Concrete reinforced with a macroscopic carbon nanotube element and method for producing the same

Abstract

A method of reinforcing concrete using a macroscopic carbon nanotube element is described. A concrete mix is poured over a macroscopic carbon nanotube element in the form of a bar, fiber, mat or related structure to prepare the reinforced concrete. The carbon nanotube element may be pretreated to improve bonding with the concrete and / or generate a stiffer element. A concrete object reinforced by the carbon nanotube element is also described. The reinforced concrete object of the invention can be used in structural health monitoring.

Description

[0001] CONCRETE REINFORCED WITH A MACROSCOPIC CARBON NANOTUBE ELEMENT AND METHOD FOR PRODUCING THE SAME

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] Reference is made to U.S. Provisional Patent Application Serial No. 63 / 669,772, filed July 11, 2024 and entitled CONCRETE REINFORCED WITH A MACROSCOPIC CARBON NANOTUBE ELEMENT AND METHOD FOR PRODUCING THE SAME, the disclosure of which is hereby incorporated by reference and priority of which is hereby claimed pursuant to 37 CFR 1.78(a)(4) and (5)(i).

[0004] FIELD OF THE INVENTION

[0005] The present invention relates to a concrete object reinforced with a macroscopic carbon nanotube element and a method for producing the same.

[0006] BACKGROUND OF THE INVENTION

[0007] Various techniques are known for preparing reinforced concrete.

[0008] SUMMARY OF THE INVENTION

[0009] The present invention seeks to provide a reinforced concrete object and a method of producing the same.

[0010] There is thus provided in accordance with a preferred embodiment of the present invention a method of producing a reinforced concrete object including: providing at least one macroscopic carbon nanotube element; and casting a concrete mix over the carbon nanotube element. Preferably, the at least one carbon nanotube element is in the form of a bar, a woven mat, a non-woven mat, a fiber, a braid, a yam, a multi-thread yam, a multi -thread twisted yarn, a strip or a twisted strip. Preferably, the at least one carbon nanotube element is produced by a floating catalyst chemical vapor deposition process. In accordance with a preferred embodiment of the present invention, the at least one carbon nanotube element includes ultra-long carbon nanotube (ULCNT) bundles. Preferably, the bundles have a diameter of about 5 to about 150 nm, more preferably about 15 to about 30 nm. Each bundle preferably includes about 10 to about 200 carbon nanotubes, more preferably about 10 to about 50 carbon nanotubes. Preferably, the bundles have an aspect ratio of at least 10,000.

[0011] In accordance with a preferred embodiment of the present invention, the at least one carbon nanotube element consists essentially of carbon nanotubes. In accordance with another preferred embodiment of the present invention, the at least one carbon nanotube element includes carbon nanotubes and a matrix. Preferably, the matrix is organic, ceramic, metallic, or a mixture thereof.

[0012] In accordance with a preferred embodiment of the present invention, the method further includes stretching the at least one carbon nanotube element, to align the CNTs within the at least one element to enhance its tensile strength and increase its rigidness, prior to casting the concrete mix. Preferably, the stretching is performed at a rate of 0.1- 50 mm / min. In accordance with a preferred embodiment of the present invention, the stretching is performed in a lubricant. The lubricant is preferably selected from the group consisting of organic solvents, polymeric resins prior to the addition of a hardener, polymeric resins following the addition of a hardener, acids, bases, ionic liquids, or a mixture thereof. Preferably, the at least one carbon nanotube element is stretched to at least 120% of its original length, more preferably at least 150% of its original length, and most preferably at least 200% of its original length.

[0013] In accordance with a preferred embodiment of the present invention, the method further includes functionalizing the at least one carbon nanotube element with functional groups prior to casting the concrete mix. Preferably, the functional groups include hydroxylic, carboxylic, and aminic groups or any combination thereof. In accordance with a preferred embodiment of the present invention, the hydroxylic and carboxylic groups are added by thermal oxidation, chemical oxidation or electrochemical means, and the amine groups are added covalently by the use of hyponitrous acid (H2N2O2), ammonia (NH3), hydroxylamine (NH2OH), nitric acid (HNO3), ammonium persulfate ((NH4)2S2OS), nitrogen dioxide (NO2), nitrous acid (HNO2), nitrogen triiodide (NI3), nitrogen tetroxide (N2O4), nitro compounds, nitrosylsulfuric acid (HSO4NO2), or hydroxylamine-O-sulfonic acid (H2NOOSO3H), or non-covalently with amine containing moi eties such as3 -aminopropyltri ethoxy silane, ethylenediamine, di ethylenetriamine, polyethyleneimine, poly dopamine, N-hydroxysuccinimide and 1- ethyl-3-(3-dimethylaminopropyl)carbodiimide, 4,4' -diaminodiphenylmethane, diaminobenzidine and diazonium salts.

[0014] There is also provided in accordance with another preferred embodiment of the present invention, a reinforced concrete object including: concrete and at least one macroscopic carbon nanotube element. Preferably, the at least one carbon nanotube element is in the form of a bar, a woven mat, a non-woven mat, a fiber, a braid, a yarn, a multi-thread yarn, a multi-thread twisted yarn, a strip or a twisted strip.

[0015] In accordance with a preferred embodiment of the present invention, the at least one carbon nanotube element includes ultra-long carbon nanotube (ULCNT) bundles. Preferably, the bundles have a diameter of about 5 to about 150 nm, more preferably about 15 to about 30 nm. Each bundle preferably includes about 10 to about 200 carbon nanotubes, more preferably about 10 to about 50 carbon nanotubes. Preferably, the bundles have an aspect ratio of at least 10,000.

[0016] In accordance with a preferred embodiment of the present invention, the at least one carbon nanotube element consists essentially of carbon nanotubes. In accordance with another preferred embodiment of the present invention, the at least one carbon nanotube element includes carbon nanotubes and a matrix. Preferably, the matrix is organic, ceramic, metallic, or a mixture thereof. In accordance with a preferred embodiment of the present invention, the hydroxy succinimide at least one carbon nanotube element includes functional groups. Preferably, the functional groups include hydroxylic, carboxylic, and aminic groups or any combination thereof.

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Fig. l is a simplified schematic of a framework for preparing a concrete sample in accordance with one embodiment of the present invention.

[0019] Fig. 2 is a simplified schematic of a framework for preparing a concrete sample in accordance with another embodiment of the present invention. Fig. 3 is a simplified schematic of a metal segment useful for facilitating tensile testing of a is a simplified schematic of a framework for preparing a concrete object in accordance with one embodiment of the present invention.

[0020] Fig. 4 is a simplified schematic of a concrete object in accordance with one embodiment of the present invention with metal segments attached for facilitating tensile testing.

[0021] Fig. 5 is a simplified schematic of a concrete object in accordance with one embodiment of the present invention attached to a tensile testing apparatus.

[0022] Fig. 6 is a graph showing the results of tensile testing of un-reinforced concrete samples.

[0023] Fig. 7 is a graph showing the results of tensile testing of carbon nanotube fiber reinforced concrete samples.

