All right, let's get this thing going. So welcome back to the Post-Tensioning Institute's monthly webinar for February. My name is Kyle Boyd, and I'm the moderator of today's session. I'm also the chair of the Education Committee for the Post-Tensioning Institute, and that's EDC 130. And that's the committee that sponsors this monthly webinar and tries to get content from within the post-tensioning industry out to the general industry there. For those of you who it's your first time joining, we do host this webinar every single month at the same exact time. So it's the second Wednesday of the month at 1 o'clock Eastern, 10 o'clock Pacific. And so if it's your first time, welcome. If you're a repeat customer, that's just awesome. As you can see, today's topic is around the evaluation of existing post-tension structures. This is a topic that we get a lot of requests on, both to do a webinar on and also a lot of technical questions through the Post-Tensioning Institute. Many of those questions are around upcoming renovation projects, changes of occupancy, inspections of existing buildings and seeing corrosion things. So it's a very popular topic that has a lot of attention. Today's speaker, Otto Schwartz, he's an industry professional who consults often on this exact type of work. Before I introduce him and go into the details, we do have our typical few housekeeping slides we got to go through. So the first one has to do with our webinar sponsor. We do offer sponsorship for this. Post-Tech Manufacturing is an awesome sponsor. They provide cable system wedges, anchors, and have a really cool tendon shear machine called the Minimax. Highly recommend you go to their website and check it out. It's a pretty neat little doohickey they got going on there. With that, we'll talk about the continuing education. So if you are on this webinar, you do get a continuing education credit. You have to be on the webinar for the entire time, and you have to be logged in through your email. If you're there, that automatically registers you for that certificate. You'll get it at the end of the webinar itself. We do our continuing education credits through RCEP, and we have AIA continuation credits for you to receive from there. So just log on, watch it, stay on there the whole time, you'll get the credit. And then as far as webinar protocol, just a couple things, or copyright material, sorry. With the copyright material, we just have to put this up to keep the lawyers happy. If you guys want any information from us, just reach out, we'd be happy to try and assist you with whatever unique case you have going on. And then for the webinar itself, all attendees, you're muted, camera's off, we can't hear you, we can't see you. If you have questions, ask them through the Q&A feature, and we'll go through them at the very end. There's a little button with Q&A, you can ask your questions in there, and we can see them. So just ask them there. We'll do our best to answer them all at the end. I already talked about you need to attend the entire webinar to get that certificate. However, if you do have to step away, or you miss it, you can go online and we record all the webinars. You can watch the webinar, take a quick quiz, and also get that continuing issue credit for there. So with that, I think I can introduce our speaker for today. Otto is a structural engineering consultant with a very reputable group out in New York, Ryan Biggs Clark Davis. He's a senior associate with the firm, and he's licensed in 13 states. So what that means is he too benefits from getting the continuation credit for watching these webinars. And he can go back and watch the previous ones if he's trying to stay up to speed with all those 13 states being licensed. He works with all the conventional building materials, but he does have a specialty and a desire to work in post-tensioning and repair and evaluation. So we thought for today's webinar, we'd bring in a working professional who does this day in and day out as one of the most qualified people to speak on this topic. You're hearing it from somebody who this is their career, this is what they do. He's very involved in the Post-Tensioning Institute, especially on DC20, which is the design committee, and DC80, which is the repair and retrofit, i.e. today's presentation. He's got well over 90 minutes worth of material to go through today, and he's going to try and squeeze that into an hour. So I'm going to turn it over to him so he can get going, and then we'll take questions at the end. Good morning or good afternoon, I guess, depending on where you are in the country. Welcome to the webinar, Evaluation of Existing Post-Tension Concrete Structures. This webinar is a product of the work of the PTI DC80 Committee on Repair, Rehabilitation and Strengthening, and is very closely based on the DC80.3 publication, The Guide for Repair and Evaluation of Unbonded Post-Tension Concrete Structures. It's available for purchase on PTI. It will go much more into depth on the topics that I will go over very quickly today to get everything in the time period that we have. This webinar is an overview of the process. It's based or aimed at engineers having, say, three to five years of experience in structural engineering and a touch on post-tension concrete theory. Hopefully some of the more experienced practitioners will also take something positive from this. Before we cover the main topic, we'll spend some time reviewing the basics of post-tensioning systems so that we know where we started, how we got here, and where we are today, and give you an idea of what you might encounter in the field. From there, we'll go into the steps, not necessarily sequential, but they can be taken in the order we present them, for the evaluation of post-tension structures, document review, review of service conditions, visual and non-destructive evaluation, exploratory evaluation, or what I'll say destructive evaluation, kind of the fun stuff, what laboratory testing can tell us to help us determine what is in fact wrong with the project or the problem, structural analysis to determine use, serviceability, and strength conditions, and then end up with a well-rounded evaluation report. Very quickly before we get too far into this, just to give you an idea of my perspective of this, I came right out of school and started designing in post-tensioning in the late 90s. We were still dealing with non-encapsulated systems, doing new construction in the southeast. I did design for about 10 years or so before I started to get into repair and rehabilitation of existing structures, starting primarily with conventionally reinforced, but then into post-tension structures and their evaluation. Honestly, the unique behavior of these structures is kind of what gets me out of bed in the morning. It's a fun topic to go through. Let's see if my, there we go. So we'll begin with the basics of post-tensioning, pre-stress concrete, primarily materials and systems. It is important to understand how pre-stress works, however, to really understand why these systems are where they're vulnerable, when they do fail, why they fail. Post-tensioning applies an active