Having looked into all the properties of concrete the production process and related issues.
Now, we can look into how you select material for concrete making. So, this lecture module
1 module 8 lecture 1.
We will discuss about cement aggregate and water selection, and therefore obviously the
outline of our discussion to do would be. First, we will discuss with cement selection.
Then we will have a discussion on water selection aggregate selection.
So, ingredients of concrete are selected based on performance requirement of concrete like
cement also you select based on what is the performance requirement of concrete. What
do you want from the concrete? And as we have mentioned earlier several times; Performance
of concrete in fresh state that is this hardening state hardened state and long term performance
are kept in mind. So, we select cement based on all this because you know we have seen
So, when we looked into types of cement then properties of cement, you know the cement
itself can actually influence the properties of the concrete. In terms of its setting,
in terms of hardening and then when it as hardened and in the long term strength development
and durability etc. Therefore, cement we select according to the need. You know,
as the as required you know like as we as the performance we want from concrete based
on that, requirement we select the cement we select the cement.
Well; the most common of course is ordinary port-line cement that, we have understood by now and used in.
You know, and it must satisfy of course the requirements of relevant code, there are 3
codes I have mentioned here; I S- 269 meant of 33 gates. This is meant of 430 gates and
this is meant of 53 gates. Now, you see this obviously is as we have seen this grade means
that it will give you 33; strength on 28 days. When tested in the standard huh manner. You
know, cement motor cases. So, when we test standard cement motor cube in a standard manner.
Then the strength we get would be getting 33 MPA here. 33 this is 43 MPA 28 days and
this is 53 MPA 28 days.
Now, this 3 cements all are ordinary cements, obviously you can understand that this gives
a little bit of high 28 days cement itself. And here, its somewhere in-between and
this is huh the least;, so obviously how- how do you get it. If you recall then possibly
if you recall in the beginning, we discussed about compounds of cement C3S would gives
you high cement. C2S gives you, long time strength, and if you grind it more the physical
fineness of the cement also, enhances the early strength.
So, that is what has been done here actually you grind it more increase the C3S content
reduce the C2S content compare to this let us say to get high strength high early strength.
o there is no specific need where they should be used. In fact any one of them is good enough,
but if you think that 28 days strength somewhat early strength you desire this could be 1
thing, but it doesnt serve much of a purpose any one of them can be selected. This will
have obviously long strength, obviously; this will have better- compared to this. And when,
it comes to production of calcium hydroxide this will have large amount of calcium hydroxide
produced compared to this.
The reason being this has got more C3S compared to C2S and we know, C S produces more compared
to C2S. Anyway, but there is no specific guidelines or specific places where 1 should use 53 should
use 33. The 2 can be used everywhere, but this can also lead to high drain shrinkage
and things like that. So, that is it, but you can get a little bit earlier 28 days strength
will be slightly more, and so will be 7 days strength because 7 days strength is two-third
of the 28 days strength.
So, to that extent this will have some advantage otherwise this does not have much of an advantage.
Most commonly, used are these days are 43, 33; was used earlier when nothing else was
available. So, this is when it comes to the grade of cement grade of cement, but then
OPC is used everywhere, most commonly well. Today, of course; use of OPC possibly in many,
you know; small scale usage has reduced because of the availability of cement and mainly cement.
You know, blended cement, but most OPC; you can use, in most of the places that is the
first thing OPC you can use, in most of the places without any problem and as the most
Well, if I am looking at early age performance, then what kind of cement shall I select? This
table tells. You these cement selection ideas of cement selection. For example, this column
gives you performance required. This gives you cement type this gives you an application
example and other effects may be side effects you can say. For example if I want you know
repair of runways I mentioned; where you cannot stop your traffic or even road sometimes,
where I cant stop my traffic then the cement I would be using should have the performance
that is instead of very high early setting.
In such cases; I use quick setting cement or the jet cement, I mentioned earlier; when
we were discussing about, type of cement, which acts very fast. So, if I require very
high early setting, then I can use jet cement thats an extraordinarily very high surface
area and you remember that, when raw materials are different and it sets very fast. Similarly
quick setting cement; so they can set very fast, but they would generate heat of hydration
So, when only you require very early setting you cant stop the traffic this cost they
will be costlier surely they will be costlier cement in fact this as to be imported. Now,
when you need you know when the extra expenditure is justified from, the point of view of let
us say, traffic flow that; you have stopped because that would if you stop traffic flow
especially, in airport runways it would huge loss to various organization. So, if you,
if the additional cost is justified, then obviously 1 could go for this ones.
So, you need quick setting very early setting, then you use this sort of cement some places.
You might need high early strength, for example: in pre cast element because where you have
or pre-cast element. Usually you know, in concrete bridge construction. Let us say,
you have bridge guiders, which are now, are actually cast in casting. If it is post tension,
I mean pre tension are cast in casting and with a with the pre tensioning. Wire tensioned
in the beginning itself and then cast the beam.
