Resma Final Report PDF 11

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1. INTRODUCTION

1.1 Nanotechnology

The idea behind nano-science started at a talk entitled Theres Plenty of Room at

the Bottom by physicist Richard Feynman at an American Physical Society meeting at

the California Institute of Technology on December 29, 1959. This was long before the

term nanotechnology came into use. Feynman talked about a process by which scientists

would be able to manipulate and control individual atoms and molecules.

Nano science involves the ability to control individual atoms and molecules. By

convention, nanotechnology is taken as the scale range of one to hundred nanometers

(following the definition used by the National Nanotechnology Initiative in the US). The

lower limit is set by the size of atoms (which is approximately a quarter of an nanometer

in diameter) while the upper limit is more or less arbitrary.

One nanometer constitutes about a billionth of a meter. For example, a sheet of paper is

about 1, 00,000 nanometers thick. On a comparative scale, if the diameter of earth were a

meter, then a nanometer would be the diameter of a marble.

Nanotechnology considers two main approaches as shown if Fig.1:

The Top-Down Approach- large structures are simply reduced in size to the nano

scale while maintaining their original material properties. Here the materials are

deconstructed to their smaller composite parts without any atomic level control.

The Bottom-Up Approach- materials are engineered from atoms or molecules through

a process of self-assembly. This approach is also called molecular nanotechnology or

molecular manufacturing.

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Fig.1 Top Down and Bottom Up Approaches in Nanotechnology

(Source: Florence and Konstantin 2010)

Nano engineered materials have a relatively larger surface area when compared to

their original form which can be understood from Fig.2. When modified at the nano

scale, matter shall exhibit certain extraordinary and useful properties, different from

before. This is because the properties of a material are size-dependent in this scale range.

In some cases, inert materials may become reactive when produced in their nano scale

form. Some nanostructured materials may also become stronger and have different

magnetic properties compared to other forms or sizes of the same material. Some may be

better at conducting heat or electricity, while some may reflect light better or change

colour when their size or structure is altered.

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Fig.2 Variation of Surface Area with Particle Size


Therefore, nanotechnology does not simply involve working with smaller

particles; it rather involves the utilization of the different physical, chemical, optical and

mechanical properties that occur at the nano scale. For instance, nano scale gold particles

are not the yellow colour which as in macro-scale; rather nano scale gold appears red or

purple. At the nano scale, the motion of the golds electrons is confined. Because of this

restriction, gold nanoparticles react differently with light when compared to the larger-

scale gold particles. These phenomena can be put to practical use. For instance, nano

scale gold particles selectively accumulate in tumors. This can be used for precise

imaging and targeted laser destruction of the tumor without harming the healthy cells.

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2. COMMONLY USED NANO MATERIALS

2.1 Carbon nanotubes (CNTs)

Carbon nanotubes have a cylindrical shape and can be up to several millimeters

long. They maybe of two types single walled CNTs (SWCNTs) or multi walled CNTs

(MWCNTs). CNTs are known to have around 5 times the Youngs Modulus and 8 times

the strength of the strongest steel while having only one-sixth their density. They are also

efficient thermal conductors. Improved flexural, shear and compressive strengths,

durability, and improved resistance to corrosion and crack formation can be achieved by

the addition of CNTs to cementitious composites. Baoguo et al. experimentally found that

addition of CNTs in concrete reduces the water permeability, sorptivity and gas

permeability of concrete which in turn reduces corrosion effects. Schematic

representation of SWCNTs and MWCNTS is shown in Fig.3.

Fig.3 Schematic Representation of (a) SWCNT and (b) MWCNT

(Source: Bryan 2010)

2.2 Zinc oxide nanoparticles (ZnO)

Zinc oxide is used in the manufacture of concrete. It improves the processing time

and resistance of concrete against water permeability. Zinc oxide is also added to plastics,

ceramics, glass, rubber, paints, adhesives, sealants, pigments, etc., for its semiconductor

and piezoelectric properties.

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2.3 Silver nanoparticles (Ag)

Nano silver on contact with bacteria, viruses, fungi, etc., has the ability to affect

their cellular metabolism and inhibit cell growth. Thus nano silver can be utilized for

reducing or inhibiting the growth and multiplication of bacteria and fungi which may

otherwise cause infections, odours, etc.