internal force to the concrete element and is designed to counteract the effects of applied loads on the structure. So where dead load and live load will cause deflection of a structure in certain areas, what post-tensioning will do will actually crank the structure the other direction. And it's the balance of that force of the alchemy of design that gets you to a successful post-tension concrete structure. Post-tensioning is a subset or a specialization of pre-stressed concrete, which falls under the umbrella of the entire reinforced concrete element. So up until now, of the understanding that there is a new document coming out to govern post-tension design, but certainly through the 19 ACI 318 code, post-tension concrete is covered with pre-stressed concrete through the ACI 318 code. Pre-stressed concrete is designed to control cracks and deflections in addition to optimizing that structural efficiency. One of the things that professors used to like to say when I would sit in the class is that pre-stressing or post-tensioning gives you your dead load for free. A typical way of looking at the proportioning of post-tensioning force or pre-stressing force is to balance some percentage of the dead load such that in its unsuperimposed load state, your structure behaves as if it were weight free. In other words, the deflected condition is that there is no deflected condition. You brought the structure back to a zero tension condition under dead load. That's kind of an older way of looking at it. With more efficient designs, more and more flexural tension is allowed in pre-stressing. For example, in going from the older ACI 318 codes used to limit design flexural stress to 12 square root, now essentially you can go to just about any value that you can rationalize is safe in design and satisfies statics. Unlike pre-stressed concrete, for PT concrete, the force is applied to unbonded tendons after the concrete has reached strength. Pre-stressed concrete stresses the tendons in the bed, fresh concrete is placed around it, and as soon as the concrete is cured, those tendons are bonded hard to the system. PT concrete, or at least the majority of applications in building structures of post-tension, utilize unbonded strands that are then stressed after the concrete has reached a certain initial strength. The problems with PT structures are not unique, and again, they're a subset of pre-stressed of reinforced concrete, and many of the problems are shared across conventional and pre-stressed construction, be that corrosion from environmental exposure, thermal and volumetric changes, or just simply damage from unintended loading. The two main types of post-tensioning that are found in building structures, and that is what we're focusing on today, not necessarily – we'll cover a little bit about some of the systems in bridges and foundations and larger structures, but in building structures such as office buildings or parking garages, started with a button head wire system, which are parallel strands that are put together in a tendon, so you have a multi-strand system with an anchor head that's typically anchored through a large steel plate. The system is unprotected around the anchor, it's all bare steel cast into the section. The 1960s, 1970s, this evolved into a seven-wire wrapped strand system, governed by ASTM A416, where the ultimate strength of these strands is 250 now, in modern times 270 KSI, and again, the five-tenths unbonded PT is the typical for building systems. Six-tenths, I have not seen an unbonded, but those are in multi-strand, multi-anchorage bonded systems for post-tensioning. So, a second summary, two that we've just recently discussed and a couple others. While this presentation will primarily discuss PT and monostrand systems, bar systems, multi-strand and grouted systems, or other types of PT that provide for greater post-tensioning force when required, we won't go too far into them other than to say bar systems are very commonly applied in, say, post-tension rock anchor systems. I've personally used them in post-tension trusses or anchorage for buckling restraint braced frames in high seismic applications. They're quite handy for that. The multi-strand tend to be more common in segmental bridge construction, and those are typically bonded grouted systems, but we'll be looking at the monostrand and wire systems. So, the evolution. The first post-tension systems were the buttonhead parallel wire strand. These were wrapped in a paper sheathing, greased and wrapped in a paper sheathing, such that when the concrete cured, there was no bond between the strand and the concrete, and it could be stressed at one end, and that stress would be transmitted through the tendon all the way to the opposite anchorage. In the 1960s, the plastic sheathing systems were developed. 1970s, that went from a loose-fitting strand to a multi-strand and an extruded sheathing with a corrosion-protected or protectant resystem, and then beginning in 1985, the fully encapsulated systems started showing up on the market. Again, personal experience, we were still using for non-critical structures, in other words, structures that weren't subjected to heavy de-icing salts, lots of freeze-thaw deterioration or coastal structures. The unbonded non-encapsulated system could be found easily into the early 2000s. So what do we mean paper wrap, push-through, heat seal? The approximate dates where these existed in construction are shown beneath each, but the paper wrap system achieved its goal. In other words, it provided a bond break between the concrete and the strand, which allowed for the stressing after the concrete had reached a certain strength, but the paper was not particularly a good barrier for corrosion protection or moisture. So the strand was pretty much left to the whims of whatever got to it through cracking and exposure of the structure. Push-through strands were an improvement on that. It was a larger plastic sheath. The post-tensioning strand was greased and simply pressed through, but this left a large annular open spaces of air, and if there was damage to the system or if water ingress in the system through the anchorage occurred, this allowed an avenue for that water to travel down the length strand and continue to cause, to wreak havoc, so to speak. The heat seal systems were a little bit better, reduced annular space, less airspace, less continuity, where the greased strand was placed in a essentially heat shrink plastic wrap. Modern systems, the extruded systems where the strand is now coated with a corrosion inhibiting coating, the plastic sheathing is extruded directly onto the strand for a very tight fit, and there's very little annular space between the strand and the sheathing. Again, the early PT systems provided friction resistance, but they did not have an intentional corrosion resistance capability, and personal experience, this has been the Achilles heel of a lot of the older PT structures up through, say, the mid-1980s or in certain cases, the 1990s. Today, if you design a new post-tension structure by ACI 3814, you've got to provide a totally encapsulated system. These systems have a tightly extruded