The Red Bucket - Episode 9. Coating Concrete (Pt. 1)


It's been said that if you want to have problems, coat concrete. But if you know the intricacies of the concrete and the surrounding environment, the job should go smoothly. Join us this week for part one of our talk with Paul Kennington, AMPP Instructor and Coatings Consultant, about concrete. All of that and more are coming up next on The Red Bucket.


Click to follow along with the transcript:


Intro and Introduction to Paul Kennington

Jack Walker: All right, Paul, here we are again in Houston, Texas. And we've had some great conversations over the last couple of days.

Paul Atzemis: We sure have.

Jack Walker: One of the conversations that we had was with our friend Michael Pardo, and we were talking about mentors, and I am lucky enough to do a podcast every month with one of my mentors. You took me under your wing when you became the boss in Tech Service and things like that. So, that's lucky enough for me. But, today, we're lucky enough to have one of the early mentors I had at Carboline on the show and…

Paul Atzemis: And one whom I've relied on extensively through my career. We didn't work together as long as you and he did, but he's always been a resource.

Jack Walker: Anytime this guy would come to town, I would just follow him around, be his shadow, and just try to soak up any information that I could from him when he was in St. Louis. Any advice that I would give to any young person out in our industry is if you're around somebody who has experience, who's been in the industry 10, 20, 30 years, listen to that person. Be that person's shadow. Figure out what you can learn, because that was always my policy when I got in the industry. I knew I didn't know anything, and it was, "Let's find these guys who know stuff and let me just absorb as much as I can."

Paul Atzemis: Because one of the things you find is when you're following that person, they're doing things naturally.

Jack Walker: Right!

Paul Atzemis: It is thoughts and questions and actions and movements that are natural to them. Even when they're putting together "I'm gonna help train you," those thoughts don't enter their head because it's not even second nature. It's first nature. And, as the new person, you need to be able to follow them and say, "Why did you do that?" And when you find a good one, they are more than happy to stop and tell you all of the thoughts that led to that.

Jack Walker: And this guy has always been there. He's a teacher by nature. He teaches AMPP CIP courses, and I'm going to stop beating around the bush.

Paul Atzemis: He has a fantastic name.

Jack Walker: Yes, he does have a good name. We are going to stop beating around the bush. We're going to introduce my good friend Paul Kennington. How's it going, Paul?

Paul Kennington: Oh, fine, thank you. It's good to be retired, I think.

Jack Walker: Well, semi-retired, right? Because you're still teaching the AMPP CIP courses and, he's being modest, but I think he's forgotten more about coatings than I know.

Paul Kennington: At least the bad cases.

Concrete: The Enemy of Coatings

Jack Walker: What we really wanted to talk to Paul about today, because he is an expert in this area, is concrete. And concrete can be, he said it before we started, the "Enemy of coatings." And that's kind of an interesting concept. I've never put it so bluntly, but if you want to have problems…

Paul Atzemis: … paint concrete.

Jack Walker: Paint concrete. Especially if you're used to only coating steel. It is an entirely different world. Problems can and will happen. They're guaranteed. And you have to be quick on your feet and follow proper procedures in order to prevent any kind of problems.

Paul Atzemis: And one of the things that you think about is it's everywhere. It's hard, it's there, it's strong. You don't think about the problems that are inherent in that product.

Jack Walker: It is the most widely used construction material on the face of the planet.

Paul Kennington: Yes, it is.

Jack Walker: And numbers that I saw, that are probably more than a decade old, were in the hundreds of billions of dollars. And so, I would feel like it's safe to say that we're in the trillions at this point.

What Makes Up Concrete?

Jack Walker: So, I think it's important to start with, and it's important to understand, what concrete is made up of before we start to talk about coating concrete. So, Paul, tell us a little bit about what makes up concrete.

