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Fibers: How They Help Keep Things Together

First, what are synthetic fibers? A bag of fibers contains millions of virgin polypropylene fibers designed to uniformly disperse throughout a concrete mix. They are chemically inert and they have no absorption properties. The action of the fibers are strictly mechanical and therefore they do not affect the mix design and additives in most situations.

When added to concrete, the special packaging dissolves allowing the fibers to mix in with the concrete easily. Synthetic fibers should be added at least 5 to 10 minutes before discharge to ensure equal distribution throughout the mix.

Synthetic fibers provide a secondary reinforcement to both strengthen concrete and increase its durability. It does this by controlling internal cracks caused by gravity and drying conditions. If untreated, these cracks increase the permeability of the concrete allowing salts and other harmful chemicals to take hold. That spells disaster for concrete in the long run.

The first 24 hours after placement of concrete is generally when these cracks occur. Because of moisture loss, the cement hydration process, and temperature changes, concrete dramatically changes in volume. Because most concrete is restrained these volume changes create stress on the concrete and cause cracking. Restraint is caused b the difference in shrinkage between the surface and the underlying concrete area.

Synthetic fibers reduce the amount of cracking of concrete in three ways. One, it provides more strength to resist volume changes. Two, it bonds with the fresh concrete and distributes stress more evenly. Three, it reduces evaporation of water by reducing the bleed rate.

Synthetic fibers are easy to use compared to welded wire fabric due to placement issues. (If placed on the bottom of the slab, wire provides no benefit.) Each help control cracking. Wire fabric helps control cracks after they are formed, while fibers help prevent cracks from forming.

Note: Synthetic fibers are not recommended to increase joint spacing or as a substitute for any reinforcement required by building codes and standards. Adding fibers does not justify changing the mix designs in most situations.

Product information provided by the manufactures of synthetic fibers.

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