All right, we're a little past one, two past hour, we're going to get started. So good morning to the West Coast and good afternoon to those out East. Welcome back again to the Post-Tensioning Institute's monthly webinar for September already. My name is Kyle Boyd, and I'm the moderator of today's session, and I'm also the chair of the Education Committee, EDC 130. And EDC 130 is a committee within the Post-Tensioning Institute that sponsors this monthly webinar that we have. For some of you that are returning, welcome back. For those who it's your first time, we do host this webinar every single month and it's at the same exact time, which is the second Wednesday of the month at one o'clock Eastern, 10 o'clock Pacific, or anywhere in between, depending on where you live out there in the world. So as you can see, today's topic is high-strength PT bar, which is often used for rock and soil anchors, however, is definitely also applicable and widely used to concrete structures as well. Last month's topic was very much focused on building structures. This month we wanted to get a little more into material, and we also wanted to get into some of the underground stuff, the soil retention, and different sides than your just traditional vertical building unbonding of it. So that's why we're diving into this PT bar, and there's a lot of great information, new research that's been done out there, and we're gonna dive into that with both Pete and Andy who will be presenting, but before I introduce those two, I do have to go through just some of our basic housekeeping items. The first one is continuing education. Everybody who's logged in with your email, you will get one hour of continuing education credit through RCEP. If you're watching in a conference room, our recommendation is still log in on your computer under your email so you can still get that credit that's through there on that. The second one is copyright material, and this is just our standard slide that says everything that we're showing here is copyright protected. If you do want to see any information, please reach out to us. We'd be happy to potentially share some stuff on a situational basis. And the next slide is our webinar protocol. So y'all that are listening, you're all in muted, cameras off, and in listen-only mode, so we can't hear you, we can't see you. If you have any questions, you need to ask them through the Q&A chat feature that we have, and then we'll be able to see them on our end and answer them at the very end of it. The webinar is being recorded, so if anybody misses it or you do have to step away for any reason, you can go online, you can watch it, and upon completion of it, there's a little quiz you can take. Once you take that quiz, you'll get that same one hour continuing education credit that's out there for free. Wow, so that's a great, great thing. All right, jumping into our speakers that we have today. The first one is Pete Spire. He's a very senior individual. We're very lucky to have him presenting with us today. He's vice chair of DC35, which is our Pre-Stress Rock and Soil Committee. He's vice president for Williams Form Engineering, VP of Engineering. He's out of the San Diego office. As you can see by his background here, he has a very deep background in both construction and engineering of anchored earth retention systems, which is obviously very applicable to today's presentation. So we have a senior professional presenting on a material bar that's used often every day in his career. The second presenter we have is Andy Baxter. He's a professional geologist and professional engineer with GIA Consultants. He specializes, too, in the design of low-grade and temporary hydraulic structures. You can also see his very impressive and deep background that's listed here. I'm not going to go through and read every bit of it because it's so extensive. He's authored many reports and manuals, and he is the chair of DC35, which is, once again, that Pre-Stress Rock and Soil Committee that we have. So between both Pete and Andy, you're hearing directly from the source that is the industry lead for this particular topic. And so with that, I'm going to hand it over to Andy, who's going to kick things off for you. Thanks, Kyle. Can you hear me okay? Very good. All right. So as Kyle said, we're focusing today on ground anchors, and DC35 focuses on ground anchors, portions from the Post-Tensioning Institute. What we are focusing today specifically is on A722 bar, pre-stressing bar. We produce two TechNotes, TechNote 23 and 24. The first one is related to non-A722 bars. I'll get into more detail on what those are, but in general, those are bars that don't meet either physical or behavioral properties that are specified in A722, but are often used in practice in scenarios that we would typically use A722 bars. TechNote 24 is for A722-like bars. These are bars that meet either physical or behavioral properties associated with the A722 bars, but not everything. And so we'll get into detail on what the elements are not met in those like bars, and how that can affect design, construction, and testing of below-grade structures. All right. Hey, Andy, this is Pete, real quick. Just to mention too, these TechNotes are available on the PTI website at no cost. You can download them at any time. Thanks, Pete. Okay. So, TechNotes, they are part of the DC35 committee. They were produced by the committee as a whole, voted on, and approved. And like I said, that these are applicable to the soil and rock anchors, however, there are applicable, as Kyle said, to other prestressed concrete applications. We'll mention a little bit of the correlations as we go along. So looking first at A722 bar, right, so this is an ASTM requirement. It has physical and behavioral properties listed in the ASTM, and it has a process portion of the specification. So when I say physical and behavioral portions, those are, you know, size, thread dimensions, and then behavioral are, you know, yield and ultimate strengths. And then the process specification includes an element that is done at the mill, which is, in this case, the cold stressing to 80% of the minimum tensile strength and the stress relieving. And what does that do specifically? That starts to kind of guarantee stress strength relationship elements, proof test loads from the mill, and low relaxation conditions. All of those are not specifically laid out in the ASTM 722 as a behavioral requirement. They're intended to be achieved through this process portion of the specification. So what is a non-A722 bar? Non-A722 