Now, once the beam has attained sufficient strength it can with stand some amount of
compressive strength, compressive stresses due to the you must release it and you must
release the bed as early as possible because that, would give you less cycle time. You
can produce more at a faster rate from economy it is justified to release the bed as early
as possible. Similarly, when in post tension system where you have pre cast member- segments
in case of bridges.
Now, those ones you would like to release it as early as possible the casting bed you
would like, to release it as early as possible, because the next element, you can start casting
them. So, in such situation where you would like to get some high early strength,
so you can, so that you can apply some load the handling load etcetera. There you need
high early strength. So, pre cast element is one of them tunnel form construction of
building. You know, in tunnel form construction of building
you can possibly finish structural system of a floor in just about, 3, 4 days time.
Now, what is done you have a monolithic mono lithic wall and construction and the form
work can be shifted, it looks like tunnel and it can be shifted from 1 floor to another
very easily, then with a crane and then you pump the concrete in and quickly do the concreting
and try to get gain early, strength. So, that you can release fast, because shifting process
and fixing process is very short during that period of you know, in this type of tunnel
form building construction.
Let us say so to get advantage if you want to get advantage of this first rate construction.
You must also do your allow you know release the as fast as possible. So, to do that 1
can use of course various means; you can use OPC with some admix RHPC rapid hard cement
or you can use OPC stream, curing heat curing or any combination of them. So, my point is
of course today's discussion is we are trying to select the cement. So, if I need high early
strength some examples I have given you, then I would possibly use RHPC or other solutions
are also available, combinations are available.
So, I can use all of them but RHPC would be used in such situation rapid hard cement would
be used in such situations where you know that, I need high early strength. And obviously;
we have seen earlier that if you get high early strength, this might have some effect
on long term strength. You won not get as good as long term strength compared to OPC
or you know other some other cement. So, when I am interested in high early strength concrete.
Then I will be actually using rapid hard cement type 3 ASTM type.
So, still look at the same early age performance if I am looking at higher workability, then
cement alone may not do the job. That we have seen that, cement may not do the job. We will
use actually OPC plus admix. You know this is useful for all application. As I said today,
the modern engineered concrete construction cannot be third of without some out of workability,
enhancing plasticizer or admixes, which are plasticizers or water reducing agent and they
are also called they reduce the water demand. They reduce the water demand of the concrete
for the same or same workability.
So, 1 solution is used OPC with some sort of admix the other solution would be of course
use OPC fly ash. As we shall see later on when we do mix design that, fly ash can reduce
some amount of water? And we discussed this earlier also in connection with use of material.
So, when you have OPC with edition there can be reduction in water demand. Of course, if
it is, in the cement itself then in any case you dont know whether, further reduction
if you want, because if you have given water reduction further water reduction you want
then you have to use an admix.
So, you can have combination of all of them to reduce the water demand and this has got
huh application. Now, if you use only OPC plus fly ash not for this. You know, not for
this is longer curing time would be a former time, will increase if you use fly ash combination.
So, this is only true for FA; you know this is true only for FA, F A plus OPC; this is
only true for FA plus OPC situations, where you might need a longer curing time. You have
seen that you need 14 days instead of 10; and you need 10 instead of 7. Well; just OPC
you can do with 7 here you would need curing time 10 days, when you are using fly ash.
So, curing time would be longer and so will be the case with because strength gain is relatively less
in this case and in the case, of OPC and fly ash and therefore, you needs a longer period
of time before you could remove the former. So, this is the other effect, then this is
another case, low heat of hydration this is required in mass concrete let us say or in
dam construction as we have discussed earlier, many other places where heat of hydration
is a problem.
So, if you have excessive heat of hydration that heat would be locked inside the locked
inside the concrete in the core of the concrete, because it may not be depending upon the thickness,
volume to surface area ratio of the volume to surface area ratio of the concrete. You
know, in a thick section your volume is large surface area is less. So, volume to surface
area ratio is large in such situation, evaporation rate from the surface is relatively slow and
all the heat from inside really do not get out. I mean the cooling rate from the surface
is slow and all the heat from the core of the concrete is not dissipated in 1 it remains
and as a result there may be internal. You know, it is it might be of high temperature
the surface is at a lower temperature, which would like to contract.
So, when it is a thermal contraction occur at the surface, where inside still warm and
no contraction takes place. As a result there will be a restrain to the surface con- contraction
resulting in cracks. So, you would rather like that low- heat of hydration here the
heat of heat generated itself is low. So, that you know dissipation process is low,
of course you can have other means of cooling them and things like that but heat of hydration
wherever it is a problem, especially mass concrete thick raft or, you know roads pavement
etcetera. In such cases, one option is used OPC plus
fly ash because we know the fly ash reduces down slow down the reaction. It will have
when you use OPC plus fly ash some amount of, OPC is replaced by fly ash, which means;
this heat of hydration will come only from OPC; fly ash do not contribute. So, much to
the heat of hydration and in fact it will be initially not reacting till some calcium
hydroxide is produced.
