Last
aspects of workability that is what we are
going to discuss today are: segregation and
bleeding. So far we have looked into workability
with reference to fresh concrete. So, this
is the last aspect of fresh concrete are segregation
and bleeding. These are 2 phenomena which
takes place in fresh concrete.
So, we shall first look into segregation.
We will discuss about the mechanism, something
about the test possible test for segregation
and then what factors affect segregation.
Then we will look into the other phenomena,
that is, bleeding and then we will look into
its mechanism, tests and what factors affects
or how we can control bleeding. That is what
we will look into. So, let us start with segregation.
Let us define it. Well we have seen through
several of our earlier lectures that, concrete
from the time of mixing and finishing. It
is subjected to transportation, placing and
compaction and during this process it must
remain uniform. If it does not remain uniform,
then we call it to segregate. Does not remaining
uniform implies that, ingredients are separating
out and that is what we call as segregating.
Well, results some of the results we have
earlier seen namely: we have seen something
honey-combing. That takes place that could
be the result of segregation, because the
coarse segregates remains and the fine aggregate
separated off from them and that is what 1
kind of segregation is, it can results in
honey-combing. And in general this can result
in low performances.
So, that is what the result of segregation
is. Then this is therefore, if you define
it; is the process by which well mixed concrete,
you know fresh concrete becomes non uniform
within a batch. So, in a batch which you have
mixed properly together uniform concrete,
this process of segregation is the process
by which, the concrete will become non uniform,
it will become non uniform for the same batch.
And it may mean variation of coarse aggregate
content at places, basically when you were
talking of segregation, this phenomena essentially
means that, coarse aggregate content would
vary, because ingredients they are non uniform
now. So, in some places they will have 1 material,
some other material place other material.
Since, aggregates are the large part of the
material, you know bulk of the material is
the aggregate and that to coarse aggregate
from bulk of the volume of concrete compared
to other ingredients, so quite often the proportions
of coarse aggregate in 1 place, would be different
then other place when it has segregated. Segregation
results in coarse aggregate proportion being
different at difference location. So, that
is what we are saying; that is how we define
segregation right. So, that is how we can
define segregation.
Then let us see what are the types of segregation?
Two types of segregations are defined; 1 is
called internal segregation, where coarsest
or heaviest part of the aggregate, tends to
accumulate at the bottom, you see tend to
accumulate at the bottom and lighter and if
it is usually the finer ones, that tend to
accumulate at the top of the mix, usually
during compactions.
So, you have a section, you have a section
of the concrete let us say and the bottom
will have heavier, you know in the bottom
will have heavier aggregates right; this portion.
And the top aggregate would tend to accumulate
at the top, you know top. So, this is lighter,
this is light this is heavy coarse aggregate,
usually coarse aggregate are heavier and when
this happens, we call it internal segregation,
usually the aggregates, the coarse aggregate
which are heavier will have a tendency to
settle at the bottom, compared to the lighter
finer aggregate you know and motor containing
the finer aggregate, that will have a tendency
to set. So, that is the result of an internal
segregation as opposed to external segregation.
Now external segregation if you look at it,
that is due to some sought of external forces,
say improper handling and this we have discussed
earlier. We have seen that, if you do not
discharge a concrete from the mixture in the
right manner, then large size aggregate particle
may be placed somewhere whereas, the fine
you know like for example, is a drop from
a very height more than 2 meter or dropping
directly you know or through an inclined very
stiff slope if you are dropping it over the
bottom opening is very small or inclined etc.,
several of them we discussed earlier. We have
seen that, when you are not handling properly,
it can result in segregation, because and
larger particle will have a tendency to move
away from the discharge point and the motor
will have a tendency to settle very close
to the discharge point. And that kind of segregation
you have looked earlier in 1 of our earlier
discussion in the context of placing of concrete
and handling and placing of concrete.
So, external segregation are results of such
handling; improper handling such improper
handling. Or due to compaction you know in
it vibration etcetera, this kind of thing
can also lead to external segregation. If
you have inadequate cohesiveness of the cement,
let us say high water cement. So, you do not
have you remember that, we said that, cement
paste shall be cohesive, because it is supposed
to disperse the aggregate, it forms a matrix
and in which the aggregates are dispersed.
And then it must have the therefore, it must
have the dispersion capability. And it must
have also the capability to hold it together,
it should be sufficiently cohesive.
But, supposing we have high water cement ratio,
it is thin, the paste itself is thin and not
able to hold the system properly, it can results
in water just flowing away it is too thin.
So, water just goes away alone in the mix
leaving the rest of the thing and therefore,
this can result in what is known as wet segregation.
If you have insufficient water cement ratio;
that means, now it is not again you know it
cannot hold the aggregate, the paste cannot
hold the aggregate, because the water is less
it is dry, this may results in dry segregation.
So, what will happen; this can result in usually
these are external segregation, you know external
segregation while handling. So, this could
dry segregation. So, the stones etcetera will
go away, the coarse aggregate will go away
leaving the dry mortar closed you know near
by the discharge point. So, this can wet segregation
and dry segregations are the other 2 and these
are variety of external segregation. So, this
is the types of segregations. Why does concrete
segregate mechanism of segregation?
