Abstract
Commercially available 'E' glass fibres in the form of strands have been successfully used to reinforce gypsum plaster. Small quantities of fibres (2-7 % by volume) were added to produce glass reinforced gypsum (GRG) composites having much improved strength and fracture toughness. The properties of GRG made by two different fabrication techniques have been studied and are described in this Thesis. The premixing technique utilises the conventional methods of mixing chopped strands and plaster slurry and produces composites with a random three-dimensional orientation of fibres. The upper limit of the fibre addition by this technique was found to be 4 volume %. At this low level of fibre addition, the only significant improvement observed was in the impact strength of the composites. The spray-suction technique developed in this programme of work produces composites with random two-dimensional fibre orientation. The optimum amount of fibre that can be efficiently incorporated by this technique in the plaster matrix was found to be 7 % by volume. Further increase in the glass content increased the porosity of the composite which resulted in a significant deterioration in the mechanical properties. The strength of the composites increased with increase in glass content and length. The maximum strength obtained with 7 volume % fibre addition was of the order of 30-38 MN/m[2] for flexural strength, 15-22 MN/m[2] for tensile, 35-45 MN/m[2] for compressive and 20-45 Nmm/mm[2] for impact strength. Incorporation of glass fibre modifies the tensile failure mode of the gypsum plaster from brittle to quasi-plastic by multiple cracking of the matrix. The behaviour under compression remains similar to that of the unreinforced plaster. For fibre contents of 2 - 3 % by volume the composite fractured in a brittle manner whereas at fibre contents of 3-7 volume %, the composite fractured in a quasi-plastic manner with extensive fibre pull-out. There was very little improvement in the stiffness of the composite at the small fibre volume fractions used, due to the low fibre matrix modular ratio. The modulus of elasticity of the GRG produced in this work was of the order of 16-20 GN/m[2] as compared to the modulus of 14-18 GN/m[2] for the unreinforced matrix. The theoretical estimation of the strength and stiffness of the composite based on simple mixture law, taking into account the length, orientation efficiency factors and porosity effects, was in reasonable agreement with the experimental results.