Charles Hanskat, P.E., the Executive Director of the American Shotcrete Association (ASA) joins the show to discuss shotcrete and what should be expected when constructing a concrete pool shell.
[00:00] - Introducing Charles Hanskat, P.E.
[01:26] - Shotcrete History
[06:39] - Shotcrete depends on velocity for compaction
[09:44] - Wet mix vs. Dry mix. What's right for you?
[11:38] - Water, Cement, and Time
[15:39] - Slump
[17:58] - Compressive strength
[19:46] - Aggregates
[21:25] - Standards and Minimums
[24:36] - Shrinkage and Cracks
[32:10] - Ideal conditions and Curing
[39:21] - Weepers
[42:29] - WU and ASA Shotcrete Certifications
[46:15] - Closing
173. Shotcrete Science (w/ Charles Hanskat, P.E.)
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[00:00:00] Introducing Charles Hanskat
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Eric Knight: Hi everybody and welcome back to the Rule Your Pool podcast. I'm your host Eric Knight with Watershape University. This is episode 173 and today, as promised in the New Rule Your Pool podcast, we are branching way beyond pool chemistry.
Now, if you've been listening to this show for any amount of time. Uh, we talked pretty much exclusively about pool chemistry for the last six years, and now we're going to start talking about other topics that affect swimming pool builders, designers, engineers. And who better to have on here to talk about these foundational issues than Charles Hanskat, a professional engineer in civil and structural. He's the executive director of the American Shotcrete Association. We work very closely with him at Watershape University. He has helped us develop some classes. In fact, they have a certification for nozzlemen, which is separate from Watershape, but they also have a certification for shotcrete and shotcrete inspection.
So Charles, thank you so much for joining our podcast today.
Charles Hanskat: Well, Eric, thank you for the invite. It's great to be able to talk about shotcrete and concrete, especially for pools. Thank you.
[00:01:26] Shotcrete History
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Eric Knight: You know, there are other types of pools. There's vinyl liners, there's fiberglass, of course, and, and those types of pools will get their chance on this podcast as well. But for right now, we're going to talk about concrete. Because in my experience, Charles, uh, with all my years at Orenda, we had so many calls that came in from homeowners in particular.
And yes, you had the chemistry ones like, oh, there's algae or cloudy water. But the main theme was my plaster looks bad. We had so many cement issues. Is it safe to say that you are a subject matter expert on cement chemistry?
Charles Hanskat: Well, cement used in concrete and shotcrete is a placement method for concrete. So I would say I'm very experienced
Eric Knight: man. Yeah, he's, he's a humble man, you know, being a civil engineer and a structural engineer with more letters and certifications after your name than most doctors. I am glad to have you on the show, which is undoubtedly the low point in your career. So thank you for being here.
Charles Hanskat: I wouldn't say it's a low point, but it's an
Eric Knight: Okay,
Charles Hanskat: interesting point. Let's call it that.
Eric Knight: well, yeah, you don't, you don't know our audience, Charles. Okay. So let me get right into the questions of what our listeners have submitted. So the first thing is. Let's just define Shotcrete because one of the questions that we get is, what is the difference between Gunite and Shotcrete?
Charles Hanskat: Okay, let's talk about shotcrete. Shotcrete is a placement method for concrete. Concrete's been around since the Roman days. They use concrete to build structures. we've refined it certainly over the last a hundred years. Shotcrete actually started in, uh, 1907, so we got over a hundred years of experience with shotcrete. The pool industry uses the term gunite for what we, in technical documents called dry mix shotcrete.
It comes from the original shotcrete, which was taking dry material, conveying it with air through a hose and adding water at the nozzle. And it was developed in 1907. And then there was a, uh, patent that was established in 1914 and a company called the Cement Gun Company bought the rights to the placement, uh, with this gun. The, what they called the, the shotcrete or the Gunite. Anything that came out of it was it was a trademark that they established and so they called everything that came out Gunite.
Eric Knight: Interesting. I didn't know that's where it came from. That's cool.
Charles Hanskat: Fifties and sixties we started to have concrete pumps that could pump wet concrete instead of with the dry mix or gunite where we had water at the nozzle. This was pre-mixed concrete and they could pump it through a hose and add air to give it velocity. And we wrote technical documents at ACI. We had to come up with a new term. So that's when we use dry mix for the old gunite and wet mix for the newer material where it's pre-mixed concrete, that's shot out of the nozzle. Adding air to give it velocity.
Shotcrete depends on high velocity. If you've ever seen it going in place, you'll notice that it it's coming out of that nozzle very fast. And honestly, it's about 60 to 80 miles per hour.
Eric Knight: Oh wow.
Charles Hanskat: if you think about, uh, you're driving your car down a freeway 70 miles an hour. You hit a concrete column. That is the impact that we get with every cubic foot concrete that's being shotcreted, whether it's dry mix gunite, or wet mix. We depend on high velocity. And part of that's achieved by the size of the air compressor, maintenance of the air compressor, uh, the type of pump that's used, the type of gun that's used in dry mix.
Eric Knight: Okay, thank you for that. Let me clarify here. So if I'm hearing you right, gunite is a dry mix that has air in the machinery to pump it through the hose and then it adds water at the nozzle. Whereas wet mix shotcrete is already pre-blended. It's pumped, I guess, pneumatically or somehow from the truck, and then you add velocity by adding air at the hose instead of water. Is that correct?
