CTSP Episode 195 - Saturated Surface Dry (SSD)


Carboline Market Manager - Water & Wastewater, Jeremy Sukola joins Jack and Paul to discuss saturated surface dry (SSD). The trio discusses the factors that make SSD crucial to a successful concrete resurfacing job.


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Jack Walker: Joining us this week on the Carboline Tech Service Podcast is Jeremy Sukola. Jeremy is part of our engineering resources group. Jeremy, how's it going?

Jeremy Sukola: Going well guys, how you doing?

Paul Atzemis: We're doing well up here. I'm happy to say I finally got my voice back. It took about two weeks of sinus infection and just no voice. So I'm happy to have it back.

Jack: You know, the real interesting part about that is nobody really figured Paul to be a woo girl. We had our sales conference two weeks ago, and Paul lost his voice because of all the wooing. Yeah. Anyway, so Jeremy's not on here to talk about Paul's woo girl status. Jeremy is here to talk about saturated surface dry. This has been a topic around Carboline recently. Jeremy, why don't you go ahead and explain to us what saturated service dry actually means?

What is Saturated Surface Dry (SSD)?

Jeremy: Absolutely, I know it is a topic of interest, and everybody is super excited to learn about what SSD actually stands for. As you just stated, SSD stands for saturated surface dry. What we really use the term SSD for is to describe concrete substrates. It's really what we use to describe the moisture saturation of aggregates that go on concrete. It's one of four terms that we use. Oven dry, air dry, saturated surface dry, and then wet. So, we use that term just to describe the amount of moisture that's in the concrete substrate before it's going to be repaired, resurfaced, or recoded.

Paul: So that was a lot of information. Why don't you tell us a little bit about why SSD is important when we're talking about concrete and concrete coating?

Why is Saturated Surface Dry Important?

Jeremy: Sure. We look at substrates when we are going to repair or coat them. When we look at surface preparation, let's say, for instance, steel. With steel, we're looking at surface profile, surface profile depth, peak density, salt contamination, all kinds of things. Well, with concrete, we've got some things that we've got to pay attention to as well. Obviously, we're looking for a clean substrate. We're looking for a surface profile, what we call a concrete surface profile or CSP. We're looking at pH. But SSD is one of those things that's often overlooked. A concrete substrate that is in an SSD state basically means that the moisture's at an equilibrium. Right. It minimizes that moisture transfer that's going to happen between the substrate and a cementitious repair material. So if a concrete substrate's in that SSD state, it should not give off any moisture or add any moisture to the repair material. And conversely, it shouldn't take away or remove any moisture from the repair material. And that's super important for a proper cure or a proper hydration of that cementitious repair material. It's what helps it keep its strength and achieve adhesion.

What Coatings Require Saturated Surface Dry?

Paul: I think a really important thing that I don't want anybody to miss or to get overlooked, you said when you're using cementitious repair materials?

Jeremy: Correct.

Paul: This is truly when we're talking about repairing cement products with cement products. This is not what we look for if you're using epoxies or polyurias, these aren't the cases then. This is truly for concrete repair products.

Jeremy: Right. So coatings can tolerate SSD substrates, right? Some better than others. Epoxy linings. These high-build epoxies that we use for pretty aggressive environments. They're great with SSD surfaces. As long as there's no active water on the substrate, they do great. But things like polyurias and polyurethanes, they don't do very great. We actually use epoxy primers underneath them because of the water that's in the substrate. So, for this, we truly are talking about cementitious repair materials, or those hybrid epoxy cementitious repair materials, anything that has water in the repair material. So, micro silica mortars, standard Portland-based or hydraulic cement, mortars, things like that that require hydration to cure. That's really where SSD is important to really pay attention to and get correct.

The Science Behind Saturated Surface Dry

Jack: Well, it's an interesting thing that you bring up because one of the things that we've talked about a lot on this show is about how things are constantly trying to seek equilibrium, that that's just kind of the nature of the environment that we live in. With the saturated surface dry, you are creating a surface that is going to be more equal to your coating, so there's less of that attempt. So what happens if saturated surface dry isn't achieved, Jeremy?

