Evaluation of fibre distribution in concrete using AC impedance technique

Distribution and orientation of fibres in concrete has a profound effect on the hardened mechanical properties. AC Impedance Spectroscopy (AC-IS) is a useful tool to characterize bulk changes in the electrical properties of materials; thus, its application to fibre reinforced concrete is only natural. In this study, the impedance characteristics of fibre reinforced self compacting concrete and normal concrete were measured using an impedance analyzer. Ultrasonic signals were also collected for the same specimens. Beam specimens were used to create a condition of one-directional flow of concrete during casting. The impedance response was measured in the direction of pouring concrete, in the direction of flow and in the orthogonal direction. The results from the response were presented in terms of the normalized matrix conductivity, the fractional function, the equivalent single fibre representation, and the dispersion factor. All results indicate a preferential alignment of the fibres in the direction of the flow, and in the plane normal to the direction of pouring. As the fibre volume fraction decreased, the degree of orientation in the flow direction reduced, while the dispersion of fibres increased. The reduction of specimen width in the orthogonal direction led to the increase in alignment in the pouring direction. Similar trends were observed for self compacting and normal fibre reinforced concrete. The ultrasonic method was not found to be sensitive enough at low fibre volume fractions (below 1%) to detect significant changes in the amplitude of signals in the direction of flow. The results of impedance spectroscopy were corroborated by the evidence from image analysis and manual counting, which indicated a higher orientation number in the direction of flow. Furthermore, the load carrying capacity and deformability of the composite was also found to be higher in the direction of flow. © 2009 Taylor & Francis Group.