239 - Assessing post-fire structural damage in tunnels with Negar Elhami-Khorasani

A tunnel can ride out a fire without collapsing (or even critical visible structural damage), but a question whether it is safe for operations, and what is its long-term residual fire resistance remains. With repair bills being in high seven-eight figures, this is more than just a theoretical question... In this episode we dig into the hard middle ground of fire damage post mild/large fires, and cover where modeling and fire science can help reducing the uncertainty and guiding decisions. With Professor Negar Elhami-Khorasani from University at Buffalo, we map how ventilation settings, tunnel slope, and fuel push temperatures into either safe or punishing regimes, and why spalling can turn a survivable event into a structural headache.

We break spalling down to first principles—vapor pressure, thermal gradients, and restraint—then translate that into a practical method: update the section as concrete “disappears” so the thermal boundary moves and heat penetrates realistically. From there, we track damage you can act on: concrete volumes beyond 300°C, steel temperatures that risk incomplete recovery, and bond loss that forces major repairs. Just as important, we model through cooling, when heat keeps migrating and residual capacity sinks. The result isn’t a guess; it’s a bounded map of what to replace and why.

We also take on the tactical questions that matter: How long would an extreme fire need to threaten collapse, given different soils and depths? What’s the real value of polypropylene fibers in high-strength mixes? How should owners structure a fast, post-fire workflow—quick checks for reopening within days, followed by a deeper, simulation-informed durability plan? By pairing observed spalling and known operations with targeted heat transfer and mechanical analysis, you can reconstruct the event, communicate risk clearly, and spend repair budgets where they return the most resilience.

If you care about structural fire engineering, tunnel safety, spalling mitigation, and performance-based design that reduces downtime, this conversation delivers a roadmap you can use.

Further reading - recommended papers by Negar Elhami-Khorasani and her team:

Structural fire behavior of tunnel sections: assessing the effects of full burnout and spalling effects

Numerical modeling of the fire behavior of reinforced concrete tunnel slabs during heating and cooling

Fire Damage Assessment of Reinforced Concrete Tunnel Linings

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Transcript

Framing The Post-Fire Problem

Wojciech Wegrzynski 0:00

Hello everybody, welcome to the Fire Science Show. Today we're going into the world of structural fire engineering, and while most of the considerations in structural fire engineering, at least the ones that I usually participate in, they consider whether the fire can destroy, collapse a structure, how safe the structure overall is in case of fire. In today's episode, the discussion went to a little different avenue, which I actually quite appreciate because what if the fire was not big enough to destroy a structure, but it's been there and it's done some damage. How do we know that the damage is significant and how do we know the damage needs to be repaired? That's a really, really, really huge question in a post-fire assessment. And uh because we're dealing with tunnels, this question is really valued at many, many millions of dollars. To answer this question, uh I have a special guest, Professor Negar Elhami-Khorasani from University at Buffalo. She's an expert in structural fire safety, and she's also a recipient of IFSS Magnuson Award. That's a very prestigious early career award, and it's very fitting because Professor Sven Magnussen was a pioneer of structural fire engineering, so I'm really happy that uh this award went uh her way. In this episode, we cover a lot of things related to structural fire safety of tunnels. We cover spalling a lot. Finally, I have a spalling episode. I've I was planning a spalling episode for such a long time in the podcast, and finally we talk in-depth on spalling, but that's not everything. We also talk about probabilistic approaches uh to structural fire safety risk and also the issue of diagnosing uh the structural damage and and fixing it. So it's uh packed with great knowledge, I've learned a lot, I hope you will so as well. Let's spin the intro and jump into the episode. Welcome to the Fire science Show. My name is Wojciech Wegrzynski, and I will be your host. The Firest Show podcast is brought to you in partnership with OFR Consultants. OFR is the UK's leading independent multi-award-winning fire engineering consultancy with a reputation for delivering innovative safety-driven solutions. We've been on this journey together for three years so far, and here it begins the fourth year of collaboration between the Fire Science Show and the OFR. So far, we've brought through more than 150 episodes which translate into nearly 150 hours of educational content available, free, accessible all over the planet without any paywalls, advertisements, or hidden agendas. This makes me very proud and I am super thankful to OFR for this long-lasting partnership. I'm extremely happy that we've just started the year 4, and I hope there will be many years after that to come. So, big thanks OFR for your support to the fire science show and the support to the fire safety community at large that we can deliver together. And for you, the listener, if you would like to learn more or perhaps even become a part of OFR, they always have opportunities awaiting. Check their website at OFRconsultants.com. And now let's head back to the episode. Hello, everybody. I am joined today by Negar Elhami-Khorasani from University at Buffalo. Hey Negar.

Negar Elhami-Khorasani 3:47

Hello, thank you so much for having me. It's a real pleasure to be here. And I just mentioned this very quickly that I've been a longtime fan of your podcast and the conversations you've been bringing to the community. And I often recommend episodes to my students.

Wojciech Wegrzynski 4:01

So that explains some spikes in the US recently. Thank you, Negar. Well, but but thank you. Thank you very much. I appreciate that. And uh hopefully today we create some great content uh for for those who come after us. And uh I cannot start the episode with anything else than congratulating you on the Magnuson Award from the ISS. Wow, amazing! Like great job there. I'm really, really happy that it went your way.

Negar Elhami-Khorasani 4:26

Thank you also. Thank you, thank you. And I should say that these are collective efforts, you know, my collaborators, my students, and I really appreciate everybody in my network basically um contributing all these years to the research we're doing.

Why Tunnels And Post-Fire Decisions

Wojciech Wegrzynski 4:43

Yeah, we're we're all blessed with people around us who allow us to do great science and great science, let's talk about it. Uh, we're about to talk about post-fire structural assessments, and as I understand, we're gonna mostly focus on tunnels. So maybe uh let's start with how how did you get to work on this? What triggered this interest uh for you?

Negar Elhami-Khorasani 5:04

Right. So basically, a couple of years ago, there was this research question that we know tunnel fires, you know, it potentially could happen. And when they happen, there could be you know extreme events. We have had historical cases, but uh not all of these fires are extreme. And when we looked at the literature, we couldn't find much um solid guidance on post-fire damage assessment. Uh, there are um you know qualitative guidance, but uh we wanted more uh quantitative assessment, a little bit more solid step-by-step approach, and that was something that was missing. And in parallel, so that's something that fire happens, then an engineer goes into the tunnel looking around, inspecting, and trying to decide, okay, is it safe to reopen? What's the decision, especially for mild fires, something that is like a gray area? You don't know if because you're looking, if there's not much damage, you're like, okay, if there's extreme damage, there's no way that you can reopen, but somewhere in the gray area. And then in parallel, in my research group, we've been working on in general performance-based design of structures. And in this case, we thought that okay, if we can frame the research question in a way that, okay, what do you do afterwards? But then use the same procedure to quantify the potential downtime and functionality of the structure during the design phase, then you can also uh design for basically cases where you minimize losses because of that downtime.

