Saulo Rocha Ferreira

Effect of Sisal Fiber Hornification on the Fiber-Matrix Bonding Characteristics and Bending Behavior of Cement Based Composites

Cycles of wetting and drying can change the microstructure of vegetable fibers through a mechanism known as hornification, which modifies the polymeric structure of the fiber-cells resulting in a higher dimensional stability. In the present work the influence of hornification on the sisal fiber-matrix bond adhesion as well as in the sisal fiber dimensional stability and mechanical behaviour under direct tension was evaluated. Furthermore, cementitious composites reinforced with randomly dispersed hornified sisal fibers were developed and characterized under bending loads. The results show that the tensile strength and strain at failure of the hornified sisal fibers were increased by about 5% and 39%, respectively, whereas the modulus of elasticity was reduced by 9%. The fibers also presented higher dimensional stability with the hornification process. The fiber-matrix bonding was improved and the pull-out resistance of the fibers submitted to ten cycles of wetting and drying was increased by about 40% to 50%. The higher fiber-matrix bond strength contributed to an increase in the ductility and post-cracking behaviour of the composite. The fracture process was characterized by the formation of multiple cracks with the hornified sisal fibers presenting a higher ability to bridge and arrest the cracks.

Influência de ciclos molhagem-secagem em fibras de sisal sobre a aderência com matrizes de cimento Portland

RESUMO Visando a reducao da capacidade de absorcao de agua de fibras ligno-celulosicas, ciclos de molhagem e secagem sao usualmente utilizados na industria de papel e celulose. Esse procedimento enrijece a estrutura polimerica das fibro-celulas (processo conhecido como hornificacao) resultando assim em maior estabilidade dimensional da fibra. No presente estudo foi avaliada a influencia da hornificacao de fibras de sisal no seu comportamento fisico (variacoes dimensionais e absorcao de agua), mecânico (comportamento sob cargas de tracao direta) e microestrutural (modificacoes superficiais da fibra e da estrutura das fibro-celulas). Ensaios de arrancamento da fibra de sisal em matriz de cimento portland foram realizados, utilizando comprimentos de embebimento de 25 mm e 50 mm, com o objetivo de verificar se a possivel estabilidade dimensional decorrente da hornificacao aumentava a adesao fibra-matriz. Os resultados indicaram maior estabilidade dimensional, reducao na capacidade de absorcao de agua, aumento na resistencia a tracao e capacidade de deformacao e reducao no modulo de elasticidade da fibra de sisal com a hornificacao. Acrescimos na carga de arrancamento foram observados indicando uma maior aderencia da fibra hornificada a matriz de cimento. Palavras-chave: Fibras naturais, Sisal, Interface, Resistencia ao arrancamento, Hornificacao

CHARACTERIZATION AND TREATMENT OF SISAL FIBER RESIDUES FOR CEMENT-BASED COMPOSITE APPLICATION

Sisal fiber is an important agricultural product used in the manufacture of ropes, rugs and also as a reinforcement of polymeric or cement-based composites. However, during the fiber production process a large amount of residues is generated which currently have a low potential for commercial use. The aim of this study is to characterize the agricultural residues by the production and improvement of sisal fiber, called field bush and refugo and verify the potentiality of their use in the reinforcement of cement-based composites. The residues were treated with wet-dry cycles and evaluated using tensile testing of fibers, scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. Compatibility with the cement-based matrix was evaluated through the fiber pull-out test and flexural test in composites reinforced with 2 % of sisal residues. The results indicate that the use of treated residue allows the production of composites with good mechanical properties that are superior to the traditional composites reinforced with natural sisal fibers.

Effect of Natural Fiber Hornification on the Fiber Matrix Interface in Cement Based Composite Systems

Several fiber treatments can be applied to mitigate the high water absorption of vegetal fibers. Wetting and drying cycles are usually performed in the industry of paper and cellulose to reduce the volume variation of the natural fibers. This procedure stiffens the polymeric structure of the fiber-cells (process known as hornification) resulting in a higher dimensional stability. The aim of this study is to determine the effect of the hornification on the interface of natural fibers. For this purpose, cycles of wet and drying was applied on Sisal, Curaua and Jute fibers. Fiber pull-out tests were performed in embedment lengths of 25mm. Furthermore, the influence of the hornification in the fibers mechanical (under tensile loading) and microstructural (surface modifications of the fiber and changes in the fiber-cell structure) behavior were investigated. The results indicate changes on the tensile strength and strain capacity of the studied fibers, showing that morphology and chemical composition play an important role on the efficiency rate of hornification.

Effect of Cellulose Nanopulp on Autogenous and Drying Shrinkage of Cement Based Composites

Cellulose based materials, such as natural fibres, when added into a cement based matrix, can greatly affect the properties of the composite in its fresh state, early age and hardened state, including self healing capacity. This is due to their porous structure, hydrophilic character and water retention capability, which are likely to create additional moisture paths inside cement matrices. This paper focuses on the effect of nanoscale addition on the autogeneous and drying shrinkage of cement mortars formulated from HPFRCC mixes. Two other different kinds of cellulose based additions were also studied for comparison, respectively eucalyptus micro fibers and natural sisal fibers, together with “conventional” steel fibres. This study is a part of a larger investigation undertaken by the authors in the framework of the EU-FP7 research project EnCoRe (www.encore-fp7.unisa.it) aimed at assessing the possibility of using cellulose based materials and natural fibers as promoters of self healing processes in advanced cementitious composites.

Cementitious Composites Reinforced with Natural Fibres

Natural fibres as dispersed reinforcement in cement-based materials may represent an interesting alternative to industrial fibres, because of their good mechanical properties and inborn sustainability signature. In this section the mechanical properties of Natural Fibre Reinforced Cementitious Composites (NFRCCs) will be reviewed. Cutting-edge topics will be finally addressed, focusing, on the one hand, on the use of nano-sized cellulose-based constituents in cementitious composites, and, on the other, on the self-healing capacity that natural fibres may bring to cementitious composites. An appendix summarizing the main properties of the natural fibres so far most commonly employed in cementitious composites will complement this information.

Inverse Identification of the Bond-Slip Law for Sisal Fibers in High-Performance Cementitious Matrices

The use of Natural Fibers (NFs) in Fiber-Reinforced Cementitious Composites (FRCCs) is an innovative technical solution, which has been recently employed also in High-Performance FRCCs. However, NFs are generally characterized by complex microstructure and significant heterogeneity, which influence their interaction with cementitious matrices, whose identification requires further advances in the current state of knowledge. This paper presents the results of pull-out tests carried out on sisal fibers embedded in a cementitious mortar. These results are considered for identifying the bond-slip law that describes the interaction between the sisal fibers and the cementitious matrix. A theoretical model, capable of simulating the various stages of a pull-out test, is employed as part of an inverse identification procedure of the bond-slip law. The accuracy of the resulting simulations demonstrates the soundness of the proposed theoretical model for sisal fibers embedded in a cementitious matrix.