Having looked into the other process of the concrete
production; namely the mixing, batching, transportation placing last of it that is the curing; the
most important part will be looked after today. Today we shall look into curing, this is the
most important. Quite often people tend to ignore this, but this is most important as
we shall see in a short while. Also we will discuss about prefabrication right. So, let
us see what we what is going to be our general outline of our discussion today. First of all we will look into curing as I
mentioned, then we will look into what is the need of curing essentially and then we
will look into details of various methods of curing. In prefabrication; we will look
into the process and we will mention something called modular coordination. So, let us look
into curing. Why do we need curing? Now, if you remember when we talked about
cement; we said that, it is you know is produced from lime and clay binding both of them together.
Now, such material when bind together, forms a kind of solid solution about which we will
discuss in later on and we said that, when you mix water with it, there is a reaction
and we call this reaction as hydration reaction. And then through this reaction it becomes
solid. Now this reaction is never instantaneous, it takes time. And therefore, this reaction
since it takes time, the condition must be conducive for reaction to continue.
Now, these reactions since it is a reaction with water, it can takes place only in presence
of moisture, that is, vapor. In other words, in case of you know the 100 percent relative
humidity in the pores of the cement based system is required for this reaction to continue.
So, hydration takes place at only at 100 percent relative humidity. Now when you mix water
with the cement, what happens is; since the reaction is not instantaneous, only a part
of this water reacts, rest all remains there. But if you expose it to atmosphere, most of
it will leave operate. So, there is a loss due to evaporation. So, there is loss due
to evaporation. So, therefore, there is 100 percent relative
humidity condition will not be maintained. And that is the saturated; you know pores
will not be saturated with moisture, unless you replenish the water that has been lost
by evaporation. Besides, chemical reaction of cement with water also consumes some amount
of cement. So, supposing I stopped the evaporation process, even then there will be some amount
of loss of this moisture, due to the chemical reaction of the cement with water, because
some of the water will be consumed. And that we call as self desiccation and it is actually
internal loss of water and we must replenish this water, if the pore has to remain saturated
with moisture vapor. So, therefore, this is the, these are the basically the needs.
Now some time it may be also needed to maintain a temperature, because you see the concrete
being at very high temperature; it is not desirable you do not want to concrete at a
very high temperature. Or even after we have cast the concrete, we would like to control
the temperature; especially in tropical areas or hot conditions hot and dry condition hot
conditions particularly. And there we might may have to you know maintain the temperature
of the concrete, through replenishment of water or by supplying water.
So, curing therefore is needed to continue the hydration process, to rather avoid loss
of water or reduction of this relative humidity. So, what we do; we actually add water to it.
So, curing is the process in which we add water or prevent the loss of water from the
concrete or the cement paste or whatever it is of the similar sort of thing. You know
right now, we are mostly talking of the cement paste loss of water, although its concrete
has got mainly water would be lost reacted with cements, because aggregates as I mentioned
earlier they are inert. So, cement paste to which actually water reacts
that consume some amount of water, other water is lost by evaporation. So, this water has
to be replenished or supplied from outside and its evaporation loss should be restricted.
So, this process by which actually we prevent loss of water through evaporation or drying
of concrete and also supply additional water to replenish the loss of water; that process
we call as curing and it takes place over a period of time.
So, we have seen the needs of curing and also you know how you do it. 1 more additional
important point related to curing is; you see as we shall see in 1 of our later lectures,
that hydraulic cement when it when reacts with water, since all water do not react instantaneously,
some of the water will evaporate, we replenish this water alright, but the space that was
originally occupied by water; it has to be gradually filled by the product of the reaction.
Now, this takes time: it takes days, months and up to even year or more. So, this space
which was originally occupied by water must be filled in by the reaction product. Now,
this only can occur if curing is taking place or if reaction is taking place, curing is
being done and reaction is taking place,. Now if I do not cure it, the space that was
originally occupied by water; that will remain as capillary pores, capillary sized pores.
Sizes we will discuss later on, but they are capillary pores, because water can enter there
by capillary suction. Now this capillary pores will allow water
to come in and as again we will see later that, most of the degradation process in case
of concrete is due to ingress of aggressive chemical agents from outside, mostly through
water. Therefore, this pores, their presence is harmful from point of view of long time
durability of concrete. The capillary pores that is formed by the space occupied by the
water that has not reacted in the long run; these pores pole space allows water to come
in and therefore, allows aggressive chemicals to come in, which we shall discuss again.
And therefore, they are harmful to long term durability.
So, if you do curing sufficient curing, this capillary pore space will be filled up. But
even though if it is not filled up, at least they will get segmented and will not remain
interconnected. So, segmentation of the pores, that is, separation of 1 pore to another pore;
that means, they should not remain interconnected, is most important from durability point of
view of concrete right, you know this capillary segmentation, this is very, very important.
Capillary segmentation is very, very important from durability point of view of concrete.