Paul Kennington: Well, let's start out with the most basic concrete that we're aware of because concrete can be Portland cement, it could be epoxies, it could be vinyl esters, it could be all kinds of different materials. But concrete that we're most familiar with is on our highways and our structural parts of our building, and it's Portland cement, then it's rock, sand, admixtures, and water. And so, ideally, as a person that has coated many square miles of concrete in my time, I would tell you that we would like the water-to-cement ratio to be as low as possible, because this is where we have a problem with bleed water coming through and making our surface "punky," as we speak. So, surface tensile strength is the only thing that we have going for us with coatings because their bond to the substrate is based on how strong that surface tensile strength is. And, as a concept, you could think if I poured 3000 PSI concrete and I was to do a pull test, I would get around 300 PSI pull strength on ASTM D4541, because it's about a 10% value of its compressive. Although concrete's very good in compressive, it tends to be a little weak in flexural and in tensile strength itself.

Paul Atzemis: Naturally, concrete's not flexible.

Paul Kennington: Even steel tends to be a little more flexible than concrete. All you have to do is look at our overpasses, our highways, and our structural material, and you'll find that it's widely, widely used, and probably the most coated surface in the world, whether it comes from aesthetics, all the way to protection.

The Different Types of Concrete

Jack Walker: It's good that you brought up that there are lots of different types of concrete, that you can make a concrete out of almost anything with epoxy polyurethane. But really, today, we're going to focus in on that cement-based. So, when we talk about that, and you talked about the water-to-cement ratio, and things like that. Concrete's a sponge. Let's talk a little bit about the air and the capillaries that are in that final part of the concrete and why that is important.

Paul Kennington: Well, a lot has to do with the concrete mix design, because, after you get past your Portland cement, your amount of water that you put in it and your rock and sand, and different gradations of rock, different gradations of sand, you also have different types of admixtures that you add to it to give you certain physical properties in its environment. So, with that in mind, we sometimes have what we call our five basic types, "1, 2, 3, 4, 5." And then, if you see a little letter following it, capital A, like, "1A, 2A, 3A," you see it's air-entrained, which even makes a problem of coating concrete far greater because it's made to be a sponge because concrete is absorbent. And when it absorbs, and you're in northern climates where it freezes, it expands. So, just as we use a freeze plug in an engine to try to keep from cracking steel, we try to use the capillary action that it created during the mix design to allow the water to go, to keep it from spalling. This is the common word that we use for concrete. Cracking. Spalling. Disintegrating.

Paul Atzemis: And spalling is one of those other things. We see it, it happens a lot with either water, or when your rebar starts to corrode, so it's real common if you're driving down through the northern areas and you see it on a bridge, and you see that rusty piece of metal on the background of that bridge support. The removal of the concrete, that process is also spalling, and it's the same thing. Whether it's steel that's expanding or water that's expanding, it's when that inward force is pushing the concrete off, and it causes it to pop off into a larger area, usually.

Paul Kennington: And that's where concrete has some of its problems, it's that even though it's very good and compressive, the properties tend to drop significantly for the modules of elasticity and the flexural part of it. So, concrete being hard, fall on it, you'll find out how hard it is, and so, what happens is, it'll tend to crack.

Jack Walker: Which can cause problems for coatings as well.

Paul Kennington: Right, and one of the reasons they sometimes just seal concrete. Let's talk about, not all concrete. In our area, we mainly look at chemical resistance. But in all areas, it could be from dust proofing in a warehouse. It could be for wear resistance as traffic passes. So, there are many reasons why we try to better the physical properties of concrete. Still, the most economical product we can find that can do what it does.

Type 1 Concrete

Jack Walker: So, I think you brought it up a little bit, but there are five basic types of concrete, and I think while we have the concrete guru here, we should get into the differences between the five different types of concrete. Well, let's start with type one. What is type one concrete, Paul?

Paul Kennington: Well, it's what we call general purpose. It's what we use typically unless certain environments require something else. Typically, they are our house foundations, our brick-and-mortar type materials, and they're on our structural surfaces, because it's the most economical material, and it's just all around a general purpose, has good physical properties, and in about 28 to 30 days it develops the properties of the design of the civil engineer.

Type 2 Concrete

Paul Kennington: Then we have type two. There are certain areas that we know by pH that, if we have an acidic soil, we know that concrete's highly alkaline, and we know that, eventually, they'll neutralize each other. So, we have what we call "moderate sulfate resistant." Type two is pretty common in California and in the western coastal areas because of the acidic nature of the soil.