Jeremy: Well, it depends. It depends on if you have too much water or not enough water. If there's not enough moisture in the concrete substrate, and you don't achieve that SSD, you don't achieve that equilibrium in that substrate. What's going to happen is that substrate is going to want to pull water out of that cementitious or that epoxy cementitious resurfacing material. Right? So, there's capillary action going on, and it's literally going to suck the water out of it. So, that can lead to several issues. One that may not be as critical would be some shrinkage cracks might form. You'll see that a lot. They'll look like spiderwebs all over the substrate. That's usually caused by water being pulled out of that cementitious repair or resurfacing material. A big one is an incomplete hydration of the repair material itself. If there's not enough water. That water-cement ratio is calculated. That's why we put mix a certain amount of water with this repair material. No more, no less. That water-cement ratio is important. So, if it pulls that cement out and you get that incomplete hydration, you actually can lower the strength of that repair material considerably. And one of the most critical things that we have to really pay attention to, if there's not enough water in the substrate - we don't have that SSD condition - is there's a weak substrate repair material interface there. Right? So, where the resurfacing material actually meets the substrate, you get a weak interface, and you have substandard adhesion, which we're always trying to avoid. Now, on the other end, if you've got too much water in the substrate and the surface itself is not drying. Okay. So when we say SSD, we want the substrate to be full, but we want the surface to be dry. If you're in a condition where the substrate is full of water, and you've got some excess moisture that remains on the surface, you've got some ponding, you've got some shiny spots where there's extra water, you run into some of the similar issues as you do with not enough water. Poor adhesion being the main culprit there. Concrete pores. They're full of water. So you don't get that capillary action to help suck that repair material into the pores of the substrate. So you get a really weak interface and low adhesion. Again, once we start messing with that water-cement ratio, you get a weak repair material again. So, instead of having a super high compressive strength, we have a pretty weak material there. Now, if you are lucky enough with a wet substrate to actually get the mortar to stick, what can happen is as water wants to come through that, it can actually start to blister that repair material. So, too much water, not enough water, both bad things when we're trying to achieve SSD.

Paul: So, what are some of the ways we might use to actually achieve the right amount of moisture on the surface as an SSD state?

A concrete substrate at a saturated surface dry (SSD) state

Concrete at a dry state (lighter color) and at a saturated surface dry state (darker color)

Project Factors That Drive Saturated Surface Dry

Jeremy: Again, it depends because concrete is a very dynamic material in a very dynamic substrate. Right? It's always changing. So, there are some things that we have to look at that are pretty project-specific. So, first, the concrete structure itself, where is it? Is it above ground? Is it below ground? Because if it's above ground, it's probably in direct contact with sun, which means that it's probably drying out at a different rate than a structure that's below ground that's in constant contact with the groundwater. What are the temperatures? I happen to be in Orlando, Florida, today. I'm sorry, guys, to tell you this. It's 75° and sunny. So, a concrete substrate at 75° and sunny is completely different than a concrete substrate at 95° and sunny, which it usually is here in Orlando, Florida.

Paul: Right.

Jack: Sure. Sure.

Jeremy: Right? What's the humidity? Is the air saturated, or is it not? Is there wind? If it's an above-ground structure, is there wind blowing on it? There's a lot of factors that can really affect that, what we call effective absorption rate of the concrete substrate itself. And to throw another wrench into it, concrete's all different. Where I live in the Southeast, in Atlanta, and where you guys are in St. Louis, we use different local materials to make up our concrete. Right? There's different types and sizes of aggregates. The porosity of the concrete may be different. The amount of air in the concrete may be more or less. The age of the concrete. Was anything added to it? Surface hardeners or additives to the concrete? These all affect how we can achieve an SSD condition. For a substrate that's underground, it may be at SSD, and if not, it may take just a spritzing of water with a Hudson sprayer or something like that real quick to get it to reach that equilibrium. For concrete structures that are above ground, it's a little more challenging. We have to take all those things into consideration. Sometimes it means using a garden hose and soaking the concrete multiple times, and then hitting it again just as you're going to apply the resurfacer. Once you see that darker substrate compared to the dryer substrates, you know you've got an SSD.