If, you have density of the aggregate different,
you know you have different densities of the
aggregate, say coarse aggregate has got a
different density than the mortar, and then
internal segregation will occur. Because we
said that what is internal segregation? The
tendency of the coarse aggregate which are
heavier, as to settle down at the bottom and
the finer mortar which has got actually fine
aggregate its specific gravity is different,
will have a tendency to relatively remain
in the top.
So, when you are compacting, if something
heavier will have a tendency to go down, something
lighter will not go down, so easily it have
a tendency to remain in the top. And if the
specific gravities are significantly different,
say in case of something like heavy concrete.
Heavy concrete is used in radiation chain;
hematite is used as the aggregate which could
have a very high specific gravity, order of
4 4.5 and so on. And if you use a finer aggregate
or mortar, which has got much lower density
or specific gravity of the sand is very low,
this can lead to the sand remaining at the
top and the mortar remaining at the top and
aggregates going down and result in internal
segregation.
So, internal segregation can no occur when
there is a specific gravity difference between
the 2, because it is internal it is not due
to any external agents, not due to handling,
just because the specific gravities are different;
aggregate will have a tendency to remain separate
than the mortar. Let us see what could be
the other 1. In case of jolting and vibration
and other forces, dissimilar ingredients may
get separated.
Now, remember when you are talking about conveyor
belt sometime, you know transporting concrete
through belt conveyor. So, when you are transporting
concrete through belt conveyor, you have jolting,
you know when you are trying to transport
concrete through belt conveyor, you can have
jolting, jolting you can have joltin you know.
So, over the belt rollers it can jolt, because
you remember we had belt something like this,
the belt will travel like this. And when this
concrete on top is a heap of concrete travels
to this, it will have a jolting, you know
it travels concrete is placed here. So, over
the belt conveyor, this is in the direction
of travel of concrete, it can cause; it can
be jolting there can be jolting over the concrete.
So, in such situations since the concrete
is been jolted up and down up and down. So,
it can so happen that, the larger particle
moves in a different way, if the cohesiveness
of the mix is not proper; large particle moves
up you know does not move moves more, the
mortar remains sticking to the sticking to
the concrete. So, if you have a too much of
aggregate in the system such a thing can happen.
So, similarly vibration can results in dissimilar
aggregates ingredients, that is, aggregates
and the mortar, dissimilar there both in shape
and sizes may get separated they might get
separated. So, jolting etcetera can results
in jolting and vibration that is external.
So, that is the mechanism of segregation.
The factors affecting internal segregations
are the friction, internal friction and bond
and friction between paste and aggregate;
the cohesiveness and ability of the plastic
deformation of the paste and mortar and interference
between coarse aggregate particles. That means,
what would stop the aggregation.
If the aggregate is held in position, you
know by or in this location by the paste or
within the paste, because the paste forms
the matrix, aggregates disperse within that.
And if you see as slightly ahead, the mortars
forms the matrix and coarse aggregate particles
are dispersed in that embedded or you can
say they are dispersed in that they are inclusions
sort of dispersed in that of course, they
cannot just not embedded they are large quantity.
So, they are dispersed in the mortar.
Now, if the mortar is cohesive, it is effectively
dispersed in the aggregate system, then it
will be uniform. But it must have also the
holding capacity, should form proper bond
with the aggregate, should have the right
kind of holding capacity of the aggregate
so that, there will be no segregation. So,
if this bond is not there, there will be segregation
and that is being talked about right. So,
this acting is a internal friction right that
is the bond and friction between paste and
aggregate. So, this is what is you know this
bond is bond is 1 thing plus friction. So,
if something is moving the friction would
have held that movement.
So, friction between the paste and the aggregate
system and mortar and the coarse aggregate
system right, the cohesiveness of the paste
and the mortar and ability to plastic deformation.
If it holds, then plastic deformation of the
mortar and paste would take place and simultaneously
it will also cause deformation of the movement
of the aggregate together with it. And that
is what actually will stop, you know if this
is not happening; then there will be segregation.
So, this is these are the resistances or you
can say the factors affecting against the
internal segregation. So, the factor factors
affecting factors, which oppose this internal
segregation, are the bond, the cohesiveness
the friction between paste and the rest of
the aggregate, mortar and coarse aggregate.
So, therefore, all this can resist, this is
not internal friction; this should be internal
segregation. So, internal segregation can
be resisted by cohesiveness of the mix.
So, if the mix is cohesive and you can see
the solve points out, if you have a good workable
mix, which can deform easily, would actually
give you proper resistance against segregation
right. So, that is the mechanism of segregation
and that is how you can see that what opposes
the segregation.
Continuing with the mechanism of segregation,
you see as I mentioned all the above factors
points to the fact that, you know it points
out to the fact that, workability has got
a strong role. If you have a good workable
mix, which is capable of deforming and moved
uniformly cohesively; this will have, this
will actually resist segregation. So, a strong
role a workable mix is less prone to segregation;
that is the idea. So, workable mix properly
workable mix is less prone to segregation,
you know segregation and separation. So, something
has to the paste has to hold that aggregates,
the mortar has to hold the coarse aggregate
and then there will be no segregation. So,
if they mix is that, there will be very little
internal segregation.