Charles Hanskat: Dry mix Gunite uses air to convey dry, relatively dry concrete materials through a hose.
Eric Knight: Mm-hmm.
Charles Hanskat: that air is also projecting it at the 60 to 80 miles per hour. We're adding water to make it concrete right at the nozzle. Wet
Eric Knight: Oh, got it. Okay.
Charles Hanskat: delivered in a ready mix truck, sometimes in a volumetric truck, a concrete mobile, where it is wet concrete that's already got all the water added to make it concrete and then pushed through the hose by hydraulic cylinders. If you've ever seen a concrete pump, you'll hear it kind of clunking back and forth. It actually allows it to push the concrete into the delivery line while two hydraulic cylinders are working in opposing directions. But it takes wet concrete, pushes it through, and then has to add air at the nozzle to give it the velocity.
[00:06:39] Shotcrete depends on velocity for compaction
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Charles Hanskat: Once again, we depend on that 60 to 80 miles per hour velocity for compaction of the concrete, uh, encasement of the reinforcing steel. The velocity is a key to shotcrete.
Eric Knight: So it by getting that velocity, instead of pouring, because when I think of concrete, I think of what they do on a driveway, whereas they bring that truck out and they just pour it and they move it around with shovels and and rakes like that. So the compaction, I mean, I understand some of this because I've been with Watershape since April now. I've learned a lot of this. But for the sake of our audience, why do we need that compaction? What does it do? Does it increase the strength of it? Why?
Charles Hanskat: So concrete needs to be consolidated to be in place. Even a driveway, they should be tamping it, they should be vibrating it to consolidate it, to give it its full density. Uh, if you just pour it out of the back of the truck, it'd be a pile of concrete. It would get hard, but it'd have voids in it. It wouldn't be as strong as what you have when you consolidate that concrete.
to, if You had a concrete pool that was formed. That was poured in place, uh, they typically would need to use a vibrator to provide that consolidation and to make sure that you get full encasement of the reinforcing steel that's in the wall section. it basically provides a lot of vibration and shakes the concrete to make this concrete consolidate in place.
Eric Knight: And it forces the air bubbles out.
Charles Hanskat: Mostly air bubbles. We still have some air bubbles that stay in there, but uh, those tend to be relatively small.
Eric Knight: Okay, so, uh, high velocity shotcrete, you said encapsulation. So I'm envisioning when a swimming pool is being built, you've got this cage of rebar and there's some plumbing in the walls, maybe there's, uh, certainly inlets and all these things. Does that mean you have to be shooting kind of behind things at different angles? What do you mean by encapsulation and why is it so important?
Charles Hanskat: Well, as an engineer when you design a concrete section, concrete's really good in compression, but not good in tension. So we put reinforcing steel to be able to carry the loads. The water loads, the soil loads that could be on the back of a pool wall. And it uh, depends on concrete fully encasing that bar. If you've ever seen a reinforcing bar, you'll notice that it's got deformations on it. Those deformations kind of lock that steel bar in place.
Now, if you have a PVC pipe, that doesn't have deformations. We don't get a lot of bond, but it's basically something we can wrap concrete around and get consolidation around the back. If it's a big pipe, the shotcreter, we used to call them nozzlemen. Now the ACI has changed the term to shotcreter. They have to have their expertise to actually think about how do, how am I pushing concrete around big pipes. uh, get around the rebar at the floor wall joint, when you're building up from the base of the wall up. Um, you need expertise of the nozzleman and to, to really be placing that concrete properly.
[00:09:44] Wet mix vs. Dry mix. What's right for you?
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Eric Knight: Hmm. Okay. Now let's get back to wet mix versus dry mix. Is there a big difference in quality? Like if I'm a pool builder, how do I make the choice what's right for me?
Charles Hanskat: Uh, Both of them are equivalent when done with the proper materials, equipment, and the expertise of the shotcreter. I started the industry in 1977, building water tanks. Water tanks, up to 20 million gallons of storage, a hundred feet tall. 350 feet in diameter, guaranteed to be water tight.
They were done entire. The entire wall was done in dry mix. So you have to do it right. Uh, the expertise of the shotcreter for dry mix is a little bit higher than required for wet mix. In wet mix, the shotcreter opens the air valve to give it velocity. They don't control the mix. In dry mix or gunite, shotcreter is actually controlling the water into the concrete right at the nozzle. That's where we were making that wet concrete. And so it takes more expertise on the part of the shotceter, but with a good company who's got a proven track record, both will give you equivalent strength, permeability, and ultimately the the long durability of concrete. You know, concrete should easily last 50 to a hundred years when done properly.
Eric Knight: The colisseum is still standing here, Charles.
Charles Hanskat: But they, they didn't have reinforcing steel and a lot of the old, uh, that's something that we developed as engineers so that we could do, um, slabs. You know, a flat slab in between columns because there's bending. Coliseum, you'll see a lot of those old structures are arches, right?
Eric Knight: Mm-hmm.
Charles Hanskat: are very good because they can span over an area without having reinforcing 'cause they stay in compression.