Onsite Saturated Surface Dry Testing

Jeremy: There's not really a test for it other than your bare hand. If you walk up there and you've got puddling water, or you've got standing ponding water on the surface, you should probably give it a minute or introduce some air. It really is the best way to tell.

Jack: What about a paper test? Ten seconds with the paper?

Jeremy: You could take a dry piece of paper and hold it on there. One thing with using, a piece of paper or a piece of cloth to do that, is water wants to wick. So, if you put something on the surface that easily absorbs water, it may pull some water from deeper down in the substate that may make it look like it's too wet when it may not actually be. So, usually, just a quick hand on the substrate is really the best way to tell. If your hand comes out wet, probably should wait a couple of minutes.

Natural or Accidental Achievement of Saturated Surface Dry

Jack: An interesting thing as we were talking about this that came to mind is, I wonder how many people over the years have achieved this phenomenon completely by accident. Because if you think about this, you follow your SSPC coating guidelines. You're going to abrade your concrete. Well, first, you're going to SP-1 your concrete. You're going to clean it, get rid of contaminants.

Jeremy: That's right.

Jack: Then you're going to abrade it. Then you get to clean it again. SP-1. A lot of times with concrete, water. So, you're introducing water to remove your blasting contaminants, and therefore, a lot of cases probably taking dry concrete and bringing it closer to its equilibrium.

Jeremy: That's exactly right. There's a lot of times where when you go through the process of surface preparation just by following the standards the way that they're written. Just as you said, you arrive at a place that you didn't know you should be. And that's a good thing. A lot of times with concrete, too, it's sometimes we overthink it. Sometimes we overthink the state of the concrete. We want to do such a good job of cleaning it, and getting the contaminants off, and preparing it, and getting it ready for a resurface or a repair material that we stay away from water. I was on a project recently where my contractor was going to apply an epoxy cementitious repair material. And they had spent two days blasting the concrete and another full day vacuuming the substrate off to get all the dust out of it. And when I showed up, I said, okay, where's the hose? What hose? Well, I need you guys to spray these walls down with water. And they, you would have thought I had an arm growing out of my head because they had taken so much time to get the concrete clean that we had forgotten a step, which is this SSD condition. And it's such an important step. So, when we abrade concrete, a good way to clean all that dust off and all that grit off is with water. And in the process, you're probably getting that concrete to an SSD state. So, water is your friend when repairing concrete.


Jack: So, I think we talked about this briefly, but I think we should just summarize real quickly, saturated surface dry doesn't mean you're wet at the surface. It means you're dry at the surface, but the concrete itself is saturated with moisture. Well, I want to talk about the types of coatings that we're talking about one more time. Jeremy, just clarify coatings that do well and require the saturated surface dry.

Jeremy: Right. Cementitious repair and resurfacing products. Okay? Anything that has cement in it that requires water for hydration. Right? So, it can be any Portland-based cement, hydraulic cement, micro silica mortars, calcium aluminum mortars. All of these mortars require an SSD substrate prior to repair. Now, when it comes to polymer coatings, epoxies can withstand SSD substrates. We don't actively look for SSD when we're using epoxies, but if it's an SSD substrate, we're okay. Aromatic polyurethanes and polyurias, they do not like any moisture around. As a matter of fact, when those materials are applied to concrete substrates, we use epoxy primers because of the water that's in there, and we're usually introducing some sort of heat or forced air into them to aid in the drying. Any of the polys, the polyurethanes, the polyurias, and water, they don't mix.

Jack: Yeah, and if you remember back to one of our episodes, we talked about the fact that water interacts with the isocyanate, and then that isocyanate isn't able to interact with the Part A or polyol. So, that's great information, Jeremy. Thank you very much for joining us.

Jeremy: Absolutely. It's good to hear your voice. Good to hear your voice, Paul.

Paul: Yeah, I have it back, so there you go.

Jack: And I hope you guys like hearing from Jeremy because we're not going to take that long of a break from him. He'll be back in two episodes as we begin an exploration into building a specification. So for Jeremy and Paul, I'm Jack, and we'll see you guys in a couple of weeks.