[0024] Fig. 8 is a graph showing the results of tensile testing of pre-treated carbon nanotube fiber reinforced concrete samples.

[0025] Fig. 9 is a SEM image of a concrete object reinforced with carbon nanotube fibers.

[0026] Fig. 10 is a SEM image of a concrete object reinforced with pre-treated carbon nanotube fibers.

[0027] Fig. 11 is a photograph of a concrete object reinforced with carbon nanotube fibers after breaking during tensile testing.

[0028] Fig. 12 is a photograph of a concrete object reinforced with pre-treated carbon nanotube fibers after breaking during tensile testing.

[0029] DETAILED DESCRIPTION OF THE INVENTION

[0030] Concrete is the most commonly used building material in the world. Due to concrete’s low tensile strength, it is commonly reinforced with a higher tensile strength material. The most commonly used reinforcement is steel bars (rebars), although other reinforcements are known, such as fiber-reinforced polymer and glass-reinforced polymer. One of the disadvantages of using steel rebars in concrete is the susceptibility of the steel to corrosion.

[0031] It is also known to add nanomaterials to the concrete mix to improve the strength of the concrete. Oxides of silicon, aluminum, zinc and chromium have been used for this purpose. More recently, carbon nanotubes (CNTs) have been added to concrete mixes. While the use of microscale CNTs dispersed in the concrete is known, there have been no reports of using macroscale CNT elements to reinforce concrete in the same manner as steel rebars. Macroscopic CNT elements do not have the same susceptibility to corrosion as steel. They also have high tensile strength, low density, high surface area to bond with the concrete moieties and possible routes to expedite and / or enhance the concrete hydration process, and therefore are suitable for use as reinforcing elements in concrete.

[0032] There is thus provided in accordance with a preferred embodiment of the invention, a method of producing a reinforced concrete object comprising: providing at least one macroscopic carbon nanotube element, and casting a concrete mix over the carbon nanotube element.

[0033] The macroscopic carbon nanotube element is preferably formed by a floating catalyst chemical vapor deposition process. For example the macroscopic carbon nanotube element can be prepared by a method disclosed in WO 2005 / 007926, which is incorporated by reference herein in its entirety. Briefly, a carbon source and a catalyst are fed to a reactor at high temperature (500-1600 °C) to form a carbon nanotube (CNT) aerogel, and a mechanical force is applied to displace the CNTs from the reactor. In one preferred embodiment, the reactants include a sulfur compound and an iron compound in a solvent or as a vapor. The sulfur compound is preferably thiophene or elemental sulfur. The iron compound is preferably ferrocene, iron penta-carbonyl, or elemental iron. The solvent is preferably chosen from alcohols, ketones, aromatic compounds and short hydrocarbons. In the gas phase, the reactants can be carried in a short hydrocarbon. The mechanical force preferably includes spinning on a rotating spindle.

[0034] The resulting CNTs are preferably ultra-long CNTs (ULCNTs). The single ULCNTs are mostly made of few-walled CNTs consisting of 3-5 walls with diameters of about 5-8 nm. The ULCNTs may consist (up to 50%) of single-walled and doublewalled CNTs with diameters of 0.6-2 nm and 1.2-4 nm, respectively. The ULCNTs may consist (up to 95%) of multi -walled CNTs with 6-70 walls and diameters of 8-150 nm. Raman spectroscopy properties show G / D peak intensities (IG / D) of 2 to 100, more typically 5 to 15. BET results show a specific surface area of 50 - 1000 m2 / g, more commonly 100-300 m2 / g. The single ULCNTs self-assemble to create ULCNT bundles consisting of typically 5-50 CNTs with a bundle diameter of 5-150 nm but can also consist 2-100 CNTs with diameters of up to 300 nm. The CNT bundles have lengths of 10 to 2000 pm resulting in aspect ratios in the order of 104to 105.

[0035] The two primary macroscopic ULCNT morphologies produced by the direct spinning process are a CNT mat (CNTM) and a CNT fiber (CNTF). These morphologies can be further post-processed to produce sub-morphologies.

[0036] The CNTF is a macroscopic entity having an unlimited length, typically with a diameter of 10 pm to 1 mm. The density of the CNTF is variable, ranging from 0.1 to 2 g / cm3with a linear density of 0.1-50 g / km (tex). The specific tensile strength of the CNTF is in the range of 0.1-4 N / tex (ultimate tensile strength 50-2500 MPa). The density can be controlled by the amount of pressure applied to the surface of the CNTF. The means of applying pressure can be mechanical, using a manual, electrical, or a hydraulic plate or roll press. Other means can be based on capillary forces by immersing the CNTF in an organic solvent such as acetone, ethanol, or iso-propyl alcohol and rapidly drying it in air or a heated environment. Several CTNFs can be spun to produce a yam. The yam can consist of two to 100 single fibers. The number of twists of the fibers within the yarn can vary between 1 to 100 twists / cm.

[0037] The CNTM is a macroscopic entity with a typical thickness of 1-300 pm. There is no theoretical limit to the length and width of the CNTM. Typical dimensions are 2 m x 1 m. The CNTM has an extensive range of densities, from 0.01 to 1.5 g cm'3. The density can be controlled by the amount of pressure applied to the surface of the CNTM as described above with respect to the CNTF. The CNTM can be cut into thinner strips. Cutting the strips can be done mechanically with scissors, a utility knife, a hacksaw, waterjet cutting etc., or by optical means such as a laser cutting machine. The strips can be cut to any width from 0.1 mm or higher. The strips can be cut to any length from 0.1 mm or higher. To maximize the physical properties of the strip, the strip is cut along the collection axis of the CNTM. This will result in tenacity values of 0.05 to 0.5 N / tex (ultimate tensile strength 10-1000 MPa). The CNT strip can be twisted along its axis to produce a twisted CNT strip. The number of twists can vary between 1 to 100 twists / cm. The strips can be rolled perpendicular to their length axis (aka “cigar rolling”) to produce pseudo CNTFs. These pseudo CNTFs will retain the length of the original strip but will have an altered diameter (instead of width) of 10 pm to 10 mm. The pseudo CNTFs can remain pristine or be compacted by mechanical means or capillary forces, as described above.

[0038] The macroscopic CNT element preferably consists essentially of ULCNTs, in a typical composition of 70-95%. The macroscopic CNT element may also comprise up to about 10% amorphous carbon, up to about 25% metal catalyst nanoparticles, such as iron nanoparticles of 1-200 nm, and various impurities at concentrations of 100 ppm or less.

