Corrosion in prestressed concrete

Today in this module on corrosion of embedded metal, we are going to cover corrosion in prestressed concrete systems which is most of the time it’s not there are something special about the corrosion mechanisms in prestressed concrete as compared to the conventional concrete systems Now, first I am going to list; show you very classical illustration of prestressing, you know, using some of stack of books you can see this is Professor Gustav Magnel’s demonstration He used to do it in class, you know, take a stack of books, you know, and then press it from the side so that you can carry more load So this is same principle applied for prestressed concrete systems also where, you know, you have prestressing And there are two types of prestressed concrete systems, one is pre-tensioned system and the other one is post-tensioned system The one on the left is a pre-tensioned system There are two types of prestressed concrete systems based on the time at which the strands are stressed if it is pretensioned concrete system then the strands are stressed before the concrete is cast In the post-tensioned concrete system, the strands are actually stressed after the concrete has achieved its strength or after the concrete has hardened So, one on the left which you might see these kind of bridges most of the flyovers in the city you might see this kind of system most of them are actually pre-tensioned concrete And this is a typical metro on the right side It’s a typical metro bridge or elevated, you know, metro system where you will mostly see post-tensioned segmental concrete construction This time I would also like to mention if you see an I-girder it is not necessary that it’s always like these kind of I-girders It’s not necessary that they are always post-tensioned sorry pre-tensioned but there could also be post-tensioned system Some of the photos in the remaining of the lecture you will see that concrete So, what is the corrosion protection strategy in the pretension concrete? Here you can see in this close up you have the black dots which are the strands and the grey represents the concrete The chloride penetrate through the concrete cover and reaches the steel strand through diffusion or the typical, you know, chloride ingress processes which are also present in the conventional reinforced concrete systems And then once the chloride builds up to a sufficient level at the steel surface or the right here you can say that the corrosion will initiate And the pitting corrosion on strands and deformed bars are of 2 types, ok or the type of corrosion which you see in the case of rebars and in the case of strands are little different especially the, you know, corrosion cracking patterns So, for example in the case of typical rebar or a conventional deformed bar like in this case what will happen is? You can see that some pits here and there But there are also region which is not pitted much but the point here is there is no space like in the case of strands It’s a single solid bar like this single piece and then the only surface which is corroding is actually in contact with the concrete So, whatever the rust is formed it will very fast exert expansive stresses on to the concrete nearby leading to cracking and then followed by, you know, the oozing out of this rust through the concrete cover and then you can see that brown stains on the concrete surface But in the case of prestressed concrete system it’s a little different what happens is? First the because of the stress the pitting can be very significant or very localized and you can see very severe pits whereas some region are not at all corroding or you know very limited corrosion But you can actually have severe pits which might lead to the fracture of that particular wire like you know if in this case this particular wire will fracture much earlier than the others which might lead to structural problems

And also on top of that whatever the rust is formed in the beginning they will occupy these small triangular kind shape spaces between the 7-wires Once this space is fully filled with rust only after that the strand or the rust will actually exert pressure on to the concrete under the surrounding concrete So that is the and because of this what will happen is? There may be prolonged corrosion before you actually see some brown stains or something So, visual observation is not really a good; it is not really a good idea when you talk about post tensioned systems So there could be in the case of strands there could be a delayed expensive stresses, cracking and manifestation of corrosion products Now and another thing which can happen in the case of post-tensioned system or prestressed concrete system is that as you see on this image here or if you take 1 but 1 pitch length of the strand that’s shown in this picture here This picture here or sketch you can see that this, all the 6 wires are actually getting corroded not only the wire in other words in 1 pitch all the wires reaches the tops are, I mean, are at the top surface and those portions are getting corroded So effectively what you can see is? You know, when you look at the cross sectional loss over a pitch length it is 6 times approximately 6 times the cross sectional loss on 1 wire So effectively you lose significantly as compared to a rebar If it is in the case of a rebar let’s say you have a rebar like this which is a solid bar only the top portion will be corroding and then this region will not be actually corroding and the picture on the bottom left you can see, severe pitting like this here You can see a pit form So severe pitting whereas some region is not at all, this region there is no corrosion at all Now this is how, you know, because of this there will be significant reduction in the structural capacity and that is a significant concern So actually in this case of prestressed concrete we don’t even want to get into the corrosion propagation phase In fact, we want to enhance or increase the duration of the initiation phase as much as possible That is the because once it starts corroding it becomes very dangerous in the case of prestressed concrete systems Now let’s look at what is the strand protection strategy or system in post-tensioned segmental concrete As you see in this sketch here you have several box girders or the segmental segments like in this picture you have 2 diaphragms and then 4 segments just to show but there will be more just to make it more clear to understand I have put it something like this And then you have tendons which run from 1 pillar to the other Typical simply supported systems will be something like this And when you look at the strand alone, you know, each of the tendon is made of, you know, about 19 or depending on the diameter of the tendon you will see that multiple strands are placed inside a duct; inside a plastic duct or high density polyethylene duct which is this one and then the space in between the strand and the duct is filled with the cementitious grout Now, this highly congested strand and that is where the challenge is, you know, how to get this space between the strand and the plastic duct filled with cementitious grout properly filled That’s the biggest challenge and then if it is not filled properly you can actually experience significant amount of corrosion or early corrosion of the system Now there are 2 types of tendon systems one is internal and another one is the external The internal is when the duct or the entire tendon is inside the concrete when the external means it is kept outside the concrete, okay Depending on various reasons designers might choose either to go with internal or external So here, it shows a profile, typical profile of tendons on the top drawing you can see it is a straight profile like this and then it goes like that In the bottom one you will see a curvature; curved profile mainly for as you see here it is a continuous beam system and in the top one typically when you have simply supported system you go for that kind of profile

