Use of Carbon Nanofilaments in Producing Cementitious Composites with Improved Mechanical and Durability Performance

摘要

Reinforcement of cementitious materials is a common technique for improving mechanical performance and preventing crack propagation, and is typically applied at the macro-scale, meso-scale (millimetre scale) and/or at the micro-scale using macrofibres and microfibres, respectively. Cementitious material failure is a multi-scale process, however, and also occurs at the nano-scale (10-9 m). The use of nano-additives may therefore be valuable in reinforcing cement hydration products at the nano-scale, bridging nano and micro cracks to prevent initial crack propagation, and refining the pore structure to densify the cement matrix. udThis research focuses on the use of carbon based nano-additives as nano-reinforcement agents in cementitious composites, with the aim of producing novel high performance nanocomposite materials for practical structural application (e.g. as repair materials). Four types of nano-additives were investigated: multiwall carbon nanotubes (MWCNTs), functionalised MWCNTs, carbon nanofibres (CNFs), and “few layer graphene oxides” (FLGO). The unique geometrical characteristics of these additives in particular, as well as their mechanical properties such as high strength, ductility and stiffness, were the motivation for this study. udIn this work, extensive experimental studies have been conducted to develop practical and effective dispersion techniques for carbon nano-additives for cementitious application, and to produce novel cementitious composites with nano-additives (i.e. Nanofilaments Reinforced Cementitious Composites (NRCC)) and with hybrid nano-and-micro fibres (i.e Multiscale Hybrid Reinforced Cementitious Composites (MHRCC). The work has also focused on evaluating the efficiency of nanofilaments as nano reinforcement agents, controlling cracking and its impact on the durability of the produced composites. More specifically, test methods have been used to assess the shrinkage and cracking response of thin composite layers exposed to restrained shrinkage, and their sulfuric acid resistance. Furthermore, the potential application of the developed hybrid composites (MHRCC) as a layer repair/strengthening material was examined. udCarbon nano-additives (MWCNTs, functionalised MWCNTs, CNFs, and GO) have a high tendency to agglomerate due to their strong Van der Waals self-attraction and hydrophobic surfaces. To date, ensuring a uniform dispersion in water and in the cementitious composite is the main challenge that hinders their effective use as a nano reinforcing agent. The present study focuses on a novel dispersion technique for dispersing the carbon nano-additives in water and in the cementitious composite. The effect of various intensities of sonication and treatment times, and the effect of surfactants and mineral admixtures on the dispersion behaviour were investigated. Nano-additive suspensions were semi-quantitatively and qualitatively analysed analysed using Ultraviolet–visible spectroscopy (Uv-vis) and Transmission Electron Microscopy (TEM). The mechanical properties and microstructure of the resulting composite material were characterised through compressive, direct tensile strength, and Scanning Election microscopy (SEM) tests. It was found that periodic and short-duration, high-intensity sonication achieved superior dispersion of the agglomerated carbon nano-additives at different concentrations, and led to cementitious composites with improved mechanical performance. The new composite material overcomes a number of issues associated with conventional cementitious composites, in particular their tendency to crack at the nanoscale under loading, and during shrinkage. udThe obtained results on dispersion show that it is possible to produce composites reinforced (i) at the nano scale by incorporating a very low percentage of nanofilaments at 0.025%(by cement weight) and (ii) at nano-and-micro scale by incorporating a low percentage of nanofilaments at 0.025% (by cement weight) together with micro steel fibres (volume fraction of 2%). Addition of nanofilaments resulted in cementitious composites with improved tensile strength, drying and restrained shrinkage performance. Hybrid fibre composites exploit the synergistic effect between nano-and micro additives and can potentially lead to significant improvements in toughness and other mechanical properties. Encouraging results are reported suggesting that incorporating effectively dispersed nanotubes/fibres has the potential to produce composites with simultaneously improved mechanical performance and long-term durability. Repairing of reinforced concrete beams by using a thin layer (35 mm) of Multiscale Hybrid Reinforced Cementitious Composites(MHRCC) was found to have many advantages, such as increasing the ultimate load, stiffness, extending the service life, and potentially delaying crack formation and propagation.