bars that we're seeing in the ground anchor industry are, you know, A615 bars, which don't meet either the physical or behavioral properties of the A722 bars. This is your standard rebar specification, in this case used as a thread bar. And then hollow core bars, once again, same scenario, do not have the, don't meet behavioral physical properties, but they're still being used in these applications. And why are they being used? Because they're economically beneficial to the project, right? And they're being used for temporary applications, lower loads, hollow bar has production values to essentially be able to do the project at lower cost. So TECDOTE 23, we're focusing in on the use of the concerns of the use of those materials when referencing PTI, PTI as the assumption that you're using A722 bar in the design, construction, and testing. And you'll see how there are concerns when you're using the recommendations that you could create unsafe or especially unexpected conditions when using them. We'll look at specifically the stress-strain behavior under tensile load, elastic modulus, stress relaxation and creep, and corrosion protection. What we'll find is that some of these elements are significantly different, and some of them are very similar. And so, you know, when you're looking at these other materials, you can see whether or not this is a critical concern or not. All right, diving right into stress-strain behavior. So we're looking specifically at this table has the ASTM 615 bar in the first four rows. It has the ASTM 722 bar in the fifth row, and then the hollow bar in the sixth row. And what you're looking at, you have yield strength, ultimate strength, and then ratios associated those. We have 60% and 80% ultimate compared to yield. And if you've designed with these systems and you understand it and use the PTI guidance document, you'll understand that 60% is our design load and 80% is our maximum test load. And so what do these ratios indicate? And we'll look at it in specific. Specifically, we have grade 60 bar compared to a 722 bar grade 150. Just a reminder here is that in 615 bar, when we talk about grade, we're talking about yield strength. When we look at 722 bar, we're talking about ultimate strength. That can often be confused, but that's very, very simple difference here. So it must be understood. So at the 80% ultimate versus yield condition, that number is greater than one. And what does that mean? What that means is that the test load that's part of the PTI system is 80% of ultimate. And if you pull to that full test load, you're going to yield the bar or very likely will yield the bar, right? Because that's your minimum ultimate. Your minimum ultimate is going to be 60 versus 64. And would that result in failure? Maybe. Would it result in other issues? Most likely. For example, you would definitely not pass the criteria associated when pulling at that test load. And similarly is that it's significantly greater than 90% of yield, which is what's recommended in other design guidelines as a maximum load you'd ever want to put on these materials. So looking at that and jumping back for a second, you can see where some of these other materials line up in those ratios. You're at a scenario of one and then 0.9. So in some cases, you can safely pull to those values, but you'll have very unintended consequences. So let's look at some other materials that are used in concrete PT work. These materials we're not going to cover in detail. Some of them have similar minimum strength requirements and other behavioral characteristics that are similar to the A722 bar. So each one of these materials should be looked at specifically considering at least the criteria that we've laid out in the tech note in here. Okay. Moving to the actual stress-strain behavior associated with the 722 bars versus the 615 bars. This plot, what we're looking at is, you know, is your classic strain and stress plot. You've got, obviously, as you increase the load, you're increasing the strain, and you have your measured for elastic modulus is a function of this slope. Now that said, is that A615 bar and A722 bar actually have very similar elastic modulus as they're only off by about 700 KSI. And so what you're going to find is that you're not going to have a big difference. So if you're measuring and estimating your predicted elongation of these bars, it's going to be similar. But that only applies up to around the 60% FPU mark. Now this plot is only of A722 bar. On the left side, you have a cold-stress bar, and on the right side, you have before cold-stressing. But so the plot on the right is more indicative of an A615 bar where you have this curve diverting away from a linear relationship at that 65% mark. So you would expect, what you would have is you'd have this elongation occurring as you approach the 80% load that is going to be greater than expected. And so all of your calculations associated with testing and elongation are not going to reflect the actual field conditions. So pre-stressing steel is used because it has the ability to hold a high force and maintain that stress and load throughout the design life of the structure. What you don't see in the ASTM spec, but you do find indirectly in ACI documents and FHWA documents, is that there is a low relaxation expectation. And that value for that low relaxation expectation is that it's around 4% or less. And we'll talk about this relaxation and the values and where they come from in more detail later. But the expectation is at 4%. And non-A722 bars, we have very little data of that relaxation because it's not a requirement in that material. And especially at higher stress loads, excessive pre-stresses can have, as we said, unintended consequences because we can have extreme load loss in these bars at higher stresses, being held at higher stresses, that can result in either failure or deformation that is not accounted for in the design. So let's dive into stress relaxation a little more and understand what that is versus creep. Because when we're doing designs with DC, with the recommendations from PTI, we understand that creep is an element of the design that if you have a creep-susceptible material, meaning the ground creep, you have to reduce your loads. In other scenarios where you'd have creep, you are, sorry, when you're doing the testing, at that higher load, we have creep criteria associated with it. So relaxation is different than creep in that it's the actual load loss holding the anchor at the same length. So when we do a creep test, we know we're supposed