NEW MATERIALS IN CONSTRUCTION (CONCRETE)The development and
application of newmaterials in constructioncontinually adds to the
choices and decisions facing clients, designers and all responsible
forbuilding and construction. They continually seek greater and
more reliable information about the serviceability in order that
they meet more stringent design, safety and economic criteria.Ever
sinceThomas Edisonpatentedportland cementin 1907, it has been used
for a variety of different uses. Sidewalks, buildings, sinks, and
furniture are but a few of the productsmade fromcement in the form
of concrete. Cement is the dry powder that when mixed with other
additives and water makes concrete. Over the past decade, new types
of concrete and cement have been formulated that do everything from
bend, togrow plants, and let light through.In 2005, researchers at
theUniversity of Michigancreated a bendable form of concrete that
is "500 times more resistant to cracking and 40 percent lighter in
weight." This new type of concrete has substituted the gross
aggregate normally used in the making of concrete, for thin fibers.
Projects in Japan, Korea, Switzerland, and Australia have already
used this new bendable concrete. Unfortunately, the country in
which it was created has been slow to adopt its use.BETO ORGNICOwas
created in 2005 by "Lisbon-based architects and designerse-studio."
This organic concrete blends organic and inorganic material
together to create a living surface. Concrete retains water, as
suchthe concreteis used as a "battery" to provide water during dry
spells for the plant life growing on it. Rather than having grass
growing between concrete slabs, it is now possible to have the
grass grow onthe concreteslabs. These slabs could be added to
outside walls to create living siding and provide plants to soak up
CO2.LiTraConis a Hungarian concrete product developed seven years
ago by architect Ron Losonczi. By impregnatingthe concretewith
optical glass fibers, light can be transmitted from the outside in
or inside out. This concrete has the same strength as regular
concrete and will continue to transmit light through walls up to
twenty meters (twenty-two feet) thick.Finally,Tececohas developed
an eco-cement that absorbs CO2 from the environment. By adding
reactive magnesia to the cement, water and CO2 are absorbed and
harden. Other waste products, such as "fly and bottom ash, slags,
plastics, paper glass etc" can also be added to the cement without
affecting the CO2 absorption.These new types of cement and concrete
give architects and designers more choices for creating truly
different looks. Normally, you think of uglyconcrete wallsor slabs.
Now concrete can not only be bent, but used as a basis for plants
and light effects.Development of new types of concrete with
improved performance is a very important issue for the whole
building industry. This development is based on the optimisation
ofthe concretemix design, with an emphasis not only to the
workability and mechanical properties but also to the durability
and the reliability ofthe concretestructures in general. Appearance
of the new types of concrete requires a revision andimprovementof
existing structural systems and actual building technologies. The
economical aspect are of importance as well.BASIC CONCEPT ABOUT
CONCRETEConcreteis a construction material composed
ofcement(commonlyPortland cement) as well as other cementitious
materials such asfly ashandslag cement,aggregate(generally a coarse
aggregate such asgravellimestoneorgranite, plus a fine aggregate
such assand),water, andchemicaladmixtures. The word concrete comes
from the Latin word "concretus", which means "hardened" or
"hard".Concrete solidifies and hardens after mixing with water and
placement due to achemical processknown ashydration. The water
reacts with the cement, which bonds the other components together,
eventually creating a stone-like material. Concrete is used to
makepavements,architectural
structures,foundations,motorways/roads,bridges/overpasses,parkingstructures,brick/blockwalls
andfootingsfor gates,fencesandpoles.More concrete is used than any
other man-made material in the world.[1]As of 2006, about 7 cubic
kilometres of concrete are made each yearmore than one cubic metre
for every person on Earth.[2]Concrete powers a$US35-billion
industry which employs more than two million workers in theUnited
Statesalone.[citation needed]More than 55,000miles (89,000km)
ofhighwaysin America are paved with this material. ThePeople's
Republic of Chinacurrently consumes 40% of the world's
cement/concrete production.A superplasticizer is one of a class of
admixtures called water-reducers that are used to lower the mix
water requirement of concrete. Normal water-reducers based on
lignosulphonic acids, hydroxycarboxylic acids or processed
carbohydrates are capable of reducing water requirements by about
10 to 15 per cent. Incorporating larger amounts to produce higher
water reductionsresultsin undesirable effects on setting, air
content, bleeding, segregation and hardening characteristics.
Superplasticizers are chemically different from normal
water-reducers, and are capable of reducing water contents by about
30 per cent. They are variously known as superplasticizers,
superfluidizers, superfluidifiers, super water-reducers or high
range water-reducers. Since they were first introduced in Japan
about 15 years ago they have been used to produce several million
cubic metres of concrete; in the construction of the Olympic
stadium in Montreal alone, 5000 precast concrete units were
produced utilizing superplasticizers.The basic advantages of
superplasticizers include, (1) high workability of concrete,
resulting in easy placement without reduction in cement content and
strength; (2) high strength concrete with normal workability but
lower water content; and (3) a concrete mix with less cement but
normal strength and workability.Superplasticizers are broadly
classified in four groups, viz, sulphonated melamine-formaldehyde
condensates (SMF), sulphonated naphthalene-formaldehyde condensates
(SNF), modified lignosulphonates (MLS), and others including
sulphonic acid esters, carbohydrate esters, etc. variations exist
in each of these classes and some formulations may contain a second
ingredient. Most available data, however, pertain to SMF- and
SNF-based admixtures. They are supplied either as solids or as
aqueous solutions. In this Digest the dosage refers to the solid as
a percentage of the weight of cement.TYPES OF CONCRETEEver
sinceThomas EdisonpatentedPortland cementin 1907, it has been used
for a variety of different uses. Sidewalks, buildings, sinks, and
furniture are but a few of the productsmade fromcement in the form
of concrete. Cement is the dry powder that when mixed with other
additives and water makes concrete. Over the past decade, new types
of concrete and cement have been formulated that do everything from
bend, togrow plants, and let light through.BENDABLE CONCRETE :The
new concrete is 500 times more resistant to cracking and 40 percent
lighter in weight. The materials inthe concreteitself are designed
for maximum flexibility. The Engineered Cement Composites
technology has been used already on projects in Japan, Korea,
Switzerland and Australia, but has had slow adoption in the US.
