Ensuring the safety of concreting structures does not just involve creating strong structures. It also involves an in-depth understanding as to how concrete degrades over time, and what factors are involved in causing that corrosion.
Our two presenters, Jaden Texeira and Sarah De Carufel, will help you better understand these factors and how to accommodate for them in this exclusive on-demand webinar from Giatec.
Technical Account Manager, Giatec Scientific
Product Owner, Giatec Scientific
Hello everyone, welcome to Giatec webinar Understanding Corrosion in Reinforced Concrete Structure. I already see that we have a chat in from Irvie saying hello from Saudi Arabia. So hi everyone. Today in the webinar, so my first of all my name is Sarah. I manage the Technical Support here at Giatec and today I’m just going to be the moderator are for this presentation, and Jayden, which is our technical account manager for non destructive testing, will be conducting the training. Before we start and quickly jump on the agenda for today, I just want to remind everyone that there is a chat in a Q and A bot button at the bottom of the screen, please feel free to ask any questions using those two options and we’ll try to get to you with an answer either during the presentation or at the end during our Q&A session. So today we’re going to be focusing mostly on corrosion, which you guys would be aware by the title, but we’re going to go into what is corrosion, what causes corrosion, and why is it important to understand the corrosion and at the end and we’re just going to cover how to do to actually measure that corrosion there’s different methods to do so. We’re going to go in details through some of them, and we’re just going to finish with an example of what you can do with corrosion rate measurements. So Jayden off you go.
Thank you Sarah, and thank you again everyone for joining us today. So first we’ll start off with what exactly is corrosion? So, corrosion is an inevitable process that occurs when refined metals return to their more stable forms as oxides, carbonates and sulfides. Corrosion is an electrochemical reaction that happens after surface of rebar, where at the anode, ferrous ions and electrons are oxidized and released. And these electrons travel over to the cathode, where there’s an availability of moisture and oxygen, and these reduce to form hydroxyl ions. These ions then transfer over back to the anode where to react with the ferric ions to form ferric compounds. These are more commonly known as rust and rust is the byproduct of corrosion. This is what can occur on the steel or the surface of the reinforcing steel. It’s, it’s very important to note that with this by product that is formed, it holds a greater volume than the steel before it, so in terms of the original rebar, reducing in diameter and degrading the byproduct of this also has a greater volume up to 6%, and this can cause internal stresses on the reinforced concrete.
It’s important to note that there are a few things that we have to take into consideration when understanding corrosion for this chemical reaction to occur, there needs to be a sufficient amount of oxygen at the surface of the rebar, enough moisture. We need iron. The steel reinforcement with the potential difference in continuity between the anode and the cathode. When we’re thinking of reinforcements in concrete, they’re two different systems and we have to understand exactly how they react with each other with concrete. Of course, in the initial stages we mix in water, but after it’s had time to mix, set and hydrate, that is now a concrete pour solution and it’s very high in alkalis, namely sodium hydroxide and potassium hydroxide, and this pour solution typically has a pH of greater than 12.6. So now that not just that we have plain concrete, but we have this reinforcement. With this reinforcement, the steel reacts with this high alkaline environment and a metal oxide is actually produced on the outer layer of the reinforcement, and this serves as a protective layer to corrosion, more commonly known as the passive layer. It’s important to note that with this environment that this reinforcement is in this high alkali environment, this is what this passive layer thrives in, so there are circumstances which will get into where this passive layer can breakdown known as de-passivation, and this can be a reduction in the alkalinity of the pour solution or the presence of chloride ions, which is very popular in the world of corrosion.
When we’re thinking of corrosion on reinforcement, it can be classified into two simple ways, the first one being microcell corrosion, where its corrosion occurring on the same bar. So, you’ll have your anode in your cathode very close together. The second type of classification of corrosion would be macrocell corrosion, so this is where the anode and the cathode there, corn occurring on two different bars, where they can be on the same bar, but at a significance significant distance away from each other. In this example, you can see that the anode and cathode are on two different bars, but they’re connected with the third bar, which allows that transfer electrons to take place. Now, understanding a little bit of the electron flow, what, how corrosion occurs. Corrosion may cause a few things after this byproduct of rust forms and it’s exerting that internal pressure on the concrete. So first and foremost, once there’s enough byproduct of rust, it can cause cracking, delamination’s, spalling. It’ll reduce the diameter or a loss of section in the rebar and it will hinder the bond between concrete and reinforcement, so it’s very important to note that if we can catch this early, then we can reduce the potential of cracking, delamination and spalling.
