcement or of the joints specified in the IBC (ACI 318-14 Sections 24.4 and 14.3.4, respectively)" that is for ICC ES-4282. Others have the same statement which appears to be in conflict with ACI 360.
 
As far as I know, synthetic fibers have not been approved for use for structural purposes, yet. I believe it is in the research phase. I have used them on slabs on ground in warehouses for several years. I typically design the slab as non-reinforced and then add the minimum quantity of macro fibers. My preferred mix is 3 lbs per cubic yard of Tuf-Strand SF. The performance of the slabs has been great. So, basically, I do what you are calling the fail safe method.

I did use them for some non-structural mass walls. The walls were 3' thick. The minimum amount of reinforcing would have been significant. The general contractor asked if the same fibers could be substituted in the walls. We reviewed the design as if the wall were not reinforced and allowed them to substitute at 5 lbs per cubic yard.
 
structSU10,

"fiber must not be used to replace any structural reinforcement or of the joints specified in the IBC (ACI 318-14 Sections 24.4 and 14.3.4, respectively)"

I don't have access to ACI318-14, so I have no clue what are the "joints specified in the IBC" means, can you provide?

Note that ACI360 did not call to "replace" structural reinforcement, but allows the use of fiber reinforcement to, per ACI360 10.3, "reinforce concrete slabs-on-ground to provide increased strain capacity, impact resistance, flexural toughness, fatigue endurance, and tensile strength". Also note on "10.1—Introduction Polymeric and steel fibers have been used in concrete slabs-on-ground for over 30 years to improve concrete’s plastic (early-age) and hardened properties." I don't see there is an approval issue.

You need to go through the entire sections that cover the utilization of fiber reinforcement in SOG, the intended and permissible uses including strength gain (higher flexural tensile stress), and permissible design methods (see 10.3.3. I missed the 10 in yesterday's response).
 
structSU10 said:
...procedures treat it as strengthed plain concrete with an improved modulus of rupture...

I think that you nailed it with that statement. Conventional SOG fibers:

1) Do limit the size of any cracks that develop and improve concrete properties with respect to an unreinforced concrete design.

2) Cannot be viewed as providing a continuous load path for tensile stresses within a concrete member.

I believe that it is #2 that the disclaimer verbiage is intended to speak to.
 
I got ACI318-14.

14.3.4 - Contraction and isolation joints in plain concrete member. Why reinforcing the joints? You need smooth dowels, if necessary.

R24.4.1 Shrinkage and temperature reinforcement is required at right angles to the principal reinforcement to
minimize cracking and to tie the structure together to ensure it is acting as assumed in the design. The provisions of this
section are intended for structural slabs only; they are not intended for slabs-on-ground.
 
OHIOMatt said:
I believe it is in the research phase.
That's rich. They've been researching this since I was in college in the 70's. It's a solution in search for a problem.
 
Synthetic fibers are not reinforcement. They are intended to enhance the properties of concrete, not replace structural reinforcement. For quite a few years I was a member of ACI 544 Committee on Fiber "Reinforced" Concrete. It was during the 80's when research was early and synthetic fibers were coming into vogue. Steel fibers had been around for quite a while, but glass and polypropylene fibers were starting up. We learned a lot about fiber enhancement and one thing we did learn was that it did not replace structural reinforcement. It does increase the modulus of rupture and the compressive strength. Steel fibers provide more modulus of rupture enhancement than synthetic fibers.

 
I understand they work by improving the bleedwater... and enhance the concrete in that fashion.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
ACI 360-06

10.2—Polymeric fiber reinforcement

Polymeric fibers are used to reinforce concrete against plastic shrinkage and drying shrinkage stresses. Fine monofilament (denier less than 100) or fibrillated polymeric fibers are typically added at low volume addition (LVA) rates of 0.1% or less of concrete volume for plastic shrinkage crack control. Macropolymeric fibers (denier greater than 1000) are typically added at high volume addition (HVA) rates of 0.3 to 1% by volume for drying shrinkage crack control. The length of fibers used for slab-on-ground applications can range between 1/2 to 2.0 in. (13 to 51 mm).

10.2.2 Design principles—The design principles for micropolymeric FRC are the same as those used for unreinforced concrete. Macropolymeric fibers provide increased post-cracking residual strength to concrete slabs-on-ground. The same design principles in Section 10.3.3 can be used for macropolymeric FRC.