[00:11:38] Water, cement, and time
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Eric Knight: Interesting. Well, there's something to be said because something that I've learned is the older concrete gets, presuming it was shot correctly and mixed correctly, the stronger it gets, because over time it continues to harden. Is that correct?
Charles Hanskat: Yes. typically has plenty of cement, and part of that cement that is, that's what gives it strength. The cement and water react together making a hydration product. And there's generally un hydrated cement that is in the concrete. And so if it's exposed to water, it will get stronger and stronger.
Now, if you are out in the desert and you never see any moisture in the concrete, it won't gain much strength. But in pools or tanks, like I started in, uh, we, we have water there continuously and it, so it will get stronger and stronger.
Eric Knight: So the concrete in effect is kind of, I, I'm going to use the term weeping in or drinking a little bit of that pool water? 'Cause it's water tight, but it's not waterproof. There's a difference, right?
Charles Hanskat: Yeah, we actually get, don't get a flow of water through, excavated your pool wall and on the backside. When you have good concrete, you're not going to feel water coming through. But there are small pores in the concrete that water can get into and will kind of saturate it, but we're not. We're going to lose water and that'll flow through the wall. And that moisture saturating the concrete can lead to additional hydration of cement particles in the concrete.
Eric Knight: So let's get a little bit technical here. Some of the questions that are brought up in our construction school construction 2111. I wrote them down because I've now taken it twice, uh, sitting in the back. When I listen to questions from students, I write them down because this is clearly what people are coming to the classes for. So one of them was saying, how do I control the water to cement ratio on a wet mix truck that may have to drive a long way out to the job? Because that operator of the truck may add water to keep that big drum spinning, uh, with the concrete so it doesn't, you know, solidify in the drum.
What do you say to that? Because it sounds like if I'm controlling, using a dry mix gunite, I'm controlling the water cement ratio or the nozzle man is, or the shotcreter is. How do you do that on wet mix? Is it even possible?
Charles Hanskat: So. Most of the wet mix concrete is delivered in ready mix trucks. There is a mix design that will say to the concrete supplier. Hey, I need this many pounds of cement or cement, like materials like fly ash, silica fume, slag, uh, the course aggregate, that's the rock and the sand and the water. And they have a specified water to cement ratio.
The water cement means the weight of water divided by the weight of cement or cement like material. And it's a direct correlation to the strength of the concrete. And ultimately the the permeability and durability. There are people doing driveways that say, hey, I want a 0.5 or a 0.55 water cement ratio because they want it to be like soup. We can't do that in shotcrete. Our water cement ratios are typically lower than what you see in cast in place because we have to shoot it in place and have it hang vertically. Uh, without, generally it is not a two-sided form. It's a one-sided form in pools, sometimes no form. Uh, we can shoot shotcrete overhead. that's not a pool issue unless maybe you're doing a grotto or something that's attached to a pool
Eric Knight: That's like a tunnel, right? For a subway or something like that?
Charles Hanskat: Yeah, we, well, we can do underground, uh, we can do a lot of architectural and carved rock types of things, which you do see in some pools. So it is controlled by the ready mix supplier. They will, say, have a 0.42 water cement ratio. The truck should be delivered with that water in it.
[00:15:39] Slump
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Charles Hanskat: Now if the driver is trying to be super helpful, oh, everybody wants a six inch slump, you know, a really flowable concrete, and it shows up to the job, we can't shoot it. It's not going to stay in place.
So. It is a matter of quality control on the part of the concrete supplier. Slump is, it's a measure of the flowability of concrete. So higher slump, it flows more, it spreads out. If you have it coming out of the back of the truck, it, it flows a lot more. If you have a six inch slump, it's going to flow more than a three inch slump, and we're typically three, to
Eric Knight: is that like six inch flow per second or something? How, how's that
Charles Hanskat: Slump is a measure of, it's, it's a very kind of crude, but we have a cone that we fill full of concrete, uh, 12 inches high. We uh, consolidate it a little bit with a tamping rod, and then we take the metal cone off, and it slumps down and we measure how far does that pile of concrete slump down?
And it can be, know, one inch, it can be six inches, it can be eight inches. It, uh, depends on how the flowability of the concrete. How much water is in there. If we add more water every time we're adding water, we're reducing the strength of the concrete and increasing the permeability, which are key
Eric Knight: and increasing the slump.
Charles Hanskat: So slump is a measure of that, though a lot of times in shotcrete, uh, our experience, shotcrete contractors can look at the concrete coming down the chute and know whether that's the right slump or not. So we may not need to do a test. But it is the slump, is the, the predominant test to the flowability of concrete.
Eric Knight: Nice. So for the pool builders listening to this, what should they be asking their shotcrete subcontractor for? What slump, what water cement ratio? For wet mix and for dry.
Charles Hanskat: Typically our water cement ratio is 0.45 or less. Typically, a 0.42 is pretty common for shooting walls. We're looking at a three to four inch slump. it came in at one to two inches, it's too dry. It's hard for us to get concrete to flow around the bars. Dry mix, I can't really tell you the water cement ratio because the water's added right at the nozzle.
[00:17:58] Compressive Strength
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Eric Knight: But what should he, what should the shotcreter be trying to achieve? Is 0.42 is what it sounds like?