2.4 Aluminium oxide nanoparticles (Al2O3)

Aluminium compounds react with calcium hydroxide produced by hydration of

calcium silicates. The rate of pozzolanic reaction is proportional to the amount of surface

area available for reaction. Thus when added to concrete, it may improve the split tensile

strength and flexural strength of concrete.

2.5 Zirconium oxide nanoparticles (ZrO2)

Zirconium oxide nanoparticles are white in colour with high surface area and

typical dimensions of five to hundred nanometers. They show good translucency, high

physical resistance, chemical resistance, and are also good insulators.

2.6 Titanium dioxide nanoparticles (TiO2)

Titanium dioxide is a white pigment that can be used as an excellent reflective

coating. It can also be added to paints, cement, windows, etc., for its sterilizing

properties. Titanium dioxide when applied to outdoor surfaces breaks down organic

pollutants, volatile organic compounds and bacterial membranes through powerful photo-

catalytic reactions, thus reducing air pollution. These pollutants can now be washed off

using water (rain) since titanium dioxide nanoparticles are hydrophilic in nature. Water

particles shall accumulate and form sheets of water which dissolves the organic pollutants

in it. As the water moves down, it takes the pollutants along with it. Therefore it also

imparts self-cleansing properties to the applied surface. An example of such a structure is

shown in Fig.4; Church Dives in Misericordia, Rome, Italy was the first structure

where the application of the self-cleansing concrete using TiO2 was done.

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Fig.4 Church Dives in Misericordia, Rome, Italy

(Source: Pacheco and Said 2011)

2.7 Silicon dioxide nanoparticles (SiO2)

These are mainly useful in concrete having large volumes of fly-ash. Nano silicon

dioxide increases the compressive strength of concrete at an early stage by filling the

pores between the large fly-ash particles and cement particles. It decreases the setting

time of mortar and reduces the bleeding of water and segregation by improving the

cohesiveness.

2.8 Wolfram (tungsten) oxide nanoparticles (WO3)

In recent years, tungsten dioxide has been employed in the production of smart

windows which are electrically switchable glasses that changes light transmission

properties with an applied voltage. This allows the user to tint their windows as required,

thus changing the amount of heat or light passing through.

2.9 Nano ferrous oxide (Fe2O3)

Inclusion of nano phase ferrous oxide particles in cement improves the properties

like split tensile strength and flexural strength. It is also found to increase the setting time

of cement.

2.10 Nano silica

Nano silica has been found to increase the strength, durability, flexibility and

workability of concrete. Nano silica particles increase the viscosity of the fluid phase of

concrete and fill the voids between cement grains.

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3. NANOTECHNOLOGY IN CONSTRUCTION INDUSTRY

By the proper manipulation of nanoparticles, the properties of currently available

construction materials can be improved. The various construction materials in which

nanotechnology can be applied include:

3.1 Nanotechnology for concrete

Concrete is a macro-material. It can be strongly influenced by its nano-scale

properties. The degradation of calcium silicate hydrate reaction caused by calcium

leaching in water can be controlled by the addition of nano silica to cement based

materials thereby blocking the penetration of water into concrete and thus leading to

improvement of durability of concrete.

Also carbon nanotubes are known to increase the compressive strength of cement mortar

specimens and change their electrical properties. This in turn becomes useful in damage

detection. Addition of small amounts of carbon nanotubes can improve the mechanical

properties of mixture of Portland cement and water. Oxidised multi walled carbon

nanotubes have been found to improve the compressive and flexural strength of concrete

by a large factor. In addition there is also a great potential for the use of nano sensors in

concrete structures as they can be used for monitoring quality and durability of concrete.

3.2 Nanotechnology in steel

The properties of steel can also be improved by efficient utilization of

nanoparticles. The addition of copper nanoparticles can reduce the surface unevenness of

steel surfaces. Vanadium and molybdenum nanoparticles improve the delayed fracture

problems associated with high strength bolts, improving the steel microstructure and

reducing the effects of embrittlement.