Traditional concrete presents many problems: lack of durability and
sustainability, failure under severe loading, and the resulting
expenses of repair. ECC should address most of those problems. The
ductile, or bendable, concrete is made mainly of the same
ingredients in regular concrete minus the coarse aggregate. It
looks exactly like regular concrete, but under excessive strain,
the ECC concrete gives because the network of fibers veining the
cement is allowed to slide within the cement, thus avoiding the
inflexibility that causes brittleness and breakage.The Michigan
Department of Transportation (MDOT) used the ECC to replace part of
a bridge that crosses Interstate 94. The slab eliminated the need
for expansion joints, which are moveable steel teeth that separate
sections of regular concrete. With the ECC, a longer continuous
slab is possible. The Mihara Bridge, a new structure in Hokkaido,
Japan, has a deck of ECC that is a mere 2 inches (5 centimeters)
thick.Studies suggest ECC should last twice as long as regular
concrete, but the researchers said more tests are needed to confirm
that claim. Professor Victor Li estimates that over the course of
60 years, with servicing and replacement costs considered, a bridge
made of ECC could be cost 37 percent less than a traditional
span.LITRACON :LiTraCon("light transmitting concrete") is a
translucent concrete building material made of fine concrete
embedded with 5% by weight of optical glass fibers. It was
developed in 2001 by Hungarian architect Aron Losonczi working with
scientists at the Technical University of Budapest. The days of
dull, grey concrete could be about to end. The Hungarian architect
has combined the world's most popular building material with
optical fibre from Schott to create a new type of concrete that
transmits light.A wall made of "LitraCon" allegedly has the
strength of traditional concrete and an embedded array of glass
fibers that can display a view of the outside world, such as the
silhouette of a tree, for example. Thousands of optical glass
fibres form a matrix and run parallel to each other between the two
main surfaces of every block. Shadows on the lighter side will
appear with sharp outlines on the darker one. Even the colours
remain the same. This special effect creates the general impression
that the thickness and weight of a concrete wall will disappear.
The hope is that the new material will transform the interior
appearance of concrete buildings by making them feel light and airy
rather than dark and heavy.In theory, a wall structure built out of
the light-transmitting concrete can be a couple of meters thick as
the fibers work without any loss in light up to 20 m. Load-bearing
structures can also be built from the blocks as glass fibers do not
have a negative effect on the well-known high compressive strength
of concrete. The blocks can be produced in various sizes with
embedded heat isolation too. Thousands of optical glass fibers form
a matrix and run parallel to each other between the two main
surfaces of each block. The proportion of the fibers is very small
(4%) compared to the total volume of the blocks. Moreover, these
fibres mingle in the concrete because of their insignificant size,
and they become a structural component as a kind of modest
aggregate. Therefore, the surface of blocks remains homogeneous
concrete. It can be produced as prefabricated building blocks and
panels. Due to the small size of the fibers, they blend into
concrete becoming a component of the material like small pieces of
aggregates. In this manner, the result is not only two materials-
glass in concrete- mixed, but a third new material which is
homogeneous in its inner structure and on its main surfaces as
well. The glass fibers lead light by points between the two sides
of the blocks. Because of their parallel position, the
light-information on the brighter side of such a wall appears
unchanged on the darker side. The most interesting form of this
phenomenon is probably the sharp display of shadows on the opposing
side of the wall.If more and more buildings begin using this
technology, more natural light can be used to light offices and
stores. This could lead to huge drops in the amount of electricity
used to light buildings, since they'd be naturally lit during the
day. Also, people who get exposure to the sun are generally happier
and more productive, so that is another reason for businesses to
use this light-transmitting concrete.
TECECO (porecocrete porous concrete) :One particular
eco-friendly product that is generating much attention is -
Porecocrete Porous Concrete from Asset Rehabilitation / TecEco. By
adding reactive magnesia to the cement, water and CO2 are absorbed
and harden. Other waste products, such as "fly and bottom ash,
slag, plastics, paper glass etc" can also be added to the cement
without affecting the CO2 absorption. TecEco porecocretes represent
a large-scale market for eco-cement. Porecocrete porous pavements
mimic nature. Eco-cement sets by absorbing carbon dioxide, as by
design it allows the entry of abundant quantities of the gas
through what is an open pore structure. Using recycled aggregates,
concrete cannot get much more sustainable. The main potential use
for porecocretes is to make porous pavement in cities so that
people are less affected by drought. These are pavements with lots
of holes in them, and with subsurface drainage and usually a
capacity to store water underneath or in a reservoir. Surface
runoff water either soaks into an aquifer in suitable terrain or is
captured above an impervious layer and drained preferably to
underground storage for further use. Before infiltrating into the
subsoil or sub-surface drainage the process improves water quality
by providing surface area and aerobic conditions for cleansing.
Some of the main advantages of Porecocrete Porous Concrete are that
water penetrates through quickly leaving drier and safer surfaces
with no standing water, and a reduction in noise pollution as
porous pavements also absorb noise. Then it leads to less
maintenance on nearby buildings and superstructure, as aquifers
would be more regularly replenished resulting in less variable
ground moisture content, reduced ground movement with wet dry
cycles. Porous pavements made with TecEco Eco-Cements would not be
attacked by salts and would last considerably longer than
conventional binders such as bitumen (or asphalt) and Portland
cement.Heat is absorbed by pavements during hot, sunny days and due
to the fact that we have paved all the ground, large cities just
get hotter and hotter. The solution is to let the ground breathe
and porous pavements do just that.In Australia, some parts of the
US and several other places in the world, it has been noted that
subdivisions made with porous pavements that also have street trees
can be several degrees cooler than surrounding suburbs.How do
Eco-Cements Work?Eco-Cements are made by blending reactive magnesia
with conventional hydraulic cements like Portland cement. It is not
recommended that large amounts of pozzolan are added to an
Eco-Cement as the pozzolan will compete with the carbonation
reaction of lime and tend to block the carbonation affect slowing
it down. Eco-Cements are environmentally friendly because in
permeable substrates the magnesium oxide will first hydrate using
mix water and then carbonate forming significant amounts of
strength giving minerals in a low alkali matrix. Many different
wastes can be used as aggregates and fillers without reaction
problems. The reactive magnesium oxide used in Eco-Cements is
currently made from magnesite (a carbonate compound of magnesium)
found in abundance. In future TecEco hope to make it from abundant
magnesium in sea water using the Greensols process.When added to
concrete magnesia hydrates to magnesium hydroxide, but only in
permeable materials like bricks, blocks, pavers and pervious
pavements will it absorb CO2 and carbonate. The greater proportion
of the elongated minerals that form is water and carbon dioxide.
These minerals bond aggregates such as sand and gravel and wastes
such as saw dust, slag, bottom ash, plastics, paper etc. Eco-Cement
can include more waste than other hydraulic cements like Portland
cement because it is much less alkaline, reducing the incidence of
delayed reactions that would reduce the strength of the concrete.