Charles Hanskat: Uh, 0.35 to 0.45. And so in that range is generally good. The minimum strength we should be looking for, compressive strength, is 4,000 PSI at 28 days. Most shotcretes, because of the high paste content, we'll get, 5,000 PSI. Sometimes 7 to 14 days. Well before that 28 day span.
Dry mix generally has less water because we don't have to push wet concrete through a hose. We're just adding water at the nozzle. So dry mix the water cement ratios tend to be less a good mix design. Uh, once again, a, a good shotcreter, they can generally get as good if not better strength and low permeability than wet mix. But once again, it is dependent on the experience of the shotcreter at the, you know, controlling the water at the nozzle.
Um, so as a pool builder, which we do, a shotcrete inspector class. We have a full day seminar and have had a lot of pool builders come in. And, uh, give a lot of guidance to, to what to look for, uh, for your shotcrete contractor. If you as a pool builder are buying the concrete, get the mix design. Make sure that it is less than 0.45 water cement ratio, that the strength is 4,000 PSI minimum at 28 days. And the slump, if they're asking for a six or eight inch slump that's too high, it should be a lower slump. And the shotcrete contractor should know that. Don't just call up the Ready Mix plant and yeah, send, send out the shotcrete mix. You know, try to get documentation of the actual concrete mix design.
[00:19:46] Aggregates
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Eric Knight: Thank you. Is there any truth to dry mix Gunite having smaller aggregates or needing to have smaller aggregates rather than having this nice gravel aggregate in the mix? Because I, I've heard it's more sand.
Charles Hanskat: No. Uh, both will have about, both of them are more, there's generally more sand or fine aggregate in shotcrete mixes because we're, we're pumping it in wet mix through a small hose. The hoses are typically two inch hose, so sometimes they're going down to an inch and a half. We're doing the same in dry mix, we can shoot a three eight inch aggregate in dry mix the same as you can in wet mix. And in dry mix, there are pre-packaged materials that come in with fiber, they have the rock, and, and they have uh, just as large an aggregate. We can't, you know, we wet mix, we can't shoot an inch and a half 'cause we're trying to push it through a two inch hose. So that's
Eric Knight: Yeah, it would get stuck.
Charles Hanskat: we'll typically have a smaller aggregate. Three eight inch is the normal limit that we see for size, and that might mean that you occasionally get a half inch rock in there. But that's good enough to push through that two inch hose.
Eric Knight: Right. Well, the reason I ask is because in our plaster class construction 3611, which by the way, we are teaching at, the Phoenix Education Vacation, I believe you will be in that class teaching the shotcrete section with Bill. And, also in San Antonio at the Southwest Show. But in that shotcrete section, there's a lot of photos actually of way too much sand in the mix. And it looks brown, and then you could just, you know, if the concrete fails, you could just literally take a hammer and just tap it and it just crumbles apart.
[00:21:25] Standards and Minimums
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Eric Knight: Beyond water to cement these aggregates, how do they need to be blended in? And does the American shotcrete Association that you're the director of, do you have specifics of what a pool builder should be giving to their shotcrete contractor and say, I need this.
This is the quality mix that I demand. Do you have a document like that?
Charles Hanskat: Okay. There are pool builders, uh, some pool designers who say you only need 2,500 PSI concrete at 28 days to be able to build a pool. Honestly, that's not enough paste in the concrete mix to provide good encasement of the bars to provide you low permeability. That's why we stress that the strength needs to be at least 4,000 PSI. So you get enough cement paste in the concrete to wrap around the bars. So wet mix, have a mix design, you, you get what's in the truck or volumetric. They have a mixed design too for the volumetric. Um. Dry mix, we say that you should have at least one part cement to four parts sand, or you can do one part cement to three part sand and one part rock. if somebody comes in and I have seen pool shotcreters not, not the top of the line, who are using seven parts sand to one cement. And that is, and they're saying, well, I only need 2,500 PSI, but you don't have enough cement paste to really provide good compaction, good encasement of the reinforcing bars.
So ask your, know, if it's, if it is dry mix gunite, ask them, well, you know, what mixed proportions are you using? And they can use rock. the rock does help to, uh, reduce the shrinkage a little bit. But, um. There are a number of them that in the past have just done sand and cement. Uh, some
Eric Knight: Well hold on.
Charles Hanskat: Of dry mix now is actually, sorry I cut you off, but a lot of dry mix now is using volumetric mixers where they actually can do a mix design out of the truck. It's a concrete mobile where they mix the sand and the rock and the cement together and then put it into the gun and add water at the nozzle. Sorry.
Eric Knight: Nice. Oh no, I, you're, you're here. I, I'm going to, I'm going to defer to you. Is this 4,000 PSIA standard from the ACI or ASA? Because I know that's what Watershape teaches, but what's the standard?
Charles Hanskat: At ASA, we are not a standards developing organization like an American Concrete Institute or ASTM. But we have put together position statements and one is that we really stressed that the concrete needs to have at least a 4,000 PSI strength.
And that's for not only strength, but lower permeability. Well, just think about it. If instead of a four-part sand to one cement, you do seven or eight parts, you got twice as much sand with the same amount of cement. It's going to be more porous. It's not going to have the strength, it's not going to have the low permeability of good concrete.