strand where the steel is completely encapsulated, the strand is protected the entire length with sheathing and watertight transitions, so the ones I've seen, there's actually an O-ring greased seal between the sleeve and the coated strand. Similar seal between the sleeve and the anchor, the anchor is completely plastic dipped, with the exception of a small section of the bearing of the wedges, all of this is greased after the strand has been stressed and cut, and a watertight screw-on cap is placed at the end. Theoretically, although I wouldn't necessarily recommend it, you could then take this, drop it in the ocean, and the water's not going to get to it. So, evaluation of post-tension concrete structures. We'll start with a bit of a disclaimer. The evaluation should always be performed by qualified licensed design professionals in the jurisdiction of the project who are experienced with PT construction and design. I'll call it philosophy for bringing up young design engineers into this type of work, is that it's very important that you know how the building goes together before you can diagnose what's wrong with it. After having done design work for various elements of a post-tension system, you're much better able to recognize what different types of distress mean, and whether they are due to the post-tensioning, due to the mild steel, due to thermal constraint, and allows you to diagnose the system with a, I'll call it an educated background. The field personnel performing or supervising repair operations of unbonded PT systems should be PTI certified. PTI has a certification process. If I recall correctly, the International Building Code, IBC Chapter 17, requires that PT operations are inspected, that the special inspector also be PTI certified. So, as I alluded a little bit earlier, this is not necessarily chronological, but the general evaluation of post-tension structures should follow this roadmap left to right. In the state of New York, there is a legislation that has been passed that every parking garage structure needs to be evaluated every three to five years, depending on its age, for deterioration. If not legislated, such as what I've just described, you typically won't be called out to take a look at a structure unless there's some evidence that something's gone wrong. That's when the owner picks up the phone or the client picks up the phone. So, the first thing that you should ask, other than where is the structure, what's the nature of the structure, is to find out, do some study on the history of the structure, document review, and figure out its exposure and service conditions. Field investigation can certainly follow that primary method for doing so, and the initial investigation is visual, but we'll go over non-destructive evaluation techniques and exploratory or destructive evaluation. The results of that evaluation or the causes of whatever distress you're seeing can be further illuminated through laboratory investigations of properties of the concrete, the post-tension, corrosion, coating, and steel. Structural analysis, I've often found, precedes some of this, so that you know the severity of what you're looking at before you actually go out and take a look at it. But in the cases of anything but trivial damage, a structural analysis should certainly be consisted or conducted on the initial structure, and then, of course, a summary report when you're all done. So, history of the structure, what can you find? If you are lucky, the owner has the original construction drawings. A set of structural drawings for a post-tension system, unless it was 100% delegated, which I have seen in older buildings, will contain the drapes of the strand, the force per foot of the strand or the concentrated force of the strand, and the layout. This information is used by the post-tension system provider to put together shop drawings, and the snip over on the right is a little piece of what looks like a two-way plate, banded tendon in one direction, uniform tendon in the other direction, shop drawing. So, the step between simply an arrow that says 260 kips, and what that means in terms of the number of tendons, location of tendons and drape, is cleared up through the post-tension shop drawings. So, if you're even luckier, the owner will have and have kept the post-tension shop drawings, which will give location, number, and spacing that you should expect during construction, certainly help you verify that what was built in the field is what was supposed to have been built in the field. Additional information that I tend to find less available, but sometimes get lucky on, are the stressing records, the elongations and gauge forces that prove that the tendons were stranded and anchored to the correct force. Previous investigation reports can be very valuable and save a whole lot of time during investigation. Someone else may have already done concrete properties, may have already located broken strands, you may be coming in as a secondary or second opinion. The loading history, use of the structure, whether it has undergone adaptive reuse, whether it's still being used for the purpose for which it was designed, very important. And then lastly, any opportunity you have to speak to the owner or maintenance staff who were around when the building was constructed and have a history of it. But I think you should be very careful not to lead these people with your question. Make sure that you get an unbiased answer. In other words, don't approach them and say, ah, I bet that crack's been there since two weeks after construction. Am I right? Because the owner will tend to want to there's a general feeling that you want to agree with people and and make them happy and they'll go oh yeah it could have been it could have been two weeks after it's much better to say to ask a question blankly without any leading details and get an honest recollection of when events occurred because they can certainly certainly change your diagnosis of building the history of a structure when you're doing the document review take a look at or know what to expect before you go out in the field in terms of what the exposure of that structure is a office building that is completely enclosed and watertight and sealed with an environmental barrier of a current wall system is going to show a very different type of behavior than a parking garage than a coastal loading dock or some other structure that is exposed to moisture and chlorides on a regular basis knowing whether or not that structure has undergone adaptive reuse or remodeling will give you some idea as to the likelihood that things may have been damaged during that remodeling process so after the i'll call it somewhat cursory historic review because you'll probably want to go back to those after you see what's in the field field investigation is the next step and the most value can be maintained simply by wandering the structure and taking a look at what is there and what you think should and shouldn't be there visual observation can help you detect crack deteriorations note deflections water staining or grease linkage from the strand corrosion impact damage from vehicles etc