[0039] The macroscopic CNT element can optionally be impregnated with a thermoset or thermoplastic polymer system. A thermoset polymer system can undergo a controlled heat and pressure treatment to initiate polymerization but remain in a semi -solid, tacky state. This partial curing provides the impregnated element (prepreg) with stability, enabling it to be handled and stored without sticking to itself or other surfaces. The prepreg can either be readily cured to standard reinforcing morphologies such as bars or meshes with customizable dimensions or adequately stored to be cured later to desired configurations, including unique features such as curves, kinks, and twists for more elaborate construction needs. Once cured and formed, thermoset systems cannot be melted or reshaped by heating. Irreversible crosslinking occurs during polymerization, creating a three-dimensional network structure that provides the material with high strength, durability, and resistance to heat and chemicals. The thermoset systems that can be used to produce prepregs and rigid reinforcing bars, meshes, or tailor-made structures may include:

[0040] • Epoxy resin systems: have excellent mechanical properties, high strength-to- weight ratio, and good adhesion to carbon fibers. They offer high stiffness, chemical resistance, and dimensional stability. • Phenolic resin systems: these offer high thermal stability, flame resistance, and low smoke emission properties,

[0041] • Polyester resin systems: cost-effective thermoset matrices for applications where high mechanical properties are not critical yet offering good corrosion resistance.

[0042] • Vinyl Ester resin systems: combine the properties of epoxy and polyester resins, offering improved toughness, fatigue resistance, and chemical resistance.

[0043] • Bismaleimide (BMI) resin systems: offer high temperature resistance, excellent mechanical properties at elevated temperatures, and low moisture absorption.

[0044] A thermoplastic polymer is a type of polymer that softens and becomes moldable when heated and solidifies when cooled, allowing it to be reshaped multiple times without undergoing chemical degradation. The thermoplastic polymers that can be used may include:

[0045] • Polyetheretherketone (PEEK): PEEK is a high-performance thermoplastic matrix known for its exceptional mechanical properties, chemical resistance, and thermal stability

[0046] • Polyetherketoneketone (PEKK): PEKK offers similar properties to PEEK but with improved processability and lower melt viscosity.

[0047] • Polyamide (PA): Polyamides, such as nylon, offer good toughness, impact resistance, and wear resistance.

[0048] • Polyphenylene Sulfide (PPS): PPS offers high-temperature resistance, chemical resistance, and dimensional stability.

[0049] • Polyethylene (PE) and Polypropylene (PP): These thermoplastics offer low cost, lightweight, and good chemical resistance.

[0050] • Polyetherimide (PEI): PEI offers high temperature resistance, flame retardancy, and excellent mechanical properties.

[0051] • Polyvinylidene Fluoride (PVDF): PVDF offers excellent chemical resistance, weatherability, and UV resistance.

[0052] • Polycarbonate (PC): PC offers high impact resistance, transparency, and dimensional stability. The macroscopic ULCNT element can optionally be functionalized with functional groups to improve bonding to concrete. In one embodiment, the functional groups are hydroxyl groups. Hydroxyl groups on reinforcing ULCNT bars can form hydrogen bonds with the hydroxyl groups in a cementitious matrix, leading to improved adhesion. Cementitious matrices contain numerous hydroxyl groups, especially in hydrated cement phases such as calcium silicate hydrates, calcium hydroxide (portlandite), and other hydration products. These hydroxyl groups are available on the surface of the matrix and can participate in hydrogen bonding.

[0053] These hydrogen bonds are directional and create a strong physical attraction between the ULCNT bar’s surface and the matrix surface. Multiple hydrogen bonds can form at the interface between the ULCNT and the matrix, creating a network of interactions that collectively enhance adhesion. This network of hydrogen bonds helps to "anchor" the ULCNTs into the matrix, reducing the likelihood of fiber pull-out and increasing the mechanical interlocking. Enhanced adhesion due to hydrogen bonding improves the transfer of stress and load between the ULCNTs and the cementitious matrix. This is crucial for reinforcing applications, where the goal is to improve the composite material’s overall strength and durability. By forming strong bonds at the interface, the reinforcing fibers can more effectively share the load with the cementitious matrix, leading to better mechanical performance. Hydrogen bonding also increases the surface energy at the interface, which can lead to better wetting of the ULCNTs by the cement mix. Better wetting ensures more intimate contact and, thus, stronger adhesion.

[0054] In another embodiment, the functional groups are carboxyl groups. Carboxyl groups on reinforcing ULCNT elements can interact with calcium ions in the cement matrix, forming strong ionic or covalent bonds. Cementitious matrices contain significant amounts of calcium ions, primarily from calcium silicate hydrates, calcium hydroxide (portlandite), and other calcium-containing phases. When carboxyl groups deprotonate to form carboxylate anions, these negatively charged sites can attract and bind to the positively charged calcium ions present in the cement matrix. This interaction leads to the formation of strong ionic bonds, where the electrostatic attraction between the negatively charged carboxylate group and the positively charged calcium ion creates a stable bond. In some cases, carboxylate groups can form covalent bonds with calcium ions through coordination chemistry. This occurs when the lone pairs of electrons on the oxygen atoms of the carboxylate group coordinate with the calcium ion, forming a complex. These covalent interactions are generally stronger and more stable than purely ionic interactions, leading to enhanced adhesion. Carboxylate groups can have multiple bonding interactions with calcium ions. Each carboxylate group can interact with more than one calcium ion, creating a network of bonds that increases the strength and stability of the adhesion.

[0055] The formation of ionic and covalent bonds between the carboxyl groups on the ULCNT bars and the calcium ions in the cement matrix leads to strong interfacial adhesion. This enhances the mechanical interlocking between the CNTs and the matrix. Improved adhesion reduces the likelihood of fiber pull-out and increases the load transfer efficiency, contributing to the overall mechanical strength and durability of the composite material. It must be noted that CNTs have significantly more surface area (in the range of 200-1200 m2 / gr) in comparison to standard bulk steel rebars which are between 0.01-0.1 m2 / gr) thus dramatically enhancing the quantity and total strength of the interaction between the ULCNT elements in comparison to steel rebars.

[0056] The ULCNT surface can be functionalized with hydroxylic and carboxylic side groups by thermal or chemical oxidation. The oxidative methods can be used individually or can be combined.

[0057] Thermal oxidation can be facilitated by heating the CNT element to 300-600 °C in air for a period of 0.5 to 96 hours. This facilitates the partial oxidation of defect containing moieties present on the CNT backbone, or even the oxidation of the pristine CNT backbone. The higher the temperature and / or the length of the oxidation period the higher the defect density (represented by hydroxylic and carboxylic functional groups). The CNT element can be externally heated, such as in a muffle furnace, tube furnace or convection furnace. These can be heated, for example, using standard heating coils, IR lamps, hot flue gases or steam, flame-based heating, or oxygen based hot or cold plasma. As the CNTs are electrically conductive they can also be heated by a radiofrequency (RF) based convection furnace or by applying voltage which generates joule heating. All of the heating methods require a continuous flow of air (at rates of 0.1-50 slpm) to ensure that enough oxygen is available to provide complete oxidation of the CNT elements. Chemical oxidation can be achieved by reacting oxidating chemical agents with the CNT element. The oxidative chemical agents can be applied by numerous methods such as dipping, immersing (in liquid, gas or vapor), spraying, etc. The time period for applying these chemicals on the material can be from 0.5 hour to a week. Several iterations (e.g. up to seven) can be used. The concentration of the different chemical oxidizers should be in the range of 0.1-12 M. After the chemical oxidation the CNT elements should be neutralized by rinsing the material with deionized water until the material’s pH becomes neutral.

