[ ] July 20, 2012

Independent Study Survey:

Sustainable, “Green” Concrete July 20, 2012

Mentor: Dr. Fouad Fouad

Student: Austin Dada

University of Alabama at Birmingham

Department of Civil, Construction, and Environmental Engineering

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Objective of Research

The purpose of researching “green”

concrete materials is to determine a more

sustainable solution to the future population

growth and infrastructure demands that the

world will require. Concrete is the second most

used material in the world after water and in

order to ensure that the cost of manufacturing

and transporting concrete and cement does not

dramatically increase, solutions must be found

which promote the use of recycled aggregates

which are closer to construction sites and

alternatives to portland cement must also be

researched. Also, mining large quantities of

aggregates and minerals for the production of

raw concrete materials results in extreme

deforestation and denudation of topsoils

around the world. The amount of carbon

dioxide released due to the manufacture of

ordinary fresh concrete due to fossil fuel

combustion and calcination of limestone is in

the order of one ton for every ton of concrete

produced (Raijiwala, Patil 2010). The Kyoto

Protocol calls for the world’s industrialized

nations to reduce their respective carbon

footprints by 5.2% below 1990 levels by the

year 2012 and research into the solutions for

green concrete will greatly aid in these efforts.

Also, stress on current landfills and

accompanying taxes would be alleviated should

recycled concrete aggregates become more

prominently used in the construction industry,

especially in structural applications.

Introduction

Concrete sustainability has long been

an important issue in the construction industry

for environmental stewardship and economic

reasons. With concrete being the second most

used material in the world, after water, and

with the durability and strength benefits that

come with concrete as a building material, it is

evident that emerging sustainability practices

would place a heavy emphasis on the

manufacture and placement of concrete.

Concrete, however, already possesses inherent

qualities which make it an already sustainable

building material. It is very durable, recyclable,

and has a high energy performance. It can be

used to reduce noise pollution and the urban

heat-island effects. Concrete also improves

indoor air quality and lighting efficiency.

Cement, one of concrete’s ingredients, is

usually manufactured very close to the site at

which it will be used in construction which

diminishes the amount of transportation

needed to get materials to any given

construction site, thereby reducing fuel

consumption and costs.

However, issues plaguing the

sustainability of concrete are also present. The

process which is employed to create the most

commonly used cement, portland cement, is an

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extremely energy intensive process, consuming

a large amount of resources and releasing very

harmful air pollutants into the atmosphere.

Carbon dioxide is emitted in very large

quantities at cement manufacturing facilities, a

greenhouse gas which is seen as threatening to

the sustainability of modern civilization. Cement

manufacture accounts for about 5% of the

world’s carbon dioxide emissions. Around 50%

of the world’s population resides in urban

locales and as populations continue to increase,

it is only reasonable to assume that urban

areas, and hence construction, will begin to

increase as well. This will, in turn, cause more

cement to be manufactured as well, and in

order for the cement and concrete industry to

be sustainable in the future, solutions must be

found to make concrete a more sustainable

material.

Various efforts are being employed to

ensure that concrete becomes a more

sustainable material. Three areas of interest for

the increased sustainability of concrete covered

herein include the replacement of portland

cement using sialite technology, the use of

recycled concrete aggregates, and the use of

concrete as a vessel for carbon sequestration.

Also the use if nanotechnology in concrete

structures was studied and presented.