So, it is it has a dilution effect diluted; you know the OPC is now less. So, this results
in less heat of hydration because OPC if the heat of hydration is directly proportional
to huh quantum of C3A present; C2S, C4F and C3S present mainly C4 C3 A, then C3S etcetera.
We have seen the amount of heat of hydration that comes out. So, mainly C3 A; so now quantity
of C3A present in the cement. So, heat of hydration will get reduced and do not contribute
so much to the heat of hydration.
You can have low heat cement; this is I S 1260 low heat cement essentially it will have
less. It is would be relatively coarser, then let us say, hardening cement or OPC and we
have seen, the strength development of different cements. This will belong to actually type
4 ASTM cement. You know, it is just opposite to the depict cement, this doesnt harden
So, its initial strength gain is lower but heat of hydration is also lower. So, it will
have more C2S component possible then C3S component; eventually, this would generate
less heat of hydration, but you might require more curing and then form removal if you are
using it in situation where form removal is required, but normally when nobody will use
them in such situations. They will be used mostly for mass concrete where you dont
require bottom to be removed. Possibly it is onto the ground like think raft and similar
situations;, so or you know mass concrete situations.
So, there you will be using this so this 1 as to keep in mind; in case it is being used
by chance, but normally; in such situations bottom you know bottom ring is not to be removed.
So, usually because it is will be on the ground pavement or let us say, dam thick raft that,
comes into the ground straight away. So, but curing time required is longer curing time
required is longer.
So, that is what is the early stage performances are we are talking about let us see, the matured
age performance. You remember we talked of maturity matured when it has hardened. Note
the early stages it has hardened long durability strength etcetera. We are talking of them
talking of those ones. Now, if I now if I need you know the normal strength, then I
can use OPC even I can use PC etcetera. There is no problem provided I know, the rate of
you know its behavior- which is known to us
So, high strength is desired, then I usually it is preferred that, a low C3A cement is
used together with silica and then super plasticizer. This can be used for long span tall structure
etcetera, high strength concrete, something like 100 MPA will you define, the high strength
concrete above 60 MPA. We said it is a high strength concrete. So, when you are looking
for high strength concrete;, so let us say above 100, below 100 instead of this you can
even use fly ash and as an as an filler material and p- filler material and use super plasticizer
to reduce down the water cement ratio significantly.
So, while you are using get trying to get high strength; the performance is high strength
OPC with preferably with low C3A component. And silica fume and admix that is what is
used and this is. You see, the example is in Malaysia where they have used high strength
concrete in India; I think J hospital flyover in Mumbai, and there are several other structures
which, are where this is being now used. The nuclear power propulsion you know that they
use it first time in. So, this high strength concrete above 60 MPA concrete that, there
you can use materials and this can be used, in very long span bridges or very tall structures.
So, like I said in Kuala Lumpur thats 1 of the examples, then we have seen this actually
the sulphate resisting cement. So, where I want sulphate resistances, where you know,
where I am interested in sulphate resistance, then I would use OPC together with PPC or
you know preferably, some cases OPC in low. You can even use OPC in low sulphate, but
the code gives you IS 4562 gives you the details. You can use this also but with certain additional
thing. But, then I can use PPC post cement and sulphate resistance sulphate resisting
cement and in very high, content I can use super sulphate d cement.
I can use super sulphate d cement, you know I can use super sulphate d cement.
So, in sulphate environment this 1 can use this you know I S 456, 2000 gives you the
guideline. This guidelines for this is available in IS 462 2000, which at what sulphate concentration
which of this to be used. At low sulphate concentration of course you can use OPC and
some cases super sulphate d cement some cases even may be a coating or something onto the
So, depending upon the sulphate that you are encountering in the environment 1 might select
according to the and you remember sulphate resistance cement is 1 where I will have less
C3 content cement is 1; which, is useful and super sulphate d cement is produced from we
mentioned this sometime earlier 85 percent with OPC and little of gypsum. Thats how
we use, we produce with super sulphate d cement, which is highly sulphate resisting cement.
So, this when you have sulphate resistance you know sulphate required desired performance
is that it should be able to resist sulphate. We can be using 1 of this cements sea water,
again let us remember back we talked about, sea water situation and in sea water we can
use some cases, we can use OPC, but PPC and PSC will perform better sulphate resisting,
cement is not the not the solution. There because you remember we talked about, that
sulphate resistance cements. It doesnt bind the chloride as much as possibly select
cement does PPC and both, by their virtual their low. You know, they give a better structure
micro structure of the concrete both PPC and PSC and thereby they actually show you improvement.
As far as durability is concerned
So, in sea water environment sea water environment, 1 would 1 would possibly use any 1 of this
cement, OPC also in certain cases, but then we will have certain thing additional things
also. You know, there choice are many, but I can use PPC and PSC so thats what it
is for selection sea water. Now, long term strength we have already seen that PPC and
PSC actually improves the micro structure of concrete, the reduces.
