1. Behavior of reinforced concrete

plantcalicobeansUrban and Civil

Nov 29, 2013 (3 years and 10 months ago)

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5 MARK
S:


1.

Behavior of reinforced concrete

Concrete is a mixture of coarse (stone or brick chips) and fine (generally sand or crushed stone)
aggregates with a binder material (usually

Portland cement
). When mixed with a small amount of water,
the cement

hydrates

to form microscopic opaque crystal lattices encapsulatin
g and locking the aggregate
into a rigid structure. Typical concrete mixes have high resistance to

compressive

stresses

(about
4,000

psi (28

MPa)); however, any appreciable

tension
(
e.g.,

due to

bending
) will break the microscopic
rigid lattice, resulting in cracking and separation of the concrete. For this reason, typical non
-
reinforced
concrete must be well supported to prevent the development of tension.

If a material with hig
h strength in tension, such as

steel
, is placed in concrete, then the composite
material,

reinforced concrete
, resists not only compression but also bending and other direct tensile
actions. A r
einforced concrete section where the concrete resists the compression and steel resists the
tension can be made into almost any shape and size for the construction industry.



Key characteristics

Three physical characteristics give reinforced concrete its sp
ecial properties:

1.

the

coefficient of thermal expansion

of concrete is similar to that of steel, eliminating large internal
stresses due to d
ifferences in
thermal

expansion or contraction.

2.

when the cement paste within the concrete hardens, this conforms to the surface details of the
steel, permitting any stress to be transmitted efficie
ntly between the different materials. Usually
steel bars are roughened or corrugated to further improve the

bond
or cohesion between the
concrete and steel.

3.

the

alkaline

chemical environment provided by the

alkali

reserve (KOH, NaOH) and
the

portlandite

(
calcium hydroxide
) contained in the hardened cement paste causes
a

passivating

fil
m to form on the surface of the steel, making it much more resistant
to
corrosion

than it would be in neutral or acidic conditions. When the cement paste exposed to
the air and meteoric w
ater reacts with the atmospheric CO
2
, portlandite and the

Calcium Silicate
Hydrate

(CSH) of the hardened cement paste become progressively carbonated and th
e high pH
gradually decreases from 13.5


12.5 to 8.5, the pH of water in equilibrium with

calcite
(
calciu
m
carbonate
) and the steel is no longer passivated.

As a rule of thumb, only to give an idea on orders of magnitude, steel is protected at pH above ~11 but
starts to corrode below ~10 depending on steel characteristics and local physico
-
chemical condition
s
when concrete becomes carbonated.

Carbonation of concrete
along with

chloride

ingress are amongst the
chief reasons for the failure of

reinforcement bars

in concrete.
[
1]

The relative cross
-
sectional

area

of steel required for typical reinforced concrete is usually quite small and
varies from 1% for most beams and slabs to 6% for some columns.

Reinforcing bars

are normally round
in cross
-
section and vary in diameter. Reinforced concrete structures sometimes have provisions such as
ventilated hollow cores to control their moisture & humidity.

Distribut
ion of concrete (in spite of reinforcement) strength characteristics along the cross
-
section of
vertical reinforced concrete elements is inhomogeneous

2.

Reinforcement and terminology of Beams

A beam bends under

bending moment
, resulting in a small curvature. At the outer face (
tensile face
) of
the curvature the concrete experiences tensile stress, while at the inner face (
compressive face
) it
experiences compressive stress.

A

s
ingly reinforced

beam is one in which the concrete element is only reinforced near the tensile face
and the reinforcement, called tension steel, is designed to resist the tension.

A

doubly reinforced

beam is one in which besides the tensile reinforcement t
he concrete element is also
reinforced near the compressive face to help the concrete resist compression. The latter reinforcement is
called compression steel. When the compression zone of a concrete is inadequate to resist the
compressive Moment(positive
moment), extra reinforcement has to be provided if the architect limits the
dimensions of the section.

An

under
-
reinforced

beam is one in which the tension capacity of the tensile reinforcement
is

smaller

than the combined compression capacity of the concr
ete and the compression steel (under
-
reinforced at tensile face). When the reinforced concrete element is subject to increasing bending
moment, the tension steel yields while the concrete does not reach its ultimate failure condition. As the
tension steel
yields and stretches, an "under
-
reinforced" concrete also yields in a ductile manner,
exhibiting a large deformation and warning before its ultimate failure. In this case the yield stress of the
steel governs the design.

An

over
-
reinforced

beam is one in w
hich the tension capacity of the tension steel is

greater

than the
combined compression capacity of the concrete and the compression steel (over
-
reinforced at tensile
face). So the "over
-
reinforced concrete" beam fails by crushing of the compressive
-
zone c
oncrete and
before the tension zone steel yields, which does not provide any warning before failure as the failure is
instantaneous.

A

balanced
-
reinforced

beam is one in which both the compressive and tensile zones reach yielding at
the same imposed load o
n the beam, and the concrete will crush and the tensile steel will yield at the
same time. This design criterion is however as risky as over
-
reinforced concrete, because failure is
sudden as the concrete crushes at the same time of the tensile steel yields
, which gives a very little
warning of distress in tension failure.
[4]

Steel
-
reinforced concrete moment
-
carrying elements should normally be designed to be under
-
reinforced
s
o that users of the structure will receive warning of impending collapse.

The

characteristic strength

is the strength of a material where less than 5% of the specimen shows
lower strength.

The

design strength

or

nominal strength

is the strength of a materi
al, including a material
-
safety factor.
The value of the safety factor generally ranges from 0.75 to 0.85 in

Allowable Stress Design
.

The

ultimate limit state

is the theoretical failure point with a certain probability. It is stated under factored
loads and factored resistances.

3.