then following up on that non-destructive evaluation methods and there's quite a few of them listed here we'll go through these and we'll go through what each of these can add to your investigation and what they might be used for not every non-destructive evaluation technique is applicable to every type of distress that you might see in a structure field investigation and visual examination cracking and leakage may be indicators of structural distress brought on by construction or design era issues or remodeling issues, for example, if tendons were damaged or cord removing penetrations in a bathroom per se excessive cracking in PT structures is a warning sign of problems but it's important to note what those cracks indicate whether those cracks indicate a global failure whether those cracks indicate a potential for a loss of strength or ultimate strength capacity when walking a structure take note of deflections vibrations in areas of floor slabs or spans or other evidence of potential structural distress because again if you've been called by the owner to take a look at it there is certainly something that has caught their eye there's a reason you're getting called to begin with the important part is to not simply focus on that reason to look at what other symptoms may be surrounding that reason that will help explain what's wrong with the structure if you are i'll say lucky or unlucky enough to be in a structure where you can find exposed strand this happens a lot of times in one-way slab systems where the slab tendons come up over the beam there's not enough cover and you can then perform a visual examination of exposed tendon sheathing in some cases the sheathing is even gone evaluate the tendon itself without having to do any destructive removals if water infiltration has gotten into the strand and cause deterioration and corrosion of the strand and damage the tendon sheathing at low points you'll note grease linkage or other type of salt deposits as shown in the photograph on the right i've mentioned adaptive reuse a few times uh this is a on the left i hope they didn't need all of those or i should say i hope they did need all of those a lot of new pipe penetrations it is very very common that in a reuse or after the sale of a building that the the nature of the fact that it is pt structure may or may not have been communicated to the new owner who goes in and may unwittingly have performed modifications that damage the pt system quite often when i did the my early work in nashville and was designing office buildings that changed hands quite frequently i had the number of a local contractor on speed dial so that we could go in and repair cut strands that occurred due to coring installation of a dog bone and re-stress and repair the system and kind of get it back up to speed but those techniques will be covered in part two of this of the dc80.3 presentations certainly but not always corrosion is an issue all of these pictures i think are kind of put forth to scare you this this is by far worse in a state of corrosion that i think i've seen in any structure that i've had to evaluate the upper left photo is a heavily corroded and deteriorated and pitted seven wire strand the lower left corner is a button head system an anchor that looks as if it came off the titanic perhaps the right hand side is an anchor zone multi-strand anchor that has undergone significant corrosion so for the visual inspection it's i guess important to think about where you would expect for strand to have distress if the system is completely encapsulated i would recommend looking for signs of corrosion or signs of leakage as the first cue of where to look for distress if it is in fact the tendon system and not the mild reinforcing that has started to corrode remembering that in buildings constructed as as much as only 15 maybe 20 years ago were unencapsulated systems looking at areas where water can get through the concrete into the system and cause that distress is certainly where you should focus exposed slab edges and anchor zones are notorious slab cracks at mid spans certainly are locations where deterioration can occur but if the strand sheathing is unreached it's usually fairly well protected expansion joints and construction joints we'll talk about in the next couple of slides are great candidates for areas of deterioration high points where the strand is too close have abrasion damage etc also are typical spots where problems can occur it's certainly something you should check out as you're walking or make sure that you view as you're walking a building and doing your evaluation visual inspection of the tendon anchorages up until again late 90s even early 2000s the system on the right was fairly common in new construction and non-exposed areas i can recall doing construction administration on projects that we were doing for new construction and that strand that was shown that the bare strand that's shown between the sheathing and the anchorage the typical detail was to simply wrap it with duct tape well anchor zones are areas of very high stress of the strand and the reinforcing steel that is placed behind it is placed there to help confine the concrete behind the anchor head thus improving its bearing capacity for the seated anchor but some amount of micro cracking certainly does occur grout pockets are also problematic for having avenues for allowing water into this system and again the system in the older ones anyway were unprotected these were bare steel systems if the grout pocket is not cured properly if the grout in the pocket has a significant or a noted amount of shrinkage micro cracking around the perimeter occurs and water can simply wick in that avenue and make its way directly to the anchor zone with some apologies to my mechanical metallurgy professor back in grad school i can't explain the chemistry of it right at the moment but the potential for corrosion for steel increases with the amount of stress that's on the steel i'll be happy to look up a source or two on that if someone is curious but it has to do with the straining of the metallic crystalline microstructure makes it more susceptible so we have an area here of the highest stress in the strand if you consider friction losses across the strand the most susceptible steel and of course an area where water and chloride ingress is not uncommon expansion joints are another great place to call swimming pools and collect chlorides in the parking garage structures especially exposed structures the photograph on the left pen is pointing to the top of a unprotected post-tensioning monastrand anchor that has been exposed due to corrosion of the edge angles in the right hand photograph the metallic angles that have broken apart the concrete and allowed deleterious materials to get directly to the steel edge angles also typically have headed studs so the corrosion will go from the edge angle down the stud and provide a a wonderful wedge to break across or break apart the concrete section construction joints again problematic just the same as an end anchor you don't seem to have the problem of the stressing wedge or the pocket that gets grouted but you still