Now we’ll get into what exactly causes corrosion. What causes these alkaline environment to reduce? Or exactly how does it start this reaction? What we’ll be chatting about today are two main corrosion causes, the first being carbonation induced corrosion and the second, the more commonly known chloride, induced corrosion. Carbonation induced corrosion. But first, what exactly is carbonation? Carbonation can be classified as the reduction in alkalinity of concrete due to the introduction of atmosphere carbon dioxide, which is being gradually introduced into the concrete. This introduction of CO2 it attacks a few different hydration products. What we’re going to focus on today is sodium, potassium and calcium hydroxide, namely calcium hydroxide or commonly known as a portlandite. So, we can see here in the graph, on the schematic on the right hand side we have that CO2 being absorbed into the concrete, it’s reacting with the different hydroxide products and it’s forming calcium carbonate and, or sorry, different carbonates, sodium, potassium and calcium carbonate and also water. So with this reaction, the forming of carbonates, this is actually going to reduce the alkalinity of the concrete, and it’s very important to note again that passive layer. It needs that high alkali environment, whereas this introduction of CO2 through the carbonation it’s going to reduce that. So we really want to understand how this occurs.
With the pH of typical concrete being above 12.6, according to some research it’s shown that carbonated concrete has a pH on the scale below 9 around 8. So, we have carbonated concrete, or at least we think we do. How exactly can we determine this? So, the main method to understand what portions of concrete are carbonated or not, is by using an acid base indicator, commonly known as Phenolphthalein. So, what you can do is this is simply sprayed on the concrete. You can see on the right-hand side there depending on the pH of the concrete it will be able to turn purple or colorless depending on that threshold. In the example that you see here, we can see the same samples of concrete age from 8 to 99 days exposed to carbonation and environments around 60% relative humidity and you can see at 8 days there’s a lot of purple. Simply there’s not a lot of carbonation, but as we, as it’s continuously exposed to carbon dioxide over at 90 days, we can now see that it’s fully carbonated concrete. In these examples, you can see there’s no reinforcement, but of course with reinforced structures carbonation CO2 in the atmosphere, how exactly does this interact with the reinforcement?
So, in this schematic we have reinforcement with that metal oxide layer which has been formed and we sprayed the concrete, and we can see the pH is generally above 9.2, so there’s no carbonation and then as we can see as gradually as carbon dioxide is introduced to the concrete. It’s reacting with that calcium hydroxide to form that calcium carbonate and H2O, so it’s slowly reducing the pH level of the concrete. This is commonly known as carbonation front that’s moving through the concrete and we can see overtime under conditions that sustain that carbonation front coming through the concrete. It’ll make its way, it can make its way down to the reinforcement. So, what you can see here is now the area under the reinforcement, it still has a high alkalinity, but due to the introduction of the atmosphere, carbon dioxide, it’s been able to react with some of the hydroxide product products, namely calcium hydroxide, to form these carbonates and reduce the alkalinity environment of the concrete and the alkalinity environment around the reinforcement. And with carbonation induced corrosion, there’s a higher chance of seeing uniform corrosion along the rebar surface in comparison to pitting or localized corrosion, which we’ll see in a few moments with chloride induced corrosion.
So most commonly, the most commonly induced type of corrosion is due to the ingress of chlorides. Simply put, it’s chlorides that are traveling through the poor network, the pore structure, and making their way down to the reinforcement. Concrete can be exposed to chlorides through deicing salts, exposure to marine environments, depending where everyone is turning into the webinar today, you’ll be able to understand if some of your concrete structures are exposed to those marine environments and also during the design. So, what exactly, what kind of admixtures are being used in the initial design? Do they contain chlorides? Some of them do. So, you can see here that the chloride ions are penetrating through the pore system because concrete is that porous permeable material and making their way to the reinforcement. Here we can see that after the buildup, after the accumulation of these chloride ions on the passive layer, once they’ve reached a certain threshold known as the chloride threshold, it starts to breakdown this passive layer. This phenomenon is currently not fully understood. Exactly how these chloride ions breakdown this passive layer, but it does occur.