10.3—Steel fiber reinforcement Steel fibers are used to reinforce concrete slabs-on-ground to provide increased strain capacity, impact resistance, flexural toughness, fatigue endurance, and tensile strength (ACI 544.4R). Steel fibers are either smooth or deformed. Deformations provide mechanical anchorage in the concrete. The matrix bond and anchorage allows steel fibers to bridge cracks that develop in the hardened state and redistribute the accumulated stress caused by applied loads and shrinkage stresses. The length of steel fibers used for slab-on-ground applications can range between 3/4 to 2-1/2 in. (19 to 64 mm).

10.3.2 Elastic method—Slabs-on-ground are designed and their thickness is selected to prevent cracking due to external loading, as discussed in Chapter 6, with the following modifications. Steel fibers are accounted for by setting the allowable stress equal to the equivalent flexural strength of the composite steel FRC

fb = Re,3/100 × fr

When steel fibers are added at high rates (> 0.5% by volume), the modulus of rupture may be increased. For example, using a Re,3 = 55 and modulus of rupture = 570 psi, the allowable bending stress would be: Fb = 55/100 × modulus of rupture = 0.55 × 570 psi = 314 psi

This would be compared to a unreinforced slab that would have an allowable flexural strength of 0.50 × 570 psi = 285 psi.


10.3.3.5 Steel fibers combined with bar reinforcement— Serviceability requirements often control over strength considerations in fluid-tight slabs-on-grade. ACI 544.4R quantifies the effect of steel fibers in conjunction with bar reinforcement on serviceability. Equations are presented to estimate the reduction in reinforcing bar stress due to the presence of steel fibers. Such reductions are helpful in meeting serviceability requirements presented in ACI 318 and 350.

 
On joints.

10.2.3 Joint details

Construction and sawcut contraction joint details and spacing for micropolymeric FRC are the same as those used for unreinforced concrete. Macropolymeric fibers at quantities between 0.3 to 1% by volume increase the post-cracking residual strength of the concrete.

This material behavior permits wider sawcut contraction joint spacing; however, load transfer stability at sawn contraction joints should be considered carefully at wider joint spacing.
 
My understanding of the polypropylene type fibres (if this is what is meant) is that they improve the workability of the wet concrete & can influence early drying behaviour and that's about it really.

They are not reinforcement in the normal sense like you'd get with steel fibres which enhance both the strength and stiffness. Basically in structural terms they aren't worth anything in my experience as others have eluded to.

I'm aware of a few contractors round my location that in the long lost past (20 odd years ago) that seemed to put them into driveways as replacements for mesh with the belief it replaced steel mesh. Saw them replace a few driveways due to cracking into random chunks under light residential traffic loading as they also typically undercooked the preparation of the subgrade.

Moral of the story is I don't think these types of fibres add anything meaningful to a mix design that couldn't be achieved via using other appropriate admixtures or through adjustment of the mix properties itself.



 

Maybe WWM doesn't really work, either...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
Thanks for the thoughts. We had received a calculation to eurocode that utilized the fibers and determined a thinner slab due to a fairly large dosage - nearly 8 lbs/yd. The owner asked us to review and comment, and that is where I was unsure of what is allowed by the code. It seems to me it should just be used as a redundancy rather than to thin the construction, at least for IBC structures.
 
Can you provide IBC section that address this issue? Kind of curios that IBC would say something different than ACI has offered. Did you check ACI544.4R?
Here is a short introduction to ACI544.4R-18:

" New developments in materials technology and the addition of field experience to the engineering knowledge base have expanded the applications of fiber-reinforced concrete (FRC). Fibers are made with different materials and can provide different levels of tensile/flexural capacity for a concrete section, depending on the type, dosage, and geometry. This guide provides practicing engineers with simple, yet appropriate, design guidelines for FRC in structural and nonstructural applications. Standard tests are used for characterizing the performance of FRC and the results are used for design purposes, including flexure, shear, and crack-width control. Specific applications of fiber reinforcement have been discussed in this document, including slabs-on-ground, composite slabs-on-metal decks, pile-supported ground slabs, precast units, shotcrete, and hybrid reinforcement (reinforcing bar plus fibers)."
 
I was under the impression that, rightly or wrongly, grade supported slabs still did not fall under the purview of North American building codes.
 
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