Jack Walker: I think that's an interesting point that you bring up, that people think about concrete, but it's not the same everywhere.

Paul Kennington: No. And the environment's not the same that we place it.

Jack Walker: And some of that's due to the soil and things like that, but some of that's due to local sourcing, right?

Paul Kennington: Oh, yes. Because it's all about money.

Jack Walker: So, sand in Missouri might not be the same as sand in California.

Paul Atzemis: And that's just it! One of the most expensive parts is going to be essentially hauling dirt or rocks of some sort. And so, you want to use it as close to the source as possible. So, it's going to be different all over the country. Now, one thing that Paul brought up here that I think we need to touch on is the pH of the concrete, that we're looking at something that starts at a very high alkaline number, and, as it cures, that number comes down a little bit. But that's why we have to be concerned with acidic soil.

Paul Kennington: Absolutely, because you get a chemical reaction. We know, according to the pH scale, we have acids and alkalines on the opposite end, so they tend to neutralize each other, and I would hate to be living in a multimillion-dollar home in California, and all of a sudden ask some concrete expert, "What's going on with my foundation?" And they go, "Oh, that's just the natural reaction." You're going to lose your home.

Type 3 Concrete

Jack Walker: The next type of concrete is type three, and type three is that high early strength. Talk about the advantages there.

Paul Kennington: Well, the advantage is when you have to build a structure, and you quickly need to either perform chemical resistance, if it's in that environment, or the concrete needs to gain physical properties very quickly so that you can go on with your project. And type three is really nothing but type one in a finer grind, and it wets out so much quicker and hydrates, and that's how concrete cures. We take a powder that's gone through a rotary kiln, and we dry it out, we make it a certain size, and then we add water to it, it goes through a period of hydration, and then it cures. Unfortunately, we cannot pour concrete at the low rate of hydration. It only takes like 0.22% of water to hydrate the cement. The problem is that the workers will walk off the job. So, most of the time, we see it at a two-to-one ratio. In other words, the water goes up to 50% of the cement, and that way, it works much easier. It can be placed, and the structural is there.

Jack Walker: But what does that do to the strength of the concrete?

Paul Kennington: Well, over a period of time, it does very little if they don't overwater it. It's just like anything else. "Oh, it's easy. Let's make it easier." It's always that "Let's make it easier," and, "Add more water," and now, the water is just like an epoxy that we all know of. You add more curing agent, it doesn't necessarily make it good for you. And so, that's where we have a problem.

Jack Walker: And that more water versus less water really changes whether it's, we'll call it concrete, all the way down to mortar or grout. And then, literally, the differences between those things are the size of aggregate and, really, the amount of water.

Paul Kennington: Or the amount of cement that we put into it. If we can put more silica products into it: Sand. It's much cheaper. It's just like making a can of paint. Every manufacturer's going to make the epoxy the smallest part of that can that they can in order to be competitive and still perform. And so, same way with cement, we add water to it, which sometimes increases cure time. So that's why we go with Portland cement type three in order to give the hydration a quicker reaction. And let me throw in admixtures or additives, while we're talking about all these, and the fact that we have different types of admixtures that will actually take and pre-wet the aggregate so that it will not pull, like the fines in there, and the rock, the surface area of the rock, is great, and we want to try to keep that water from attaching to the rock and react with our cement. So, that's why the water-to-cement ratio is greater because it makes it more flowable. And so, with that in mind, if we can put an admixture on there that will pre-wet the rock and sand, then the water can readily come to the hydration period of the Portland cement.

Paul Atzemis: And that ratio is important, and that's why a lot of times you'll see commercial or professional, let's say, concrete structures, outlast a lot of times what you see a homeowner or a novice do because we're all still using the same ingredients and it's a matter of understanding how they're put together. And, when you're a homeowner doing it by yourself, you're pouring a sidewalk, you're doing whatever it is that you need to do, you may add that extra water to help smooth out the surface or to get it to flow the way you want without truly understanding the long term consequences of what that was because it's still hard when you're done, and you can still walk on it, and it's still there. But you may not have the original integrity that you thought you had because of those adjustments.

