Strength VS Durability

BRENDAN:

Well, g’day! Kia ora and good morning, everybody, and welcome to MARKHAM's latest online event. Today we're focusing on the topic of, Does stronger concrete always mean longer lasting concrete? I’ll just get this screen sharing, and introduce the team.

With us today we've got Doug and Charles who are based in our Napier office. They're an experienced team and can answer your questions as we go along. They’ll introduce themselves as they come on board. The agenda for today is: [1] Concrete strength and durability - The myth versus reality. [2] Special needs of low carbon and green concrete. [3] How to specify for durability and not just strength.

Doug, would you like to take us away and start from the point of what we mean by concrete strengths?

DOUG:

Yes, sure thing. Thanks, Brendan. My name's Doug Hamlin. I work in the research and development section here at MARKHAM. Been here for 14, 14-and-a-half years. So I have been through many roles in the business, from application on site, through to specification work with architects, engineers, construction, sales, project management. Done the lot, really. And yes, I'm very passionate about long-lasting concrete; concrete durability. And so what we wanted to have a look at first was, what we actually mean by concrete strength. Now, there's, you can see there, there's a couple of main ones, the compressive and tensile.

Firstly we'll look at the compressive tests. So that's compressive strength. So that's looking at the ability of the concrete to reduce... to withstand load. So that's load that's compressing down or pushing down on the concrete or onto the structure. And this is typically measured by loading up a concrete cylinder to the point that it fails. And that gives you a reading, MPa reading. And then that's taken as the design... as the strength of the concrete that you're designing to, or that's required for that structure.

Now, tensile strength is essentially the other way around. So it's looking... rather than looking at the pushing force, it's looking at the pulling force. So that concrete's ability to withstand being pulled apart or being, you know, being flexed. And how that's done is that same cylinder is put on a machine and pulled until the point that it breaks. And then that is known as the tensile strength. So it's normally... say if you got a 40 MPa compressive strength, your tensile strength normally about 10%, as a general of thumb, of compressive strength.

Now, a couple of other items that have been looked at a lot more in recent times, especially with the green push or with the cement replacements. The early strengths, that’s your 1-3 days’ strength, is really becoming really important in construction now.

So, if you think about being able to remove formwork on site, you know; being able to carry on the construction program; being able to pull precast panels off the casting beds, that sort of thing. And so, yes, something that's really, really measured now. And just lastly, on the unit of strength there.

That is measured in MegaPascals, which is a calculation from the amount of load in Newtons and the square millimetres of the concrete cylinder, the height, all those dimensions. That's all calculated, and you get given a MPa reading there for the strength.

So if we could go on to the next one there, Brendan. So with compressive strength, that's really probably the... it's really the most common factor that's used, when you're specifying and designing concrete, that's one of the key bits of information that is looked for when you're building a structure.

And with that, a lot of the time it's used as kind of the indicator or the sole factor to determine what's a suitable concrete for the conditions that you're designing in. And so if you say, OK, we need it to be highly durable concrete. Let's go for as high an MPa as we can.

But therein lies the problem, in that it's not just the MPa; or, the higher the MPa doesn't mean higher durability in the concrete. So there's things like specific mix design. So, you know, you could be having a lower MPa concrete. So 40 MPa concrete with the right conditions, right mix design, that could actually outperform a higher MPa concrete.

Now this is not a complete blanket statement, but it's very true of many construction sites; because they're not taking into account... they’re not doing quality curing on site. They're maybe not doing quality finishing. They're not... The long term site environment is not being considered. And most importantly, the impermeability has not been considered.

So you can have... you could have a 60 MPa concrete gets poured on a bad weather day, you're resulting in a whole lot of drying shrinkage or plastic shrinkage cracking. And that structure is much more susceptible to corrosion damage, to deterioration, than, say, a lower MPa concrete that's poured with right conditions, right admixtures, everything's been thought out there; and it's a quality concrete that you end up with there. So yes, just really want to want to challenge that thought that you may have; that compressive strength is really the only measure of durability in concrete; because it really isn't.

There's... As we can see, there's five factors on the screen there; I'm sure there's more, that need to be considered when you're looking at durability of concrete. I'll hand over to Charles now. He'll go diving into this a little bit deeper.