Wojciech Wegrzynski 6:36

So that's actually quite an interesting question now that you pose that, because obviously if the tunnel collapsed, you have your answer. Uh the post fire damage is huge. If the if it was like a single passenger vehicle, I would guess it's just dirty, water spray it, and then you're done. But uh, we had a fire in in Warsaw which involved uh heavy goods vehicle. Well, it was like uh a smaller type of truck, and I would estimate the the hit release rate of that fire in the range of 10, 20-ish megawatts, more probably closer to 10-ish megawatts. It was not part of the assessment after the fire, but they reopened the tunnel very quickly. But I must imagine if the fire was a little bit larger, and if there was like some obvious discoloring damage to the concrete, I don't think the decision would be that easy to be made. So that that that's a quite a valid research question. How is how big is the scatter of the fires in the tunnels? Because I assume the extreme fires are extremely rare, but are they a big part of all fires or fires are just rare in tunnels?

Negar Elhami-Khorasani 7:40

I think uh working with the US stakeholders, and in the US, the small fires happen, but again, they're quick and very small, smaller than, for example, the example you mentioned. Um, and then we haven't had a major extreme event, but they are also building new tunnels. Uh, and they did exactly have this question for us. So statistics maybe doesn't necessarily show that there's so many moderate level uh fires, but the question that authorities have here is that what if it happens, what do we do?

Wojciech Wegrzynski 8:21

Preemptively, uh before another.

Negar Elhami-Khorasani 8:23

Right.

Fire Severity, Ventilation, And Slope

Wojciech Wegrzynski 8:24

Which is not a great management strategy. Arguably, like you worry when it happens, but uh okay, for question to be asked. Um, in terms of what what okay, fire is a fire, but uh what what kind of temperature heat fluxes those tunnel structures are exposed to? Like maybe you can introduce the listeners to the range of fires and their consequences in the tunnels, and maybe how do you figure that out, how bad they will be.

Negar Elhami-Khorasani 8:50

Right. So we specifically in one case looked at uh, and I'll expand on it, but we specifically looked at a tunnel which was for passenger trains. So temperature inside the tunnel eventually, before I get to the uh heat release rate, and temperature will be a function of number of parameters, not just the heat release rate, but also even the slope of the tunnel and the diameter of the tunnel and the material properties um of you know the bounding basically here's concrete. So all of the and ventilation, ventilation also. If it's um there are fans, mechanical fans in the tunnel, all of this um makes a difference in terms of the type of temperatures you get and the way that the the temperature spreads, basically the fire spreads inside the tunnel. So we wanted because we were looking at damage, we didn't want to just look at the temperature, let's say in one cross section of the tunnel, but we also wanted to know how that profile changes along the length of the tunnel. So time and space, they were two variables. And when we started looking at the literature of potentially experiments for passenger trains that were set on fire so that we can get heat release rate, we actually found some data. And then we found some bounding values. And from there, we said, okay, this is the input fuel, and then we have different ventilation values, and the ventilation values are typically so the fans, if it's a passenger train, it means that people need to evacuate. So there's this concept of critical velocity. Basically, it means that if a fire happens, the fans should be set in a way that you do not get the fire spread in one direction, you don't get this back layering so that people can evacuate from the other side. So assuming that the operator is setting the ventilation in the range that is pushing the fire on one side, and then consider different slopes, we ended up uh running hundreds of simulations in FTS, and we had HRR values in the order of 40 megawatt as input. And then we had the other extreme, even something less than 10 megawatt. What happened in that whole range? Any fire, I would say, by combining then with the slope and ventilation and everything else, then we ended up having a completely bimodal type of temperatures. So anything more than I would say 30 megawatt fire would end up having temperatures in the order of, you know, median was maybe in the order of 1000 degrees Celsius. So we were pushing like very high temperatures. And then fires less than maybe 20 megawatt, we had low temperatures, like really low temperatures. Structurally wouldn't impact. I mean, for humans still need to evacuate, but structurally, we wouldn't be concerned. But then in between, that we would call it intermediate fires. That's where the ventilation could push the fire either go high or low. So we would get either we could get like 800 degrees Celsius or we could get like 400 degrees Celsius, depending on the condition of the slope and everything else.

Temperatures Along The Tunnel

Wojciech Wegrzynski 11:57

How much of that is like direct flame impingement near the fire source? And how much of that is just you know an average smoke temperature downstream, exposing a bigger part of the structure?

Negar Elhami-Khorasani 12:10

Right. So that's a great question. And I think um numbers that I reported in terms of temperature, those were the maximum temperatures. Maximum that would be right in the cross section where you have the source. When we were looking at this, we actually assumed that there will be multiple train cars, and then the ignition happens in one train car, but the actual fire spreads across multiple train cars, and then we had to also set an ignition threshold in FTS, and we did consider a range, it wasn't just a value. We said, you know, average is 300, but let's assume. So all of these values, all the parameters actually came with a distribution. If we didn't have any information, we would assume like a uniform distribution between a you know range for 300 to you know 400 degrees Celsius where an ignition could happen. So we did consider that potential for fire spit inside the tunnel. So those values I reported tells them that was the maximum T max in the cross section, but then there were hundreds of meters of the tunnel length that would be impacted, but the temperature profile could be actually uh the temperature could be a lower value as you go further away. So another thing though, I would mention that you may get so the fire starts at a car and then starts spreading. At some point, your highest temperature may not be right at where the ignition started. It may be like a car, you know, uh nearby. And that comes with a delay, right? So I'm not gonna deviate, but we did some experiments as part of the second part of this project, which we tested just concrete slap, uh tunnel slabs. And then one of the fire scenarios we considered, we actually had a delay. Like the maximum temperature was 850 degrees Celsius. It was one of the fire curves that we got out of the FDS, but it didn't reach 850 immediately. It had temperatures in the order of 300, 400 degrees Celsius for some time, and then it reached 500. So it's it's basically a section downstream of the ignition that sees the maximum temperature with a delay.

Wojciech Wegrzynski 14:20

Yeah, kind of like a traveling fire.

Negar Elhami-Khorasani 14:22

Exactly. Yeah, exactly.