have a location where you have a cold joint of the concrete construction joints and anchor zones in existing buildings are commonly designed a couple of ways the one that's shown in the upper left hand corner is a continuous strand so the intermediate anchor is stressed strand is left continuous the next concrete placement is conducted and then the tendon is stressed again so from an analysis standpoint and in terms of the p over a or the net compression across the section the force is continuous through that construction joint also commonly in post-tension structures are pore strips where a length of the concrete structure 100 feet 75 feet or so is constructed a two or three foot strip is left out the structure is continued that area is left typically on formwork and then the concrete to close that strip up is placed at a later date which means the concrete in that area is not in fact post-tensioned it's a non-post-tension link between two portions of the structure there's a a parking garage i looked at about 90 miles west of our office here that was constructed this way and because of the lack of post-tensioning in that particular area as with all reinforced concrete structures the amount of cracking required to engage the reinforcing steel is higher and thus the ability for watering chlorides to get down to that system and to begin to degrade it occurs that unprotected strand without protective sheathing is stripped back as part of the installation process that the crews would would pull that back as i talked about wrapping it with duct tape in its final condition to allow for that anchor if it's a deadhead anchor to be seated they place the the jack on that face and pull against the wedges to create the deadhead that was cast all the way within the concrete if you see an eruption the strand is without force so these are the the telltale signs if there's been a break in the strand these these 10 interruptions are not particularly common they do have they do typically only happen where the concrete cover is at or less than three quarters of an inch or so but the grease in the strand and the fact that it's not a straight profile does a great job of dissipating the energy of the tendon force release if a break in a strand is close enough to the anchorage it's not uncommon for a small portion to uh to shoot out of the anchorage and knock the the grout plug out of the end so if the slab edge is open you'll actually see the strand sticking out of the building a foot or two and again the break has to be fairly close to the edge for this to happen for there to be enough elastic energy left to cause that to occur understanding how post-tensioning works in the building getting back to this point of doing the design before you do the evaluation just looking at this photograph um i would say this is a textbook shear crack without being able to zoom out the columns on the right looks like you have some diagonal shear cracking but we know that this is a pt building and some evaluation shows us that the strand should be in this profile here well our first assumption that the columns on the right would have been incorrect columns probably on the left and a high point and it should be lifting up on the concrete about in the a parallel location to where this crack occurs so knowing this and the fact that the columns probably on the left the shear crack is now going the wrong direction so and we'll get into nde just a little bit later in the talk but as it turns out as built the tendon profile was not correctly done has a reverse curvature understanding this and knowing that you know the tendon applies a force interior to the radius direction of its curvature in other words it tries to straighten when you pull on it understanding then why this crack is there starts to make some sense so the next few slides are not associated with corrosion deterioration but deal with improper construction or improper design and then again the yellow arrows i should hit before there's the direction of force towards the inside radius of that curvature a typical parabolic profile um if you get into pt design theory they can talk about matching the inverted moment profile gives you a profile that balances out the dead load force so where the tendon is curved upward and kind of a smile it will pull up on the section as it tries to straighten the downward curvature deposits that load over the supports but what happens if the profile isn't installed correctly if the chair heights are put in wrong in the field well at that point we now have upward and downward forces that do not align with the supports and in fact on the left hand side the downward force will tend to accentuate what would have been if it's taking all that shear that should be deposited into the column and is moving it off into the span and can cause so in addition to simply a profile that is offset local variations and a smooth parabolic profile we'll call them vertical wobble for lack of a better term can also be causes of distress and cracking that wouldn't necessarily fall into what one would characterize as a smooth parabolic profile and that's what we're going to talk about today so let's go ahead and take a look at some of the examples that we're going to look at today so let's go ahead and that wouldn't necessarily fall into what one would characterize as typical shear or typical moment type cracking in a reinforced concrete structure the reverse curvature causes actually a downward force now the net force if i sum the force from left to right and do a summation of forces f y of that it's still zero but the distribution is different in other words the downward force of the anchor the upward net force of the overall curvature and the local downward force do come out and are statically determinate but we've localized that force on an area of low cover and will in fact cause a popping or cracking and we have a photograph of just this condition i'm not sure what building this was taken from but i'm sure there are textbooks that would love to have this photograph to illustrate just this point where the strand itself has a reverse curvature or a bump which could have simply been due to something as innocent as dislodgement during placement now acia does require a minimum support of tendons at four foot on center pardon me but the language of 301 also indicates that the contractor it is their responsibility to secure it such that it cannot be displaced during placement so this is a construction error i cannot fathom that the design drawings would have had the hump in it one of those things that you should check when you're walking the deck before concrete placement in addition to construction defects of improperly placed strand or profile design defects also can cause now these design defects tend to show up earlier in the in the lifespan of the building sometimes they may not manifest themselves for a year to two years but these usually occur while the contractor is still on site while slabs are still being placed details are defects and mistakes in detailing can lead to slab edge splitting tendon eruptions and again property damage and personal injury so one of these is