This is what is being reiterated; the ability
of the mortar, the ability of the motor to
hold coarse aggregate and disperse them adequately
resists segregation. This is very important.
This should be understood that, mortar should
be able to. So, mortar itself should be cohesive
and sufficient enough to hold the coarse aggregate
in position. So, quantity of this proportions
are important in that aspect.
Cohesiveness of the mix is most important,
together with appropriate handling method,
because you have seen that if you handle it
in a wrong manner, if you we have seen earlier
sometime that, if you handle it in a wrong
manner like the bottom opened bucket; that
does not you know and wide opened bottom opening,
bucket with wide bottom opening; they do not
cause segregation. If you have a batch part
of the batch loaded on a wheelbarrow and another
part is loaded onto the another wheelbarrow;
this will have definitely sort of segregated
mix.
Similarly, when you have 2 stiff chute through
which you are discharging the concrete; that
can result in segregation. Dropping the concrete
over height more than 2 meters; those can
cause segregation. So, similarly jolting over
the conveyor belt or transporting concrete
through simple truck over a undulated road;
all this can result in external segregation.
So, appropriate handling is very important
and also the mix.
How do we test? 1 of the test that was there
earlier are you know, because there is not
really these tests are yet to be standardized.
One of the tests proposed was that, you have
a cylindrical concrete mould something like
this; you have a cylindrical concrete mould.
Fill this mould with concrete in a specific
manner and vibrate it for a fixed time, then
separate it out into 2 parts; the top part
and the bottom part, upper and the lower part.
Then what you do; you measure the proportion
of coarse aggregate in this portion upper
half and proportion of coarse aggregate in
the bottom portion. So, you measure the proportions
of aggregate in two halves.
Then, the segregation factor is defined as
this is S F this is called segregation factor,
which is the ratio of coarse aggregate content
of the lower part to the coarse aggregate
content of upper. We have separated into 2
parts, if you remember last slide as I was
talking about. This is the upper half, this
is the lower half and coarse aggregate content
is measured in 2 halves and you know ratio
of the coarse aggregate to the lower part
to the upper part; that is called segregation
factor.
So, if it is not segregating, if it is uniform;
this should be equals to 1. So, 100 percent
non segregating, non segregating mix will
have this you know equals to 1, lower you
know close this value of S F to 1; the tendency
of internal segregation will be least. Tendency
of internal segregation will be least, when
this is very close to 1. So, this is one method
and very simple method of measurement of segregation.
Tendency: you know segregation tendency against
internal segregation. So, S F is a measure
of internal segregation, the closer it is
to unity the milder is the tendency of segregation;
that is what we have understood. The closer
it is unity milder will be the tendency to
segregation internal segregation.
You know this will happen if you have specific
gravity difference between the coarse aggregates.
So, you have vibrated it, if the coarse aggregate
is very heavy compared to the mortar; it will
go and settle down at the bottom. And that
is we said is a cause of internal segregation
by law, because internal segregation means
the aggregates proportions will be different
in different part of the concrete, different
proportion of the concrete. So, 1 of the major
reason is this.
For external segregation; uni-axial tensile
strength is a good measure of tendency of
external segregation, higher tensile strength
have less tendency. This is what we talked
earlier also, when we were talking about rheology
of fresh concrete and paste etc. We mentioned
that, 3 components in terms of workability
of concrete, there are 3 terms if you remember
first 1 we said was compatibility, second
1 is the mobility, and third 1 is the stability.
Stability means it does not segregate, especially
under external handling vibration etc.
So, this it will not segregate; if the tensile
strength of the fresh concrete is high.
So, uni-axial tensile strength is a good measure.
There are some ways to measure this against
you know external segregation; higher tensile
strength have less tendency. So, it is actually
if you are workable mix will have less tendency
to segregate as well, because it will have
sufficient cohesiveness to hold on to the
coarse aggregate, the mortar will have sufficient
cohesiveness, it will be of sufficient quantity
to hold the coarse aggregate in position and
also disperse them.
The other test, so external this is the uni-axial
tensile strength and that is you remember
you know this is related to workability. So,
good workable mix are generally less tendency;
they have less tendency to have segregation.
Now, other test is called heap test. A heap
test is you know there are several other tests
proposed, but I just picked up 1 more simple
test. In heap test, the concrete is allowed
to fall from a conical hopper on a flat surface
and after setting, it is photographed.
So, it is a very simple method; you have a
standard conical hopper which is same use
the same 1 for all concrete and it is allowed
to fall through it to a flat surface over
a standard height over a fixed height, I mean
I will not say standard, but fixed height
and is observed to photographed after setting.
And this photograph is observed for tendency
of external segregation. For example, when
you have drop through this hopper to the flat
plate the concrete you know, if the coarse
aggregate particles are separated of there,
you know coarse aggregate particles are separated
off there somewhere else, somewhere else coarse
aggregate and the mortar forms as a heap,
you know mortar something like forms as a
heap here mortars are something like heap
and aggregate particles are separated off
and then it has a tendency towards external
segregation.