Ferrocement

The term

ferrocement

is most commonly applied to a mixture of

Portland cement

and sand applied over
layers of woven or expanded steel mesh and closely spaced small
-
diameter steel rods

rebar
. It can be
used to form relatively thin,
compound curved sheets to make hulls for boats, shell roofs, water tanks, etc.
It has been used in a wide range of other applications including sculpture and prefabricated building
components. The term has been applied by extension to other

composite materials

including some
containing no cement and no ferrous material. These are better referred to by terms describing their
actual contents.

The term "ferrocement" was g
iven to this product by its inventor in France,

Joseph Monier
. At the time,
(1850's) he wanted to create urns, planters, and cisterns without the expense of kiln firing. In 1875
he
created the first steel and concrete bridge. The outer layer was sculpted in its wet state to mimic rustic
logs, thereby also introducing

Faux Bois

concrete into practice. (Recent tre
nds have "ferrocement" being
referred to as ferro concrete or

reinforced concrete

to better describe the end product instead of its
components. By understanding that

aggregates

mixed with

Portland cement

form

concrete
, but many
things can be called

cement
, it is hoped this may avoid the confusion of many compounds or techniques
that are not ferro concrete.)

Ferro co
ncrete has relatively good strength and resistance to impact. When used in house construction in
developing countries, it can provide better resistance to fire, earthquake, and corrosion than traditional
materials, such as wood, adobe and stone masonry. It

has been popular in developed countries for yacht
building because the technique can be learned relatively quickly, allowing people to cut costs by
supplying their own labor. In the 1930s through 1950's, it became popular in the United States as a
constru
ction and sculpting method for

novelty architecture
, examples of which created "dinosaurs in the
desert", or a "giant pair of cowboy boots and hat" for a service st
ation.

Advantages

The advantages of a well built ferro concrete construction are the low weight, maintenance costs and long
lifetime in comparison with purely steel constructions.
[
citation needed
]

However, meticulous building precision
is considered crucial here. Especially with respect to the cementitious composition and the way in which it
is applied in and on the framework, and how or if the framework ha
s been treated to resist corrosion.

When a ferro concrete sheet is mechanically overloaded, it will tend to fold instead of break or crumble
like stone or pottery. So it is not brittle. As a container, it may fail and leak but possibly hold together.
Much
depends on techniques used in the construction.

Please note: Much more information for projects of reinforced concrete has been updated regularly, than
for products that we call ferrocement or ferro concrete.

[
edit
]
Disadvantages

The disadvantage of ferro concrete construction is the labor intensive nature of it, which makes it
expensive for industrial application in the western world.

In addition, threats to degradation (rust) of the
steel components is a possibility if air voids are left in the original construction, due to too dry a mixture of
the concrete being applied, or not forcing the air out of the structure while it is in its
wet stage of
construction, through vibration, pressurized spraying techniques, or other means. These air voids can
turn to pools of water as the cured material absorbs moisture. If the voids occur where there is untreated
steel, the steel will rust and exp
and, causing the system to fail.


20 MARKS:

1.

Common failure modes of steel reinforced concrete

Reinforced concrete can fail due to inadequate strength, leading to mechanical failure, or due to a
reduction in its durability. Corrosion and freeze/thaw cycl
es may damage poorly designed or constructed
reinforced concrete. When rebar corrodes, the oxidation products (
rust
) expand and tends to flake,
cracking the concrete and unbonding the rebar from t
he concrete. Typical mechanisms leading to
durability problems are discussed below.

Mechanical failure

Cracking of the concrete section can not be prevented; however, the size and location of the cracks can
be limited and controlled by reinforcement, place
ment of control joints, the curing methodology and the
mix design of the concrete. Cracking defects can allow moisture to penetrate and corrode the
reinforcement. This is a

serviceability

failure in

limit state design
. Cracking is normally the result of an
inadequate quantity of rebar, or rebar spaced a
t too great a distance. The concrete then cracks either
under excess loading, or due to internal effects such as

early thermal shrinkage

when it cures.

Ultimate failure leading to collapse can be caused by crushing of the concrete, when compressive
stresse
s exceed its strength; by
yielding

or failure of the rebar, when bending or shear stresses exceed
the strength of the reinforcement; or by bond failure between the con
crete and the rebar.

Carbonation

Carbonation, or neutralisation, is a chemical reaction between

carbon dioxide

in the air with
calcium
hydroxide

and hydrated

calcium silicate

in the concrete.

When designing a concrete structure, it is normal to state the

concrete cover

for the rebar (the depth
within the object that the rebar will be). The minimum concrete cover is normally regulated by design
or

building codes
. If the reinforcement is too close to the surface, early failure due to corrosion may occur.
The concrete cover depth can be measured with a

cover meter
. However, carbonated concrete only
becomes a durability problem when there is also sufficient moisture and oxygen to cause electro
-
potential
corrosion of the reinforcing steel.

One method of testing a structure for carbonation is t
o

drill

a fresh hole in the surface and then treat the
cut surface with

phenolphthalein

indicator solution. This

solution will turn [pink] when in contact with
alkaline concrete, making it possible to see the depth of carbonation. An existing hole is no good because
the exposed surface will already be carbonated.

Chlorides

Chlorides
, including

sodium chloride
, can promote the corrosion of embedded

steel

rebar

if present in
sufficienty high concentration. Chloride anions induce both localized corrosion (
pitting corro
sion
) and
generalized corrosion of steel reinforcements. For this reason, one should only use fresh raw water or
potable water for mixing concrete, ensure that the coarse and fine aggregates do not contain chlorides,
and not use admixtures that contain ch
lorides.

It was once common for

calcium chloride

to be used as an admixture to promote rapid set
-
up of the
concrete. It was also mistakenly believed that it would prevent f
reezing. However, this practice has fallen
into disfavor once the deleterious effects of chlorides became known. It should be avoided when ever
possible.

The use of de
-
icing salts on roadways, used to reduce the

freezing point

of water, is probably one of the
primary causes of premature failure of reinforced or prestressed concrete bridge decks, roadways, and
parking garages. The use of
epoxy
-
coated

reinforcing bars and the application of

cathodic protection

has
mitigated this problem to some extent. Also FRP rebars are known to be

less susceptible to chlorides.
Properly designed concrete mixtures that have been allowed to cure properly are effectively impervious to
the effects of deicers.