CHARLES:

Thanks Doug.

BRENDAN:

Just before you start that, Charles, I just like to welcome those who have joined us since we started. Thank you very much. And down the bottom in your toolbar or under the ‘More’ tab, you'll find the questions button and a chat button. We’d really, really welcome your questions as we go along. We're going to have a Q&A session at the end. And if you have any suggestions or additions to what we’re saying, please do drop them in the chat. We'd be happy to discuss.

Thanks, Charles.

CHARLES:

Awesome. Thanks, team. I'm Charles Currie and I’m focused on precast and ready mix; helping a lot with ready mix to improve mixes, make them work better. Specially responding to a lot of the changes we're seeing and the drive towards lower carbon cement. I've been involved, got involved in projects that I’ve dealt with since I've been here at MARKHAM.

A lot of existing concrete infrastructure, mining infrastructure, marine; a bit of all-sorts. So been up close with a lot of the attack and interesting things that it's subjected to over the course of its life. So just wanted to dive into this subject a little bit more, some of the areas we’re separating out durability from strength, where it can be specified and, yes.

So lowering permeability, stopping the chlorides and water ingress, is obviously a biggie. Obviously the attack, the corrosion needs these free ... needs the concrete to be permeable in order to progress. Upping the reinforcement cover, the protection of the rebar is a big way to protect the attack. Once that surface is broken down, the underneath is pretty much free for all in some cases. We’re seeing a lot of SCMs, supplementary cementitious materials, being used now, such as fly-ash or silica. A lot of these are improving the resistance of the pore structure to stop some... to prevent attacks over the design life of the concrete. And responding to the environment that it's in.

A lot of attack from soils, marine exposure, harsh conditions of mining concrete, for example. So just bearing that in mind in the design of the concrete, the actual environment that it's going to be in.

DOUG:

Yes, that's good, Charles. I just want to... just what you touched on there about the permeability, that’s really, really important when you’re considering concrete. So we've seen concretes that were, you know, up to 80 MPa even. I think that's the highest we've tested here, for water permeability. We can still get moisture into that concrete, even at that high MPa. And there's a bit of a belief out there that once you hit 40 MPa in the concrete, that it's waterproof, it's not going to leak.

And it's typically in the case of, say, a water reservoir or a water tank, something like that. Now, while that's not entirely untrue, that that 40 MPa is going to withstand that, it's not going to let the water come through like, you know, like a river - what you don't... what you are forgetting there is, that is still getting through to the reinforcing steel over time.

So yes, it might be holding that water in that tank, but it's not stopping the moisture getting through the concrete, and starting... and taking contaminants through to that reinforcing steel. So yes, that's just something I wanted to highlight there.

CHARLES:

Not to mention, too, that being the higher MPa, the more chance of cracking and then it using those cracks to progress as well, hey?

DOUG:

Yes, that's right. Exactly. Carry on there.

CHARLES:

Yes, we'll go to the next slide.

So yes, a couple of key ways that this is done. Performance-based specifications is more like a shift towards design-life predictions. So yes, once again in targeting... in these real-world conditions needing to be met effectively. But do you want to share a few examples of what this looks like in the real world, Doug?

DOUG:

Yes, yes I do.

So with these low-carbon cements, they all do change - in terms of what we've mentioned, the early strengths, that's one area. They do change, in terms of workability, water demand, things like that. So the old kind of ‘to code’ prescriptive approach doesn't always work with these cements. And it's really, really important as designers to specify the performance that you want over the design life. So you might be aiming for a 50-year or 100-year design life and have a chloride diffusion requirement in there. Or, you know, whatever test it may be; that ready mix companies, construction professionals, can come up with a mix that meets those requirements.

So rather than just specifying a water-cement ratio and a strength to meet, it's really important with these low-carbon mixes is to specify the performance requirements, because they simply just don't place and finish and work on site like traditional concretes did and, you know, it just makes it even more important to have that performance-based specification there rather than prescriptive.

CHARLES:

Yes, definitely, that’s good. So the other way is to, obviously - innovative additives for durability, the nano-silica hydrogel admixtures will actually help to improve the microstructure. And in short, making the concrete work as good as ‘normal’ traditional concrete, where these SCMs have been added. As Doug mentioned, there can be a lot of changes.

