FAQs

In the marketplace one hears a lot of numbers being thrown around as to the proper or appropriate quantity of synthetic fiber to introduce as secondary reinforcement in the concrete.

There is a tendency on the part of some synthetic fiber marketing groups to play the game of 'How Low Can You Go'. In this rather interesting marketing game the players, using the rules to the old TV game show "Name that Tune" attempt to under bid the competition as to the quantity of their fiber required to perform as secondary reinforcement. Now comes the part where one term is substituted for another. Instead of secondary reinforcement the synthetic fiber marketer slips in the term plastic shrinkage reinforcement.

Here is the difference; secondary/temperature-shrinkage reinforcement is what the old wire mesh has been doing for a number of years. Plastic shrinkage reinforcement is a term coined by the synthetic fiber industry to represent what the fibers do in the first eight hours of the concrete's life. Specifically as plastic shrinkage reinforcement the synthetic fibers reduce bleeding and surface plastic shrinkage cracks resulting from rapid evaporation of water from the concrete. Furthermore, during this period of the life cycle of concrete the synthetic fibers reduce plastic settlement, which is associated with the reduction in bleed water.

Nycon has held its position from day one that the minimum quantity of nylon monofilament fiber required to perform as secondary reinforcement is 1.0 pound per cubic yard, and that 1.5 pounds of fibrillated polypropylene fiber is the proper quantity. Nycon also sees 1.0 pound the minimum quantity for monofilament polypropylene.

When 0.5 pound per cubic yard of either monofilament nylon or polypropylene fibers are introduced to the concrete, the use must be limited to plastic shrinkage/plastic settlement reinforcement. At 0.5 pound per cubic yard the fiber count and thus the critical fiber spacing are not sufficient to generate the required FRC results to meet the requirements of ICC ES AC32, Section 4.1.2 (Secondary Reinforcement). At this dosage level the fiber will potentially meet the requirements of ICC ES AC32, Section 4.1.1 (Plastic Shrinkage Reinforcement).

It is important for the engineer to clearly define the purpose of the synthetic fiber in the concrete and the appropriate dosage rate. Then the ready mix producer and the contractor, in concert, must furnish the appropriate fiber at the proper dosage level.