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Carbon nanotube and nanofiber cement based nanocomposites with advanced mechanical properties and self-sensing ability (Doctoral thesis)

Δανογλίδης, Παναγιώτης/ Danoglidis, Panagiotis

Cement based materials exhibit extremely brittle failure, low tensile strength, low strain capacity and are susceptible to cracking. These characteristics are serious shortcomings that not only impose constraints in structural design, but also affect the long term durability of structures and their performance in a life-cycle perspective. Current solutions for new concrete constructions, as recommended and enforced by design codes, has made significant progress in increasing the compressive strength of concrete leading to high strength and performance cement-based materials. However, these types of materials are characterized by increased brittleness and at the same time exhibiting low resistance to cracking.The present Doctoral Thesis entitled: “Functionally graded cement based nanocomposites with advanced mechanical properties and self-sensing ability” was highly motivated by the idea of developing high-performance and advanced self-sensing cementitious materials that could effectively be used as construction materials and at the same time exhibit self-sensing capability to monitor their strain and cracking. To increase the life of a structure, delaying the micro and macro cracking and increasing fracture resistance is very important. This can be achieved by using fibers at the nano scale: as all cracks are formed at the nano scale level, to suppress the initiation and growth of these cracks, the use of nanoscale fibers can enable notable structural concepts. The overall object of the Ph.D. thesis is to develop new high performance nanomodified cement based nanocomposites with substantially increased flexural strength, stiffness and toughness as well piezoresistive response. To achieve this, the cement based materials were reinforced at the nanoscale, by incorporating carbon nanotubes (CNTs) and carbon nanofibers (CNFs), as well as at the microscale by incorporating polypropylene (PP) microfibers.Carbon nanotubes and carbon nanofibers exhibit superior mechanical and electrical properties. Their supreme stiffness, high strength and aspect ratio make them excellent reinforcing candidate materials, offering outstanding improvements in the mechanical properties. Reinforcement at the nanoscale, in addition to providing fracture resistance, can also beneficially alter the nanostructure of cement based materials. Nanoscale carbon fibers are also highly conductive materials and when subjected to various deformations they have the ability to record electromechanical changes, expressing a linear and reversible piezoresistive response. As carbon nanoscale fibers adhere together due to van der Waals forces, agglomerates or bundles are formed that make their dispersion difficult and laborious. Konsta-Gdoutos et al. has focused on solving the major challenge associated with the incorporation of CNTs and CNFs in cementitious matrix in order to manufacture nanocomposites with good dispersion of CNTs/CNFs at a reasonably low volume fraction. At early research, cement pastes reinforced with small amount of CNTs and CNFs were successfully designed and produced exhibiting very good mechanical performance. However, the successful application of nanotechnology in the construction materials by using carbon nanoscale fibers to improve (a) the mechanical properties and (b) the piezoresistive response of the cement mortar and concrete nanocomposites, still remains a challenge. Given the limited data available on the effect of CNTs and CNFs in both the mechanical and electrical properties of cement based nanocomposites, in this Ph.D. thesis a thorough experimental approach was followed: (i) to determine the impact of well dispersed CNT and CNF nanomodification on the strength, stiffness and energy absorption capacity, (ii) to determine in detail the flexural first cracking response, and the fracture properties such as fracture toughness and strain energy release rate; (iii) and to evaluate the multi-functionality and smartness of cement mortars, reinforced with highly dispersed carbon nanotubes and nanofibers. Dispersion of CNTs and CNFs was achieved by adding the nanofibers to an aqueous polycarboxylate based surfactant solution, at a surfactant to nanofibers weight ratio of 4.0. Ultrasonic energy close to 2800 kJ/l is applied to the suspensions to achieve uniform dispersion of nanoscale carbon fibers. The quality of the nanoscale fibers’ dispersion was evaluated through Ultraviolet Visible Spectroscopy tests (UV-Vis). The effectiveness of the dispersion method is reflected by the high amounts of the ultraviolet radiation absorbed by the carbon atoms in the surface of the individual nanotubes. Following the above mentioned procedure mortars reinforced with CNTs and CNFs at amounts of 0.025% to 0.5 wt%, were produced. The effect of the addition of CNTs and CNFs at different amounts % per weight of cement on the mechanical properties of mortars was thoroughly investigated. The purpose is twofold:1.To investigate the mechanical behavior of nanocomposite cement mortars by determining the flexural and compressive strength, the modulus of elasticity, the energy absorption capability and the strength and flexural toughness at the “first crack” by conducting three-point bending and uniaxial compression experiments.2.To study the failure mechanism by a thorough fracture mechanics characterization, complementary to the conventional failure criteria. The critical values of stress intensity factor, KIC; strain energy release rate, GIC; crack tip opening displacement, CTODc; critical crack length, ac; and material length, Q, of Portland cement mortars, reinforced with well dispersed carbon nanotubes and carbon nanofibers were experimentally determined by fracture mechanics tests following the Linear Elastic Fracture Mechanics (LEFM) and Two Parameter Fracture Model (TPFM).The efficiency of nano-reinforcing ability of CNTs and CNFs and their strong interfacial adhesion with the mortar matrix resulted in an increase of the nanoreinforced mortars’ flexural strength of about 100%. Moreover, the Young’s modulus and energy absorption capacity of the nanoreinforced mortars is 1.9 and 1.7 times higher than that of the plain mortars. While the introduction of the carbon fibers