So it's helping on both the short term and the long... like, for placing etc. And in the longer term as well.

BRENDAN:

Thanks very much for the questions coming in. That's good; we’ll address those a little bit later on.

CHARLES:

Awesome. We’ll head into a few project examples and get to them, hey? No, not quite!

DOUG:

Yes, that's good. So I mentioned the performance specifications there. So how do we actually specify that? So how do we specify for long term durability?

These are a few of the parameters that we do. We have seen it, you know, being done very well. So looking at the permeability of the concrete; so specifying a depth and penetration of water under pressure. There's a standard test for that. The chloride diffusion. So looking at a number there. A lot of the actual roading authorities, particularly Transport for New South Wales does; Queensland Roads do as well; have a number there that they want you to target in the mix design; so that you have to come under that in any trial mixes. And that's a very good way... Those two factors, they are very easy to demonstrate in a test. And it doesn't take years; it more takes, you know, 1 to 2 months to show the compliance to your performance requirement there.

And that, what that also does is it gives your ready mix providers, gives your construction companies, a bit of room to move, to adjust and optimize a mix design. Because concrete is so variable from even different states within Australia, different quarries, the weather. There's so many things that go into a concrete mix; and if you can give a performance requirement for someone to meet, using their local knowledge and the materials that they're working with, you just get a lot better result. Rather than just prescribing a set number of mix requirements.

Another area we do see - not as common - is the resistivity. So that's using a device that measures resistance to electrical current through the concrete. There's not many, but there's a few, suppliers out there now with the devices for this. And that measures the inferred ability of the concrete to resist moisture transmission, and resist chlorides through the concrete, and also give you an indication of the corrosion potential of the rebar. So that can be included as well, so a level there.

The VPV is probably more of a traditional approach, used by VicRoads, WA Roads. And what that does is give you a level of voids or percentage of voids in the concrete that the concrete needs to hit. It's not an ideal test specifically when you're talking about low carbon concrete, but it is quite a fast way to make sure a concrete is compliant and it is hitting requirements there. So yes, something to look at there.

And lastly - I almost feel like this is one of the most important points from the point of view that it never... I shouldn’t say ‘never’! It rarely happens on site. And that is where a contract actually follows the curing specification. So as an engineer, you might put in that it has a three-day water cure or a curing membrane or a curing treatment equal to water caring. But to ensure that actually happens on site, is very important. I think the figures are something like, you can lose up to 50% of the durability of that concrete if you don't cure it properly.

So you could have done everything else right and then it goes on site. And it's not... that moisture loss, that curing isn't controlled, after it's been poured; and you've just undone a whole lot of great work there. So yes, I'd really stress that that's included in your checks on site, that type of thing, that it has been included; and monitor it for the first couple of pours, make sure it is actually happening there.

CHARLES:

Often can fall down the cracks a bit - no pun intended, but! And even just where there's not something specified, it just kind of becomes a bit of a blame game. No one wants to pay for it on site.

DOUG:

Yes, definitely. But at the same time, it is a contracted thing. It is in the specification. So there is a reason and a mandate to follow that up there. Right. So just want to have a look at a few projects now. Looking at what we're talking about in the real world, how we've...MARKHAM has actually helped these projects.

So first one goes back to early 2000s. The structure. I'm not sure exactly how old it was, [built 1929] but it had some severe corrosion. It wasn't supporting its own weight. And this concrete was actually trucked from Gisborne up to Tolaga Bay. I think the travel times varied from between 4 and 6 hours, by the time the last bit of concrete was being discharged. And that was wheelbarrowed out along the wharf. It wasn't strong enough to support the truck. And then it was placed down the shutters.

And what was interesting about this, this was predominantly to protect the concrete from the chlorides there. That was with the hydrogel admixture. But it also worked with the concrete itself. So the requirement to travel for four hours and then, you know, be on site mixing for another, you know, 1 to 2 hours, is very challenging to achieve.

