The Significance of Concrete Curing

NIKKI: Kia ora and g’day everyone, welcome to Markham’s latest podcast episode. I’m Nikki Horn, Markham’s Project Manager for New Zealand, and it’s great to have you listening today.

This is the third topic of our series, Invisible Strength. We’re keen to get into the science and practical challenges of The Significance of Concrete Curing.

With me as we walk through this topic is Doug Hamlin, our Research and Development Manager – let the audience hear you, Doug!

DOUG: Thanks Nikki, yes great to be on this podcast today with yourself and the team. Thanks to everyone that’s listening. I’m keen to help this series along and offer some practical insights where I can!

NIKKI: Awesome thank you, Doug. So let’s jump right in! Give us the basics of concrete curing. What does that actually mean and how does it work?

DOUG: Well, we all know that concrete is a mixture of Portland cement, aggregates, sand, and water. There will usually be other things in the mix, but that’s the fundamental parts. So you mix this together and place it where it’s going to go, either in its permanent position in situ on-site, or in precast formwork and then move it to where it needs to go, and basically, once you mix those materials together it goes hard and becomes strong. That’s really simplistic, but I want to emphasize that curing is not just a drying out process. There’s actually a chemical reaction happening between those ingredients that forms the long C-S-H molecules using the cement, sand and water. Now those molecules are what gives concrete its strength, you know, the cement paste or backbone of concrete strength.

NIKKI: OK. So in other words the strength of the concrete is tied up in that curing process, right?

DOUG: Absolutely. I’d really like to emphasize that point. The long term strength and durability of the concrete itself is dependent on the quality of the curing.

NIKKI: I guess that begs the question, what can go wrong?

DOUG: Oh, so this is a sensitive topic and I don’t want to sound like we’re trashing what people do every day. There’s a lot of science that goes into quality concrete mixes, and into structural engineering. A structure will often claim a service life of 50 or even 100 years, based on the design work. But yes, as with anything in construction, things do go wrong at that point of curing the concrete.

The most commonly discussed fail point is poor hydration, of that cement and water. There are many preferred methods in the industry to avoid the cracking that occurs from shrinkage in this situation, such as things like water ponding, black plastic or hessian or other thermal or fabric blankets. And then you’ve got your spray-on membrane curing compounds. But shrinkage cracking is very common, and when shrinkage cracking occurs, the water is escaping from the top layer of the concrete instead of being held there for the curing process. So right from Day One, you’re setting the concrete up to fail, because the micro-cracks that result from that shrinkage cracking allow contaminants to enter the concrete and set up that cycle of premature deterioration.

And I really don’t think that’s well enough understood. So obviously you do get shrinkage cracking on concrete and everyone just accepts that that’s what concrete does, and to a point it does. But shrinkage cracking, this type of cracking actually matters, it sets up the process that prevents the concrete from achieving its intended design life. So that all the mix design and engineering work, all that good work that has gone into the structure are offset by the failure to cure the concrete evenly.

Speaking of curing the concrete evenly, another thing that can go wrong is that the concrete is cured OK at the surface level, but not deep inside in the internal matrix. That is, there is uncured cement within the concrete. This tends to happen with high volume elements or pours, where the heat of hydration can reach excessive temperatures. Over 70°C, this curing process fails, leaving uncured material. Then what happens is, later in the life of the structure, that material can be stimulated to start curing again when you get some moisture migration, things like that. This delayed curing creates internal pressure and can lead to cracking from inside out of the concrete.

That’s not as common as shrinkage cracking, but is another example of how things can go wrong despite the best efforts from the design teams.

And then there’s ASR, what else … low cover issues …

NIKKI: Now you’re scaring me! So how does this play out in the field, in actual structures? You mentioned premature deterioration, and failure to achieve design life. To me, that sounds like a recipe for expensive unplanned repairs down the track.

DOUG: It is, that’s very true. What I’d like to do is invite our Sales and Marketing Manager, Hayden Prestidge, to join us. He’s seen quite a few things out in the field. Hayden, could you come on board and introduce yourself?

HAYDEN: Thanks Doug, thanks to Nikki, hey everybody. Appreciate the opportunity to join. So what’s happening? What’s our topic?