Wojciech Wegrzynski 14:23

Nice, nice, very interesting. I ballpark number, I got to similar numbers when I was doing my right railway projects, but they were usually capped at 20 megawatts for some. Okay, yeah, and and we also were observing surprisingly low temperatures, and and probably that's because you know this the perhaps there's a like a turning point where the flames engulf the entire cross section of the tunnel. Maybe there's some kind of like inflation point where where the physics kind of changes a little bit, uh yeah, right.

Negar Elhami-Khorasani 14:53

But right, and I think the slope, like we had we had steep, we considered, and it was we we have like, for example, an example tunnel in the US, so two percent. Uh, you know, so the slope uh made a difference.

Wojciech Wegrzynski 15:06

Wouldn't ventilation cancel out slope in your case?

Negar Elhami-Khorasani 15:08

So that's the thing, but if your ventilation is also on uh so we did consider there could be an error by the operator. We're not okay. So if you're not doing exactly what you know the fans are supposed to do, so everything basically what could go wrong, everything that could go wrong, let's assume one of those cases.

Wojciech Wegrzynski 15:26

Yeah, you're proper structural engineers testing all all all possibilities and running hundreds of simulations, yeah. Typical stuff. Okay, so how does high temperature damage the tunnels exactly? Like what are we worried about in here?

Modeling Traveling Fire Effects

How Heat Damages Concrete And Steel

Negar Elhami-Khorasani 15:44

So um a couple of cases, uh, and we've been expanding. So when we started first, we said, okay, um, it's a reinforced concrete, let's say uh tunnel liner, and it will be exposed to high temperatures. What is going to happen? So concrete has low thermal conductivity, but still with these fires, uh, it will pick up the temperature and there will be a uh thermal gradient inside the cross section. So a couple of things happen. A, the area of concrete that is reaching high temperatures, and so far we've been using a 300 degree Celsius threshold. So any concrete that reaches beyond 300 degrees Celsius should probably be replaced. That's one criteria for damage without looking at it structurally, just material-wise. And then there is the reinforcement, if you have you know conventional reinforcement. Uh tunnel sections are typically in design mostly because of their shape. It's the if they're circular, if it's like even this horseshoe, so you have the count, they're mostly in compression, they're taking the load in compression, but they are still against the soil. And if you want a structure, if structure is heating up, so it wants to expand, it can't, depending on the stiffness of the soil. So the behavior actually changes. If your tunnel is in a soft soil or is in rock, you're gonna see some differences because if it's in soft soil, you're gonna see more deflections, not huge, but you're gonna see more deflections compared to rock. Whereas in the in rock, you're gonna see more stresses building up because the tunnel doesn't really have the ability to move or that displacement. And therefore, you will, depending on how long this fire and how hot this fire is, you will start building up obviously stresses and then potentially yield your rebar, again, depending on how long that fire is. And then another primary, and this is something that we spent some time trying to incorporate in the model, although it's very complex, is spalling. So when we started looking at this, we said, okay, if realistically we want to analyze and evaluate damage, we need to be able to capture spalling. Spalling is basically you have these um when concrete heats up, you have these pieces of concrete that falls off. And it can continuously, you know, you're gonna you're losing your concrete basically. And it's a pretty complex process. If I go into a little bit of detail in terms of what happens, there is the basically pore pressure buildup. I'm not a material scientist, but I know enough in terms of what um what happens during spalling. So basically, concrete contains moisture. So when you heat up concrete, basically that moisture turns into steam, vapor, and then at some point you have this vapor pressure that builds up, uh, which exceeds the tensile capacity of concrete. And the tensile capacity of concrete is not much. So then you start having these pieces fall off. That's not the only mechanism that drives balling. There's also that thermal stress built up. So you get a gradient, and because of that gradient, you build you also start experiencing tensile stresses inside concrete, which again exceeds tensile capacity. So a combination of these mechanisms and how fast concrete heats up, and what's the restrain. So the amount of restraint it has, it will again make an impact in terms of the stress built-up. All of those, so the boundary conditions, they all make a difference in terms of when and how much you're gonna get smolen. Modeling this, so there's a lot of literature out there. I would classify them as experimental and numerical modeling. So there's experimental lot of you know, different scales, small scale, larger scale. And then with numerical, I would say there are these micro-level modeling where they really go into modeling the physics of the problem.

Wojciech Wegrzynski 19:40

There's also Nasser who just drops AI on it.

Negar Elhami-Khorasani 19:42

Right, right, right.

Wojciech Wegrzynski 19:44

Which I actually find brilliant. I seen that in that, but that's another topic.

Spalling Mechanisms Explained

Negar Elhami-Khorasani 19:49

And and his paper came after we were working. So I I wish his paper would have been published while we were working on this topic. When we started looking at this, we said we have these models for heat transfer and mechanical analysis, thermo mechanical analysis, finite element analysis. We're doing all these structural analysis at high temperatures. How do we actually incorporate spelling? And we couldn't really find the straightforward way to do it in the literature. So we came up with an approach, it's very simplistic, and I'll explain what it is. And we're aware of how simplistic it is, but it actually provides us, and we do not necessarily look at it as this is exactly what's going to happen, but it's more of it will give us bounding, like a lower bound of damage, upper bound of damage, to get an idea of what could be the worst case scenario. So no spalling, fine, that's like one extreme. Perfect. Yeah. And then we went and said, hey, look, let's look at the data in the literature. And we limit it, we did again, there's a lot of data. So we just looked at larger scale tests because the shape and the size of the specimen also makes a difference. If you, for example, have a reinforced concrete column. I know we're talking in the context of tunnels, but if you have a reinforced concrete column, you can have spalling in the corners because of the shape. But this is this reinforced concrete slab for a tunnel section. So we said, let's look at relevant shapes and larger scale sizes to have a to get a better feeling of how this thing behaves, and potentially data on spalling. So we looked at the test measurements, and we found that okay, a lot of these tests they use either hydrocarbon fire curves, extreme cases, and then uh so there's RWS in the experiments, there is a wrapped ZTV fire curve, and then hydrocarbon or modified hydrocarbon from URCO. So we found those, and then we saw that in the test they record the time. So they're probably like listening to when you can hear in a furnace test, for example, that there's like sound. So They're reporting the time that something happens. We're like, okay, this is when it started. And because you know the fire curve, you can actually get the fire temperature, not the material temperature, which is another thing that we know that you want the material temperature. But you get the temperature of the fi at that point because it's a standard curve. And then also they typically report the depth of spalling, which is post-fire. Which is post-fire, right? After it's done, the simulate the experiment is done, cool down, they go and measure this. We said, okay, number one, we had to pause here because the whole motivation where we started, we said these are moderate fires, we're not necessarily looking at all these like very extreme fires.

