Hi, this is the fourth lecture in the module on surface preparation and protective treatments and in this lecture we will focus on placement of repair materials and curing. In the previous 3 lectures we looked at how to prepare the surface and of both concrete and steel and then we also looked at anti corrosive or zinc coatings and then looked at sacrificial anode cathodic protection, we call it as SACP. And then in the last lecture, we talked about ICCP, then electrochemical re- alkalization, and electrochemical chloride extraction. And in today’s lecture we will be looking at placement of repair materials and curing. In other words, how to place the repair material after cleaning the concrete surface and after cleaning the reinforcement, then you have to place them and if you are doing some treatment like cathodic protection after installation of those anodes etc. How to place the repair material to cover the substrate and also looking at why curing is very important when we talk about the durability of the repairs. So, curing is very important.
Now, this is how to place these repair materials. So, we are going to talk about these 7 different techniques in this lecture. So, basic thing is, use of trowels to place repair materials, then also dry packing, then form and cast in place, then form and pump concrete, then pre placed aggregate concrete, dry mix shotcrete and wet mix shotcrete, these are the techniques we are going to talk about today. Again, very briefly we will show you some examples of this and how these things
Now for Trowel applications, repair material should be trowelable or it should be wet enough, flowable enough, but not too flowable also and it should have good consistency, in other words it should not sag. See the picture on the top right, they are using a perfect tool like you can see, if you want a 90 degree edge, the trowel itself is made of a metallic plate which is bent 90 degrees, you can get a fine finish. Why I put this picture is it is very important to show the importance of the tools which we use, you have to use right tools. That way the final product or the finishing will be very good. I will show one more slide on this with different types of tools later.
And other thing is, the picture on the bottom right, it shows, you take the material on a trowel and then you are placing it here, it should not fall off to the ground. If you are talking about a roof, which used to be a practice earlier, actually a couple of decades ago, if any concrete roof, first thing is after removal of the formwork they used to plaster the roof of the bottom surface of the roof. So it’s not very easy to do and the mason should be highly skilled to do that also. So they place this or literally throw the mortar onto the roof surface or the bottom side surface and
then the mortar should stick there, it should not fall off that is why the property of the mortar is also very important. So, it should be trowelable, it should be cohesive enough and it should also stick to the substrate concrete that means it should be having good adhesive properties also. So, fine grained materials, non-sag properties means when we apply this to the roof surface if it just falls off, then it is not really going to be a good repair that means, if it falls off, there will be some debonding or the adhesion is not good that is why it is falling off. And in some times it could be just because of cohesionlessness also, in such case it will fail in this right inside the new material which is provided. So, both cohesion and adhesion are important properties when you talk about placing of these kind of materials onto a surface where, which is horizontal and then where gravity comes into action.
Now, these are other examples where on the column on the left side, you can see, again, a smooth finish is required. And on the right side, you can see it is on ground. So, all you need to do is place it and then trowel it well. So, for example, a new floor if you are trying to trowel it immediately after placing the concrete, it will not work very well because then it will actually pull the moisture also along with your trowel. So sometimes masons they wait for some time for it to get dried enough so that it becomes trowelable mortar, or concrete. So these things are important to note when you go for it.
This is a set of photographs showing different types of trowels. If you want to talk about different shapes for specific applications, see, most of the time we don’t invest much time on selecting a good tool. But if you select a good tool, it will really save a lot of time in the finishing process. And at the same time, at the end of the work, you get a very good finish. So I request like, it is very important for us to look for good tools. Also may be you can say that it is not available in the market, but, you also have to create a demand for different things in the market.
So you have to ask, if you want a 90 degree trowel, which has that 90 degree as it is shown on the picture here., you have to ask the suppliers and then they will eventually procure those type of tools and then eventually our goal is all these things should be easily available in the market, so that we tend to use good tools. And it is very important, using good tools and specific tools for specific applications.
Now, dry packing method, as is shown in the picture on the right side, you can see this dark color region is the repair material which are putting at the bottom of this girder section. In dry packing basically, you have a very strong formwork which is attached to the substrate concrete. And then you basically hammer the relatively dry mix. It is not really a flowable material, because imagine if you hammer this on to a flowable material, the material will just skews out.