[00:24:36] Shrinkage and Cracks
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Eric Knight: Makes sense. Now, you said the word shrinkage. Let's shift gears here. Uh, I know like check cracking and spider cracking on plaster. It's, it's going to be very similar chemistry for the listeners' out there. But we're going to talk about shotcrete here. We're going to talk about the shell of the pool. All concrete cracks. I've learned that we talk about that in pretty much every Watershape class that deals with concrete. Why does it crack?
Charles Hanskat: I have a big sponge that I wash my car with. And when that sponge is wet, it's a certain size, but if I put it on a table overnight, it dries out. And what happens to that sponge? It, it gets shorter. Right?
Eric Knight: Yeah, it gets smaller.
Charles Hanskat: And what happens with concrete is the same. Because the cement and water react, it takes a little bit less space. The concrete actually shrinks like that sponge. Now if I take that sponge, that big sponge, let's say it's a foot long and nail down the two ends to the table, nail it down when it's wet. I come back the next morning. It's the same length, right? Because I nailed down the ends. But what happens now? That sponge has tension. Concrete's good in compression, not in tension. And that's why we put reinforcing steel into the concrete.
That's drying, shrinkage, that is through the entire thickness of the wall or the, the concrete, whatever concrete section you're doing.
Eric Knight: So the water that you use to hydrate it in the truck, or at the nozzle, is evaporating out. After these reactions take place, it leaves. Therefore, the volume of the material has to shrink. Therefore, it creates these little micro cracks?
Charles Hanskat: And we always get, uh, shrinkage. There's, some big words like autogenous shrinkage. That's the reaction of cement and water. It actually takes less volume. But the concept is it dries out and it tries to shrink and it creates tension and we put in steel. Now that's drying shrinkage.
There's also what we call early age plastic shrinkage cracks. And these are the craze cracks you talk about. Say I'm casting in Phoenix, Arizona. It's a hundred degrees high temperature. Dry, low humidity, maybe wind. And what happens is the water evaporates at the surface of the concrete. Concrete's wet, it's got water in there. The water evaporates from the surface, and once again, it tries to get smaller. It can't because if I just go an inch or two inches down into the concrete, it's still wet. It hasn't been exposed to that evaporation at the surface. And so that's when we will get those, what I would call early age plastic shrinkage, cracks. People call them craze cracks.
Eric Knight: Or check cracks.
Charles Hanskat: they're generally random cracks. And so that, that can be prevented. We can put, uh, a mist of water. Uh, When we, when I did tanks back in the sixties, well actually seventies and eighties, uh, we would use a pressure washer just to kind of mist the surface of the concrete so it wouldn't allow that water to evaporate. And would prevent the early age plastic shrinkage cracks. So they don't have to happen.
Drying shrinkage, on the other hand, because of concrete, the way it's made, the way the water's in there and cement hydrates, we will always get some drying shrinkage. And that's why a good designer will put in reinforcing steel accommodate that drying shrinkage. If they don't put in enough steel, concrete will crack. The steel holds it together.
Eric Knight: Crack enough that you could see it? Because micro cracks you can't see with the naked eye, correct?
Charles Hanskat: Typically a five mill, which is five thousandths of an inch crack will be very tough to see. And that's the point that it will leak. If you have a through crack that's bigger than five thousandths of an inch, it will leak. Smaller, it generally will be, the water won't penetrate, but you have to be right on top of that crack to see it. Now get a crack that's 10, you know, 10 mils twice as big, you're, you're going to see that. And, uh, if there's inadequate reinforcing in the wall, those cracks will tend to be bigger. You put in steel, what happens? Yeah, it shrinks, but it's, it holds it together. So those crack widths don't allow leakage.
Eric Knight: Makes perfect sense. Now, in the plaster class, I teach the water chemistry and the cement chemistry segments on the second day. When plaster, because it is cement based. When that shrinks, it will shrink, something has to fill those voids. Two things about this. There's this concept called the interfacial transition zone, which I'm sure you're familiar with around rock it. It's especially true in in smooth aggregates like pebbles and glass beads, because cement does not chemically bind to those aggregates, when it shrinks, it's a lot more likely to separate from that aggregate, kind of like releasing the baseball slightly than to unlock from all these, you know, cement to cement, strong binding connections that creates a void space. So my question to you is, in concrete, what fills that void space and how do we protect it?
Charles Hanskat: We don't necessarily see that much void around the rock because most of our aggregates are course, they're, they're pieces of rock, whether it's
Eric Knight: They're gritty. Yeah.
Charles Hanskat: Um, and we, we do get some bond to that. I've looked at thousands of cores taken out of concrete sections. Whether it's a structural wall or tank or a pool, and you don't see voids around the rock. Now what happens in concrete? We talked about the fact that there's un hydrated cement, so if it stays wet, what happens when we get hydration? it's a chemical reaction that causes it to kind of, like to equate it makes spikes and plates. It's like growing crystals.
I don't know if you ever had crystals when you were a kid. You grew the crystals in
Eric Knight: Mm-hmm.
Charles Hanskat: and they grew.
Eric Knight: The sugar crystal things. Yeah.
Charles Hanskat: uh, Concrete's the same way. It grows these spikes and plates then will fill those voids. We still get some small voids, but we don't typically get delamination of the aggregate.