Another important source of chloride ions is from

sea water
. Sea water contains by weight approximately
3.5

wt.%

salts
. These salts include

sodium chloride
,

magnesium sulfate
,
calcium sulfate
,
and

bicarbonates
. In water these salts dissociate in free ions (Na
+
, Mg
2+
, Cl
-
, SO
4
2
-
, HCO
3
-
) and migrate
with the water into the

capillaries

of

the concrete. Chloride ions are particularly aggressive for the
corrosion of the carbon steel reinforcement bars and make up about 50% of these ions.

In the 1960s and 1970s it was also relatively common for

Magnesite
, a

chloride

rich
carbonate mineral
, to
be used as a floor
-
topping mater
ial. This was done principally as a levelling and sound attenuating layer.
However it is now known that when these materials came into contact with moisture it produced a weak
solution of

hydrochloric acid

due to the presence of

chlorides

in the magnesite. Over a period of time
(typically decades) the solution caused

corrosion

of the embedded

steel

rebars
. This was most commonly
found in wet areas or areas repeatedly

exposed to moisture.

Alkali silica reaction

This a reaction of

amorphous

silica

(
chalcedony
,

chert
,

siliceous

limestone
) sometimes present in
the

aggregates

with the

hydroxyl
ions (OH
-
) from the c
ement pore solution. Poorly crystallized silica (SiO
2
)
dissolves and dissociates at high pH (12.5
-

13.5) in alkaline water. The soluble dissociated

silicic
acid

reacts in the pore
water with the

calcium hydroxide

(
portlandite
) present in the

cement

paste to form
an expansive

calcium silicate hydrate

(CSH). The

alkali silica reaction

(ASR)
, causes localised swelling
responsible of

tensile stress

and

cracking
. The conditions required for alkali silica reaction are threefold:
(1) aggregate containing an alkali
-
reactive constituent (amorphous silica), (2) sufficient availability of
hydroxyl ions (OH
-
), and (3) sufficient moisture, a
bove 75

%

relative humidity

(RH) within the
concrete.
[6]
[7]

This phenomenon is sometimes popularly referred to as "
concrete cancer
". This reaction
occurs independently of the prese
nce of rebars: massive concrete structures such as

dams

can be
affected.

Conversion of high alumina cement

Resistant to weak acids and especially sulfates, this cement cures quickly and reaches very

high
durability and strength. It was greatly used after

World War II

for making precast concrete objects.
However, it can lose strength with heat or time (conversion), especially
when not properly cured. With the
collapse of three roofs made of prestressed concrete beams using high alumina cement, this cement
was

banned

in the

UK

in 1976. Subsequent inquiries into the matter showed that the beams were

Sulphates

Sulfates

(SO
4
) in the soil or in groundwater, in sufficient

concentration, can react with the Portland cement
in concrete causing the formation of expansive products, e.g.

ettringite

or

thaumasite
, which can lead to
early failure of the structure. The most typical attack of this type is on concrete slabs and foundation walls
at grade where the sulfate ion, via alternate wetting and drying, can increase in concentration. As
the
concentration increases, the attack on the Portland cement can begin. For buried structures such as pipe,
this type of attack is much rarer especially in the Eastern half of the United States. The sulfate ion
concentration increases much slower in the
soil mass and is especially dependent upon the initial amount
of sulfates in the native soil. The chemical analysis of soil borings should be done during the design
phase of any project involving concrete in contact with the native soil to check for the pr
esence of
sulfates. If the concentrations are found to be aggressive, various protective coatings can be used. Also,
in the US ASTM C150 Type 5 Portland cement can be used in the mix. This type of cement is designed to
be particularly resistant to a sulfat
e attack.

Steel plate construction

In steel plate construction, stringers join parallel steel plates. The plate assemblies are fabricated off site,
and welded together on
-
site to form steel walls connected by stringers. The walls become the form into
which

concrete is poured. Steel plate construction speeds reinforced concrete construction by cutting out
the time consuming on
-
site manual steps of tying rebar and building forms. The method has excellent
strength because the steel is on the outside, where ten
sile forces are often greatest.

Fiber
-
reinforced concrete

Fiber reinforcement is mainly used in

shotcrete
, but can also be used in normal concrete. Fiber
-
reinforced
normal concrete is mo
stly used for on
-
ground floors and pavements, but can be considered for a wide
range of construction parts (beams, pillars, foundations, etc.), either alone or with hand
-
tied rebars.

Concrete reinforced with fibers (which are usually steel,

glass
, or

plastic fibers
) is less expensive than
hand
-
tied rebar, while still increasing the tensile strength

many times. Shape, dimension, and length of
fiber are important. A thin and short fiber, for example short, hair
-
shaped glass fiber, will only be effective
the first hours after pouring the concrete (reduces cracking while the concrete is stiffening) but
will not
increase the concrete tensile strength. A normal
-
size fiber for European shotcrete (1

mm diameter,
45

mm length

steel or plastic) will increase the concrete's tensile strength.

Steel is the strongest commonly
-
available fiber, and comes in differen
t lengths (30 to 80

mm in Europe)
and shapes (end
-
hooks). Steel fibers can only be used on surfaces that can tolerate or avoid corrosion
and rust stains. In some cases, a steel
-
fiber surface is faced with other materials.

Glass fiber is inexpensive and cor
rosion
-
proof, but not as ductile as steel. Recently, spun

basalt fiber
,
long available in

Eastern Eur
ope
, has become available in the U.S. and Western Europe. Basalt fibre is
stronger and less expensive than glass, but historically has not resisted the alkaline environment
of

portland cement

well enough to be used as direct reinforcement. New materials use plastic binders to
isolate the basalt fiber from the cement.

The premium fibers are

graphite
-
reinforced

plastic fibers, which are nearly as strong as steel, lighter
-
weight, and corrosion
-
proof. Some experiments have had promising early results with

carbon nanotubes
,
but the
material is still far too expensive for any building.

