Additional Benefits Derived from Fiber Reinforced Concrete
October 17, 2009
Impact Resistance – There is a test described in ACI 544.2 which uses a modified Proctor Density/Marshall Compaction set-up. A 10-pound hammer dropped 18’’ inches imparts a dynamic load on a steel ball that transfers the load to a concrete disk. This test has been used to show how the 3-dimensional network of fibers distributes this point load to a greater mass of the concrete. The cracking pattern of the fiber reinforced concrete disks is visually different than those of the plain concrete tested. Furthermore, this test shows how the fibers hold the concrete together after it cracks. Anyone who has tried to jack-hammer Fiber Reinforced Concrete (FRC) can attest to its impact resistance.
Surface Abrasion Resistance – The 3-dimensionally distributed fibers provide reinforcement to a concrete surface that resists a wide range of mechanical forces. These mechanical forces include waterborne particulates in a concrete pipe to the frictional forces of a wood pallet being pushed across a warehouse floor. Fibers have demonstrated this enhanced hardness using ASTM C779/C944.
Permeability – Water penetration of concrete is related to the concrete’s density. The fibers have demonstrated their ability to reduce permeability by reducing bleeding thus bleed channels as well as plastic settlement. Additionally, by modifying the plastic shrinkage cracking and drying shrinkage cracking, the fibers create a more uniform concrete resistance to water penetration. There are several tests that can be used to provide evidence of fibers contribution to the reduction of the concrete’s permeability.
Fatigue Strength – The contribution to the long-term durability of concrete has been a very important attribute of fiber reinforcement. The ability of the fibers to increase the service life of concrete exposed to a wide range of loading and unloading stresses has been documented using the standard fatigue tests where a set load is applied multiple times for a predetermined number of cycles. Here the 3-dimensional network of fibers distributes the stress to a greater mass of the concrete such that there is not a centrally focused zone required to resist the loading. The fiber’s configuration, length, and dosage have been shown to be factors in contributing to the results.
Freeze-Thaw – ASTM C666 has been used for years to show that fibers help resist the deteriorating effect of freezing and thawing. By reducing permeability and reducing the fatigue factor of expansion and contraction, fibers have demonstrated their value in exterior concrete.
Non-Corrosive – Synthetic fibers, polypropylene, polyolefin, and nylon are plastics and therefore are non-corrosive. Steel Fibers are non-corrosive as long as they are encapsulated in the concrete. The alkalis of the cement provided the needed protection.
Chemically Inert – The chemical handbooks list polypropylene, polyolefin, and nylon to be chemically inert when exposed to those chemicals that would be found in the components that comprise concrete, including additives and admixtures. Additionally, these plastics are resistant to concentrations of alkalis and acids that would come in contact with concrete.
R.C. Zellers, PE/PLS
Director, Engineering Services