So, 1 can use them OPC also you can use there is no problem but you know, the way where
OPC will not perform as much as you know as good as PPC or PPSC. When it comes to long
term- durability etcetera. But supposing I use PPC with slightly higher water cement
ratio OPC with slightly lower water cement ratio will perform. So, I have choices available
I can choose from, even any 1 of them. The main important issue would be what is available.
I mean easily available; you know if you have to for example: if you have to like, produces
most of our slack cement.
If you want to have small quantity of slack cement let us somewhere, in the northern part
of India. It is not very easy; it would be it would take possible time planning requires
to be better. So, what is easily available that is 1 can look into that, and with that
1 you can use you know judiciously, because if you know the behavior then using it, judiciously
is possible. But certain, things are for example: almost you know not to be used, like sulphate
resistance say it is costly cement.
So, if you dont have a sulphate environment why should you use;, so that kind of judicious
decision 1 as to take for selecting the cement, but the cost is a major factor cost is a major
factor. Finally 1 as to see what is the cost and you know, cost economy, because you cant
construct something without considering the economy. So, that is how we do cement selection,
specific cement selection. We will go for cement selection in this manner.
Let us say, let us look at the selection of water. Now, this is most important because
large part of our, you know construction. In the particularly in the northern part of
the country, let us say, NCR region of Delhi, Rajasthan, UP and Haryana. There is its it
is far away from the coast sea coast and therefore, the chloride induced corrosion should have
been a rare phenomena here coming possibly occasionally, but it has been seen in housing
sector there is a large amount of chloride induced corrosion.
In these areas too in buildings the if you remember if you can recall the photograph
that I showed, with reference to corrosion when we are talking about, durability of concrete.
You know a slab whose bottom was totally the concrete as fall of from the bottom completely.
Now, if you recall that photograph this was not for the reason was actually high chloride
content. How, where does the chloride comes from it comes from the ingredients of concrete
in such situation.
In fact, you can you can you can you can understand you can find out whether such you know, such
chloride comes from ingredient or whether they are coming from the outside. You can
find out; for example, if I determine chloride in the concrete surface. Now, if the chloride
is coming from coming from, coming from the ingredients, then supposing I take a sample
from here cut take a sample and this core sample, I have taken out. And I measure the
chloride in this I slice it off measure the chloride here chloride slice it off and measure
You know, for each of the slice etcetera. Now, I will find the profile of the chloride
if it is coming from outside. Profile of the chloride will be something like this. It will
be maximum here, and less on- the other side, but if it is coming from the ingredients you
will find- more or less similar chloride. You know, uniform chloride the distribution
of the chloride will be uniform throughout.
So, this is the chloride from inside internal chloride. If it is internal chloride internal
chloride it will be something like this. This will be the distribution, if it is external
chloride this will be the distribution. So, this is external chloride;, so 1 can actually
find out whether the chloride as come from inside or from outside. Now, by and large
if you see the places the buildings particularly buildings. It is infrastructure this problem
is not so much, because you have better quality control.
You at least you must be testing the, you know1 as to test the water quality, but housing
sector this is relative- problem construction is not fully, engineered. You know, 1 would
call like if you have, a sort of converted into a volumetric batching, and then mixing
by simple rotary mixture. And that is not fully engineered concrete, and then water
is just mixed without real full control. In such situations quite often, it has been observed
that it has it has been observed that, contamination, in you know water contamination.
The chloride comes into concrete through water and where does how does it comes from,
because water used in most of these north Indian places. You know, is ground water for
construction. So, when somebody is using ground water deep from deep ground water source.
This could contain a lot of salts soluble chloride salts, the river water will not have,
because it will all chloride soluble. I mean soluble in water would go away.
So, contamination in terms of salt concentration high if you go deeper and deeper into the
ground and this is supposing this is used without, testing quite often you may have
lot of contaminants in it and it can results in long term, durability problem and other
problems to. Now, let us see;, so first thing is contamination shall not be you know should
be should not be there. And if, it is there water contaminants are there those 1s which
hinders. The hydration process or interferes with the hydration process.
So, 1 issue could be contaminant presence in water it should not hinder the hydration
process. For example: if supposing there is a sugar factory and the sugar factory is discharged.
You know, sugar water discharge goes into the ground there and somebody is using into
a pond or some such places. Let us say, somebody using this water for concrete making; obviously
concrete is not going to set because sugar is going to be retarded
just as an example sugar is retarded.
So, contaminants supposing little bit of sugar comes from the sugar factory or more something
like that, the goes into the water that you are using for concrete making; it may not
set properly. So, contamination because sugar interferes with the hydration process. So,
contaminants should not interfere with the hydration process that is number 1; it should
not interfere with the hydration process this is number 1. Number 2 it should not induce
long term durability problem.
The earlier example that, I have given; the example that, I have given is the is of chloride.
