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Surface Preparation and Protective Treatments, Part I

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Hi, this is a module on surface preparation and protective treatments. In this, we will look at surface preparation methods for both concrete and steel. In the second lecture we will look at Anti-corrosive and or zinc coating and Sacrificial anode for cathodic protection. And then in the third lecture we will focus on impressed current cathodic protection and then electrochemical Re-alkalization, electrochemical chloride extraction. And also the placement of repair materials, curing and their importance. So, this is how the module is split. So, today we will look at surface preparation methods for concrete and steel.
 
We will look at what are the general surface preparation procedures and then later we will talk about different type of tools available for various cleaning and cutting and all that steps. Now, locate area to be repaired using hammer sounding or chain drag that is the step 1 and then you can see here whether you do some non-destructive testing or whatever way we talked about that in our previous lectures on delamination and how to detect delaminated area in concrete.
 
Then, the next step in the process is design and installing temporary shoring system. That also we talked earlier then comes the removal of the deteriorated concrete. So, as you see in this picture, you need to use a jackhammer or some kind of tool to remove and the bars which are damaged by the removal operation or have a significant section loss. So, in this process, let us say in the process of removal of the concrete, the rebar should not get damaged.
 
And in case if something like that happens whether you cut the rebar or something
whatever it is. If it happens, then you have to replace those rebar. For example, if I am cutting this region here, this part here. If I use a saw cut and then to cut the edge, in that process, if the rebars are cut, then definitely that rebar need to be replaced. Even if it is a partial cut, there will be stress concentration so you have to replace that rebar or better is to avoid such things.
 
Now, recommended geometry for defining the removal region or for defining from which location concrete should be removed and also for undercutting. So, if you are talking about a beam as you see in the top left, this is a sectional view and the right side is the elevation of the same beam, this portion has to be removed. In other words, this much region has to be removed, the hatched region has to be removed. And that ensures that the new concrete or the repair material will also go and fill in this space inside the stirrups that ensures better mechanical grip and the concrete or the new repair material will not fall downward.
 
 
So, this is the repair material. Now, if you are talking about an element something like this, this is the number 2, this is number 1, this is number 3. If you are talking about something like this a corner or something, then again you have to make sure that there is an under cutting process or in other words, the concrete from behind the stirrups or the rebars need to be removed.
 
In case of a slab again here we can see that the concrete below the reinforcement like this concrete here is removed so that the proper grip is provided. So, undercutting is very much important. This is an example showing, how undercutting can be done. You can see various applications where these are the rebars which were actually embedded and now completely the rebars are exposed and the concrete behind the rebar is also removed. And different tools are available for this to remove concrete like water jet. So, you can see this is a water jet here which is used to remove the concrete until the concrete behind the rebars are also removed.
 
Now, step 3 is preparing the surface boundaries to prevent further edged condition. Geometry of boundary should minimize edge length. So, you can see here saw cuts are used to cut the region.
 
Now, if I say how do we cut that, how to define this boundary? Let us say you have a patch repair but as you see in the left side it is an irregular shape. Let us say the concrete damaged area is irregular in shape. But when you talk about repair, if the shape is irregular that means, let us look at the bottom left, there are lot of regions where the shape is so irregular and there are lot of regions where stress concentration can happen.
 
And also the perimeter of this surface here in this case is much larger than if I actually make an assumption and cut something like this as you see in the bottom right. If the perimeter is very large, the amount of bonding material required also will be much higher, but if you make a cut like this L shape it looks neat. So, aesthetically also it will be pleasing to see and also we will need less bonding material and at the same time, there will be no or limited region where stress concentration can happen.
 
In this case, probably this region you may have stress concentration but again it is much limited as compared to the case on the left side. So, I am showing here 2 cases where more perimeters and this is the recommended layout with less perimeter and a well defined shape like a rectangle and an L shape. And also in this case if you follow the well defined space, the estimation of the amount of materials required to repair is also easy to calculate.
 
Because surface area I can define very well and amount of material required also can be defined very well which is good for both contractor and the client. And at the end, we get a nice or a very simple shape and also aesthetically pleasing.
 
Now, these are some of the examples where you can see in all these, there is a straight cut which you will see here also you can see it is cut in a very straight well defined manner. And here on the right side image also you can see a cut something like this is made in a well defined manner. So, this is how repair work should be done. Now when you make these cuts, make sure that you don’t cut the rebar as you see on the right side image.
 
If you cut here, if the cut is deep, then you may cut the rebar also that must be avoided. So that is something very important. So, the depth of the saw which you use is very important when you talk about this.
 
This is another example of a recommended layout of edge cutting for surface repair. As you see on the photograph on the left side, you can see some region, it is not that very well defined only some region it is spalled. But may be the corrosion is happening in the other region also and to avoid a future problem it is better in these cases to cut something like this. So, instead of something like this or defining the repair work something like this, we better go for something like this.
 