3.3 Nanotechnology for wood

Wood is composed of nanotubes or nano fibrils. Highly water repellent coatings

can be made for coating wood surfaces using silica and alumina nanoparticles and

hydrophilic polymers.

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3.4 Nanotechnology for glass

Titanium dioxide nanoparticles can impart self-cleansing properties when used in

glass. The organic pollutants and bacteria present on the surface shall be decomposed and

broken down, which can be easily washed down by rain or water as titanium dioxide

nanoparticles are hydrophilic in nature. Fumed silica (SiO2) nanoparticles can also be

used in the making of fire protective glasses. This forms an interlayer sandwiched

between two glass panels and turns into a rigid and opaque fire shield if heated.

3.5 Nanotechnology for coatings and paints

Corrosion protection can be assured by the addition of hydrophobic nanoparticles.

Being hydrophobic, they repel water from metal pipes. It can also protect metal from salt

water attack.

3.6 Nanotechnology for thermal insulation

Silica based nanoparticles can be used for transparent insulation, which leads to

the possibility of super insulating windows.

Also micro and nano electromechanical systems offer the possibility of controlling and

monitoring the internal environment of buildings. This in turn leads to energy savings.

3.7 Nanotechnology for fire protection

Fire resistance of steel structures is usually done by providing a coating using a

spray-on cementitious process. By mixing the carbon nanotubes with cementitious

materials, fibre composites having some of the good properties of carbon nanotubes can

be made. Thus nano cement has the potential to create tough, durable and high

temperature coatings.

3.8 Nanotechnology for structural monitoring

Nano sensors can be utilised for monitoring and controlling the environmental

conditions and structural performance of concrete. They range from 1 to 100 nanometers.

Nano sensors can be embedded into the structure during the construction phase.

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4. CONCRETE AND NANOTECHNOLOGY

Fig.5 Particle Size and Specific Surface Areas related to Concrete Materials


The mechanical behaviour of concrete materials depend on the structural elements

and the phenomena that are effective at micro and nano scales. The size of the calcium

silicate hydrate phase which is the primary component responsible for the strength lies in

the range of nanometers. The particle size and specific surface areas of various materials

are shown in Fig.5.

The global cement production is around 800million tonnes/year. The calcination

of limestone produces approximately 0.97 tonnes of CO2 for each tonne of clinker

produced. Around 900kg of clinker is used in each 1000kg of cement produced. So the

global cement industry produces around 1.4 tonnes of CO2 each year. This represents

about 6% of the total worldwide man-made CO2 production. In order to reduce these

emissions and also to improve the mechanical properties of cementitious composites,

various researches are going on especially in the field of nanotechnology.

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5. CARBON NANOTUBES

Carbon nanotubes are hollow tubular channels formed either by a single wall or

multiple walls of rolled graphene sheets. The former being called single walled carbon

nanotubes (SWCNTs) and the latter being multi walled carbon nano tubes (MWCNTs).

Their ends are capped by dome shaped half fullerene molecules. MWCNTs are formed

by concentric SWCNTs placed concentrically one inside the other. SWCNTs have a

diameter of 0.4 to10 nm while MWCNTs have a diameter ranging from 4 to 100 nm. The

mechanical properties of nanotubes greatly depend on the arrangement of atoms of nano

structure. The atomic structure of nanotubes is defined by the tube chirality. Based on this

there are mainly two types of CNTs namely, zigzag shaped which has a zero chirality and

arm chair shaped with a chiral angle of 30 degrees as shown in Fig.6.

Fig.6 (a) Armchair shaped and (b) Zig-zag Nanotubes

(Source: Saptarishi et al. 2013)

CNTs form a 2-D lattice structure where the carbon atom is attached to 3 other

carbon atoms. One of the carbon-carbon bonds is a double bond. This 2-D hexagonal

structure gives CNTs stronger bond than diamond along the plane and relatively weak

inter-planar bonds. This allows the different tubes within the MWCNT to slide relative to

one another. The lengths of carbon nano tubes are not restricted. They usually range from

micro to even millimeter ranges. CNTs are produced usually by a growth process from

one end. The main force that holds the nanotubes in place is the Van der Waals force.

