Introduction to Reinforced Concrete Video Lecture - Civil Engineering (CE)

hi this is the first video in the
reinforced concrete series and I'll be
briefly introducing what reinforced
concrete is and what is the role of
concrete and steel in the reinforced
concrete structures as we have discussed
before concrete is very strong in
compression and that is the one of the
reason why concrete is so widely used in
construction industries and in
structural members but the the one of
the big disadvantage of concrete is that
it is very weak in tension the
compressive strength of concrete is very
high for the normal strength concrete it
can be it can range from 20 mega Pascal
to say 60 mega Pascal but for high
strength concrete it can be more than
even more than 100 mega Pascal as well
however the tensile strength in concrete
is very low it is usually around
one-tenth of the compressive strength
but unfortunately most of our structure
members will have both compressive
stress and tensile stress developed due
to the load acting on it for example
let's take an example offer of a beam
simply supported beam subjected to
transverse loading the transverse
loading can be in the form of uniformly
distributed loads as shown in the figure
or it can be point load as shown in the
figure again due to this transverse
loading the reinforce come be mean have
compressive stress as well as the
tensile stress as well so as you know
due to the transverse loading the beam
will deflect like this one so there will
be tension develop at the bottom phase
and there would be compression develop
at the top phase so as you can see here
if you take a section at CC there will
be compressive stresses develop at the
top and the
stresses develop at the bottom due to
the bending moment that arises there so
as we have discussed before for the
compressive stress we don't have any
problem with concrete because concrete
is very strong in compression but
tensile stress is a big problem in
concrete as soon as the tensile stress
resists the tensile strength of the
concrete which is very low the concrete
will crack and immediately fail if it is
a plain concrete now this picture shows
the four point loading for a plain
concrete beam as you can see there are
two supports here and the load is
applied through these two points and it
is called as four point loading if you
draw the bending moment diagram we know
that it will look like
you look like this so this is a positive
bending moment diagram and this is the
moment span here and the maximum tensile
stresses will be developing at the
bottom phase now as soon as the tensile
stress will reach the tensile strength
of the concrete the concrete will
immediately crack as seen here and it
will fail immediately now this is
another example of the tensile failure
of the beam tensile failure in the
concrete this is what we call as a split
cylinder test or Brassard cylinder test
for concrete now the load is applied at
the horizontal face of the cylinder here
so due to the load there will be tension
develop here the tensile stresses
develop here as soon as this tensile
stress reached the tensile strength of
the concrete the cylinder will just
split like sone here so the plain
concrete fails immediately after it
reaches its tensile strength now as we
discussed before most of the structure
member will develop both the compressive
stress and tensile stresses and as this
tensile stress will exceed the tensile
strength of the concrete the concrete
will fail the plain concrete will
immediately fail so a very clever
solution to solve this problem is to
introduce a reinforcing steel in the
tensile region as you can see in this
figure if we put a reinforcing bar in
the tensile reason but what it does is
that once the tensile stress in the
tension zone reaches the tensile
strength of the concrete we will see a
minor cracks coming here in the concrete
as soon as the cracks appears this
tensile stress will now be transferred
to the steel now so that means all the
tensile stresses now is taken up by the
steel and the compressive stress is
still taken by the concrete because
concrete is still very good in
compression so as you can see here this
is how the reinforced concrete will
behave all the tensile stresses will be
by the steel and all the compressive
stress now is taken by the concrete as
as before as well so this be this beam
will not fail because the tensile
stresses are not taken by the steel so
as you can see here these are the
tencel's dents and reinforcements
provided in in the concrete section so
having the tensile reinforcement now the
tensions steel it provides inside stop
to the reinforced concrete member and
also as you know this steel is very
ductile so it provides the ductility to
the structure as well
however the the steel is vulnerable to
corrosion and also steel members are
usually small thin sections so it can it
can buckle in compressive loading and
also when subjected to heat it can lose
strength as well so these are the
disadvantage for the steel but when we
combined the concrete and the steel we
get a very good construction material
call as reinforced concrete and these
two materials the concrete and the steel
complements each other very well as we
discussed before concrete is very strong
in compression but very weak in tension
but the steel is very strong in tension
and also strong in compression as well
so this the all the tensile stresses
that develops can be taken by the steel
now and also this steel is vulnerable to
corrosion and heat damage but if we
provide if we cover the steel with the
concrete it can prevent corrosion and
fire damage or heat damage in the steel
as well so so the steel and concrete
very complements very well with each
other and it proved it gives us a very
good construction material which we call
it as a reinforced concrete and also as
concrete is very cheap and all the raw
materials required for the concrete is
widely available so the reinforced
concrete is a very cheap construction