Now, due to you know the enforcement corrosion due to chloride that is the enforcement corrosion
due to chloride. Now, that is a long term durability problem. Now, contaminant actually
induced a long term durability problem, whereas if I have some sort of sugar coming as a contaminant
into the water, the extreme case. Of course; this means that, it will interfere with the
hydration process. It will interfere with the hydration process.
So, neither the contaminants are the contaminants in water should not do both. You know neither
it should do this nor that also, it should not have too much of alkali content, which
may lead to actually may be help in alkali aggregate reaction? Although that is that
is not very well known but such alkalis which can react with some of those silica presence
in the aggregate. So, those are the issues contaminants should not be there; so, what
is code we will discuss. The code is given as we will discuss that
the code has given us guidelines. What should be the best sort of water which will not actually
lead to either of this problem or you know, problem of long term durability or problem
of problem of hindrance with the hydration process slowing down or interfering with in
some manner. So, the code as given us guideline that how to tackle this. So, it has given
us the range of contaminants that is permissible in water thats we will discuss.
But in cases it may happen that you are little bit doubtful, because you have not been able
to you have tested and you are not able to test properly and you are doubtful that it
might actually hinder the process of hydration. So, in such cases 2 things 2 tests has been
processed by the code. one is with reference to setting you know the contaminants present
in water should not disturb the setting process. So, 1 has been 1 is with respect to setting
and the other is with respect to strength development. So, it is suggested that in case
you have some doubt. In case you have some doubt regarding development of strength.
You can cast cubes with- proposed water and distilled water and you can compare them
simple, because where you have doubts so best thing is to do what well; take the most suitable
water that is distilled water and use the proposed water, and compare the compare the
cube strength results. Let us see what we compare; we actually compare 28 days cube
We actually compare 28 days cube compressive strength and you know strength of concrete
cast with proposed water and that with the distilled water and the 28 strength should
not be, you know it should be it should not be distilled water.
You know, this should be more than 90 percent of the distilled water it shouldnt be less
than, In fact this is less than should not be less than 90 percent of that cast with
distilled water. It should be greater than 90 percent of that, cast with distilled water.
So, that is the idea. You know; so if it is coming 90 percent, then we are not really
worried about, but if it is coming that, is if I say fc proposed water of 3 samples etcetera.
Proposed water it should be greater than, 90 percent greater than, equal to 90 percent
of fc 0.9. You know, 90; I mean 0.09 I will say, 0.90 f c of distilled water and then
we can accept this water; then we can accept this water otherwise we cant accept. So,
thats the idea -then initial setting time.
We also test the initial setting time with proposed water and shall not be less than
30 minutes obviously;, then you have a problem, if it is less than 30 minutes then you have
a little bit of problem. If it is less than 30 minutes, then we have problem because you
see the setting time should not be in any case should not be more less than, 30 minutes.
And shall not differ by plus minus 30 minutes from that, of paste prepared with distilled
water. So, you prepare distilled water measure their
initial setting time and initial setting time in no case should be less than, 30 minutes
because that is the minimum requirement for any cement and this also shall not differ
by, 30 minutes from that of, paste prepared with distilled water. For example, if you
get with the distilled water let us say you get 60 minutes or let us say just 30 minutes.
Now, you shouldnt get you know like- the difference between this you get let us say
90 minutes. So, 93 this difference is too large so thats what is the point, the difference
between the 2 should not be also plus minus 30 minutes from that, of paste prepared because
then it is disturbing the setting process, hydration process and hardening process etcetera.
So, these are the tests required. Now let us see, what are the guidelines given the
code gives us a guideline that, pH should be less than 6 thats the guideline given
in code but, it is generally said that pH should be between 8 and 6. It should not be
too alkaline it should not be too acidic. Because in such acidic environment the reaction
process may not progress in the right way; the chemistry of course is not really of our
I mean, we dont really need into the goal, need to go into the chemistry of it. How cement
reacts the mechanism, but at page between, you knows it; the reaction proceeds properly
in a neutral environment; in high alkaline or low alkaline or high acidity acid condition.
The reaction couldnt do not go properly also they might also impart to you know in
the I mean; basically, impart or induce some durability problem in future. So, that is
why the page suggested page in the code is that it should not be less than 6, but additionally
you say that it should not be greater than 8 either. And that is why the code as given
you test and this test are for example, for 100 milliliter of water,
You need the sodium hydroxide 0.02 normal. You know you take 100 milliliter of water.
The amount of sodium hydroxide needed to neutralize this 100 milliliter of water. We use 0.02
normal sodium hydroxide solution and this should not be more than 5 milliliter.
If it is more than 5 milliliter that means it is more acidic then we really because this
will contain some acidic material, dissolved acidic material and this is neutralized by
sodium hydroxide. You know, so if I should not require more than 5 milliliter of sodium
hydroxide of strength 0 to normality to you know, neutralize 100 milliliter of 100 milliliter
of water. So, that if it is more than this it means that it is more acidic than I require
really 1 and thats not desirable.
