News Room

Let’s Talk about defining industry terms and a little about Fiber Reinforced Concrete (FRC) testing.

There are some terms being used when we are defining synthetic and/or steel fibers as well as their properties and applications that create some confusion for those that may be considering using FRC.

One of the terms that seem to find frequent use is…synthetic fibers replace reinforcing steel. As a standalone phrase one could question the limits of the statement. If read loosely it would seem to say that synthetic fibers will replace both structural/primary steel and secondary/temperature-shrinkage steel. Furthermore, there are no limits or stipulations that accompany this statement. It is a very general, all-encompassing statement when standing alone. There are no specific provisions attached to the statement as to the type of synthetic fiber, the dosage rate, the length or the configuration.

Nowhere in ACI 318 can one find any reference to the substitution of structural steel with microsynthetic or macrosynthetic fibers. Plainly stated when the area of the steel is used in the structural design of the cement concrete-steel composite element then the synthetic fibers cannot replace the steel in question. None of the standard design equations in ACI 318 can be converted to use ASTM C1399 or C1609 data. The fact remains the single most important application for the synthetic fibers when replacing wire mesh is to hold the self- inflicted cracks together after they form.

Only when the steel in question is used as secondary/temperature-shrinkage reinforcement, typically wire mesh, can the synthetic fibers replace the steel. Here we will define the synthetic fiber as polypropylene, nylon or polyolefin. Other synthetic fiber types or coextruded blends may in the future prove to contribute other properties to the concrete.

There are at least three factors to consider when contemplating the use of synthetic fibers as structural reinforcement. These factors are Modulus of Elasticity, Poisson’s Ratio, and Creep. When viewed next -to-next-to with these same properties of steel the synthetic fibers pale in comparison. Thus the synthetic fibers are not ‘structural synthetic fibers’ as some would promote. ACI, ASTM, and others have agreed to label these new generation synthetic fibers, macrosynthetic fibers.

This relatively new synthetic fiber has become a major market success due to the physical properties enhanced in the cement concrete. The benefits of the macrosynthetic fibers are measured using ASTM Test Methods identified as C1399, C1550, and C1609. Both C1399 and C1609 utilize a concrete beam to measure the engineering properties of the macrosynthetic fiber reinforced concrete post-first crack. Although there are some similarities in the tests there are major differences. The principal differences being that C1609 uses closed-loop loading whereas C1399 uses open-loop loading as does C78, the standard flexural beam test. C1399 uses a steel plate inserted beneath the concrete beam up to and including the point of the first crack. At this point, the steel plate is removed and the beam is reloaded and the test is continued. ASTM C1609 is a continuously run test from initial loading to failure.

It has been determined that there are parameters either found in the testing apparatus or testing sequence that may dramatically affect the results of the test. A number of these parameters have been identified and the Fiber Reinforced Concrete Association is sponsoring a test program that will yield data that will be provided to ASTM Subcommittee C09.42. Hopefully, the Subcommittee will utilize these data in refining the test method thus reducing the standard deviation/coefficient of variation of the test method. The initial phase of the research looked at the supports of the test specimen (beam). This facet of the program proved that freely rotating supports yielded different results (lower) than supports that were restrained by friction typically caused by rust. The second phase of the research will commence shortly.

What will this research mean to the industry and the engineers? There is a very important need for C1609 and later C1399 to be reworked/refined so that the data generated will reflect the true properties of the macrosynthetic fibers, not the properties of the testing equipment and how the test is conducted. ABC Polymer Industries is a strong supporter of this project and has been and will continue to be very selective when choosing a commercial laboratory to conduct our product testing.

– R.C. Zellers, PE/PLS, Director, Engineering Services