So now with these aggressive chloride ions flowing through the concrete, penetrating this passive layer, Now the rebar is exposed to that electrochemical reaction that can occur which can produce rust and deteriorate the rebar. As mentioned, with carbonation induced corrosion, it can be more commonly seen as uniform corrosion, whereas pitting corrosion is commonly seen with chloride induced corrosion. This is because when those ferris ions are being released at the anode, there are those chloride ions present and they react to form iron chloride, and these travel a certain distance away from the reinforcement up to a more alkali area where there’s a little bit more oxygen as well. And this reacts with water and forms the iron oxide, which precipitates, and these chloride ions are again released and come back down to the corroding area so it’s a regenerative process where it’s happening in the same location, and this is where you can see pitting, pitting, corrosion occur, a real-life example just on the left-hand side there.
So, there may be a few thoughts already going on of why exactly do we need to understand corrosion, but it really is evident, especially with some of the older structures that have been built in the 1950s, 60s and 70s. A lot of building owners are trying to understand which repair rehabilitation needs they need to go through to maintain the service life of their structure so you can see we want to understand critical locations of a structure where visual inspection isn’t enough. Yes, visual inspection is the go to. It’s the easiest, most cost effective, but there is a lot that we cannot see with the eye. We can understand where cracks are or cracks can occur due to corrosion because there has to be a certain amount of rust. Internal pressure on the concrete floor to crack. If we can catch it before then we can be proactive instead of reactive and that can save a lot of money for building owners in n terms of rehabilitation for small or big structures. Again, we can estimate the time to cracking an estimate service life. So, with this information that will get into in terms of what kind of measurements will be taken from the reinforcement, we’re able to formulate these, put them into different softwares, different equations to understand time to cracking estimates, service life, and so on.
Now you may be thinking, can’t we just prevent it somehow and there are different methods out there to control corrosion. A few are during the design stages, material selection, any inhibitors, coatings, and then more of the cathodic protection and some sacrificial anodes. A list of the few different methods to help control corrosion in the initial stages and throughout the life of the structure. Now we’ll get into measurement techniques, so we understand the destructive process of corrosion. Two different ways that can occur, carbonation and chlorides, but how exactly do we understand what’s happening at the surface of the rebar with either destructive or non destructive techniques to be able to get that information, namely back to the building owner so that they can understand what they want to do with the structure. Today we’ll talk about three different things, half-cell potential, which you may all be familiar, or at least have heard of in the industry. Electrical resistivity, how that relates and corrosion rate, of course.
So, with half-cell potential as I mentioned in the beginning, there needs to be some potential energy for corrosion to occur and with the transfer of electrons hydroxyl ions there is potential energy that’s generated on the surface of the rebar, especially because of this reinforcement being submerged in the electrolyte, which is the concrete pour solution. So, knowing that there’s this potential energy on the surface of the rebar, and if we’re able to measure that, then we can get some information on corrosion activity, and that’s actually what ASTM has put into a standard, we’ll see in the next slide of what those results would look like. So, for half-cell potential, which is the potential of the half-cell, which is the reinforcement, we would typically need another half-cell to complete this system to be able to understand what’s going on. In the diagram on the right-hand side, you’ll see that we have a reference electrode, and this has unknown potential and some of the common reference electrodes out there have a solution of copper, copper sulfate or silver silver chloride with known potentials that are stable enough to get those measurements from the reinforcement and to be stable enough to be able to put into a device itself.
So, what would happen is you would have a reference electrode on the surface of concrete. You would require connection to the reinforcement to complete the circuit and then measure the potential difference between the reinforcement and this reference electrode using a voltmeter. Again, for understanding corrosion, half-cell potential is the only standardized technique to understand corrosion and this falls under ASTM C876. You can see on the left hand side, ASTM has been able to tabulate this data so we’re getting these potential measurements from the surface of the concrete. Trying to understand what’s happening at the surface of the rebar and ASTM has outputed measure potential values with a probability of steel corrosion less than 10% uncertain and more than 90% chance of steel corrosion. On the right hand side, you’ll notice another table, and this is from the RILEM guidelines and standards and they took part in numerous tests on numerous different samples under the conditions that you see listed in the table and they wanted to understand the potential values that one might see in the specific locations. If, for example, we look on the right table, the 3rd row down, we’re looking at water saturated concrete without oxygen. And we’re reading potential values from negative 1000 to negative 900.
But if we go over to the table from ASTM, this would mean that we’re seeing a high chance, more than 90% probability of corrosion. But if it’s water saturated concrete with no oxygen, is there really corrosion happening? So that’s there. So that leads to some considerations that we need to understand when using the half cell potential test. So again, we’re using that half-cell electrode on the concrete surface and we’re traveling through a concrete cover down to the rebar surface to understand exactly what’s going on. So, you can see on the right-hand side what we actually measure versus, what we measure versus what is actually happening at the surface of the rebar. So, with the half cell potential tests, we cannot detect localized corrosion. It’s more of a zonal measurement due to the temperature. The resistivity of the concrete, the cover thickness and also the anode to cathode ratio and the biggest one being the availability of oxygen from that RILEM table where there was saturated concrete without oxygen, so it’s very important to understand these different parameters when doing half-cell potential and interpret your measurements accordingly.