[0058] Possible oxidative chemical agents include: hydrogen peroxide (H2O2), potassium permanganate (KMnCk), sodium hypochlorite (NaClO), chlorine (Ch), ozone (O3), Sodium Peroxide (Na2C>2), Potassium Dichromate (BCC O?), Bromine (B ), ferric chloride (FeCL), chlorine dioxide (CIO2), peracetic acid (CH3CO3H), sodium chlorate (NaClOs), phosphoric acid (H3PO4), sulfurous acid (H2SO3), formic acid (HCOOH), boric acid (H3BO3), hydrochloric acid (HC1), sulfuric acid (H2SO4), perchloric acid (HCIO4) and hydrofluoric acid (HF).

[0059] Amino groups enhance the interaction between reinforcing ULCNT elements and cementitious matrices through several mechanisms due to their chemical nature. Amino groups are basic and can interact with acidic sites within the cement matrix. Cementitious matrices contain various acidic sites, such as hydroxyl groups on the surface of hydrated cement particles. The lone pair of electrons on the amino group can form hydrogen bonds with these hydroxyl groups, enhancing adhesion. Additional acidic sites include unreacted or partially hydrated silicate and aluminate phases that can interact with the amino groups through protonation.

[0060] Amino groups can also interact with basic sites within the cement matrix, such as calcium ions. The lone pair of electrons on the nitrogen can coordinate with these metal ions, forming ionic bonds. These interactions are particularly strong because calcium ions can form stable complexes with nitrogen atoms, leading to enhanced mechanical interlocking and chemical bonding between the ULCNT elements and the cement matrix.

[0061] Most amino functionalization processes require prior functionalization with hydroxyl or carboxyl groups on the CNT surface to form a covalent linkage, followed by introducing amino groups. The process of introducing the amino groups includes dipping, immersing (in liquid, gas or vapor), spraying etc. The time period for applying these chemicals on the material can be from 0.5 hour to a week. Several iterations (e.g. up to seven) can be used. The concentration of the different chemical amino functionalizing agents should be in the range of 0.01-10 M. Chemical amino functionalizing agents include: 3 -aminopropyltri ethoxy silane, ethylenediamine, di ethylenetriamine, polyethyleneimine, poly dopamine, N-hydroxysuccinimide and 1- ethyl-3-(3-dimethylaminopropyl)carbodiimide, 4,4' -diaminodiphenylmethane, diaminobenzidine and diazonium salts.

[0062] Other oxidizers have the potential of directly functionalizing the CNT surface with amino groups without the need for hydroxyl or carboxyl groups to be present. The process includes dipping, immersing (in liquid, gas or vapor), spraying etc. The time period for applying these chemicals on the material can be from 0.5 hour to a week. Several iterations (e.g. up to seven) can be used. The concentration of the different nitrogen containing oxidizers should be in the range of 1-12 M. Such oxidizers include: hydroxylamine (NH2OH), nitric acid (HNO3), ammonium persulfate ((NH4)2S2OS), nitrogen dioxide (NO2), nitrous acid (HNO2), nitrogen triiodide (NI3), nitrogen tetroxide (N2O4), nitro compounds, nitrosylsulfuric acid (HSO4NO2), and hydroxylamine-O-sulfonic acid (H2NOOSO3H).

[0063] Another method of creating bonding between the ULCNT element and the cement matrix is by reaction of the polymer matrix with which the ULCNT is impregnated with the cement matrix. For example, epoxy groups can form strong covalent bonds with the cement matrix through reaction with amines or hydroxyl groups. This can be effected by introducing partially cured ULCNT elements to the concrete mold to considerably increase the bonding strength between the reinforcing element and the cementitious matrix.

[0064] The macroscopic ULCNT element in the pristine or rigid (polymer infiltrated) formation typically has a bar or mesh morphology, but can also be designed to take the form of the concrete object to be produced. One or more elements are added to concrete as reinforcements to provide tensile and flexural strength. These reinforcements are typically placed strategically within the concrete structure to resist tensile and flexural stresses.

[0065] As the CNT elements in their natural form are not rigid, it is sometimes desired to stretch the CNT elements to increase their rigidity and tensile strength prior to casting the concrete mix. Stretching is typically performed at a rate of 0.1-50 mm / min. In order to protect the CNT element during stretching, it is possible to add a lubricant. The lubricant wets the surface of the CNT bundles within the macroscopic element and reduces friction between adjacent bundles, thus minimizing mechanical wear and damage to the CNTs during the stretching and alignment procedure. The lubricant also has the potential to reduce CNT damage due to overheating and improves the efficiency of the alignment process by reducing the energy consumption in stretching systems.

[0066] As CNTs are hydrophobic, most organic solvents and oils can serve as lubricants. The polymeric resins described above for use in providing strength to the CNT elements can also be used as lubricants for stretching prior to strengthening the elements. Superacids can also act as lubricants by protonating the CNTs thus creating a positive charge on their surfaces and repulsion forces between them.

[0067] Suitable organic solvent lubricants include: acetone, ethanol (ethyl alcohol), methanol (methyl alcohol), isopropanol (isopropyl alcohol), toluene, xylene, ethyl acetate, methyl ethyl ketone, hexane, chloroform, dichloromethane (methylene chloride), benzene, carbon tetrachloride, trichloroethylene, acetonitrile, dimethylformamide, dimethyl sulfoxide, tetrahydrofuran, diethyl ether, petroleum ether, cyclohexane, propylene glycol, butanol (butyl alcohol), cyclohexanone, naphtha, propylene carbonate, 1,4-di oxane, ethylbenzene, isopropyl acetate and styrene.

[0068] Suitable oil lubricants include: vegetable oils, animal oils, paraffin oil (white mineral oil), liquid paraffin (mineral oil), light mineral oil, heavy mineral oil, technical mineral oil, synthetic rubber process oil, synthetic ester oil, polyalphaolefin synthetic oil, diester synthetic oil, polyalkylene glycol synthetic oil, alkylated naphthalene synthetic oil, phosphate ester synthetic oil, silicone-based synthetic oil, fluorinated synthetic oil, polyolester synthetic oil, polyvinyl ether synthetic oil and polyphenyl ether synthetic oil.

[0069] Suitable superacid lubricants include: fluoroantimonic acid (HSbFe), magic acid (a mixture of fluorosulfuric acid (HSO3F) and antimony pentafluoride (SbFs)), carborane superacids (derivatives of carborane, such as H(CHBuClu)), perfluorinated acids (such as trifluoromethanesulfonic (triflic) acid, CF3SO3H), fluorosulfuric acid (HSO3F), fluorophosphoric acid (H2PO3F), Perchloric acid (HCIO4) in combination with certain metal halides, sulfuric acid (H2SO4) in combination with certain metal halides or fluorides, chlorosulfonic acid (HSO3CI), nitric acid (HNO3) in combination with triflic acid or certain metal fluorides, azelaic acid (C9H16O4), methanesulfonic acid (CH3SO3H), boron trifluoride (BF3) complexes with Lewis acids, zirconium tetrachloride (ZrCL) in combination with certain metal fluorides, boron trifluoride etherate (BFs’EtzO) in combination with certain metal halides, titanium tetrachloride (TiCL) in combination with certain metal fluorides, iodine pentafluoride (IF5) in combination with certain metal fluorides, antimony pentafluoride (SbFs), chlorine trifluoride (CIF3) in combination with certain metal fluorides, and sulfuric acid and oleum (fuming sulfuric acid) mixed with fluorosulfuric acid or antimony pentafluoride.