Non
-
steel reinforcement

There is considerable overlap between the subjects of non
-
steel reinforcement and fiber
-
reinforcment of
concrete. The introduction of non
-
steel reinforcement of concrete is relat
ively recent; it takes two major
forms: non
-
metallic rebar rods, and non
-
steel (usually also non
-
metallic) fibres incorporated into the
cement matrix. For example there is increasing interest in

glass fiber reinforced concrete (
GFRC
)

and in
various applications of polymer fibres incorporated into concrete. Although currently there is not much
suggestion that such materials will in general replac
e metal rebar, some of them have major advantages
in specific applications, and there also are new applications in which metal rebar simply is not an option.
However, the design and application of non
-
steel reinforcing is fraught with challenges; for one t
hing,
concrete is a highly alkaline environment, in which many materials, including most kinds of glass, have a
poor
service life
. Also, the behaviour of such reinforcing materials
differ from the behaviour of metals, for
instance in terms of shear strength, creep and elasticity.
[9]
[10]

Fibre
-
Reinforced Polymer (FRP) (
Fibre
-
reinforced plastic

or FRP) and

Glass
-
reinforced plastic

(GRP)
consist of fibres of

polymer
, glass, carbon, aramid or other polymers or high
-
strength fibres set in a resin
matrix to form a rebar

rod or grid or fibres. These rebars are installed in much the same manner as steel.
The cost is higher but, suitably applied, the structures have advantages, in particular a dramatic reduction
in problems related to

corrosion
, either by intrinsic concrete alkalinity or by external corrosive fluids that
might penetrate the concrete. These structures can be significantly lighter and usually have a
longer

service life
. The cost of these materials has dropped dramatically since their widespread adoption
in the aerospace industry and by the military.


2.

Types of concrete

Regular concrete

Regular concrete

is the lay te
rm describing concrete that is produced by following the mixing instructions
that are commonly published on packets of cement, typically using sand or other common material as the
aggregate, and often mixed in improvised containers. This concrete can be pr
oduced to yield a varying
strength from about 10 MPa (1450 psi) to about 40 MPa (5800 psi), depending on the purpose, ranging
from blinding to structural concrete respectively. Many types of pre
-
mixed concrete are available which
include powdered cement mi
xed with an aggregate, needing only water.

Typically, a batch of concrete can be made by using 1 part Portland cement, 2 parts dry sand, 3 parts dry
stone, 1/2 part water. The parts are in terms of weight


not volume. For example, 1
-
cubic
-
foot (0.028

m
3
)
of concrete would be made using 22

lb (10.0

kg) cement, 10

lb (4.5

kg) water, 41

lb (19

kg) dry sand,
70

lb (32

kg) dry stone (1/2" to 3/4" stone). This would make 1
-
cubic
-
foot (0.028

m
3
) of concrete and
would weigh about 143

lb (65

kg). The sand should be

mortar or brick sand (washed and filtered if
possible) and the stone should be washed if possible. Organic materials (leaves, twigs, etc.) should be
removed from the sand and stone to ensure the highest strength.

High
-
strength concrete

High
-
strength concr
ete

has a compressive strength generally greater than 6,000 pounds per square inch
(40 MPa = 5800 psi). High
-
strength concrete is made by lowering the water
-
cement (W/C) ratio to 0.35 or
lower. Often silica fume is added to prevent the formation of free ca
lcium hydroxide crystals in the cement
matrix, which might reduce the strength at the cement
-
aggregate bond.

Low W/C ratios and the use of silica fume make concrete mixes significantly less workable, which is
particularly likely to be a problem in high
-
str
ength concrete applications where dense rebar cages are
likely to be used. To compensate for the reduced workability,
superplasticizers

are commonly added to
high
-
strength m
ixtures. Aggregate must be selected carefully for high
-
strength mixes, as weaker
aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to
start in the aggregate rather than in the matrix or at a void, as normally

occurs in regular concrete.

In some applications of high
-
strength concrete the design criterion is the

elastic modulus

rather than the
ultimate compressive strength.

Stamped

concrete

Stamped concrete

is an architectural concrete which has a superior surface finish. After a concrete floor
has been laid, floor hardeners (can be pigmented) are impregnated on the surface and a mold which may
be textured to replicate a stone / bri
ck or even wood is stamped on to give an attractive textured surface
finish. After sufficient hardening the surface is cleaned and generally sealed to give a protection. The
wear resistance of stamped concrete is generally excellent and hence found in appl
ications like parking
lots, pavements, walkways etc.

High
-
performance concrete

High
-
performance concrete

(HPC) 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 limit
ed 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 placement



Compaction without

segregation



Early age strength



Long
-
term mechanical properties



Permeability



Density



Heat of hydration



Toughness



Volume stability



Long life in severe environments



Depending on its
implementation, environmental

[1]

Ultra
-
high
-
performance concrete

Ultra
-
high
-
performance concrete is a new type of concrete that is being developed by agencies
concerned with i
nfrastructure protection. UHPC is characterized by being a steel fibre
-
reinforced cement
composite material with compressive strengths in excess of 150 MPa, up to and possibly exceeding 250
MPa.
[2]

UHPC is also characterized by its constituent material make
-
up: typically fine
-
grained sand, silica
fume, small steel fibers, and special blends of high
-
strength Portland cement. Note that there is no large
aggregate. The current type
s in production (Ductal, Taktl, etc.) differ from normal concrete in compression
by their strain hardening, followed by sudden brittle failure. Ongoing research into UHPC failure via
tensile and shear failure is being conducted by multiple government agenc
ies and universities around the
world.