Supposing nothing happens like this and all
concrete are forming as a heap of this form,
then it has got less tendency to external
segregation. So, that is how we can measure
the external segregation as well as internal
segregation, you know tendency. So, tendency
to segregation, but these are not usually
these are not very often done, when being
in laboratory studies 1 may do it to find
out, because this is not possible to do day
and out in a quality control situation.
But this is how 1 can test whether it has,
1 faces the segregation problem if the mix
is not proper, visually you can observe whether
segregation is taking place or not and of
course, 1 has to be careful as far as internal
segregation is concerned, when specific gravity
of aggregates are quite different. But otherwise,
well 1 has to just check it. So, these are
meant for laboratory testing not really quality
control tests.
Let us see what is the factors which affects
the segregation and this factors are; if you
have large size aggregate, maximum size of
aggregate nominal maximum size of aggregate
over 25 millimeter has a tendency to segregate
more, because larger the size this is the
tendency to go away from rest of the thing
. And higher quantity of large size particles
or there is something called gap grading.
What is gap grading? We talked about grading
in the, sometime we said grading know we talked
about grading. What is grading? Grading is
having difference sizes in right proportion.
Supposing one of the size is missing, one
of the size has got 0, it can happen one can
design good concrete with that as well, but
1 must keep in mind what are the possibilities
than can happen. Gap grading means 1 size
in between is missing. That is not available
at the site. So, you do gap grading, right.
Now, such gap graded system will have a tendency
for the larger size above the gap to move
away, because you see gap graded means 1 particular
size in your grading is missing, because we
said the final next size will go inside this.
And the next size will go still inside, this
in the finer size and so on.
Supposing 1 of the size is missing, the cohesiveness
of the mix would that would be there inside
may be relatively less and holding capacity
might be less. So, the largest size aggregate
will have a tendency to go up, but that does
not mean you cannot have a gap grade concrete,
you can have ensure that you have cohesiveness
of the rest of the you know mortar or rest
of the mix, rest of the mix having sizes that
contain sizes finer than the larger size.
So, 1 can still design there is no problem.
The other issue is of course, the specific
gravity of the coarse aggregate, relative
to the fine aggregate. This is very important.
So, when all this are there, you have to look
for measures to reduce down the low fine aggregate
fraction in the sand and low cement content;
so also, unfavorable shape of aggregate, and
too dry or too wet concrete. Now these are
the factors which affect the segregation.
Now; that means, you know if it is gap graded,
if you have this sizes larger than this, it
does not mean that, you cannot make concrete
without them what you have to do is possibly
you have to increase the fine content, may
be you have to increase the paste content
and cement content and makes the paste cohesive
and its holding capacity more.
Similarly, high quantity of large size particles
if it is there and gap grading is there, you
have to again possibly increase the fines
or paste and increase the cohesiveness of
the paste to hold this 1. Specific gravity
difference if that is there, then again you
have to see same thing there, may be the large
size aggregate may be quantity can be reduced
and you can increase the mortar content relatively
more so that, it can hold and disperse.
So, accordingly 1 can design them, keep 1
has to keep this in mind, since we know these
are the factors, low cement content is a factor,
so you might have to increase it. Low fine
is a factor, so you may have to may have to
increase it. And, then unfavorable shape of
aggregate if it is there may be you have to
increase all this. Too dry or too wet mix
that we talked about; that means, water cement
ratio are to be checked, if I have to dry
less water too water cement ratio, there has
to be other measures by which we have to actually
reduce down the segregation.
So, that is how 1 can control the segregation,
if 1 knows factors affecting. Well that is
I think all about segregation all right. 1
additional thing is that, air entrainment
can usually reduce down the segregation, how
does it do it, because in air entrainment
when you use air entraining mixtures, air
bubbles are formed uniformly throughout the
structure, it is very very uniform throughout
and it maintains. This air bubble maintains
cohesive mix, maintains the mix paste cohesive
and therefore, it can hold on to the aggregate
system pattern.
So, by increasing the cohesiveness of the
paste system and then of course, the mortar
system, because paste is included in the mortar
and that forms a you know the sensor dispersed
in the matrix; paste matrix. This can improve
the cohesiveness of the overall mortar and
then allow you know maintain or other can
contain the aggregate within it in a disperse
form hold it together. And therefore, air
entrainment can lower the segregation. So,
that is all about segregation.
Let us look into the next aspect of the fresh
concrete. So, segregation tendency has to
be reduced. The other aspect similar aspect
is bleeding; bleeding of segregation I mean
bleeding of concrete. Bleeding is nothing,
but special case of segregation, segregation
perhaps is a general term where an ingredient
separates out. So, far I have been talking
about separation of the coarse aggregate from
the rest of the material; that means is the
aggregate system, which can separate out from
rest of the rest of the matrix, rest of the
you know concrete system. Bleeding is where
water gets separated from rest of the material.
So, generally segregation is separation of
the material, in that sense, bleeding is also
part of overall segregation. Let us define
bleeding. Now after compaction, when plastic
concrete stands without agitation, we are
not vibrating it anymore; water appears at
the surface, this emergence of water at the
top from freshly placed concrete, paste or
mortar; we call it bleeding, it is also called
sometimes water gain.