And that's something else to consider is that workability of the concrete on site, getting that quality finish, quality consolidation, that type of thing, does have an impact on that long-term durability. So, yes, that's performing very well. One of our contractors went back there, just in the last year, took that photo down on the right there. So you can see there's no corrosion evident there. And we're coming up 25 years. So yes, that's a good testament really to that durability being thought about right at the very start and a successful result. Charles will let you take next one? No, this one's me, sorry!

So Carapateena. This is a mine site in South Australia. Very, very hot conditions. They can get it tropical. They can get it very, very humid. And then it can be very dry. So pretty harsh that way. And then the groundwater that they're using is also hypersaline.

So all the wash-down water can be up to seven times the salinity of seawater. So you can imagine what that does to the concrete. It not only permeates through the concrete and attacks the reinforcing steel, but because it's so strong, it actually attacks the surface of the concrete as well. And they get degradation within five years there.

So this is a perfect example of looking at the environment, looking at the conditions that a structure is going to be in and designing for that. Now this particular site, they used an admixture in the concrete. They were using a traditional admixture, but having real problems with the workability and set times of it.

So they switched across to our hydrogel admixture, which is neutral on set times and did essentially the same waterproofing job. But because you could get a better quality result or pour, the finishers preferred placing it, everything like that. We're seeing a lot longer service life coming out of that environment than what they were using.

And then coupled with that, they had a very strict curing regime as well. So it was using a curing membrane, and ensuring that all that concrete was protected over those... that early life. So yes, that's that's a good success story there. Again, making something that actually works with what your contractor’s doing; and meets the requirement, performance requirements set by the client there. That's all to say on that one. Over to you, Charles.

CHARLES:

Next one - the Ports of Auckland car park. We got involved. Added... extra durability was specified to this one. So obviously the normal attack mechanisms for a car park: the carbonation from the exhaust fumes, the contamination brought by the tyres and the constant wear and tear.

But what was of extra significance about this one was the exposed marine zone that it was in, the sea right there. So this was a good example of something that was extra; extra work was put into it; and yes, so far so good.

DOUG:

Yes, that's good. Sorry, Brendan?

BRENDAN:

No, I was just going to say, did you want to mention the wharf was built or built adjacent to, which we had also worked on some years back there?

CHARLES:

Bledisloe Wharf, yes. You’d be more intelligent on that than me. The one that was like... the remediation?

BRENDAN:

Yes. One of our very early projects.

CHARLES:

Yes. It's obviously adjacent to the Bledisloe Wharf that we got involved in, in the early 2000s. There had been a lot of corrosion and damage over the years. It was actually right at the end of its design life, and it was one of the first large wharves that MARKHAM got involved in. And there were significant repairs done, a lot of spalling. And then what we did was protect the areas around those repairs, from further breakout and contamination. And that worked really well. Going in, it was about eight, nine years later, there was no further corrosion and break-out from... this is the underneath of the wharf. So that was pretty awesome.

I'll move on to the Manawatū Gorge Highway, known as Te Ahu a Turanga. This is a recent project. We were involved in the bridge superstructure. There's quite a bit going on there, but one of the main reasons we got involved with a durability admix was for workability.

There was a micro-silica content - I think was about 8%, off the top of my head - and a number of challenges with this, namely the stickiness of the mix. They did a large site... like a pre-trial on site, pouring hollow-core beams. And that went really well on the side of workability.

The advantage for the long term is that... obviously this is a real significant asset as part of the new highway. And with making that concrete impermeable, they get the advantages of that long-term durability. Yes. Is there anything you wanted to add?

DOUG:

That's good. Yes, just to reinforce that, the hollow-core moulds that they were pouring, they needed to have that micro-silica in there to meet the durability requirements of the code, but also have it workable, but not too flowable or workable that it would put too much pressure and actually break the mould there.

So with the hydrogels, they really excel at... the official term is ‘thixotropic’, which basically means that it has very good workability when energy is applied to it. So, while it's being vibrated, while it's being pumped, then it has... flows around the rebar and everything nicely.

And soon as you stop applying that energy it sets up, and it doesn't keep on putting that pressure on the moulds there. So yes, that was, again, really an example of not just achieving high durability in the concrete, but actually working with the site conditions, and the challenges they had there.

CHARLES:

That's awesome.

DOUG:

So, yes, that brings us to the questions. Brendan.