So, as you can see for strength development 1 thing is for you know the curing is necessary
to prevent loss of water and this in the process actually hydration reaction will take place
and this will actually ensure that, capillaries are segmented and strength developmental properties
development of concrete is possible. So, that is what is the need of curing and importance
of curing right. So, next we look at the next slide. If I do not do curing then, what will
happen? Let us look at them. First of all since pores are the inherent
flow, in the structure of concrete, this will be explained in detail later on when we talk
of strength of concrete. Therefore, if I do not reduce down this pores, strength will
not be proper adequate. So, it will result in core strength development this will get.
So, poor curing will result in poor strength development right. Then if I allow water to
evaporate from the concrete surface, what will take will happen; it will get dried up.
And when concrete dries, especially in the early stage even later on of course, it can
take place. So, drying of concrete can result in its shrinkage.
Phenomena of shrinkage we will look in details, but let us understand for the time being that
drying, when it dries it will shrink it can shrink. And this shrinkage since the surface
dries may be the inner core inside the concrete specimen, it may not dry so much. So, there
can be differential shrinkages; top shrinks more, bottom does not shrink so much resulting
in formation of cracks at the surfaces. So, drying shrinkages cracks can come, if you
have allowed the top surfaces to dry rapidly. And to prevent that again, we do the curing
or replenish the water at the surface especial in the early age. Once things have become
stable and the capillary are segmented well, the water you know like the drying effect
would be relatively less. So, in the beginning it is important that
we avoid drying shrinkage and could result in crack. And therefore, if you do not do
pre curing, this can actually result in drying and result at shrinkage and this by aggregated
created by thermal effects. For example in hot arid sort of climate in the northern north
western part of the country India, we have high ambient temperature, low relative humidity
in summer, very low relative humidity. You know hot dried dessert sort of climate, you
may have about 40 percent relative humidity even in the morning and in the peak temperature
time; it can be 10 percent also. And the temperature could be as high as 40 plus. And in such a
situation, if you have not replenished the water or not at least covered the surface
of the concrete, then it will quickly dry off and both shrinkage and even you know shrinkage
cracks can result in formation of you know defects in the on the surface of the concrete.
So, this is important issue, if I do not do curing properly I can have drying shrinkage,
where sometimes only fine cracks placing at the surface, but aesthetically they are poor.
Sometimes in thin sections they can be flowing through cracks, because of drying shrinkage,
you know very thin section drying shrinkage cracks can be flowing through of course, there
are other ways of actually tackling the cracks, but curing is you know it helps in preventing
such sort of crack formation. Now, last 1 I mentioned earlier that, if you
do not curing, you know if you do not do proper curing; the capillaries will not be segmented,
they will remain in interconnected. And interconnected capillary pores are not good for long term
durability of concrete, because water can come in through this pores and result in deterioration
of the concrete along with aggressive chemicals water will come in. So, interconnectivity
of the capillaries in concrete system should be prevented and that can be done by segmentation
of capillaries through proper curing. So, curing of course, there are other means,,
but curing is also helps in segmentation, 1 must remember that, if you do pore curing
there could be no segmentation. And therefore, it may result in poor durability of the concrete
right. So, let us see diagrammatically what happens. So, how this strength development
issue like people have done experiment Past experiments have shown that, you know
the strength development of concrete takes place in this manner. Say let us look at this
say; if you keep continuously near this is the curve, this is curve continuously near.
See this side is the composite strength of concrete in MPa, this is same thing in psi
and this is the age. So, with age you see the strength of concrete increases, but if
you do not supply replenish this water, the strength do not increase beyond a point. In
fact, it will have a slight decrease. Now, you do curing for 3 days. So, we can
see that initial of course, the strength here also 7 days strength increase takes place
just look into a continuous year. So, some amount of loss would be there, but it would
be it would be dependent on the type of test that was done. So, in a specific case it shown
that, actually temporary you know it increases up to about 14 days and after that you have
studied. But it will depend on what is the condition of the air; what is the temperature,
what is relative humidity, is there any wind and things like that. So, in a very dry condition,
high temperature this may not be even this much, there may be much less reaction and
therefore, you know, but then qualitatively this is fine.
Now see this case the same concrete when it was kept in water for 3 days; that means,
you have done a moist curing for 3 days. Then it is shown at increasing strength much higher,
even in the same time much higher strength increase. Then of course, beyond that it is
shown reduction and so on so forth. And if I keep it for 7 days, it is something like
this, if I keep it for 14 days in water the design curing for 14 days, for 20 days this
is how it is. So, you can see that, more the number of days
I keep it in water or I cure it, actually there is an increase in strength development.
Now, if I keep it continuously moist forever. So, here I started you know I keep kept it
for 28 days beyond that I put in air, there is a slight increase after that the strength
increases slightly, but then finally, it decreases. But if I keep continuously in moist condition
the strength will go on continuously increasing of course, you can see that, this is asymptotic
to certain value, I mean it is just not infinitely increasing something of that kind.