Portland cement concretes on the other hand can't include large
amounts of waste because the alkaline lime that forms causes
delayed and disruptive reactionsEco-Cement CarbonationThe more
magnesia added to Eco-Cement and the more permeable it is, the more
CO2 that is absorbed. The rate of absorption of CO2 varies with the
degree of permeability. Carbonation occurs quickly at first and
more slowly towards completion. A typical Eco-Cement concrete block
would be expected to fully carbonate within a year. Eco-Cement also
has the ability to be almost fully recycled back into cement,
should the concrete structure become obsolete. .Steps involved in
makingEco-Cement1. Magnesite (a compound of magnesium) is heated in
a kiln to around 600 to 750 degrees C.The lower firing temperature
of the Tec-Kiln makes it easier to use free energy such as wind or
solar or even waste energy and TecEco plan to make a kiln that does
not use fossil fuels and in which the CO2 gases produced from the
magnesium carbonate as it decomposes is captured and contained for
further use or safe disposal.2. Grinding in the hot area of the
Tec-Kiln will result in increased efficiency.3. The heating process
produces reactive magnesium oxide (magnesia).4. The reactive
magnesia powder is added to a pre-determined, but variable amount
of hydraulic cement such as Portland cement, and if desired,
supplementary cementitious materials like fly ash.5. The resulting
blended powder is Eco-Cement.6. When mixed with water and
aggregates such as sand, gravel and wastes, Eco-Cement concretes
are ready for pouring into concrete, pressing into blocks or other
uses.BETAO ORGANICO:This type of concrete was created in 2005 by
"Lisbon-based architects and designers e-studio." This organic
concrete blends organic and inorganic material together to create a
living surface. Concrete retains water, as such the concrete is
used as a "battery" to provide water during dry spells for the
plant life growing on it. Rather than having grass growing between
concrete slabs, it is now possible to have the grass grow on the
concrete slabs. These slabs could be added to outside walls to
create living siding and provide plants to soak up CO2.
SPRAYED CONCRETE : The technique of sprayed concrete has been in
use for over 50 years in construction, structural repairs and a
variety of other applications. The use of properly applied sprayed
concrete is now recognised as being a technically sound and
economic method of applying concrete both for effective repairs and
for new constructions. The sprayed concrete forms and excellent
bond with itself, concrete and masonry. The material is compacted
onto the substrate under its own momentum, resulting in a strong,
dense product with good resistance to abrasion and weathering.
Sprayed concrete is extremely versatile and as a free forming
material lends itself to use in the construction industry. This
imperviousness and low water cement ratio gives a durable concrete
with a host of proven applications. In many cases sprayed concrete
will out-perform traditional concrete both in strength and
permeability. The elimination of form work, the speed of
application, the small access required and the ability to have the
spraying machine and materials over 200 metres from the point of
application, result in a large cost saving over other techniques.
With the ever-increasing structural loadings, the technique has
proven particularly suitable for the strengthening of bridges,
tunnels and culverts.The Phaeno Science Center, designed by Zaha
Hadid, is the largest building in Europe constructed from
self-consolidating concrete, which requires no vibration to
eliminate air pockets and even out distribution of aggregates. SCC
can be placed at a faster rate with no mechanical vibration and
less screeding, allows shorter construction periods, permits
structural and architectural shapes and surface, not achievable
with conventional concrete.
Emmanuel Combarel and Dominique Marrec, two French architects,
used Ductal, a high-performance concrete created by Lafarge in
2001, to build the RATP Bus Center in Thiais,HIGH PERFORMANCE
CONCRETE:It is a relatively new term used to describe concrete that
conforms to a set of standards above those of the most common
applications, but not limited to strength. While all high-strength
concrete is also high-performance, not all high-performance
concrete is high-strength. Some examples of such standards
currently used in relation to HPC are:Ease of placementCompaction
without segregationEarly age strengthLong-term mechanical
propertiesPermeabilityDensityHeat of hydrationToughnessVolume
stabilityLong life in severe environmentsSHOTCRETE : Shotcreteuses
compressed air to shoot (cast) concrete onto (or into) a frame or
structure. Shotcrete is frequently used against vertical soil or
rock surfaces, as it eliminates the need for formwork. It is
sometimes used for rock support, especially in tunnelling. Today
there are two application methods for shotcrete: the dry-mix and
the wet-mix procedure. Indry-mixthe dry mixture of cement and
aggregates is filled into the machine and conveyed with compressed
air through the hoses. The water needed for the hydration is added
at the nozzle. Inwet-mix, the mixes are prepared with all necessary
water for hydration. The mixes are pumped through the hoses. At the
nozzle compressed air is added for spraying. For both methods
additives such as accelerators and fiber reinforcement may be
used.The termGuniteis occasionally used for shotcrete, but properly
refers only to dry-mix shotcrete, and once was a proprietary
name.PERVIOUS CONCRETE: Pervious concreteis sometimes specified by
engineers and architects when porosity is required to allow some
air movement or to facillitate the drainage and flow of water
through structures. Pervious concrete is referred to as "no fines"
concrete because it is manufactured by leaving out the sand or
"fine aggregate". A pervious concrete mixture contains little or no
sand (fines), creating a substantial void content. Using sufficient
paste to coat and bind the aggregate particles together creates a
system of highly permeable, interconnected voids that drains
quickly. Typically, between 15% and 25% voids are achieved in the
hardened concrete, and flow rates for water through pervious
concrete are typically around 480 in./hr (0.34 cm/s, which is 5
gal/ft/ min or 200 L/m/min), although they can be much higher.
Both the low mortar content and high porosity also reduce
strength compared to conventional concrete mixtures, but sufficient
strength for many applications is readily achieved. Pervious
concrete pavement is a unique and effective means to address
important environmental issues and support sustainable growth. By
capturing rainwater and allowing it to seep into the ground, porous
concrete is instrumental in recharging groundwater, reducing
stormwater runoff, and meeting US Environmental Protection Agency
(EPA) stormwater regulations. The use of pervious concrete is among
the Best Management Practices (BMPs) recommended by the EPA, and by
other agencies and geotechnical engineers across the country, for
the management of stormwater runoff on a regional and local basis.
This pavement technology creates more efficient land use by
eliminating the need for retention ponds, swales, and other
stormwater management devices. In doing so, pervious concrete has
the ability to lower overall project costs on a first-cost
basis.ROLLER-COMPACTED CONCRETE :It is sometimes calledrollcrete,
is a low-cement-content stiff concrete placed using techniques
borrowed from earthmoving and paving work. The concrete is placed
on the surface to be covered, and is compacted in place using large
heavy rollers typically used in earthwork. The concrete mix
achieves a high density and cures over time into a strong
monolithic block. Roller-compacted concrete is typically used for
concrete pavement, but has also been used to build concrete dams,
as the low cement content causes less heat to be generated while
curing than typical for conventionally placed massive concrete
pours.GLASS CONCRETE:The use of recycled glass as aggregate in
concrete has become popular in modern times, with large scale
research being carried out at Columbia University in New York. This
greatly enhances the aesthetic appeal of the concrete.ASPHALT
CONCRETE Strictly speaking, asphalt is a form of concrete as well,
with bituminous materials replacing cement as the binder.Base layer
of asphalt concrete in a road under constructionRAPID STRENGTH
CONCRETE This type of concrete is able to develop high resistance
within few hours after been manufactured. This feature has
advantages such as removing the formwork early and to move forward
in the building process at record time, repair road surfaces that
become fully operational in just few hours.RUBBERIZED CONCRETE
While " rubberised concrete" is common, rubberized Portland cement
concrete ("rubberized PCC") is still undergoing experimental tests,
as of 2007 .POLYMER CONCRETEPolymer concreteis concrete which uses
polymers to bind the aggregate. Polymer concrete can gain a lot of
strength in a short amount of time. For example, a polymer mix may
reach 5000 psi in only four hours. Polymer concrete is generally
more expensive than conventional concretes.Polymer concrete
coatingHigh-Strength ConcreteA concrete of high strength can be
made without admixtures provided it is mixed with low amounts of
water and has desirable workability characteristics. At low water.
cement ratios however, it is not easy to achieve good workability.