Similarly; for alkalinity I have a test and this test is 100 milliliter of water when
I am trying to neutralize, with Sulphuric acid 0 to -normal strength, then I should
not require more than 25 milliliter. So, the amount of 0 to normal Sulphuric acid required
for neutralization should be less than, 25 milliliter that means; it is properly alkaline.
So these are the 2 tests adopted in the code to ensure that it is really not as too much
acidic not highly alkaline. You know, it is within the bounds and this
is to test in addition other prescription those are given or other recommendation the
organic solid should not be less than 200 milligrams per liter should be less than 200
milligrams per liter to 100 PPM. Because you know, milligram per liter would mean also
PPM; I mean, in organic solids 3000 milligram per liter. Now, this is important SO3 sulphate
400 milligram per liter, although you know, sulphate in really doesnt create any problem.
Because sulphate in the is not problematic if it is only high. Because SO4 in you know,
gypsum in the initial phase actually reacts with C3A, but excess sulphate as got no meaning.
So; therefore, there is a control on the sulphate as well but most important I would say, is
this aspect. The chloride most chloride and you see, these values are restricted to 500
milligram per liter for rain concrete and 2000 milligram per liter for plain concrete.
So, this is most important because this can induce river corrosion. Just this and in addition
to this if you also remember that, we said that the total chloride present in the concrete
should not be value certain kg per meter cube, of concrete. You, know we said point 6 kg
whatever it is we just mentioned earlier overall but at the same time water should not have
So, this there is bound water should not have too much of chloride total chloride some more
chloride might come from aggregate etcetera. So, this is most important part because chloride
presence can really, create problem in long term durability.
So, that is these are the these are the requirements as far as water is concerned as far as water
is concerned curing water suitable for mixing is suitable for curing as well. It is suitable
for curing as well, but 1 has 1 has to ensure that, curing water does not produce any stain
same problem curing water of course will not affect the initial hydration process, but
it can surely affect the long term durability.
So, you cant use chloride contaminated or sulphate contaminated water for curing;,
so water suitable for mixing its also suitable for curing purposes and 1 additional feature
is very much there that it should not produce any stain no color. Because if there are some
you know some amount of iron present soluble iron salt is present, then its permissible
as for as, the previous you know, recommendation of the code is concerned; IS 400 and 56 2000,
but it might give a red colored stain.
So, that may not be statically acceptable that may not be statically acceptable. So,
therefore, 1 has to take additional care that it is not statically it does not create any
stain as far as curing water is concerned. I have already mentioned that excess carbon
di oxide or chloride which can attack the concrete they should not be present in curing
water. One important issue is, this normally it is said portable water which is fit for
drinking is also fit for construction or concrete mixing.
Well; by and large this is true because bacterias all affect human being it doesnt affect
the concrete by and large this is true, but- more important is the guidelines that, has
been just stated or given in code and 1 has to follow the code actually. This is only
some from the code to give the understanding of how should go on selecting, the water and
what is the importance of water proper quality water in concrete construction.
So, 1 has to go by the code although loosely speaking people say portable water, if it
is fit for drinking, it is also fit for mixing, but that is not may not be always true. Second
issue is of course, the sea water well sea water should not be used preferably not to
be used without, but if you are know you have no other way, you have no water. You have
to use that water, then you have got to take adequate precautions, because then you know
you are risking your concrete you have already embedded some sort of a kind of a germ into
the concrete a contaminant, into the concrete right in the beginning.
So; therefore, 1 as to take care of this aspect that means you should see that it remains
free of water during service period, should not come in contact with water etcetera. There
can be more than 1 solution possible but 1 as to be careful;, so sea water is not debarred
fully because if nothing is available what to do, but you must test the water whatever
is available. They may be difficult that we have no water available. So, what to do well
whatever is available, you test it and accordingly take right kind of precaution.
So, I mean if you cant treatment may be very costly;, so accordingly, then you take
kind of precautions that, is the idea. So, that is how we go about selection of water;
let us see, how we go about, selecting aggregate first thing is aggregate should be satisfying
the requirements of IS 300 and 83, which is the code meant for aggregate.
So, coarse aggregate or fine aggregate it should satisfy the requirements of IS 383;
that is the first requirement; let us see, what are the specific requirements that we
are talking about well, if you remember packing characteristics of aggregate is most important.
Therefore, grading is important because we got to control the packing characteristics.
And as I said that you do not have really a model whereby by finding out proportions
of various components of the aggregate various sizes in the aggregate. I am able to find
out what is my packing density what maximize or gives me the optimal packing density which,
I require since we did not have that kind of packing models available anyway now there
is only and not in codes.
So, most of the codes gives you boundaries of grading you know the zones or ranges of
grading, which I think, I have shown you sometime earlier. So, aggregate grading ranges are
usually given some ideas. I would have given you earlier, but you can also understand the
fact that IS 383 gives you such guidelines you know the ranges of the aggregate permissible
range zone. What are the permissible ranges of sizes, various sizes proportions of the
sizes allowable sizes, that is what is the grading and it is important.
