The main aim of this paper is to measure the induced torque needed to rotate a steel fiber, hence an experimental and parametrical analysis of factors governing steel fiber alignment in cement pastes and mortar, rotating from static position and rotating in a dynamic fluid is here presented. To aim this objective, a set of rheological tests has been conducted to assess the torque necessary to rotate steel fibers immersed into different fresh cement paste and mortar mixes with Bingham fluid behaviour. Fibers of different aspect ratios (length/width) and different geometry, straight and hooked-end, have been evaluated as they are the more commonly used. On the other hand, different parameters (type of mixture, size of aggregates, volume fraction of aggregates) affecting cement mixtures are also analysed and their influence in fiber orientation discussed. Fiber alignment depends on external torques applied to fibers, immersed into a cement-water-aggregate viscous system, that can be produced during or after casting. The flowability of the fresh suspension with fibers produces a load/pressure that generates a torque that can align them. Fiber alignment is a main goal to pump the fresh material. Hence, the factors that govern fiber alignment are studied which increase the post-cracking strength of cement-based composites under load along its life service due to casting or pumping. To that end, a set of tests has been conducted to assess the torque necessary to rotate steel fibers immersed into different fresh cement paste and mortar mixes with Bingham fluid behaviour. Fibers of different aspect ratios(length/width) and different geometry (straight and hooked-end) have been evaluated. On the other hand, different parameters (type of mixture, size of aggregates, volume fraction of aggregates) affecting cement mixtures are also analysed and their influence on fiber orientation is discussed. The values obtained here are between 1 and 14 N . mm min of dynamic yield torque and 0.1 and 0.5 N . mm min for viscoplastic torque per fiber, depending on fiber geometry, are helpful to improve the fiber alignment in cement-based composites reinforced with fibers through a design and production based on these parameters.

Special acknowledgment, appreciation and recognition to the IETcc-CSIC collaboration using the Viskomat™ NT rheometer within the framework of the Excellence Project MINECO BIA2013-47876-C2-1P. The authors want also acknowledge the laboratory of Materials of Architecture, Universidad Poiltécnica de Madrid.

Peer reviewed