Sialite Technology

Sialite technology is being touted as a

sustainable alternative to portland cement. It

requires less energy to produce and is gathered

from natural resources which are found in

abundance. Compared to portland cement,

sialite cementitious technology also emits a

miniscule amount of pollution (Sun, Jain,

Nguyen, and Zuckerman 2009). Sialite is rich in

silica and alumina. These two elements are

where the name “sialite” is derived. This

technology was developed using principles

present in the formation natural rock

formations in the field of earth science. Sialite

contains two main ingredients, solid alumina ad

silica rich industrial wastes which are grounded

into powder. Sialite also contains a rock forming

agent which acts as a catalyst in helping the

material form the natural rock formations. This

concept of natural rock formation is derived

from the process that occurs when volcanic ash,

formed by high temperatures and volcanic

activity, is converted into rock. This process

usually takes millions of years to complete but it

has been observed that volcanic ash that comes

into contact with seawater and the minerals

present therein, is converted into rock at a

much faster rate. Rock forming agent contains

the minerals present in seawater and the

volcanic ash is replaced with industrial waste

silica and alumina powder. The rock forming

agent converts the silica and alumina products

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in a matter of minutes as compared to the

natural process. The process of the production

of sialite can be seen to be more

environmentally friendly than that of portland

cement and the raw materials used to create

sialite are present in abundance. The

framework that sialite employs is an

aluminosilicate structure which is a material

that comprises 70% of the earth’s crust while

portland cement, calcium oxide, only comprises

5% of the earth’s crust. A large portion of sialite

components comes from solid waste material as

well, 95%. Sialite also poses a great economic

advantage in its production over portland

cement due to the small amount of energy that

is produced. Its capital investment per ton is

$15 compared to the $45 for portland cement.

It also produces in the range of 30 – 90% less

CO2 than portland cement manufacture. Its

production cost is also 30% lower than that of

portland cement’s.

In the realm of material properties and

performance, sialite can be engineered to be

denser and less porous than portland cement

which in turn, would increase the strength of

concrete made with sialite as a component. The

framework of the microstructure for sialite

allows more bonds between its silicon and

oxygen molecules than portland cement allows

between its calcium and oxygen molecules, 4

directions verses 2 directions respectively (Sun,

Jain, Nguyen, and Zuckerman 2009). These

extra bonds make the structure stronger and

less likely to have fractures occur. Also, the

interface between portland cement and sand is

weaker than the interface between sialite and

sand. This does not make a great difference in

compressive strength when natural sand

aggregates are used but when recycled iron ore

tailings were used, sialite concrete showed

significant increases in its compressive strength

over portland cement. Sialite has been tested to

be more durable than portland cement as well,

with a longer service life due to its corrosion

resistance and sialite also has a much lower

heat of hydration which can help to mitigate

cracking due to uneven thermal expansion.

Disadvantages to the use of sialite

include lack of experience with the use of the

material in field applications and the lack of

standardized publications detailing proper use.

The rock forming agent which is employed in

the formation of sialite is dependent upon the

type of industrial waste which would be used. If

the wrong proportion of rock forming agent is

used or the wrong type of rock forming agent

mineral is used, then the compressive strength

of a sialite concrete mixture would be greatly

compromised. More testing is required in order

to determine the correct proportions and

combinations of ingredients to optimize

material properties of sialite concrete.

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Geopolymer concrete is another form

of concrete which employs alternatives to

portland cement. It is fly ash-based and does

not require the presence of portland cement as

a binder material. Geopolymer concrete is an

amorphous alumino-hydroxide product that

exhibits the ideal characteristics of rock-forming

elements, similar to sialite technology. This

technology has been determined to be able to

save up to 80% of CO2 emissions caused by the

cement and aggregate industries if employed

on a grand scale. This is achieved because the

geopolymer binder is a low-CO2 cementitious

material and does not rely on limestone

calcination to cure (Raijiwala, Patil 2010).

Geopolymer concrete is ideal for application of

use in the precast industry since it needs to cure

at very high temperatures and can be

manipulated easily in laboratory environments.

Geopolymer concrete has been cured at

temperatures between 250C and 600C for

lengths of around 24 hours. These methods

ensure that the geopolymer concrete exceeds

compressive strength of ordinary concrete by

1.5 times as well as splitting tensile strength by

1.45 times and flexural strength by 1.6 times.