Wojciech Wegrzynski 22:41

So because the curves that you've mentioned, they are all ridiculous. Like they are like uh RWS. It's like I might maybe wrong, but it's like 1200 in five minutes.

Negar Elhami-Khorasani 22:52

Yes, and then the maximum is 1350.

Wojciech Wegrzynski 22:55

It's it's it's absolutely ridiculous. Like we're doing those experiments in our furnaces in ITB. We had to buy a special set of extremely expensive platinum thermocouples for those tests. Because they though those curves destroy your normal equipment in the furnace. That's how ridiculously high uh temperatures we're talking about for fire resistance testing. Like in the standard fire curve, you get like 830 degrees in the first 30 minutes, if I'm not wrong, something like that, maybe 900 in an hour. That's it. And then here, like bam, 1200 five minutes. Wow, it's it's it's insane.

Data Gaps And Extreme Fire Curves

Negar Elhami-Khorasani 23:28

Like exactly, totally agree. So we worked with what we could find. So we ended up expanding our data set again. Although it's not a tunnel fire curve or relevant perhaps, but because we couldn't find anything else, we ended up looking at slab tests exposed to ISO or ASTM, which has this lower again, um, heating rate. And we said, okay, this at least has a slower heating rate. So it gives us an idea. Then, so we are now talking about less than 100 tests collected, uh, sometimes maybe 60. And uh we said, okay, we have an idea of the when we again we've been expanding. So when we started, we said, let's just look at the mean or the median of the time or and corresponding temperature where the spalling happens. And then some of these extreme cases definitely reported that there's like more uniform spot. It's like if it spalls, sometimes you can have just patches, like there's a section of the tunnel or slab that is spalling, but sometimes you basically like the whole layer it just keeps spalling, and you're gonna lose basically all your cover to your rebar, even can go beyond the rebar. So we said, um, we're going to assume for the sake of getting a just uh upper bound of damage, we will have uniform spalling. So from the measurements, we said if we know the depth and we know how long it took, they also recorded that. So we can get a rate of spalling, and then we basically came up with a code, like a routine, like for loop, where we said we're going to update the cross section in our thermal analysis, where we know at what point we want to start to remove a layer, and then we continue. So you need a really fine mesh. We're talking about you know two, three millimeters per minute. So you have a very fine mesh and you start removing these layers. And the key is that you need to update the boundary condition, the basically the heat exposure, now to the layer that is exposed to fire because you've lost concrete. So you also need to update that. So with that, you get heat penetrating even further into your section, and then that feeds into also your mechanical now.

Wojciech Wegrzynski 25:50

So basically, you have a slab which is a concrete slab, you expose it to high temperature, then you assume as some criteria coming from your experiment, the spalling must have happened, and you've just lost like five millimeters of concrete, you update your numerical model, which now loses that five mils of concrete, and you continue until you eat through it. Did I get it?

Negar Elhami-Khorasani 26:11

That is correct. And in a lot of cases, we it's not we have seen that concrete can continuously spall even beyond the rebar. Uh, in our cases, we stopped spalling at the level of rebar.

Wojciech Wegrzynski 26:23

That was another that's already critical damage, so yeah.

Negar Elhami-Khorasani 26:26

Right. And and it might actually slow down because that rebar should technically provide some level of confinement. So we even if it spalls, it should hopefully be at a slower rate um for spalling. So that's how we incorporate it in the finite element process, and then putting all of this together, because going back to your question, I was about what do you how do you expect you know damage in your structure? So we said first, let's do heat transfer analysis with the heat transfer analysis without spalling and with spalling, you will get a um what's the volume of concrete that's beyond 300 degrees Celsius, just just one measure. Uh, B, what is the temperature of steel? That's in that rebar. Yeah, that gives an indication of how much, because again, if you're beyond, let's say, 500 degrees Celsius in the rebar, um, you may start not fully recovering the rebar strength. There is some literature on that. Uh, so you can decide that okay, this is like pushing, you know, uh, it's not slight damage anymore. You're you need to look into your structure. And then the other thing, um, this is more recent bond strength. So bond between the rebar and concrete will get impacted. So at that point, you're looking at major damage in the sense that it's not like it's not safe, but you have to repair. And repairing the rebar, repairing concrete, if you're just removing part of your cover and replacing it, is not as bad as getting into repairing your steel.

Wojciech Wegrzynski 27:56

I can imagine.

A Practical Spalling Model

Negar Elhami-Khorasani 27:57

And then, so that's with the heat transfer, which we argue that it's not a lot of work because conducting heat transfer analysis, we think is doable. When it gets to the mechanical analysis, you need more expertise and you need to make sure your final element is mechanically like it's everything is working, all the thermal properties, material properties. And we also argue the the reason we say yes, it's more complicated is because we strongly argue that you should not stop at the end of the heating phase and you should continue the stimulation through the cooling and let it cool down all the way because heat continues to penetrate inside concrete, even the fire is cooling down, and then the concrete doesn't recover during cooling, it actually loses more strength. So the residual um state of your structure will be different from what is experiencing at the peak, and it could be at a worse, worse than what you experience at the peak of the fire.

Wojciech Wegrzynski 28:59

I mean, I really like how you frame the research problem because uh coming from a fire laboratory, for me, the life is easy. A client comes, they want to uh build a tunnel, I invite you to the furnace, we go RWS, it's madness. If you can survive that, you're approved, you're good to go. In your case, and and you can do that with a single fire test because we have one, well, it's multiple, but but technically, let's say, let's just assume it's one scenario. In your case, like there's like a whole plenty of scenarios that end up with plenty of different outcomes in terms of the damage to the tunnel and different repair time and different downtimes. And it's it's like there is no way you could technically feasibly uh assess that with the furnace. Because you you've done hundreds of FDS simulations, uh, that's a lot of work. But can you imagine doing a hundred furnace tests? It's impossible. Yeah, so so uh it's very, very interesting that this this this methodology allows you to to find the intermediate problem, not the worst one. They just worry, will it collapse or not? And arguably, if it's it's severely damaged and it's closed for two years for renovation, the outcome is not that much different than a collapse, right?