NIKKI: Hello Hayden, great to have you along. I do understand you have seen a few things in the field in your time! Doug’s been telling us about the things that can go wrong at the curing stage, and we’re wondering how that plays out in structures over time.

HAYDEN: OK cool. So I guess Doug’s talked about how micro cracks let in moisture and contamination, into the concrete. And anything that causes cracking will set up this same situation really. Once contamination enters the concrete, you start seeing larger cracking developing, and the stains of corrosion from the reinforcing steel, all those sorts of things developing in the concrete. Little cracks become big cracks, and big cracks kill the concrete. You get a cluster of symptoms developing really that is commonly known in the industry as ‘concrete cancer’.

Look, we’re simplifying things here, there are other factors for concrete besides poor curing. So it’s not all about the curing that will cause cracking – but let’s focus on this for now. So you often are presented with a structure where the deterioration is extensive, and there is more than one thing happening at a time. But good quality curing, even hydration, ‘even’ being consistent across the whole concrete surface, and minimal cracking is an excellent way to preserve the durability of the concrete – if that makes sense.

NIKKI: Yes I think so. So this leads into another question, well I guess it’s two questions. First, at the curing stage, if people already have methods to reduce shrinkage cracking, why does it keep happening, and what can we do to stop it? And second, what do we do if we find that cracking has developed, but it’s well after the curing stage? Oh, I guess that’s three, sorry!

HAYDEN: Three questions! No, no problem, I get that. It’s two sides to the coin, and it’s where we work a lot. Can we improve the situation at the curing stage, so that’s when the concrete’s new? Or if something’s happened at that point, can we rescue or salvage the slab or the situation after the curing stage? So there are those two things we’ll focus on.

It might surprise you, but sometimes teams don’t bother with curing protection at all. They might just pour a concrete structure and it’s left. And when that happens, things like weather factors – heat, wind, even the quality of the concrete itself – all play against quality curing. Rapid construction schedules push back the time quite a bit, on what’s been allowed for curing protection. Look, we’re not here to bag people out but this is some of the reality of what’s happening in the industry.

What I’d like to talk about is capturing the concrete’s internal moisture, and reducing that evaporation that leads to uneven curing. What you need to do is retain that moisture in the concrete to provide quality curing. By immobilizing the moisture right inside the concrete, you can guarantee that high-quality curing, you can guarantee stronger concrete to last longer, and seal those concrete’s porosities at the same time. We do it with a treatment called hydrogel treatment.

And the best part is, it can be retro-applied. Like we talked about before, if you don’t get it on at the curing stage, it can be retro-applied onto an existing slab. You can’t re-start the curing process, of course, but hydrogel treatment will close micro-cracking, it will grow more calcium silicate hydrate and fill in those gaps, fill in those pores, and provide long-term protection against contamination.

So that’s a positive, I guess, to both of the key questions, Nikki that you asked. Yes, we can improve the situation at curing time and offset any deficiencies in the process or circumstances. And if that’s missed at curing time, if nothing’s done at that point, Yes, we can treat the situation to an existing slab. But I would always emphasize, as anybody would, anybody really in the industry, that’s been around, the sooner the better, when we’re talking about these sort of things.

NIKKI: And I suppose also, the sooner, the cheaper, maybe?

HAYDEN: Yes, great point, the sooner you act on remediation, the less the long-term costs will be. That’s a simple fact across the board. It’s true in anything, though, isn’t it, but how often we forget that, or we like to pretend we forget that. But there’s an old saying, what is it, a stitch in time saves nine?

NIKKI: Yes – or it once, do it right. That’s absolutely true.

Well, thank you so much, Hayden and Doug, for joining us today. And to all our listeners, we welcome any questions or feedback you have on today’s topic, or even if you have a specific project you’d like to talk about.

If you want to find out more, please go to our website, www.markhamglobal.com. We do our best to respond to all enquiries within a business day.

We are also happy to take suggestions for topics in the future. What are your concrete pain points in the field? How can we help to educate the industry?

Thank you so much for listening, and don’t miss our next episodes!