So, what happens when you are continuously in moist condition, there is of course, there
could be some sort of redistribution and more little bit of more hydration or reduction
of porosities and pore sizes, slight increase in strength and there comes and that is reduction
and. So, what we see from and this shows for water cement ratio 0.5 concrete. For another
water cement ratio, the strength exactly will not be like this. And you can see the relative
strength; this was about 40 MPa close to 40 MPa, when you have done a curing for 28 days.
So, ultimate strength the maximum strength attained was 48 MPa. And if you keep it for
continuously moist, the maximum strength attained will be something like about 20, 23, etcetera.
If you keep it for 7 days, you know it is somewhere around 30, somewhere around 32 or
33. So, this is the relative ratio of the relative ratio of the strength that it 1 attains
1 can get when it is cured. So, for another water cement ratio thing could be different,
but qualitatively we understand that, if we the more the days of are curing, I get the
long term maximum strength enable is high. We of course, mostly concerned with 28 days
strength, because concrete is specified in terms of 28 days right.
So, generally this idea is kept in mind while specifying the number of days required for
curing right. So, that is what it is, it says how the strength development is actually governed
by curing conditions right. Now, let us look now into. So, what we have understood so far;
what is the curing actually, what is the need of curing and how curing helps in strength
development and what happens you know if you do not do curing that is it. So, let us now
look into methods of curing, right. So, since we are supplying water 1 method;
obviously, would be using water or moist curing. But that can be done in several ways; we can do it by ponding, we can do it by spraying water on to the surface, we can do it by covering with wet sand wetted
Hessian clothes or absorbent covering. So, we can use moist curing you can use in different
ways, you know ponding is very suitable for flat surfaces, let us say a bridge deck or
even a slab you know the roof slab or floor slab. So, what you can do you put sort of
a barrier at the boundary of the flat surface and just pond the surface with water and keep
it ponded for the time you decide. And that is a very good way of doing curing, especially
for flat surfaces. If you can’t pond, you do not that much
of water of course, you have to go for some other sort of way and water might evaporate
at very fast rate, then ponding also becomes very difficult. So, in such situation you
can do something else; you can come cover it with wet sand, you can cover it with wet
sand or you know sand its drying off spray again water sand will hold on to the water
for longer period of time. And you repeatedly periodically you go on wetting that sand and
that is what is this is. Then wetted Hessian or absorbent covering,
so you have put some sort of you know gunny bags for example, old jute gunny bags or similar
1, which can absorb water, which can keep water in position, some jute or similar sort
of thing cotton ways or Hessian you know was the old gunny bags were used very often. So,
wet them; keep them over the surface and this can be done not only for the horizontal surface.
Sand is of course, horizontal flat surfaces, but Hessian can be done for vertical surfaces
as well. So, for example, when you call and you wrap
up the Hessian or gunny bags or the older kind and you know the cement bags those were
jute bags those days, so 1 would actually wrap it around and the column and spray water
to it sprinkle water on to it keep wet all the way, for the required number of days.
Spraying is also useful in some cases for example; where you can’t you know like wet
Hessian. That is very effective wet Hessian or something of that kind, sometime you can
spray for example, in an automatic pre cast factory pre cast factory, I will talk about
pre-cast or pre fabrication later on. In pre cast factory, 1 might have sprinklers
to continuously spray water on the pre cast element. So, you can have such sort of situation;
continuously maintain a 100 percent relative humidity while spraying water. Spraying water
can be done even, otherwise many places spraying of water is used. You have small pump which
will through which you know you can just spray water or sprinkle water on to the surface.
So, this is the other method right. Now that you can do provided you have sufficient
water available. If you do not have water available, then what you do; at least you
can do is you can cover, you can cover you know the concrete, first of all prior to setting
that curing does not start you know till the concrete is sort of solid. So, it has to be
a solid after which you start sprinkling water, but even in its plastic or watery conditions
slightly, soft condition plastic condition not solid condition the water can evaporate.
So, you can cover the surface so that, no water no moisture can evaporate from the surface
of the concrete. Then keep the form walled the mould, you know we discussed about the
form work earlier. So, we said we will put in the mould and get in the right shape for
the concrete specimen or the concrete element. And you can keep the form for example, for
a vertical member you can keep the side shuttering on for certain a long period of time. If,
you think that you can’t do curing. So, this will ensure that, there is no loss of
moisture from those surfaces. But remember, you are only blocking you know evaporation
loss, you are preventing loss due to evaporation. What about self desiccation? That water will
still be consumed by the concrete internally. So, relative humidity will not be 100 percent,
it will not be as good as spraying water or replenishing the water, but it would be better
than not having anything right. So, you can retain the form, cover the concrete. And there
are something called membrane curing. Now you see membrane curing is a again, where
you have lack of water also is also used in pre cast industry, pre cast industry like
a metro construction that was there in that is going on or that was going on in Delhi.