Self
-
consolidating concretes

After identifying the defects in concrete in Japan were mainly due to a) high water cement ratio to
increase workability, b) poor compaction mostly happened due to the need of speedy const
ruction in
1960s

70s, Professor Hajime Okamura envisioned the need of a concrete that is highly workable and
does not rely on the mechanical force for compaction. During the 1980s, Professor Okamura and his PhD
student Kazamasa Ozawa (currently professor)
at the University of Tokyo, Japan developed a concrete
called Self Compacting Concrete (SCC) that was cohesive but flowable and took the shape of the
formwork without use of any mechanical compaction. SCC is known as self
-
consolidating concrete in the
Unit
ed States. SCC is characterized by:



extreme fluidity as measured by

flow
, typically between 650

750

mm on a flow table, rather than
slump(height)



no need for

vibra
tors

to compact the concrete



placement being easier.



no bleed water, or

aggregate segregation



Increased Liquid Head Pressure, Can be detrimental to Safety an
d workmanship

SCC can save up to 50% in labor costs due to 80% faster pouring and reduced

wear and
tear

on

form
work
.

Vacuum concretes

The use of steam to produce a vacuum inside of concrete mixing truck to release air bubbles inside the
concrete is being researched. The idea is that the steam displaces the air normally over the concrete.
When the steam condenses i
nto water it will create a low pressure over the concrete that will pull air from
the concrete. This will make the concrete stronger due to there being less air in the mixture. Obviously
this needs to be done in a sealed container.

Shotcrete

Shotcrete

(als
o known by the trade name

Gunite
) uses compressed air to shoot concrete onto (or into) a
frame or structure. The greatest advantage of the process is that shotcrete can be applied overhead or on
vertical surfaces without forming. It is often used for concr
ete repairs or placement on bridges, dams,
pools, and on other applications where forming is costly or material handling and installation is difficult.
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

tunneling
. Shotcrete is also used for applications where
seepage i
s an issue to limit the amount of water entering a construction site due to a high water table or
other subterranean sources. This type of concrete is often used as a quick fix for weathering for loose soil
types in construction zones.

There are two applic
ation methods for shotcrete.



dry
-
mix



the 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.



wet
-
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.
[3]


Limecrete

Limecrete

or lime concrete is concrete where cement is

replaced by

lime
.
[4]

One successful formula was
developed in the mid 1800s by Dr. John E. Par
k
[5]
. We know that lime has been used since Roman Times
either as mass foundation concretes or as lightweight concretes using a variety of aggregates combined
with a wide range

of pozzolans (fired materials) that help to achieve increased strength and speed of set.
This meant that lime could be used in a much wider variety of applications than previously such as floors,
vaults or domes. Over the last decade, there has been a ren
ewed interest in using lime for these
applications again. This is because of environmental benefits and potential health benefits, when used
with other lime products.

Environmental Benefits



Lime is burnt at a lower temperature than cement and so has an imm
ediate energy saving of 20%
(although kilns etc are improving so figures do change). A standard lime mortar has about 60
-
70% of
the embodied energy of a cement mortar. It is also considered to be more environmentally friendly
because of its ability, throug
h carbination, to re
-
absorb its own weight in Carbon Dioxide
(compensating for that given given off during burning).



Lime mortars allow other building components such as stone, wood and bricks to be reused and
recycled because they can be easily cleaned of

mortar/limewash.



Lime enables other natural and sustainable products such as wood (including woodfibre, wood
wool boards), hemp, straw etc to be used because of its ability to control moisture (if cement were
used, these buildings would compost!).

Health
Benefits



Lime plaster is hygroscopic (literally means 'water seeking') which draws the moisture from the
internal to the external environment, this helps to regulate humidity creating a more comfortable living
environment as well as helping to control cond
ensation and mould growth which have been shown to
have links to allergies and asthmas.



Lime plasters and limewash are non
-
toxic, therefore they do not contribute to indoor air pollution
unlike some modern paints.


3.

Fibre reinforced process

FRP involves
two distinct processes, the first is the process whereby the fibrous material is manufactured
and formed, the second is the process whereby fibrous materials are bonded with the matrix during the
moulding process.
[2]

Fibre process

The manufacture of fibre fabric

Reinforcing Fibre is manufactured in both two dimensional and three dimensional orientations

1.

Two Dimensional Fibre Reinforced Polymer are characterized by
a laminated structure in which
the fibres are only aligned along the plane in

x
-
direction and y
-
direction

of the material. This
means that no fibres a
re aligned in the through thickness or the

z
-
direction
, this lack of alignment
in the through thickness can create a disadvantage in cost and processi
ng. Costs and labour
increase because conventional processing techniques used to fabricate composites, such as wet
hand lay
-
up, autoclave and resin transfer moulding, require a high amount of skilled labour to
cut, stack and consolidate into a preformed co
mponent.

2.

Three
-
dimensional Fibre Reinforced Polymer composites are materials with three dimensional
fibre structures that incorporate fibres in the

x
-
d
irection, y
-
direction and z
-
direction
. The
development of three
-
dimensional orientations arose from industry's need to reduce fabrication
costs, to increase through
-
thickness mechanical properties, and to improve impact damage
tolerance; all were problems

associated with two dimensional fibre reinforced polymers.

The manufacture of fibre preforms

Fibre preforms are how the fibres are manufactured before being bonded to the matrix. Fibre preforms
are often manufactured in sheets, continuous mats, or as cont
inuous filaments for spray applications. The
four major ways to manufacture the fibre preform is though the textile processing techniques
of

Weaving
,

knitting
,

braiding

and

stitching
.

1.

Weaving can be done in a conventional manner to produce
two
-
dimensional fibres as well in a
multilayer weaving that can create three
-
dimensional fibres. However, multilayer weaving is
required to have multiple layers of warp yarns to create fibres in the z
-

direction creating a few
disadvantages in manufacturin
g,namely the time to set up all the

warp

yarns on the

loom
.
Therefore most multilayer weaving is currently used to p
roduce relatively narrow width products,
or high value products where the cost of the preform production is acceptable. Another one of
the main problems facing the use of multilayer woven fabrics is the difficulty in producing a fabric
that contains fibres

oriented with angles other than 0" and 90" to each other respectively.

2.