material and very versatile construction
material as well
and it can basically be used for
constructing any kind of structure so
for example for a typical house as you
can see here the reinforced concrete can
be used for constructing columns it can
be used for constructing in both them
suspended slab or slabs on the ground it
can be used for constructing foundations
staircases beams retaining walls shear
walls and for all of these structure
members we can use reinforced concrete
as a construction material now let's go
a little bit into detail about each of
this component of the reinforced
concrete that means first let's look
into the concrete then we'll look into
the reinforcing steel as well the
material properties of these two
materials now looking into the concrete
as we said the compression strength of
the concrete is the main advantage so
the compressive strength strength we
have - we measure it as at run at 28
days is taken as the standard test and
it is take is specified as nominal
strength of the concrete when we prepare
concrete by mixing cements sand stones
and water as soon as we mix cement in
water the hydration reaction takes place
and it will start to gain strength this
strength gaining process will go on from
what many quite a long time but will
take 28 days as them as the standard you
know for none for measuring the nominal
strength of the concrete because at that
time by that time most of the strength
of the concrete has developed and we use
characteristic compressive strength and
that is denoted as FC - for all of our
design calculations now if it tests a
hundreds of concrete cylinders we
subject to compressive force and test
the compressive strength of the concrete
we'll see a very varying strength from
these cylinders because concrete is a
mixture of different materials and it is
very hard to
the same strain from all the all these
different cylinders it's most likely
that if it is a lost number of cylinders
your compressive strength that you get
from this large number of cylinder
chests will be following this normal
distribution pattern some cylinders
scheme might be giving a very high
strength so for example in this curve
it's around 55 mega pascal most of the
majority of cylinders say will fail
around 40 mega pascal but there will
also be some cylinders that might fail
at very low strength that is 25 mega
pascal will have only very low strength
so now the question is which trends
should be used for our design
calculations if we use this maximum
strength that we get it will be very
risky for us because most of our
cylinders are failing we're below that
load that stress if you use this mean
stress that we get from a mean stress
means strain that we get from this all
the cylinders chest it is still risky
because as we see here 50 percent of the
cylinders are actually giving us less
friend than this main strength so it is
still very risky to use the main
strength for the design purpose so what
we do for the design purpose is that we
use the characteristic strength
compressive strength rather than the
mean strength or the highest value of
the strength so what characteristic
strength says is that for safer
typically for example Australian design
code we use 95 percentile value that
means so we will be using this strength
as the characteristic strength which
means that 95 percent of the cylinder
specimens we get higher strength than
this value and that means that 5 percent
of this specimen may fail below that
value but we are happy to take that risk
because we have that 95 percent
confidence level in those in the
strength that we are using so that is
what is called as the characteristic
compressive strength and we use it for
all the design purpose but for mix
design when you are ordering for the
concrete
we instead of ordering for this
characteristic value will be ordering
concrete for this mean strength value
for this example even though we are
using 30 mega Pascal as a complete
characteristic compressive strength will
we we will be ordering mix or target in
the mix design for 40 mega Pascal and
the normal characteristic conquer
concrete usually available are 20 mega
Pascal 25 32 or 40 or 50 mega Pascal and
also that we can we can get a concrete
for high strength concrete like 80 or
hundred or even greater than that as
well and Australian design code gives
this table for computing the modulus of
elasticity for the concrete for
different characteristic compressive
strength the modulus of elasticity for
concrete are given in by this table and
the density for concrete is taken as
2400 kg per meter cube for the plain
concrete this graph shows the stress
strain curve of compressive stress
strain curve for concrete cylinders now
this is for the difference strength
compressive strength cylinders for
example for example this is the curve
for a 20 mega Pascal's strength concrete
and this is the core for 100 mega Pascal
concrete cylinder now risking it we can
see that as we as we increase as we
increase the compressive strength of the
concrete the concrete starts to become
more brittle that means as soon as we
reach the maximum strain the concrete
will fail abruptly and also the
confining effect of the concrete members
will tend to increase the compressive
strength of the cylinder as well for
example in a reinforced concrete member
reinforced concrete column there will be
stirrups like these ones these are the
stirrups which confine the concrete
or and this confinement can increase the
compressive strength of the concrete as
well now there can be two kinds of
stirrups one is square stirrups or the
circular stirrups as well and the
circular stirrups has more confining
effect to do the hoop stress that's that
develops in the circular members
compared to the square Hoops so in the
square hoops if the confining effect is
smaller than the circular hoop because
these straight members in this in the
square hoop can buckle like outwards
when the compressive when the bursting
force is coming out from the core the
