Will Synthetic Fibers Replace Reinforcing Steel in the Concrete?
April 19, 2012
Nowhere in ACI’s 318 can one find any reference to the substitution of structural steel with micro or macro synthetic 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 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 (see Chapter 7 in ACI 318) is used as secondary/temperature-shrinkage reinforcement, typically wire mesh, can the synthetic fibers replace the steel. We define the synthetic fiber as polypropylene, nylon or a blend of polypropylene and polyethylene. Other synthetic fiber types or coextruded blends may in the future prove to contribute additional beneficial properties to the concrete.
There are at least three factors to review when considering the use of synthetic fibers or more accurately stated macrosynthetic fibers as structural reinforcement. These factors or engineering properties are Modulus of Elasticity, Poisson’s Ratio and Creep. When viewed next-to these same properties of steel, the macrosynthetic fibers pale in comparison. Thus the macrosynthetic 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 best 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 difference is that C1609 uses closed-loop loading whereas C1399 uses open-loop loading as does C78, the standard flexural beam test. C1399 also requires the use of a steel plate, which is 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 the conclusion of the test.
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 (FRCA) is sponsoring a test program that will produce data provided to ASTM Subcommittee C09.42. The goal is for the Subcommittee to utilize these data in refining the test method thus reducing the standard deviation/coefficient variation of the test method.
There are multiple phases to the test program, which is being conducted at TEC Services in Atlanta, GA.
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.