[0070] The concrete mix can be any standard or non-standard concrete mix. A concrete mix comprises cement, aggregate, water and optional additives.

[0071] In a preferred embodiment, the cement is Portland cement. Preferably, the cement is Type I Portland cement, which is the most widely used cement in concrete construction. It is suitable for general construction purposes and is used in a variety of applications, including reinforced concrete structures such as buildings, bridges, and highways. Other varieties of Portland cement than can be used in the present invention include:

[0072] • Portland-Limestone Cement (PLC), a variant of Portland cement that includes interground limestone. It offers similar performance to Portland cement but with lower carbon dioxide emissions during production. PLC can be used in reinforced concrete applications where sustainability and environmental considerations are important.

[0073] • Portland Pozzolan Cement (PPC), a type of Portland cement that contains pozzolanic materials such as fly ash, silica fume, or volcanic ash. Pozzolanic materials react with calcium hydroxide in the presence of water to form cementitious compounds, enhancing the strength and durability of concrete. PPC is suitable for reinforced concrete construction and is often used in structures exposed to aggressive environments.

[0074] • Sulfate-Resistant Portland Cement (Type II): Type II Portland cement is formulated to resist attack by sulfates, making it suitable for use in environments where the concrete may be exposed to sulfate-rich soils or waters. It is commonly used in reinforced concrete construction for foundations, marine structures, and wastewater treatment facilities. • Moderate Sulfate-Resistant Portland Cement (Type II (M)): Type II (M) Portland cement offers moderate resistance to sulfate attack and is suitable for use in reinforced concrete structures exposed to moderate sulfate concentrations. It provides enhanced durability in environments where sulfate exposure is a concern.

[0075] • Low Heat Portland Cement (Type IV): Type IV Portland cement generates less heat during hydration compared to other types of cement. It is often used in massive concrete structures, such as dams and large foundations, where controlling heat of hydration is critical to prevent thermal cracking. Type IV Portland cement can be used in reinforced concrete construction where heat buildup needs to be minimized.

[0076] • High-Early- Strength Portland Cement (Type III): Type III Portland cement is designed to develop early strength rapidly, allowing for faster formwork removal and construction schedules. It is suitable for reinforced concrete applications where early strength gain is required, such as in cold weather concreting or in fast-track construction projects.

[0077] The aggregate can include coarse aggregates, such as gravel or crushed stone, and / or fine aggregates, such as sand. The ratio of coarse to fine aggregate can vary based on the desired properties of the concrete mix.

[0078] Optional additives can be included to modify the properties of the concrete mix. Examples of the optional additives include:

[0079] • Plasticizers (Water Reducers): Plasticizers are additives that improve the workability of concrete without increasing the water content. They reduce the amount of water required for a given level of workability, resulting in increased strength, improved finishability, and reduced permeability. This can be particularly beneficial for reinforced concrete where proper consolidation around the reinforcement is crucial. Some examples include but are not limited to: lignosulfonates such as calcium lignosulfonate or polyethylene glycol such as PEG 400.

[0080] • Superplasticizers: Superplasticizers are a type of plasticizer that provides even greater water reduction and workability enhancement compared to standard plasticizers. They are often used in high-performance concrete mixes to achieve very high strength, improve flowability for easier placement, and reduce the risk of segregation and bleeding. Some examples include but are not limited to: polycarboxylate ethers (PCEs) such as Glenium® 7500, naphthalene sulfonates such as sulfonated naphthalene formaldehyde, or melamine sulfonates such as Melment® F10.

[0081] • Retarders: Retarders are admixtures that delay the setting time of concrete. They are useful in hot weather conditions or when extended workability is required. In reinforced concrete construction, retarders can help ensure proper placement and consolidation around reinforcement without premature setting. Some examples include but are not limited to: calcium lignosulfonate, sugars (e.g., sucrose), hydroxycarboxylic acids, phosphates (sodium phosphate and potassium phosphate), boric acid and gypsum.

[0082] • Accelerators: Accelerators are admixtures that speed up the setting and early strength development of concrete. They are particularly useful in cold weather conditions or when rapid construction schedules are required. In reinforced concrete, accelerators can help achieve early strength gain, allowing for faster formwork removal and progression of construction. Some examples include but are not limited to: calcium chloride, calcium nitrate, sodium nitrate, calcium formate, triethanolamine (TEA), potassium carbonate and silica fume.

[0083] • Air-Entraining Agents: Air-entraining agents are admixtures that introduce microscopic air bubbles into the concrete mix. These air bubbles improve the durability of concrete by enhancing its resistance to freeze-thaw cycles, reducing segregation and bleeding, and improving workability. In reinforced concrete, airentraining agents can help prevent damage to the concrete and reinforcement caused by freezing and thawing in cold climates. Some examples include but are not limited to: vinsol resin, wood rosin, sodium lauryl sulfate, calcium stearate, alkylbenzene sulfonates, Darex® AEA, Micro Air® and Sika® Air.

[0084] • Shrinkage Reducers: Shrinkage reducers are admixtures that mitigate the shrinkage of concrete as it cures, reducing the risk of cracking. In reinforced concrete, shrinkage reducers can help minimize the potential for cracks that could compromise the integrity of the structure and affect the bond between the concrete and the reinforcement. Some examples include but are not limited to: Sika® Control-40, MasterLife SRA 20, Fritz-Pak Control Finish, DEY® Addiment SRA and ConShield® Shrinkage-Reducing Admixture.

[0085] Typical concrete mixes that can be used with the present invention include 1 part cement, 0.4-0.6 parts water, 1-2 parts fine aggregate (sand) and 2-3 parts coarse aggregate (gravel or crushed stone). Additives can be included as necessary. Some specific examples of concrete mixes include (all parts are by weight):

[0086] • Standard Mix for General Construction (max. compressive stress: 3000 psi)

[0087] Cement: 1 part

[0088] Water: 0.5 parts

[0089] Fine aggregate: 2 parts

[0090] Coarse aggregate: 3 parts

[0091] Additives: As needed

[0092] • High Strength Mix for Structural Elements (max. compressive stress: 5000 psi)

[0093] Cement: 1 part

[0094] Water: 0.4 parts

[0095] Fine aggregate: 1.5 parts

[0096] Coarse aggregate: 2.5 parts

[0097] Additives: As needed for specific performance enhancements

[0098] • Special Mix for Seismic Resistance (max. compressive stress: 3500 psi)

[0099] Cement: 1 part

[0100] Water: 0.45 parts

[0101] Fine aggregate: 2 parts

[0102] Coarse aggregate: 2.5 parts

[0103] Additives: As needed for enhanced seismic performance, such as shrinkage reducers and plasticizers

[0104] Lightweight Mix for Reduced Dead Load

[0105] Cement: 1 part

[0106] Water: 0.5 parts Lightweight fine aggregate (expanded clay or shale): 1 parts

[0107] Coarse aggregate: 2 parts

[0108] Additives: As needed for workability and enhanced properties specific to lightweight concrete

[0109] Casting the concrete mix over the macroscopic CNT element can be performed using a pre-cast method or a cast-in-place method. In the pre-cast method, the macroscopic CNT element or elements are set in a frame of the desired size and shape and the concrete mix is poured into the frame. After curing, the resulting reinforced concrete object can be transported to a desired location. In the cast-in-place method, the concrete mix is cast over the macroscopic CNT element in the location in which the reinforced concrete object is needed.