So, after you have compacted it, the concrete
when it is standing alone without any agitation,
you know till it is set what will happen;
water generally tends to come up at the surface.
And this tendency of water to come up at the
surface and accumulate we call it bleeding
or water grading. I have already mentioned,
bleeding takes place prior to setting.
Now, we can identify 2 types of bleeding,
you see one is called uniform bleeding, other
is called channeled bleeding. You can identify
2 types of bleeding; 1 is called channeled
bleeding other is called uniform bleeding.
What is uniform bleeding? It takes place by
uniform seepage over entire surface, you know
water comes up at the top; that means, there
is some seepage of water from the fresh concrete,
the green concrete it is also called green
concrete fresh concrete, you know it is a
fresh state, wet state, and water comes up.
You can understand that, this is easily understandable
as we shall see later on; the specific gravity
of the materials in concrete, the ingredients
of concrete are different and water has the
least specific gravity.
So, it generally tends to come up it seeps
through. And when it is uniformly seeping
through the entire surface of the concrete,
right this is called 
uniform bleeding. But sometime what can happen
in addition to this uniform bleeding, some
places the water might come out selectively,
very high rate at a very high rate water might
come out selectively, through some channels
that are formed localized channels that are
formed and this is called channeled bleeding.
These are called channeled bleeding. So, 2
types of bleeding we have recognized; 1 is
uniform bleeding, other is channeled bleeding.
So, what is like this is your let us say this
is my concrete. So, when this is my concrete
surface, when water comes up throughout uniformly
that is uniform bleeding. But sometime you
might see that, water comes up uniformly alright,
but some place there are you know some places
water has water is trying to come out more
and getting dispersed all round. So, this
is, this place you know water is trying to
come out more, rest of the places relatively
dry. This place is wet and water is trying
to spread from this places.
So, channeled bleeding, because on channel
might have formed inside, through which the
water is coming seeping through at the top
and that is called channeled bleeding. So,
channeled bleeding is localized through channels
and uniform bleeding is all over the surface.
Now when water comes up, you have bleeding
and the solid subsides, because overall volume
is same; total wet volume contains solids
and the water.
Now, water is coming up, water has come up
and at the top water has come up at the top.
This was the original total volume, now water
you will have here and the rest of the solid
is there. So, the solid would have subsided,
the water would have come and that is what
is, you know that is what happens in case
of bleeding, so solid comes. Total volumes
remain same, if there is no evaporation loss.
So, if there is no evaporation loss, total
volume will remain same. When there is no
evaporation loss total volume will remain
same right. So, in case of bleeding, total
volume will remain same, but if evaporation
takes place, actually there is a decrease
in volume. Now, therefore, bleeding can be
quantified in terms of subsidence as well,
because the volume is changing is 1 thing
and second thing is there is subsidence. So,
when you want to quantify bleeding, we can
quantify bleeding in terms of subsidence that
is taking place, subsidence you know quantified
in terms of subsidence and can also in terms
of volume of bleed water.
So, you can quantify either in terms of subsidence
or in terms of volume of bleed water that
is there. So, this is the 2 way we actually,
we quantify bleeding. We quantify bleeding
in 2 different ways; 1 in terms of subsidence
and second in terms of volume of water that
has come up. We will quantify it in terms
of in 3 ways actually and that is what will
be mentioning in the next slides.
But let us understand a little bit about how
the bleeding takes place. You can see, I mentioned
the specific gravity of ingredients are different.
If you see cement, the specific gravity is
3.15 which has the highest. I think I have,
I am repeating this I must have mentioned
earlier. And this 1 must realize in concrete,
the cement has got a specific gravity of 3.15.
The specific gravity of normal aggregate would
range from this, but sometime it can be higher
also, depending upon the minerals or mineralogical
composition of the aggregate. For example,
if you are using iron ore as iron ore as a
aggregate then; obviously, this will be much
higher. But cement is normal piece of cement
is around this and water has got a specific
gravity of 1.
So, you can see the water is the lightest
of all. And this is inherent to concrete.
Water will have a tendency to come up. So,
these are your aggregate system etcetera,
in the mortar water will have a tendency to
come up. And it when it comes up, it brings
in some amount of cements also with it if
especially very fine particles they can come
together with the water. Even in case of uniform
bleeding some amount of slur will come, in
case of channeled bleeding of course, large
quantity of slur will come. So, this surface
laitance is formed with this mortar which
has come up and water which has come up, with
the fine you know material it fill form a
slur. What about cement? Cement will try to
go down also aggregates, because it is somewhere
intermediate.
But 1 additional feature which we will discuss
again is that, water might get accumulated
you know it can trap, bleed water can get
trapped below that aggregates larger aggregates
system. So, this can be water rich pockets.
And this water rich pockets have got their
implications in many other things including
the strength of the concrete. So, these are
water and durability as well. So, these are
water rich pockets, bleed water can stuck
here.
So, basically why does bleeding takes place
and it is somewhat inherent of course, you
can control it, reduce it, keep it in an you
know keep it in an situation, keep it in a
way or to that level where it is not so harmful,
but water will definitely will have a tendency
to come in. So in fact, we will have higher
water here and less cement. So, water cement
ratio is higher very high compared to that.
