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Application of impact resonance method for estimation of steel fiber distribution in concrete

Fiber reinforced concrete (FRC) shows improved performance in cracking and shrinkage, as well as high ductility and tensile strength. However, one of the major drawbacks of FRC is determination of fiber volume fraction as well as achievement of uniform distribution of fibers in the concrete mixtures. Non-uniform distribution can cause weak zones with just a few fibers or even none. On the other hand, if fiber content is very high or fibers are too large, during concrete casting they can easily group into fiber balls and form areas only filled with fibers and air. Purpose of this work is experimental investigation of steel fiber distribution in concrete with impact resonance method (IRM) [1] in order to assess casting defects such as fiber segregation, non- uniform distribution of fibers or different kind of cracks. The IRM is based on determination of resonant frequencies of vibration on specimens excited by impact hammer. Experimental tests are performed on steel fiber reinforced concrete (SFRC) prismatic specimens of different concrete mixtures and with cracks known in advance. Additionally, possible internal cracks and other defects in SFRC mixtures are explored. Prior to testing, SFRC prisms are exposed to x- rays in order to validate reliability of test results. By use of discrete Fourier transform, acceleration in time domain is transformed into frequency domain. Further, resonant frequencies are determined as frequency response peaks. From fundamental resonant frequency, dynamic modulus of elasticity of FRC is calculated according to standards ASTM C215 [2] and EN 14146 [3]. Based on the reduction of dynamic modulus of elasticity and change of frequency response curve some conclusions about local damage or defects are made. Smaller defects (e.g. air voids, inhomogeneity) are indicated with change in frequency response curve shape, while larger defects (such as cracks) are indicated with reduction in resonant frequency, and consequently reduced dynamic modulus. Further, with softer impact hammer tip smaller defects and voids can be detected. On the contrary, stiffer tip is more convenient for detection of local failures. These conclusions are verified with x-ray scans of SFRC prisms.

steel fiber distribution, impact resonance method, impact hammer, dynamic modulus of elasticity

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