پایداری کامپوزیت های بتنی فیبری طبیعی با میکرو سازه ها و خواص مکانیکی

ABSTRACT Study on durability of natural fibre concrete composites using mechanical strength and microstructural properties

Natural fibres are prospective reinforcing materials and their use until now has been more traditional than technical. They have long served many useful purposes but the application of materials technology for the utilization of natural fibres as the reinforcement in concrete has only taken place in comparatively recent years. The distinctive properties of natural fibre reinforced concretes are improved tensile and bending strength, greater ductility, greater resistance to cracking and hence improved impact strength and toughness. Besides its ability to sustain loads, natural fibre reinforced concrete is also required to be durable. Durability relates to its resistance to deterioration resulting from external causes as well as internal causes (Aziz et al 1984).

Earlier, mechanical characterization and impact behaviour of concrete reinforced with natural fibres were studied (Al-Oraimi and Seibi 1995). Here an experimental study was done using glass and palm tree fibres on high strength concrete. Mechanical strength properties such as compressive, split tensile, flexural strengths and post cracking toughness were studied. It was concluded that natural fibres are comparable with glass fibres. A finite element analysis was also done using ANSYS software. Both the analytical and experimental results were compared and found acceptable. Comparison between theoretical and experimental investigations on the compressive strength and elastic modulus of coir and sisal fibre reinforced concretes for various volume fractions was also carried out (Ramakrishna and Sundararajan 2002). It was observed that both the experimental and analytical values of elastic modulus had shown 15% discrepancy, which can be regarded as comparatively small.

Rheological properties of coir fibre reinforced cement mortar in terms of flow value, cohesion and angle of internal friction were determined for three different mix ratios and four different aspect ratios and fibre contents (Ramakrishna and Sundararajan 2002). Based on the rheological properties of fresh mortar, it was recommended to use shorter fibres with low fibre-content for achieving workability and higher fibre content for better cohesiveness in wet state. Previously sugarcane bagasse fibre reinforced cement composites were studied (Bilba et al 2003). Various bagasse fibre–cement composites were prepared and their influence on various parameters on the setting of the composite materials were studied. Botanical components, thermal and chemical treatment of bagasse fibres were also studied.

The natural fibre composites may undergo a reduction in strength and toughness as a result of weakening of fibres by the combination of alkali attack and mineralization through the migration of hydrogen products to lumens and spaces. Earlier researchers (Toledo Filho et al 2003) reported their study on development of vegetable fibre– mortar composites of improved durability. Several approaches were proposed to improve the durability of vegetable fibre–cement composites. These included carbonation of the matrix in a CO2-rich environment; the immersion of fibres in slurried silica fume prior to incorporation in ordinary Portland cement matrix; partial replacement of ordinary Portland cement by undensified silicafume or blast furnace slag. The performance of modified vegetable fibre–mortar composites was analysed in terms of effects of aging in water, exposure to cycles of wetting and drying and open air weathering on the microstructures and flexural behaviour. It was suggested that immersion of natural fibres in a silica fume slurry before the addition to the cement based composites was found to be an effective means of reducing embrittlement of the composite in the environment. Also early cure composites in a CO2- rich environment and the partial replacement of OPC by undensified silicafume were the efficient approaches in obtaining natural fibres with improved durability. The capability to absorb energy, called toughness, is important in actual service conditions. For that purpose, an experimental investigation was carried out (Ramakrishna and Sundararajan 2005) on impact strength of a few natural fibre reinforced cement mortar slabs. Four types of natural fibres such as coir, sisal, jute and Hibiscus cannebinus with four different fibre contents such as 0·5%, 1·0%, 1·5% and 2·0% by weight of cement were used. The tests were carried out using repeated projectile test apparatus and the performance of specimens was ascertained based on the parameters viz. impact resistance, residual impact ratio, crack resistance ratio and the condition of fibre at ultimate. From this elaborative test results, it was concluded that coir fibres absorb more energy i.e. 253·5 J at 2% fibre content and a fibre length of 40 mm. Microstructure and mechanical properties of waste fibre–cement composites were already studied on 28 days cured specimens (Savastano Jr et al 2005).

At normal 28 days curing ages, some studies have been reported already (Sivaraja and Kandasamy 2007, 2008, 2009). Mechanical strength properties of companion specimens, flexural properties of beams under static and cyclic loading conditions and behaviour of beam-column joints under cyclic loading have been carried out and proved that rural fibres including coir and sugarcane natural fibres exhibit better performance than conventional concrete.

Hence the past research activities on natural fibrous concrete focussed on mechanical strength and microstructural studies at 28 days curing period only. But no elaborative studies have been carried out so far on accelerated curing methods. So this paper focuses on this for getting a rough idea about the life of natural fibres in concrete.