Studies on mechanics of fibro Reinforced cement Based Composites

Abstract

Fibre reinforced cement-based composites are now well recognized to be a promising new construction material. The use of short discrete fibres is relatively new in spite of the concept of fibre reinforcement being recognized more than fifty years ago. Much of the impetus for developing fibre reinforced cement-based composites was derived from successful development over the past thirty years of fibre and whisker reinforcement to improve the properties of metals, resins, and ceramics.

The present investigation on fibre reinforced cement-based composites is aimed at gaining a better understanding of the mechanical properties of composite material. Analytical approaches have been considered to predict some of the basic characteristics of the composites from the known characteristics of the constituents, and the validity of the same has been examined from the experimental data generated for this investigation.

Chapter 2 provides a review of the literature pertaining to the present investigation. The review presents various analytical and experimental investigations regarding elastic properties, strength and deformation, crack arresting and toughness characteristics, and other related aspects of composites.

In Chapter 3, the scope of the investigation is presented.

Experimental investigation and results are presented in Chapter 4. Cement mortar as matrix reinforced with fibres of distinctly different ductility characteristics formed two systems in this experimental investigation. Test data pertaining to matrix and fibres have also been provided. Uniaxial tension and compression tests are conducted on specimens having randomly distributed and aligned fibres of different aspect ratios in the range of volume fractions of 1 to 4 percent in cement mortar matrix.

Chapter 5 presents analytical equations formulated, based on energy principles, to compute elastic constants, such as Young’s modulus and Rigidity modulus. The effect of random orientation of the fibres is taken into consideration by the introduction of dimensionless factors derived on the basis of energy considerations. Three models have been proposed to predict the Young’s modulus and Rigidity modulus of composites. The experimental data closely agree with the Effective Iso-strain model.

Strength and deformation of fibre reinforced composites are discussed in Chapter 6. Analytical expressions have been proposed to predict the ultimate strength of composites in terms of strength and deformation of constituents. The equations thus obtained can be employed to predict the ultimate strength of composites in both tension and compression. Examination in relation to experimental data of this investigation reveals that an approach wherein the strain at ultimate stress of the matrix is assumed to be equal to that of the composite predicts values closer to the observed values.

Chapter 7 deals with fracture and toughness properties of fibre reinforced composites. A review of analysis of failure mechanism of fibrous composites is presented. The mechanical and geometric characteristics of fibres which lead to the unique failure modes of these composites are discussed. Analytical expressions are derived to predict the cracking/fracture strength of composites both in tension and compression. The agreement with experimental findings is reasonably good. Toughness of the material, which is a measure of its capacity to absorb energy, is expressed in the form of dimensionless factors, for relative comparison of composites. It has been found that the toughness increases with an increase in either volume fraction or aspect ratio of fibres and is not very sensitive to the ductility of the fibres.

Analytical expression has been proposed to predict stress-strain response in compression of composite in Chapter 8. The prediction is based on the stress-strain characteristics of individual constituents. In addition, volume fraction of fibres, bond efficiency, and orientation factors are considered. A close agreement between predicted and measured response has been obtained.

In Chapter 9, the important conclusions of this investigation are reexamined and summarized.