And in their pre-cast elements they were using this. So, many in similar pre-cast situation,
one can use this membrane curing. So, it could be either spray or hand applied; that is with
through brush, some sort of a coating like thing, usually could be resins or waxes, which
will prevent loss of moisture from surfaces. So, 1 has to be this are actually chemicals
or waxes or resins as I said, they are applied on to the surface of the concrete to prevent
your proportion less. But remember, they their temperature stability has to be seen, there
should not melt away or peel off from the surfaces, because of the effect of the temperature.
The curing compound as they are called which might be effective in let us say; a cold climate
of Europe may not be effective in tropical climate. So, 1 has to be careful about such
things, because tropical climate the temperature at the surface effective temperature, what
we call sole layer temperature, because the radiation coming on to the surface of the
concrete, in addition to the surrounding temperature can be as good as 60 degree centigrade.
So, in northern Indian, say dry summer season, it could be as good as that. So, in such situation,
if you are curing compound is not stable, it will not be effective at all. So, 1 has
to be careful and cautious about such curing methods 1 being applied, the compound has
to be chosen appropriately. So, membrane curing is other kind, it is also again not replenishing
the water, but essentially what it is doing; it is actually preventing water loss, where
you have no way you are not getting water, you know in dessert say. This could be a good
useful thing provided you have taken care of the situation of the temperature.
Second aspect is quickly you apply and leave it there. So, therefore, you can you know
you do not have to employ people for long term curing. So, that is an advantage quickly
you can apply and if you have taken care of it design properly, you have taken you know
taken sort of cushion for lack of strength development and so on properly, it can be
used. So, depending upon the situation this can be used this can find its solution. If
possible, there is nothing like better than moist curing. So, that is it.
Then you can have plastic sheet: covering polythene covers. You can use polythene covers
or something of that kind, in order to prevent loss of water. Where you have no water what
we do? We use either the membrane curing compounds or water proof plastic sheeting. So, that
is the other curing. Another way of curing is using steam because; from Le Chatelaines
principle; the chemical reaction this particular chemical reaction which is exothermic in nature,
is favored by high temperature condition. Now, steam is at higher temperature, when
cement reacts at a faster rate with water at higher temperature.
So, steam curing is again keeping it 100 percent saturated, relative humidity remains 100 percent
and ensuring the temperature itself is also high. So, that is what is steam curing. Now
that can be used if you want quick strength development, early strength development. You
want to release the moulds or you want to do pre stressing or similar situations, where
you want early strength development steam curing is used, because you can attain the
same strength early strength and by doing this curing. So, steam curing is used specifically
in certain places where you want early strength development right. So, we look into details
of steam curing a little bit more, because that process you should see. Now let us see what happens further effect
of moist curing. Now this is for cylinder compressive strength against water cement
ratio This side is water cement ratio and you can say; this is pounded now this is pounded,
you can see that, this is line this corresponds to dotted line corresponds to pounded, that
is, this 1 is pounded. And the thick line this 1 corresponds to the Hessian. Now pounded
is a continuous supplier pounded there is 100 percent water. Hessian means well it would
become dry and then you make it wet again. Now, it has been observed that, this result
is 1 day result, this is 3 days and this is 28 days result. The cylinder compresses strength
when clotted against water cement ratio, pounded tends to give you always higher strength than
the Hessian. And this difference increases with the age. So, 1 day age you see this difference
is relatively less and it is, but what happens when you have high water cement ratio, this
affects our relatively less pronounced. So, at high water cement ratio, this affects
our less pronounced, because after all very high water cement ratio, this is somewhere
around 0.42 or 4 3 may be somewhere around that kind of situations, well pounded at Hessian
tends to do the same, because there are you know like capillaries would be relatively
more and generally the effects of curing by pounded and Hessian methods and. So, we can
see that the pounded is better especially when you have low water cement ratio, but
anyway any case pounded means 100 percent equation you see. Hessian is quite good.
So, this idea one can use, commonly Hessian is used in vertical surfaces and also on horizontal
surfaces, but it is still whatever is possible one can do, both will give you equally good
results right. Now let see other methods. I mentioned to you curing by form work; covering
with membranes and you know all this would actually covering form work and covering with
the membranes etcetera, covering with you know this covering membranes form work, all
this actually stops evaporation loss, but do not replenish the moisture loss due to
self desiccation. So, they can be as good as the moist here right. So, that is I have
mentioned already and explained. Now, the code specifies the minimum number
of curing days. Code specifies minimum number of curing days, because as we shall see higher
the number of days of curing, you will have strength better strength development and also
segmentation would be its other criteria, also initially few days you must do curing,
otherwise drying shrinkages cracks might appear. So, based on these considerations the code
specifies number of days of curing. Now, it has been seen that, for segmentation
of the capillary, the amount the number of curing days required is a function of water
cement ratio as well. For example, with water cement ratio 0.4 when you have, because 0.4
means less water fill space in the beginning. So, less capillaries to start with you know,
because it is the water excess water which does not react. That forms capillaries. So,
we will come to this again and again, because this is very important from the point of view
of concrete. So, higher the water cement ratio more will be the capillaries. And therefore,
you require longer period of time to segment these capillaries.
