Coal-tar pitch is a promising carbon matrix precursor for carbon-carbon composites. It has a suitable viscosity, high carbon yield, and it forms graphitic structures. In addition, pitch is a relatively cheap raw material. This thesis is a study on the use of coal-tar pitch as carbon matrix precursor in carbon–carbon composites.

Carbon fibre reinforced carbon (C/C) composites are advanced materials which exhibit good performance in high temperature inert environments. These materials are generally produced from carbon fibre reinforced polymers (CFRP) by carbonisation and a long series of densification treatments using chemical vapour deposition (CVD) or polymer impregnation. The carbonisation of CFRP results in a highly porous structure with low strength and stiffness so that the aim of the densification is to fill the voids to improve mechanical properties. It is however known that an understanding of void structure in the composites and its relation to raw materials, processing conditions and mechanical properties is important.

A technique is developed whereby Silicon Carbide whiskers may be incorporated uniformly into epoxy resin using an ethanol transport medium. Volume fractions up to 10% are achieved. The mechanical properties of the whisker loaded resin are studied using uniaxial tensile coupons and compact tension test methods. With increasing fraction of whisker, the tensile modulus, tensile strength and fracture toughness are all found to increase significantly. A method for producing hybrid E-glass fibre/epoxy resin/SiC whisker crossply laminates is developed from an existing frame winding technique. Scanning electron microscopy of polished sections of the resultant laminates confirm that an reasonable distribution of the whisker in the matrix is achieved. Laminates containing 0%, 2% 5% and 10% matrix volume fraction of SiC whisker are fabricated. Tensile coupons taken from the laminates were tested under quasi-static and tension-tension fatigue loading conditions. Under both static and fatigue loading, the residual stiffness and accumulation of cracks in the transverse ply are monitored. It is shown that the presence of whisker delays the onset of matrix cracking and the subsequent multiplication of transverse ply cracks under both static loading and fatigue loading conditions. A model to describe the transverse ply crack spacing-applied stress relationship for each laminate type under static loading conditions is developed. The model is based on a combination of a shear-lag stress analysis with a two parameter Weibull statistical description of the transverse ply strength variation. A computer simulation is developed which simulates the propagation of transverse ply cracks across the width of a laminate based on a fracture mechanics analysis. Using a Paris law expression to describe the growth of individual cracks modified to allow for crack interaction the model predicts the variation in residual stiffness with increasing fatigue cycling.

The experimental performance together with the theoretical predictions of strains and deflections of thin fibre cement corrugated sheets are principally reported in this thesis. Existing theories of tensile behaviour of fibre reinforced cement are reviewed. Asbestos cement and alternative fibre reinforced cement technologies associated with these sheeting materials are discussed. Asbestos cement and polypropylene network reinforced cement corrugated sheeting have been tested under a variety of loading regimes and support conditions. Asbestos-free commercial products have also been tested. Finite element analyses based on the LUSAS system have been used to predict the shape of the uncracked load- deflection curves and deflection and strain profiles of various corrugated sheets with different geometrical profiles. Wetting and drying tests on uncracked asbestos cement and polypropylene network reinforced cement sheets have been carried out and a prediction made using the Lusas program of the warping stresses caused by moisture gradients. A new approach has been developed to predict the load-deflection curves and deflection profiles of semi-cracked and severely cracked polypropylene network reinforced cement sheets under various loading regimes. In this approach, cyclic loading characteristics of the tensile specimens were used to estimate the elastic moduli of the cracked composite for various numbers of cracks at two different load levels. Generally, the theoretical predictions show good agreement with the experimental results for uncracked and cracked sheets and hence the mathematical models could, with confidence, be used to assess the performance of a range of corrugated sheeting with various loading configurations.

Foamed concrete is produced by combining foam and slurry thus entrapping numerous small bubbles of air in the cement paste or mortar. The density of foamed concrete is a function of the volume of foam added to the slurry and the strength decreases with decreasing density. In this study the effect on the properties of foamed concrete, of replacing large volumes of cement with both classified and unclassified fly ash (pfa) was investigated. The casting densities of the materials used in this investigation varied between 1000 and 1500 kg/m3 and 50%, 66.7% and 75% of the cement (by weight) was replaced with pfa. The properties measured included compressive strength, dry density, porosity, permeability, water absorption, drying shrinkage, elastic deformation, creep, and void size distribution. The use of high volumes of unclassified ash did not appear to have any significant detrimental effects on the measured properties of the foamed concrete. Although the rate of gain in strength was reduced by the use of large volumes of ash, up to 67% of the cement can be replaced by ash without any significant reduction in, the long-term strength. The permeability increased with increased air content but the water absorption did not appear to be influenced by the volume of air entrained. The elastic modulus of foamed concrete reduced while the creep increased with reducing density. The drying shrinkage of foamed concrete does not seem to be a function of density and the shrinkage values were similar to those of cement paste with similar water/binder ratios; these values were between two to three times greater than those of conventional concrete. An increase in ash content lead to a slight reduction in the drying shrinkage. A mathematical model was developed relating the compressive strength of the foamed concrete to age, porosity and ash/cement ratio.

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