[0110] In order to prepare a reinforced concrete object, a macroscopic CNT element or elements and a framework into which the concrete will be poured are prepared in the days prior to production. On the day of production, a concrete mix is prepared on-site by mixing cement, aggregates, water and any additives. The CNT elements are assembled within the framework, and the concrete mix is poured into the framework.

[0111] Immediately after pouring, the reinforced concrete object is covered or sprayed with curing compounds to prevent moisture loss and maintain a favorable environment for hydration. The covering may be wet burlap or plastic sheeting. This initial curing stage lasts for the first few days after pouring and is crucial for preventing premature drying and ensuring adequate strength development. After the initial curing period, the concrete continues to cure over an extended period, typically several days to weeks, depending on project specifications and environmental conditions. Various curing methods may be employed, including water curing (sprinkling or ponding), wet covering, steam curing, or application of curing compounds. These methods help maintain moisture levels within the concrete and promote complete hydration of the cement, resulting in improved strength, durability, and resistance to cracking.

[0112] Throughout the curing stage, the concrete element is regularly monitored for temperature, moisture content, and strength development. Quality control checks are conducted to ensure that curing conditions meet project specifications and industry standards. Once the concrete has sufficiently cured and gained strength, the framework is removed carefully to expose the reinforced concrete object. Surface finishing operations may be performed to achieve the desired appearance and texture of the concrete element.

[0113] The thickness of concrete in reinforced concrete structures varies depending on several factors, including the structural design requirements, intended use of the structure, and local building codes. The following are some general guidelines for the typical thickness of concrete in different structural elements of steel reinforced concrete buildings.

[0114] For suspended floor slabs in residential and commercial buildings, typical thicknesses range from 100 mm to 150 mm (4 inches to 6 inches). For elevated slabs in parking garages or industrial buildings, thicknesses may range from 150 mm to 300 mm (6 inches to 12 inches) or more, depending on the required load -bearing capacity and span. The thickness of reinforced concrete beams and columns is typically larger than the slab thickness, ranging from 200 mm to 600 mm (8 inches to 24 inches) or more, depending on the structural requirements. The concrete cover, which is the thickness of concrete surrounding the reinforcement in beams and columns, is typically specified by design codes and standards. It may vary depending on factors such as the size of the reinforcement and the exposure conditions.

[0115] The thickness of concrete walls in reinforced concrete structures varies depending on factors such as the height of the wall, the lateral loads it must resist and the desired level of durability. For exterior walls in residential and commercial buildings, thicknesses may range from 150 mm to 300 mm (6 inches to 12 inches) or more, depending on structural requirements and insulation needs. Interior walls may have smaller thicknesses, typically ranging from 100 mm to 200 mm (4 inches to 8 inches), depending on the load-bearing capacity and design considerations.

[0116] The thickness of concrete foundations depends on factors such as the type of foundation (e.g., spread footing, mat foundation), the soil conditions, and the structural loads. For spread footings supporting columns or walls, typical thicknesses range from 300 mm to 900 mm (12 inches to 36 inches) or more, depending on the size of the loads and soil bearing capacity. Mat foundations, which distribute loads over a large area, may have thicknesses ranging from 600 mm to 1200 mm (24 inches to 48 inches) or more, depending on the structural requirements and soil conditions.

[0117] One of the advantages of CNT reinforcement elements is that they are non- corrosive, unlike steel. In steel -reinforced concrete, the concrete acts as a corrosion barrier for the steel reinforcement elements. Accordingly, the concrete must be sufficiently thick to prevent corrosion of the steel. Since CNT elements do not require a corrosion barrier, the thickness of the concrete in CNT-reinforced concrete objects may be decreased by 30 to 70% relative to the thickness of the concrete in the equivalent steel-reinforced object.

[0118] Furthermore, the mechanical properties of ULCNT reinforcement elements are equivalent and up to 5 times better than steel reinforcement bars in tensile stress, as shown in Table 1.

[0119] Table 1

[0120] Therefore, it is expected that the number of ULCNT reinforcement elements will be equal or up to 5 times smaller than the number of steel bars. Alternatively, the number of ULCNT elements can remain the same but with a cross-section that is similar or to up to 5 times smaller than steel reinforcement bars. In addition, the density of ULCNT elements is typically about 1 g / cm3, whereas the typical density of steel rebars is about 8 g / cm3. Thus, the mass of ULCNT elements in a reinforced concrete object can be 8 to 40 times lower than if steel rebars are used. The lower weight of the reinforcement together with the lower thickness of concrete required leads to a significant overall reduction in weight for a ULCNT reinforced concrete object relative to a steel- reinforced object.

[0121] The conductive nature of CNT fibers allows them to function as both a reinforcement and a sensor element. The ULCNT elements may be used as an embedded resistive strain gauge for continuous structural health monitoring within the reinforced concrete object. The elements can be used either in their pristine form or after post treatment in the form of a prepreg or a fully cured ULCNT element.

[0122] A resistive strain gauge is a sensor that utilizes the principle that the electrical resistance of a conductor changes when it experiences physical deformation (strain). Strain refers to the relative change in length of an object due to applied stress (force). When a conductor is stretched or compressed, its length and cross-sectional area change. This, in turn, affects the ease with which electricity flows through it, altering its electrical resistance via two main mechanisms: (1) Length change - as the object stretches, the ULCNT fiber (or rod) lengthens, increasing its resistance. Conversely, compression shortens the fiber (or rod), decreasing resistance. (2) Cross-sectional change - deformation can also alter the fiber’s cross-sectional area, further affecting resistance. When the concrete object strains or flexes, the ULCNT element experiences the same strain and deforms accordingly. By measuring the change in resistance of the strain gauge, the strain experienced by the object can be inferred. Optionally, a Wheatstone bridge circuit can be used to amplify the change in resistance for more accurate measurement.

[0123] As the ULCNT elements are excellent, corrosive-resistant conductors, they can be directly connected to an ohm-meter like a standard electrical lead without any additional modifications. The expected change in resistance of the ULCNT element embedded in concrete is in the range of up to 5%, preferably up to 1%. In this range, the resistance and strain have a linear relationship. Changes in resistance above this range indicate a defect in the concrete object.

