The use of Steel and Polymer Fibers as Concrete Reinforcement by Desi J. Kiss, MS,PE

Concrete by itself is naturally very brittle, lacking ductility when tension or impact loads are imposed.   Traditionally concrete has been successfully reinforced with steel bars and/or welded wire fabric set into the concrete where analysis indicates high tensile stress or high impact loads.   Even well designed bar-reinforced concrete systems will manifest cracking over time.

Today a new concrete reinforcing system is evolving that attains superior crack control and very impressive impact resistance.  The system entails the introduction of randomly distributed, engineered steel fibers into the concrete mix.   This results in a hardened concrete product that offers a much more flexible composite system.    The new system is equally suited to all types of concrete flatwork and to precast wall panels.   Since the concrete mixture is fiber reinforced throughout its section in multiple directions, cracks developing in any specific direction have “nowhere to go.”    Assuming normal design loading, cracks may appear; however, their width and length will be markedly limited.

Steel-fiber-reinforced concrete is a state-of-the-art composite material made of hydraulic cements, fine or fine and coarse aggregate, and a dispersion of discontinuous steel fibers.  It may contain pozzolans and additives commonly used with conventional concrete.  

The addition of the steel fibers to the concrete mix does not significantly increase compressive strength but does increase the compressive strain at ultimate load.   The changes in mixture proportions to accommodate the steel fibers do have a significant influence on the compressive strength.

The steel fibers are manufactured specifically for concrete reinforcement by deformation process in corrugating cold drawn steel wire segments. The material is in accordance with ASTM A 820, Type I, low carbon, cold drawn, deformed steel wire, with a minimum ultimate tensile strength of 120,000 psi (827 Mpa).   The steel fibers come in several sizes.  For example the steel fibers  with a length of 2 inches have an average equivalent diameter of 0.040 inch and average aspect ratio = 50.   In addition, the fibers are corrugated the full length for increased mechanical anchorage.

ACI Committee Report 544, section 3 R specifies that the fibers may be introduced into the concrete at any time at the batch plant or job site. 

The steel fiber is an alternative to conventionally reinforced slabs-on-grade as it provides superior performance as well as elimination of conventional reinforcement, faster placement of slab, lower life cycle costs and reduction in labor costs.

Polymer fibers reinforced concrete

Deterioration and failure of concrete are closely related to the formation of cracks due to load, creep and environmental effects.  Cracks in concrete allow ready access to the reinforcing steel of corrosion-inducing elements such as moisture, chlorides, carbon dioxide and oxygen.   The soils available on many of the project sites become highly corrosive when water saturated. 

Most corrosion of reinforcing steel in concrete is caused by the migration of chloride ions reaching the surface of the steel.   The steel in concrete is protected from corrosion by a combination of the chemical reactions at the steel surface, known as passivation, and the protection from the environment provided by the concrete cover itself.   Thermal and moisture movement in the cement paste produces microcracks prior to loading and is concentrated at the interface of coarse aggregates.    As a result of environmental effects and perhaps premature loading, these microcracks will propagate and eventually group to form significant cracks. 

In addition to the main steel rebar reinforcing, polypropylene fibers can be used as secondary reinforcement to ensure state-of-the-industry systems free of cracks and to decrease permeability and absorption of corrosive elements.  These fibers enhance the durability of concrete through suppression and stabilization of microcracks as well as resistance to the widening of cracks.

The polymer fibers in concrete serve two major functions. The first is to prevent plastic shrinkage cracking. The second is to reduce the segregation of the concrete components.   A balance of mechanical, physical, and durability characteristics determines the effectiveness of the polymer fibers.  Unrestrained plastic and drying shrinkage movements are both significant causes of cracking in newly placed concrete systems.   Plastic shrinkage cracking occurs within a few hours after placement, while drying shrinkage cracks appear in hardened concrete.

When concrete is placed, the aggregate starts to settle and water rises toward the surface. Once the concrete surface has attained some initial rigidity it may not be able to accommodate plastic shrinkage by plastic flow, allowing the development of shrinkage cracks.  This and a number of other physical properties are improved by the addition of the polypropylene fibers to the concrete mix.    The reduction in permeability of the concrete is an attribute of primary importance with regard to the protective measures against corrosion.  The high toughness index, which is the ability of concrete to sustain a load under the initial crack, is very important as it relates to spalling and to a continuing bond to the steel reinforcement.

Polypropylene fibers are used because polypropylene is inert -- these fibers do not degrade and are not affected by the alkalis found in the concrete mixture.  ICBO Report No 4811 specifies that the fibers may be introduced into the concrete at any time at the batch plant or job site. This advantage enhances the efficiency of many construction schedules.   

A rate of 1.5 pounds (per cubic yard of concrete) of 100% virgin polypropylene fibrillated, MD graded fibers can be used in the concrete mix.  The fibers are specified to have a specific gravity of .91 and contain no reprocessed olefin materials.  The mix specifically designed for use as secondary reinforcement can be used throughout in the walls and floor systems.  The rate can be increased to 2.0 pounds per cubic yard for more stringent applications such as: underground tunnels, precast walls or large retaining walls.   

Normal concrete mixing action and mixing time is sufficient for complete fiber distribution. Concrete containing polypropylene fiber addition can be pumped with no difficulty. 

The advantages of using the polypropylene fibers as secondary reinforcement are numerous, and they will ultimately increase the useful, crack-free life span of the structural systems. 

This is an example of the new building environment pressing the construction industry to new levels of value-added performance, resulting in superior solutions. 

Desi J. Kiss, M.S., P.E., is the Principal of DJK Engineering and served as Lecturer at the California State University Fullerton, Civil and Mechanical Engineering Departments.   Mr. Kiss obtained his Master Degree from CSUF and has over 30 years of practical experience.  He can be reached at 949/458-9169. 

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