Recycled Concrete Aggregates

A growing number of concrete

structures are experiencing obsolescence and

require demolition or repair. Recycled concrete

has been used for years to ensure that

demolished concrete won’t be wasted and that

construction practices and project s stay

sustainable and economical. In New Zealand

and many other nations however, these

recycled concrete aggregates have been mostly

used in low-end and nonstructural applications

rather than large-scale structural projects.

Research and projects have been shown to

prove recycled concrete aggregates for the

creation of high strength concrete. Past

research has shown that a film of water coating

concrete aggregates can produce a weak spot in

the production of concrete and certain methods

must be used to mitigate these weaknesses.

This water coated zone is known as the

interfacial transition zone and can increase the

w/c ratio at certain points in the concrete

thereby, undermining the target strength of the

mix. When compared to normal concrete,

recycled aggregate concrete has two interfacial

transition zones rather than the one present in

normal aggregate concrete. Methods such as

soaking recycled concrete aggregates in water

before mixing, improve the interaction between

cement paste and aggregate and can help to

reduce the problems caused by the extra

interfacial transition zone present in recycled

aggregate concrete. Also a two stage approach

can be used in the mixing of recycled aggregate

concrete, where half of the mix water is added

and mixed for 60 seconds and the other half of

the mix water is then added and mixed for an

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additional 120 seconds. This allows for micro-

cracks that were present in the recycled

concrete to be filled and decreases the ability

for the mix’s strength to be diminished. The

quality of the interfacial transition zone is also

improved in this method.

Research has shown that the

compressive strength of recycled aggregate

concrete faces a gradual reduction as the

recycled concrete aggregate content is

increased above 30%. General consensus

shows that compressive strength decreases by

up to 35% with the replacement of natural

aggregates with recycled concrete aggregates

(Zhang and Ingham 2010). However, when

correct proportions of recycled concrete

aggregate are put into place, it has been

observed that it can lead to higher compressive

strengths than those achieved by natural

aggregate concrete. This depends upon many

factors including the compressive strength of

the source concrete from which the recycled

concrete aggregate was garnered. Generally

the source compressive strength was found to

be required to be equal to or higher than the

target compressive strength of a new recycled

concrete aggregate mix. This same concept also

applies to flexural properties of concrete where,

concrete with a significantly lower flexural or

compressive strength than the target strength

was not suitable for production. Slump in

recycled concrete aggregate mixes was found to

be much smaller than that of natural aggregate

concrete.

Recycled concrete aggregate regularly is

treated with the same standards and

procedures which were written in order to

pertain to natural aggregate concrete. This

regularly results in mechanical properties which

are inferior to that of natural aggregate

concrete. For this and many other reasons, a

property of recycled aggregate concrete which

is observed closely is the residual mortar

content of the mixture. When this property is

taken into account, it can be seen that there is a

larger total mortar volume in recycled

aggregate concrete than natural aggregate

concrete. The “equivalent mortar volume”

method ensures that the mortar volumes of

recycled aggregate concrete and a

corresponding natural aggregate mix would be

equal and therefore, strength and other

properties would be equal.

Concrete Carbon Dioxide Sequestration

Carbon capture technology focuses on

the storage of carbon dioxide emissions in

depleted underground mines or wells or in

man-made vessels. Cement and concrete

production produces a great amount of carbon

dioxide gas as a bi-product of manufacture and

in order to make concrete a more sustainable

material, it has been seen as necessary to curb

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these emissions (Haselbach 2009). Carbon

capture in concrete has been viewed as a viable

option in order to increase sustainability. Non-

hydraulic cements usually use carbon dioxide as

a curing agent but early stage concrete which

employs portland cement can also sequester

carbon dioxide for curing purposes. A major

issue present in the realm of carbon

sequestration is search for application of the

captured carbon in industry or commercial

venues to produce value added economic

benefits. Carbon sequestration in concrete

makes concrete stronger through curing and

can pose as one of the many solutions to the

problem of value added benefits of carbon

capture technology (Monkman and Shao 2010).