probably a construction defect one of them that looks to be a design defect unless the documents simply just weren't followed the first photograph is a blowout of an anchor zone which could have been caused by poor consolidation lack of proper placement of backup steel or hair pins the photograph on the right is a horizontal sweep of the strands where the strands have again tried to straighten and burst from the concrete section horizontal sweeps of tendons are a horizontal sweeps of tendons one of the advantages of a post-tension system is that the continuity in the strands can be displaced and swept horizontally as they travel from end to end of slab but that sweep and the forces that that will impose on the structure must be explicitly accounted for in the design there are if i do recall sections of 318 that indicate approximately what force and especially at construction joints needs to occur to link those strands back into the concrete section tendon blowout anchorages i've seen a couple of these in my time in new construction again rarely does this occur late in the lifespan of a building but can be caused to misplace bursting steel that confining steel right behind the anchor zone low concrete strength or voids or poor consolidation which personally have found to be the most common these normally occur during the stressing operation but deterioration can also cause these type of blowouts to occur if the reinforcing steel behind the anchorage begins to deteriorate the concrete in its capacity to take that compressive load as a supplement and refinement to visual observation we're going to go through quite a number of methods pardon me to help determine why what happened did happen with each of these not every non-destructive technique is applicable to every investigation that you might be on so i'll try to give an idea when each of these can be helpful and could be used acoustic emissions or as i like to to talk to owners when they say well you know how are you going to investigate this i said i'm going to drive out to the field and i'm going to proceed to hit your structure with a hammer it's very low tech but very very effective for determining voids and delaminations in the concrete section and it's simply based on the fact that a strike and a metallic hammer obviously works best for this the sound that the concrete emits against when hammer struck in areas where it's sound or areas when it's hollow is vastly different from a ting or a ringing sound to a dull thud or in some cases if the delamination is big enough it sounds like a bass drum this is not only conducted with hammers this can be conducted on large slab surfaces with a chain drag i've also done it by bouncing a four foot foot piece of rebar chain drag tends to pick up more shallow delaminations makes a sound very similar to pulling a chain across paper if there's a shallow delamination in the concrete section as opposed to the rattle of the chain on a on an intact surface pocket meter or a electromagnetic locator of steel, basically a metal detector, is very, very handy if you're doing removals or attempting to locate the position of the strand through the electromagnetic method. The drawback of this, I believe, is simply the depth of penetration. So if your steel is within a couple three inches of the top surface, and you're attempting to find a safe place to do destructive evaluation or verify the drape of your post-tensioning tendon, a pocket meter certainly can come in handy. The next level up for that is GPR. We happen to have a unit or two just like the one in the photograph that I have used off and on over the past decade and a half or so. And GPR is essentially a sonar. It's sending an energy signal into the concrete, and it creates a picture based on the reflection of that signal from items within the concrete that have a different sound speed velocity. So the picture you see, the GPR sample screen that's shown here, is nothing more than a calculation of the GPR system of the time it takes for the single signal to go from send to receive off the reflected item. And that sound speed is different for each material. So the GPR unit has to be set for the approximate moisture content or age of the concrete because that affects the signal speed. A few drawbacks with this is that if there is a dense layer of reinforcing steel or something fairly shallow, it will shield so that you can't see anything below it. So if you have a four-by-four wire mesh and a 12-inch thick sample or so, you may not be able to see the bottom of that. The reflections are indicative of what you're seeing. If the sound speed is significantly different in your faster or slower, you'll get either a white or a black reflection. So what we're looking at in the photograph here, the top of each hump tends to indicate the location of a bar. And the black reflection that you're seeing at about, oh, say 12 inches or so on the back surface is actually the reflection off the back of that section to air. So going to a void will provide a reflection. GPR can also be used to find other internal items. It cannot be used to find, at least not very well, PVC or plastic because the sound speed and velocity in both of those medium are very, very similar to concrete. But if that plastic or PVC is open, if there's a void, you will see the void. So I've used GPR before in hydronic slab systems to locate the tubing for drilling just as much as I have reinforced concrete or post-tension concrete. The next step up in the cost level, but certainly provides a great picture of what's going on in this section for location of the reinforcing X-ray. It's a great tool if your section is thin, for example, taking a look at a damaged prestressed double T. The thicker your section gets, some degree of caution needs to be taken in considering the results. The X-ray sample emits from a point source. So the further the reflective element is, say the rebar is from the point source, the film that's on the back face will show that element in a different location because you're taking a triangle from the point source to the reflection to the film. I was doing a repair on a building again in South Nashville after a tenant, and they were very, very diligent. They hired someone. They came out, X-rayed the slab, drew the cables out on the slab, and promptly drilled through the cable because the slab was thick enough and the cable was at about mid-depth at that point, and the picture showed it two inches over from where they thought it was. They didn't take into account the spreading of the X-ray signal to the film, but that was good. I got to use the dog bone fix and dig a little bit into that building. So the other issues with X-ray are that, of course, you have to evacuate the building. There are safety considerations, et cetera, but applications where X-ray can give you a great picture and see high detail as much as sheathing breaks and breaks in the steel will show up in X-ray imaging. Impact echo method uses mechanical impact to generate a stress pulse in the concrete. The reflection of these pulses is then read at a transducer. This system won't help in the location of reinforcing or post-tensioning