to maintain that load and continue to increase the pressure on the jack or elongate the cylinder. And so you're actually measuring creep, not relaxation in that scenario. But that relaxation will result in creep, that structural relaxation in the bar. Now that relaxation is not affected by the tendon bond length, but since if you increase the cylinder stroke, then that creep then is a function now of the bonded length. So a little confusing, but keep in mind that basically that relaxation is a structural, is a modification to the actual structure of the steel, and it is a function of that, and that load gets lost regardless of a one-foot tendon or a hundred-foot tendon. So reminding again that relaxation on its own is not a standard, is not part of the standard specification, but it's an expectation of the behavior of the steel when subject to that process. So let's look at it in detail. What does this mean? So I've got two examples. The first example is related to a dam tie-down. So in this scenario, your first condition, you have a true A722 bar. Your design load would be 232 kips. If you have a load loss at that design load, if you've locked off at that design load, you've reduced down to 223 kips. Now keeping in mind that you would lock this off at 70%, so you're well within the range of that load loss occurring and still maintaining your design load. If you use a grade 615 bar or grade 80 bar, excuse me, an ASTM 615 bar, and you lock off at these high stresses, you could get 20 to 33% load loss in this material. Now if that occurs in a dam tie-down where you have very little movement in the structure, assuming that it's on rock, but you have this continual load loss, this would likely move the resultant of the load in the structure outside of the middle third and potentially outside of the structure. And then if the design condition occurred, you could lift the rock or pressures raised underneath and fail the structure. So that load loss is significant when we're looking at a dam tie-down. When we look at a soldier pile and tie-back wall, however, you have, we're going to look at deformation specifically. So you have, you're considering the load loss in using the A722 bar of 0.5 inches versus using the grade 80 bar, which would be on the order of a third of an inch. That is a significant difference. And if your requirements are, you know, to maintain deformation at the top of the wall below an inch, that's a third of it already right there, taken off just from structural load loss in the, in the anchor, because that results in the, in the face of the soldier pile and lagging wall to, to move out or sheet pile wall to move out, to maintain that, that original lock off load that won't happen right away. It'll happen over time as the load, um, is initially applied. And then the ground follows it as a follow on load and, and keeps and keeps moving that anchor. So you can see in this scenario, that 0.33 inches may actually be okay. Uh, but, uh, but it needs to be understood that that's in that's on top of, uh, other deformations that are expected, uh, when using these systems. Now, lastly, we have corrosion protection, uh, corrosion protection. Uh, we have very specific requirements and PTI associated with using PT materials in the ground, and that is there, uh, because these are, these are steels that are at high stress. Now noting the difference between susceptibility to, uh, stress, corrosion, cracking, or, um, or bigger picture, uh, hydrogen embrittlement. They're about the same. Ultimately the 615 steel and the A722 steels are, are both relatively equally susceptible to the, to the, uh, stress, corrosion, cracking. However, now you're at that. You have the, it did it, the added component of the high tensile stress. And so if you're, if you're stressing these A615 bars to higher than, than what they're expected or designed to hold, then you've got them in a position with this high stress condition and in the presence of hydrogen, meaning that you have corrosion occurring on the outside, then you can have a scenario where a hydrogen embrittlement has been a real concern and that applies, of course, to the same, uh, to our corrosion for the 722 bars, which is why for permanent anchors, we have encapsulation recommended for those so that in that scenario, you would need to look at this and say, all right, if that's the case, then I need to, um, really consider the use of, of, of encapsulation of these bars, if I'm going to keep them for long-term and at high stress, because they're under the same conditions. All right. So in summary for non-A722 bars, um, if you use the PTI, uh, recommendations directly, uh, we've shown that you can basically have an unsafe condition, um, which is a great concern. Uh, you could have unintended consequences, uh, when using them, you need to understand, uh, the, you know, the stress strain behavior, uh, the relaxation, the corrosion protection are all critical components that need to be well understood when using non-A722 bars. Uh, we are not, uh, we are not providing recommendations for how to use those, but alerting you to the concerns and the things to look for when designing and installing these systems. All right. That switches us off over to the, uh, Techno 24. So that's our A722-like material. Uh, these bars are, as I said in the, at the introduction, uh, these bars are, are, have similar behavioral properties in that they have, uh, the yield and the ultimate stress capable, but they're not necessarily, uh, meeting the, the, the process portion of the specification, the cold stress and the stress relief. And so as in the end, you have a similar bar, but you have bar that may behave differently, uh, under load under long-term stressing. So revisiting, uh, our stress relaxation and creep, right? So, uh, uh, a proper pre-stressing steel needs to be able to hold a high load, uh, and a high, and a high force with minimal load loss, right? So we looked at that as, you know, a 4% point, um, in this case and the A7 by, by subjecting the A722 bars to the cold stressing and stress relief at the 80%, you've, you've created that, that low relaxation bar. And that's been the process specification. That's part of A722 since it's, uh, I'm pretty sure since it's beginning. Now, what that, you know, reiterating it once more time is that, you know, there's no max relaxation requirement there as part of the ASTM. So as we look at the table of physical properties for the deformed type two bars, right, so type one bars are not deformed, type two bars are deformed bars. We have, uh, we