Now, in a plaster, it's such a thin coating. There may be more of an issue there, and if it's a smoother aggregate, it may be more of an issue. But, concrete, generally, you know, you look at a core, it's monolithic material. To really see the bubbles, you almost have to use a microscope. There are tests that we do to establish how much air is in a given concrete.
Eric Knight: Yeah. And the hydration process, like once water has added to it, I've felt shotcrete. It gets hot.
Charles Hanskat: Right?
Eric Knight: It gets pretty hot, and that accelerates the evaporation, right?
Charles Hanskat: To some extent. Um, if you have a really thick section, you may approach 150 degrees Fahrenheit inside that section. Still maybe not enough for evaporation. Usually the evaporation is only at the surface, and that's from solar gain, wind, lower humidity.
If I'm doing a, a concrete slab in Puerto Rico and it's 90% humidity, it's not going to get nearly the number of cracks, those craze cracks that you would see if you were in Arizona. Very low humidity, and that basically pushes up the evaporation that you get from the water from the surface.
[00:32:10] Ideal conditions and Curing
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Eric Knight: Well, real quick, I didn't have this question written down, but what would be the ideal weather conditions for shooting a shotcrete Pool? In Charles Hanskat's opinion. What's the ideal? Obviously people listening to this can't control the weather. Charles can, but nobody else can. No, I'm just kidding.
Charles Hanskat: I, I wish I
Eric Knight: What would be the perfect day? I.
Charles Hanskat: be a wealthy man,
Eric Knight: Yeah. Yeah, indeed.
Charles Hanskat: cloudy day, you know, um, not low, low humidity.
Eric Knight: Mm-hmm.
Charles Hanskat: you know, as long as it's more than 50 degrees, I'd say 60 to 80 degrees would be the ideal.
Eric Knight: you shoot in light rain?
Charles Hanskat: You, you can, and as long as it's not ponding up. Now if I'm in Florida in a thunderstorm and it's ponding up, no. If I'm in Seattle, with a misty rain, yes, you probably could. Though you have to be careful because when you're finishing, more moisture at the surface, you could actually be weakening that surface if the moisture collected.
Eric Knight: That makes sense. But the process of curing I learned in your position paper. Well, I don't know if you wrote it, I presume you were involved with it, but ASA's position paper, I learned something that I did not realize, just in the last few months. I always thought curing was a natural process that happened within the concrete. But apparently curing is an action that is external to the concrete that we do to it to actually try to enhance strength gain and keep it cool and, and, you know, keep the water from leaving too fast. Is that correct?
Charles Hanskat: Yeah, curing is something that's standard in all concrete construction. Basically, you provide moisture to the concrete to, once again, take that unrated cement, give it water, let it hydrate, and it fills the pores. It makes it stronger. We recommend a minimum of seven days of continuous wet curing. I do see in the pool industry, people tell the homeowner, oh, we'll just water it twice a day. You know what, in Phoenix?
Eric Knight: Hm, that's not enough.
Charles Hanskat: of summer, they can water it. It's going to look gray for about 15 minutes, and then it's going to be just like it's bone dry.
Eric Knight: Right?
Charles Hanskat: reason you're doing that, you're giving it more water to hydrate the unhydrated cement that is in the concrete mix. And you're getting more for your money. Whatever you paid for that yard of concrete, if you just cure it for another seven days wet curing, you're going to make it stronger and less permeable. It's going to last longer. You're getting more for your money by just providing that curing. Some pool builders will flood the pool. Uh, some may put a sprinkler on, where they actually will kind of sprinkle that entire wall to keep it damp. Uh dripper hoses at the top. But the idea is to keep it continuously wet.
Eric Knight: Where is the dividing line between doing that, the wet cure and affecting the water to cement ratio? I have had that question before on plaster especially. But on shotcrete, if I add water to it, sprinklers or whatever, is that not affecting the water to cement ratio? No,
Charles Hanskat: concrete's kind of weird. If you add it while it's liquid, it's flowable, that raises the water cement ratio. But once it's set. We're not affecting water cement ratio, we're basically just hydrating the cement that is un hydrated within the concrete section. So we're not reducing the strength, we're actually adding to the strength.
So concrete's kind of weird, 'cause if I put it in the truck into the wet concrete, that's bad.
Eric Knight: Mm-hmm. Yeah.
Charles Hanskat: but if I put water on it after it's hard, then it's good.
Eric Knight: That's good.
Charles Hanskat: So it's.
Eric Knight: I know in plaster, like wet troweling is a big problem. Now, I'm not a plasterer, but I've talked to enough of them and I've talked to enough builders that see like white smear marks all over the floor because maybe it's set too fast. So they'll get their trowel wet, they'll throw some water on it, and then maybe they don't get the same kind of compression, but they change because it's still, you know, malleable. They change the water to cement ratio and you get all these check cracks on those areas.
Charles Hanskat: Yeah.
Eric Knight: I know it's a visible problem and you know where it comes down to. One of the main reasons I had you on this show, Charles, and thank you by the way, for being on here again. Um, you could have this amazing project as a homeowner and you invest a hundred thousand plus dollars, maybe it's 200,000.