at the nanoscale cannot produce a post-elastic change, a significant improvement of the linear stage of the material before crack initiation, at which stress is directly proportional to strain, (first crack) allows the material to greatly improve its elastic response and toughness performance.The strong interfaces between the mortar matrix and well dispersed carbon fibers at the nanoscale substantially enhance both strength and stiffness. The evaluation however of the fracture parameters of the nanoreinforced mortars indicated that besides strength and stiffness the values of fracture toughness (Stress Intensity Factor, KIC) and fracture energy (Strain Energy Release Rate, GIC) are approximately 85-100% higher over the plain mortar. The material length value Q, calculated using the TPFM, is the index of the fracture process zone above the crack tip and can be used to assess the brittleness of a material. The Q values of both the CNT and CNF reinforced mortars are higher than those of plain mortar; reflecting a less brittle behavior of the nanoreinforced mixes.The contribution of carbon nanotubes and nanofibers on the mechanical performance of concrete, is also investigated. It should be noted that research available in the literature on the effect of nanoscale fibers on concrete’s performance is very limited. Flexural and compressive strength, Young`s modulus and energy absorption capability of CNT and CNF/reinforced concrete were determined through four point bending and uniaxial compression tests. The addition of a very small amount of 0.1 wt% of both carbon nanotubes and nanofibers in conventional concrete (w/c=0.51) does not affect the compressive strength but leads to very high increases in the flexural strength of 50% and the elastic modulus of 67%, increasing the stiffness of concrete matrix from 29 GPa to 48 GPa. It is noted that elastic modulus values of ≈40 GPa correspond to high-strength concrete with compressive strength above 120 MPa. Finally, the energy absorption capacity was improved by 47%.One major objective of this Ph.D. thesis was to improve the pre-peak and post-crack mechanical behavior of mortars by using a hybrid (ladder scale) reinforcement at the nano and the micro scale. Network of CNTs and polypropylene (PP) microfibers were used as reinforcement. Three point bending tests were conducted on the nano (CNT) and micro scale (PP) reinforced mortars and a thorough analysis of the load to mid-deflection curves took place to evaluate their mechanical response at the elastic stage (pre-peak) and after the “first crack” (post-crack). The hybridization was found to enhance simultaneously the flexural strength, Young’s modulus and energy absorption capability of the cementitious matrix. Over all the incorporation of CNTs and PP microfibers have resulted in remarkable increases of toughness indices (I5, I10, I20) hence into a stronger and tougher composite compared to the singly-reinforced composites or plain mortar. Carbon nanotubes and nanofibers also exhibit remarkable intrinsic properties including electrical conductivity. Thus, appear to be ideal candidates as nanoscale reinforcement to improve the mechanical performance of cement-based materials, while providing novel properties such as electromechanical response and self-sensing ability. In this Thesis, Electrochemical Impedance Spectroscopy (EIS) measurements were conducted to evaluate the electrical properties; the real and imaginary part of resistance; and the capacitance of mortars reinforced with CNTs at different amounts wt% of cement. The real part of resistance can determine the critical amount of CNTs required for the formation of a continuous conductive network. Both the imaginary part of resistance (reactance) and capacitance values indicate the state of dispersion and the existence of agglomerations in the matrix. It was found that the nanotube content of 0.1 wt% is associated with the onset of percolation. For higher CNT amounts the conductive nanotube pathways are already established and the resistive phase reaches a plateau demonstrating an independence of the CNT content. Also, the 0.1 wt% CNT nanocomposites exhibit the lowest reactance and energy storage capacity values, expressed as capacitance. The electrical current can easily pass through the established conductive network of individual CNTs; therefore either very small or very few CNT agglomerates exist in the mortar matrix. The flexural strength and the Young’s modulus of the 0.1 wt% CNT-nanocomposites also reaches the highest improvements compared to the mortar without the nanoscale reinforcement.To evaluate the impact of nanomodification on the multifunctionality and the smartness of the CNT and CNF reinforced mortar and concrete, piezoresistivity experiments were conducted by measuring the electrical resistance (AC) of the specimens under monotonic and cyclic compressive loading. Consistently with the flexural strength, stiffness and toughness results, and electrochemical capacitance the nanocomposites reinforced CNTs and CNFs yielded high fractional changes in resistivity, (up to ≈15%) under cyclic compressive loading in the elastic region (up to 5 MPa), which is indicative of the amplified sensitivity of the material. To further evaluate the effect of the nanoscale fibers’ dispersion in the cementitious matrix, the piezoresistive response of nanocomposites reinforced with ‘‘as received” CNTs was investigated. These nanocomposites do not exhibit any piezoresistive response, indicating this way the necessity for the presence of effectively dispersed nanotubes and nanofibers, able to create an effective conductive network that allows the material to identify the change in the mechanical deformation. The self-sensing ability of CNT and CNF reinforced mortars and concretes, was also investigated by applying monotonic compressive load up to failure. From the experimental findings, it is safely attributed that the nanomodified mortars are able to perfectly recognize their mechanical deformation and the cracking at the nano, micro and macroscale, throughout the application of the compressive load. The excellent piezoresistive response of nanomodified mortars and concretes clearly indicates their capability to be used as sensors that can detect strain variation and cracking in concrete, through their resistivity measurements, leading into a new era of monitoring and evaluating in real time the condition of any concrete element.