Use of Near-Field Microwave Reflectometry to Evaluate Steel Fiber Distribution in Cement-Based Mortars

Nonuniform distribution of fibers in steel fiber-reinforced cement-based materials can lead to heterogeneous hardened properties directly impacting mechanical properties. Given the highly conductive and strong scattering characteristics of steel fibers at microwave frequencies, near-field microwave reflectometry can offer an effective and robust methodology for evaluating steel fiber content and distribution in these materials. This paper evaluates the steel fiber distribution in fiber-reinforced cement-based mortar (FRCM) using near-field microwave reflectometry. The statistical properties of the microwave reflection experiments conducted at 3 and 10 GHz were correlated with fiber content and distribution in FRCM samples made with various fiber contents ranging from 0 to 3%. Image analysis techniques were also implemented to assess fiber distribution on cut surfaces at different thicknesses. The results of the microwave reflection properties are found to correlate well with fiber density determined from image analysis on hardened samples and fiber homogeneity obtained from freshly cast prism samples. A 3D microwave image of sample with 3% fiber content exhibited relatively high nonuniform image intensity.