So, it has to be dry enough so that the material does not skew out like this. Otherwise, if you have a flowable material, it just skews out and your hammer will go in, that does not mean that we will pack so you need relatively dry material to pack it that is very important. And the mortar with proper consistency capable of being molded, you check whether it is packable or not, you take it to mortar and then make a ball out of that in hand, if you are able to make a ball, then it is
probably good enough to be able to pack it like as shown in the picture, either you push it into by hand or by a hammer or another tools available.
And where these kinds of things are are applicable. You can see on the left side, if you have some small holes or took a core and then you want to pack that core back in. And as it is shown here, it is an end of a tendon system, again you can see these circular regions where actually use the dry packing is a very good idea. Also when you talk about dry packing, typically the water binder ratio of that material will be relatively less and that will also ensure that it is a non- shrinking material. Because you want that once it is packed it should not shrink. So, typically with non-shrink grout or a relatively dry mortar, we can actually go for this packing. Again when I say non-shrink grout, but still you can hammer it depending on the depth to which you have to fill. So, these are some examples on where we can apply this dry packing. Whichever structure, if it is a vertical element, if you take a flowable mortar or a very flowable concrete, you cannot
pour it into that vertical element, I mean for example, in that picture on the left side, if you have a hole which is horizontal, if you pour the material will just flow out. So, in such cases you have to use relatively dry material and even if it is non-shrink grout material you can fill it and provided a port or something on that so that it does not flow out. But anyway the point is wherever there is a possibility of flow of the material because of gravity action, you can use this dry pack technique to fill the cavity or void.
And another example here underside of a girder, if you make very flowable material, it may not fill very well. It will just flow out it will be very difficult to even finish the repair work. So, people generally go with dry material here, which will have good adhesion and cohesion. So, by hand also you can pack it, pack the spaces.
Now, when you do this dry packing, one very important thing to note down is that it should be completely packing the space around the steel reinforcement. If it does not happen, let us say you are just superficially placing the material over there instead of really packing it, what will happen is as you see here on the picture, on the right side, you can see there is an air void or a space which is left behind the rebar or the undercut region. So, that space also should be filled by the
mortar so that the steel gets a uniform chemical environment around the surface. So, in this particular case some portion of the steel is exposed to air whereas the remaining portion is exposed to the repair material, but ideally what we should do is this region also should be filled with mortar. So, there should be no air void as it is shown or marked right now.
Now, there is another technique is form and pump concrete. Form which helps in defining the space or how much area should be filled up in what shape and then a pump is a technique by which you place the concrete. So, as you see in this picture here you have a concrete pump which fills or pumps the concrete into the formwork region, you can see the formwork region here.
Also you notice an outlet is very much required, an outlet is kept on the bottom portion of the space which needs to be filled and also you see an inlet of the concrete which comes out is at the bottom right. So whenever you talk about pumping, it is always good to go for an inlet which is placed at the bottom portion and let the air pressure or the pump pressure push the concrete from bottom up, that is how the idea. In this case you can see the concrete is flowing this way from
right corner to the left and it is displacing the air which is present and then air comes out through like this. So, finally, the entire the cavity or predefined spaces filled with repair material.
Again, the concrete or the repair material should be pumpable. What do you mean by pumpable is mainly it should be segregation resistant and it should flow well without much of pressure requirement. So, this is very important, because the material should not segregate if it is segregated, it will get choked or clogged inside the pump itself. Good flow characteristics, self- bonding, it should be cohesive and aggregate size must be smaller.
So, when you talk about repair there will be undercutting and the space behind the rebar also will be very limited, in such cases you should ensure that the size of the aggregate is relatively or is smaller than the clear spacing between the reinforcements and concrete so that you get an integral concrete.
This is an example or an image showing similar thing where if you are talking about a long beam or something, it is the side view of the similar beam, if you can think you can also say here, because all the time you may not be able to provide a outlet at the top, depending on the direction in which it flows. So, in this case, if I am saying a long beam or a girder where I start pumping in and let the concrete flow this way and until the concrete reaches here, then I will close this portion and then start pumping here. So, when you have a long sections available, you can fill
from one end to the other by moving the outlets because otherwise it creates layers which eventually does not help in filling the portion above the rebar.