As water reductions of about 25 to 30 per cent can be achieved by
using superplasticizers without loss in workability
characteristics, significantly higher initial and ultimate
strengths are realized. Although high cement content may also be
used to obtain high initial strengths in concrete, the higher heat
developed by the chemical reactions produces undesirable cracks and
shrinkage.High early strength development, a characteristic of
concrete made using superplasticizers at low water:cement ratios,
is particularly advantageous in the production of precast units.
For prestressed beams and units, which need overnight heat-curing,
use of superplasticizers allows reduction in curing time and curing
temperatures. High early strengths are particularly useful for
placing concrete in traffic areas such as city roads and airport
runways. Pumping at reduced water content is also facilitated by
the use of superplasticizers.In the early 1970s, experts predicted
that the practical limit of ready-mixed concrete would be unlikely
to exceed a compressive strength greater than 11,000 psi (76 MPa).
Over the past two decades, the development of high-strength
concrete has enabled builders to easily meet and surpass this
estimate. Two buildings in Seattle, Washington, contain concrete
with a compressive strength of 19,000 psi (131 MPa).The primary
difference between high-strength concrete and normal-strength
concrete relates to the compressive strength that refers to the
maximum resistance of a concrete sample to applied pressure.
Although there is no precise point of separation between
high-strength concrete and normal-strength concrete, theAmerican
Concrete Institutedefines high-strength concrete as concrete with a
compressive strength greater than 6000 psi (41 MPa).Manufacture of
high-strength concrete involves making optimal use of the basic
ingredients that constitute normal-strength concrete. Producers of
high-strength concrete know what factors affect compressive
strength and know how to manipulate those factors to achieve the
required strength. In addition to selecting a high-quality portland
cement, producers optimize aggregates, then optimize the
combination of materials by varying the proportions of cement,
water, aggregates, and admixtures.When selecting aggregates for
high-strength concrete, producers consider the strength of the
aggregate, the optimum size of the aggregate, the bond between the
cement paste and the aggregate, and the surface characteristics of
the aggregate. Any of these properties could limit the ultimate
strength of high-strength concrete.Admixtures
Pozzolans, such as fly ash and silica fume, are the most
commonly used mineral admixtures in high-strength concrete. These
materials impart additional strength to the concrete by reacting
with portland cement hydration products to create additional C-S-H
gel, the part of the paste responsible for concrete strength.It
would be difficult to produce high-strength concrete mixtures
without using chemical admixtures. A common practice is to use a
superplasticizer in combination with a water-reducing retarder. The
superplasticizer gives the concrete adequate workability at low
water-cement ratios, leading to concrete with greater strength. The
water-reducing retarder slows the hydration of the cement and
allows workers more time to place the concrete.High-strength
concrete is specified where reduced weight is important or where
architectural considerations call for small support elements. By
carrying loads more efficiently than normal-strength concrete,
high-strength concrete also reduces the total amount of material
placed and lowers the overall cost of the structure.The most common
use of high-strength concrete is for construction of high-rise
buildings. At 969 ft (295 m), Chicago's 311 South Wacker Drive uses
concrete with compressive strengths up to 12,000 psi (83 MPa) and
is the tallest concrete building in the United States.HIGH SRENGTH
LIGHT WEIGHT CONCRETELightweight concrete has been used
successfully for many years for structural members in high-rise
buildings. In addition to its lighter weight, which permits saving
in dead load and this concrete provides better heat insulation than
normal weight concrete. In recent years, the applications of
high-strength concrete have increased, and high-strength concrete
has now been used in many part of the world. However, not enough
significant data of high-strength lightweight concrete with
compressive strength in excess of 60 N/mm2 have been obtained. This
report summarizes results of an experimental study of the
properties of hardened high-strength lightweight concrete such as
strength, drying shrinkage, durability and porosity, and provides
important new information on the mix proportion and curing method
of this concrete. These results are as follows; (a) In regard to
porosity of lightweight aggregate, it was observed the tendency
that expanded shale type has a lager radius than that of sintered
fly ash type, it depends on aggregate characteristics, surface
texture and void connection. (b) Different water content of
lightweight aggregate gives influence to porosity of mortar matrix
under drying condition. (c) Resistance of freezing and thawing or
fire of light weight aggregate concrete is not necessary to advance
under moisture condition, because of light weight aggregates, due
to their cellular structure, capable of containing more water than
normal weight aggregate. (d) As the consideration of the porosities
and water content of hardened concrete, it was evaluated the
properties of high-strength lightweight concrete. (author
abst.)Part of the results of an ongoing laboratory work carried out
to design a structural lightweight high strength concrete ( SLWHSC
) made with and without mineral admixtures is presented.
Basaltic-pumice ( scoria ) was used as lightweight aggregate.A
control lightweight concrete mixture made with lightweight
basaltic-pumice (scoria) containing normal Portland cement as the
binder was prepared. The control lightweight concrete mixture was
modified by replacing 20% of the cement with fly ash and by
replacing 10% of the cement with silica fume. A ternary lightweight
concrete mixture was also prepared modifying the control
lightweight concrete by replacing 20% of cement with fly ash and
10% of cement with silica fume. Two normal weight concretes (NWC)
were also prepared for comparison purpose.Fly ash and silica fume
are used for economical and environmental concerns. Cylinder
specimens were cast from the fresh mixtures to measure compressive
and flexural tensile strength. The concrete samples were cured at
65% relative humidity with 20 C temperature. The density and slump
workability of fresh concrete mixtures were also measured.
Laboratory test results showed that structural lightweight
concrete (SLWC) can be produced by the use of scoria. However, the
use of mineral additives seems to be mandatory for production of
SLWHSC. The use of ternary mixture was recommended due to its
satisfactory strength development and environmental
friendliness.Future construction of concrete floating platforms for
offshore oil exploration off the east coast of Canada will lead to
a substantial increase in the use of high-strength lightweight
(HSLW) concrete. HSLW concrete has been extensively used in Norway
and other parts of Europe. HSLW concrete with its high durability
and lightweight characteristics is a very much sought after
material in the construction of concrete floating platforms.SELF
CONSOLIDATING CONCRETESelf-consolidating concrete, also known as
self-compacting concrete and SCC, is a highly flowable,
non-segregating concrete that can spread into place, fill formwork
and encapsulate even the most congested reinforcement, all without
any mechanical vibration. As a high-performance concrete, SCC
delivers these attractive benefits while maintaining all of
concrete's customary mechanical properties and durability
characteristics. SCC is defined as a concrete mixture that can be
placed purely by means of its own weight, with little or no
vibration. Adjustments to traditional mix designs and the use of
superplasticizers creates flowing concrete that meets tough
performance requirements. If needed, low dosages of viscosity
modifier can be used to eliminate unwanted bleeding and
segregation. Since its inception in the 1980s, the use of SCC has
grown tremendously. Thedevelopment of high performance
polycarboxylate polymers and viscosity modifiers have made it
possible to create "flowing" concrete without compromising
durability, cohesiveness, or compressive strength. The flowability
of SCC is measured in terms of spread when using a modified version
of the slump test (ASTM C 143). The spread (slump flow) of SCC
typically ranges from 18 to 32 inches (455 to 810 mm) depending on
the requirements for the project. The viscosity, as visually
observed by the rate at which concrete spreads, is an important
characteristic of plastic SCC and can be controlled when designing
the mix to suit the type of application being constructed.