The second major way of manufacturing fibre preforms is Braiding. Braiding is suited to the
manufacture of narrow width flat or tubular fabric and is not as capable as weaving in the
p
roduction of large volumes of wide fabrics. Braiding is done over top of mandrels that vary in
cross
-
sectional shape or dimension along their length. Braiding is limited to objects about a brick
in size. Unlike the standard weaving process, braiding can pr
oduce fabric that contains fibres at
45 degrees angles to one another. Braiding three
-
dimensional fibres can be done using four
step, two
-
step or Multilayer Interlock Braiding.Four step or row and column braiding utilizes a flat
bed containing rows and col
umns of yarn carriers that form the shape of the desired preform.
Additional carriers are added to the outside of the array, the precise location and quantity of
which depends upon the exact preform shape and structure required. There are four separate
seq
uences of row and column motion, which act to interlock the yarns and produce the braided
preform. The yarns are mechanically forced into the structure between each step to consolidate
the structure in a similar process to the use of a reed in weaving.Two
-
step braiding is unlike the
four step process because the two
-
step includes a large number of yarns fixed in the axial
direction and a fewer number of braiding yarns. The process consists of two steps in which the
braiding carriers move completely through
the structure between the axial carriers. This relatively
simple sequence of motions is capable of forming preforms of essentially any shape, including
circular and hollow shapes. Unlike the four step process the two step process does not require
mechanica
l compaction the motions involved in the process allows the braid to be pulled tight by
yarn tension alone. The last type of braiding is multi
-
layer interlocking braiding that consists of a
number of standard circular braiders being joined together to form

a cylindrical braiding frame.
This frame has a number of parallel braiding tracks around the circumference of the cylinder but
the mechanism allows the transfer of yarn carriers between adjacent tracks forming a multilayer
braided fabric with yarns interl
ocking to adjacent layers. The multilayer interlock braid differs from
both the four step and two
-
step braids in that the interlocking yarns are primarily in the plane of
the structure and thus do not significantly reduce the in
-
plane properties of the pre
form. The four
step and two step processes produce a greater degree of interlinking as the braiding yarns travel
through the thickness of the preform, but therefore contribute less to the in
-
plane performance of
the preform. A disadvantage of the multilaye
r interlock equipment is that due to the conventional
sinusoidal movement of the yarn carriers to form the preform, the equipment is not able to have
the density of yarn carriers that is possible with the two step and four step machines.

3.

Knitting fibre pre
forms can be done with the traditional methods of Warp and [Weft] Knitting, and
the fabric produced is often regarded by many as two
-
dimensional fabric, but machines with two
or more needle beds are capable of producing multilayer fabrics with yams that tr
averse between
the layers. Developments in electronic controls for needle selection and knit loop transfer, and in
the sophisticated mechanisms that allow specific areas of the fabric to be held and their
movement controlled. This has allowed the fabric to

form itself into the required three
-
dimensional preform shape with a minimum of material wastage.

4.

Stitching is arguably the simplest of the four main textile manufacturing techniques and one that
can be performed with the smallest investment in specialize
d machinery. Basically the stitching
process consists of inserting a needle, carrying the stitch thread, through a stack of fabric layers
to form a 3D structure. The advantages of stitching are that it is possible to stitch both dry and
prepreg fabric, alt
hough the tackiness of the prepreg makes the process difficult and generally
creates more damage within the prepreg material than in the dry fabric. Stitching also utilizes the
standard two
-
dimensional fabrics that are commonly in use within the composite
industry
therefore there is a sense of familiarity concerning the material systems. The use of standard
fabric also allows a greater degree of flexibility in the fabric lay
-
up of the component than is
possible with the other textile processes, which have r
estrictions on the fibre orientations that can
be produced.
[15]

Moulding processes

There are two distinct categories of

moulding processes

using FRP plastics; this includes composite
moulding and wet moulding. Composite moulding uses Prepreg FRP, meaning the plastics are fibre
reinforced before being put through further moulding proce
sses. Sheets of Prepreg FRP are heated or
compressed in different ways to create geometric shapes. Wet moulding combines fibre reinforcement
and the matrix or resist during the moulding process.
[2]

The different forms of composite and wet
moulding, are listed below.

Composite moulding

Bladder moulding

Individual sheets of prepreg material are laid
-
up and placed in a female
-
style mould along with a balloon
-
like bla
dder. The mould is closed and placed in a heated press. Finally, the bladder is pressurized forcing
the layers of material against the mould walls. The part is cured and removed from the hot mould. Bladder
moulding is a closed moulding process with a relat
ively short cure cycle between 15 and 60 minutes
making it ideal for making complex hollow geometric shapes at competitive costs.
[16]

Compression moulding

A "pre
form" or "charge", of

SMC
, BMC or sometimes prepreg fabric, is placed into mould cavity. The
mould is closed and the material is compacted & cured inside by p
ressure and heat. Compression
moulding offers excellent detailing for geometric shapes ranging from pattern and relief detailing to
complex curves and creative forms, to

precision engineering

all within a maximum curing time of 20
minutes.
[16]

Autoclave / vacuum bag

Individual sheets of prepreg material are laid
-
up and pl
aced in an open mold. The material is covered with
release film, bleeder/breather material and a

vacuum bag
. A vacuum is pulled on part and the

entire
mould is placed into an autoclave (heated pressure vessel). The part is cured with a continuous vacuum
to extract entrapped gasses from laminate. This is a very common process in the aerospace industry
because it affords precise control over the mo
ulding process due to a long slow cure cycle that is
anywhere from one to two hours. This precise control creates the exact laminate geometric forms needed
to ensure strength and safety in the aerospace industry, but it is also slow and labour intensive, m
eaning
costs often confine it to the aerospace industry.
[16]

Mandrel wrapping

Sheets of prepreg material are wrapped around a steel or aluminium mandrel. The pre
preg material is
compacted by nylon or polypropylene cello tape. Parts are typically batch cured by hanging in an oven.
After cure the cello and mandrel are removed leaving a hollow carbon tube. This process creates strong
and robust hollow carbon tubes.
[16]