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5.1 Mechanical properties

Nanotubes can be either metallic or semiconducting depending on the tube

chirality. SWCNTs have a hollow center making it lighter. CNTs have a Youngs

Modulus of around 1TPa , an average tensile strength of 60GPa , average ultimate strain

of 12% and a density of 1.3-1.4 g/cc. MWCNTs also have a large aspect ratio of up to

2500000:1, carbon purity greater than 90%, length of 1-10 micrometers and 3-15

nanotube walls.

5.2 Synthesis of CNTs

CNTs are synthesized chiefly by three methods: electric arc discharge, laser

ablation, and thermal or plasma assisted chemical vapor deposition (CVD).

5.2.1 Electric arc Discharge- here an electric arc is passed between two carbon

electrodes, through an inert gas like argon or helium. The high temperature caused by the

arc causes carbon to sublimate and re-solidify into the highly organized CNT structure.

This setup is shown in Fig.7. Electric arc discharge produces CNTs with purity around

30% by weight and can be used to produce both SWCNTs and MWCNTs.

Fig.7 Electric Arc Discharge Method

(Source: www.nanoscience.com)

5.2.2 Laser Ablation technique-as shown in Fig.8, it uses a pulsing laser to vaporize a

piece of graphite within an inert gas inside a furnace at 2000C. The vaporized graphite

solidifies onto the cooler walls of the reaction chamber and forms CNT. Up to 70% purity

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by weight (SWCNTs) can be obtained by this process. The diameter of CNT is controlled

by controlling the temperature of the chamber. A very high and pure form of CNT may

be obtained here but it is an expensive process.

Fig.8 Laser Ablation Technique


The main disadvantage regarding the above method is that CNTs cannot be

produced continuously by this process. During nanotube synthesis, impurities in the form

of catalyst particles, amorphous carbon, etc., are also produced. Therefore, for these

processes, an additional purification step is necessary to separate the tubes.

5.2.3 Chemical Vapor Deposition (CVD)-it is a technique where nanotubes are formed

by the decomposition of a carbon containing gas. The carbon based gas

(ethanol/methane) along with a metal catalyst is used to initiate the growth of CNT. Since

carbon source is continually replaced by flowing gas, amount of impurities are also lesser

than electric arc and laser ablation techniques. Fig.9 shows the CVD setup.

Fig.9 Chemical Vapor Deposition Method


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This is a widely used method for up-scale to industrial production mainly because

of its low cost/unit rate. Also the reactor can be readily designed to operate in continuous

mode. Other advantages also include higher yield and purity of CNT and the possibility

of synthesizing long CNTs of controlled diameter, length and alignment.

5.3 Requirements for a good CNT reinforcement

5.3.1 Good Dispersion

Because of the large surface area of CNTs, there are chances of agglomeration of

particles due to strong Wan der Waals forces between the nanotubes. The carbon

nanotubes may agglomerate in the form of ropes and clamps. These are very difficult to

entangle. These agglomerations can form large voids within the cement matrix and are

unable to transfer stresses across the bundles. This along with the hydrophobicity, lack of

solubility, and nano dimensions of CNTs, makes their handling and dispersion into

cement very difficult. Thus it is necessary to have a well dispersed matrix of CNTs. Only

then the stresses can be uniformly dispersed from the cement matrix to the nano

filaments. Otherwise stress concentrations may occur due to uneven distribution of nano

filaments.

5.3.2 Uniform Alignment of Nano filaments

Alignment is necessary to produce a uniform stress transfer when under axial

loads. This is because nano filaments perpendicular to primary stresses cannot transfer

stresses along its axis efficiently. Therefore, if aligned parallel, full efficiency can be

achieved.

5.3.3 Large Aspect Ratio

To optimize the bond between cement and nano filaments, surface area should be

very high.

5.3.4 Optimal Bond

Optimal bond between cement and nano filaments is necessary because if the

bonding is weak, it can cause the filaments to slide out of the cement matrix under a load

much less than the strength of individual nanotubes. Thus proper adhesion must be

ensured.

