Assessment of fiber alignment through combined driven oscillatory and directional magnetic fields in matrix with similar rheological behavior to cementitious materials

Abstract

In this paper a novel method to improve the effectiveness of the aligning of fibers used as reinforcement in Fiber Reinforced Cementitious Composites (FRCC) through directional homogeneous magnetic fields is proposed. A combination of homogeneous magnetic fields, oscillatory and directional, are assessed as a new method for fiber aligning which increase the aligning index and mechanical efficiency index of the fiber in cementitious composites. Fresh cementitious materials present a Static Yield Stress (T₀^s), which must be overpassed to allow the rotation of the fiber immersed in the matrix. To reduce the T₀^s of the matrix, oscillatory magnetic fields (OMF) are applied firstly, as they generate a vibrational strain of the fiber. Later, fibers are aligned through a directional magnetic field which finds a lower opposition of the rheological torque. The fibers are subjected to an orientation rotation caused by the magnetic field, which is caused by a magnetic torque. This torque is opposed by two torques, one of rheological origin (Yield stress and viscosity) and another of inertial type (geometry and mass distribution of the fiber). The rheological torque is the torque that opposes to the rotation of the fiber and is a function of the rheological properties of the matrix. Two pairs of Helmholt coils have been used in this research, to be able to generate orthogonal magnetic fields to directional magnetic fields. When the OMF was generated, one of the pair of coils were connected to the net, with a frequency of 50 Hz. OMF was performed in 0.64mT-1.25mT-2.18mT. To evaluate its effectiveness each of them was applied in periods of time of 2–5–7–10 seconds before the directional magnetic field of 30 mT. To assess the method steel fibers has been submerged in a metaphor fluid to reproduce the rheological properties of cement materials. The initial and final angles of the batch of fibers have been determined through photography and Computer-aided design. The results obtained shown that the use of OMF increases the alignment index up to 35 % and the mechanical efficiency rate up to 24.35 % in the batches studied. The research followed also showed that previous vibration can be applied during periods of 5 seconds, with the same pair of coils that for aligning but also the use of the two pair of coils can be used simultaneously.