Chapter 30 MWCNT and CNF Cementitious Nanocomposites for Enhanced Strength and Toughness P.A. Danoglidis, M.G. Falara, M.K. Katotriotou, M.S. Konsta-Gdoutos, and E.E. Gdoutos Abstract Cementitious nanocomposites reinforced with carbon fibers at the nanoscale were fabricated and tested, exhibiting remarkably improved mechanical and fracture properties. The cementitious nanocomposites were reinforced with well dispersed multiwall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs). A dispersion method involving the application of ultrasonic energy and the use of a superplasticizer was employed to prepare the nanoscale fiber suspensions. Flexural strength and Young’s modulus were experimentally investigated and compared with similarly processed reference cement based mixes without the nano-reinforcement. The nanocomposites’ fracture properties were also determined using the two parameter fracture model (TPFM). The excellent reinforcing capability of MWCNTs and CNFs is demonstrated by a significant improvement in flexural strength (87 % for MWCNTs and 106 % for CNFs reinforcement), Young’s modulus (100 %), and fracture toughness (86 % for MWCNTs and 119 % for CNFs reinforcement). Keywords Multi-walled carbon nanotubes • Carbon nanofibers • Mortars • Fracture mechanics • Young’s modulus 30.1 Introduction Cementitious materials suffer from low tensile strength and limited strain capacity, which gives rise to the formation of nanocracks under relatively low tensile loads. The use of new reinforcing materials like multiwall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) makes it possible to produce cement based nanocomposites with revolutionary properties. These materials, with aspect ratios greater than 1000, have Young’s modulus around 1 TPa [1] tensile strength of 65–93 GPa, and maximum strain of 10–15 %. However MWCNTs form agglomerates or bundles and adhere together with van der Waals forces, which make it particularly difficult to separate. Early attempts to add MWCNTs to cementitious materials have failed due to poor adhesion. Konsta-Gdoutos and coworkers [2, 3] developed a revolutionary method for effective dispersion of MWCNTs in cement pastes, mortars and concrete. The dispersion of different lengths of MWCNTs and CNFs in water was achieved by applying ultrasonic energy and using a surfactant. Recent and ongoing research by the same authors have focused on developing multifunctional cement paste nanocomposites with high strength and advanced strain detection ability that can be used as a novel self-sensing nanoreinforced cementitious structural material. The incorporation of highly dispersed MWCNTs and/or CNFs at low concentrations has been shown to effectively arrest the cracks at the nanoscale, imposing significant improvements in the mechanical properties of the nanocomposites [4–8]. While a few studies on the mechanical strength of cementitious systems containing MWCNTs and CNFs exist, there haven’t been any studies on the fracture performance of MWCNT and CNF mortar nanocomposites. The incorporation of fibers at the nanoscale and their influence on the interfaces and the pore structure of the mortar matrix are likely to render significant differences to the fracture response of the nanoreinforced mortars as compared to conventional ordinary Portland cement (OPC) systems. The objective of this study is to investigate the mechanical and fracture properties of MWCNT and CNF reinforced mortars. Mortar nanocomposites reinforced with well dispersed MWCNTs and CNFs at an amount of 0.1 wt% of cement were fabricated. A detailed investigation on the effect of the nanoscale fibers on the mechanical properties and fracture toughness of the mortar matrix was performed by conducting three-point bending and fracture mechanics experiments. Results compared with similarly fabricated reference mortars without the nano-reinforcement are clearly showing that the incorporation of MWCNTs and CNFs in the mortar matrix significantly improves the mechanical and fracture properties of the mortar matrix. P.A. Danoglidis • M.G. Falara • M.K. Katotriotou • M.S. Konsta-Gdoutos • E.E. Gdoutos (*) Department of Civil Engineering, Democritus University of Thrace, 12 Vas. Sofias, 671 00 Xanthi, Greece e-mail: egdoutos@civil.duth.gr #The Society for Experimental Mechanics, Inc. 2016 C. Ralph et al. (eds.), Mechanics of Composite and Multi-functional Materials, Volume 7, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-21762-8_30 241
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