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5.4 Dispersion of CNTs

Dispersion of CNTs in concrete is critical in deciding the properties of cement

based nano composites. Dispersion of CNTs directly within the cement paste during

mixing is not feasible as the thickening of cement paste begins within a short period after

the addition of water. Otherwise, it may result in large CNT clusters within the hydrated

paste. Therefore to avoid this, nano materials are first dispersed in water and then the

nano material-water dispersion is mixed with cement using a conventional mixer. It has

been found that homogeneous distribution of nano materials in cement is possible only

when cement particles are also distributed homogeneously without any agglomeration.

5.4.1 Dispersion techniques

5.4.1.1 Physical Techniques

5.4.1.1.1 Ultrasonication-In an ultrasonic processor, electrical voltage is converted to

mechanical vibrations, which are transferred to the liquid medium (water or solvent)

which leads to the formation and collapse of microscopic bubbles. During this process

(known as cavitation), millions of shock waves are created and a high level of energy is

released, leading to dispersion of nano materials in the liquid. Fig.10 shows the effect of

CNT dispersion when mixed by hand stirring, hand shaking and ultra-sonic mixing.

Fig.10 Effect of CNT Dispersion when Mixed with Water and a Surfactant by (a)

Hand Stirring, (b) Hand Shaking, and (c) Ultrasonic Mixing

(Source: Bryan 2010)

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5.4.1.2 Chemical techniques

5.4.1.2.1 Use of surfactants- surfactants can be used to improve the aqueous dispersion

of nano materials by reducing surface tension of water. It can also lead to stable

dispersion as a result of electrostatic or steric repulsions between the surfactant molecules

adsorbed on the nano material surface. However, the dispersion capability of surfactants

strongly depends on their concentration, and an optimum surfactant to nano materials

ratio should be used for preparing cementitious composites.

5.4.1.2.2 Use of cement admixtures- poly-carboxylate which is commonly used as a

super plasticizer with cement paste, was found to be effective in the dispersal of CNT.

5.4.1.2.3 Covalent functionalization- the most common approach to improve the

dispersion ability of CNTs in water is the covalent functionalization. Covalent

functionalization using acid mixture has been found to be successful in dispersing CNTs

individually within the cementitious matrix. Moreover, CNTs become tightly wrapped by

the C-S-H phase of cement, due to covalent bonding between COOH or C-OH groups of

nanotubes and C-S-H.

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6. ADVANCES IN INSTRUMENTATION

The development of instrumentation with high spatial resolution has made it

possible to observe the structure of concrete at the nano-scale and to measure the

physical, chemical, and mechanical properties of its microscopic and nanoscopic phases.

The characterization of materials and their mechanical properties can be done by:

6.1 Scanning electron microscopy (SEM)

The electron microscope was developed when the wavelength became the limiting

factor in light microscopes. It was found that electrons having much shorter wavelengths

enable better resolution. A scanning electron microscope scans a focused electron beam

over a surface to create an image. The electrons in the beam interact with the sample,

thereby producing various signals that can be used to obtain information about the

surface topography and composition. SEMs are widely used in a number of industries and

laboratory es to investigate the microstructure and chemistry of a range of organic and

inorganic materials. The main components in an SEM include:

A source of electron

A column down which the electrons travel

An electron detector

A sample chamber

Computers and displays to view the images

The samples are mounted and placed in the sample chamber. The sample chamber can

include translation stage, tilt, and rotation devices. The electrons are produced at the

source by thermionic heating. These are then accelerated to a voltage of 1-40kV and

condensed into a narrow beam. The main types of electron sources include:- tungsten

filament consisting of a v-shaped wire of tungsten which is heated to produce electrons,

solid state crystal which is a thermionic emission gun and field emission gun consisting

of a wire of tungsten with a sharp tip, that uses field electron emission to produce the

electron beam. Fig.11 shows the various electron sources.

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Fig.11 Electron Sources namely, Tungsten filament, Solid State Crystal and Field

Emission Gun


The electrons produced at the top of the column are accelerated down and passed

through a combination of lenses and apertures to produce a focused beam of electrons as

shown in Fig.12. These electrons hit the surface of the sample which is mounted on a

stage in the chamber area. Both the column and the chamber are evacuated by using

pumps. The level of vacuum shall depend on the design of the microscope.