have a list of, of bars that go all the way from five eighths to three inches in diameter. Now it's important to know that base, that, that the larger diameter bars were added over time, uh, to the ASTM and through revisions, but what we're finding is that, and what we found is that domestically the two and a half and the three inch bars are not available with the process portion of the specification, uh, performed. So they, they may, they meet the yield and the, and the ultimate strength requirements, but there are, but, you know, as we looked at that chart at that top end from the 65 to 80%, that cold stressing changes a little bit, the behavior of that bar at the high, at the high stress condition. So reiterating some of these items, you know, those larger bars were added, you know, over 25 years ago and that domestic producers frankly have never had the equipment to cold stress these bars. So they've been, they've been in there and they, and they, you know, the intent is hopefully that, you know, people were using them with the understanding that the cold stressing hadn't occurred, uh, or at least understood what the, what the, uh, the, what the, what the result of that would be by not cold stressing. Um, but they've always been available, um, in the market with the other properties available. So, um, this has come up to light recently. Um, there's been advances in relaxation testing, uh, providing more reliable data. Um, and there's a, and let's see, I think that's where I stop. Um, Pete, you're going to jump into and talk about the relaxation testing and, um, and get really in depth into the relaxation. So I'm going to jump off there and let Pete start. Yeah. So thanks, uh, Andy, even staying on this slide before we move on to the next one, just a couple of things. You know, we talk about why, why is that right? Like why, why haven't, uh, uh, producers been able to cold stress these larger bars and it's pretty obvious that the larger bars, they just have, they have so much higher load if you're going to cold stress to 80%. So, um, you know, so the, the mills that are the mill that currently has stretching tables really just recently upgraded to include the inch and three quarter size. So, um, you know, so obviously these larger bars have always been really provided, um, you know, as a, as an, uh, we call it a seven to two like, right. So, uh, um, but yeah, so, but like, yeah, and again, you know, the advancement and relaxation testing, uh, let's see, where am I at here? Um, um, no, I get, do I have a control here? You should be able to just press the down arrow on your keyboard. Yeah. You mean it there, but so, uh, kind of delayed here. There you go. Sorry about that. So, so what, you know, so I'm going to kind of dive in a little bit more on the relaxation testing setup and, and, and really kind of get into the, the, you know, nuance of, of relaxation. Um, one of the reasons why, you know, we really never, I think one of the reasons why relaxation was never added to the spec and, and a seven to two is because of all the nuance and, and that sort of thing. And the process that was added really kind of created a low relaxation bar. So it was never really seen to be needed. Um, but, uh, you know, the other thing about relaxation testing is it's a, you know, it's a really fine tuned technical, uh, technical test you, what you're doing is you're holding these bars at a, you know, at a, at a pretty high load for a period of a thousand hours, which is essentially almost 42 days. Um, and obviously when you get up to the larger diameter bars, it requires a pretty elaborate and technical test frame. So this test frame here is pretty much the only one that, that, that I know of, uh, then, you know, when it was designed and intended to produce really good, smooth data, when you're taking data over, you know, 42 days, you got, you really have to have some precise instrumentation. So, um, RJ Peterman labs built this, uh, built this frame a few years ago. Uh, one of the things that's also critical in relaxation testing is you got to have, you got to have a constant temperature and maintaining a constant temperature for 42 days is, you know, you need a really, a climate controlled room. Um, so this frame can test up to three inch bars. One of the unique things about relaxation, you know, you want to, as Andy said earlier, you want to hold the gauge length constant throughout the 42 days with very minimal, you know, so you got to have real fine tune adjustments. This makes the use of hydraulic equipment to, to load the bars, really not really adequate for relaxation testing. So Bob kind of developed this system and he, and he's got, uh, you know, he's got basically these screw jacks in gray that you can see in the photo and they're, they're basically a hundred ton screw jacks. And he's got four of them. So he can, so he has the capacity to go up to 400 ton, a little over 800,000 pounds actually. Um, and that allows really fine tune adjustments, uh, uh, you know, throughout the life of this, of the, of the relaxation test and being able to take really good data. So, um, so let's, uh, move it on with that. So I guess, you know, through this test, through this, you know, relaxation testing that we've done, much of it, much has been learned in the past couple of years. Uh, and really there's a general consensus within the A722 committees, uh, that we really need to update A722 to reflect this knowledge because of the differences we're finding it between the A722 and the A722 light bar. So, you know, this was all led by some research that was done by the U.S., uh, U.S. Army Corps of Engineer. Uh, it's a dam modification or dam safety modification, uh, mandatory center of expertise is what that acronym stands for. So, uh, but it was largely, you know, in an effort to look at, uh, you know, look at the Bluestone Dam and kind of analyze it. Uh, and they were looking at a lot of three inch ground anchor bars were used for that. And, uh, you know, so they, they kind of want to really look at, you know, what, what they had with, because of, they were using, they use three inch A722 light bars. Want to understand really what, what, uh, some potential risks there could be. Um, so, you know, the ASTM committee we've been working, I am on that committee. We've been working to update A722 now for gosh, a couple of years. Um, there's been a lot of debate and, you know, a lot of, a lot of things, but it seems like we're getting pretty close to the addition of a relaxation threshold requirement, um, as well as including specific