Now, when you have this profile there are the strands will not stay at the center of the duct, okay When you pull the strands depending on the profile the strand will try to, you know, occupy a shortest distance and it will get a little bit eccentric to either one of the side of the duct as you see here on the left image if the strand is pulled then it is moving towards one direction as you see here both in both these tendons the strands are more congested in the top portion of the duct than at the bottom portion And in this case over here you can see that strands are pulled toward the bottom In this case also it is actually at the bottom Here it is at the top, you know, moving towards the top So it is not necessary that always the strands will be at the center So, because of this what is actually happening is? There is some region here like this as I am drawing where you have less congested region and the grout which you fill flow through that region and then it will reach the other end And then people generally conclude that if the grout is pumped from one end of the tendon and if it is reaching the other end it is actually completely filled But that is not the right conclusion to make because you have to really make sure that this congested region like this here is actually filled up that is very very important For achieving that we have to have a very good grout and also allow enough time for the grout to flow and best way is instead of pumping the grout from one end of the duct to the other end you pick the lowermost points as per the, you know, Federal Highway Administration this is the recommendation especially based this is based on a study conducted by Florida Department of Transportation So as you see here they pick the lowermost points on the profile and then try to pump in This is the inlet, pump the grout into the duct and then the grout is allowed to flow upward and through this vent it is supposed to come out and then vents here also So the filling from the bottom most point let the grout flow upward and that is how it is supposed that will actually ensure much better filling than filling from one end and letting the grout flow downward etc Now other thing is, you know, to prevent the void formation at the end or the anchorage you can also, you know, do a little bit to prevent the void formation and entry of moisture You can see that slight inclination of the tendon profile at the end can also so, for example if this is the end of the end anchorage zone if water is coming through like this it will never go upward like this But in this case you may see that water going like this So that is kind of this is not good and this is good practice to follow slight change in the profile Now, nowadays you will see that most of the, you know, duct sorry most of the tendon systems you have this cementitious cap like this, what you seeing on this What is happening is in this? Cement has an inherent property of absorbing moisture it is hydrophilic in nature So it will absorb the moisture and then if that anchorage region has sufficient moisture Definitely it is going to lead to corrosion of the strands at the near anchorage zones And you might see strands corroding something like this in very short period of time which is not really a good thing to happen Especially when we think that these type of structures are supposed to last for 100 years or more Now, recently people are also practicing trying to put some kind of polymeric coating on this you know like, you know, water proofing coating But remember that these structures when they are exposed to sunlight these polymeric coating might actually degrade in very short period of time maybe few years down the line they might degrade unless it is very, you know, prevented or protected from sunlight So I would say the metallic caps are the best preventive measure for these kind of systems And on top of the metallic cap you also should have very, you know, good another concrete

element which will protect it, you know, you need to really take care of the anchorage zones otherwise these structures will have severe corrosion problem in very short period of time Now, also the joints you know, these are the segmental joints, the joints we can see here So these joints usually are supposed to be filled we know earlier few decades ago people used to go for dry joints means there is nothing in between the 2 segments You just keep the 2 segments together and then you know press it or due to the prestressing action itself and then rely on the shear keys for the resistance However, now later on people found that it leads to severe corrosion because through this dry joint oxygen and moisture can penetrate and which will lead to localized corrosion of the strands which are going like this So this region will actually corrode So to prevent that, you know, recently people are actually using epoxy joints So that means you put epoxy on the inside surface or the facing surface of the both the segments and then push them put them together and squeeze it Because of prestressing it will get squeezed and it is supposed to fill the entire region and to prevent the entry of moisture and oxygen it is very very important But we have seen if you do not really put a lot of, you know a good amount of epoxy it will actually lead to further leakage and corrosion later on This is an example showing that inside the duct you can actually also have stagnant water and it should be, it is very important to make sure that these kind of things does not happen This is not good for the strands So how do we prevent that? You know there is a design procedure where they suggest to provide a, you know drain hole I mean you know hole on the segment for draining the water whatever possible water stagnation it will drain and then let the water out It is very important to keep these systems away from water This is an example showing, you know, I want to show where the water can come in So like this is an expansion joint and are 2 girders Water can come in and then enter here to this way, this way So this is a severe issue So this must be avoided by making sure that there is a very good protection mechanism to prevent the entry of water, okay Now there was there used to be a practice of filling the duct with water before the grout is filled in Now it definitely and then expecting that the grout will actually displace the water and prevent the formation of air void This was the idea for going for practicing filling the duct with water But it is not really a good idea because the strands might get corroded and the type of water which you use if it has a lot of chlorides that might also affect So, because of this it is not nowadays recommended to fill the duct with water before filling the grout, okay So you can see here this picture was taken from one of the sites where this was actually practiced as you see here the color of this grout it is not grey it has a lot of rust, you know, getting mixed with it So it is a proof that the strand inside has corroded to some extent So that is something which need to be avoided Now also like, you know, we have to have, you know good number of inspectors and qualified persons at the site This is a picture taken, you know in almost 9 years ago but you can what you can see is there is no inspector at this site when the grouting was, you know being the bridge was being grouted Because the grouting in essence is not given sufficient importance and that is what the problem lies I think And then also the shear keys it is leading to, you know, this you can see an example where a shear key is lost and you know it does not really have a good shear resistance, might not it might this really affect the shear capacity of this Now we generally think that the post-tensioned concrete bridges are having long, you know, it is are all performing well Why we have this feeling is? Mainly because most of the structures which we see around are less than 10 years old or