Next, looking into corrosion rate or corrosion current density and this is one of the most sought-after measurements that practitioners are looking for in the field to not understand the probability corrosion but also the kinetics of corrosion. At what rate is this rebar corroding? What is the mass loss? How much time do we have left in this structure? So when the corrosion is occurring at the surface of the rebar, this transfer of electrons there is a current that’s created on the surface of the rebar, known as the corrosion current and this is what we want to understand with different measurement techniques and devices out there on the market in order to use devices they have to go off certain equations and this,the devices that will go through, they all employ this Stern Geary equation for corrosion of electrochemical systems.
So, you can see on the right-hand side we have icorr numerator B Tafel constant to a corrosion constant, typically 26 millivolts. And then what we’re trying to understand as a practitioner on site is that area of polarization and that polarization resistance of the rebar. With these two values we’ll be able to understand the corrosion rate of the reinforcing steel. The biggest one here is that polarization resistance, so it’s the change in potential energy over the change in current. So that’s where I’m going with the different measurement techniques, we’re from the surface of the concrete, and we’re going to be emitting an electrical current down to the rebar, understanding this change in current and then receiving this change in potential energy from the rebar, and this can all be calculated into polarization resistance RP.
We can understand what area we’re polarizing and this can all be put into the equation, the Stern Geary equation to understand the corrosion rate. Once we get the measurements, what exactly do we do with them? Or how do we interpret them? And there are some classifications in terms of corrosion current density that can be converted to corrosion rate microns per year, and then you can see on the right hand side classifications of passive low to moderate corrosion, moderate to high and high corrosion rate. So, for today’s webinar we’ll be touching on four different techniques, the first being electrochemical impedance spectroscopy. This will help us understand the little bit of the theory and the influence of current and frequency on electrochemical systems, linear polarization resistance, LPR, which may be a technique that everybody’s familiar with. Galvanostatic and connectionless electrical pulse response analysis system, which is a mouthful, so we’ll short term short form that as CEPRA.
So, with electrochemical impedance spectroscopy, short form EIS, this helps us understand the impedance resistance of electrical chemical systems using different frequencies. And how does this apply to corroding rebar? So, on the left-hand side you can see, we’re fluctuating frequency, low to high frequency, we’re getting that voltage response, and this can be correlated to corroding rebar or noncorroding rebar and this is some of the theory that’s put into the different methods, and this is how we interpret the data, understand what kind of corrosion status is happening at the surface of the rebar. So, I had mentioned before that concrete resistivity had a part of it. If we’re doing measurements from the surface of the rebar, or if and if we’re understanding rebar reinforcement being in concrete where you have two different materials, we need to take them both into consideration when doing measurements, and we can apply this with a simple Randall circuit, where we have resistance of concrete, which could be ohmic resistance and then in parallel we can have impedance of the electrode which would be rebar and then also the capacitance of that electrode.
So, we’re trying to understand what’s the resistance of the concrete and what’s that impedance which would be RP in our corrosion rate equation to understand exactly what this corrosion rate is. Saying this, you can see on the bottom we have capacitance of the electrode, with this capacitance effect there is an imaginary impedance that we must take into consideration, and we can. The goal is to minimize this capacitance effect. So exactly how do we get rid of this double layer capacitance effect and just focus on the resistance of concrete and the impedance of the electrode which what we truly care about, so this can be done by fluctuating the frequency from low to high frequencies we’re able to minimize this effect and this can be illustrated in the Nyquist plot for the Randall circuit. So, on the X axis you have the real impedance which we’re looking for, and on the Y axis we have this imaginary impedance due to the capacitance. And you can see on this curve we have decreasing frequency to the left and subsequently increasing frequency to the left and on the bottom right you can see resistance of concrete plus the resistance of the rebar, so that’s what we’re looking for, and we’re able to do this by fluctuating the frequency to 0.