SCC's unique properties give it significant economic,
constructability, aesthetic and engineering advantages. SCC is an
increasingly attractive choice for optimizing site manpower
(through reduction of labor and possibly skill level), lowering
noise levels, and allowing for a safer working environment. SCC
allows easier pumping (even from bottom up), flows into complex
shapes, transitions through inaccessible spots, and minimizes voids
around embedded items to produce a high degree of homogeneity and
uniformity. That's why SCC allows for denser reinforcement,
optimized concrete sections and shapes, and greater freedom of
design while producing superior surface finishes and textures.HIGH
PERFORMANCE CONCRETEIn recent years, the terminology
"High-Performance Concrete" has been introduced into the
construction industry. This edition of Technical Talk explains
high-performance concrete and how it differs from conventional
concrete.The American Concrete Institute (ACI) defines
high-performance concrete as concrete meeting special combinations
of performance and uniformity requirements that cannot always be
achieved routinely when using conventional constituents and normal
mixing, placing and curing practices. A commentary to the
definition states that a high-performance concrete is one in which
certain characteristics are developed for a particular application
and environment. Examples of characteristics that may be considered
critical for an application are:* Ease of placement* Compaction
without segregation* Early age strength* Long-term mechanical
properties* Permeability* Density* Heat of hydration* Toughness*
Volume stability* Long life in severe environmentsBecause many
characteristics of high-performance concrete are interrelated, a
change in one usually results in changes in one or more of the
other characteristics. Consequently, if several characteristics
have to be taken into account in producing a concrete for the
intended application, each must be clearly specified in the
contract documents.A high-performance concrete is something more
than is achieved on a routine basis and involves a specification
that often requires the concrete to meet several criteria. For
example, on the Lacey V. Murrow floating bridge in Washington
State, the concrete was specified to meet compressive strength,
shrinkage and permeability requirements. The latter two
requirements controlled the mix proportions so that the actual
strength was well in excess of the specified strength. This
occurred because of the interrelation between the three
characteristics. Other recent commercial examples where more than
one characteristic was required are given in Table 1.High-strength
concrete A high-strength concrete is always a high-performance
concrete, but a high-performance concrete is not always a
high-strength concrete. ACI defines a high-strength concrete as
concrete that has a specified compressive strength for design of
6,000 psi (41 MPa) or greater. According to a paper(1) by Paul Zia
of North Carolina State University, other countries use a higher
compressive strength in their definitions of high-strength concrete
with 7,000 psi (48 MPa) minimum. Other countries also specify a
maximum compressive strength, whereas the ACI definition is
open-ended.The specification of high-strength concrete generally
results in a true performance specification in which the
performance is specified for the intended application, and the
performance can be measured using a well-accepted standard test
procedure. The same is not always true for a concrete whose primary
requirement is durability.Durable concrete Specifying a
high-strength concrete does not ensure that a durable concrete will
be achieved. In addition to requiring a minimum strength, concrete
that needs to be durable must have other characteristics specified
to ensure durability. In the past, durable concrete was obtained by
specifying air content, minimum cement content and maximum
water-cement ratio. Today, performance characteristics may include
permeability, deicer scaling resistance, freeze-thaw resistance,
abrasion resistance or any combination of these characteristics.
Given that the required durability characteristics are more
difficult to define than strength characteristics, specifications
often use a combination of performance and prescriptive
requirements, such as permeability and a maximum water-cementitious
material ratio to achieve a durable concrete. The end result may be
a high-strength concrete, but this only comes as a by-product of
requiring a durable concrete.Concrete materials Most
high-performance concretes produced today contain materials in
addition to portland cement to help achieve the compressive
strength or durability performance. These materials include fly
ash, silica fume and ground-granulated blast furnace slag used
separately or in combination. At the same time, chemical admixtures
such as high-range water-reducers are needed to ensure that the
concrete is easy to transport, place and finish. For high-strength
concretes, a combination of mineral and chemical admixtures is
nearly always essential to ensure achievement of the required
strength. Examples of concrete mixes for durable and high-strength
concrete are shown in Table 2.Most high-performance concretes have
a high cementitious content and a water-cementitious material ratio
of 0.40 or less. However, the proportions of the individual
constituents vary depending on local preferences and local
materials. Mix proportions developed in one part of the country do
not necessarily work in a different location. Many trial batches
are usually necessary before a successful mix is
developed.High-performance concretes are also more sensitive to
changes in constituent material properties than conventional
concretes. Variations in the chemical and physical properties of
the cementitious materials and chemical admixtures need to be
carefully monitored. Substitutions of alternate materials can
result in changes in the performance characteristics that may not
be acceptable for high-performance concrete. This means that a
greater degree of quality control is required for the successful
production of high-performance concrete.HIGH VOLUME FLY ASH
CONCRETEWhat is fly ash?Fly ash is a by-product from coal-fired
electricity generating power plants. The coal used in these power
plants is mainly composed of combustible elements such as carbon,
hydrogen and oxygen (nitrogen and sulfur being minor elements), and
non-combustible impurities (10 to 40%) usually present in the form
of clay, shale, quartz, feldspar and limestone. As the coal travels
through the high-temperature zone in the furnace, the combustible
elements of the coal are burnt off, whereas the mineral impurities
of the coal fuse and chemically recombine to produce various
crystalline phases of the molten ash. The molten ash is entrained
in the flue gas and cools rapidly, when leaving the combustion zone
(e.g. from 1500C to 200C in few seconds), into spherical, glassy
particles. Most of these particles fly out with the flue gas stream
and are therefore called fly ash. The fly ash is then collected in
electrostatic precipitators or bag houses and the fineness of the
fly ash can be controlled by how and where the particles are
collected.TopThe use of fly ash in concreteFly ash can be used in
concrete as a partial replacement for ordinary portland cement
(opc). Fly ash can be introduced in concrete directly, as a
separate ingredient at the concrete batch plant or, can be blended
with the opc to produce blended cement, usually called
portland-pozzolana cement (ppc) in India. Fly ash blended cements
are produced by several cement companies in India.Generally
speaking, currently in the concrete industry, the percentage of fly
ash as part of the total cementing materials in concrete normally
ranges from 15 to 25%, although it can go up to 30-35% in some
applications. The use of fly ash in concrete will improve some
aspects of the performance of the concrete provided the concrete is
properly designed. The main aspects of the concrete performance
that will be improved by the use of fly ash are increased long-term
strength and reduced permeability of the concrete resulting in
potentially better durability. The use of fly ash in concrete can
also address some specific durability issues such as sulphate
attack and alkali silica reaction. However, a few additional
precautions have to be taken to insure that the fly ash concrete
will meet all the performance criteria.The table given below is a
paper presented by Dr Wilbert Langley and Dr Gordon Leaman at the
sixth CANMET/ ACI / JCI International Conference, held May 31 -
June 5, 1998. These are the actual mixes used in demonstration
projects throughout Canada to prove the practicality of using
high-volume fly ash concrete for a variety of projects. The
Parklane Development in Halifax, Nova Scotia, Canada is a seven
story structure and was built with55% high-volume fly ash
concrete(high strength mix given in the table below) .