So, bleeding actually takes place, because
water has a tendency to come up, being the
least specific gravity material and cement
has a tendency to go down. So, that is what
it is. And if you define them, we can now
quantify what is you know in different way
the bleeding.
First way quantify is called bleeding capacity,
bleeding capacity right, this is what we are
calling. So, bleeding capacity we can define
in terms of volume of water, released from
unit volume of concrete; total volume of water
released from unit volume of concrete, you
know during the throughout the bleeding, because
bleeding does not take place in go. It takes
place over a period of time, right from the
time you have placed the concrete in the fill
the mould to the time it has set the bleeding
will continue of course, not necessarily up
to the point of setting, but setting may arrest
generally may arrest the bleeding process,
because once it is solidified the water has
reacted it can come out.
So, as the setting process progresses, we
will stop gradually. So, during this whole
period of bleeding, the duration of bleeding,
the amount of total volume of water that has
come in from unit volume of concrete that
is what we call as bleeding capacity. And
it might be expressed in c c per c c of fresh
mixture, the amount of you know c c is the
volume of water in 1 unit c c of fresh mix
mixture of paste concrete mortar everything,
whatever you are in the context you are talking.
Or it can also be defined in terms of settlement
because; we said the subsidence is another
way of subsidence is another way of defining
the bleeding.
So, quantifying bleeding in terms of bleeding
capacity, we can talk in terms of settlement
amount of settlement divided by the original
height or settlement per unit height. What
we are saying is settlement per unit height
is the setting you know arrest still setting
arrests bleeding. So, during the total process
of bleeding, the bleeding that has taken place
divided by the original height in which bleeding
per unit height in centimeter per centimeter
is the bleeding capacity. So, this is 1 way
we quantified bleeding.
The other way of quantifying bleeding is in
terms of what is called bleeding rate. Bleeding
rate is defined as the rate at which water
is released and it is expressed in terms of
volume of water that is in centimeter cube
c c per second the rate per centimeter square
of the surface area, per centimeter square
of the surface area of concrete. So, it is
the volume of water released per unit time
per unit surface area of the concrete. Or
it can be talked in terms of subsidence at
a point on the surface of the concrete in
terms of centimeter per second.
So, the rate at which it is subsiding, at
any point of time is called you know bleeding
rate. This rate is; obviously, cannot be constant,
you know it will be constant possibly for
certain period of time, but it will reduce
down with time. We shall have a diagram to
see this. So, bleeding rate would change,
but we have we are defining, next slide will
define that in details.
Now let us see the third way how we can quantify
bleeding duration of bleeding. It is the periods
elapsed from concrete placing to the time,
when bleeding has become negligible. Well
in terms of subsidence if you are looking
at it, this diagram will show you how we can
quantify this.
So, you can see this through this diagram
that, this diagram shows for a specific type
of you know water cement ratio 1.32, because
1 would like to have high bleeding by absolute
volume, this by volume. And the sample has
been taken as 4.25 centimeter and then bleeding
was measured by what is called float method
in terms of the subsidence. So, the volume
is not there, it is measured in terms of subsidence.
This x gives you time, so this gives you time
in minute. And then what has been done; subsidence
has been measured.
So, originally it was 0, then subsidence increases
along this direction; 0.1 0.2 0.3 0.4. So,
subsidence has been measured in terms of you
know subsidence in centimeter; 0.1 centimeter
2 etc, subsidence has been measured with
time. So, when you measure the subsidence
with time, it is observed that, initially
it has got a almost linearly it is decreasing.
So, this is the period of constant rate.
Then you can see that bleeding gradually reduces
down, the rate of bleeding gradually reduces
down. So, this is the rate of bleeding. The
initial rate of bleeding we call it rate of
bleeding, you know in centimeter per second.
So, that is the rate of bleeding. The total
subsidence is this and you remember bleeding
capacity has been defined in terms of total
subsidence right per unit origin of height.
So, this is related to total subsidence. So,
bleeding capacity can be defined in terms
of this.
So, bleeding rate orders are given for example,
103 into 10 to the power minus 6 centimeter
per second etcetera, the rate has been defined.
And period of constant rate beyond that of
course, bleeding rate reduces. So, duration
of bleeding which is the third measure of
bleeding is shown here. So, you can see all
three quantifying terms of bleeding are defined
through this, through this curve. These are
the actually characteristics through which
you quantify bleeding; total amount, rate
and duration. You know these shows are how
much would be the volume change, total volume
change of once the evaporation has taken,
you know once the top water has evaporated.
So, how much would be the volume change that
will take place.
This tells us what is the rate, because initial
rate is important, I should have a slow initial
rate may be and then; obviously, it will be
faster. So, high rate of this is the rate
and of course, total duration that is what
we mentioned, total duration of bleeding.
So, that is how we can quantify bleeding.
What happens when bleeding takes place?
Normal bleeding results in subsidence, although
may not be visible in dry windy situations,
because if this dries lot of evaporation is
taking place. If the rate of evaporation is
higher than the bleeding rate, then one may
not see it, but it never delays the bleeding
is taking place. So, in such situation, the
reduction in the volume reduction would be
taking place.