the straight members can buckle out like
this one so the effective confining in
this square hoops is only this area only
so the circular spiral is better in
confining the concrete but nevertheless
both the square and circular stirrups
will provide the confinement in the
concrete and this confinement will
increase the effective compressive
strength in the concrete as well and for
example these comparing these two
columns in this column this the distance
between the stirrups is higher than this
one that means this one has higher
confinement
then this one the the first column so
the concrete in here will have higher
compressive strength now as you can see
in this curve this dotted line here as
is without any confinement so you have
the compressive strength of around say
35 mega Pascal but as we reduce the
spacing that means as we increase the
confinement as you can see the the the
compressive strength is increasing as
well so that is the effect of confining
or confinement on the stress-strain
curve of the concrete now regarding the
tensile strength of the concrete as we
discussed before the concrete is very
weak in tension that means it has a very
low tensile strength which is usually
around 1/10 of the compressive strength
tensile strength can be obtained from in
the empirical formula from the
compressive strength itself as you can
see the tensile strength is given by
this equation empirical formula in
Australian design code 9 by clause three
point one point one point three and it
can also be obtained from direct tensile
test or other tests that we will be
discussing later on now coming to the
reinforcement SCO steel is usually used
as their enforcing material to take the
tensile stresses in the concrete
concrete sections and most of the steel
that we use in the reinforced concrete
members are deformed bars like this
fostering in the deform bars we have
this deformation in the bar and that
will take that will improve or enhance
the bond between the steel and the
concrete and and these are the plane
bars but the bonding in the deforms bar
are much better than that of the plane
bar
now the stress-strain relationship of
the steel is very similar in tension and
compression as well so so if we
subjected if we subject a reinforcing
steel to tensile force the stress will
first increase linearly with strain and
this is a linear part or the elastic
portion of the stress-strain curve for
steel and at certain point this stress
the steel will eat it and that is called
as ill point and the corresponding
stress is the eel strength of the steel
that is fsy and the corresponding strain
in that that particular point is the
yield strain in the steel now this the
slope of this linear portion of the
stress-strain curve for steel is the
young modulus of the steel which is
given by this formula and the stress
over strain which is 200 mega pascal
yeah two hundred thousand mega pascal
and after ill point the strain in steel
will increase without the increase in
the stress as you can see here this
strain is increasing without increasing
in increase in the stress and that is
called as the eel platoon the ilkka tube
will represent the ductility in the
steel member the larger the eel patio
the more ductile the steel is and it is
the in plate itself is the function of
the strength of the steel as well the
high strength steel with high percentage
of carbon will have smaller platy
whereas the low blow percent is carbon
steel will have lowest strength but will
have high longer plate you as well now
due to the yielding or classic
deformation that goes during this
process and the inter-atomic bond
between in the industry will break and
rearrange in the solid state and this
will cause the increase in for the
increase in the stress and
is called as strain hardening and this
is very typical property of a ductile
material like steel and eventually at
certain stress the steel will fail or it
will break but for most of the design
purpose we do not rely off the strain
hardening
so the stress-strain curve for a steel
is usually idealized by a by linear
curve like right like shown in the right
hand side so it is a linear and then so
elastic perfectly plastic curve we can
assume the behavior of the steel will be
like this one for most of our design
purpose so these are the some yield
strength values for commonly used steel
in different countries for Australia so
these these are the commonly used steel
here you can see this TJ notes for
deform bars and this letter denotes firm
the ductility Class L stands for load
Oakley G and instance for a normal
doctor lateen and both of this steel can
have the yield strength value of 500
mega Pascal and these are the different
diameters usually available in Australia
now comparing the load doclet is steel
that is Class L steel compared with the
class in or the normal dr. D system we
can see the stress-strain curve for
these two types the first one SL 82 is
the low ductility steel this is this one
and SN 12 is them normal dr. the steel
which is this one so as you can see here
for the load dr. this still the platy
itself is very small that means the
doclet it is very brittle kind of steel
whereas SN 2 hours is the normal
doctor-doctor this steel has a very flat
plate - that means it it it can deform
to a large extent and it
as a much better duck in it as well and
instead of reinforcing steel we can use
different other materials as well as a
reinforcing material for example fiber
reinforced polymer as shown here can
also be used as the reinforcing material
some of the big advantages of using
fiber reinforced polymers are that it is
core it has a it doesn't corrode and it
has a high tensile strength as well and
it is very light and it provides the
electric and magnetic neutral neutrality
as well but the dig but the big
disadvantage is that even though it has
a very high tensile strength it is very
brittle material compared to the
reinforcing steel so it cannot provide
the required dr. DT to the steel to the
reinforced concrete members