Carbonation in concrete occurs when a

reaction between carbon dioxide and calcium

compounds results in a thermodynamically

stable calcium carbonate. Many calcium based

materials have the capacity to bind with carbon

dioxide. It has been researched and theorized

that this type of carbonation process can be

used heavily in the curing of precast concrete.

This carbonation process was found to reduce

concrete shrinkage by as much as 50%. The

amount of carbon dioxide used depends on

many factors including the amount and type of

cement used and the quantity of water used.

Usually, early-age concrete is used in order to

more effectively utilize as much carbon dioxide

as possible, to bind with as unstable molecules.

There has not been a large industrial-grade

implementation of carbon capture in concrete.

Calcination is the release of carbon

dioxide during the process of cement

production calcium carbonate is converted to

calcium oxide and CO2 is emitted. A reverse

calcination can occur if the cement paste in

concrete is exposed to carbon dioxide at a

certain humidity level. It is discouraged to

perform carbon capture methods on reinforced

concrete since it is theorized that the gas would

have an adverse affect on the reinforcing bars.

Nanotechnology use in Concrete Applications

The use of nanotechnology in concrete

is an emerging practice which promises to

expand the limits of concrete strength,

durability, and sustainability. Nanotechnology is

commonly defined as the, “understanding,

control, and restructuring of matter on the

order of nanometer to create materials with

fundamentally new properties” (et al. Sanchez,

Sobolev 2). Although nanotechnology is poised

to benefit the properties and understanding of

concrete structures at the nanoscale level, the

use of nanotechnology in industry concretes

remains an elusive practice and the results of

nanotechnology testing in concrete materials

rarely ever is converted to a marketable

solution. The primary advances in

nanotechnology have resided in its use of

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augmenting and understanding cementitious

materials. With the increased use of

nanotechnology in the field of concrete, not

only can cementitious materials be augmented

to display improved properties, but the

understanding of the molecular and structure of

concrete can be better understood. An example

of nanotechnology’s contribution to the

fundamental understanding of concrete

materials can be found in the knowledge of the

nanomaterials which compose concrete. The

amorphous phase, calcium-silicate-hydrate (C-S-

H) is the paste which largely holds concrete

materials together and has been greatly

researched with the use of nanotechnology.

Using a bottom-up approach to the viewing of

concrete materials, it can be seen that concrete

is composite of molecular group, surfaces, and

chemical bonds at the nanoscale. These physical

groups interact with each other through

chemical reactions, intermolecular forces, and

intraphase diffusion (et al. Sanchez, Sobolev

2010). It is known that processes and reactions

which occur at the nanoscale ultimately affect

the properties of the macroscale and the

performance of the bulk material.

Nanoscience and nano-engineering of

concretes refer to the two main ways in which

nanotechnology is exploited in order to analyze

and implement applications which improve the

properties and/or understanding of the nano-

structure of concrete materials. Nanoscience

deals with the measurement of the nanoscale

levels of concrete with the use of different

forms microscopy specialized for observation of

nanomaterials. This in turn, provides a greater

understanding of the makeup of concrete and

the bonds which occur at the nanoscale level

which strengthen or weaken the overall

structure of the material. Through the use of

nanoscience and instrumentation, it is now

clear that the some of the most important

facets of the C-S-H bonds in concrete exist at

the nanoscale and a fundamental

understanding of these small scale view of

concrete material is required in order to make

more accurate assessments of concrete

properties and how to augment them to one’s

liking. Nano-engineering refers to the

techniques used to augment the mechanical

properties of a cementitious material or to

provide the concrete the ability to self-monitor,

self heal, control cracking, and numerous other

abilities. Nano-engineering has recently been

achieved through the addition of nanosized

materials and building blocks to the concrete

mix (otherwise known as nanoparticles or

nanotubules). These nanomaterials are mainly

employed to induce or control certain behaviors

or to add properties to the bulk material among

other things. From these technologies, more

strict and accurate design methods can be

employed for concrete structures rather than

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the empirical approaches that have been

implemented thus far.