So, if we’re able to bring this frequency to 0, then the effect that would have on the capacitor is that it would create an open circuit so that of course that electrical current is trying to flow on the path of least resistance, so then it would flow through the concrete resistance up to the impedance of the electrode, and that’s what we want to measure. Of course, now we have both of them, but we want to isolate the two. So then on the flip side of the Nyquist plot, we can sweep that frequency to a high frequency. And then this would cause the capacitance or the impedance due to the capacity effects to be negligible. So then now we’ll have that current flowing through the concrete resistance through the double layer capacitance, because we’re able to mitigate that imaginary impedance by going to the higher frequency. That was a mouthful about frequency and circuits, but this information is what’s utilized in the different devices to mitigate this capacitance of fact and output the correct and accurate corrosion rate values.
The first one that will touch into is linear polarization resistance, so again, I’ll bring your attention to the top right. What we’re focusing again is how do we get those AP area of polarization and that polarization resistance? So we’re going to have to understand difference in potential energy, but then also the fluctuation of current. In the diagram on the right hand side, you could see an individual with an LPR testing equipment, and there’s three electrodes there, so the first one being the working electrode, which is simply the reinforcement. We have a reference electrode, so this reference electrode is typically a half-cell and this understands the potential energy on the surface of the rebar before the measurement and after the measurement, so it’s able to monitor that Delta E that we’re looking for, for the RP value. Then we have the counter electrode. This is what is emitting that alternating current down to the reinforcement, and then as we’re alternating this current polarizing the rebar, we can get that change in potential energy, plug it into that our P value back into the Stern Geary equation and calculate that corrosion rate.
Two things to note for LPR. Based on the instrument technique that you’re using it estimates an area of polarization. So just as important as RP, AP is just as important to be able to understand the true corrosion rate and with LPR it does require a connection to the reinforcement. Next, we’ll move on to the Galvanostatic Pulse technique, first introduced for field application in 1988 and its rapid nondestructive test for corrosion rate assessment so you can see that in the diagram it looks a little bit similar to the LPR technique where you have the mandatory connection to the rebar. You’ll have your reference electrode which measures the change in potential energy you’ll have your counter electrode which is emitting and polarizing the current or polarizing the rebar with the current, but you can also notice that we have this guard ring. So, as we’re polarizing the rebar, we want to understand again that area of polarization. So, with the Galvanostatic method instead of a current application, standard application of the current it actually uses a pulse that polarizes the rebar for anywhere from 30 to 100 seconds, and then these guard rings affectively, their role is to emit a countercurrent that is able to confine the current from the counter electrode.
So, if we are able to understand exactly the area that this current is going towards, we can get more accurate information. The efficacy of the guard rings is still up in the air based on exactly how the guard ring that confines this current is still there’s still some question marks around the accuracy of the effect of the guard ring. So, we chatted about half cell potential. The potential energy on the surface of the rebar corrosion rate, that corrosion current that the practitioners are looking for to understand the connects of corrosion and then electrical resistivity. Not as popular in terms of buzzword when you think about corrosion maybe, but it has its importance. So as we know, concrete is a porous permeable material and with electrical resistivity, we were able to understand that connectivity of the pour solution, degree of porosity, connectivity of pores, all that not carbonation or carbonation, but also these aggressive chloride ions are traveling through. So, if we have a more durable, less permeable concrete, there’s less chance or the very slower rate of corrosion. And it’s a very important to understand resistivity of the concrete reinforcement concrete cover that’s protecting the reinforcement.
So, rule of thumb, low porosity, high electro resistivity, durable concrete. And this brings us to the last measurement technique for corrosion rate, but not only corrosion rate but also electrical resistivity as well. So, with the Giatec iCOR® this device can measure corrosion potential. Corrosion rate and electro resistivity, all with the same device and output these results to the practitioner in the field. So, you can see from the diagram on the right-hand side at the top right. This does not require a connection to the rebar, so only device in the field of corrosion that does not require connection to the rebar to be able to get those corrosion rate measurements. If you are using it for half-cell potential that follows the ASTM C876, which will require connection but for corrosion rate, it’s on its own. No connection to the rebar, and for this it uses the four probe Wenner-Array similar to the test method for surface resistivity. So, what you can see is with the theory of frequency, still employed with the Giatec iCOR, of course, we use a higher complexity in terms of the electrical circuit that is used in the iCOR, so you can see in the diagram on the bottom right. This is the electrical circuit that we can put up in a schematic for the iCOR in terms of where the flow paths are for the current so you can see the two outer electrodes. The two outer blue dots. That is what is emitting a current pulse similar to the galvanostatic pulse into the concrete.