Paul Kennington: And so, if you prepare your substrate properly, and the American Concrete Institute loves to see a layer of sand over your Visqueen so the excess moisture has a place to go and it's not trapped and because not all of it's going to come to the surface. And so, that's why they like the layer of sand.

Jack Walker: Well, that would probably help because one of the phenomenons you see with that, when you put down that moisture barrier, sometimes the concrete, because the water can only go in one direction, will curl and stuff in the corner. So, the idea of having that sand and stuff down first, that's genius.

Vapor Barriers

Paul Atzemis: Okay, so wait, let's go back a step. A vapor barrier. Truly, how many times have we all had to work with a substrate and said, "Is there a vapor barrier present?" That's usually what the question is, but the idea of "Yes, that truly is part of a good system, a vapor barrier, a base for the concrete." Those are all integral steps, that it's not just a concrete truck pulling up and dumping a cement mix out on your ground. The preparation that goes behind it to make everything else successful is truly important.

Paul Kennington: Absolutely, because if the proper ingredients are not coming from the manufacturer of the concrete, the truck itself, and the people who have built the forms and put the chairs in to support the steel, it all has to be in concert in order for it to be good. And that's why, sometimes, we see concrete come apart in the field. And no two concrete pours are the same, unfortunately, because humans have a part.

Type 4 Concrete

Jack Walker: So, coming back around, we are now at type four concrete. What is type four concrete?

Paul Kennington: Well, that's your low heat of hydration. Anytime you have massive pours, if anybody has ever seen 100% solids in a five-gallon bucket, that got mixed and did not get used, you'll see it go through an exothermic destruction. And so, type four is low heat of hydration. So, when they're pouring large pours, many times what they'll do is they'll isolate this large pour from another pour, and then they'll pour one in the middle in order to keep the heat of reaction down, such as the Hoover Dam.

Paul Atzemis: That's what I was just going say, the Hoover Dam is a classic example of this.

Paul Kennington: And the chemists will modify, and, bless the chemists because they're the ones who get into the down and dirty. They will modify, either they'll put tricalcium sulfate in it, or they'll take it out. And so that's what they do with low heat of hydration. They take some of the properties out that will slow the heat of reaction down and keep it as low as possible so that when you have a massive pour, you don't have a structural integrity problem.

Type 5 Concrete

Jack Walker: So, now let's go ahead and talk about type five. It's closely related to type two, I would assume by the name, but let's talk a little bit about it.

Paul Kennington: Yeah, well, it's high sulfate resistant, meaning that if you're in an environment where you have high acidic and certain clays and certain other materials can have high sulfate, which is acidic in nature many times, it will attack and neutralize the concrete. And so, we call that carbonization when the concrete starts reacting with its properties around it, and it'll start to weaken. As the pH drops of concrete, so does the physical properties of concrete.

Jack Walker: So, hypothetically, thinking about this, we know that if you put concrete in and it's at a high pH during its cure, right? So, new concrete should always have a high pH. But, if one were to come by and test that new concrete and find out that it's a 7 pH, we have problems. But could it be possibly due to using the wrong type of concrete for the soil?

Paul Kennington: No, because when it goes through all its chemical reactions, it forms a rock, per se, and the calcium hydroxide in it, which is one of the major binders of concrete, will actually always have a high pH. And when you start taking any hydroxyl group and taking it down as far as pH, you start weakening the solution. Because as, lye, let's say, sodium hydroxide, 50% of it, it's 14 plus. Okay? If you start watering it down or you start changing the physical properties, it no longer will have the properties. And that's just like making soaps, saponification. We bring in, animal fats or acids and react with the lye or sodium hydroxide to get a soap so that it will not harm us, as a general rule. So, what you do is you start weakening the structure, and that's why they so often do petrographic studies in concrete to find out where the problems are.

Jack Walker: So, then, what could cause something like that? Is just something going on with the chemistry within the concrete itself?

Paul Kennington: It could be the soils that you set it on. It could be parking garages with the exhaust fumes because you got your different vapors coming from the exhaust pipes. So, if they don't seal it properly and they allow that to react with the environment because we know that anything can go from a solid to a liquid to a gas. So, with that in mind, we know that we can start, we call it carbonization as the general rule. We start adding other properties to it that we don't want. And so now the concrete starts to become soft and starts losing its physical properties.