Examples of nanoscience and instrumentation

advances include nanoindentation, atomic force

microscopy, small-angle neutron scattering,

ultrasmall-angle X-ray scattering, quasi elastic

neutron scattering, nuclear magnetic resonance

spectroscopy, and nuclear resonance reaction

analysis. Atomic force microscopy (AFM) uses a

small (on the scale of micrometers) cantilevered

silicon tip brought in contact with surfaces,

which causes the cantilevered portion to deflect

slightly depending on its proximity to the

material and the materials properties, in order

to take precise measurements of surfaces at an

extremely small scale (fractions of a

nanometer). This tool has proved to be very

useful in the measure of the surface structure

of hydrating cements, cement paste

morphology changes, the carbonation process

of cements, and forces which produce cement

cohesion. AFM imaging has shown that C-S-H is

a well-ordered structure of nanoparticles,

rather than an amorphous phase. AFM has also

observed the formation of the C-S-H phases

during hydration and it has been observed that

this phase is made up of a group of identical

clusters with sizes close to 60 x 30 x 5 nm3 (et. al

Sanchez, Sobolev 2010). Nanoindentation

techniques made it possible to measure the

local mechanical properties of a cement paste

at the nanoscale. Using this approach in

instrumentation, it was found that the C-S-H in

cement pastes exists in low density, high

density, and ultra-high density forms and that

high density C-S-H degrades much less during

decalcification than the lower density C-S-H

clusters. This shows that the ratio of low to high

density C-S-H may have an effect on the design

of future concrete structures. Nanoindentation

has also been used to observe the interfacial

transition zone around concrete aggregates and

has been used to find that the elastic modulus

of the interfacial transition zone is not

proportional to the distance from the interface.

Nuclear magnetic resonance spectroscopy can

be used to quantify the silicon and aluminum

substituted tetrahedral chain lengths and

structural positioning of the elements which

comprise cement matrices. The technique has

contributed greatly to the debate over what the

true molecular structure of C-S-H is. Nuclear

resonance reaction analysis (NRRA) has

provided new insight into the mechanisms

which determine the setting and curing

processes of concrete and also measures the

effect of environmental factors on these

mechanical processes and the processes of

cement hydration. Neutron and X-ray

scattering techniques has allowed researchers

to be able to view the microstructure of cement

during the hydration process. This provides data

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on the morphology of the cement matrix during

hydration.

Nano-engineering is also known as

nanomodification and is a promising new field

of research for use in concrete and

cementitious matrices. Examples of nano-

engineering techniques that are currently being

researched and employed are the addition of

novel superplasticizers, nanoparticles, and

nanoreinforcements. Methodologies are

available which would allow for

nanotechnologies like these to be mixed

successfully with concrete in order to institute a

bottom up approach to the augmentation of

cement matrices, which will promote many new

and exciting properties for the increased

strength, weathering and self healing processes

of concrete. Engineering of concrete at the

nanoscale can take place in three places: in the

solid phase, in the liquid phase, and at

interfaces (et al. Sanchez, Sobolev 2010).

Nanosized cement particles and nanobinders

are generally being used and researched in

order to find an alternative to conventional

admixtures while at the same time reducing

carbon emissions and lowering carbon

footprint. Nanomaterials exist in three

groupings based on their geometry or shape.

These include quantum well, quantum wire, and

quantum dot. One of the principal structural

units in nanotechnology is the quantum dot

which can be configured into a myriad of

crystals and shapes. They have immense

amounts of surface area which allows for their

mechanical and electrical properties to be

greatly different from macroscale materials and

more sensitive to small changes in stress.

Nano-SO2 have been found to improve

the workability of concrete mixtures while also

increasing resistance to water penetration.