Damage Metrics And Repair Triggers

Negar Elhami-Khorasani 30:10

Right. And it's interesting, you brought up collapse. So I will mention this that while we were presenting this project as in one of the conferences in the US, we actually received a question that later on one of the stakeholders also showed interest. It wasn't easy, we just had to run a few additional uh simulations. So it the question came from a firefighter. And the question was okay, we understand that you're not looking at collapse and potentially you know you really need a long fire for the structure to collapse. But can you at least, as a firefighter, I would be interested in knowing how long that fire should be before there is a chance of collapse? Because I want to know about the safety of my crew. And is it like 100 hours? Is it 150 hours? Is it like 60 hours? Give me a number so that I at least know uh what sort of magnitude I'm looking at. And that that gives me a sense of, and it was, I think, a fair question. Yeah, fair question, yeah. And for that, because of that question, and because of uh the nature of the question, we ended up running one of our scenarios, an RWS care. We switched to an RWS care, which was the most extreme, and then we said, okay, let's continuously like as ridiculous as it is, like how hot it can be. We let's just continuously run this model for, I don't know, 60 hours and still to see what that structure actually just gives it. And it's given it so in two, so we looked at four different soil types, two cases eventually failed, and that was a um deep rock, so it was very deep, and then it was in rock, so it built up a lot of stresses, and the other one was um moderate um depth, so we're talking about 50 meters maybe, and then it was moderate soft soil, so moderate soft soil meant deflections, and the the the fact that they actually failed at different locations. One failed from the crown, the other one failed from the spring line from the sides because of the stress built-up. So one was the deflections that were too much and then eventually failed. The other one uh in the rock, it wasn't too much deflection, it was all the stresses that were building up that eventually uh the structure just uh you got like concrete crushing and rebar yielding and all of that.

Wojciech Wegrzynski 32:37

How big factor is the diameter of the tunnel in this case?

Negar Elhami-Khorasani 32:40

We were these ones were large diameter, okay. These ones were large diameter tunnels, so uh and everything we're they're building, and that because the question came from Fire River who was involved in the large diameter, so large diameter we're talking about, I don't know, 12, 10, 12 meters beyond. Uh so we're having like two lanes of traffic or multiple levels, like two-level train, and these are like large tunnels.

Wojciech Wegrzynski 33:04

Will it be much different if you have a cut-and-cover tunnel with just you know uh concrete port? Uh I'm not sure maybe maybe it's not common, but I know a technique where you just cut the first the walls, then you pour the the ceiling, and then you uncover the soil from underneath the ceiling to get to the bottom of your tunnel and you feel the bottom of the tunnel. Those tunnels with rectangular sections, that they're definitely not like working in compression as well as the round ones, right? So will there be a big difference in that behavior?

Negar Elhami-Khorasani 33:34

We haven't so we have we considered shallow uh soft soil.

Wojciech Wegrzynski 33:40

Okay.

Negar Elhami-Khorasani 33:40

There it didn't fail, but it was a circular shape, so it's more in compression. My guess with the rectangular shape, the culvert type shape, I think the shallow um depth will help. It won't push it because there's not so much um rate on it, but the shape is not in favor because now you're looking at things in bending. Yeah, yeah. So it would be the balance of the two. So because it's in bending, it might eventually um start, you know, and that's where you're probably gonna get deflections, and will depending on how much rebar you have and how much bending it can take. It may not the whole thing may not collapse, but you're definitely going to get some.

Wojciech Wegrzynski 34:20

You may also have a submerse tunnel where they build a section on the land, they carry it into the sea, they submerse and connect them, and they're also rectangular, and then you could have a hundred meters of water on top of that.

Negar Elhami-Khorasani 34:31

Right. So the pressure, so in some of these cases, because we want it to get some realistic, we actually have so we had soil pressure and we had water pressure.

Cooling Phase And Residual State

Wojciech Wegrzynski 34:40

And we test them all the same in the tunnel. In the in the yes, yeah. How much the conditions in an interlocked circular tube under solid rock pressure will differentiate from just uh uh furnace test where we perhaps apply a single point loading or just loading to the single element which is not locked with everything else.

Negar Elhami-Khorasani 35:02

So I think the there are two things here. One is one thing is the pressure, and the other thing is again the stiffness. That that basically the the fact that the the tunnel section is not really free to move, right? Because it's going against something, it's basically expanding against something that is really stiff, and the rock is also pushing back, right? Um, so in combination, compared to what we have in the um and then I'll mention one more thing, one more thing, is that in a furnace we are testing a section.

Wojciech Wegrzynski 35:39

Yeah, yeah, yeah. Section, not a whole ring.

Negar Elhami-Khorasani 35:41

Uh-huh. And it's a whole ring. And it's typically, I don't know, and yours is uh they're not um the you apply a load from the top, but are they also restrained? How much restraint do you apply? That's another thing.

Wojciech Wegrzynski 35:52

I I would have to ask my colleagues because I was uh ignorant observer of those uh at best. I so I I have no clue what exactly is the restraint where they connected to the further. I know they're loaded.

Negar Elhami-Khorasani 36:02

Right. So I the combination of all of these, it makes a difference. It actually makes a difference. And when we were doing our we did four slab tests, they were flat, they weren't the curved, um, like real, they were designed, the slabs were designed as a tunnel slab. So amount of reinforcement, section, thickness, everything, but it was a flat slab. And we tested four. So the way we um we applied the load from the top from within actuator, but we wanted to provide some restraint from the side because this is part of again a full cross section of a tunnel. And again, if it wants to expand inside the plane, it can't because it's there's the rest of the section. So we applied post-tensioning.

Wojciech Wegrzynski 36:50

I mean, we obviously okay, load bearing that that that that's gonna be different. Will that kind of interlocking force distribution, etc. will the loss will this also influence the spelling itself?

Collapse Time For Firefighter Safety

Negar Elhami-Khorasani 37:03

Yes, I think so. In our tests, we we had two different levels of restrain, and one was more reflective of circular, and the other one we lowered it just to be reflective, going back to your question. We we in none of our models we really worked with a rectangular cross-section, but we just lowered it because we thought in a low in a rectangular section you would have less of that axial effect. It didn't make that much of a difference in our test, but I would say because our maximum temperature was in the order of 850 degrees Celsius. So many parameters here. Sorry.

Wojciech Wegrzynski 37:37

Yeah, I mean, spalling is is such an interesting phenomenon because there is such a wealth of parameters and also interactions of those parameters between each other that play a role that it's very, very difficult to like predict quantitatively, say with high degree of confidence, confidence that I in this case it's no spawning, in this case, yes, it's spawning, and how much? It's it's all it's it's unbelievably complicated, but but still it's a problem, like practical engineering problem, a million-dollar problem, a 10 million dollar problem.