Now, you dont have the for example; if you are mixing 2 different sizes you can mix
them proportion in, proportion to get the overall grading. This is very easy, this is
not a it is not a very complex job 1 can find out. Supposing you have a binary mixture consisting
of 2 you know x1 of 1 particular and x2 of other particular, then what should be you
know, the proportions in the overall aggregate system because we know the proportion of 1
should be between let us say, 90 to 100 another should be from let us say 75 to 95 and so
So, knowing the proportions that is required if I know how much of 2; I am mixing and the
proportion of each of the aggregate in this, 1 and 2; I can actually simple arithmetic
1 can find out how 1 can satisfy the grading. So, grading it must satisfy and you can mix
aggregates from different. Let us say or different size aggregates to get the right kind of grading,
fine aggregate to coarse aggregate proportions, 1 of course decide through. So, grading is
most important parameter 1 as to look into then aggregate is selected from the point
of view of strength and durabil. You cant select any aggregate; you have to select such
aggregates, which will satisfy the requirement of strength and durability. Durability means,
alkali aggregate reaction 1 as to look into and some strength it should have sufficient
strength. It may not be very strong, but it has to have sufficient strength. And we measure
this strength by what is called; you know, direct strength measurement we dont do.
We measure it through what is called aggregate crashing value.
Now, this aggregate crashing value is nothing you take specific amount of aggregate passing
through particular sieve and retained on a particular sieve, then crash it in a standard
manner like, something like applying 4 tons load in 10 minutes or so on, with specific
rate of loading. Now, when you crush this aggregates you know, crush the aggregates
then it will be it some powders will be formed. So, sieve it through sieve it through say
2 point 336 millimeter sieve. The percentage passing through that is an indication how
much has been crashed more crashed means, it is weaker.
So, we actually make a relative kind of test on aggregates, things like aggregate crashing
value test and it must have a minimum value or it must have a maximum value to be selected.
As suitable for structural concrete; so it is usually not a big thing most of used aggregates
are, you know used whichever already are in use aggregates are further requirement are
crashing value requirement.
So, that is 1 issue the strength should be sufficient; you just cant pickup let us
say, broken bit pieces bit pieces and put it in the aggregate that will bring down the
strength of the normal strength concrete. We are talking of normal strength concrete
at the moment maximum of size of aggregate MSA nominal MSA, we talked about, MSA shall
be maximum possible, because it will improve the packing.
So, as much as possible M SA I should like to use because that, will improve the packing.
You remember we talked of more the number of you knows, number of I will have better
packing single size packs do not pack so well. So, 2 will pack better and more the number
of sizes are a better packing and you we also said that, I RMSA would improve my you know,
it reduced down my water requirement, for mixing water for a given you remember that,
when we talked about aggregate. Earlier when, we talked about we mentioned about this more
we also discussed the strength.
So, when we were discussing about strength effect we said that MSA higher MSA is suit-
suitable for, leaner mixes. And smaller MSA is required for rich mixes or high strength
mixes. So, if you if you look from that point of view strength of the concrete reduce as
MSA increases as I increase the MSA. So, we have seen that 150 75mm MSA for you know,
it is used for mass concrete and 2 to 4 millimeter MSA is usually, suitable for structural concrete
and 10 millimeter for high strength concrete or fiber reinforced concrete. Because the
fibers spacing is related to the size of the aggregate, workability of the fiber concrete
fiber reinforced concrete is related to the size of the aggregate.
So, this is rough guideline normal strength concrete maximum MSA 2 millimeter or 4 millimeter
usually 2 millimeter 40 millimeter aggregate. If it is available you might use and it is
suitable but this is this is roughly the idea. So, MSA selected depending upon actually strength
and also on the and larger size I would like use, but 1 more issue is related to the aspect
of the structural element itself that, is the maximum MSA should be cover minus 5 millimeter
because it has to go between the steel bar and the shuttering.
So, it should be less than it should be less than cover minus 5 millimeter or bar spacing
minus 5 millimeter. You know, this should have a bracket here bar spacing minus 5 millimeter,
whichever is lower. So, MSA cannot be maximum MSA; you use shall be lesser than, this e
practical construction criteria which governs in addition to the web criteria.
So, you select aggregate according to maximum size of the aggregate according to this. But
remember higher the MSA lesser will be water requirement. So, you try to find out an optimal
from consideration of this whichever whatever is maximum possible you use that, both natural
rounded gravel or crushed aggregate can be used and used effectively.
You know, the behavior then there is no problem because we know that rounded gravel it will
reduce down my water requirement significantly, but if its surface character is such that
it is not rough, it can reduce down the strength a little bit. So, it is smooth surface of
the nature. If it is there then it can reduce down the bone strength, but this might be
adequately compensated due to the fact that, you will have significant amount of reduction
in water content water demand for the concrete crashed aggregate on the other hand.