tendons, but it will give you a good idea for discontinuities in the slab. Member thickness, obviously, in the back face is a discontinuity or cracks or delamination system. If you decide to do investigations with impact echo, my only word of caution would be is that it is sensitive to the dimensions of an element, in other words, the thickness to width, because the reflected signal that it reads not only bounces off the back face or any discontinuity within the section, but can also bounce off the edge conditions. So if I have a four foot by four foot square pedestal and I'm trying to read something that's 12 inches or eight inches in, typically the reflections from the sides of the pedestal will tend to muddy the image and the information that I get back. Works great on large expanses of slabs and walls. Similar to impact echo, the impulse response locates delaminations in voids, again, by creating an impact, inducing a sound wave, and reading it at a transducer. This again is much better for relatively shallow depths and location of discontinuities. A qualitative idea of the quality of concrete can be obtained through rebar hammer testing. This is not quantitative. If you know the strength of concrete and can zero the rebound hammer and the concrete that you're testing is at the same orientation, in other words, you're not going vertical to horizontal, a rebound hammer will give you an idea that the concrete in the second location is relatively similar. This is very sensitive, however, to surface quality of what you're bouncing the rebound hammer off of. So a good tool for a gut feel of concrete quality, but certainly not the final tool that I would use in giving the thumbs up to a concrete section. Windsor probe, a little bit more reliable. This is bordering on destructive evaluation. It involves driving a power actuated probe into the concrete, and the concrete strength is then estimated based on the depth of penetration of the probe and the calibrated probe. The word of caution with this, of course, is to scan the concrete section prior to usage because this does penetrate a given depth into the concrete, and there's always the possibility of bouncing off rebar or damaging a post-tensioning tendon. Acoustic monitoring. If you're doing a long-term evaluation of a structure, acoustic monitoring won't tell you the status of the strands or the system as they exist, but it's a great tool for monitoring potential deterioration over time. A series of piezoelectric accelerometers detect the acoustic energy released when a tensioning wire or strand fails. So if working with an owner to analyze a structure and work out a master plan for its repair, acoustic monitoring can offer a wonderful tool to tell you how the structure is faring and whether it is continuing to deteriorate. Corrosion potential mapping. Again, the PT itself, if it's an encapsulated system, is not in high danger of this, but the corrosion of the mild reinforcing and minimum reinforcing that accompanies that post-tensioning system, the potential for its occurrence can be determined through corrosion potential mapping. What this does is it measures the electrical differential potential. Corrosion within concrete, I like to think of it as kind of a battery. If all of your concrete is at the same potential and there's no change, there's nothing to drive the current. And if you don't drive a current, you don't get the movement of electrons and you don't get the reaction and formation of the Fe plus three ions that are susceptible to corrosion and oxidation. So what corrosion and half-cell potential monitoring tells you is the potential that exists within the concrete section, the differing in the chemical properties across the slab, and as shown in the table on the left, depending on the level of potential difference, you go from a 5% probability of corrosion occurring to 95%. LPR, by contrast, doesn't give you the potential difference. It tells you the rate that corrosion is in fact occurring, but the LPR testing is much more localized and works on a similar idea, but it drives a current and potential shift through the reinforcing and measures the polarization resistance. So kind of went through what we can do non-destructively. As you see each problem in the concrete structure and you want to determine what caused that structure, that first step of NDE will help you determine what may have gone wrong and why that may have gone wrong. Destructive evaluation also has its uses. The destructive evaluation can tell you a little bit about the structure and the chemical compounds or chemical properties of what you're dealing with. Inspections should follow approximately the same locations that you would look at for visual inspections. The first step of this is ascertaining or determining the health of the tendon system. So locating a spot where the tendon system is accessible, and at that inspection point, you can evaluate the condition of the sheathing, breach the sheathing, and evaluate the condition of post-tensioning corrosion coating. Take a look at any possible damage, corrosion, or section loss to the strand, and then finally test and see if that strand is in fact still under force or whether deterioration and damage has caused it to have a loss of force. So kind of step one, after breaching the tendon sheathing, take a look at the condition of the grease. The photograph on the left is a typical uncontaminated grease. It's a nice amber translucent color, uncontaminated. The photograph on the right is a heavily emulsified grease with heavy water contamination. At this point, the grease is no longer protecting the strand, and in fact, water has gotten into the system and certainly created a condition where corrosion can occur and loss of section can occur. After evaluation of the grease, clean that off and take a look at the strand itself. PCI put out a scale for this, and then I'm not sure that this is exactly the same one. This is not used to determine the acceptability of what's still in place, but instead used to catalog the degree of deterioration anywhere from an undamaged non-corroded strand on the left to varying degrees and depth and extent of pitting that can occur. The tendon that's shown on the right, even with severe corrosion, can still have force on it and can still be functional. We can halt the corrosion process and ensure that it is not continuing to degrade. Once you've characterized the strand, probably the easiest and quickest way to determine if the strand itself is still stressed is the screwdriver penetration test. It's a simple test to assist the investigator in determining whether the PT has force. The screwdriver, the flathead screwdriver, is driven with hammer taps between various pairs of wires. What it boils down to is if you can drive the screwdriver into the strand, there is no longer stress. Now, this has to be done between each pair because it is not uncommon for only certain wires of a 7-wire strand to break. Oftentimes, they'll ping and go one at a time. So you may test two of the strand, and they may be stressed, go around 180 degrees to the other side of the strand, and you may encounter wires that