conditions for the relaxation task procedure. Uh, current ballot proposals are going to be discussed here in November and changes could happen as early as next year. So that could be interesting. Uh, real quick, taking a little look at some other relaxation standards around the world. One of the things that you can see pretty constant is every single one of them has relaxation values and they're all, they're all pretty similar. They're all the same condition, right? Like in terms of holding at 70% of FPU for a thousand hour test, um, very little difference other than the British standard has a slightly lower relaxation threshold of three and a half percent. All the others are really at four. So when you kind of look at this, so, you know, keep in mind that's a thousand hour test, right? So, you know, when you look at ACI 423, it has a methodology, uh, PCI also uses really the same methodology for estimating relaxation losses. And, you know, you can use it in strands. They also have charts for bars and that sort of thing, but there's a pretty simple equation. If you use a base relaxation value of 6,000 PSI, uh, and that's multiplied by a factor based on the initial pre-stress force. So let's say for PTI, the max pre-stress force is 0.7 times FPU. Um, that factor is 0.75. So what that really equates to is the anticipated relaxation loss is only 4.3% over the entire life of the structure. We're talking 50, 50 to a hundred years, maybe, or in some cases, maybe even longer. Um, so if you look at back at this previous slide, uh, you know, if you're at 4% after a thousand hours, I don't know, where are you going to be in, you know, in, in a hundred years, um, problem is we don't have, you know, it's really difficult as you can imagine to get data on bars beyond a thousand hours. Right. So, um, so there's, you know, we kind of, we kind of looked at some ways on how we could possibly do that. Uh, but one of the things here, so like what we found, what has been found in a lot of this testing that's been done between the a seven to two and a seven to two, like bar, if you look at the, the, the chart here for a thousand hours, you can get, you know, the current, current a seven to two bars on the market, really thousand hour relaxation values really between, you know, as a sum as low as like one, one and a half percent after a thousand hours up to four, which is kind of consistent with the other European codes. Um, but when we started testing these a seven to two light bars, they were pretty much all over the map. Uh, you know, some maybe fell a little below 4%, but, uh, you know, others were as high as 10% when held at 70% for a thousand hours. So 10% is pretty, pretty major relaxation loss. And obviously that was some concern that we really wanted to kind of look into from the DC 35 committee as well. Uh, so back more onto the stress, relaxation and creep and the mechanisms of what causes it. This illustration really, what, you know, what causes relaxation? Um, this is a, this is a picture of a fracture face of a bar that's pulled a failure, uh, scanning electron microscope, really high magnification. And what you can see here are these, you know, you can see these little grain structures that kind of go in between and, and all these things are kind of, you know, they're obviously microscopic grain structures, but you can kind of see when you stress a bar, these grain structures are going to allow some interactive movement, uh, of the individual grains in the steel. And what that, what that does is it allows that load loss as they, as they kind of realign and, you know, think of it on a, you know, on a real micro level. Um, but I, unfortunately we can't really get a picture of the relaxation or of the grain structures that, you know, are pre-failure, um, we don't have any good pitch or I don't have any good pictures of that, but this kind of helps illustrate uh, you know, what would happen after, uh, you know, after a large load loss. You can see that you can see the grain structures pretty clearly in this one. Uh, so kind of taking a look here, this is a, this is a typical relaxation curve, right? So of an A722 like bar, I'll qualify that, you know, this is, this was a bar that was held at 70% of FPU for a thousand hours. Um, typical curve, uh, what you have is a linear scale, right? So down on the bottom here, linear scale thousand hours, like I said, just under 42 days, and this is showing, it got a 5.02% relaxation value. So question is, you know, how do we use this data to estimate relaxation losses over the lifespan of a structure? That's 50 to a hundred years. So if you, if you put, if you kind of take the X axis out to 50 to a hundred years, right, later in 76,000 hours, um, this is pretty, pretty tough to do it like this, right? You can see on the far left side of the graph, you've got this blue curve. That's basically the curve that, that where the data range is a thousand hours, doesn't even register on this. Uh, so pretty difficult to kind of use the same thing, the same sort of scale. So what we do is we can plot this on a log curve and log curve, as you can see what, what used to be sort of, you know, uh, you know, high, high, high relaxation at the beginning and, and it kind of levels off the log curve really kind of represents more of a linear relationship when plotted on a log scale. So this allows us when we, uh, when we extend it out to 50 to a hundred years, a million, almost a million hours, you can, you can really extrapolate the values, right? So, you know, as you can see the curves on this particular, uh, test from one to a thousand hours, it's really quite linear. Um, so if you, you can project that out and you can kind of see after 50 years, you might see a seven and a half percent loss. If you started at 5%, um, at a hundred years, it's 8%. So think about that. You're like, you know, uh, your thousand hour loss, uh, and fit in a 50 year lifespan could actually be 150% of that. Um, so if you think back to the ACI criteria that I had mentioned, it's, uh, you know, that you're assuming a 4.23% loss, you know, this bar would be well beyond that. So, uh, you might be underestimating your total losses. If you use the ACI equation, especially with ACA, a seven to two light bars. So this is sort of a compilation thing. So we added a, you know, an a seven to two bar and to kind of show what it is. So say here, here, you've got an a seven to two bar. We put a curve in, um, for this one at 3.9%. Um, obviously the a seven to two range, if you, if you look at your thousand hour range, that somewhere