The Cure Mechanism (Pt. 1)

Paul Atzemis: Okay. So, we've kind of tiptoed all around it a little bit as we've gone through. But the actual cure mechanism and the cure performance, the timing of concrete, we kind of talk about it as a general 28 days. Can you talk a little bit about where that came from, what we're looking for, what that means?

Paul Kennington: Well, when a civil engineer, let's say, or somebody that's making a mix design, and we go by sacks, five and a half, six and a half, seven, eight, and it's just the amount of Portland cement per cubic yard. And, so, we know that we can get a higher compressor strength if we add more Portland cement. But it's a matter of variety of how much can you afford?

Paul Atzemis: And that's the most expensive part of the cement.

Concrete vs. Cement

Jack Walker: Well, I think, real quick, before we get too much further, a lot of people in the real world will interchange the word cement and concrete. And we keep saying cement, so I think we should clarify. What's the difference?

Paul Kennington: Well, Portland cement is what we all call concrete, and concrete's just a blend of different materials. That's why you can have polymer concretes, you can have different types of concrete. But when you use Portland cement, or a marl, or limestone, it chemically reacts. You take it out of the earth, grind it, go through a rotary kiln, make a clinker out of it, and then add it, and then you add water to it to make it right back to where we took it from.

Jack Walker: Yeah. I always say cement's the glue that holds everything together, and concrete is the sum of all the parts.

Paul Kennington: That's exactly it. So, that's why as a generic term, we all think of concrete as Portland cement concrete, and it's probably the most widely coated in the world. If you don't believe me, go to Disneyland, or Disney World, or Six Flags. You'll see that it's very important to them because it's not only aesthetics, but it's a structure.

The Cure Mechanism (Pt. 2)

Jack Walker: One of the things that we can look at, and we talk about the 28 days, and I think you were just talking about that before I went on the tangent of cement.

Paul Kennington: Well, 28, 30 days is what most of the industries, and that's, if you design 3,000 psi, that's about the time that it should reach. And that's a generic term because sometimes it'll reach its physical properties much sooner. Sometimes it'll take a little bit longer depending on the environment: If it's cold or hot, and the admixtures that were placed in it. We used to use admixtures—chloride elements—that we found out later worked against us because even though it worked very well for concrete, it did not work well for the rebar, and so when moisture permeated, it started the corrosion cells. And we know that that quarter-inch rebar can grow as much as two inches, or 50 times its size, in the right circumstances. So, when it does, it exerts a force, and that's what structurally cracks our concrete.

Jack Walker: And a lot of the times we used to talk about that 28 days in terms of that's how long you had to wait before you can coat it, and that's not necessarily the case anymore in 2023.

Paul Kennington: Well, actually, many times you can put additives, or you can go to type three Portland cement, which is a finer grind, and then you can put admixtures in that will keep the moisture from going to the rock and sand and hydrate the cement much quicker.

Jack Walker: And, on top of that, you could even use specific coatings that are designed to go on before the 28 days, too.

Paul Kennington: Yeah, it all has to do with physical properties because if you talk about the surface tensile strength of the concrete, it's generally about a 10% value as a rule of thumb. And so, if you can test it, there are other methods we can test from moisture, see if it's chemically reacted. But the big thing is, you'll see that the epoxies and other materials we put on have a far greater surface tensile strength than the concrete has within its own structure. That's why surface prep, that's not always done properly, has many problems with coatings because the coating's going to bond to what you put it on. And just as you have mil scale to steel, we have laitance to concrete.

Jack Walker: Well, what's that laitance?

Paul Kennington: Laitance's the bleed water, the watered down cement, and calcium salts and anything else that rises to the surface as the concrete's being worked at the day of the placement.

Jack Walker: And that's a very brittle layer, too.

Paul Kennington: Yes. It's just like, well, you look at mil scale to steel if you understand that, it's hard and dense, but poorly adhered. Same way with laitance. It's poorly adhered. So, you want to remove mil scale so you can bond to the actual steel, and you want to remove laitance so that you can bond to the actual concrete.

