Bond Strength Improves Polymer Fiber’s Crack Control

At a dosage of one pound per cubic yard of mix, Kuralon RSC reportedly reduces long-term drying shrinkage cracking in concrete by 40 percent or more.

At a dosage of one pound per cubic yard of mix, Kuralon RSC reportedly reduces long-term drying shrinkage cracking in concrete by 40 percent or more. Introduced by Nycon, Inc., and Kuraray America, Inc., the new high-performance, polyvinyl alcohol (PVA) structural-fiber concrete reinforcement Û called Kuralon RSC to denote Îreduction of shrinkage crackingÌ Û is said to offer a 50 percent improvement over other fibers at half the dosage.

The improvement in crack reduction is attributed to PVA fibers’ improved bond strength. According to product developers, molecular bonding between the newly engineered fibers and concrete is 300 percent stronger than that of polypropylene, fiberglass or steel fiber. Both steel and glass have high tensile strength, they add, but low bond strength with concrete and mortars. As concrete dries, these fibers tend to slip out, losing the benefit of their tensile strength, Nycon and Kuraray staff contend. By contrast, they add, PVA fibers form a molecular bond with concrete during hydration; characterized by low elongation and ultra-high bond strength, they are not easily pulled from the concrete.

Also contributing to Kuralon PVA fibers’ reinforcing effectiveness is a high modulus of elasticity. Typically, polypropylene fibers are used to reduce early plastic shrinkage, says Richard McCabe, senior advisor to Kuraray America. After 24 hours, their work is largely done because their modulus of elasticity is less than that of concrete. That means they are ÎstretchierÌ than concrete, where a true structural fiber is as stiff as the concrete it reinforces.

Other PVA fiber characteristics he cites include high resistance to alkali environments, exceeding that of polypropylene or glass fibers; high abrasion resistance; and, a density of 1.3 for improved finishability. The ?-in. (8-mm)-long, chemically stable, resin bundled chopped fibers feature a tensile strength of 203,000 psi (1,400 MPa).

At recommended dosages of one to two pounds per yard, Kuralon RSC fibers reportedly reduce both plastic and long-term drying shrinkage. By thus minimizing water penetration, they provide a safeguard against rusting and expansion of steel reinforcement that can result in permanent damage to concrete from spalling, plus premature deterioration in a freeze-thaw environment.

A replacement for glass-fiber reinforcement, Kuralon PVA fibers are said to be designed for cast-in-place and precast structures, as well as mortar or stucco mixes. Typical applications include new construction and repair of airport aprons and taxiways; security and seismic safeguards in new structures as well as upgrades to existing facilities; industrial and warehouse slabs; tunnel linings; and, septic tanks, vaults, architectural and specialty precast shapes. The fibers come prepackaged in disposable one-pound bags or larger packaging. Whether added at the ready mixed plant or job site, normal mixing time is three minutes.

Ed Dornn, owner of Excel Design and Construction, used PVA fibers for a 10,000-sq.-ft. car wash in Brandon, Manitoba. The PVA fibers were used to produce a durable mix that would hold up under heavy load, exposure to chemicals and moisture, and the extreme freeze/thaw cycles typical of Canadian weather patterns, he reports. We needed a reinforcing fiber that would create a tough surface and prevent delamination Û one that would minimize shrinkage cracking, permeability, and spalling of the slab.

The fibers improved workability, placement, and finishability of the mix, and played an important role in preventing aggregates and fines from settling out. The fines, which typically contribute to shrinkage cracking by rising to the surface and thereby weakening it, were tied up in the matrix. Even with vibration, the mix stayed together, showing no segregation of the aggregates. Û Nycon Inc., 800/456-9266,; or, Kuraray America Inc., 707/486-5631,


Stork/Twin Cities Laboratories of St. Paul, Minn., has established that Kuralon RSC fibers at a dosage of 1 lb. per cu. yd. meet or exceed ICC AC32 requirements. Test results are summarized in the table below.

Flexural 4.83 MPa
(700 psi)
4.83 MPa
(700 psi)
100 Control
Compressive 44.10 MPa
(6,400 psi)
44.15 MPa
(6,400 psi)
100 Control
Freeze-thaw 71.8 71.8 100 Control
Bond Strength 56.90 KN
(12,790 lb.)
66.25 KN
(14,890 lb.)
116 Control
Plastic Shrink 86.4 mm2
(0.025 in.2)
16.1 mm2
(0.134 in.2)
75* Min. 40%*
* Percent reduction in plastic shrinkage cracking compared to control