Cracking mechanisms in durable sisal fiber reinforced cement composites
Introduction
The need for economical, sustainable, safe, and secure shelter is an inherent global problem and numerous challenges remain in order to produce environmentally friendly construction products which are structurally safe and durable. This paper addresses the development of strain hardening cement composites using sisal, a natural fiber with an average tensile strength of 400 MPa and strain at failure of 3% as reinforcement in concrete. These composites provide an exciting opportunity to the housing construction industry and may generate economic incentives particularly in developing countries since the availability and production of composite reinforcement requires a low degree of industrialization. Furthermore, in comparison to the most common synthetic reinforcing fibers, natural fibers require less energy to produce and are the ultimate green products.
Natural fibers have been traditionally used as a substitute of asbestos in the form of chopped, short, and/or in a pulp form for the production of thin elements for roofing and cladding. An increased use of these materials for applications such as cladding, internal, and external partitioning walls is possible and may lead towards the development of low cost-sustainable materials [1], [2], [3], [4], [5], [6], [7], [8]. Natural fiber cement composites have been mainly reinforced by short or pulp cellulose fibers. Nevertheless, their application in the construction industry is still quite limited due to the lack of understanding in how to improve the durability while making ductile materials.
Continuous fiber reinforced cement based composites are a new class of sustainable construction materials with superior tensile strength and ductility [9], [10]. The enhanced strength and ductility is primarily governed by the composite action that exists such that the fibers bridge the matrix cracks and transfer the loads, allowing a distributed microcrack system to develop. These materials are strong enough to be used as load bearing structural members, in applications such as structural panels, impact and blast resistance, repair and retrofit, earthquake remediation, strengthening of unreinforced masonry walls, and beam-column connections [11].
Vegetable fiber cement composites produced with ordinary Portland cement matrices undergo an aging process in humid environments during which they may suffer a reduction in post-cracking strength and toughness. The aging process is due to fiber mineralization and results in reducing the tensile strength of fibers and decreasing the fiber pullout ligament after fracture. This mineralization process is a result of migration of hydration products (mainly Ca(OH)2) to the fiber structure. Efforts to develop durable and structural cement composite laminates reinforced with long sisal fibers has shown much promise recently [12], [13]. A recently developed matrix that lowers Calcium Hydroxide production (only 50% Portland cement as compared to conventional systems) increases the long term durability of natural fiber, reduces CO2 emissions, and presents an economical and sustainable approach. The modified matrix has shown no strength and toughness reduction in accelerated aging tests [13]. The present study is focused on the implementation of this matrix in lieu of a Portland cement matrix which invariably results in a low durability performance record [13].
A fundamental understanding in toughening mechanisms and how cracks form and propagate in the brittle matrix composites is important for optimization, analysis, and design. The cracking mechanism in a multilayer sisal fiber reinforced composite is addressed in this paper. An experimental procedure was used to monitor and analyze the distributed cracking formation, the stiffness degradation, and the crack spacing distribution functions during flexural bending and tensile tests. Photographs of the crack formation at periodic strain levels were processed by image-analysis techniques and correlated with the applied stress levels. The differences between the flexural results and tensile results were studied by comparing the strain distribution profiles in the loading configuration. In the tension test, the strain distribution was verified by the uniform nature of cracking across the specimen, whereas in the flexural test, the movement of the neutral axis was experimentally determined by strain-gages attached to compression and tensile composite surfaces. Analysis of the flexural strains allows a direct comparison between material properties obtained from tensile and flexural data.
Section snippets
The sisal fiber
The sisal fibers used in this investigation were extracted from the sisal plant in a farm located in the city of Valente, state of Bahia – Brazil. The sisal plant leaf is a functionally graded composite structure which is reinforced by three types of fibers: structural, arch, and xylem fibers. The first occurs in the periphery of the leaf providing resistance to tensile loads (see Fig. 1). The others present secondary reinforcement, occurring in the middle of the leaf, as well as, a path for
Mechanical performance
Direct tensile tests were performed in a closed loop servohydraulic testing machine with a capacity of 500 kN (see Fig. 3a). The tests were controlled by the cross-head displacement at a rate of 0.1 mm/min. Six specimens measuring 400 mm × 50 mm × 12 mm (length × width × thickness) were tested using a gage length of 300 mm with fixed–fixed boundary conditions. Aluminum thin sheets were glued on both ends of the specimen and the pressure of the hydraulic grips was adjusted to 1.37 MPa (200 psi) in order to
Mechanical tests
Fig. 5 shows a typical tensile stress strain response of the sisal fiber reinforced composite system. Two measures of tensile strain are used including the localized strain measured from the electrical resistance gage and the nominal strain defined by dividing the cross-head stroke displacement by the specimen length. Comparison of the two strain measurements allows one to evaluate the early age cracking phase and make a distinction among all stages of cracking. These responses differ at
Conclusion
A study on the cracking formation under tensile and bending load of continuous sisal fiber cement composites was presented and mechanisms of crack formation and toughening were addressed. Use of continuous sisal fabrics that are formed by aligning and stitching the fibers in a multilayer cement composite system present a new perspective for the use of natural fiber reinforced composites in the construction industry. A matrix with a low content of Portland cement and Calcium Hydroxide was used
Acknowledgement
The authors acknowledge the Brazilian Agency CNPq for its partial financial support.
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