So these are some of the intricacies associated so, multiple inlets might need to be designed. Depending on the size of the repair, dimensions of the repair work you are talking, you have to decide what type of formwork is required and how it can be pumped efficiently. How the material can be pumped into the cavity efficiently. When I say efficiently it also means that effectively because you have to really fill the space behind the rebar also. So, both efficiency of the entire work and effectiveness of the work must be considered.
Now, these are again some more examples showing where this kind of pumping is adopted, especially when you talk about column jacketing or enlarging the size of the column there it is widely used. And this is again a micro concrete which is being pumped as an overlay or a screed concrete or any such applications wherever micro concrete is used, you have a funnel type setup here or a tank for the material which is then pumped into the space where it need to be filled.
This is another example showing column jacketing work where again it can see here the people are actually standing on floating platforms and it is not that easy to work environment, so, you create a encasement and then you pump the concrete into the space in between the existing column and the formwork. And you can see here, this is the formwork made out of wood, and you have a concrete column and then the space in between is filled with the pump, you can see this person here holding the hose, concrete hose. Why I am showing all these pictures to you is to
tell you that these kinds of things are widely used in construction and there are a lot of challenges in construction. It is not just getting any concrete, getting any pump and you pump, it will not work. You have to really look at pump pressure, how the quality of the material, the flow characteristics of the material, and what is the dimension of the area which needs to be filled all these have to be looked at so, you really need to engineer the materials, adopt good construction practices, and select good tools.
In the previous slide I showed about square columns actually, but we also see a lot of rectangular columns in our structures. I just wanted to show because this is not something which is widely used in our country, when you talk about circular columns. Nowadays, these type of column formwork which are made out of cardboard are available, and it they even come in with more than a feet in diameter. So, once you put this then you can place the concrete. One typical example where you can use this is a electric lamp post or wherever you need a pedestal, you need a circular concrete element there, and also for columns. So wherever you think of a cylindrical shape, you can use this type of formwork. You place it and place the concrete inside and remove the paper cardboard just after the concrete get sufficient curing. So, as you see on the picture on the right side, you can easily remove this thing and it is a very environment friendly product also because it is just cardboard.
Now, these are some examples to show when you talk about repair or even construction of new con columns. What are the types of form works which can be used, on bottom left picture you can see one type of setup where there is a slit and at which you can open the formwork, you can see and it is two pieces. So, after the construction you can remove that, also if you want a specific shape of your column like some kind of architectural design, like you see here the grooves and etc. then you can make your form or mold your formwork accordingly and then
make it. It is because otherwise what you will be doing in this particular case is you will make a cylindrical column and then ask another person to come and make those grooves which are going to be more time consuming, but if you have a, mold which makes that shape inside then it is much easier. So, in the long run if you have large number of columns like this to be made, these kind of tools or this specially designed formworks will be of very good use in completing a good job in short period of time and get away good finish at the end, that is also very important getting a good finish. And also other pictures, you can see here that again a two piece column formwork and you can see how you connect these pieces like a lock and which will form something like this. So, you can see here there are a lock here and another lock here. So, these kind of things are in a form work can be utilized to speed up the work and at the same time get good finish of the concrete surface because in circular columns, you cannot really rely on a mason to get a perfect circle and also is it is very difficult and time consuming process. But if you adopt these kinds of tools, or this kind of specially designed formwork, you can save a lot of time.
If you are talking about a jacketing then also you can adopt this kind of formworks for repair. Make this kind of formwork and then fill the repair material into the space between the form and the existing columns, as I showed on the bottom left.
I am showing you all these, so that you can think about these different ideas and then try to implement such things in the construction projects.
Now, form and cast in place, in previous slide we were talking about pumped concrete and here we are talking about cast in place, in other words, you are basically pouring or placing the concrete in the formwork. As you see here, the gray color is substrate and the dark color is the repair material and then here is the cavity and then you can see the formwork made like this so that you provide sufficient space to flow, there is also a dotted line here that indicates that some other portions of the same column because the shape has to be well defined. So, wherever the you are placing the concrete only that portion need to be deviated like towards the left, as you see on the for sketch. So, the remaining portion can still form the same shape as it is required. Wherever you need basically some openings at an interval of let us say 1 meter or 2 meter depending on the case, you can make an opening through which you can place the concrete and remaining portions can have this continuous formwork like this. So, again material should be workable, once you pour or place the concrete it should flow and reach and fill all the cavities that is very important. If it is not having good flow again you will see that a lot of pockets here and there will be formed which is again not a good repair.