And there’s two flow paths for this current. One, the first flow path is from electrode to electrode, so which would be RC2. RC1 is just the contact resistance from the electrode. So, part of this current is going between electrodes. So, we can measure RC2. Which would be typically the surface resistivity of the concrete and then the second flow path for the current is RC3, which is able to polarise the rebar mitigate that effect of the double layer capacitance and output a voltage response, and then we’re able to calculate the RC4, which in theory would be the polarization resistance. So again, we’re emitting that polarizing current. Understanding the voltage response and we’re able to solve this circuit and output of polarization resistance and with the iCOR we can definitively say we know what the area of polarization is based on the circuit and based on the test method that’s being used.
Now it’s all great when we see all these schematics, the theory behind it. But then when we actually get out into field, how does everything work and come together? So, there’s a case study done in on the Three Nations Bridge in Cornwall, Ontario. There’s a lot, this was one of the many bridges that connects Canada to America, and the objective of this was just to evaluate the bridge for deterioration. This you can see just in the images on the right-hand side there was an asphalt where layer which was removed for the purpose of the evaluation and the methods that were used to understand the deterioration and corrosion were the typical visual inspection and change dragging to understand the laminations and encroaching potential, corrosion rate and electrical resistivity were used with the Giatec iCOR.
After this asphalt where a layer was removed, damaged ranged from minor cracks to severe spalling in different areas. Not all due to corrosion propagation, but also from salt scaling and freeze thaw. So, what was understood from this case study on this bridge? So, on the right-hand side there’s a few figures. The first one, the top one being the information received from change ragging. So, the areas where there was delamination’s below that, you’ll have a potential map of the bridge corrosion right values below this, and then electrical resistivity. So, all this will be able to give the practitioner a thorough understanding of exactly what’s happening on not only in layers or in areas of the bridge that had visual damage, but most importantly in areas that didn’t have any visual damage or rust stains or delamination’s, so this is where electrochemical methods come in very handy to be able to understand what’s happening at the surface of the rebar when there are no cracks. Again, going back to the beginning of the webinar, it takes a certain amount of rust byproduct of the corrosion to cause cracking, cracking all the way to the surface, spalling, delamination’s, so we go back to that theory of proactive instead of reactive.
Understanding the subsurface conditions of the rebar with electrochemical methods can truly help reduce the cost rehabilitation plan for future repair and have a better estimate on service life and when repair will be needed. More information on this case that can be found in Concrete International as well. And that’s it for today folks. I appreciate everyone again taking some time out of their lunch if they’re from North America or there is a few members from overseas as well. I hope all the information was clear. Now we can open it up for some questions.
Thank you very much, Jayden. I’ve been monitoring the chat and Q&A but on my side, I’m not seeing any questions. Do you have anything on your side?
Let me just take a look.
And as a reminder, please feel free to put any comments in the chat or the QA box at the bottom of the screen, we’ll be more than happy to answer any questions that you have.
Just a few comments from the different attendees asking for the PowerPoint slides. So yes, the PowerPoint slides will be sent out to the attendees. Question from Basil. Is it true that the carbonation corrosion layer formed after the breakdown of the passive layer becomes a secondary protective layer?
Yeah, I’m not sorry, Basil, like I’m not 100% sure about the answer to this question so I will revert back to you with an answer. But what is for sure is that you’re not, the corrosion, if you look at it in one way, you have that by product that forms it creates, it starts cracking your concrete, so even though if it protects, one location is going to most likely cause cracking your concrete, and it is going to make it worse on the other location of your reinforcement. The thing with carbonation is a front, you wouldn’t have like a little bit of carbonation on one side and like a really big amount on the other side. Most of the time. So, it’s going to be that uniform situation and you know having those cracks and everything you could propagate corrosion and every single around the circumference of the reinforcement. So even though there is some kind of if this true that there is some kind of protection from this corrosion byproduct, it would not stop your corrosion activity in your structure. And if nothing is done it is still going to have to, you know, do the repairs or anything that is required to make sure that the service life of the structures is good for another 10-15 years if needed.
Thank you, Sarah. A few more questions coming in. The next one in case of strands and not rebar, we consider always the normal area in the iCOR. So, in terms of, that’s from Pierre Claudio, so in terms of strand is being very important to understand the diameter of these strands and that information can be inputted into the iCOR, similar to the diameter of the rebar. If there are oncoded one of the big questions are PT cables, where’s the there in plastic tubes and with the different systems, there’s no system that can understand corrosion rate just because it’s an insulative layer that’s on top of the reinforcement or reinforcing strands.