So, normal bleeding results in subsidence.
That is most important. This takes place and
therefore, there will be reduction in volume.
So, whether you see it or not there can be
some reduction in volume of course, in dry
and windy situation we do not see it, dry
and windy situation you do not see it, sometime
you may not see it. Normal bleeding is also
harmful in certain situations, but let us
look into more harmful bleeding; it is the
channeled bleeding. Channeled bleeding is
more harmful, it is really harmful, because
it will bring laitance at the top resulting
in porous internal concrete. So, it will bring
large laitance, because it is locally there
is a channel formation. And lot of bleed material
you know water bleed water will come and it
will bring in lot of cement, because large
quantity is coming it has formed a channel.
So, such channeled bleeding can result in
large laintance amount of laitance coming
at the top. And therefore, loss of laitance
from the inside, which means there will be
actually porous internal concrete, because
material has come out come out from inside
and you can have porous internal structure.
So, therefore, this is harmful. So, channeled
bleeding is definitely harmful.
Now, excessive normal bleeding is also harmful,
because it may result in non uniform concrete.
Top will have higher water cement ratio and
bottom will have much lower water cement ratio,
lot of water has come out, so top will have
high water cement ratio,. Top layer would
be relatively weak layer and that is why weak
plane, it will actually create weak plane
at the surface not planer, weak plane at the
surface. So, it will create weak plane at
the surface, because the water has come in,
water cement ratio is very high and if some
sort of fine laitance or water layer has formed
at the top, you can allow it to evaporate,
if it evaporates then do finishing then of
course, it is not so harmful. If you are casting
another lift on top of it, another set of
concrete is being placed on top of it, then
the water must dry out, otherwise it will
create weak planes what is called cold joints.
So, the construction joints would be weak
the weaker joints would be there. So, of course,
a skillful man, a skillful mason would not
finish, until all the bleed water has actually
gone out from the surface. So, it can be harmful,
because it can result in non uniform concrete
bringing in some amount of laitance, but otherwise
just coming some evaporative evaporation at
the top, does not create any, some bleed water
at the top does not create problem if you
allow it to evaporate and then put the next
lift and or finishing work, you do only after
that and then this is not very a little bit
of bleeding is not really harmful to the concrete,
if it is take care of properly. In fact, it
can reduce down the overall water cement ratio.
But such bleeding can be significantly reduced
by special mechanized way with control permeability,
form outliner etcetera although that is not
part of our discussion at the moment, but
there are techniques to reduce that. So, that
is what effect of bleedings are.
Then let us see; what are the mechanisms.
We have already mentioned that, specific gravity
of water being lower than other ingredients,
results in bleeding. Channeled bleeding essentially
is a result of mechanical rupture, of horizontal
particular layer you see, when water is trying
to come out. If the bleeding rate is high,
it might cause rupture of the solid system,
there a mat like surface you know layer, which
is formed it can cause rupture and take out
some material. Once it takes out those materials,
the channel is formed. And when this materials
complete channel is formed, you know little
bit formed it can make exactly bigger channel
and finally, a complete channel might form
and through that channel lot of water will
have a tendency to come out.
So, the channel bleeding is basically rupture
of the particulate system or particulate mat
that is formed and forms the channel. And
that is that is really harmful. So, now, what
people have tried to do; they have tried to
look into the bleeding process more a little
bit more in a systematic manner from scientific
approach. And 1 can think in terms of bleeding
as a settlement of solids in a suspension
of you know water suspension like; if you
have suspension of water say soil, not what
you call inert solid in a liquid, the soil
in water, what happens; the particles depending
upon their sizes, particles try to settle
down at the bottom, the coarser particles
settle down at the bottom, the finer takes
longer time to settle down. And that is governed
by Stokes law, depending upon the property
of the liquid, viscosity etcetera, diameter
of the particle and so on.
So, 1 can look in terms of bleeding as a phenomena
in a solid actually, settled in a suspension,
solid settles at the bottom, you know gradually.
The coarser particles trying to settle earlier,
etcetera, heavier and coarser particles trying
to settle earlier. So, 1 can look that process
in this manner. The other way of looking it,
you have a wet concrete which is you know
wet system, wet concrete and in which you
have inter connected pore systems created
by the water. So, if you have interconnected
capillary system, through this water will
try to move out and this is something like
flow through pipes as defined by Poisseeuille’s
law.
Well the modification of this Kozeny-Carman
equation which is used in soil, people have
tried to model bleeding through this. As if
you have a porous system, porous solid system
with capillary pores and through that water
tries come out. So, this is through this water
tries come out. So, this is what this is what
is you know Poisseeuille’s law. There are
2 ways people have tried to model this system
right.
So, 1 way if you look at suspension in a solid
suspension, the solid is trying to settle
down and other way is as if through pipes,
you know capillary pipes very fine pipes water
is trying to coming out. So, that is how people
have modeled. Any way we will not look into
the equations in our discussion, but this
is how people have tried to model and try
to understand the phenomena of bleeding, well
the later the Kozeny-Carmans sort of equation
seems to be more reliable.