Paul Atzemis: You know, I use that as an example. Every once in a while, you know, you try, not a trick question, but a loaded question, the question being, "When you guys poured the concrete, when everything was all done, did you get a nice smooth finish? Make sure you had everything just smooth as can be and beautifully troweled out? Nice, smooth?" They're like, "Yep, yep." "Okay, take that off."

Paul Kennington: Or even one further for that is they say, "Oh, well, what's the purpose of preparing the concrete, isn't it to get a roughened surface?" That's only one part. Because they'll do a broom finish and say, "Well, we got a rough surface." But you still haven't removed the laitance.

Paul Atzemis: Right. And, honestly, you can tell them to just go tap on it a little bit with a hammer and see. That's going to crumble and fall off, and you're going to hit a spot where now it doesn't do that anymore.

Paul Kennington: Yeah, we used to do the knife test or the powder test. The instrument that I liked the best was the smit hammer. It was a rebound hammer that you could go to the concrete surface, and you could hit it, and it'll give you approximately the strength of the concrete. And then I'd say, "Let's grind it and see what we get." And, many times, our physical properties would go up anywhere from a hundred to maybe up to 500 psi simply because we took that weak layer off.

Paul Atzemis: So, on that path, the 28 days that we're looking at, and we had mentioned, sometimes there are coatings that can go on earlier. And typically, what we're looking at for coatings in that area are coatings that will still allow the hydrolysis process to continue. So, most of the time, those high-solids epoxies, those aren't great candidates for that because they stop that process. But there's a lot of very thin film things or a lot of things that are designed to be breathable, which will still let that happen. Talk to us a little bit about, as this "28 days" goes, I'm using air quotes, that "28 days," where we're looking to get to and what that tells us.

Paul Kennington: Well, many civil engineers know the factors of Portland cement concrete, and one of the things that they'll specify, as soon as it can support traffic, will put a curing membrane on it. And many times, it is dissipative, where what it'll do is it goes onto the surface, it seals the surface for a short period of time, and allows the moisture to remain in the concrete, because, when you blow wind, or anything comes across that surface, you remove the moisture, and so then it turns back to a powder. And that's why we sometimes can actually grind the concrete and get higher physical properties. And so, that's why so many people use a curing membrane because that is that breathable membrane. You've just got to make sure that if you're the person on the job, that it's not detrimental to your coating as far as it doesn't allow it to hear. And, so, sometimes people like to use diesel fuel or motor oil because the forms will come off readily. But now, the coating won't stick. And, so, with that in mind, we do have to sometimes use something and allow the concrete surface to fully react even at the surface. Because, if you don't, you'll get a thin layer. And that's why surface prep is so important.

Paul Atzemis: That's right. What we look at is the process that we're looking to do is to get to something sound enough to, like you said, support vehicle traffic, or support the surface prep method that you need to do before you start. Because, honestly, I mean the fundamentals of surface prep is we're destroying part of it. And it's a defined part. We know how much we want to remove, take it off. So, the rest of the concrete needs to be able to support that method of aggression.

Paul Kennington: Right. Just as we take mil scale off to get to good steel, we want to take laitance off to get to good concrete. It's part of the physical structure.

Paul Atzemis: It'd be like trying to do abrasive blasting on molten steel that's still glowing hot. Concrete is not as hot, but it's still curing, hardening, drying, whatever that process, and when you relate those two like that, you would never do it on hot steel. You can't do it on concrete before it's reached some level of cure that it can support this. And we use 28 days as that benchmark. And, like you said, the people who formulate it know, frequently, to the hour of when it's going to reach that level because they understand the chemistry of what they're working with.

Paul Kennington: Yeah, but that's if it's at 70 degrees for 28 days with 50% relative humidity. Yeah, we can predict something that precise. The problem is Mother Nature doesn't cooperate with us. I've been in many a discussion with people who say, "You say that you can bond to the concrete in 30 days." I say, "We probably can, but the problem is how good are the physical properties of the concrete."

Closing Remarks

Jack Walker: So, that’s it for part one of our deep dive into concrete with Paul Kennington. Join us next month for part two. Until then, we’ll see you next time.