Cast-in-place columns and beams were poured with concrete specified
to meet design strengths of between 4,350 psi at 28 days and 7,250
psi at 120 days. Actual strengths developed exceeded required
strengths by 30%-40% on an averageHIGH VOLUME FLY ASH CONCRETEAll
mixes contained air entraining admixtures and
superplasticizersConventional MixLow Strength 55% ReplacementMedium
Strength 55% ReplacementHigh Strength 55% Replacement
Total Cementitious Content (c+fa)(lb/cu.yd) 483374566660
Cement (lb)483166250300
Class F Fly Ash (lb)0208316360
Sand (lb)1334146712501266
Stone (lb)1700183418341850
Water (lb)220185198185
Water to Cement Ratio0.460.490.350.28
Compressive Strength (psi)
3 day4,6001,2502,3203,190
7 day5,0001,7503,0404,900
28 day6,5003,3505,5008,300
91 day7,1004,0507,70010,900
365 day7,5507,40010,000-
Set Time (hours:minutes)
Initial6:258:255:35-
Final7:5011:157:40-
In the US, the State of Wisconsin has been using a 60% Class F
fly ash in concrete mix since 1989. HVFA concrete has now found a
commercial niche in the Sydney construction market and is being
trialed for theSydney Olympicfacilities. For the Crown Casino
project, Connell Wagner required highly durable and low drying
shrinkage concrete for the construction of the 55,000 square meter
basement that was locatedbelow the water tableAnother benefit of
using fly ash in concrete is that fly ash makesbeautiful,
"architectural" concrete. Fly ash of today is light in color and
its extreme workability ensures smoother finishes. That most famous
of architecturally exposed concrete buildings, theJonas
SalkInstitute, was built with fly ash concrete. I have seen the
NCCBM building located at Ballabhgarh & it still looks
beautiful even after having weathered so many years.Addition of fly
ash in plaster virtually eliminates defects like crazing, map
cracking, drying shrinkage cracks, debonding, grinning, expansion
& popping.SHRINKAGE COMPENSATION CONCRETEUse of Shrinkage
Compensating Concrete (SCC) In Pre-Stressed Concrete
PRE-STRESSED CONCRETE:
There are characteristics of shrinkage compensating concrete
(SCC) that are similar to the objectives and methodology behind
pre-stressed concrete. Pre-stressed concrete is defined as a
concrete member with a pre-determined compressive force, or moment,
built into the member so that the internal stresses, designed as a
result of the members intended use, will be equal to or less then
the pre-stressing stresses built into the member. Post-tensioning
and pre-tensioning are methods of achieving pre-stressed
concrete.
The objectives and methodology of using SCC to enhance the
properties of concrete is very similar to the objectives and
methodology of using pre-stressed to accommodate structural
loadings. Shrinkage cracking control, combined with the other
inherent advantages of SCC, make SCC a better material for
pre-stressed concrete members.
SHRINKAGE COMPENSATING COMPONENT
ACI recognizes two methods of achieving SCC, ettringite crystal
development or calcium hydroxide platelet crystal development. The
inherent characteristics of the calcium hydroxide platelet system,
developed by the use of CONEX, is the better system of the two due
to its inherent likeness of chemical action during hydration of the
cement in use. The advantages of the platelet SCC method makes it
well suited for use in pre-stressed concrete in general and in
post-tensioned concrete in particular.
DESIGN CRITERIA:
The primary design objective of pre-stressed concrete is to
place a compressive force in the concrete member that would prevent
the concrete from going into tension and failing under design load
conditions. Failure usually being defined as tension or stress
cracking occurring in the concrete member. The primary objective
for using pre-stressed concrete is economic. This is due to the
fact that a pre-stressed member, of the same physical dimensions as
a conventional reinforced concrete member, will have a greater load
carrying capability. Conversely, for a given design loading, a
pre-stressed member will be smaller in dimension and weight then a
conventional concrete reinforced member.
SCC - HOW IT WORKS:
Shrinkage compensating concrete (SCC) has a case history of
placements free of shrinkage cracks due to the "pre-stressing"
action (restrained expansion or RE) created within the concrete
during hydration and curing. During this period several phenomena
are taking place simultaneously within the concrete. The most
important being expansion of the concrete matrix due to the
chemical reaction of CONEX creating development of calcium
hydroxide crystals, and bonding of the concrete to the reinforcing.
While this is occurring, the RE created causes the concrete to go
into compression. The calcium hydroxide system of expansion, that
is the formation of the platelets that create the expansion, is
approximately at the same rate as the curing of the concrete. As
long as compressive stresses within the concrete are greater then
the tensile stresses the concrete will remain in compression, and
tension cracks will not appear. This characteristic is taken
advantage of by using SCC in the construction of cast in place
slabs on grade (warehouse floors, pavements, secondary containment
structures for hazardous materials) and structural members (bridge
decks, primary containment structures, buildings, etc.). The
benefit of SCC in slabs on grade is the ability to place larger
sections (i.e. 20,000 ft2 / 2,000 m2) without contraction joints
and with a reduction, and often elimination, of edge curling. The
advantage of using SCC in bridge deck and containment structures is
the increase in imperviousness of the concrete as well as the lack
of shrinkage cracking.
SCC AS APPLIED TO PRE-STRESSED CONCRETE:
Currently the use of SCC is expanding into the pre-cast industry
and also into pre-stressed concrete applications. While there are
examples of cast in place pre-stressed applications of SCC, it is
still a long way off from being in general use. This is perhaps due
to the lack of published data detailing the use of SCC in
post-tensioned applications. Tests should be done to establish the
required expansion for desired results. These tests would establish
the amount of compressive stress in the member resulting from
different dosages of CONEX and related degrees of post-tensioning.
This would allow other member characteristics to be adjusted
accordingly. An obvious goal of using CONEX is to increase the
quality of an existing concrete product and/or reduce the
production cost of that concrete product. The addition of CONEX
will increase the internal compressive stresses in the
post-tensioned concrete member(s) if the proper restraint is
provided. How this will impact the member design and/or production
methods will need to be developed, but it presents a different look
at the potential of shrinkage compensation in pre-stressed
concrete.