BRENDAN:

Yes, that's really good. Really appreciate what you've gone through there. And we've got a question on the chat, from Peter: “When is galvanised rebar used? I have seen it but it is rare.” Did you want to chat about that? Galvanized, and other forms too.

DOUG:

Yes. That's a good topic. It does come up from time to time. I’d agree it's not common. It's rare. And again it depends on the site conditions. I guess the thing to consider is, it's only as good as the care that's taken while it's being placed.

So if you get a pinhole or abrasion on that rebar and you break down that protective layer, you’re through to the steel anyway. And it only takes one of those sites to start that corrosion process happening. Now what I would say there is it's just as important if you're using galvanized rebar or epoxy coated rebar to still protect that concrete or still pay attention to the mix design of that concrete, and the curing; everything that's involved there.

It's definitely not a “get-out-of-jail-free” card and it does does come at cost. So it's worth, if you're going to do that, go to those levels, it's worth spending time on the actual concrete mix itself, including an admixture in there if it's needed; or making sure that the curing, everything is happening on site, to achieve that long term durability.

BRENDAN:

Yes, that's good, thank you. That helps with the question. But just be free, everybody, to drop your questions or comments into the chat, questions in the chat. We’d love to hear from you. So one of the thing the crossed my mind, Doug, while we’re on this section is, you mentioned a number of things to specify on site for the durability as opposed to the strength. Is there something that you can look for on site fairly readily? As to, if you're doing trials or tests, what would you particularly look for?

DOUG:

Yes, that's a good point. So yes, we ran through a few of the examples there. But I really feel that, specifying, Number One would be the chloride diffusion of the concrete. That not only tells you the resistance to chlorides in the concrete, but it gives you an indication of the permeability as well.

So yes, that that would be my Number One. And that can be done... there's a couple of different versions of the test. The shorter one is done at 28 days old and is finished inside two days. So yes, it's not long. And when you're talking about a big project construction timeline, it doesn't take a long time to conduct that test.

Then the diffusion one is a slightly... it’s more like two months before you get the results there. But yes, that chloride permeability, I guess you call it. And there's some really good guidance out there, like the B80 specification. Some of the others... I just can't rememeber them off the top of my head, but we can get them out later - where they've set limits, for you know, 100-year or 50-year design life, and what diffusion limit that you should be hitting. And that can be demonstrated by the ready mix plant there. Another area is water permeability. So there is on-site testing that can be done, as well as laboratory testing. It doesn't take long to do. And you can set a level there, that you want the concrete to meet.

But yes, I would say, first and foremost is to get that chloride diffusion, chloride permeability testing into your specifications, and then from there you can kind of build your framework there.

CHARLES:

As well as the curing, eh?

DOUG:

Yes. So the curing is important. That's more something you need to ensure happens on site too. Because if you run a laboratory test for chloride diffusion, that's a water-cured sample that gets given a good life, so to speak. So if you're wanting to repeat that same process on site, you really want to make sure that the same... the concrete's getting the best cure it can on site as well.

If that makes sense?

CHARLES:

That's good.

BRENDAN:

That's good. We’ve got a question from Rebecca. “If you use admix do you still need to apply AQURON later?” That's a practical one.

DOUG:

Yes! Do you want a go, Charles?

CHARLES:

It just depends on the environment, I suppose, and what else is going on. It might be best to go over little bit more detail after this with you, Rebecca.

But in some cases it's just the admixture will be used and that will suffice. In other situations where there's like a harsher, more corrosion - wastewater is a good example of that - we'll do both. And then when the spray-on is included, that also - from the beginning - that also acts as curing as well.

So it's a really good way to ensure that that is done, and there’s QA to prove that curing as well. So yes, if that helps?

BRENDAN:

Yes, very good. And Matteo says “What can be done to mitigate carbonation in concrete?”

DOUG:

Yes, that's a good topic there, Matteo. With carbonation, you’re really looking at, again, the permeability of the concrete. And why that is is when you... CO2 by itself doesn't attack concrete per se.

It's when it mixes with moisture and forms carbonic acid that it actually starts to break the microstructure of the concrete down. So really, the best thing you can do against carbonation is to restrict that moisture entry at the surface of the concrete, or through that matrix of the concrete. Because if you can stop that moisture, that acid, from penetrating into the concrete, you restrict that attack really to the top surface.