they are very young Babies always look good so that is one reason why we are having if you know general feeling that all these post-tensioned structures or prestressed concrete structures are of good quality But as time passes we will see that this size of this yellow region is going to reduce and then the green, white and blue it is going to increase So, I mean this is also a pie chart here, okay So let us not judge the long term performance of these structures based on the short term performance It is very important to understand this and we really have to do things So that, these young structures will actually last as long as they are decided to last So the protection of strands from corrosion is very very important So this is a bar chart showing the, you know, premature corrosion on prestressed concrete bridge post-tensioned bridges So the numbers in the parenthesis shows the number of years before they exhibited or showed corrosion Now, what are the issues? Mainly the tendons are inadequately grouted and hence they corrode prematurely In this picture here this bridge actually failed after the tendons were refilled with grout and you know and very short period you can see here within about 15 years they started experiencing corrosion which is not at all expected in these kind of bridges which are designed for 100+ years Now, where is the corrosion actually happening? So, you see the profile here and then what is happening is, the cement in the grout will segregate, cement will move downward and water will move upward So cement will move downward; water will move upward and then basically it segregates and then you see lot of bleed water will form at the top portion of the tendon like it is shown in this close up And then eventually that bleed water will evaporate So you can see this unshaded or the white region is supposed to be filled with; this white region is supposed to be filled with grout but it is not So that will lead to additional problems This is another bridge where you can see a vertical tendon like in this pillar here where this portion here is not at all grouted So that is also leading to severe corrosion as it was visible in about 8 years this strand system completely corroded which is again very early corrosion Other kind of problems which can be observed is the broken ducts, if the plastic is of not high quality broken duct and then, you know, lot of voids present and then water stagnation and then a lot, you know, even there are regions where there is no grout at all This is a picture taken from the center span But there is no very clearly indication that there was no grout reached at this location So these and then this is another failure which happened in just 33 years due to poor grouting This is another demonstration of poor grouting being used As you see in this bridge here, on this end you can see a semi-circular region of grout If the tendon was completely filled; the tendon goes like this, you know, horizontal tendon on this girder, you know, the pier cap if it goes, if it is not filled properly you will see the semi-circular or partially filled If it is properly filled this grout should be a perfect circle and which is not visible it is very clear that the grout used in this What is the problem with this is that you know after some time as you see here if there are regions with no grout and with grout The top portion of this grout this indicated by this red line; horizontal red line the top portion will get very easily carbonated Now when you have carbonation like that and you know chlorides can enter and CO2 can also enter through this and it will carbonate this region Now you have a strand which is passing through a good grout and it is exposed to a highly carbonated grout, okay

That leads to accelerated corrosion of the strands and this is also an example picture here which says that, you know, partially filled region and in this picture if you look very carefully you can see that the top portion is actually showing more corrosion than the bottom portion These are all evidences showing that very severe strand corrosion due to inadequate grouting Now in the market today, okay Due to the significant void formation people have worked on developing better quality grouts however even better quality means grouts which can actually fill the tendons much better But even in some of those grouts we see a new type of problem which is the soft grout formation where the inert materials which are used in this grouts were floating and it forms this soft grout as you see here very porous in nature and which can actually absorb the moisture and keep it there So it is like froth in a coffee cup I mean on top of the coffee very porous, very low strength which can absorb moisture keep it there and then lead to corrosion of the strand Now the tendons are really the backbone of post-tensioned systems and it is very very essential to, you know, make sure that they are protected well from corrosion otherwise we will have severe strand corrosion related issues So we looked at prestressed concrete system and they can be vulnerable How they corrode? We should not look at rely on the visual inspection or visual, you know, manifestation of the rust on the concrete surface that is not a good idea And that in the pre-tensioned and post-tensioned concrete systems the corrosion mechanisms are different rather the mechanism leading to corrosion is different In the case of pre-tension system, it is mainly through the chloride diffusion through the cover whereas in the post-tension system it is the grout you know inadequate filling or inadequate grouting is something which relates to which leads to premature corrosion And these are the references list of all the references used in this presentation okay