Negar Elhami-Khorasani 38:14

So because of that, we ended up doing something else. We haven't published it yet, it's coming, but we said this is so uncertain, and we really simplified the process, and it's so complicated, complicated. So we are randomly generating all these other parameters and we're running 540 simulations of FDS and doing all these like F prime C of concrete changes. Now, what was following? So instead of now selecting in in our analysis, we were adding uncertainty to that. We expanded a little bit our database because um colleagues keep testing, which is really beneficial for us. So we collected more data and we said, now let's look at a range of potential, and you'd really see the difference. Now we split the data into just looking at, and you can see the difference between a cases exposed to ISO versus cases exposed to hydrocaron, and you see, well, it starts later, obviously, and the rate of spalling is also slower. And then we said, Okay, now we have enough data, it's not great, but we have enough data to fit a distribution to each of these cases. It's actually Weebull distribution. And we said, okay, let's instead of doing one case, let's do a series of cases. There is a correlation between you know when it starts and how fast it goes. But for now, we said, given how complicated this is and all these parameters, let's just be captured, let's say it starts late, but it goes really fast. Let's not let's make it independent and run again all of these potential scenarios. And heat transfer analysis is relatively easy. Let's at least get that part into the heat transfer analysis to get the potential ranges because otherwise, however, I've we said all of this, but I we should also talk about how to mitigate spalling because oh, we'll we'll get there. I yeah, yeah. So because there is a there is a solution to some extent there.

Wojciech Wegrzynski 40:14

No, no, we we will we'll get there in just a few minutes. I I'll ask you, let's let's try a round of quick questions, quick answers. Uh, because I was collecting questions about spalling for the five years of running this podcast, and I finally have someone who's willing to talk about it. So let's do it. Now, uh, roll of moisture.

Negar Elhami-Khorasani 40:30

Uh-huh. Moisture content is another parameter. So the obviously the more moisture you have, you will have uh more vapor pressure and higher likelihood of spalling. Uh, in reality, another thing is also high strength concrete versus normal strength concrete. All of these tunnels are my job.

Wojciech Wegrzynski 40:50

I ask questions. You get the next one on the answers.

Negar Elhami-Khorasani 40:53

Quick answers.

Wojciech Wegrzynski 40:54

Yeah, no, no, okay, yeah, good high strength versus low strength. That was the next on the on the next one.

Negar Elhami-Khorasani 40:58

Okay. High strength concrete, you have a denser structure, right? Because you have high strength. So then there's less space inside your concrete matrix for those wave pressure to build up. So you definitely get more spalling again.

Wojciech Wegrzynski 41:12

So more, more, more porosity is is better in this case.

Negar Elhami-Khorasani 41:15

So permeability, yeah. The permeability of concrete will make a difference, and uh higher permeability basically let your vapor to escape.

Geometry, Soil, And Failure Modes

Wojciech Wegrzynski 41:25

Okay. Back to moisture. One more thing. Is there a safe level of moisture, like three percent, whatever? Is there a number that if my concrete is dry enough, the spalling will not occur?

Negar Elhami-Khorasani 41:36

Um, I don't know because we had low moisture in one of our tests. We had low moisture content and high strength concrete. We had two percent, we had high strength concrete. It spalled.

Wojciech Wegrzynski 41:48

Is there a safe temperature? Like threshold temperature. If I never cross 300 degrees, will it never spall?

Negar Elhami-Khorasani 41:54

Yeah, I would say even like 400. 500 pushing it. 500 might you will start seeing, but 300, 400, I haven't seen.

Wojciech Wegrzynski 42:02

So it's like degree of probability of the spawning.

Negar Elhami-Khorasani 42:06

It's pretty low. It's pretty low.

Wojciech Wegrzynski 42:08

What about stiffness of the curve? Like uh increase of temperature per minute, like if I go 100 degrees per minute versus I go 10 degrees per minute, will that make a difference?

Negar Elhami-Khorasani 42:18

I think if so, again, I haven't seen if you remain below 300 for even if you like, I mean it's just three minutes, shoot up 300. I you're not gonna get, I haven't seen um any uh uh spalling. Uh but well, let me give you one this example. So we reached 800 degrees, 850 in 20 minutes. That's not super fast compared to these extreme cases. Beyond 550, we started seeing spalling.

Wojciech Wegrzynski 42:49

I I mean in the fire laboratory, I've seen spalling happen at the ISO curves. Like I once had an experiment where I had reinforced concrete blocks which were half a meter by half a meter, and boy, that was a stupid decision because they were literally jumping from spalling, you know, on all the corners.

Negar Elhami-Khorasani 43:05

Or okay.

Wojciech Wegrzynski 43:06

Yeah, yeah, yeah. So so so again, maybe the concrete was perfect, maybe the everything was fine, maybe the temperatures were not that severe, but just you know, the shape factor and the gradients promoted it. So it's it's really, really interesting.

Negar Elhami-Khorasani 43:19

Exactly.

Wojciech Wegrzynski 43:20

How about subsurface meshes? Because that's also a common thing that people would put, like a mesh two centimeters into the concrete uh to slow down the the eating process because the mesh I I I don't know exactly what the mesh is supposed to do in here.

Negar Elhami-Khorasani 43:35

It probably helps with um not as some sort of confinement in that layer, uh but I don't think it will prevent.

Restraint And Test Limitations

Wojciech Wegrzynski 43:45

No, I I I it I think it's a mitigation strategy to reduce the the spalling to the mesh, presumably. That is my assumption why why people put that. Okay, let's go further mitigation because I know the common mitigation. Strategy is to use polypropylene uh flakes or fibers. Yeah, yes. How does that work? What what is this?

Negar Elhami-Khorasani 44:09

So basically, the idea is that because you are one of the again theories behind sprawling was the built-up of the moisture, the baby per pressure, and how you need to add permeability to concrete. But you're dealing with, for example, high strength concrete, a very dense uh structure. So these polypropylene fibers basically melt at relatively low temperatures below 200 degrees Celsius, they melt. So when they melt, it's like inside your concrete material, you're um opening up space. So when then you get that vapor pressure or thermal stresses, you you have this um uh space, additional space where the vapor can travel. So it reduces the build-up pressure uh and the likelihood of spalling. We looked into the literature and we said, okay, how much should we add of these fibers to our mix? And so of the four slabs we tested, three had fibers and one was without fiber, same concrete batch. So first we poured the concrete without the fibers and the same concrete truck, then we added the fibers and then poured the other three. So everything pretty much was exactly the same. Um, we added so after looking at the literature, uh looking at uh you don't want to add too much because basically your flowability of concrete gets impacted with these fibers, and then you have to add mixtures and all of that. So two kilograms per meter cube was the amount that we decided after reading the literature. We said it looks like all the tests that they've done, this seems like for like it working, it should work. And in our test, it worked, it did not spell.