So, you can see that, with water cement ratio 0.4, we need only 3 days for capillary segmentation,
where as with water cement ratio 0.7 you need about 1 year of curing to segment this capillary,
because you have large amount of capillary when the water cement ratio is so high. When
your water cement ratio is so small like 0.4, in fact, below 0.4, you would require very
little, because there is an capillary pores would tend to be 0 at much lower water cement
ratios. So, we can see, theoretically will do like this you can do like this actually.
So, segmentation refer curing days for segmentation, would depend upon the water cement ratio as
well right. But then, you can’t you know develop practices or recommend practices based
on this kind of details, the code give you simple formulae or or as per IS 456 Indian
standard code of practice IS 456 2000, it simply gives you number of days required for
of curing. So, that is it. And then, initial information 1 more thing;
when we talked of cement, we talked of some other you know when just introducing concrete,
I said there are admixtures and the admixtures of the kind of chemical admixtures. And the
other kinds of admixtures are the mineral admixtures. Now this is mineral admixtures
we call them pozzalonas, perhaps this was mentioned earlier. We will come to this again
in relatively more detail. Now, this pozzolanas are actually pozzolana,
this are materials silica essentially Si O2 silica, which can react with lime forming
same product as cement with water, similar not same, similar product as cement and water.
Now, this reaction; however, is relatively slow. This reaction is relatively slow compared
to that of cement and water. And not only that, when you some amount of this material
is used in conjunction with some amount of cement and then add water to it, cement itself
produces some amount of lime, so its chemical reaction. Again this will be discussed we
will talk about it. And this lime can react with the pozzolana.
Now this reaction is a slow process, so this requires longer time more number of curing
days for, this requires more number of curing days for strength development. And therefore,
our code suggests following values; 7 days for ordinary Portland cement no blending,
10 days for blended cement; that means, where you have used pozzolana together with the
ordinary Portland cement clinker or you have used mineral pozzolanic admixtures, but it
does something more, it says if it is a dry and hot condition, corresponding periods will
be 10 and 14, because you know evaporation rate will be higher and it gives you higher.
Now British code of course, goes in a slightly more elaborate way, but does not matter I
think we are concerned about the Indian codes, other codes some other codes, we will give
some formulae etcetera all. That is not our consent at the moment, but Indian code does
it in this manner. So, that is how based on the need of curing the codes to its equation. Now let us look at high temperature situation,
high temperature curing. You see if I subject concrete to high temperature, that is concrete
cast and cured at the indicated temperature, as you can see That is let us say at 30 degrees
it is cast and cured for 1 day right, this is 30 degree centigrade right, its cast at
30 degree centigrade and cured for 1 day is that is shown. Its cast at 50 degree centigrade
and cured for 1 day, you get this line. Same concrete at 3 days, you will get this strength,
7 days and 20 days. Now you cast at 20 degree centigrade, this
is 10, 20 would be somewhere here. And you will find that the strength here is nearly
the maximum. So, if you cast a concrete at higher temperature, I mean cast and cure at
higher temperature, especially cure of course, the casting has also the effect. You do not
really recommend casting of concrete at high temperature, similar reasons basically. So,
cast and cure the concrete at high temperature, what happens is; initial strength is high,
but later strength is low up to 20 degree centigrade or around 20 23 centigrade perhaps,
this strength is relatively high, beyond that if you increase the temperature 12, it does
not show you high strength at 28 days. And if long term, in other words you can conclude
from this that, if you want high early strength, you can go for high temperature, but you want
high long term strength high temperature is no good may be up to 23 degree centigrade
20 23 degree centigrade is good. So, high early strength if you desire, then you can
do high temperature curing, but at the cost of long term strength. This is, because the
hydration process you know which is a chemical reaction and favored by high temperature;
it is non uniform when you do at high temperature, in fact, this effectively very pronounced
if you cast the concrete at high temperature. Curing after few hours of casting at higher
temperature still is better than casting and curing at the same temperature.