[0124] ULCNT elements embedded within the concrete are less susceptible to damage compared to surface-mounted strain gauges that are used in some structures. The exceptional strength and chemical stability of ULCNT elements contribute to the longterm functionality of the strain gauge system. This distinguishes the ULCNT elements from steel rebars that are susceptible to corrosion, which affects their electrical resistance. It is possibly for this reason that steel rebars are not used for structural health monitoring, thus highlighting another advantage of ULCNT elements over steel rebars for reinforcing concrete.

[0125] EXAMPLES

[0126] Example 1

[0127] A carbon nanotube fiber was prepared by the method disclosed in WO 2005 / 007926. The fiber was cut into six fiber strands which were held taut along the length of a framework measuring 450 mm x 150 mm x 6 mm. A concrete mix was poured into the framework and allowed to dry overnight. The thus-formed concrete slab was removed from the framework and stored in a water bath at 20 °C for six days. The slab was then cut with a diamond-tipped saw to dimensions 400 mm x 42 mm x 6 mm and stored in a climate chamber at 20 °C and 65% relative humidity. The concrete cover of the fibers was 3 mm.

[0128] Example 2

[0129] A concrete mix that is appropriate for thin (10-11 mm) samples was sought. Such a mix must contain only fine aggregates, since larger aggregates have dimensions larger than the desired slab. A disadvantage of using only fine aggregates is that a large amount of water is required, which reduces the strength of the concrete. One method of reducing the amount of water needed is to add a superplasticizer additive. A superplasticizer from the poly carboxylate family, Visocrete® 3110 (a poly carboxylatebased superplasticizer, Sika AG, Switzerland) was chosen. Water to cement ratios of 0.42-0.50 were tested. The properties of the optimal mix found are set forth in Table 2.

[0130] Table 2

[0131] In this mix, the water to cement ratio was 0.46 and the cement to sand ratio was 0.44.

[0132] The mix was formed by adding the superplasticizer to water and then adding the cement and mixing until homogeneous. Then, sand was added in portions and the mixture was mixed until homogeneity was reached. The resulting viscous mix was poured into a framework 10 as shown in Fig. 1. Framework 10 comprises wooden beams 12 resting on a plate of Teflon coated glass 14. Cross beams 16 create voids 18 that are about 200 mm long and 45 mm wide. The mix was poured into framework 10 to a thickness of about 10 mm and allowed to set for 24 hours. Upon removal from the framework, the samples were placed in a water bath for seven days to achieve complete hydration of the concrete. After seven days, the samples were removed, washed and air dried.

[0133] Additional samples were prepared, wherein carbon nanotube fibers prepared by the method disclosed in WO 2005 / 007926, were embedded in the samples. CNT fibers 20 were placed in framework 22 (see Fig. 2), which includes screws 24 to hold the fibers taut. Each fiber end was attached to a first end of a terminal crimp 26. The second end of terminal crimp 26 was attached to a rubber band 28, which was connected to one of screws 24. The rest of the process was the same as for the unreinforced samples. The illustrated embodiment shows five fibers per sample. However, any number of fibers may be used in each sample.

[0134] Additional CNT fibers 20 were prepared and pretreated prior to being placed in framework 22. The CNT fibers were subjected to thermal oxidation at a temperature of approximately 400 °C for a duration of two hours. During this treatment, selective oxidation occurs at defect sites and along the nanotube surface, leading to the formation of carbonyl (C=O) and carboxyl (-COOH) groups. These functional groups promote hydrophilic interactions with chemical moieties present in the concrete, including calcium-rich and silicate phases. Furthermore, the functionalized surfaces enable localized hydrolytic interactions with the cement bulk, allowing the CNTs to participate in hydration-related processes. This facilitates stronger interfacial bonding and mechanical integration of the CNT network within the hardened concrete microstructure, contributing to improved reinforcement and durability. Two samples with untreated fibers and three samples using fibers with thermal pretreatment were prepared.

[0135] To each sample 30 were attached four metal segments 32, shown in Fig 3, to facilitate tensile testing. Two segments were glued to both the top and bottom of the sample 30 at each end, covering about 50 mm of the ends of the sample, such that the portion of the sample between the segments is constant at 95-100 mm, as shown in Fig. 4. Sika AnchorFix® 3001 glue was used. The two segments at either end were held together using clamps 34 for at least seven hours. Clamps 34 were removed and a further metal segment 36 was attached at each end of sample 30 between the ends of two segments 32 not attached to sample 30 using a screw 38, as shown in Fig. 5. The other end of further segment 36 was attached to a headless screw 40 of a Shimadzu® AGS-X tensile testing machine. The results of the tensile testing are shown in Tables 3, 4 and 5. The stress-strain curves are shown in Figs. 6 (pristine concrete), 7 (non-treated fibers) and 8 (treated fibers).

[0136] Table 3

[0137] Table 4 Table 5

[0138] The treated fibers give superior results. The highest tolerated stress for the treated fibers was 3.4 MPa and the lowest was 2.5 MPa, whereas for the pristine concrete the highest tolerated stress was only 2.4 MPa, and for the untreated fibers the highest tolerated stress was 2.1 MPa.

[0139] The improved performance of the treated fibers can be attributed to improved interaction with the concrete due to the creation of hydrophilic groups on the fibers, as confirmed by SEM imaging. Fig. 9 shows a SEM image of an untreated fiber in concrete. It can be seen that there is little interaction between the fiber and the concrete. It appears that the CNT fiber surface is not wetted by the concrete matrix, as only some concrete particle debris are seen on the fiber surface. This de-wetting effect leads to the poor adhesion as shown. On the other hand, the SEM image of the treated fiber in concrete in Fig. 10 shows significant interaction between the fiber and the concrete. It appears that concrete nanoparticles (10-50 nm) are generated within the fiber bulk. This strong interaction leads to a strengthening of the concrete as evidenced by the improved tensile testing results.

[0140] The improved interaction between the treated fibers and the concrete can also be observed based on the breaking of the concrete samples. Fig. 11 shows a sample with untreated fibers at the end of the tensile testing. Fig. 12 shows a sample with treated fibers at the end of the tensile testing. The untreated fibers remained intact after the concrete fractured, suggesting poor bonding of the fibers to the concrete. This allowed the fibers to slip through the matrix during failure. However, the treated fibers broke together with the concrete, showing that they were strongly connected to the concrete.

[0141] The results of Example 2 demonstrate that carbon nanotube fibers, when subjected to thermal oxidation to introduce hydrophilic functional groups, significantly improved the tensile performance of reinforced concrete. Treated fibers exhibited higher maximum stress and greater mechanical integration with the concrete matrix, as shown by SEM imaging and fracture behavior. These findings support the use of functionalized CNT elements as superior reinforcement materials in concrete due to the enhanced bonding and mechanical strength.

[0142] It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as modifications thereof which would occur to a person of skill in the art upon reading the foregoing description and which are not in the prior art.

