in this lecture
i am going to address some important
questions which come to our mind
regarding concrete technology
these questions are
why do we need to provide the
reinforcement in concrete
and
why do we prefer steel as a
reinforcement
why is concrete weak in tension
and what makes the concrete very strong
in compression
i am also going to discuss the various
properties of steel at the end i will
introduce you to the coefficient of
thermal expansion along with a numerical
and
the factors affecting the coefficient of
thermal expansion
to make it easy for you
let's discuss a little bit about the
mechanics of materials
that is
how materials behave under stress
stress in this case refers to the
mechanical stress that is
the internal resistance offered by the
body to the external load applied to it
there are three types of stresses
the first one is compressive stress
which comes into play by compression
forces
the second one is tensile stress which
occurs due to tension forces
and the third one is shear stress
which occurs due to sliding a body or
another body
it has been observed that
not all the materials can take each type
of stress equally
if we talk about concrete
it's very strong in compression and very
weak in tension
now the question arises
why is concrete weak in tension
concrete is weak in tension because of
the presence of a weak link within the
concrete matrix known as interfacial
transition zone or
its
concrete is mainly composed of
aggregates that are glued together by a
cement paste
which is a mixture of cement and water
the outer surface of large solid
aggregates is normally filled with
calcium hydroxide
which is a byproduct of hydration
reaction
that takes place inside the concrete
this calcium hydroxide layer is weak and
porous
contributing to a weak tensile strength
of the overall structure
hence the tensile strength of the
concrete is almost zero
the next question is
what makes the concrete very strong in
compression
the remarkable compressive strength of
concrete is attributed to how it's made
it consists of various aggregate
materials and cement which gives it the
quality of adhesiveness
these aggregates are strong enough to
carry the compression loads efficiently
the cement holding the aggregates in
position can crack
if it is subjected to tension
leading to the failure of structure
hence to counter this problem
steel bars are added inside the concrete
structures
which add to the tensile strength of the
concrete
now you might be thinking why steel bars
are specifically used
although there are many other metals
available like aluminium
cast iron
copper for reinforcing concrete but
steel is most widely used as a
reinforcing material in concrete
the reason behind this being the two
materials bind together very well
without any slippage thus act together
as a one unit in resisting the applied
loads
apart from this
the steel possesses some other important
properties
which make it a superior metal in
comparison to other metals
when it comes to reinforcing the
concrete
the various properties of steel are
it provides good strength for economic
design of rcc structures
and it shows a good resistance against
corrosion for higher durability
the steel is highly ductile and can
deform without losing its toughness and
is non-brittle
it provides good grip with concrete
due to the presence of ribs and lugs
hence providing a strong anchorage bond
the most important property being the
coefficient of thermal expansion which
is approximately same for both steel and
concrete that is
1.2 into 10 raised power minus 5 degree
centigrade
the coefficient of thermal expansion
describes how the size of an object
changes with the change in temperature
in simple words
with the changes in temperature
both the concrete and steel would expand
or contract with the same amount
which is not true for other metals
the coefficient of thermal expansion is
given by delta l equal to alpha lt
where delta l is the change in length l
t is the temperature difference and
alpha is the coefficient of thermal
expansion which varies slightly with
temperature
we have a question based on coefficient
of thermal expansion in which the span
of a member is given as 20 meter
and the difference between maximum and
minimum temperature is 10 degree
centigrade
we need to find the length by which the
member would expand or contract
so we have the formula for coefficient
of thermal expansion
delta l equal to alpha l delta t
as we know alpha is equal to 2 into 10
raised to the power -5 l is given as 20
meter which is equal to 20 000 mm
and t is equal to 10
on subshitting these values
the length by which the member would
expand or contract will be equal to 4 mm
in is 456 2000
it's clearly mentioned that if the span
exceeds 45 meter then we must provide
the expansion joint
the various factors which affect the
coefficient of thermal expansion are
type of aggregate
size of section
nature of cement
and relative humidity
so this was all about this lecture
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