Q. How do the uses of steel fibers and synthetic fibers compare/differ as reinforcement for concrete?
Both steel and synthetic fibers can be used as secondary reinforcement in concrete and both will enhance long term durability properties. The principle differences distinguishing steel fibers from synthetic fibers are Modulus of Elasticity and steel fibers contribution to the flexural strength of the concrete.

Physically the major difference between steel and synthetic fibers is the Modulus of Elasticity or resistance to deformation under tensile stress. Steel fibers are deemed high modulus whereas synthetic fibers (nylon and polypropylene) are known as low modulus. As with rebar this means the steel fibers can contribute to structural properties of the concrete composite. In the case of steel fibers used in plain structural concrete this means a contribution to the flexural strength of the concrete.

For the record, the Modulus of Elasticity of steel is 29 million-psi and for nylon and polypropylene the range is 400,000 to 500,000 psi.

Another way to view this important technical property is Poisson's Ration. Here we look at the deformation of a material under tensile loading. Steel will resist deformation much better than nylon or polypropylene. Poisson's Ration can be related to creep, which is the long-term deformation of a material under load. Standard compression and flexural tests at 28-days cannot determine these properties.

Finally, the steel fibers at dosage levels above 40 pounds per cubic year (25-kg/cubic meter) will increase the flexural strength of the concrete. This can be used to reduce the cross-section of slabs-on-ground or increase the load capacity of the originally designed cross-section.

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