around that one and a half percent up to 4%, you get this range and you're going to be somewhere between 2.2 and 6%. Um, and still be within like the European spec. So there is a potential, you know, uh, potential concern. If you're underestimating your losses, if you're using even a seven to two bars, it might have a real thousand hour relaxation value in that, you know, closer to the three and a half to 4% range. So, you know, where is stress relaxation accounted for in the design? Um, now, you know, like it for PTI, especially, you know, for really all applications, but specifically ground anchors, you know, we have the max test load at 80%, but your, your, your lockoff forces are at 70% of FPU and they, yeah. And that's really to account for long-term pre-stress losses. And that includes stress relaxation and steel. For ground anchors particularly, we look at its stress relaxation and ground creep or the creep between, like Andy was talking about that earlier, the grout to ground creep when you're in a creep susceptible soil, you'll get some load loss for that. And then your maximum design load is 60% FPU. So if lock-off is at 70% of FPU, your total long-term losses should not exceed 14.3% to maintain a pre-stress force of 60% of FPU. So 14.3% sounds like it could be a lot. That's a pretty decent amount. But let's back it to kind of point out some things aside from ground anchor design. In ground anchors, common sources of pre-stress losses that are applicable in pre-stress concrete are not there. So concrete creep, shrinkage, elastic shortening, they're not typically major factors of losses in many pre-stress ground anchor applications. So if you look at 14.3%, if most of that's coming from stress relaxation and steel, you might be okay. One thing, you know, talking about this a little bit here is, you know, stress relaxation and creep, Andy had talked about it on the non-A722 material. You know, relaxation and creep behavior are similar in the sense that bars with higher relaxation will experience higher creep. So, you know, when he was talking creep, changes deformation, changes strain. So when we test creep in a lab, we test it and we basically are looking at, you know, strain that occurs over a constant load. So here's a couple of graphs of some creep testing that I did, that I had done. And it was some research that was funded by the ADFC and the DFI groups. And we've got, you know, on the top curve, you got an A722 light bar creep and an A722 regular is represented in red. Huge difference in the creep characteristics of these two bars. These were inch and three quarter bars, both were inch and three quarters. They were held at 80% of FPU for a period of 60 minutes. So what's important to kind of look at here is you've got, you know, if you look at, you got 266 microstrain between 10 minutes, that's basically 377 minus the 111. So if you apply that to the PTI acceptance criteria for a creep test in a ground anchor, if you have an unbonded length of only 12 and a half feet, you'd fail a PTI criteria, the one to 10 minute criteria, 0.04 inches. So, you know, and that's just from the metal, not even any creep contribution from the soil, you know, from the grout to ground bond stress. So pretty interesting when you kind of look at it that way and why, you know, you know, the big difference is there. So something we'd be looking to address as we move forward with the DC35 committee. PTI also says if you fail the creep criteria in the first 10 minutes, you can also hold it for an additional 60, you know, for an additional 50 minutes and do a 60 minute creep test. And you measure the creep between six and 60 minutes, which is a logarithmic cycle. So this one shows that, you know, slightly, you get 375 microstrain between six and 60 minutes. You get a slightly longer unbonded length, actually, the unbonded length would have to be 17 feet, nine inches to fail the creep criteria of 0.08 inches between six and 60. So the big question that I always had when I looked at this graph, and when I first found this, I'm like, well, so why are, you know, why are creep failures common when using a 722 light bar? And I think the answer really is, it boils down to the fact that, you know, lab conditions are much more, you know, controlled than they are in the field when you're stressing with hydraulic ramps, you're overstressing the bar a little bit. PTI allows a 50 PSI plus minus ratio on your, you know, on your load when you're loading at creep. So I think a lot of those things factor into the fact I think, you know, basically people, you know, in the field, they kind of maybe, you know, stress the limits of the criteria, and you may not be getting all this creep from the metal in the bar that you know, that you're really that you're really measuring. So that's why I don't think we're failing a lot of them. So. So accounting for the impact, you know, with all these differences here, when you look at the difference between a 722 and a 722 light, obviously, there's some, you know, some current concerns and considerations that we really need to think about, about, you know, when using these, you know, these a 722 light bars. So there's four items here we can, you know, that we're that we're that I'm going to go through, I don't need to read them to you here, because I'm going to cover them in the next slides. But we're going to basically talk about how we didn't count. How do we account? How are we going to account for the large relaxation losses on relaxation sensitive applications? Now, when I say relaxation sensitive applications, kind of refer back to the two examples that Andy presented, you know, you had the dam tie down and, you know, obviously, relaxation losses can really be a problem in a dam in a dam tie down application. Now, in a, you know, in a temporary ground anchor, a temporary soil soldier pile wall or retaining wall, it not those are those tend to not be as relaxation sensitive, because, you know, the relaxation loss relatively results in relatively small movements of the wall that are typically within the allowable tolerances. So, so, you know, the first item that you can do that an engineer can do is, you know, he can obtain specific bar relaxation properties from the bar manufacturer and adjust calculations accordingly. Now, obviously, the relaxation testing that we've kind of developed over the last couple of years, there's, you know, there's still not a ton of data. We've got some pretty good data, but, you know, like, obviously, for each bar size, every each bar size, every, every little, any little