Fig.12 Schematic Representation of Column and Chamber of an SEM


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The position of the electron beam on the sample can be controlled by the scan

coils provided above the objective lens. These coils allow the beam to scan over defined

areas of the sample. The resulting signals produced by the electron-beam interactions are

detected by one or more detectors to form images which are then displayed on the

computer screen.

6.2 Scanning tunnelling microscopy (STM)

The STM works by scanning a very sharp metal wire tip over a surface. By

bringing the tip very close to the surface, and applying an electric voltage to the sample

or the tip, the image surface can be obtained at an extremely small scale i.e. even down to

the individual atoms. Fig.13 shows the level of resolution that can be achieved using an

AFM.

Fig.13 Atomic Resolution with STM


The STM works mainly on three principles-

The quantum mechanical effect of tunnelling which helps to see the surface, the

piezoelectric effect which allows the precise scanning of the tip with angstrom level

control, and the feedback loop which monitors the tunnelling current and coordinates it

and also positions the tip. Tunnelling is a quantum mechanical effect. In quantum

mechanics, electrons also have wavelike properties. A tunnelling current is said to occur

when electrons move through a barrier which they classically should not be able to move

through. In classical terms, if there is not enough energy to move across a barrier, such

movement does not occur. However, in the quantum mechanical world, electrons have

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wave like properties. These waves dont end abruptly at a wall or barrier, but taper off

quickly. But if the barrier is thin enough, the probability function may extend into the

next region through the barrier. Thus there is a small probability of an electron being on

the other side of the barrier and therefore, if there are enough electrons, some electrons

will be able to move across the barrier and reach the other side. When an electron moves

through a barrier in this manner, it is called as tunnelling. For a thick barrier this does not

happen. The current through the barrier drops off exponentially with barrier thickness.

This effect has been shown in Fig.14.

Fig.14 Electron Wave function


In case of an STM, the tip or the sample acts as the starting point of the electron.

The barrier is the gap between them (air, vacuum, or liquid) and the second region will be

the tip or the sample depending on which one was the starting point. By monitoring the

current through the gap, the tip-sample distance can be controlled. The piezoelectric

effect is usually created by squeezing the sides of certain crystals, such as quartz, lead

zirconium titanate or barium titanate. This creates opposite charges on the sides. This

effect can also be reversed if necessary. The feedback loop shall constantly monitor the

tunnelling current and make necessary adjustments to the tip in order to maintain a

constant current. These adjustments are recorded by the computer and using them, the

image of the sample can be obtained. This is called constant current image. For very

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flat surfaces, the feedback loop can be turned off and only the current will be displayed.

This is called constant height image.

6.3 Atomic force microscopy (AFM)

The AFM was developed to overcome the basic drawback with STM i.e., an STM

can image only conducting or semiconducting surfaces. The AFM has the advantage of

imaging almost any type of surface, including polymers, ceramics, composites, glass, and

biological samples. AFM is also referred to as scanning probe microscopy (SPM). In an

AFM instead of using the quantum mechanical effect of tunnelling, atomic forces are

used to map the tip-sample interaction. Today most AFMs use a laser beam deflection

system where a laser is deflected from the back of the reflective AFN lever and onto a

position-sensitive detector as shown in Fig.15.

Fig.15 Laser Beam Deflection System


The AFM relies on the forces between the tip and the sample. As shown in Fig.16,

these forces are not measured directly. Instead they are measured based on the deflection

of the lever.

By Hookes Law, (1)

Where, F is the force, k is the stiffness of the lever and z is the distance the lever is bent.

Based on these data, the sample surface can be reconstructed.

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Fig.16 Force-Distance curve for AFM


6.4 Profilometry

This is a technique used to generate topographical data from a surface.

Profilometry scans can be at a single point, line scan or even a 3-d scan. Profilometry

may be used to obtain the surface morphology, step heights and surface roughness.

There are mainly two parts for a profilometer namely a detector stage and a sample stage.

The sample stage holds the sample and the detector stage determines where the required

points on the sample are. Either the sample stage or both the detector and the sample

stage may move in order to allow for measurement. There are mainly two types of

profilometers: stylus and optical.