Maybe you're going big and it's half a million dollars. I don't know. But you invest in this backyard dream. That pool contractor and landscape contractor could do everything right. And then a plaster goes in and it could be exposed wrong, or there could be wet troweling, or there could be all these white marks, uneven carbonation, all the just cosmetic problems with plaster that can ruin the perception of that project. According to Bill Drake Lee, our founder, basically bad plaster starts with bad shotcrete. In other words, your plaster can only be as good as the shotcrete it's binding to. What do you say to that?
Charles Hanskat: The plaster is a coating and it is the final coating. So it's what you see and you want to do
Eric Knight: Right.
Charles Hanskat: but the water tightness, the liquid tightness of the pool shell is fundamentally in the concrete itself. And if you have cracks, if you have low strength, concrete, if you have cracks in the concrete, those will probably mirror through your plaster. Because what happens, at least, most pools, there are some indoor pools, but most outdoor pools, get summer, winter cycles, right? And so in the summer the concrete expands. In the winter it contracts. And so if there's a crack in the winter, it's going to be bigger than it would be in the summer. And that's a moving crack that will probably mirror through your plaster coating and probably almost any coating. Uh it's hard to span over that crack that moves as it expands and contracts summer to winter.
Eric Knight: I've also seen rebar grids coming through the floor of pools. Mostly commercial pools, but I could clearly see orange grids through it. I've seen it dozens of times. What causes that?
Charles Hanskat: On the floors?
Eric Knight: Mm-hmm.
Charles Hanskat: That's typically bleed water. What happens if they cast the floor? They're using a little bit higher water cement ratio, typically, but what happens is water does tend to flow up through the concrete. When it hits the bar, it goes around the bar and it kind of makes, it does reduce the water cement ratio.
And so you can sometimes see that bar pattern. Especially if you have less than, I'd say less than two inches of cover, on that bar. You can see that and it'll be a darker area that, uh. May not be much lower in strength, but it, it's just that it's changed kind of the water cement ratio in that concrete right above the bar.
If you see that in the wall from shotcreting, that's a sign that the shotcreter, uh, may not have been paying as much attention to the placement as they needed to. Because in a vertical wall, that bleed water goes up. It doesn't come out towards the surface like you see on a floor.
[00:39:21] Weepers
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Eric Knight: Interesting. Okay. One last question and then we'll talk about the classes that we're involved in. Weepers Weepers are when moisture come from the outside of the pool, shell into the pool. They follow weaknesses and void spaces. What's the most, or what are the most common reasons that swimming pools have weepers?
Charles Hanskat: Inadequate strength of the concrete. Now concrete is about seven times weaker in tension than compression. And so that's why they put in reinforcing steel. If the pool designer didn't put in enough steel to hold the concrete against the shrinkage, it can crack. And those weepers are through cracks. And that's typically from drying shrinkage. The early age plastic shrinkage cracking we talked about, that's craze cracking. It looks terrible on the surface, but if you core that, they only go about an inch down into the concrete. So the weepers happen from drying shrinkage. Um, it can also happen because Sometimes a designer has designed it properly, but say you have a 10 inch wall, but then you had your excavation. All of a sudden, oh, it's 12 or 14 inches, same amount of steel.
That's not going to be adequate reinforcing. They're going to have problems. They can have cracking from that. Improper consolidation of the concrete. Uh, I can't tell you how many times I've seen videos where they're. Uh, basically cutting the surface to get down to, you know, finished surface. Those trimmings go to the ground.
What do they do? They throw it up into the bench, they throw it to the top of the wall, they throw it
Eric Knight: That's the, that's rebound, right?
Charles Hanskat: no, there's trimmings. Now
Eric Knight: Oh, trimmings. Okay.
Charles Hanskat: happens while we're shooting. That's something that hits and actually bounces off. And so typically our rebound is more of the rock and we leave behind the cement, the, the sand. So actually our in place concrete is stronger than what's delivered in the truck because some of the rock comes out and we're leaving the a very, very paste rich concrete in place. But rebound is what hits and falls off. Over spray is what sticks. You know, when you're shooting your nozzle stream is not, it's not laser focused, so it, it comes out and so we get a little bit of over spray. That's something that should be removed when they're shooting. Uh, these are all things we go through with the shotcreters. Um, and then once again, those trimmings, you can't just throw it with a trowel into your cove, into your bench, into your steps and say that's consolidated. If you're going to do that, then you probably have to go fall back like you would in a cast in place job where you'd have a vibrator.
Eric Knight: To make sure it gets consolidated. Yeah.
Charles Hanskat: But then that might be a problem 'cause you're vibrating shotcrete wall underneath that maybe isn't set up. So are all the rebounds should be removed from a pool. You shouldn't incorporate it into the pool. Trimmings the same, unless they're somehow compacted, consolidated properly, it, it's not good concrete. It's not what the homeowner or the pool builder is paying for to be in that concrete pool shell .
[00:42:29] WU and ASA Shotcrete Certifications
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Eric Knight: Thank you for that. In the last few minutes that we have, Charles, let's talk about the certification that you've helped develop with Watershape. And you are the one who teaches it. So let's talk about what that class is and what would be the benefit for either a pool builder going through it or a shotcrete subcontractor.