tweak you make to your recipe is going to have, you know, is going to affect the relaxation of the, of the bar. So, you know, we've got, we've got some pretty good data, though, but as a, as you can see, relaxation is affected by the chemical makeup or the alloy, the process, whether it's heat treatment, or, you know, or, you know, hot rolled, cold stress, stress relief, if you're talking a 722, and the threading method, which is hot rolled, you know, hot rolled bar versus a cold rolled thread bar. The second item that you can do as an engineer is you can really, you can specify the threshold of the relaxation of the bar that you want to use, right? So, since a 722 doesn't have a relaxation threshold yet, you know, if you say, all right, I need a, you know, I want to have a bar that has 4% or less or something like that. You can do that. And, you know, and I, you know, as a supplier, I know how to, I know what I can do to do, you know, to get that, to get that relaxation value down. I can, I can tweak my recipe. I can do some other different things, additional heat treat processes and things like that to get the, get that relaxation down. However, you know, I always caution, you want to be a little careful when you, if you do this, and don't just blindly pick a value, like I want 4% less, or I want 4% relaxation. Just really consider the actual application. Cause what happens if you, if you don't, you can, you can specify a relaxation value that's lower than you need, but you know, so that may kind of make it more or less economical for certain producers to produce that bar. So ideally, if you, you know, kind of look at really what you need, you can increase competition. You can get a more economic design and save your owners some money. So the larger A722-like bars, the inch and three quarter and larger, with low or specified relaxation values, they, we can produce, we can produce them domestically and they wouldn't be A7, or they wouldn't be cold stress and stress relief, but you know, they may be available at an increased cost, you know, but they are available. So, and, you know, obviously if you're looking to use these, you can always confirm availability from suppliers. It gets a little bit difficult when you get into those larger bars, the two and a quarter, two and a quarter, two and a half and three inch. Again, specifying the relaxation, the one thing that's important to note is that the thousand hour relaxation losses done in a lab may not reflect the actual loss in the field or the application. When I say this, I specifically talk about ground anchors. You know, ground anchors, the lab data does not consider that all ground anchors are creep tested for 10 minutes to a higher load. Typically with PTI, you take, you know, the PTI recommendations, or you take your design load and you test that anchor to 1.33 times that, which is, you know, if your design load is at 0.6 FPU, you're testing to 0.8 FPU and you're holding that for 10 minutes at a higher load, and you're locked off at a lower load. So what that whole load hold does at a higher load it'll significantly reduce your relaxation loss when you lock it off at a lower load. Here's a graph that, you know, we did some relaxation testing and one of the things I really wanted to test this. So I held these bars, I held these bars at seven, you know, on the top curve, you can see this is a typical virgin bar, right? It's held at 70% FPU for a thousand or for, well, this, and I should probably point this out at the bottom, all values here, what we did was we held them at 120, for 120 hours, it was a little over five days, and then we extrapolated out. As you can see, the curve starts to become linear and it's pretty easy to present these out within, you know, it might be a couple of tenths of a percent off, but this really illustrates the differences. So with this blue curve down at the bottom is basically that same bar that had a 9.3% relaxation that was held for 10 minutes at 80% FPU, and then a relaxation test at 0.6 FPU was done for that 120 hours, and you can see the difference in the relaxation all the way down to 2.3%, much lower than even the 4% or that. So it's one thing that I look at is like, it's not, you know, when you see these bars that are higher relaxation, it's another reason why we've been using these bars for years in ground anchor applications. We've never really had any issue with relaxation losses in ground anchors, and this is why, in my opinion. So it's an interesting point to point out. This middle curve, I'll talk about that on the next slide, but just note that that curve was basically the same bar held at 0.6 FPU, and you can see how relaxation varies with the pre-stress force. If you hold that at 0.6 FPU, it's significantly lower than the 0.7. And what that means is, okay, so I can lower the pre-stress level of the bar. If I'm an engineer and I'm like, I'm a little concerned about relaxation, I can lower that pre-stress level so that the relaxation would be lower, and it's, you know, like I said, it's significantly lower. One thing, if you look at, there's not a lot of data on it, you know, there'll be some more data to back this up in the coming months, but the European standards suggest that lock-off loads less than 0.5 FPU are going to experience very little relaxation loss. And so, you know, we would expect, you know, and you kind of expect that if you really look at the stress-strain curves that we were looking at earlier, you know, once you, at 0.5 FPU, you really don't have, you know, you have a very linear stress-strain curve, even on non-A722 materials, they stayed pretty linear on that. If you go back to the picture that I showed earlier, it kind of makes sense that that, the lower loads you get when you're in that linear elastic range, you're not going to get, you know, you're not going to expect much relaxation because there's not much grain realignment there. So another thing to put it in perspective is ACI code for non-low relaxation bars is your A615 bars. You know, your typical design load is on a factor dead load is compared to about 0.48 FPU. So when you use those bars, they rarely see, you know, over 50%, you know, in a sustained loading application. So, you know, you might have, you might exceed that load in a, you know, in live loading and temporary loading situations of wind loads or seismic loads or something like that. But, you know, relaxation wouldn't become a concern in those types of, in those kinds of instances. I guess the fourth thing is you can specify a monitoring and retention