Stylus profilometry is the earliest form of profilometry and is usually used for

hard surfaces. It uses a probe made of a hard material like diamond and shall have the

ability to scratch or intend the surface of the sample. This method involves physically

moving the probe along the surface. The changes in the position of the arm holder can

then be used to reconstruct the surface. While the technique is extremely sensitive and

provides the highest resolution, it is also sensitive to soft surfaces and the probe may

become contaminated by the surface. Also it can be destructive to some soft materials

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and is a slow process. Schematic representation of a stylus profilometer is given in

Fig.17.

Fig.17 Schematic Representation of a Stylus Profilometer


Optical profilometry uses light instead of a physical probe. The key component to

this technique is directing the light in such a way that it can detect the surfaces in 3

dimensions. Optical profilometry is faster than contact profilometry with sacrifices in

lateral resolution. It is completely non-destructive to samples that are not sensitive to

light and can scan soft surfaces. Fig.18 gives the schematic representation of an optical

profilometer.

Fig.18 Schematic Representation of an Optical Profilometer


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7. CASE STUDIES

Nur and Vinoth (2014) conducted a comparative study on the compressive

strength, flexural strength and flow values of cement mortar reinforced with CNTs and

carbon nanofibres (CNFs). Ordinary Portland Cement (OPC) with a compressive strength

of 24 MPa was used for the experiment. The samples were tested for water-cement ratios

(w/c) of 0.35, 0.4, 0.45 and 0.5 and dosage rates of 0.1% and 0.2% CNT/CNF by weight

of cement. Superplasticizer was also added in the ratio of 0.008 by weight of cement.

7.1 Compressive strength

ASTM C109 test procedure was used to determine the compressive strength of

cement mortar using 50mm cubes. The mortar was prepared with 1 part cement and 2.75

parts of graded sand. After sonication, the nano particles were mixed with cement and

sand for 4 minutes. The mortar was cast, demoulded after 24 hours and stored in lime

saturated water tanks. The compressive strength of the cubes were tested at 7, 14 and 28

days using a universal testing machine at the rate of 890 to 1800 N/s. Comparative

representation of the above was made which is shown in Fig.19.

It was found that among the CNT samples, the maximum compressive strengths were

shown by those samples with 0.45-0.5 w/c ratios for both dosages.

Fig.19 Compressive Strength at 28 days

(Source: Nur and Vinoth 2014)

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7.2 Flexural strength

Flexural strength was evaluated as per the ASTM C348 test procedure. A sample

of 40mm40mm160mm was prepared in the same manner as done for compressive

strength test. A three point loading set up was used with a loading rate of 2640110 N. A

similar comparative study was done using cement mortar reinforced with different

dosages of CNTs and CNFs (0.1% and 0.2%) and for various water cement ratios (0.35,

0.4, 0.45, 0.5). The 28-day flexural strengths achieved by the 0.1% and 0.2% CNT/CNF

samples were presented as shown in Fig.20. It was observed that the nano reinforced

samples performed better in flexural strength than the corresponding control samples

except when a water cement ratio of 0.5 was used where CNF reinforced samples were

not as efficient. But CNT reinforced samples were found to exhibit higher flexural

strength for both concentrations as well as for all the w/c ratios used.

Fig.20 Flexural Strength at 28 days

(Source: Nur and Vinoth, 2014)

7.3 Flow test

The ASTM C1437 test procedure was used to calculate the flow of mortar. The

mortar was placed 25mm from the bottom of the mould and compacted 20 times in 2

layers. The mortar was then compacted and flushed with the surface of the mould. The

flow mould was removed and the flow table was dropped 25 times in 15 seconds. The

diameter of this mortar was recorded and the percentage increase in base diameter of the

mortar was calculated which gives the flow value. The flow of mortar after the drop has

been shown in Fig.21. The graphs showing the variation of compressive strength with

flow values where plotted. The compressive strength of CNT cement composites where

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found to increase at higher flow values, especially at the 0.2% mixing ratio as shown in

Fig. 22.

