Influence of natural fibers characteristics on the interface mechanics with cement based matrices

doi:10.1016/j.compositesb.2017.12.016

Composites Part B: Engineering

Volume 140, 1 May 2018, Pages 183-196

https://doi.org/10.1016/j.compositesb.2017.12.016 Get rights and content

Abstract

The use of Natural Fibers in cementitious composites is an innovative technical solution but, they are characterized by a complex microstructure and significant heterogeneity, which influence their interaction with cementitious matrices, whose identification requires further advances in the current state of knowledge. The present study summarizes the results of a wide series of pull-out tests carried out on sisal, curaua and jute fibers. Then, the experimental results are employed in an inverse identification procedure aimed at unveiling the key features of the aforementioned bond-slip laws. Morphological, chemical, physical and mechanical characterization of the natural fibers were correlated with the resulting bond properties within the embedding matrix. The obtained results in terms of relevant parameters, such as bond strength and fracture energy (under pull-out stresses) of the fiber-matrix interface, pave the way for future studies intended for a better understanding of the structural response of Natural Fiber Reinforced Cementitious Composites.

Introduction

The use of fibers as a dispersed reinforcement in cement-based materials is a common solution for enhancing post-cracking response of cementitious matrices in a class of materials generally referred to as Fiber-Reinforced Cementitious Composites (FRCCs) [1]. In fact, FRCCs are generally characterized by higher tensile and flexural strength, tougher and more ductile post-cracking behavior and superior durability performance [2], as fibers promote a “bridging effect” across the opening cracks and lead to enhanced response in the post-peak branch of stress-strain relationships [3]. This effect is controlled by several relevant parameters, such as fiber geometry [4], shape [5], dosage [6], orientation and distribution within the matrix, mechanical properties of fibers and bond interaction between fiber and cementitious matrix [7].

On the other hand, nowadays, the construction sector is being challenged to “go green” in many aspects including to find more environmental friendly processes and innovation technologies by using biodegradable materials [8,9]. For these reasons, research activities have been recently developed for demonstrating the potential of using natural fibers [10,11], in a class of materials often referred to as Natural Fiber Reinforced Cementitious Composites (N-FRCCs) [12,13]. In fact, N-FRCCs have the potentiality to become the ultimate green construction material option, minimizing the use of natural resources and overall lifetime impact [14]. As a matter of principle, due to many advantages, it's expected that natural fiber-cement based composites will expand their usage in the near coming future. This new trend is motivated by the lower cost of these fibers with respect to the “ordinary” industrial fibers, either made of steel or plastic materials [15].

The natural fibers are derived from vegetable raw materials and are generally characterized by a complex microstructure [[16], [17], [18]] but they present excellent mechanical performance being characterized by tensile strength, generally, higher than 200 MPa [19,20]. For instance, sisal fibers are among the most promising, in fact, investigation about their morphology (in terms of transverse section area and shape) and mechanical properties have been recently developed [21]: these studies demonstrate that the relevant geometric and physical parameters of these fibers are comparable to the ones of various types of industrial fibers [21]. On the other hand, in order to promote the use of natural reinforcement for cement-based construction materials, the interface mechanism occurring between natural fibers and cement-based matrix should be more deeply investigated [[22], [23], [24]].

In this context, the present study is mainly intended at identifying the bond-slip laws capable to describe the interaction of natural fibers embedded within cementitious matrices. First, it summarizes the results of a wide series of pull-out tests carried out on sisal, curauá and jute fibers. Then, the experimental results are employed in an inverse identification procedure aimed at unveiling the key features of the aforementioned bond-slip laws [25]. Morphological, physical, chemical and mechanical characterization of the natural fibers were also performed and these results are correlated with the resulting bond properties within the embedding matrix. Finally, some possible correlations between the main parameters (i.e., bond strength and fracture energy) of the aforementioned bond-slip laws and the fiber type and morphology are figured out.

It is worth highlighting that, despite the intrinsic heterogeneity that is expected to characterize natural fibers (in terms of both geometric and mechanical properties), the present study aims at identifying the parameters of a bond-slip law whose formulation has been chosen with the aim to balance accuracy and simplicity.

Section snippets

Fibers

This study considers three types of natural fibers: sisal, curauá and jute. These fibers are characterized by lengths ranging from 40 cm to 100 cm, depending on the original source (either leaves or stalk) and extraction process.

All fibers underwent a preliminary washing treatment in hot water (80–100 °C) intended at removing any existing residues, such as fats, waxes and mucilage, from their surface. Heating was performed in a still pan filled with tap water warmed up at a gradient of

Morphological characterization of natural fibers

Fig. 2 shows the typical microstructure of natural fibers: they present individual fiber-cells, linked together by means of the middle lamella (ML), which consist of hemicellulose and lignin. Each individual fiber-cell consists of four main parts, namely the primary wall, the thick secondary wall, the tertiary wall and the lumen. The natural fibers, generally, present similar morphology, but they differ from each other in terms of various parameters, such as the internal area of lumens, number

Outline of the analytical model

An analytical model has been formulated for simulating the interaction between fiber and matrix in a pull-out process [25,41]. It assumes that:

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the fiber behaves in a linear elastic way;
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the matrix is supposed to be perfectly stiff;
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the interaction between fiber and matrix is based on a bond-slip law τ-s, invariant throughout the fiber length.

As for the last point, the model assumes the following bilinear bond-slip law (Fig. 11): $τ (s) = {\begin{matrix} k_{e l} \cdot s & i f & | s | \leq s_{e l} \\ τ_{r} - k_{i n} \cdot (s - s_{e l}) & i f & s_{e l} < | s | \leq s_{u} \\ 0 & i f & | s | > s_{u} \end{matrix}$ where k_el = τ

Conclusions

This paper is intended as a contribution towards better understanding the interaction between natural fibers and cement-based matrices, which is the main feature controlling the mechanical behavior of natural fiber-reinforced cementitious composites (N-FRCC). The following main points can be remarked:

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the results of the geometric and mechanical characterization of three types of fibers, obtained from different plants, are outlined, along with the results of a series of pull-out tests intended at

Acknowledgements

The present study is part of the activities carried out by the Authors within the “SUPERCONCRETE” Project (www.superconcrete-h2020.unisa.it) funded by the European Union as part of Horizon 2020 (H2020-MSCA-RISE-2014 n°645704).

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Influence of natural fibers characteristics on the interface mechanics with cement based matrices

Abstract

Introduction

Section snippets

Fibers

Morphological characterization of natural fibers

Outline of the analytical model

Conclusions

Acknowledgements

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