Wojciech Wegrzynski 45:57

I don't know the exact numbers, even if new uh that you have a hefty NDA, I cannot talk about it. But uh, we we had clients who who've been experimenting with that. I would just say it's not straightforward and super easy, it's not just like take a bucket, put it to the no, no, it's not and and and you get it because it's it's an outcome of so many other elements that that go into the fire test. But indeed, we we have seen uh concrete uh blocks that perfectly work in uh in even in RWS, they they just passed the test.

Negar Elhami-Khorasani 46:26

They passed. I will say something though, which is I I don't have an I I'm actually um bringing up a question where I do not have an answer for uh well, potentially have a half answer. The question uh when again we were looking into this research, the whole philosophy here for us was damage assessment. So if a tunnel experiences a fire and then you have these polypropylene fibers inside your concrete section, great. It doesn't swallow great, but then after the event, the fibers are gone. They're melted.

Wojciech Wegrzynski 46:59

Well, but you care about the pores, and they're also created.

Uncertainty And Probabilistic Spalling

Negar Elhami-Khorasani 47:01

Yes, but I so the thing is that would you replace though that like that concrete that experienced this is like below 200 degrees Celsius, but would you go and replace it as part of repair? You could potentially you may want to replace that concrete.

Wojciech Wegrzynski 47:15

This is like also, you know, do you need to replace I that's a deeper question because why are you fixing this structure so it doesn't collapse in a future or it can survive another fire, for example? If it didn't split, even let's assume you had your concrete heated above 300 degrees, it's technically damaged, but it's still there. Your polypropellant burnt out, but it's the pores are still there. Like in the case of a next fire, it would still provide you, like uh, you know, uh enough. Perhaps, like, I mean, it it would be a layer that takes the first uh attack of the heat uh on it, and the heat will have to penetrate through it, and the bulkness, the thermal bulk of the concrete is not gonna change that much post-fire, right? Uh I mean I'm not sure. I'm not sure.

Negar Elhami-Khorasani 48:11

No, no, no, yeah, you're absolutely right. Yeah, yeah, yeah.

Wojciech Wegrzynski 48:13

So I wonder, like, but do you really need to fix it in that case? Like it'd be because uh what what I mean is that perhaps your structure was overdesigned by a factor of two, and you lost 0.3 out of that, and you still have 1.7, which is more than enough, right?

Negar Elhami-Khorasani 48:30

Definitely. I think, yeah.

Wojciech Wegrzynski 48:31

So you can argue, but I mean it's it's a I I know it's a difficult question, and and fires happen, and the road road administration has to take eventually, someone has to take responsibility for that, and it's a million dollar question. But the same question can be posed when you are trying to design the protection of the of the of the concrete. If I want to encapsulate it, do I want to spray mortar it? Uh what I'm supposed to like it, does it make a big it's an interesting question? Did you consider in your affinity lemon modeling any sort of protection layers like calcium silicate boards or something?

Negar Elhami-Khorasani 49:07

Not yet. Not yet. We are not there yet. But we did get so I'm sharing with you all the questions we got. We one of the other questions we are working on it, we have looked into it, but another primary question we got is you can see if you enter the tunnel after fire, you can see the surface, but you can't see the other side that's facing soil. Is there potential for cracking on the other side? Nobody will be able to inspect that, technically.

Wojciech Wegrzynski 49:38

This is an interesting thing because you could see that in the fire test. Actually, the fire test is the place where you would be able to do that, and you would also could observe the moisture forming on the unexposed side of the furnace to figure out some transient uh things.

Quickfire Spalling Q&A

Negar Elhami-Khorasani 49:54

That's exactly so the fire test is there, but in an actual fire, because they can't, and they would worry about the long-term durability of the concrete if of the structure, basically, if there are cracks on the other side and potentially you know moisture can get in, corrosion, all that stuff. And in a real case, they wouldn't know from any observations. So if then someone really, and again, these things take time. If you really want to be able to properly model, you have to first collect information on what happened during that event, the fire hazard part of it. And then yes, you have the information on the geometry you can get from the this the tunnel owner, you know, the state agencies, they have the the information, give it to you, model it. It takes time though. And a lot of the time, this so it would be I the way the way we're looking at it is that first decision is that whether you want to open it or not, like how how safe is it so that you get the traffic going because the downtime translates into losses. But then there will be secondary, there will be secondary decisions and repairs potentially where you would need to take time to properly analyze and figure out whether you need to do secondary repairs or upgrades if for longer term durability slash because these tunnels you want them to last for many years.

Wojciech Wegrzynski 51:20

Do do you know are there any good non-invasive uh diagnostics for that? I know this coloring and rubber hammer, that I know that just because Frankini Franchini was here and he was talking about uh uncertainties, would we use the hammer in in the in the lab, like the most perfect like condition?

Negar Elhami-Khorasani 51:37

And we still we were thinking, okay, how do they actually get this done in?

Wojciech Wegrzynski 51:42

I'm curious, I I don't know this field of of fire science that well, but uh did anyone try to do something crazy like I don't know, uh tomography of piece of concrete under this exposure to see how spot ink falls from inside. I know battery people did that. Pole shooting did that for batteries, and he was doing CT when the nail was penetrating the batteries and you could see all the interesting things inside.

Negar Elhami-Khorasani 52:04

I haven't seen any. I mean, as part of the diagnosis afterwards, you can definitely take, I mean, you do take samples like cores and things like that, but I don't think you can still get the full picture of what happens across your process, and that makes it hard.

Wojciech Wegrzynski 52:20

Okay, uh let's go up, close up. So your investigation was about what's the extent of damage. Right. What are our what are our chances to fix? What how does the fix look like? Is it replacing the concrete with new concrete and it's done, or is it more complicated?

Negar Elhami-Khorasani 52:39

No, that's pretty much what we are looking at. That's pretty much what for right now, at least, that's what we are looking at, and just basically identifying um for now if the structural safe and how much concrete we need to replace, that's pretty much it for now.

Wojciech Wegrzynski 52:54

And while doing the those assessments on sites, do people measure like displacements on the site? I don't know, run a 3D scan of the structure.

Negar Elhami-Khorasani 53:03

They do, but from a practical point of view, they pointed out that uh you really need to know what was the condition before. Uh-huh. And typically they don't have that. It's like if you even if you do the 3D scan, you really don't. I mean, you would have it from perhaps at one time originally, but not just what was the condition before the fire. So um there will be some uncertainty there that they pointed out.

Wojciech Wegrzynski 53:28

Well, also looking at your research or papers, you're comfortable doing hundreds of simulations for a research paper. I guess in case of a real fire, it would not be in a realm of impossibility to run uh dozens of simulations for fire growth informed by what has burned.

