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Q: How Does One Go About Selecting the Correct Microsynthetic or Macrosynthetic Fiber for a Given Application?

A: First we need to look at the application and the specification for secondary reinforcement provided by the engineer. We will define the secondary reinforcement as the non-structural reinforcement or the temperature-shrinkage reinforcement. Another way to define secondary reinforcement is when the cross-sectional area of the steel specified is NOT used in the structural calculations of the concrete slab or concrete element.

The following table provides the basic parameters for selecting the correct synthetic fiber: 

 

Purpose Fiber Category Fiber Dosage

Residential

Plastic Shrinkage Microsynthetic Fibers 0.5 – 1.0 lb/cy

Crack Reinforcement (Typically Monofilament Polypropylene) 

Plastic Settlement Microsynthetic Fibers 0.5 – 1.0 lb/cy

Reinforcement (Typically Monofilament Polypropylene)

Drying Shrinkage Microsynthetic Fibers 1.0 – 1.5 lbs/cy

Crack Reinforcement (Typically Fibrillate Polypropylene)

Impact Resistance, Microsynthetic Fibers 1.0 – 1.5 lbs/cy

Surface Abrasion (Typically Fibrillated

Resistance, Volume Polypropylene)

Change Reduction

Definitions:

Plastic Shrinkage Crack Reinforcement would be appropriate for slabs-on-grade and on elevated deck and thin precast concrete elements where the weather conditions (temperature, wind speed, and humidity) could produce plastic shrinkage cracks. The cause of plastic shrinkage cracking is the rapid loss of moisture from the surface of the concrete element.

Plastic Settlement Crack Reinforcement reduces the cracking directly over rebars by reducing the plastic settlement of the concrete. The 3-dimensional fiber reinforcement network restricts the settlement of the coarse and fine aggregate and cement reducing the displacement of the free water in the concrete.

Drying Shrinkage Cracking Reinforcement provides for the cracking that occurs with volume change in the hardened concrete at the moisture leaves the concrete as it ages. The three-dimensional reinforcement provided by the fibers modifies the stress distribution resulting from this volume change thus producing micro cracking versus macro cracking, which is common when WWF is used. There is also a companion reduction in the volume change.  

 

Commercial/Industrial/Warehouse/Special

Ultra-Thin Microsynthetic Fibers 3.0 lbs/cy per

White topping (Typically Fibrillated FHWA specification Polypropylene)

Plastic Shrinkage/ Macrosynthetic Fibers 3.0 lbs/cy Minimum

Plastic Shrinkage/

Drying Shrinkage

Reinforcement

Impact Resistance/ Macrosynthetic Fibers 3.0 lbs/cy Minimum

Surface Abrasion

Resistance/Volume

Change Reduction

 Post-First Crack Macrosynthetic Fiber 3.0 lbs/cy Minimum

Reinforcement (Typical range 3.0 to ASTM C1399/7.0 lbs/cy)

C1550/C1609