Cementitious materials reinforced with well dispersed multiwall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) at the nanoscale were fabricated and tested. The MWCNTs and CNFs were dispersed ...by the application of ultrasonic energy and the use of a superplasticizer. Mechanical and fracture properties including flexural strength, Young’s modulus, flexural and fracture toughness were measured and compared with similarly processed reference cement based mixes without the nano-reinforcement. The MWCNTs and CNFs reinforced mortars exhibited superior properties demonstrated by a significant improvement in flexural strength (106%), Young’s modulus (95%), flexural toughness (105%), effective crack length (30%) and fracture toughness (120%).
A thorough fracture mechanics characterization of Portland cement mortars reinforced with multi wall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) took place. The critical values of stress ...intensity factor, KICS; strain energy release rate, GICS; crack tip opening displacement, CTODc; and critical crack length, ac of 3, 7, and 28 days Portland cement mortars, reinforced with well dispersed carbon nanotubes and carbon nanofibers were experimentally determined. Prismatic notched specimens of neat mortars and mortars reinforced with 0.1 wt.% CNFs, and 0.1 and 0.2 wt.% MWCNTs were subjected to a three point closed loop bending test, using the crack mouth opening displacement, CMOD, as the feedback signal. The fracture parameters of the nanoreinforced mortars were then determined using the two parameter fracture model. The excellent reinforcing and toughening efficiency of MWCNTs and CNFs is demonstrated by a significant improvement in the critical stress intensity factor/fracture toughness (128.6%), critical strain energy release rate (154.9%), and critical crack tip opening displacement (39%). These results allow us to conclude that the MWCNTs and CNFs beneficially alter the nanostructure of the mortar matrix, resulting to a significant enhancement of all fracture and mechanical properties and provide the material, with the ability of performing multiple structural functions.
Due to their exceptional mechanical properties, carbon nanotubes (CNTs) are considered to be one of the most promising reinforcing materials for the next generation of high-performance ...nanocomposites. In this study, the reinforcing effect of highly dispersed multiwall carbon nanotubes (MWCNTs) in cement paste matrix has been investigated. The MWCNTs were effectively dispersed in the mixing water by using a simple, one step method utilizing ultrasonic energy and a commercially available surfactant. A detailed study on the effects of MWCNTs concentration and aspect ratio was conducted. The excellent reinforcing capabilities of the MWCNTs are demonstrated by the enhanced fracture resistance properties of the cementitious matrix. Additionally, nanoindentation results suggest that the use of MWCNTs can increase the amount of high stiffness C–S–H and decrease the porosity. Besides the benefits of the reinforcing effect, autogenous shrinkage test results indicate that MWCNTs can also have a beneficial effect on the early strain capacity of the cementitious matrix, improving this way the early age and long term durability of the cementitious nanocomposites.
The remarkable mechanical properties of carbon nanotubes (CNT) suggest that they are ideal candidates for high performance cementitious composites. The major challenge however, associated with the ...incorporation of CNTs in cement based materials is poor dispersion. In this study, effective dispersion of different length multiwall carbon nanotubes (MWCNTs) in water was achieved by applying ultrasonic energy and in combination with the use of a surfactant. The effects of ultrasonic energy and surfactant concentration on the dispersion of MWCNTs at an amount of 0.08
wt.% of cement were investigated. It is shown that for proper dispersion the application of ultrasonic energy is absolutely required and for complete dispersion there exists an optimum weight ratio of surfactant to CNTs. For a constant ratio of surfactant to MWCNTs, the effects of MWCNT type (short and long) and concentration on the fracture properties, nanoscale properties and microstructure of nanocomposite materials were also studied. Results suggest that MWCNTs improve the nano- and macromechanical properties of cement paste.
Utilizing the unique properties of CNTs and CNFs to enhance the mechanical and fracture properties of cement based materials and develop smart cementitious nanocomposites can be a challenge in terms ...of developing scalable manufacturing methods. Scaling up the manufacturing size of CNT and CNF reinforced cement based materials and produce multifunctional concrete that exhibits exceptional strength, stiffness and toughness and multifunctionality requires optimization of dispersion procedure. The effectiveness of successfully using CNTs and CNFs in concrete depends on the fiber count, the volume fraction of sand and coarse aggregates. In this work, we present the flexural strength and stiffness, fracture toughness and brittleness of nanomodified pastes and mortars reinforced at amount of 0.08 and 0.1 wt% and an investigation on the optimization of the fiber count proportioning of concrete. The addition of a very low amount, 0.1 wt%, of both CNTs and CNFs, increases approximately 1.5 times the flexural strength and the Young`s modulus of concrete nanocomposites. The nanomodified concrete also exhibits 60% higher energy absorption capability.
The homogeneous dispersion of carbon nanotubes, (CNTs), in a cementitious matrix is a crucial procedure that affects both mechanical properties and electrical conductivity of the nanocomposite. ...Electrochemical Impedance Spectroscopy, (EIS), can be very useful in studying and characterizing the impact of the CNT content in nanoreinforced cementitious materials in a quantitative way. In this study, measurements of the resistance, reactance and capacitance reveal the nature of circuit elements formed by the nanotube network in cement mortars reinforced with various amounts of carbon nanotubes and provide insight of the nanotubes’ dispersion state. Resistivity results show that percolation threshold was reached between CNT weight fractions of 0.1% and 0.15 wt%. Below percolation, a consistency between resistive and capacitive phases exists, i.e., resistivity, reactance and capacitance were found to decrease as the nanotube content increases. After the continuous conductive network was formed, and percolative behavior was achieved, resistivity values show a little dependence on the CNT content. However, the presence of CNT entanglements was found to contribute to an amplified energy storage ability, as both the imaginary part of impedance (reactance) and capacitance were increased. A correlation between capacitance, flexural strength and modulus of elasticity was observed for the first time. Capacitance values provide valuable information on the energy storage ability of the material and how the actual CNT dispersion state affects the mechanical properties of percolative nanoreinforced cementitious materials. Finally, a general micromechanics model, modified by taking into account the conductive mechanisms below and above percolation threshold, was successfully implemented. The theoretically determined values of the overall electrical conductivity are in good agreement with the experimental results.