In general, CONEX SCC is similar to and compatible with
pre-stressed concrete, and acts interdependently with the cement in
use, developing the following advantages:
1. A pre-stressed member with a higher degree of internal
compression to assure greater crack control.2. More impermeable
concrete.3. Better edge finishing.4. Possibly a way to reduce
production costs through less breakage and discard.FIBER
REINFORCEMENTSNON-STEEL REINFORCEMENT :Some construction cannot
tolerate the use of steel. For example, MRI machines have huge
magnets, and require nonmagnetic buildings. Another example are
toll-booths that read radio tags, and need reinforced concrete that
is transparent to radio.In some instances, the lifetime of the
concrete structure is more important than its strength. Since
corrosion is the main cause of failure of reinforced concrete, a
corrosion-proof reinforcement can extend a structure's life
substantially.For these purposes some structures have been
constructed using fiber-reinforced plastic rebar, grids or fibers.
The "plastic" reinforcement can be as strong as steel. Because it
resists corrosion, it does not need a protective concrete cover of
30 to 50 mm or more as steel reinforcement does. This means that
FRP-reinforced structures can be lighter, have longer lifetime and
for some applications be price-competitive to steel-reinforced
concrete.The main barrier to use of FRP reinforcement is the fact
that it is neither ductile nor fire resistant. Structures employing
FRP rebars may therefore exhibit a less ductile structural
response, and decreased fire resistance.However, the addition of
short monofilament polypropylene fibers to the concrete during
mixing may have the beneficial effect of reducing spalling during a
fire. In a severe fire, such as the Channel Tunnel fire,
conventionally reinforced concrete can suffer severe spalling
leading to failure. This is in part due to the pore water remaining
within the concrete boiling explosively; the steam pressure then
causes the spallingHISTORICAL PERSPECTIVE:The concept of using
fibers as reinforcement is not new. Fibers have been used as
reinforcement since ancient times. Historically, horsehair was used
in mortar and straw in mud bricks. In the early 1900s, asbestos
fibers were used in concrete, and in the 1950s the concept of
composite materials came into being and fiber reinforced concrete
was one of the topics of interest. There was a need to find a
replacement for the asbestos used in concrete and other building
materials once the health risks associated with the substance were
discovered. By the 1960s, steel, glass (GFRC), and synthetic fibers
such as polypropylene fibers were used in concrete, and research
into new fiber reinforced concretes continues today.EFFECTS OF
FIBER IN CONCRETE:Fibers are usually used in concrete to control
plastic shrinkage cracking and drying shrinkage cracking. They also
lower the permeability of concrete and thus reduce bleeding of
water. Some types of fibers produce greater impact, abrasion and
shatter resistance in concrete. Generally fibers do not increase
the flexural strength of concrete, so it can not replace moment
resisting or structural steel reinforcement. Some fibers reduce the
strength of concrete.The amount of fibers added to a concrete mix
is measured as a percentage of the total volume of the composite
(concrete and fibers) termed volume fraction (Vf). Vftypically
ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividing
fiber length (l) by its diameter (d). Fibers with a non-circular
cross section use an equivalent diameter for the calculation of
aspect ratio. If the modulus of elasticity of the fiber is higher
than the matrix (concrete or mortar binder), they help to carry the
load by increasing the tensile strength of the material. Increase
in the aspect ratio of the fiber usually segments the flexural
strength and toughness of the matrix. However, fibers which are too
long tend to "ball" in the mix and create workability problems.Some
recent research indicated that using fibers in concrete has limited
effect on the impact resistance of concrete materials. This finding
is very important since traditionally people think the ductility
increases when concrete is reinforced with fibers. The results also
pointed out that the micro fibers is better in impact resistance
compared with the longer fibers.The High Speed tunnel linings
incorporated concrete containing 1 kg/m of polypropylene fibers, of
diameter 18 & 32 m, giving the benefits noted
below.Polypropylene fibers can:Improve mix cohesion, improving
pumpability over long distancesImprove freeze-thaw
resistanceImprove resistance to explosive spalling in case of a
severe fireImprove impact resistanceIncrease resistance to
plasticDEVELOPMENTS IN FIBER REINFORCED CONCRETE: A new kind of
natural fiber reinforced concrete (NFRC) made of cellulose fibers
processed from genetically modified slash pine trees is giving good
results. The cellulose fibers are longer and greater in diameter
than other timber sources. Some studies were performed using waste
carpet fibers in concrete as an environmentally friendly use of
recycled carpet waste. A carpet typically consists of two layers of
backing (usually fabric from polypropylene tape yarns), joined by
CaCO3filled styrene-butadiene latex rubber (SBR), and face fibers
(majority being nylon 6 and nylon 66 textured yarns). Such nylon
and polypropylene fibers can be used for concrete reinforcement
High performance fiber reinforced cementitious composites
(HPFRCCs): This particular class of concrete was developed with the
goal of solving the structural problems inherent with today's
typical concrete, such as its tendency to fail in a brittle manner
under excessive loading and its lack of long term durability. The
two important properties of HPFRCC's areThe remarkable ability to
strain harden under excessive loading. In layman's terms, this
means they have the ability to flex or deform before fracturing, a
behavior similar to that exhibited by most metals under tensile or
bending stresses. Because of this capability, HPFRCCs are more
resistant to cracking and last considerably longer than normal
concrete.Their low density.A less dense, and hence lighter material
means that HPFRCCs could eventually require much less energy to
produce and handle, deeming them a more economic building material.
Because of HPFRCCs' lightweight composition and ability to strain
harden, it has been proposed that they could eventually become a
more durable and efficient alternative to typical concrete.HPFRCCs
include the following ingredients: fine aggregates, a
superplasticizer, polymeric or metallic fibers, cement, and water.
Thus the principal difference between HPFRCC and typical concrete
composition lies in HPFRCCs' lack of coarse aggregates. Typically,
a fine aggregate such as silica sand is used in HPFRCCs.One aspect
of HPFRCC design involves preventing crack propagation, or the
tendency of a crack to increase in length, ultimately leading to
material fracture. This occurrence is hindered by the presence of
fiber bridging, a property that most HPFRCCs are specifically
designed to possess. Fiber bridging is the act of several fibers
exerting a force across the width of a crack in an attempt to
prevent the crack from developing further. This capability is what
gives bendable concrete its ductile properties.Proposed uses for
HPFRCCs include bridge decks, concrete pipes, roads, structures
subjected to seismic and non-seismic loads, and other applications
where a lightweight, strong and durable building material is
desired. Though HPFRCCs have been tested extensively in the lab and
been employed in a few commercial building projects, further long
term research and real world application is needed to prove the
true benefits of this material. The newly developed fiber
reinforced concrete isnamed asEngineered Cementitious
Composite(ECC).It is 500 times more resistant to crackingIt is 40
percent lighter than traditional concreteIt can sustain
strain-hardening up to several percent strain, resulting in a
material ductility of at least two orders of magnitude higher when
compared to normal concrete.Ithas unique cracking behavior . When
loaded to beyond the elastic range, ECC maintains crack width to
below 100 m, even when deformed to several percent tensile
strains.The basic mechanical properties of ECC are : ECC Material
Properties
Ultimate Tensile Strength ( CU )4.6 MPa
Ultimate Strain ( CU )5.6 %
First Crack Stress ( fc )2.5 MPa
First Crack Strain ( fc ).021 %
Modulus of Elasticity ( E )22 GPa
ECC's tensile strain hardening behavior has a capacity in the
range of 3-7%,which means that unlike common concrete, which is
brittle and breaks under that amount of strain, ECC will bend under
the same stress, like a piece of sheet metal. The high ductility is
achieved by optimizing the microstructure of the composite
employing micromechanical models. ECC looks exactly like regular
concrete, but under excessive strain, the ECC concrete bends
because the distinctively coated matrix of fibers in the cement is
allowed to slide within the cement. ECC is made using the same
ingredients of regular concrete but without the use of coarse
aggregate.ECC has already been used by the Michigan Department of
Transportation to patch a portion of the Grove Street Bridge deck
over Interstate 94. The ECC patch was used as a replacement to the
previously existent expansion joint that linked two deck slabs.