have lost force. The slide hammer test was a variation of this in an attempt to standardize the amount of force in driving that screwdriver. There's been some criticism of the SPT, the screwdriver penetration test, that is, in fact, heavily reliant on the skill of the operator. So it's not a 100% conclusive test, but it's certainly a fairly reliable one. If that doesn't do the trick, though, institute tendon testing and deflection testing can occur, where you open up a larger length of strand, apply a load in a triangular load frame, and the force in the strand, just through summation of forces and triangular deflection, can be determined based on the amount of force and the degree of deflection of the strand between the two pinpoints. Evaluation of the anchorage zone, destructive evaluation removal of the grout plug, is kind of the next step if you assume, or if you suspect, I should say, that a strand has lost stress, and anchors just observe the general condition of the grout plug and the grout material, the bond of the grout to the concrete, again, talked about that being an avenue for water to get to an unprotected strand. After excavation, the area should be dry, free of rust, the wedges should appear free of corrosion. Some degree of surface corrosion is fairly normal. And also, cracking of the wedges is normal. That's not an indication, necessarily, that they're not still holding the strand in place. A freshly stressed strand, sometimes you can see it on post-tension anchor railings, if you look at how they've anchored those, the strands themselves will crack, but they're simply acting as they should, forming a wedge and a friction bond of the strand into the anchor lug. Mono-strand inspection at intermediate anchor zones, kind of the same thing. We've talked about situations where that force is continuous through a construction joint, or where that force is discontinuous, and there's a reinforced, conventionally reinforced strip or pore strip between the two. So intermediate anchorages, again, a source of, or I should say a susceptible point of deterioration. If doing investigations around these, a word of caution that if it's a continuous strand and a portion of this has failed, that anchor can shift and slide if you don't know what side of the anchor the force is still live upon. So some extreme caution in taking a look and excavating around these. In all cases for anchor zones, if you, especially at end anchors, no chipping in front of the anchorhead should occur unless that anchor has been locked off and is safely restrained so that it doesn't shift or pop when removing concrete. For new construction, liftoff testing can be conducted. For this to occur, the tail still has to be on the strand. Tails are typically cut, routed, and capped in existing construction, but where I have seen this used and have used it myself is if there are questions in the elongation records that are reported to the engineer during initial stressing, sometimes going back and making sure that force is stressed, can verify that you have a cable that is functioning properly. Liftoffs should never occur to a damaged strand. In other words, if there seems to be distress to the anchorage, movement to the anchorage, or some other, I'll say, symptom that is questionable, don't apply the jack back to it. Laboratory testing can give us further information and make our diagnosis and prescribe a cure. We're getting close here. I need to speed up just a little bit. A few more slides. Concrete cores should be taken in areas, obviously, where we know we're not going to core through a tendon or piece of rebar. Concrete testing can tell us, obviously, F prime C can give us compressive strength, but also information on the chloride ion content, which goes hand in hand with the corrosion potential and the protection of the mild reinforcing steel. Petrographic analysis can give us information on the degree of hydration, what the concrete was made of, if there are reactive aggregates that have caused deterioration and issues with the system, or the depth of carbonation, where the pH of the system has risen and gotten to a point where it no longer has the passivation to protect mild reinforcing. Laboratory testing of strand, obviously, can tell us a little bit about the ultimate strength of the strand, whether the strand has been damaged or unbridled, et cetera. If we're doing analysis for capacity and the difference between 250 and 270 KSI matters, laboratory testing of that will provide those results. Last step here, it's 2 o'clock and I'm on the last two slides. Structural analysis, again, to give us an opportunity to know whether the deteriorated condition needs to be fixed, should be fixed, and how it could be fixed. The original structural drawings are very helpful in creating these models. I would recommend that the method of analysis try to mirror the method of design that one suspects was conducted at the time of construction, because this helps us get into the mind of the original designer and what was intended. Load testing, if the questions still exist and occur, ACI 318 gives guidance for setting up conducting and acceptability criteria for load testing. And then, of course, after the investigation is complete, a well-documented report. Think of it as trying to convince your owner that what you did and why you did it is necessary, and explain to them the meanings of your various testing and results. For further reading and resources, present the following. This presentation was based on DC80.3. There are many other documents within the PTI library that can help you out. Awesome. All right, guys. Like we said, we were trying to fit 90 minutes worth of information into a 60-minute session, and so we hit right at the top of the hour there. We are out of time for questions, unfortunately. We do have those questions. We'll try and reach out to you guys individually. If there's a question that has a great response to it, we'll send it out to the group as well. With that, I want to move forward and just kind of showcase the next three webinars. Like I said, this presentation is given at the same exact time every single month, so it's that second Wednesday of the month. March is going to be the new ACI PTI 320 code, so it's a joint code between ACI and PTI. That's a great code for designers to come into and watch that presentation. April, we're going down to slab-on-grounds again, construction maintenance of post-tension slab-on-grounds. We just get so many requests for that. We're going to continue on the slab-on-ground topic throughout the year, as we've been doing earlier this year. Then May is the first part of a two-part presentation for barrier cable design. Barrier cable design in May, and then the following presentation in June is going to be barrier cable installation and construction. So with that, we look forward to seeing you guys next month, second Wednesday of the month. Thanks so much for joining. Bye.

post-tensioning

structural engineering

corrosion issues

PTI DC80 Committee

unbonded post-tension

non-destructive evaluation

acoustic emissions

ground-penetrating radar

structural integrity

ACI PTI 320 code