Wet layup

Fibre reinforcing fabric is placed in an open mould and then saturated with a wet [resin] by pouring it over
the fabric and working it into the fabr
ic and mould. The mould is then left so that the resin will cure,
usually at room temperature, though heat is sometimes used to ensure a proper curing process. Glass
fibres are most commonly used for this process, the results are widely known as fibreglass
, and is used to
make common products like skis, canoes, kayaks and surf boards.
[16]

Chopper gun

Continuous strand of fibreglass are pushed through a hand
-
held g
un that both chops the strands and
combines them with a catalysed resin such as polyester. The impregnated chopped glass is shot onto the
mould surface in whatever thickness the design and human operator think is appropriate. This process is
good for large

production runs at economical cost, but produces geometric shapes with less strength than
other moulding processes and has poor dimensional tolerance.
[16]

Filam
ent winding

Machines

pull fibre bundles through a wet bath of resin and wound over a rotating steel mandrel in
specific orientations P
arts are cured either room temperature or elevated temperatures. Mandrel is
extracted, leaving a final geometric shape but can be left in some cases.
[16]


4.

Typ
es of reinforcement

B.F. Skinner
, the researcher who articulated the major theoretical constructs of reinforcement
and

behaviorism
, defined reinforcers according to the change in response strength rather than to more
subjective criteria, such as what is pleasurable or valuable to someone. Accordingly, activities, foods or
items considered pleasant or enjoyable

may not necessarily be reinforcing (because they produce no
increase in the response preceding them). Stimuli, settings, and activities only fit the definition of
reinforcers if the behavior that immediately precedes the potential reinforcer increases in
similar situations
in the future, for example, a child who receives a cookie when he or she asks for one. If the frequency of
"cookie
-
requesting behavior" increases, the cookie can be seen as reinforcing "cookie
-
requesting
behavior". If however, "cookie
-
re
questing behavior" does not increase the cookie cannot be considered
reinforcing.

Reinforcement theory is one of the motivation theories; it states that reinforced behavior will be repeated,
and behavior that is not reinforced is less likely to be repeated
.
[1]

The sole criterion that determines if an item, activity, or food is reinforcing is the change in probability of a
behavior after administration of that potential reinforcer. O
ther theories may focus on additional factors
such as whether the person expected the strategy to work at some point, but in the behavioral theory,
reinforcement is descriptive of an increased probability of a response.

The study of reinforcement has produ
ced an enormous body of

reproducible

experimental results.
Reinforcement is the central concept and procedure in

special education
,

applied behavior analysis
, and
the

experimental analysis of behavior
.

Positive and negative reinforcement

As Skinner discussed, positive reinforcement is superior to

punishment

in altering behavior. He
maintained that punishment was

not
simply the opposite of positive reinforcement; positive reinforcement
results in lasting behavioral modification, whereas punishment changes behavior on
ly temporarily and
presents many detrimental side effects.
[2]

The accepted model of reinforcement began shifting in 1966 when Azrin and Holz contributed a
chapter
[3]

to Honig's volume on operant conditioning. Skinner defined reinforcement as creating situations
that a person likes or removing a situation he doesn't like, and punishmen
t as removing a situation a
person likes or setting up one he doesn't like.
[2]

Thus the distinction was based on the appetitive or
aversive nature of the stimulus. Azrin
and Holz defined punishment "as 'a reduction of the future
probability of a specific response as a result of the immediate delivery of a stimulus for that
response'."
[4
]

This new definition of punishment encroached on Skinner's definition of reinforcement, but
most textbooks now only present examples of the 1966 model summarized below:

Helpful definitions:



Appetitive stimulus: a pleasant outcome



Aversive stimulus: an un
pleasant outcome

A positive reinforcer is a consequence that increases the frequency of a behavior or maintains the
frequency. What is reinforcing is defined by what happens to the frequency of the behavior. It has nothing
to do with whether the organism f
inds the reinforcer "pleasant" or not. For example, if a child gets slapped
for saying a "naughty" word but the frequency of naughty words increases, the slap is a positive
reinforcer.

A "pleasant" consequence is not necessarily a positive reinforcer.
[5]

Getting a birthday gift is not a positive
reinforcer. There is no behavior that will increase (or be maintained) in frequency. When deciding whether
or not something is a reinforcer
, the basic criteria is the frequency of occurrence of a behavior.

Consequences are not universally reinforcing. For example, happy face stickers may be effective
reinforcers for some children. Other children may find them silly.
[
citation needed
]

A negative reinforcer increases the frequency of a behavior or maintains the frequency. It is not
punishment. These terms are often confused. A negative reinforcer

increases or maintains the frequency
of the behavior that terminates the negative reinforcer. In this case the negative reinforcer is present
before the behavior. The organism performs a behavior that terminates the negative reinforcer. The
behavior that
terminates the negative reinforcer is likely to increase or be maintained in frequency.
Suppose someone has a headache (negative reinforcer). The person takes two aspirin but nothing
happens. Then the person takes two Tylenol tablets and the headache goes
away. The next time the
person has a headache it is likely the person will take Tylenol. That is the behavior that has been
reinforced.

Forms of operant conditioning:



Positive reinforcement
: the adding of an appetitive stimulus to increase a certain behavi
or or
response.

Example: Father gives candy to his daughter when she picks up her toys. If the frequency of picking
up the toys increases or stays the same, the candy is a positive reinforcer.



Positive punishment
: the adding of an aversive stimulus to decr
ease a certain behavior or
response.

Example: Mother yells at a child when running into the street. If the child stops running into the street
the yelling is positive punishment.



Negative reinforcement:

the taking away of an aversive stimulus to increase c
ertain behavior or
response.

Example: Turning off distracting music when trying to work. If the work increases when the music is
turned off, turning off the music is a negative reinforcer.



Negative punishment (omission training)
: the taking away of an appe
titive stimulus to
decrease a certain behavior.