Well; but natural round- gravel may not be available I mean finally it is available because
you cant supposing; you are constructing something in say Chandigarh. You cant get
your aggregate from Chennai thats not possible, even though that must be that might be, the
best one- for your purpose. So, that is not the thing thats never done wherever you
have the construction you try to get the nearest aggregates possible, because the cost and
it is a natural material mostly you just process a little bit a- best you crash it.
So, crashed aggregates locally available if it is sufficiently strong you know, it has
doesnt have it is not potentially prone to alkali aggregate reaction, then you can
use this crashed aggregate. So, usually 1 uses crashed aggregate or natural gravel whatever
is easily available, whatever is easily available.
Special cases if you have radiation shield; you know like for example nuclear reactor.
Now, it is need radiation shielding; you dont want radiation to come out of it. In such
situation, we use heavy aggregate something like magnetite or very heavy aggregate density
is very because it is the high density and the thickness. Of course, high density is
required to absorb the radiation not allowing the radiation to go and this concrete we call
heavy concrete. So, when you have heavy concrete heavy density aggregates are used. In very
tall buildings in very tall buildings the wall does not carry any load, it is the frame
structures whatever, is you know they take the load.
So, the envelope the actually the closing is you know this enveloped is closed. The
space is closed by the walls. So, walls serve the purpose of the envelope and it doesnt
have to take load it has to actually withstand its self load and any horizontal load. Horizontal
load it should not itself collapse. So, it as to withstand transverse loading and some
sort of self load, that is all the load it has to take;, so load is not an important
criteria in very tall buildings the walls are not meant for carrying loads 56 storey
buildings; many cases and nothing else.
But wall has got other functions to do; it may require you know you might require thermal
insulation and acoustic insulation and in such cases light weight aggregate does the
job, because thermal conductivity is an important parameter and light weight aggregate, when
you use them if t- natural or synthetic synthetic aggregate could be there produced from let
us say, fly ash or slag or similar sort of thing or various aggregates are produced by
processes, this are you know, this they are they are chosen because of thermal and acoustic
properties and not much from the load carrying point of view.
So, that is it we select aggregate synthetic aggregate from fly ash and slag can also be
used for nonstructural purpose, but you they can take some structural load as well. So,
but they will be less you know but load bearing wall of course you need, but load bearing
wall if it is concrete monolithic wall as in case of tunnel form construction. Now,
that requires strength also here the wall itself is monolithic to slag and it carries
the load from, the slag is transferred to the wall and next to the next level and so
on so forth.
So, there the strength is required where strength is not necessary you can use lightweight again
this is most important the alkali aggregate reaction potential and other durability considerations
are primal, when it comes to aggregate selection, these are the primal issues.
I think we have discussed most of it you know aggregate- selection earlier itself when we
were talking of properties of concrete. So, now that you know it so only we are compiling
the back again looking re-looking into them while selecting cement or selecting aggregate,
water of course you didnt look much into water quality. So, thats what we are doing.
Now, one important issue is that when you look at sustainable concrete in future not
in this country at the moment we are not really using recycled aggregate, but aggregate resources
are not infinite and hence you cant go on using the natural aggregates.
So, this concepts of recycled aggregate is gaining popularity somewhere elsewhere in
the world and 1 may you know, think in this direction as well; recycled aggregate is popular
not yet on the other hand, because you know the building you have to demolish after 30,
40 years for various reasons, because it must have done away with what is known as functional.
Since such situation you demolish this debris what do you do? You have to actually do it
is another kind of environmental problem.
So, you try to recycle them and use them in construction again, admixture most important
when you are selecting admixtures it should be compatible with the concrete and it is
you know, all solid content etcetera requirement should be seen as for IS 9103 code shall not
contain any material harmful to concrete durability thats important, but the effectiveness
are determined only through trials at site, there is no other way you can do it, admixture
finally its compatibility, whether it as got any side effect or not that has to be tested
with the concrete that you are using.
So, once tested once you have satisfied then you can use this there are various because
these are all commercial product, the blended product the commercial product and when you
have to test them, to actually see their compatibility what might be compatible with 1 particular
cement, may not be compatible with cement. So, this is an important issue selection is
based on systematic procedure of testing and nothing else. So, 1 as to go by testing; in
fact there are compatible test for high strength test is done then there are mini test and
so on. So, there are several test for testing compatibility of admixtures, finally you do
the test see whether it is actually reducing your water demand or not and does it have
any side effect or not. So, that is how we select materials for concrete construction.
Well, so what we have seen so far we have seen cement selection how do you go about
selecting cement and we have also seen, how do select water the contaminants in water
and lastly we have looked into aggregate selection and admixture selection. So, that is the first
step in concrete making, but once you have decided then next step is of course is to
go for designing the mix and that is it. I think this summarizes our discussion on concrete