Fig.21 Flow of Mortar after the Drop


Fig.22 Flow Values vs. Compressive Strength of CNT and CNF Composites


Madhavi et al. conducted studies on the strength characteristics and durability of

concrete reinforced with MWCNTs. A total of 36 concrete specimens of M30 grade were

casted with a w/c ratio of 0.4. Of these, 27 specimens were mixed with MWCNTs in

proportions 0.015%, 0.03% and 0.045% by weight of cement. The specimens were water

cured for 28 days and tested for water absorption test and split tensile strength test using

cubes of size 150150150 mm and cylindrical specimens of diameter 150mm and

height 300mm.

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To avoid agglomeration of the MWCNTs, sonication process was carried out by

adding MWCNTs with surfactants, at 0.25% by weight of cement. Ultrasound energy

was applied for a sonicated period of 30 minutes. The sample was kept for magnetic

stirring for another half an hour to get a uniform mixture. After sonication, the MWCNTs

were mixed with water. Then the cement, coarse aggregate and fine aggregate were dry

mixed and the MWCNT-water mixture was added to this dry mix and rapid mixing was

done.

7.4 Water absorption test

The water absorption test for concrete was done after 28 days as per ASTM C642-

8. The test results indicate that by increasing the proportion of MWCNTs in concrete, the

percentage of water absorption can be decreased. About 17.76% decrease of water

absorption was obtained by the addition of 0.045% of MWCNT. Table 1 shows the

variation of % water absorption with % of MWCNTs.

Table 1 Variation of Water Absorption with Addition of MWCNTs

Sl No. % of

MWCNT

% water

absorption

% reduction in

water

absorption

1

Conventional

concrete 0.5873

2 0.015 0.5273 10.22

3 0.03 0.5027 14.41

4 0.045 0.483 17.76

(Source: Madhavi et al. 2013)

7.5 Split tensile strength

The split tensile strength tests were done as per IS 5816-1999. Cylindrical

specimens of height 300 mm and diameter 150 mm were used for testing. The breaking

load (P) and failure patterns were noted. It was found that with the addition of

functionalised MWCNTs, the compressive strength of concrete and the split tensile

strength increased as shown in Table 2.

27

Table 2 Variation of Split Tensile Strength with Addition of MWCNTs

Specimen

Split tensile

failure load

(kN)

Split tensile

strength

(N/mm2)

% increase

Conventional

concrete 160 2.27 _

0.015%

MWCNT 210 2.97 30.84

0.030%

MWCNT 235 3.3 45.37

0.045%

MWCNT 265 3.775 66.3

(Source: Madhavi et al. 2013)

28

8. CONCLUSIONS

Nanoparticles have a tendency to agglomerate due to van der Waals forces between

the particles. Thus in order to efficiently disperse the nanotubes an effective dispersion

technique like sonication must be employed.

Addition of carbon nanotubes results in an increase in the mechanical properties of

the composites. An effective w/c ratio of 0.45 produced higher compressive and flexural

strengths. Composites prepared using CNTs at 0.1% by weight of cement attained 54%

and14% higher compressive and flexural strength than plain cement mortar.

The split tensile strengths of concrete have also exhibited better performance upon the

addition of CNTs. The flow tests indicate that compressive strength of CNT composites

increased when the flow value was higher, especially at the 0.2% mixing ratio. CNT

mortar mixes with high compressive strengths would also be easier to work with in the

field. The water absorption tests also indicate that addition of MWCNTs results in a

reduction in the water absorption. This helps the concrete to be more durable and water

resistant.

8.1 Interpretation by the author

The mechanical properties of concrete can be greatly improved by the addition of

carbon nanotubes. Also addition of carbon nanotubes results in composites with better

durability. The reduction in water permeability indicates that the corrosion resistance of

concrete and reinforcing steel can be increased. Addition of carbon nanotubes will also

result in a decrease in the carbon dioxide emissions. Thus concrete structures can be

made more eco-friendly and also structures conforming to international green building

norms can be constructed.

However, for proper utilisation of these advantages, the CNTs must be dispersed

uniformly in the cement matrix. Thus preparation of CNT reinforced concrete must be

done with utmost care. Techniques for industrial scale production of CNTs have to be

developed so that their costs can be reduced considerably. The environmental effects of

CNT synthesis and utilisation must also be studied. If properly utilised, CNTs has a great

potential to revolutionise the traditional practices followed in the construction industry.

29

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