So, you know this gives you the composite strength in MPa as same here. So, concrete
should be cured at high temperature, casting of course, beyond a particular temperature
you do not really know but recommends that, you know this ways high temperature high temperature
concreting is done, but temperature is usually controlled. But curing can be at high temperature
to attain high early strength. But long term strength it may not be as good. So, this is
where the role of steam curing comes. You know steam curing gives you high early strength. A formula is there which is called maturity
right So, when its being absorbed that maturity is defined as sum total of temperature into
the product of the time for which the temperature was actually maintained. For example, if you
have 2 hours at 20 degree centigrade. So, 20 into 2 plus say 8 hours at 80 degree centigrade
8 into 80, that is total mature of the concrete which has been subjected to 2 hours of 20
degree centigrade in the initial period, then 80 degree centigrade for next 10 hours or
whatever it is. So, this assumption is made that, if you have
such concrete with equal maturity, they will develop same strength. This strength is a
function of maturity and that is what a curve shown through this a sort of curve, you see
this side is a maturity log scale degree centigrade days and that side is composite 1. So, what
you assume; if you have the same maturity, you will have the same strength. It has been
observed that curve is something like this and this is log of maturity versus composite
strength, there is a kind of linear relationship follows beyond certain point with the maturity.
So, that is the idea about steam curing. Now this maturity is measured as T about minus
11 degree centigrade, datum is taken as minus 11 degree centigrade, because below that concrete
will never hydrate, the cement will never hydrate and strength of concrete development,
you know strength development of concrete will not take place. So, this is the concept
of maturity. And based on this actually steam curing is done right. And as I said that initially
we do not prefer. You know it is this can this diagram show
you what happens steam cured concrete at water to cement ratio is 0.5, curing will take immediately
after casting. And again if you see this it will tell us that, this is the, this is at
23 degree centigrade, this is at 91, this is at 85 etc, you know this curve is
for 85, corresponding temperatures are shown here and this is at 21. So, this goes on increasing,
this strength goes on and this is the age 72 hours. So, its 3 days strength actually
we are looking at and this is at 54 degree centigrade etc.
Now, if you go on increasing the temperature; 85 degree centigrade 91, so beyond a point
actually when the 3 days starts coming down. So, very high temperature again is not desirable.
This is the effect of steam curing on water cement ratio point 0.5 and this is applied
immediately after casting. So, this sort of several results are available in literature,
it has done by various kind of people. So, the idea followed from this is of course,
the steam curing cycle that is been recommended in many places, is something like this. First 2 hours you do not touch, because if
you heat up the concrete at that period of time the hours strength development and as
well as other properties of concrete performance of concrete in the long term, will be affected
totally. So, first delay period, first about 1 hours about 2 and half hours, you know even
3 to 4 hours you have a delay period, then of course, you heat it up through using steam
right and temperature increases. So, this is the heating period; steam is circulated
usually through pipe or even through a cover, you just put a cover and steam is just circulated
through pipes within that cover even, people will start the link covering the element in
pre-cast industry especially. And then supply steam just below the turbulent. So, it will
just steam is just passed on to the surface of the concrete. So, this is heating period;
temperature increases, maintain this period maximum temperature period for about 7 8 hours.
You see for example, 5 to it would be around 7 hours, that 7 and 7 to about 20. So, about
13 hours, this has been maintained it could be different; depending upon the situation
should be different the maximum temperature period.
And then you allow it to natural decrement. So, naturally cools of course, it will rarely
come to 20, it will be asymptotically going down to 20 over a long period of time, but
it will require at least 7 8 hours to come down to close to the ambient temperature.
So, based on the fact that, if you use too much of high temperature during the early
stage of concrete is long term properties are affected, long term performances in not
good if you heat it up. And but, if you want high early strength, then you can use steam
curing. So, therefore, high early strength supposing
you want to release the mould or you want to do prestressing; steam curing can be good
solution. You want to place the pre-cast element right on to the you know erect it straight
away, very early in 3 days or 4 days time or release its prestressing release it from
prestressing bed. So, such situation where you want high early strength development steam
curing is a good solution. In normal courses it should not be it is costly also, your investment
would be there; boilers and steam generations system, supply of the steam there lot of losses.
So, all those part is there. So, therefore, steam curing is adapted where
you want high early strength and this could be the typical regime. Remember if you do
high temperature curing, this can affect the long term performances right. So, that is
all about curing. .
Let us look at the next part of our topic, that is, pre-fabrication. Prefabrication refers
to production away from actual site of construction, which refers to production away from actual
site of construction right. And idea is that, you cast in 1 place and use it elsewhere and
use it and this and can be used elsewhere right, you can use it elsewhere. What are
the advantages? Advantages of pre-cast concrete, you see parallel activities first of all;
that means, you do the casting, when you are doing the casting of let us say in case of
a bridge, when you are doing the casting of the deck portion, those could be beams or
segments or whatever it is. Advantages of pre-cast concrete, you see parallel
activities first of all; that means, you do the casting, when you are doing the casting
of let us say in case of a bridge, when you are doing the casting of the deck portion,
those could be beams or segments or whatever in casting what will happen, we will cast
the foundation and then you can only cats the super structure, you know substructure
cast the foundation, then the substructure you know like the peer or something like that
in case you have bridge. And then top deck after that, so it is sequential.