This is the issue I would like to mention,
because this is important from the point of
view of aggregate, durable you know strength
and durability. I have mentioned earlier that,
bleed water can get trapped below the aggregate
system. So, the water can get trapped below
the aggregate system. Now this can form water
filled pores and between the water can get
trapped below aggregate.
Now, this is for example, this is a water
trapped, this is another water trapped. So,
as the water is trying to come up, it can
get trapped below the aggregate system and
this remains porous. So, aggregate and mortar
interface are quite often may be porous, because
of bleed water trap of course, the other reasons
are also there which we shall see later on.
This is more porous than the bulk paste we
shall see that later on. But bleed water is
an important issue which can leave this as
porous. So, this I specifically mentioned,
because this has got some relation with strength
and durability. How do we test float method?
Actually used in research purpose float method.
In this you have simply, the concrete you
have a float. If this is your concrete let
us say you have a float, which will be floating
on top, you know you have a float which will
float on to the top. Now this float is such
that, it is actually heavier than water specific
gravity more than 1, but less than that of
the concrete solid; that means, less than
the bulk concrete 2.4 or something you know
I mean you can think in terms of and combined
aggregate specific gravity of solids, in case
of concrete to be closed above 2.4 or so2
point you know around that.
So, you put a put a float such that, it can
it does not it sinks in water, but floats
over the concrete. And then what you do is
the bleed water comes at the top. So, this
was in the beginning it will be at the top
and as this settles down, its height of settlement
you can measure. Bakelite could be 1 such
material, which actually sinks in water, but
floats over solid concrete. So, its subsidence
can be measured using this float method. Subsidence
of concrete can be measured using float method
and that is what power used earlier used earlier
in his experiments and through which that
subsidence diagram was obtained.
The ASTM method which is the other method
and being used as a standard method, it uses
a cylindrical container. A collecting ring
and burette is placed on top. So, that is
the collecting ring and burette is placed
on the top of the sample. Then carbon tetrachloride
is put on to the top surface of the sample
as well as in the burette. So, part of the
burette is filled with carbon tetrachloride
and this ring actually you now traps the bleed
water and comes into the burette right, through
a funnel. And comes right on top of the carbon
tetrachloride and as the concrete settles,
released water accumulates over the carbon
tetrachloride and the in the burette and there
burette is graduated and where you can measure
it at regular intervals.
So, that is how 1 can measure the volume of
the water that has come out. This is of course,
a way of measuring the subsidence; this can
measure the volume of water that has come
out. That is the standard method ASTM: American
Society of Testing Material method and this
can be utilized to measure the bleeding. Again
this cannot be a filled method it is mainly
meant for laboratory and studies and things
like that. So, test methods, details of this
test method are outside the scope of our work.
Let us see how we reduce bleeding.
Increasing the fineness of the cement, you
see again if the cement and paste system you
know solid system and the concrete is such
that, the interstitials pore space between
them is very small. So, the capillary sizes
are very small, water really cannot come up.
If they are not interconnected, then water
cannot come up. So, therefore, the basic idea
is to reduce down the fineness, you know if
you have increase the fineness of the cement
so that, the interstitial space sizes are
smaller, interstitial pore sizes are smaller.
You have a finer particles, this space will
be smaller, you have a larger particles this
space will be larger.
So, cement is a finest particle in the system.
And if you have fineness of the cement is
slightly high, it will reduce down bleeding.
Well that may not be possible it may have
other effect, but you can add fine pozzolana
to the system and that would reduce down the
bleeding. Increase the cement content or cementitious
content; that means, add pozzolana, fine pozzolana
fineness of the pozzolana and more quantity
of this cementitious material, will reduce
down the bleeding. Decrease the water content;
too much of water would bleed to high capillary
system and therefore, the bleed channels can
form.
If you have less water content less, such
water cement ratio and you know low water
cement ratio will have less bleeding tendency,
because capillaries will be relatively less
and through which it cannot go. Air entrainment
reduces the bleeding, increasing fines and
fineness of the fines again, makes the capillary
pore sizes smaller, bleed capillary pore sizes
and the wet concrete smaller. We are not talking
capillary in the final hardened concrete,
but in the bleed wet concrete itself, interconnected
capillary should be you know lesser of diameter
and there should be less interconnected. And
this is possible if you increase the fines,
because the spaces within the fines would
be smaller. Increase the cement content or
cementitious content, decrease the water content
water cement ratio and slump. Some admixtures
like viscosity modifier will reduce the bleeding,
paraffin wax emulsions they reduce and bentonite
suspensions also reduce bleeding. So, these
are the means of reducing bleeding.
Well I think that, should that would enough,
that would finish out discussion on segregation
you know segregation and bleeding. So, today
we have discussed 2 phenomena; 1 of them is
segregation, the other is bleeding. And overall,
we have discussed in fresh concrete the issues
of workability, their measurement, the rheology
of cement paste and concrete a little bit.
We tried to give idea about rheology of cement
paste and concrete. And then we have now looked
into lastly, we have looked into segregation
and bleeding. So with this, we conclude the
discussion on fresh concrete. The next turn,
next lecture onwards, we shall be discussing
on strength aspects of concretes.
Thank you.