And just to add to that, Jayden, there is for all the devices you need to understand what is your rebar diameter, because the area of polarization actually uses the circumference, so there is an input from the end user. To get a proper AP, this is not the devices are not going to figure out by themselves what the rebar size is. There is some devices out there that can do that for you, or blueprints also can be used to get that information.
Thank you, Sarah, another question from Summer, in which corrosion stage can we apply corrosion inhibitor? Sarah correct me if I’m wrong, it typically during mixed design stage or surface codings where inhibitor can be applied.
But there’s, as Jaden mentioned, in the presentation, there is different ways to protect your reinforcement. You know throughout the lifecycle just one you know if you can always put some protection layer later on. You know if you see if you themes that you know corrosion is propagating. One thing that you can keep in mind is that you know we’re trying to prevent oxygen and water and carbonation from going through. So, an effective way would be to block those components from going through. Another more extreme would be to, like, you know, do some kind of repair and put some anode and cathode protection in there, but most of the techniques are all that are out there for corrosion protection are usually you know something that you want to incorporate in the design stage of your project. You know anodic protection, kinetic protection on any type of epoxy coated reinforcement, or any type of galvanized reinforcement or conversion inhibitors in your mix design. This is all things that need to be taken in consideration in the early stages of design.
A couple of questions in Q&A one was just referencing the Concrete International article. We can make that available as well and put you in the right direction to see more information. Question from Ram. Is there a way to obtain the grade of existing rebar going back to Sarah? Your comments in terms of area, polarization and diameter that would be something from engineering drawings or other devices. To understand exactly what rebar location, but also the rebar grade in terms of diameter.
Another one, how many locations did how many measurement locations with the iCOR to generate the map on the bridge jacks you showed? 1000 Anonymous attendee. I don’t have that information regarding that case study. Sarah if you have any more comments. I wasn’t a part of that.
I don’t know the amount of measurements that were exactly done on that project. The only thing I would say is that the measurements between two points, which was probably around half a meter because of, you know you want to be able to localize your measurements, but at the same time, if you expand to like 2 meters or 3 meters are not going to be able to get that localized corrosion. On top of that, since the iCOR is a three in one device, half-cell potentials are typically taken within the meter from each other because of the slide that Jayden showed that it kind of takes an average around a certain area, so I would assume around .5 meters would be the spacing and just I don’t know the exact like I don’t recall the exact length of the bridge, width and length of the average deck. But with any of those devices, especially the ones that are actually measuring corrosion and or outputting a map, you can always, you know, do a survey at a certain location and go back and add more measurements at specific locations where you want to have more information. So, if you find that everything is green and then suddenly you have this red zone, maybe you want to just have a denser measurement on that area just to really capture what you want to measure.
Thank you, Sarah. A few more questions here and then we’ll wrap it up. So, what should the sample size be of concrete before for EIS testing in a lab condition? Different examples can be taken from a lot of the papers out there, Maria. They would have a better answer than I do in terms of the size of samples that you would want to create in your lab, Sarah, if you have anymore comments on that as well.
Yeah, so one thing I would suggest, I don’t know if there is a limitation in terms of minimum length per se, but one thing that you want to make sure is that if you’re using EIS, which is usually a lab, it’s not using the fields lab specific method, and when this method is used, typically it’s to understand what’s to correlate mass loss and things like that. So, the bigger sample is, the more concrete chipping you’re going to have to do. The more measurements you’re going to have to do, the harder it is for you to find the mass loss of your reinforcement, so a bit smaller elements would probably something that you would be looking into, typically in terms of corrosion measurements, the measurements, the sample size that we see around a foot long, so 15 centimeters roughly or 30 centimeters, and the range, but with EIS technically you should be able to have something smaller. It really depends on the, your research as a whole, but if you need help, we can help you with that.
Thank you, Sarah. Another one from Basil. When rebar gets the site as it’s being a cell, sometimes there’s the brownish color on it, just as a result of the atmosphere. Is this not useful anymore or should more of a technical analysis as once you described in the webinar?
So, I’m just going to put this out there so if you it’s normal to get corrosion on your reinforcement. The reason is that we mentioned the two things that you need is air, sorry, oxygen and water, so the humidity in the air. So any piece of metal that you put out there is going to start corroding, that corrosion is not going to be really excessive and by putting it in the concrete itself you should stop that corrosion propagation. As a whole, for sure, if you have, like really, really, really bad corrosion, you know we’re talking like a lot of pits and things like that. But if it’s just like surface corrosion that you can kind of brush off, don’t worry about it. Everything should be fine.