Claims

CLAIMS1. A method of producing a reinforced concrete object comprising: providing at least one macroscopic carbon nanotube element; and casting a concrete mix over the carbon nanotube element.

2. The method of claim 1, wherein the at least one carbon nanotube element form of a bar.

3. The method of claim 1, wherein the at least one carbon nanotube element form of a woven mat or a non-woven mat.

4. The method of claim 1, wherein the at least one carbon nanotube element form of a fiber.

5. The method of claim 1, wherein the at least one carbon nanotube element form of a braid.

6. The method of claim 1, wherein the at least one carbon nanotube element form of a yarn.

7. The method of claim 1, wherein the at least one carbon nanotube element form of a multi-thread yarn.

8. The method of claim 1, wherein the at least one carbon nanotube element form of a multi-thread twisted yarn.

9. The method of claim 1, wherein the at least one carbon nanotube element form of a strip.

10. The method of claim 1, wherein the at least one carbon nanotube element form of a twisted strip.

11. The method of any one of the preceding claims, wherein the at least one carbon nanotube element is produced by a floating catalyst chemical vapor deposition process.

12. The method of any one of the preceding claims, wherein the at least one carbon nanotube element comprises ultra-long carbon nanotube (ULCNT) bundles.

13. The method of claim 12, wherein the bundles have a diameter of about 5 to about 150 nm.

14. The method of claim 12, wherein the bundles have a diameter of about 15 to about 30 nm.

15. The method of any one of claims 12-14, wherein each bundle comprises about 10 to about 200 carbon nanotubes.

16. The method of any one of claims 12-14, wherein each bundle comprises about 10 to about 50 carbon nanotubes.

17. The method of any one of claims 12-16, wherein the bundles have an aspect ratio of at least 10,000.

18. The method of any one of claims 1-17, wherein the at least one carbon nanotube element consists essentially of carbon nanotubes.

19. The method of any one of claims 1-17, wherein the at least one carbon nanotube element comprises carbon nanotubes and a matrix.

20. The method of claim 19, wherein the matrix is organic, ceramic, metallic, or a mixture thereof.

21. The method of any one of claims 1-20, further comprising stretching the at least one carbon nanotube element, to align the CNTs within the at least one element to enhance its tensile strength and increase its rigidness, prior to casting the concrete mix.

22. The method of claim 21, wherein said stretching is performed at a rate of 0.1-50 mm / min.

23. The method of claim 21 or 22, wherein said stretching is performed in a lubricant.

24. The method of claim 23, wherein the lubricant is selected from the group consisting of organic solvents, polymeric resins prior to the addition of a hardener, polymeric resins following the addition of a hardener, acids, bases, ionic liquids, or a mixture thereof.

25. The method of any one of claims 21-24, wherein said at least one carbon nanotube element is stretched to at least 120% of its original length.

26. The method of any one of claims 21-24, wherein said at least one carbon nanotube element is stretched to at least 150% of its original length.

27. The method of any one of claims 21-24, wherein said at least one carbon nanotube element is stretched to at least 200% of its original length.

28. The method of any one of claims 1-27, further comprising functionalizing the at least one carbon nanotube element with functional groups prior to casting the concrete mix.

29. The method of claim 28, wherein the functional groups include hydroxylic, carboxylic, and aminic groups or any combination thereof.

30. The method of claim 29, wherein the hydroxylic and carboxylic groups are added by thermal oxidation, chemical oxidation or electrochemical means.

31. The method of claim 29, wherein the amine groups are added covalently by the use of hyponitrous acid (H2N2O2), ammonia (NH3), hydroxylamine (NH2OH), nitric acid (HNO3), ammonium persulfate ((NH4)2S2Os), nitrogen dioxide (NO2), nitrous acid (HNO2), nitrogen triiodide (NI3), nitrogen tetroxide (N2O4), nitro compounds, nitrosylsulfuric acid (HSO4NO2), or hydroxylamine-O-sulfonic acid (H2NOOSO3H), or non-covalently with amine containing moi eties such as3 -aminopropyltri ethoxy silane, ethylenediamine, diethylenetriamine, polyethyleneimine, polydopamine, N- hydroxysuccinimide and l-ethyl-3-(3-dimethylaminopropyl)carbodiimide, 4,4'- diaminodiphenylmethane, diaminobenzidine and diazonium salts.

32. A reinforced concrete object comprising: concrete; and at least one macroscopic carbon nanotube element.

33. The reinforced concrete object of claim 32, wherein the at least one carbon nanotube element is in the form of a bar.

34. The reinforced concrete object of claim 32, wherein the at least one carbon nanotube element is in the form of a woven mat or a non-woven mat.

35. The reinforced concrete object of claim 32, wherein the at least one carbon nanotube element is in the form of a fiber.

36. The reinforced concrete object of claim 32, wherein the at least one carbon nanotube element is in the form of a braid.

37. The reinforced concrete object of claim 32, wherein the at least one carbon nanotube element is in the form of a yarn.

38. The reinforced concrete object of claim 32, wherein the at least one carbon nanotube element is in the form of a multi-thread yarn.

39. The reinforced concrete object of claim 32, wherein the at least one carbon nanotube element is in the form of a multi-thread twisted yarn.

40. The reinforced concrete object of claim 32, wherein the at least one carbon nanotube element is in the form of a strip.

41. The reinforced concrete object of claim 32, wherein the at least one carbon nanotube element is in the form of a twisted strip.

42. The reinforced concrete object of any one of claims 32-41, wherein the at least one carbon nanotube element comprises ultra-long carbon nanotube (ULCNT) bundles.

43. The reinforced concrete object of claim 42, wherein the bundles have a diameter of about 5 to about 150 nm.

44. The reinforced concrete object of claim 42, wherein the bundles have a diameter of about 15 to about 30 nm.

45. The reinforced concrete object of any one of claims 42-44, wherein each bundle comprises about 10 to about 200 carbon nanotubes.

46. The reinforced concrete object of any one of claims 42-44, wherein each bundle comprises about 10 to about 50 carbon nanotubes.

47. The reinforced concrete object of any one of claims 42-46, wherein the bundles have an aspect ratio of at least 10,000.

48. The reinforced concrete object of any one of claims 32-47, wherein the at least one carbon nanotube element consists essentially of carbon nanotubes.

49. The reinforced concrete object of any one of claims 32-47, wherein the at least one carbon nanotube element comprises carbon nanotubes and a matrix.

50. The reinforced concrete object of claim 49, wherein the matrix is organic, ceramic, metallic, or a mixture thereof.

51. The reinforced concrete object of any one of claims 32-50, wherein the at least one carbon nanotube element comprises functional groups.

52. The reinforced concrete object of claim 51, wherein the functional groups include hydroxylic, carboxylic, and aminic groups or any combination thereof.

53. A method of structural health monitoring of a reinforced concrete structure, the reinforced concrete structure including a reinforced concrete object having embedded therein at least one macroscopic carbon nanotube element, the method comprising: measuring an electrical resistance over a length of the at least one macroscopic carbon nanotube element over a period of time; and based on changes in the resistance, determining a strain endured by the reinforced concrete object.

Images