Charles Hanskat: almost 30 years ago we started a nozzleman, a shotcreter certification. 'Cause we, we realized that quality control happens right at the nozzleman. They're seeing everything going in place. About eight years ago, we said, well, you know, there's a lot of people that are inspectors that are watching shotcrete going in place, and they need to know what to look for.
So we were teaching the shotcreter, but we also developed what we called our shotcrete Inspector class. We looked at the pool market and we know there's a lot of pool builders who hire Shotcrete subs. You know, they're not the self-performing the shotcrete and how do they know if they're good in good quality concrete or not? And so we took our shotcrete inspector kind of structural program and geared it towards the pool industry. The cool thing about shotcrete is it's visual. You see it going in place. It's not hidden in a form. And so we go through all the things required for proper concrete mix design. Uh, the equipment that's required for proper shotcreting. The nozzling techniques that they should look at for the shotcreter. All these things are very visual. You can see it going in place. And so when I talk to pool builders, it's like, how many of you go to the shoot when you actually shoot your pool shell? That is the critical time. That's the time that you are establishing the quality of that pool overall. And we will give them 40 different things that contribute to, it quality concrete in place or is it crappy? And
Eric Knight: That's a very technical engineering term.
Charles Hanskat: there's a written exam, that can be done online from Watershape. Watershape provides a, I guess a badge or a certification. It's a Watershape certification. And then we also developed pool shotcrete Inspector that is geared towards people who may be hired to come to your pool shoot. And have the knowledge. They've proved that they have the knowledge, they've taken our course, but they've also done an online exam. They've submitted their experience record. They've been basically reviewed and vetted by Watershape that they have expertise and then they get a Watershape pool shotcrete inspector certification, uh, it's an additional exam. There's additional application for that. Uh, but it's something that, you know, I think a lot of homeowners would appreciate, Hey, can I hire somebody to come watch? Or even a pool builder. If the pool builder doesn't have the expertise, is there somebody they could hire to come to watch that?
Eric Knight: Just to make sure that the contractors are doing the right thing.
Charles Hanskat: and that's where, yeah. And that's, that's the time it has to happen. You don't come
Eric Knight: Yeah.
Charles Hanskat: two days later. have to do
Eric Knight: And you call it out when you see it.
Charles Hanskat: right?
Eric Knight: You, you call it out when you see it. If the slump is too bad, you get them to stop. Is that right? And then you have them deliver a different truck?
Charles Hanskat: One of the cool things about shotcrete is you see it going in bad, you stop. Figure out what's going wrong. You can cut it out and reshoot it. We don't get cold joints in shotcrete. And as a result. If there's something, uh, you know, if it was a larger section, we had to stop cut it all out.
You can come back the next day or the next week, uh, with a properly prepared surface and shoot it and have it act as if it was all done, monolithically all at the same time.
[00:46:15] Closing
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Eric Knight: Nice. Well, Charles, it has been a pleasure having you on this podcast, and I really hope that the people listening to this, if you are in concrete pool construction or you service pools with concrete shells, think about the quality of the shotcrete and if you have any questions, you can reach out to us info@watershape.org.
You can also reach out to me directly, which is ruleyourpool@gmail.com. That's more podcast related. But Charles, thank you so much and I look forward to seeing you again in our Phoenix event. December 4th through sixth, we'll be doing our construction school. We have the Tile School, we've got the Vanishing Edge class, we've got the Plaster School, and yeah, I mean there's, there's a lot going on in Phoenix and so far we've got a lot of students signed up. We're excited for it.
Charles Hanskat: So let me pitch our website. We
Eric Knight: yeah. Perfect.
Charles Hanskat: a portal for the pool industry. It's just pools.shotcrete.org. And we have there our outstanding project awards. We have an FAQ. I answer technical questions all the time. We've had questions that have come in over the last 30 years, uh, they're all available in the archive. Our past magazine articles about pools. So use that as a resource. Also, uh, direct connection to our pool position statements. So once again, it's the website in general is shotcrete.org, but the one that'll take you directly to all the pool related information is pools.shotcrete.org.
Eric Knight: Pools.shotcrete.org. Wonderful. Charles Hanskat, thank you so much for being here, the Executive director of the American Shotcrete Association, and a true expert on concrete. Maybe we will do this again if we have more questions that come in and questions do come in. And I will reach out to you if that's the case.
It's been a pleasure. Thank you.
Charles Hanskat: questions all the time. That's part of, I'm an executive technical director, so, uh,
Eric Knight: unfortunately, I get them too, and I am not a shotcrete expert, so that's why I'm really happy to have you on here. Thank you.
Charles Hanskat: send them to me.
Eric Knight: All right,
Charles Hanskat: you, Eric.
Eric Knight: will do. Thank you, Charles. This has been episode 173 of the Rule Your Pool podcast. We will do more episodes like this as promised. Uh, if you have requests, hit me up.
ruleyourpool@gmail.com. And by the way, since bringing this podcast back, I've already had a, a flood and I mean like. A lot more emails than I expected from all over the world. We got some emails from New Zealand, from Australia, from the UK, um, from Cyprus. It's, it's amazing that people are listening to this silly podcast of ours, but I think a lot of people care.
You know, if this is how you make your livelihood in this industry, education really matters. So thank you so much and, uh, take care everybody. Until next time, see ya.