Thank you, Sarah. Two more questions. Do we have a standard for acceptance or rejection of rebar which exhibits some degree of corrosion prior to installing? So, I believe Greg that that question was just answered. Let me know in the chat if Greg, there’s more.
There might be. I’m not 100% familiar with that, but you know that normal small rust there should be fine. That’s something I would need to look into to give you more information, if, you know if there is any standard, any specification to like rejecting rebar loads.
And one last question from Perush. How effective is epoxy coating? If there is any breach of coding, will it expedite the electrochemical cell rate of corrosion? Is this true?
Yes, so to be really careful with epoxy coated rebar so there’s a lot of study that are being done on them and you know some study might be, you know, I, I don’t want to advocate for any sides, but what happens is that if you have an epoxy coated rebar, if it’s perfectly epoxy coated and then you put it in your concrete, you know you’re going to get really good protection, so that’s the kind of protection you see in lab testing. But when it comes to using epoxy coated rebar on the field, this is where the research, and, uh, you know the people have kind of, ‘m looking for a word, but they’re not you know, they either like it or they don’t like it, and there’s some kind of misconception because as you know, and as everyone on this call knows, concrete when it’s made on site, job sites are messy. If you can tell me 100% sure that there’s no scratch on that epoxy on your rebar, I’m going to have a hard time believing you because of the way it’s transported place, people are walking around at all the time, tied together, so any breach in that Epoxy coated could have would be a perfect location for corrosion because you can imagine that’s the only part at the location that you can have that reaction happening so it would just everything would be drawn to there and this is where you could see a lot of pitting. Like a lot of bad pitting. So yes, epoxy coated if it’s done properly, I think it’s great, but can we make sure that it is fully, you know, epoxy coated all the way through for every rebar that you have out there.
Thank you, Sarah. Just two more questions came into the chat. How should we increase potential and LPR? Do we decrease potential when the current is cathodic? So, if I’m understanding correctly, Basil, if you’re using a corrosion technique or if it’s LPR near and you’re measuring the rebar, you will see less of a corrosion current at cathodic areas, especially if it’s like a macrocell corrosion system compared to the anode where it’s a lot easier to polarize, and you’ll see greater corrosion rate. I don’t know if that exactly answers your question there, but that’s what I understood from it.
Any iCOR device limitations when reading reinforcement of slabs on metal decking. So, go ahead Sarah.
So it depends on your slab, so if you have a metal deck and then you have reinforcement in your concrete then any type, just not limitation, for iCOR, CEPRA, or any other device there they’re going to be drawn to the first layer of reinforcement, so if you have a reinforcement between that deck and your concrete, uh, in the concrete surface, then you should be able to polarize only that. The problem, though, is if you don’t have reinforcement and you’re trying to see if your deck is corroding, if you remember that equation that Jayden showed, area of polarization, all of the devices that are out there are set to account for an area which has a diameter. So, if you have any kind of metal sheet there, getting an area would be extremely difficult. Therefore the corrosion, the iCOR, the corrosion rate value you would get would probably be an error if you take it directly from the unit measurements themselves.
Thank you, Sarah, and one more we’ll wrap it up. We’re just coming to the hour there. Appreciate all the questions. Is there an influence given the distance between rebar connection? And half-cell potential point measurement. So great question here Claudio. So it’s the main thing is you want that continuity between the rebar. So what you’ll be able to do is it’s outlined in ASTM. There is once you have the connection to the rebar, you’ll want your readings from whichever half-cell device to stabilize within a certain amount of seconds, and that will give you the understanding that there is continuity and you’re able to read the rebar beneath the concrete surface where your measurement probe is. But if you have the fluctuation, if it’s not within the guidelines or way out of the guidelines, then you’ll want to make another connection point closer to the point of measurement, and again see if the measurement stabilizes. Then you know that there’s continuity. That’s one of the ways to test continuity with the half-cell probe.
One last for Maria. Do you check how many equipment to perform Tafel polarization or EIS? Maria, we can connect with you offline as well. We do employ, of course, the Giatec iCOR.
Perfect everyone we’ll end the webinar there. We appreciate your time during the webinar and all the questions afterwards. Great questions and we’ll be able to touch base if there are any questions please reach out to Giatec with some of the information in front of you and we’ll be more than happy to assist further.
Thank you very much everyone for taking your time today, and as Jaden mentioned, feel free to reach out to us anytime will be more than happy to help. Have a great day.
Thank you everyone. Bye-bye.