Synthetic Fiber Content Measurement in Concrete Using Neutron Thermalization

The macro-fiber content in concrete cast in the field is critical to achieving the desired toughness properties and performance of the concrete structure. Currently, there exists no standard method to determine the polymer fiber content of hardened concrete. A nuclear density/moisture gauge was used on hardened concrete pavements with and without polymer fibers. Polymeric fibers are composed primarily of carbon and hydrogen. Since these elements are substantial neutron thermalizers, the neutron count detector on the nuclear gauge is sensitive to changes in the fiber content. The effects of hydrogen and carbon present in the unreinforced concrete itself were subtracted out by taking readings on specimens without fibers. Polymer fiber volume had a clear and statistically significant effect on the neutron readings. This effect was found to be linear and the polymer fiber volume of hardened concrete could be quickly and accurately be determined. The proposed non-destructive and in-situ test procedure can provide useful information for engineers conducting forensic failures and provide a method of quality assurance for determining the as-built polymeric fiber content of fiber reinforced concrete structures.