Framework to predict the orientation of fibers in FRC: A novel philosophy

Abstract

This paper aims at providing a link between fiber orientation and the properties of FRC, the structure to be built and its respective production process. Since the proposed framework is to a large extent new, the main components are described in the beginning. Then, two major subjects are approached from a theoretical perspective: fiber orientation in 3-D and the wall-effects of fibers in anisotropic conditions. Finally, in the last part, the main steps of the proposed framework are analyzed in detail and validated with experimental results. This comprehensive and straightforward model has shown to be capable of estimating the average orientation of fibers with very good accuracy. Thereby, this paper is a meaningful contribution to create an engineering toolbox for the prediction of fiber orientation in practical applications of fiber reinforced concrete.

Introduction

Adding fibers to cement-based materials has been widely studied in the last decades. Since then, the advantages of using fiber reinforced concretes (FRC) became evident and gradually attracted the interest from the construction industry. Consequently, the use of this material increased both in quantity and diversity, with innumerous examples of applications being reported in literature [1], [2], [3]. In the present, the industry is demanding to apply FRC for structural purposes, which has already been shown to be a feasible solution through some successful applications worldwide [1], [2], [3], [4]. However, although the future of FRC is promising from a technical perspective, its generalized application requires a tailored material in order to become economically competitive against traditional solutions. In this context, a cost-contained way to optimize FRC is the application of specific production processes that generate preferential fiber orientations at the locations and directions of the structural element submitted to higher stresses. In other words, fiber orientation should be predictable so that FRC could satisfy both technical and economical restraints more effectively. A framework embracing the major influencing mechanisms of fiber orientation in a rational and design-oriented approach is therefore of paramount importance.

Among the several aspects governing fiber orientation, the wall-effects introduced by the sides of the mold [5], [6], [7], [8], [9], [10], [11], [12], [13] and the fresh-state properties of FRC [14], [15], [16], [17], [18] are most affecting. The way concrete is poured into the mold has also been recognized to have a major impact on fiber orientation, both due to the casting direction and due to preferential fiber alignment induced by the casting element itself [19], [20], [21]. The effects of vibration on causing fibers to rotate into planar orientations have been reported in literature [22], [23] and, in case of self-consolidating concrete (SCC), the flow of the fresh concrete has also been identified to play a major role on fiber alignment [14], [16], [24].

Research on fiber orientation has been carried out mostly from an experimental perspective and the few advanced theoretical approaches refer uniquely to the quantification of wall-effects on idealized isotropic FRC [13]. How to quantify the combined influence of all the aforementioned aspects on fiber orientation is therefore a big question mark.

The average fiber orientation in a certain direction is generally considered through the so-called orientation number (η). This parameter is frequently applied on experimental investigations to quantify the influence of one of the aforementioned aspects on fiber orientation by isolating it from the others. For instance, the effect of the casting direction has been quantified by considering elements poured in different positions while keeping all the remaining aspects constant [19], [21]. Another example consists on inferring the effects of compaction by subtracting from the measured η the theoretical values of idealized isotropic FRC affected by the wall-effects [8].

Both procedures are straightforward and allow gaining insight on the magnitude of the influence that each aspect has on fiber alignment. Hence, up to the present, the parameters governing fiber orientation are approached independently from each other, i.e., from an isolated perspective (Fig. 1). However, considering the nature of these aspects, its determination through such isolated perspective is arguable.

Can preferential fiber alignment induced by the casting direction be quantified by disregarding the type of casting element applied?

Is it reasonable to account for the wall-effects of the mold under an isotropic fiber orientation assumption when anisotropy due to fresh-state properties, casting and compaction processes is likely to occur?

The answer to these questions seems to be obvious. By making use of an isolated procedure, fiber orientation can only be quantified for the very specific characteristics of materials and production processes applied and for the geometry of the element produced. Conclusions extracted with such a procedure are therefore limited and it is hardly possible to extrapolate for different scenarios.

In this paper, the orientation of fibers is analyzed and predicted through a novel philosophy because the properties of FRC, the structure to be built and its respective production process are included through logical and well-structured concepts. Firstly, the outline of the new framework is presented and its core concepts are highlighted. Then, the fiber orientation is evaluated in the spatial domain, both with a theoretical- and a probabilistic and experimental approach, which provides better insight compared to 1-D analysis commonly covered in literature. Subsequently, the wall-effects are studied in detail and a theoretical approach is advanced for its quantification both in isotropic and anisotropic conditions.

The framework proposed in this paper attempts to evaluate fiber orientation from a wider perspective than mere particular cases, including theoretical concepts to explain some of its major aspects and bringing together the combined influence of material properties, production processes and the structure itself. The goal of this paper is not to present a final operational framework, but rather serves, hopefully, to provide a step towards the development of a rational and design-oriented procedure. If fiber orientation could be predicted this way, comprehensive and cost-effective designs of FRC applications could be performed [25].