program. And, you know, by this, you know, obviously we all know what the monitoring is and, you know, and I'll spend a little bit of time on that, but really the retentioning program is one of those things that's pretty interesting. If you look at, here's our, here's our curve that we showed earlier that, you know, the 5.02% relaxation after a thousand hours in seven days, really you're, you experienced 4% of that relaxation in the first seven days. So if you restress that bar after seven days, you know, you're going to have a much lower relaxation value. It's not going to exactly follow the same curve here. And the reason why is because you're not, you're going to lock off back at that, you know, you've already lost 4%. You could see you're going to, you're going to readjust that load to 4% higher. So you might have a little bit, little bit more than 1% relaxation loss in a thousand hours, but it's significant. It's going to significantly reduce it. So that's, that's another good option, pretty inexpensive. If you're really concerned about relaxation, you could even try it on a couple anchors and, you know, you know, and really measure your liftoff load to see if you really, in fact, lost that much load and just readjust it. If it looks like it's a real problem, you can continue to do it with any of them. Bar anchors, really easy to restress, strand anchors, not so much. The monitoring and, and retentioning, we kind of go over this in the tech notes, pretty, pretty straightforward, you know, they're obviously we have load cells and there's, and we discuss a little bit more of the positives and negatives of using those in the, in the tech notes. So something to take a look at. I don't really want to spend a lot of time here as we get, as we wrap up the presentation and kind of hopefully leave some time for some questions. But in summary, Tech Note 24, the A722, CREEP, all that stuff. So, you know, obviously the designers got to be aware of the different properties of both on A722 bars, as well as the A722 light bars and how those properties are really going to affect the performance of the structure. One thing I say is relaxation is complex. It's a very, very nuanced type of thing, and it's affected by materials, by manufacturing processes, by pre-stress loads, and many other things. So you can, you can get it, you know, you can definitely, you know, kind of manage it as long as, as long as you know how. Relaxation, it's dependent on stress and values, like I said, and sequence and load type and, and other, other factors. So low relaxation requirements are in industry review with the, with the changes coming soon, hopefully, to A722. So with that, I'll open it up for questions. Not a lot of time, but we do have a few minutes left. Yeah, absolutely. That was great. That was very, very technical and in-depth, and, you know, you guys are trying to cram two hours of content into an hour there, and it was well done. The first question we have is, so we talked a lot about how A722 doesn't necessarily specify the relaxation requirements. Is there a plan in the future for A722 to specify that, so that you don't have to have multiple on items as an engineer when you go to spec? Yeah, there is. In fact, there's a, there's a ballot item that was just released last week. There's, there's actually two. They're kind of, they're very similar ballot items. They're, they're looking at, you know, they're proposing about, they're proposing a five percent thousand hour relaxation value at 0.7. Why the five percent? We're a little bit over. Right now, there's just not a ton of data, and what we want to reflect is what's currently being offered in the marketplace. You know, and, you know, all the, you know, we're, the data has shown that, you know, what limited data we have, it shows that everything's kind of meeting that four percent, but, you know, I don't know. We haven't tested enough to really kind of, you know, have a confidence interval of that to, you know, so we added, we added a percent at least to kind of as a, as an initial value, and then hopefully with more data, we can kind of fine-tune some things. Sure, sure. And then, so at that point, they would, the designer would essentially be able to just specify the ASTM material, A722, without the additional, nice. All right, one more question real quick before we show the next three webinars that we have, but is there a maximum jacking stress for A722 bars, whether that be a percentage of the ultimate or percentages of the yield, any industry standards there? Yeah, I'll take this one, Pete. Yeah, our recommendations say 80 percent of guts, right, or 80 percent of ultimate, you know, minimum guts, mutts, it's been transitioned through the years, but 80 percent of the minimum ultimate strength is our maximum stressing force. Yep, I do want to acknowledge that, you know, as mentioned in the slide with Pete, that the core has done a lot of the research through Bluestone, and I wanted to acknowledge that that work has really been instrumental in bringing this all to light, and appreciate that, appreciate their work there. Awesome. All right, so I think there's a few other questions, but if we didn't get to it, you can see Pete and Andy's individual emails here to reach out to them. You can also reach out to the PTI inquiry website, and go through there, and we'll be sure to respond to that, and with just one minute left to go here, just want to tease the next three webinars that we're going to have. As I told you guys at the beginning, it's the same time every single month. It's that second Wednesday of the month at one o'clock eastern, 11 o'clock or 10 o'clock west coast time. The next three presentations are going to be, we're going to go to bridges, so post-tension bridges, bonded PT durability, and then we've had a ton of requests for slab-on-ground design stuff, so that's broken up into a two-part series. First part's going to be more on the geotechnical side, and the second part's going to be on the structural side, and that'll bring us through November and December. So that, it's the top of the hour, and we'll see you guys all here again next month at the same exact time. Thanks much guys, and have a good one. you

Post-Tensioning

Dual-Banded Layout

Kyle Boyd

Asif Baxi

Tim Crissel

Jonathan Hirsch

Karen Roberts-Wolham

Analytical Modeling

Experimental Testing

ACI PTI Code

Post-Tensioning Institute

high-strength PT bars

A722 bars

rock and soil anchors

concrete structures

relaxation testing

stress-strain behavior

elastic modulus

relaxation and creep behavior