Calcium leaching is also moreso curtailed with

the use of nano-SO2 which prevents rapid

concrete degradation. It has been shown that

with the dispersal of nano-SO2 in concrete

matrices comprising 10% of the mix, 28-day

compressive strengths increased by 26%. Only a

10% increase in compressive strength was

observed with 15% silica fume (et al Sanchez,

Sobolev 2010). Even small additions of Nano-

SO2 were observed to increase flexural strength

by as much as 25% while still increasing

compressive strengths by 10%. Nano-TiO2

addition in concrete was shown to increase the

concrete’s ability to self clean and clean the

environment. A photovoltaic process is

triggered in this concrete which allows it to

degrade many environmental toxins such as

NOx, carbon monoxide, and chlorophenols.

Nano-Fe2O3 containing concrete gives concrete

the ability to “self-sense” as well as improves its

compressive strength. The volume electric

resistance of concretes with Nano-Fe2O3 was

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found to change with the load applied to the

concrete which allows for the concrete to

generally be able to determine and express the

stress it feels under compression at any given

time. Nano-Al2O3 has been shown to increase

the modulus of elasticity up to 143% with a

dosage of 5%. It does not have a great effect on

compressive strength however.

Nanoreinforcements comprise another

way in which nanotechnology can be

implemented in the production of concrete.

Nanotubes and nanofibers are the two main

methods of implementation for

nanoreinforcement. The nanomaterials have

strength on the order of terapascals for their

moduli of elasticity and tensile strength in the

range of gigapascals at around 20 times

stronger than steel (Sobolev, Gutierrez 2005) .

They also have unique electronic and chemical

properties which include the applications of

acting as transistors and tips for scanning probe

microscopes. Nanotubes have also been

proposed in reinforcing cables in long bridges

and high-rise buildings.

Experimentation with Carbon Nanotubes

Carbon nanotubes in particular have

incredible mechanical properties and have the

potential to greatly strengthen concretes

manufactured with ordinary portland cement.

The following will document the application of

nanotechnology, specifically carbon

nanotubules, in concrete through the

presentation of experimental data. In one

experimental setup, it was observed that

carbon nanotubes have extremely high van der

Waals forces between them and this fact makes

it hard for them to disperse effectively in any

given concrete mixture. This in turn, reduces

the fluidity of the concrete mixture. The

experiment centers on the best methods for

equally dispersing carbon nanotubes within

aqueous solution and also observes the

mechanical properties of concrete specimens

that reinforced by carbon nanotubes which

have been dispersed by differing methods.

Good dispersion of carbon nanotubes is a

prerequisite to creating concrete with highly

improved mechanical properties and

workability. In the aforementioned experiment,

ten differing solutions were prepared, all of

which were initially stirred magnetically with

50ml of water and .05g of carbon nanotubes

(CNTs) an a chemical admixture. The specimens

ranged from only containing CNTs, to CNT

mixtures with air entrainment and differing

additional methods of stirring, CNT mixtures

with polycarboxylate, mixtures with calcium

naphthalene sulfonate, and mixtures with

lignosulfonate. The major outcome of the

experimentation was that the mixtures with

both polycarboxylate and lignosulfonate

admixtures provided the best dispersion of

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CNTs in aqueous solution (Collins, Lambert,

Duan 2010).

Carbon nanotubes also come in two

differing configurations which are mainly known

as single wall carbon nanotubes (SWCNTs) and

multi walled carbon nanotubes (MWCNTs). In

another experimental setup, purified MWCNTs

were suspended in 0.26 wt. % solutions.

Surfactants were also added at a ratio of 4:1 of

the MWCNT weight in order to aid dispersion.