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Stress transfer between reinforcing bars and concrete is engaged through rib translation relative to concrete, and comprises longitudinal bond stresses and radial pressure. The radial pressure is ...equilibrated by hoop tension undertaken by the concrete cover. Owing to concrete’s poor tensile properties in terms of strength and deformability, the equilibrium is instantly released upon radial cracking of the cover along the anchorage with commensurate abrupt loss of the bond strength. Any improvement of the matrix tensile properties is expected to favorably affect bond in terms of strength, resilience to pullout slip, residual resistance and controlled slippage.The aim of this paper is to investigate the local bond of steel bars developed in adverse tensile stress conditions in the concrete cover. In the tests, the matrix comprises a novel, strain resilient cementitious composite (SRCC) reinforced with polypropylene fibers (PP) with the synergistic action of carbon nano-tubes (CNT). Local bond is developed over a short anchorage length occurring in the constant moment region of a four-point bending short beam. Parameters of investigation were the material structure (comprising a basic control mix, reinforced with CNTs and/or PP fibers) and the age of testing. Accompanying tests used to characterize the cementitious material were also conducted. The test results illustrate that all the benefits gained due to the synergy between PP fibers and CNTs in the matrix, namely the maintenance of the multi-cracking effect with time, the increased strength and deformability as well as the highly increased material toughness, were imparted in the recorded bond response. The local bond response curves thus obtained were marked by a resilient appearance exhibiting sustained strength up to large levels of controlled bar-slip; the elasto-plastic bond response envelope was a result of the confining synergistic effect of CNTs and the PP fibers, and it occurred even without bar yielding.
The development of multifunctional self-sensing mortars reinforced with multi wall carbon nanotubes (MWCNTs) is herein carried out. The purpose is twofold: to determine the impact of ...nano-modification on the strength, the stiffness, and the toughening effect that MWCNTs can provide; and to evaluate the multi-functionality and smartness of cement mortars, reinforced with 0.08, 0.1, 0.3 and 0.5wt% of cement well-dispersed MWCNTs. The experimental determination of the mechanical properties of 3, 7 and 28d nanomodified mortars was achieved through three point bending, uniaxial compression, and fracture mechanics experiments. The evaluation of the smartness of the nanoreinforced mortars was achieved by measuring the fractional change in the electrical resistance of specimens, induced by external cyclic compressive loading in the elastic region. The excellent reinforcing capability of MWCNTs is demonstrated by a significant improvement in flexural strength (87%), Young’s modulus (92%), flexural toughness (83%), first crack strength (64%) and first crack toughness (65%). Results from piezoresistivity experiments confirm that the nanoreinforced mortars exhibit an increased change in resistivity under cyclic compressive loading, which is indicative of the amplified sensitivity of the material in strain sensing.
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This study reports recent developments on carbon nanotube (CNT), nanofiber (CNF), and polypropylene microfiber (PP) hybrid reinforced cementitious composites. Notable characteristics ...are (i) significantly enhanced load carrying capacity; and (ii) increased energy absorption capability at both elastic and post-first cracking stages. Three point bending tests on notched and unnotched specimens have shown that the addition of 0.3 vol% CNTs and CNFs in PP reinforced mortars increases the modulus of elasticity and first crack strength and toughness by 93%, 64% and 49% respectively. The experimental values perfectly agree with the theoretically determined modulus using a micromechanics-based model. Hybrid mixes showed considerable increase in toughness, exhibited toughness index values I5 of 4 or more, and 4 times higher residual strength. Relative to the PP microfiber reinforced mortar, the developed hybrid material is able to sustain a much higher load and successfully transfer tensile stresses across cracks, without losing its tensile load carrying capacity. Such exceptional mechanical behavior is an important performance factor for serviceability and is interpreted in terms of the mechanism of a synergistic interaction between the nano and micro scale fiber reinforcement.
In this study, the influence of the addition of carbon nanotubes (CNTs) and nanofibers (CNFs) and their dispersion on the corrosion behavior, bulk electrical resistivity and mechanical properties of ...nanomodified Portland cement mortars was investigated. The experimental determination of the mechanical properties of nanocomposites reinforced with CNFs at an amount of 0.1wt% of cement, and CNTs at amounts of 0.1 and 0.5wt% took place, through three point bending and uniaxial compression experiments. The electrical properties were measured using alternating current (AC) and following the two pole method. For the evaluation of corrosion resistance the corrosion potential (Ecorr) and corrosion current density (Icorr) were measured in specimens partially immersed in a 3.5% NaCl solution, using the Linear Polarization Technique (LPR). The electrochemical mass loss was then calculated using Faraday’s Law. The addition of 0.1wt% CNTs and CNFs remarkably improves the flexural strength and the Young’s modulus, while enhancing the bulk electrical conductivity of the mortar nanocomposites. The corrosion study shows the addition of 0.1wt% CNTs and CNFs decreases the corrosion rate and significantly increases the resistance to corrosion by delaying the onset of the corrosion reaction.
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