This is again examples to show, you can see here, this is being floor, poured into this, but again you can see here this portion is actually having a flat surface which is flushed with this surface. And you can see a conical thing which is like a funnel type setup where through which the concrete is poured into and this region.
So I am going to show you the previous slide this is this is the funnel part and this is the other surface, surface flushed with concrete surface.
Now let us look at what is the effect of this curing on compressive strength. So, as you see here, the blue curve here is a moist cured condition, moist curing for the entire time, the time goes up to 1 year on this graph. So, you can see that even after 90 days up to one year, there is a slowincrease in the compressive strength if you continue to cure, which I am not saying it is practical all the time, but just for our understanding of the importance of curing. And then if we cure for 28 days, 28 days of moist curing followed by exposure to dry air, then you get this red curve which we can say a compressive strength of 43 MPa. If it is 7 days of moist curing followed by air curing, then you get about 38 MPa. And if you don’t cure at all, if you did not have moist curing, in other words if you leave it for direct exposure to air immediately after the construction, you get only 30 MPa. So, you can see that there is about 15 MPa difference from no curing to 28 day curing. So, definitely curing influences the strength, final strength of the concrete and more
than this it influences durability significantly, if you did not cure the concrete very well the near surface of the concrete will be very porous which will influence the durability of concrete.
So, I am going to show you some examples of poor curing practice. See in the two slides ago I showed this is wet burlap or ways by which you cover the concrete so that the concrete surface, the near surface gets sufficient water for continued hydration. So, here you can see as probably prescribed in the contract, there might be wet burlap like this. Yes, that is wet agree. But what about this concrete here, it does not get any sufficient moisture for continued hydration. And if we think very carefully, this particular bridge, the bottom portion was constructed earlier than the top portion. So, at any point of time if the practice is very good, the latest portion of the concrete should be having a proper curing in other words the sack should be at the top also. Now you go to the right picture you can see that the sack is at the top but it is unfortunately a dry sack. So, again when I say in the two slides earlier I said wet burlap that means this sack should be wet. It is not just to provide a shade from sunlight, but it is to provide sufficient moisture for the cover
concrete and you can see here construction is still going on. It is not that these columns were cast way early and then its curing period is already over, no. I myself took this image. So I know what is happening at that site. So many places, you will see that the process of curing neglected, if it is neglected, the full potential of the repair material will not come.
Now, these two pictures are about new construction, but the same technique or the same principles are equally valid for any repair which you do, because when you talk about repair, you are essentially talking about something which is the outside concrete or peripheral concrete. So, anything which is exposed to the atmosphere need to be very well cured and it need to have sufficient moisture and make sure that it is not just to provide a wet sack but provide them for sufficient duration and wet, not just dry.
There is another example where, again wet sacks are provided but not on the entire surface. As you see on the left side, this is a new highway being constructed and there are a lot of places on this concrete which is not getting sufficient moisture, like this point here, there are a lot of spaces in between the wet sacks. We should cover the entire concrete, so that the structure as a whole is fully durable otherwise what will happen is the portion which is in between these sacks, they will not have a very good durability and those little portions will degrade faster than the other portions which are very well cured. So, let us cure the concrete completely, entire surface should be cured not just here and there. On the right side is actually a railway platform where they put a new overlay, I took this picture also, and you can see that only some portions is repaired what about the remaining region on this concrete surface? So, the lesson here is cover the entire surface of the repair material. So, this is very important to note otherwise you will not really have long last repair, the repair will not be durable enough.
Now, to summarize we talked about different methods of placing repair materials. We talked about trowels, different types of trowels, dry packing, form and cast in place concreting and then form and pump concrete and then preplaced aggregate concrete and also talked about shortcreating with both dry and wet mixes. And towards the end, we talked about the importance of curing, how important it is to cure the repair material properly so that we get the full potential of that repair material and eventually that will help in ensuring that the repair is durable.
I think this is the references used for making this lecture. And thank you very much.
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