Ultrasonication was used in order to disperse

the nanotubes initially in all of the specimens. In

the next phase of the experimentation, differing

rotors were used in order to disperse the

mixtures even further. Swing bucket rotors

were used to disperse one half of the mixtures

and fixed angle rotors were used to disperse the

other half. Swing bucket rotors allowed the

tubes to hang from hinges as they were

centrifuged in order for the tube to become

horizontal as the acceleration of the centrifuge

initializes. Fixed angle rotors are a single block

of metal with cavities in which the tubes can be

placed. The cavities are bored at a

predetermined angle so materials are forced

against the sides of the tube as the centrifuge is

accelerating as opposed to traveling through

the media to the bottom of the tube as is seen

with the swing bucket motors. The solutions

were all then used in concrete mixes and

specimens were made. The solutions which

yielded the best concrete specimens in terms of

flexural strength were the swing rotor

suspended MWCNTs and the reference

(control) suspension MWCNT solution which

only consisted of the initial ultrasonication of

nanotubes. The results were similar for both of

these methods of dispersion at all stages of

curing for both the measurement of the Young’s

Modulus and flexural strength where flexural

strength was almost 3.25 MPa greater in both

of these dispersion methods than ordinary

concrete cement paste and traveled as far as up

to 12.6 MPa while the ordinary concrete

cement paste resides at around 9.3 MPa. The

fixed angle rotor suspension fell in between the

two best dispersion methods and the ordinary

concrete paste (Metaxa, Seo, Konsta-Gdoutos,

Hersam, Shah 2011).

Experimentation was also done on

concrete specimens with MWCNTs and differing

water/cement ratios, 0.3 and 0.5 in order to

come closer to a “crack-free” cementitious

material. Ultra high resolution field emission

scanning electron microscopy was used in order

to visually gauge the dispersion of the MWCNTs

in solution. MWCNTs with an aspect ratio of 700

were designated as short while those with an

aspect ratio of 1600 were considered as long.

Differing aqueous solutions of these types of

MWCNTs were created by sonication and

ordinary cement paste was added to each to

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achieve 0.3 and 0.5 w/c ratios. Long MWCNTs

produced concentrations of 0.025% and 0.048%

and short MWCNTs produced 0.048% and

0.08% concentrations. Scanning electron

microscopy was used to determine the

dispersion of each of the MWCNTs in the

concrete pastes. Control specimens with no

dispersion method were also created. Fracture

mechanics testing was conducted on all of the

specimens, including three-point bending tests.

A triboindenter was used to determine the

mechanical properties of the CNT composites.

The mechanical properties were greatly

increased above ordinary concrete in the

MWCNT reinforced specimens with small

amounts of CNTs (0.025 wt% - 0.08 wt%). Long

MWCNTs with 0.025% concentration and short

MWCNTs specimens with 0.08% concentration

exhibited similar increases in Young’s Modulus,

up to 55% and 35%, for nanocomposites with

w/c ratios of 0.5 and 0.3 respectively. It had

been shown through microscopy that the

MWCNTs effectively bridged gaps between

nanopores in the concrete and filled nanocracks

as well (Metaxa, Konsta-Gdoutos, Shah 2009)

Embedded carbon nanotubes can also

be used in order to detect damage in concrete

structures due to their ability to conduct

electricity and change conductivity based on the

amount of stress that they experience.

Experimentation was done on single walled

carbon nanotubes (SWCNTs) to test these

abilities. Structural integrity modeling is what

this process is formerly known as and it allows

damage in concrete to be detected at early

stages which can be used to prevent failure. The

SWCNTs were integrated in polymers to act as

strain sensors and mainly used to monitor levels

of mechanical strain. Eight cement-CNT sensors

were connected with a wireless communication

system and subjected to monotonic and cyclic

tensile testing with a universal testing machine.