Polypropylene Fibers As Mitigation

Negar Elhami-Khorasani 53:46

Exactly. And that's what we were arguing. We said, yes, it will take time. If you really want an immediate answer, uh, A, tell us or tell yourself, like whoever is running this, uh, figure out what happened during the fire, like what was the fuel, what was the condition, try to replicate as much as possible the actual demand on the structure. And then heat transfer again is quick. Do the heat transfer, you're gonna get an idea at least of the type of temperature gradients that you got in your cross-section. Then you will see spalling. This is post-event, this is not guessing, so you can actually this time you you can measure it and you can put it in the model.

Wojciech Wegrzynski 54:22

You don't have you at least have a max boundary, like how much has spalled, right?

Negar Elhami-Khorasani 54:26

Exactly. So there you don't have to take any guesses. Now you know, so replicate that again in your model. Maybe it happened during time. Yes, sure. You don't know exactly how it spalled, but you can um run a few cases and see if you're matching the observation. From there, you get your uh thermal gradient in the section, and that is not that's still a lot of work, but it's a doable and it should not take months.

Wojciech Wegrzynski 54:54

Yeah, it's like weeks, right?

Negar Elhami-Khorasani 54:56

Right. And that helps. What that's that's basically the again for us, the argument is that we are not suggesting to replace existing approaches. Existing approaches is basically observe, inspect, look at the color, discoloration, all of that, and then take some, you know, uh destructive like cores or um non-destructive testing. This is basically supplementing with more quantitative assessment of what has happened.

Wojciech Wegrzynski 55:23

I mean, the I think there are two questions. Is it gonna collapse now? Is it gonna collapse or have severe damage? I mean, collapse as a like short uh term for any significant damage, but is it gonna collapse right now or is it gonna collapse the next time a fire happens, which could be anywhere from now 50 years ahead, right? So I mean, if you are you could ask you could answer the first question with traditional means fairly quickly, and it's expected by the traffic authority. And the second question, you can take uh uh a month through three uh to answer and and give a long-term answer to that.

Negar Elhami-Khorasani 56:01

Right. And then that comes also the structural analysis, because then you can also feed it into the structural analysis. And again, you can look at okay, what's even if if you you're not doing it a very fancy model, you can still look at your tens size stresses on the side face of the section that's um basically uh facing the soil, look at potential for all the questions that you may have in terms of how this structure actually got impacted. You can get the answer. Now, um, a lot of this is basically in a real event, but the argument is also that you can use this whole framework to uh do performance-based design in advance. And we haven't done it, but we argue that then you can also use the framework to look at potential um design of your fire protection systems, right? Uh, how much fire protection you would need to limit the damage that you could potentially endure.

Wojciech Wegrzynski 56:58

Yeah. And last question will be very selfish. Is spalling a big issue during the evacuation phase? I believe me or not, I had to answer those questions. And I we were supposed to design it so it doesn't spall and people don't get hit in the head by spalling concrete when evacuating. So is it a problem in evacuation?

Negar Elhami-Khorasani 57:17

So I'm gonna go back to one of the part of the um discussion we had. What is the safe? So if you're experiencing spalling, then the temperature of fire, we're reaching you know 500 degrees Celsius beyond 400 degrees Celsius beyond. And by that time, it's like a fire is evolving. So your people should be out of the tunnel. So from this pure safety point of view, you should not have any people inside the tunnel if the concrete starts to spawn.

Wojciech Wegrzynski 57:47

Thank you.

Negar Elhami-Khorasani 57:48

Major fire.

Wojciech Wegrzynski 57:49

Thank you. No, I have a quotable. No, don't quote podcasts in your professional work. It's just I'm I'm just gonna use it uh to annoy people. Uh but yes, uh, if you have uh a fire that can cause spalling, getting hit by concrete into your head is a list of your issues if you are unfortunately there. Um Negar, thank you very much uh for uh showing us the world of uh or talking about the world of of concrete and spalling, and uh I hope to uh see you again in the fire science show not far from now.

Negar Elhami-Khorasani 58:22

Thank you so much for having me, and I really enjoyed this conversation.

Hidden Damage And Inspection Limits

Wojciech Wegrzynski 58:27

Do you intend to go to La Rochelle for ISS? Yes, I'll be give uh the Magnuson lecture.

Negar Elhami-Khorasani 58:33

Yes, I'm looking forward to that. It's a lot of pressure, but I'll I'll work hard to uh come up with some an interesting presentation, hopefully.

Wojciech Wegrzynski 58:42

Fantastic. In that case, I I'm looking forward to that and see you in person in La Rochelle and hopefully sooner than that in the podcast once again. Thanks.

Negar Elhami-Khorasani 58:51

Looking forward to that. Thank you.

Post-Fire Workflow And Timelines

Wojciech Wegrzynski 58:53

And that's it, thank you for listening. Uh I've learned a lot, and uh the one thing that I really like that I've learned is that after a fire, you can use uh simulations and the tools that we're normally using for the design to carry out quite sophisticated post-fire assessments in regards of what kind of conditions the structure was truly exposed to. You're not as uncertain as in the design phase because you know the extent of damage, you can figure out some stuff like temperatures, you can figure out if there was a spalling, well with whole certainty you know there was spalling, which you can account for. And having all those informations, you can actually perform quite sophisticated recreation of the fire, and having it in your numerical domain, you can really look inside how the heat penetrated, and from there, well that that that's already a lot that you can know. So another space where fire safety engineers are critical because no one else is really capable of doing uh recreation of a fire at such a high level. Uh besides that, I'm really happy that we have finally covered spawning. It's such a fun thing. I mean there i I I'm not sure if there are many more complicated things in the world of fire safety where the interaction between multiple variables is so so so rich. We've brought up Professor Nasser in the episode. Uh Nasser made an online machine learning tools to predict a spalling of a colon based on his research. And there's like I think twenty parameters on the model which you can play with and increase the crease and see how much that uh improves or decreases the chance that the spalling will occur in the columns. Very, very fascinating. And I uh wish all the spalling researchers all the best because it's something that actually has made its way to the law, and we we have explicit clauses of law that tells us that we need to design the structure in such a way that the s explosive spalling of concrete is prevented. We have the clause in the code and we have to the design with that in mind. Therefore, I I really appreciate all the efforts to increase our knowledge on this extremely complex phenomenon. This would be it for today's fire science show, and there is more fire science coming your way next Wednesday, as usual. I hope to see you there. Thank you very much for being here with me. Cheers. Bye.