There is a technological sequence, but in pre-cast you can do parallel activities. So,
that is the biggest advantages. So, that can be very fast. It can be very fast, it can
be very fast that is the first thing right. It is used in buildings industrialized buildings
you can use them. For example, you can have all pre-cast products, just bring them and
assemble them like steel or similar sort of material. So, reduce time very fast construction
can go for. Factory like situation casting of concrete is done in factory like situation.
Therefore, you can have really better quality control. All quality control procedure you
can adapt and therefore it can have much better quality of concrete production.
Then experience gained by the work force through replicative you know repetitive works. So,
you repeat the same works and time again and; that means, that experience is gained that,
after some time it is all set, it is factory like. So, produced quality would be much better
everything is set. So, that is what is another element is. Standardization is possible, you
can standardize things, standardize the size standardize the sizes right. Or rather you
should use it where standardization is actually possible.
So, see buildings are not necessarily say look alike, but if you can standardize on
certain things, it would be the advantageous, big advantages. And they are something called
modular coordination can realized, which I will just mention later on. It may be economical
in the long run, because you are producing same thing again and again, so it can be economically
on the long run, you can take technologically advanced industrialized construction practices
and this is realty in many countries and the western developed wall and robotics and similar
sort of advanced technologies can be used in case of pre-cast construction practices
right. Just this is an important issue, modular coordination we can actually apply here. Modular coordination is nothing, but it is
a set of dimension or rules and it reduces the variability of dimension of building components.
So, what you do you have a basic unit, basic module and everything is repetitive. So, your
layout will also be modular layout so that, you can use one part one component, you know
you can use interchangeably the components building components.
For example, if the beam sizes are modular sizes, you can put in the modular grids. So,
that will become as possible, usually you have got an unit which is called modular size
and it helps in industrialization. Industrial building construction relies you know relies
heavily on modular construction. So, everything can be standardized really, dimensions and
things like that well. So, it can change to all kind of layout design easily, you should
you know modular coordination does this. Usually M 100 millimeter is used as the basic unit
in some cases. Now let us look at of disadvantages of the pre-cast construction well. If it is a building, then business are sustainability is market dependent. So, the market likes
your product. There are examples where pre-cast factories have been closed down, because the
market did not accept it. They can introduce monotony because; everything is same replicates,
several times you producing the same thing. So, they may not you know they may not look
aesthetically nice especially in buildings, but architect architects can take care of
this by various means; there are several other advantages of using this. So, if properly
planned and designed the monotony can be 1 can get rid of, but they can introduce.
Third disadvantage is not been actually listed here; is the cost, you have initial high cost.
So, there is a breakeven point beyond which actually it will become cheaper, up to certain
point it is never cheaper. The plants types are permanent plant, field
plant and fabrication on a building site. Permanent plant is 1 particularly used in
building, which will sell components; valves, roof system, hollow core roof system, columns
linear systems and then 2 dimensional systems. So, there are varieties of system. So, permanent
plant would be producing this 1, which could be sold and erected at site. Field plants
are used let us say for large bridges, sometime you can have a field plant and this would
produce temporary for may be 4 5 bridges together. So, 1 after another it could be you know field
plant is of that kind, which is not totally permanent, but it is quite for some time it
is permanent. And sometime for a single may be a state or something of that kind, a building
site it is for project based 1 project the plant is there and then despentary. There
are 3 main groups of activities in a prefabrication plant; main production activity; that is main
concreting activity, auxiliary production activities and managerial technical and general
support. Now, let us look at these activity situations.
See this line shows the basically main activities; that is concrete production right, casting
of elements, mixing of concrete, then preparation you know and transport basically inspections
etcetera, then it is going out. So, this slide denotes mixing, calling of concrete, casting
and then calling of the elements then they are inspection and any minor repairs required
and then of course, calling of elements storage and then it goes off.
So, this is the main activity of the casting year or a general plan. Now you can see, these
are maintenance materials testing, these are auxiliary unit, and then you have got preparation
of the moulds or reinforcement etcetera, adaptation of moulds. So, these are the you know if you
remember the previous 1; we said this is the main activity, this is the auxiliary activities
and this also is also auxiliary activities, maintenance of equipment etcetera facilities
as it is shown here. And this is the supporting activities like engineering production, planning,
accounting etcetera. So, 3 levels we mentioned earlier, these are
the 3 levels in a general situation of pre-cast you know pre-fabrication casting or pre fabrication
plant right, so main functions of the pre fabrication plant is shown here. The capacity depend upon there are several
sequence for example, mixing capacity, capacity of casting, capacity of holing concrete, capacity
of holing elements. All this the overall capacity of the system would depend upon the minimum
of all, you know if Q 1 Q 2 Q 3 etcetera are the capacity of each individual components
which are in sequence, the minimum will be the overall capacity governed by the minimum.
The best situation will be when Q 1 is equals to Q 2 equals to Q 3 and all equals to Q.