Four were monotonically loaded to failure and

the other four would be cyclically loaded for a

period of about 3 min. Electrical resistance was

measured in each specimen during loading with

conventional strain gauges attached to the

middle of the sensor. Cement-CNT sensors were

also embedded in concrete beams to wirelessly

detect cracking and crack propagation inside

the specimens. These beams were subjected to

three-point loading where two were loaded to

failure and the other two were cyclically loaded

for a period of time and then monotonically

loaded to failure. It was found that the cement-

CNT sensors with SWCNT volume of 1% had

shown a resistance increase to 125 µє. Cement-

CNT sensors with a SWCNT volume of 0.5%

went up to 75 µє. The SWCNT content of 0.5%

did not produce enough conductive branches

with the matrix in order to get a higher reading

due to the smaller amount of CNTs in these

specimens (Saafi 2009).

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[ ] July 20, 2012

Future Research and Issues

Green concrete has been proven to be

beneficial to the construction industry if used in

greater and larger scale applications. It has

been determined to conserve energy, reduce

greenhouse gases, conserve natural resources

and land, and reduce overhead costs compared

to the production of ordinary concrete

produced form raw materials. Although the

benefits of these many technologies are visible

and plentiful, there are still many obstacles that

hamper the widespread use of green concrete

technologies. The main problem encountered

with the use of green concrete technology

practices is that the industrialized world is not

yet comfortable with using these technologies

in major scale structural projects. So far, green

concrete technologies have mostly been used in

low level nonstructural projects, including

architectural applications and as sub-base for

roads and bridges. The reason that these

technologies are not widely used is due to the

lack of standardized testing, design codes and

documents written to support their use in

industry. More study and research must be

done in these areas in order to develop the

necessary standards and codes to ensure that

the technology is employed in the future. Also,

commercial barriers exist in the use of green

concrete technologies as well and quality

control problems also play a large role in the

limited use of these technologies in structural

projects. Lack of experience with the

technologies and a poor image of using recycled

concrete in particular hamper the progress of

green concrete technologies as well. All of the

current problems present with the use of

unfamiliar green concrete technologies can be

eliminated by the proper implementation of

quality control practices and new standards for

use.

Some specific applications that can be

tested for the use of recycled aggregate

concrete include the use of de-icing salts. It is

not known how the continued use of de-icing

salts on concrete would affect the life of the

concrete and its potential use in further

applications as recycled material. This is an area

of study which needs to be addressed in order

to assess the long-term use of recycled

concrete.

Challenges of Nanotechnology use in Concrete

Nano-engineering, with its boundless

potential to reform and improve the mechanical

properties of cement matrices, has some

negatives and downsides to its use as well.

Several challenges will need to be addressed

before this pathway to nanotechnology in

concrete can be traversed including:

understanding how to properly disperse the

nanoparticles in the cement, showing more

large scale results of nanoparticle reinforced

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[ ] July 20, 2012

concrete, and lowering the cost-benefit ratio of

the use of nanotechnology in concrete. Future

development in the field of nanotechnology in

concrete includes the creation of biomaterials

that mimic naturally strong structures, next

generation superplasticizers which increase

workability and reduce water more effectively,

binders which feature an internal moisture

supply to decrease micro-cracking, ecobinders

which can greatly decrease the need for

portland cement by about 10%, among many

other developments (Sobolev, Gutierrez 2005).

Conclusion

Concrete is an essential material for

infrastructure and construction and in order to

increase sustainability in infrastructure,

concrete must be a major focus. It is estimated

that world-wide cement production will more

than double by 2050 compared with 2005 rates

due to increased building activities in the third

world (Nielsen and Glavind 2007). The

excavation of aggregate resources impacts

environments and habitats, affects

groundwater levels and drainage patterns,

increases the cost of transportation, and leads

to the production of large amounts of

greenhouse gases. There are many ways in

which concrete can be made sustainable,

including recycled concrete, differing cement

technologies, and carbon capture technologies

utilizing concrete. Each of these methods of

sustainability have been tested to some extent

and one of the main similarities that these

technologies have in common is that there isn’t

enough research in order to justify heavy

commercial and industrial usage. More

standards are needed in order to ensure that

the safety and mechanical properties of these

sustainable concrete products meet or exceed

that of traditionally used concrete.

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[ ] July 20, 2012

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