As you see on the right side where the repair region is much better defined, very simple one straight cut and you see the picture on the top right where such a repair has been done. In this case some fiber wrapping and lamination also installed. But the point here in this slide is just to tell you that the definition of the repair region make it as simple as possible. Uniform or the cover is at least some minimum number then up to that depth you can probably go for removing concrete up to that depth.
 
Now, this is also widely used tool in India we call it usually jack hammers. It is
essentially a hammer which keep on hitting the concrete heart and different capacity hammers are available and but again PPEs must be worn when we use this. And other thing is that it should not damage the reinforcing steel that is inside the concrete.
 
Now, when you talk about full depth removal of concrete, not just partial depth removal, then you might need a more powerful tools because you are talking about much deeper removal of concrete, deeper that means high volume also. And then usually these are all fitted on to this large construction equipment. As you see on the picture on the right side, you can see here there is a bit which is fitted on to the end of this backhoe here, this is another example.
Both are for removal of road structures and it is typically remote control because the amount of pressure you are talking is very high. And again, you should avoid bruising of the concrete, PPEs must be worn that is something which we have to really work on.
 
Now splitters here you can see usually the holes are drilled like this and then you insert something into the concrete and then you exert a lateral pressure which will split the concrete like you see here. It is splitting the concrete here also a crack is formed, something like that. So, you go deeper into and then provide lateral pressure either by a hydraulic wedge action or a fluid pressure or even some cement which you put inside.
 
And then let it expand which creates a lot of lateral pressure and then that is how we do this and in the sequence in which you apply is also very important. For example, in the case which I am showing here, if the slab is ending here then this could be the first hole which we need to do and then split it and then come here this is number 2 and then in that will remove this concrete.
 
Also rust remover, it is very easy to apply. The picture here is not about steel rebar in concrete but this is something which I found from the internet from the web but it just explains very well how it works. So, it is very easy to apply and then effective rust removal even from areas which are difficult to reach and penetrate.
It means, if you are using a mechanical tool, you cannot reach it, but even on the micro scale, if you look at the very fine pores on the steel surface, it can. Where the mechanical tools may not be effective in such case the chemical will actually penetrate and clean the entire surface as you see on the bottom right.
 
You can see this region is very well cleaned, there is not much rust left which is difficult to achieve with a mechanical tool and grayish liquid, which can so the it converts into a rusty surface will change to a grayish liquid and then which can be removed and this is very important after drying remove the loose rust particle by scrubbing so everything has to be removed.
 
And this is the most important part here wash the steel surface with water jet. Why
because these type of chemicals are essentially acidic in nature and you don’t want to leave any acidic residue on the steel surface. So, cleaning is very important when you are talking about rust removers they are not supposed to stay there on the steel surface for long period or after removing the rust they need not be there. So it has to be water washed very well. So that Future corrosion will not happen.
 
 
So, to summarize, we talked about surface preparation of both concrete and steel, it is very important to ensure durable repair and regular shape repairs last longer, because irregular shape there will be stress concentrations and it will be difficult the quality of the work will not be that good as compared to the simple shape and that also helps in estimating the amount of materials required.
 
And also aesthetically pleasing and undercutting beyond the depth that means the entire rebar surface should be exposed even the surface which you cannot see or behind the rebar that concrete also should be removed. So that, you get better mechanical grip and at the same time, the rebar will complete the full circle or the full surface of the rebar will be exposed to the same repair material.
 
So that, you don’t have any potential for corrosion cell to develop. In other words, no steel should be exposed to both the old or the substrate concrete and the repair material. It the entire steel should be covered by the repair material and at the ends or the edges you can provide cathodic protection to prevent halo effect, which we will discuss in the next lecture.
 
And then, the corroded steel also must be cleaned properly to achieve very strong bond it is not only the concrete but the steel surface also should be very well cleaned. And as much as possible the rust must be removed. And then specific tools can ensure better quality work and selection of tool is very important. We must think that if you get a proper tool, you will actually spend less time at the site.
 
Just imagine a case where you are nailing onto the wall, if you take a rock piece and then try to hammer it and in a second case, you take a proper hammer and hammer it. In which case if the work will be of good quality and definitely if you have a proper hammer, it will be much easier to do and it will be safer to do rather than using a rock piece with irregular shape because in practice we do all this things.
 
That is why I am telling so. This is the time for us to change the way we think, there should be a significant change in the mindset of the decision makers. Also to tell people on site to go for better quality tools and when you go for better quality tools your work will be easy to do more comfortable and it will take less time and at the end, the end product will also be very good and durable. And these are the references I used for this lecture. And thank you next lecture we will talk about corrosion, the treatment for the steel. How we can protect the steel from corrosion.
 
Thank you.