Expansion joints, commonly used in bridges to allow for the
seasonal expansion and contraction of the concrete decks, are an
example of a ubiquitous construction practice that could eventually
be eliminated through the use of bendable concrete. Other existent
structures composed of ECC, include the Curtis Road Bridge in Ann
Arbor, MI and the Mihara Bridge in Hokkaido, Japan. The deck of the
Mihara Bridge, composed of bendable concrete, is only five
centimeters thick and has an expected lifetime of one-hundred
years.Comparison to other composite materials:PropertiesFRCCommon
HPFRCCECC
Design MethodologyN.A.Use high VfMicromechanics based, minimize
Vf for cost and processibility
FiberAny type, Vf usually less than 2%; df for steel ~ 500
micrometerMostly steel, Vf usually > 5%; df ~ 150
micrometerTailored, polymer fibers, Vf usually less than 2%; df
< 50 micrometer
MatrixCoarse aggregatesFine aggregatesControlled for matrix
toughness, flaw size; fine sand
InterfaceNot controlledNot controlledChemical and frictional
bonds controlled for bridging properties
Mechanical
PropertiesStrain-softening:Strain-hardening:Strain-hardening:
Tensile strain0.1%3% (typical); 8% max
Crack widthUnlimitedTypically several hundred micrometers,
unlimited beyond 1.5% strainTypically < 100 micrometers during
strain-hardening[1]
Note: FRC=Fiber-Reinforced Cement. HPFRCC=High-Performance Fiber
Reinforced Cementitious CompositesNANOMATERIALSNANOTUBES AND
NANOWIRES AS REINFORCEMENT: In developing nano-composite materials,
nanotubes and nanowires are expected to greatly improve the
properties of the composites. Silicon carbide nanowires have been
regarded as an excellent reinforcement for composites due to the
high intrinsic strength of the materials. However, the silicon
carbide nanowires have a smooth surface and are easily pulled out
when the composites break because of the weak adhesion between the
nanowires and the matrix. Therefore, we need to fabricate a
contoured surface of the silicon carbide nanowires in order to
improve the adhesion.This led to the invention of a new type of
silicon carbide nanowires periodically twinned SiC nanowires, which
have a contoured surface on the nanoscale. The nanowires with a
hexagonal cross section, a diameter of 50300nm and a length of tens
to hundreds of micrometers feature a zigzag arrangement of
periodically twinned segments with a uniform thickness along the
entire growth length. Computer simulation demonstrates that the
zigzag columnar structure is formed by the stacking of hexagonal
discs of {111} planes of SiC. A minimum surface energy and strain
energy argument explain the formation of periodic twins in the SiC
nanowires.The twinning structure has made the nanowires exhibit
different luminescence and chemical stability. A Chinese group
showed that the silicon carbide nanowires with beaded morphology
can greatly enhance the tensile strength of an epoxy composite.
Therefore, the new type of twinned SiC nanowires is expected to
find important applications in nano-composites.
MODIFICATION OF GLASSS FIBER USING PLAZMA POLYMERIZATION
TECHNIQUE : The plasma treated E-glass fiber improves the
mechanical properties of acrylic resin denture base material,
polymethylmethacrlyate (PMMA). Plasma surface treatment of fibers
is used as reinforcement in composite materials to modify the
chemical and physical properties of their surfaces with tailored
fibermatrix bonding strength. Three different types of monomer
2-hydroxyethyl methacrylate (HEMA), triethyleneglycoldimethylether
(TEGDME) and ethylenediamine (EDA) were used in the plasma
polymerization modification of glass fibers. A radiofrequency
generator was used to sustain plasma in a glass vacuum chamber.
Glass fibers were modified at the same glow-discharge power of 25W
and exposure time of 30min for each monomer. Fibers were
incorporated into the acrylic with 1% (w/w) loading except control
group. Specimens were prepared using a standard mould of
3cm0.5cm0.8cm in dimension with eight specimens in each group.
Samples were subjected to a flexural strength test set up at a
crosshead speed of 5mm/min. Scanning electron microscopy (SEM) was
used to examine the microstructure and X-ray
photoelectronConcluding RemarksIt has long been a concrete
technologist's dream to discover a method of making concrete at the
lowest possible water: cement ratio while maintaining high
workability. To a considerable extent this dream may be fulfilled
with the advent of superplasticizers. They have added a new
dimension to the application of admixtures, and have made it
possible to produce concrete with compressive strength of the order
of 90 MPa.Superplasticizers have other possible applications.
Energy conservation and diminishing supplies of high quality raw
materials will increasingly necessitate the use of marginal quality
cements and aggregates. In such instances the use of
superplasticizers may permit production of concrete at low
water:cement ratios that will be strong enough to meet normal
performance requirements. There are literally countless possible
applications of superplasticizers, for example, in the production
of fly ash concrete, blast furnace slag cement concrete, composites
with various types of fibres and lightweight concrete. In addition,
the dispersing effect of superplasticizers is not limited to
portland cement and may find application in other cementitious
systems.The fact that superplasticizers show remarkable advantages
in producing concrete should not imply that there are no problems
associated with their use. A satisfactory solution to the high rate
of slump loss in superplasticized concrete is yet to be found and
the relative effects of materials, production methods and external
conditions that influence this phenomenon are not completely
understood. Further study will be necessary of the compatibility of
other admixtures such as retarders, accelerators and air-entraining
agents in combination with superplasticizers. Though surface area,
tricalcium aluminate, and sulphate contents seem to influence
slump, no definite trend has been established.Most available data
on superplasticized concrete have been obtained using SMF- and
SNF-based superplasticizers. Even within a single type, variations
in behaviour have occurred, possibly because of the differences in
molecular weight and in the type of cation associated with the
superplasticizer. Consequently it is difficult to predict exactly
the properties and behaviour of superplasticized concrete. As more
data become available, especially on the long-term behaviour of
these concretes, it will be possible to formulate standards and
codes of practice. The future use of superplasticizers will,
however, be dictated by cost factors (of admixture and operating
costs) and by acceptance by industry