And yes, obviously we've tested hydrogels, the spray-ons there. And we're working on the admixtures as well; a control versus a treated. And you can see the control, it was going in... I mean, it varies on the concrete obviously, but typically it's double the amount of penetration into the concrete than we can get when you treat it with a hydrogel admixture or a spray-applied. And with that, that doesn't just necessarily translate into double the service life. Because it's not just... slowing that down by half doesn't just translate to, say, 50 years to 100 years. There's obviously lots of other things that go into it.

But it does give you an indication of how much more service life you can get out of that concrete. Just on that, too, you can either protect the structure right from day one with the admixture, or there’s many spray-applied treatments that can be applied down the track just to stop that moisture ingress there.

CHARLES:

Awesome. It's good. So anything else, Brendan?

BRENDAN:

Not just come in at the moment... so that might be it. No, we’ve got somebody typing. Just a moment.

“Would sylane/syloxane treatment do something against carbonation?” [- Matteo]

DOUG:

Yes. In essence, in principle it does. [”Would a ‘blunt’ coating (painting) system be better? - Matteo”] So your sylane/syloxane is forming a barrier on the surface of the concrete. And that's stopping the moisture getting into the concrete. That's only effective if it's done right from the very start of the project.

Once you've got that carbonation inside the concrete, if it's approaching the steel, that silane's not really doing much for that front. In fact, when you're putting a coating on like that, and you've got moisture in the concrete, it actually excites the molecules and can move them round faster than if you did nothing.

So I would strongly suggest to look at something like the penetrating hydrogel treatments that penetrate into the concrete, and pass that first layer of reinforcing steel, and stop that moisture transmission in that critical cover zone there. So does that... would you have anything else to say on that, Charles?

CHARLES:

No, I was just going to say, in some cases, where there might be a requirement for something on the surface like that, the hydrogel is still used as back-up really, extra insurance for when there is a pinhole or a large failure, meaning the underneath isn't totally susceptible to attack. So just yes, really depends.

DOUG:

Yes. And it's the same kind of idea with a ... a silane essentially is a coating. They do penetrate in a little bit further than a coating. But yes, again, very good if you can get it on, on day one. But once you've got that damage manifesting, you do really need to look at it,

what you can do to that internal matrix of the concrete. And yes, optimizing your mix design, admixtures, everything from day one would, I would say, have more impact on that carbonation attack than applying a paint coating to the surface of that structure because...

I mean, I don't want to dive too deep in here, but the coatings also have replacement service lives as well. And that's often a lot, lot shorter. You know, might be 5, might be 10 years over against a 50-year design life. So you really want to have something... have a max design , or have thought about the durability to last for that 50 years without having to do anything, major or replenishment, coatings, repairs, that type of thing.

CHARLES:

Yes. Yes, that’s good.

DOUG:

So trust that's helpful.

CHARLES:

It's about preventing it from getting going in the first place rather than stopping it with a barrier film when it does come through. So yes, excellent. If there's any other questions we can happy to fly back. We can do it via email afterwards or reach out to either of us, we would be glad to help.

BRENDAN:

That’s good. The ones that have come in have been very good indeed. Much appreciated! And as Charles says, we’d be very pleased to collaborate, if there's any anybody that's got any projects or project questions or general questions that you're coming up with all the time, we're happy to chat to you about it. Thank you very much, everybody, for joining us today! Great to have your company. Great to have the questions and the interaction. Thanks, Charles and Doug!

CHARLES:

Thanks very much!

DOUG:

Awesome! Thanks, guys.

QUESTION SUBMITTED OFFLINE:

"If making concrete stronger may not make it last longer, what does making concrete stronger do?” - Carolyn

RESPONSE:

In terms of durability, the point is that increasing concrete strength to increase durability is not the sole answer.  There is some increase in durability with higher strengths if everything is done correctly. What we are trying to highlight is that there are other factors that need to be considered to achieve durability; such as curing, waterproofing, good mix design, good workability, quality finish.  There are cases where a lower MPa can be used, if these other factors are considered.