There are 2 types of basic system. So, 1 is called stationary mould, other is called movable
moulds. See this is stationary mould. So, my moulds
are stationary, as you can see these are stationary mould situations, these are moulds are stationary,
this are stationary moulds stationary mould and there is a overhead crane. So, the casting
is done here on to the mould, the crane takes it out and then shifts it. So, this is called
stationary mould. This is movable mould, where it is like an assembly line, assembly line
of a car or something of this kind, where this is you know work station 1, work station
2 etcetera like this. So, the same mould moves along this direction and then finally, the
for example, here the mould moves, then may be the reinforcement is placed, next it moves.
So, it is on a either on a rail supported somewhere supported, basically will get rail
supported, so that the mould can move. So, as it moves at a given speed, here the
reinforcement is placed, the concreting is possibly done, the clear repair inspection
etcetera and it is a curing and then it is actually heat, you know could be heat curing,
here its steam curing is very suitable thing, ideal thing. So, heat curing is done, then
any observable anything inspection etcetera and then it goes out. So, this is this are
the other kind of principle that is possible right. But then whoever moulds when you are doing
as a fast process that is a very fast process, highly industrialized situations, there you
can apply your planning has to be good, engineering has to be really good right. Commonly used
for building parts; industrialized building construction, high tech building constructions.
Now, you see there is 1 problem, since they are all in single line, so if there is a disruption
in any point, whole system can get stopped. So, therefore, you have station, you know
you have stations you have stations like this stations, where if there is a problem, you
shift it out, shift it out everywhere you play have station. Similarly, if there are
other arrangements. So, various kinds of there may be 1 in between sort of a dummy station
in between. So, if there is problem, it can be shifted other 1 can go on while this is
rectified and so on so forth. So, there are various ways to tackle this,
different arrangement for movable moulds right can have continuous work stations, then provisions
for reserved moulds, the second 1 is the reserved moulds, third 1 is the provision for side
tracking, so we have side tracking, fourth 1 is multipurpose station. So, many things
can be done. So, this is taken care of by you know this problem of breakage topages
of work, due to breakdown is taken care of by this sort of arrangement right. The elements handlings are generally done
by mountain mounted gentry; portal cranes, wheel mounted, ported crane gantry crane,
overhead cranes, tower cranes and mobile cranes etcetera. They are the basic ideas. So, where they can be applied? Industrialized
building construction, now this is 1 place where it has found its big use not in India
of course, at the moment, but you can produce it at a very fast rate, many countries even
in countries like Israel or say in northern Europe Sweden, they have they have industrialized
building constructions. So, it is very fast, it if the concrete has to compete with the
steel, because steel construction is very popular in many places, because you can do
it just very quickly and erection is very easy, you have all pieces brought together
and just erect them, weld them or do whatever it is .
Now, if similar system can be replicated by pre cast concrete industry. And with the advent
of high strength concrete systems and other development in concrete, this is a very useful
thing, this can make things cheaper very cheap in the long run and the production process
can be very fast, I mean the erection process can be very fast. So, you can get your house
built in may be 1 day or in a 18 storey building built in a month or 1. There are other methods,
but this is also this is a bigger method. Bridges has been used here also in India right
from 1970s, segmental construction of bridges piece you bring in cast in and casting here,
bring them up at the site, erect them and join them by pre-stressing or various means.
There can be long line short line casting. In short line casting you can cast 1 element
and then cast the next element by the side of it, what is known as match casting and
then remove that 1, but in long line, say complete 1 span, you have moulds placed in
you know the mould placed for the whole span and then each elements are cast 1 after another
together. So, that is what it is long line casting.
So, bridges this has been used in India, I know a Ramjhula bridge over Ganga is recent
bridge and is long line casting, short line casting has been used in many Ganga bridge
in Patna is 1 of them, many in Delhi, Delhi flyovers many of them are segmental construction
of this kind. Railway sleepers have been used for many years and many other miscellaneous
items, industries and sport complexes. So, these are the possible application of pre
cast industry, it has got big pre-cast concrete, you know pre-fabrication in construction.
If, the concrete construction is engineered fully, then this would be very useful thing,
but 1 has to remember that initial cost could be high, but after a number of them, it will
break even, it will generally break even after a number of them. So, therefore, advantage
is a very much there and in this 1, 1 can take advantage of the hi-technology in the
concrete material science like high performance concrete, because they requires very good
quality control, over other kind of development in cement based composite, other kind of developments in cement based composites etc1 can actually take advantage of this situations
right. There are of course, things like sport complexes built in the country like Kanteerava
stadium in Bangalore, this is made of pre-cast elements and dome structures its beams etcetera
those are pre-cast. So, this has been already in practice, industry
has building that is yet to come, but they have they big advantage in terms of the first
construction. So, that is about prefabrication and curing. With this we generally conclude
to summarize the whole thing. We looked into the curing, the methods, the need; the methods.
And we have looked into ill-effects of not doing proper curing. Also we looked into steam
curing, then we have looked into pre-fabrication; basically what are the types of plants and
advantages and disadvantages. I think with this we will conclude.