Processing and characterisation of cementitious materials reinforced with fibres

Presented in this thesis are the test results of combined processing and mechanical property characterisation studies using a developed cementitious mix reinforced by various fibre types and forms (with short and continuous lengths). The research is aimed to identify new Fibre Reinforced Cementitious (FRC) composites that have post-cracking ductility, much higher flexural strength and higher toughness than the control (matrix) material without reinforcement, and higher than traditional FRC composites. Laboratory work uses two methods to process the green forms, one by novel compression moulding and the other by hand lay-up that were both adapted from the fibre reinforced polymer industry. Results show a reduction in the hand lay-up water/binder ratio of 24 to 41% can be achieved by applying compression moulding with a pressure of 9MPa. One key processing challenge with short recycled milled carbon fibres is to make the mix uniform, even when the volume fraction is low at 2%. Microstructural investigations confirm that the carbon fibres, having mean length of 0.085 mm, always gave a very poor dispersion, and this is due to static electricity causing the fibres to form into balls (5 to 30 mm diameter). Overall, the study with short fibre reinforcements found that, by adding 2% by volume of the polyvinyl alcohol (PVA) fibres, the stress-strain curve exhibits strain-hardening behaviour accompanied by multiple cracking. Furthermore, the flexural properties show the material to possess ductility, toughness and mean strength that, at 13 MPa, is two times higher than the control material. It is observed that the hydrophilic nature of PVA and the fibres surface roughness play a significant role in an increased bonding strength with this short fibre. When introducing continuous fibre reinforcement in the form of fabrics it is shown that the volume fraction of fibres should be no more than 5%. Unsuccessful green form specimens were a consequence of having a higher volume fraction by introducing more fabric layers. Test results show that materials reinforced with carbon fabrics give an FRC material with much improved mechanical properties, in terms of post-cracking strength, strain at peak stress and toughness (energy absorption) at peak stress. Higher overall bond strength might be attributed to an apparent increase in interfacial contact area between fibres and cement matrix and improved mechanical anchoring from the fabric’s construction. Microstructural investigations confirm that good matrix penetrability between the filaments of the tow or bundle is essential in order to maximise the reinforcing efficiency of the fabric. Investigated are two novel methods for modifying the continuous unidirectional carbon fibre reinforcements to improve the overall bond strength, by enhancing matrix penetration through and across the reinforcement plane. In one method the fabric is cut into strips to leave spaces (holes) between parallel reinforcement units for the matrix material to bridge across, while in the second method the fabric receives a surface treatment by immersion in Ethanol alcohol. Test results show that, with compression moulding and the strip form of reinforcement at 5% volume fraction the FRC composite has a flexural strength of 75 MPa. This flexural strength is ten times higher than the measured strength of the control material. The experimental research reported in this thesis shows that to achieve ‘unusual’ composite action and a relative high stress at loss of proportionality requires a continuous fibre reinforcement that can be treated or non-treated. Given the considerable increase in mechanical properties achieved using such fibre reinforcement at 5% the most promising FRC materials require to be further evaluated to find suitable candidates for load bearing products.