Example: A teenager comes home an hour after curfew and the parents take away the teen's cell
phone for two days. If the frequency of coming home after curfew decreases, the removal of the
phone is negative p
unishment.

The following table illustrates that punishment and reinforcement are a function of the presentation or
removal of a stimulus and the valence of the stimulus.

Distinguishing "positive" from "negative" can be difficult, especially when there are
lots of
consequences and the necessity of the distinction is often debated.
[6]

For example, in a very warm
room, a current of external air serves as positive rei
nforcement because it is pleasantly cool or
negative reinforcement because it removes uncomfortably hot air.
[7]

Some reinforcement can be
simultaneously positive

and negative, such as a

drug addict

taking drugs for the added euphoria and
eliminating

withdrawal

symptoms.
Many behavioral psychologists simply refer to reinforcement
or

punishment

without polarity

to cover all consequent environmental changes. Others would
disagre
e with the above examples because there is no behavior that is increasing or decreasing in
frequency.

Primary reinforcers

A primary reinforcer, sometimes called an

unconditioned reinforcer
, is a stimulus that does not
require pairing to function as a reinf
orcer and most likely has obtained this function through the
evolution and its role in species' survival.
[8]

Examples of primary reinforcers include sleep, food, air,
water, and se
x. Some primary reinforcers, such as certain drugs, may mimic the effects of other
primary reinforcers. While these primary reinforcers are fairly stable through life and across
individuals, the reinforcing value of different primary reinforcers varies due

to multiple factors (e.g.,
genetics, experience). Thus, one person may prefer one type of food while another abhors it. Or one
person may eat lots of food while another eats very little. So even though food is a primary reinforcer
for both individuals, th
e value of food as a reinforcer differs between them.

Secondary reinforcers

A secondary reinforcer, sometimes called a

conditioned reinforcer
, is a stimulus or situation that has
acquired its function as a reinforcer after

pairing

with a stimulus that functions as a reinforcer. This
stimulus may be a primary reinforcer or another conditioned reinforcer (such as money). An example
of a secondary reinforcer would be the sound from a clicker, as

used in

clicker training
. The sound of
the clicker has been associated with praise or treats, and subsequently, the sound of the clicker may
function as a reinforcer. As w
ith primary reinforcers, an organism can experience satiation and
deprivation with secondary reinforcers.

Other reinforcement terms



A generalized reinforcer is a conditioned reinforcer that has obtained the reinforcing function by
pairing with many other r
einforcers (such as money, a secondary generalized reinforcer).



In reinforcer sampling, a potentially reinforcing but unfamiliar stimulus is presented to an
organism without regard to any prior behavior.



Socially
-
mediated reinforcement (direct reinforcemen
t) involves the delivery of reinforcement that
requires the behavior of another organism.



The

Premack principle

is a special case of reinforcement elaborated by

David Premack
, which
states that a highly
-
preferred activity can be used effectively as a reinforcer for a less
-
preferred
activity.



Reinforcement hierarchy is a list of actions, rank
-
orde
ring the most desirable to least desirable
consequences that may serve as a reinforcer. A reinforcement hierarchy can be used to
determine the relative frequency and desirability of different activities, and is often employed
when applying the Premack prin
ciple.
[
citation needed
]



Contingent outcomes are more likely to reinforce behavior than non
-
contingent responses.
Contingent outcomes are those directly li
nked to a

causal

behavior, such a light turning on being
contingent on flipping a switch. Note that contingent outcomes are

not
necessary to demonstrate
reinforcement, but perceived contingency

may increase learning.



Contiguous stimuli are stimuli closely associated by time and space with specific behaviors. They
reduce the amount of time needed to learn a behavior while increasing its resistance
to

extinction
. Giving a dog a piece of food immediately after sitting is more contiguous with (and
therefore more likely to reinforce) the behavior than a several minute delay in food delivery
following the
behavior.



Noncontingent reinforcement refers to response
-
independent delivery of stimuli identified as
reinforcers for some behaviors of that organism. However, this typically entails time
-
based
delivery of stimuli identified as maintaining aberrant behavi
or, which decreases the rate of the
target behavior.
[9]

As no measured behavior is identified as being strengthened, there is
controversy surrounding the use of the term nonconting
ent "reinforcement".
[10]

Natural and artificial reinforcement

In his 1967 paper,

Arbitrary and Natural Reinforcement
,

Charles Ferster

proposed classifying
reinforcement into events that increase frequency of an operant as a natural consequence of the
behavior itself, and events that are presumed to affect frequency by their requirement of human
mediati
on, such as in a

token economy

where subjects are "rewarded" for certain behavior with an
arbitrary token of a negotiable value. In 1970, Baer and Wolf created a name for the use

of natural
reinforcers called "behavior traps".
[11]

A behavior trap requires only a simple response to enter the
trap, yet once entered, the trap cannot be resisted in creating g
eneral behavior change. It is the use
of a behavioral trap that increases a person's repertoire, by exposing them to the naturally occurring
reinforcement of that behavior. Behavior traps have four characteristics:



They are "baited" with virtually irresist
ible reinforcers that "lure" the student to the trap



Only a low
-
effort response already in the repertoire is necessary to enter the trap



Interrelated contingencies of reinforcement inside the trap motivate the person to acquire, extend,
and maintain target
ed academic/social skills
[12]



They can remain effective for long periods of time because the person shows few, if any, satiation
effects

As can be seen from the above, artificial
reinforcement is in fact created to build or develop skills, and
to generalize, it is important that either a behavior trap is introduced to "capture" the skill and utilize
naturally occurring reinforcement to maintain or increase it. This behavior trap ma
y simply be a social
situation that will generally result from a specific behavior once it has met a certain criterion (e.g., if
you use edible reinforcers to train a person to say hello and smile at people when they meet them,
after that skill has been bu
ilt up, the natural reinforcer of other people smiling, and having more
friendly interactions will naturally reinforce the skill and the edibles can be faded).
[