Residential sidewalks, excluding driveways, shouldn't need reinforcement. Better self-healing of cracks would be valuable. My city has a huge backlog of sidewalk repairs.
Switching to pavers would be the trad solution, and have the added benefit of creating jobs. There are tools to speed the process of laying them down now too.
The sidewalk panel doesn't fail (as in the concrete breaking), it's still entirely intact. It's just lifted up on one side, so it no longer aligns with the adjacent panel — it fails by being a tripping hazard.
Thanks for writing about this. I happened to be in Rome when the paper came out and it was fun to visit the Pantheon with it in mind! Do you have any thoughts about fiber reinforcing and its effect on longevity?
Having spent a master’s program playing with GFRP and fiber-enhanced UHPC, I’ll venture out on a limb and say that the latter will remain uneconomical for most applications. It is used mainly for closure pours, bridge deck wearing surfaces and other high-strain/high-abrasion applications, within structures which are still mainly made of reinforced concrete with a 75- or 100-year design life.
GFRP reinforcement is becoming more common and affordable, though the sustainability of that fall in costs is debatable (mainly driven by Chinese state-owned firms selling both glass fiber and resin precursor chemicals below cost with subsidies to fill the gap).
GFRP applications are somewhat limited by both its brittle failure mode and relatively poor response to flexure compared to steel.
That said, I am seeing increased adoption and economic competitiveness for GFRP reinforcement and concrete-GFRP composite components including tub girders in infrastructure, even within a hundred year design life.
And, unlike many building structures, extending the design life of bridges and other transportation structures is of great value.
You are on to something. If you follow real estate news, you'll often see thirty year old skyscrapers being considered antiques and ripe for replacement. Before 9/11, there was a lot of talk about replacing the World Trade Center in such circles. It was considered a complex approaching obsolescence. Obviously, they were talking about tearing it down properly and safely, not crashing fuel laden airplanes into it.
Are we faced with such a binary choice? Could we not incorporate Calcium Oxide into reinforced concrete? I am no chemist, so I do not know what kind of reaction steel would have to this. Unless adding Calcium Oxide drastically alters the curing process (perhaps by slowing it down to an unacceptable degree, by modern standards), why could we not see it incorporated as an admixture in the future?
This is what I thought also, healing small cracks could prevent water from reaching the rebar and could be cheaper than using stainless steel for the rebar.
Concrete is a very porous material, and chlorides and CO2 will penetrate even without microcracking.
It's possible that this could reduce their intrusion enough to dramatically reduce the pH drop from these chemicals and therefore increase the life of the embedded reinforcement, make it worth spending the money. But it's also possible it won't make that much of a difference and therefore doesn't add value in proportion to the increased cost, or maybe only pencils out in some applications. More research will have to be done to see if it actually works. (And to make sure that there aren't hidden downsides--besides money--to this proposed addition.)
Thanks for writing this up. When I saw that tweet, my first thought was, "But this doesn't seem to address reinforcement corrosion directly?" This article saved me the time of having to dig around myself to make sure I wasn't crazy that this is still pretty far away from a practical improvement.
It'll be interesting to see if this has an effect on chloride/CO2 intrusion and consequent loss of reinforcement passivation. I don't recall that microcracking was a huge driver of that compared to porosity, but that's not something I've done much reading on in the last 20 years or so so I might be misremembering.
I find that difficult to imagine; a typical 150 foot precast girder might develop a camber of close to an inch after form release, so much so that it's common to wait a week and then "top" it with what's basically a glorified skim coat to mitigate microcracking.
But that figure will *grow* during storage and even after installation in a bridge as concrete creep allows prestressing to pull it further out of plane, routinely hitting 3-4 inches. All the microcracking thus induced is occurring in the top face, facing whatever moisture is leaching down through the deck or joints, and any drainage failures.
Now, of course the whole point of prestressing is that the vast majority of the section depth is under compression, so it's not like there's actual section cracking anywhere near the top face, but microcracking from camber application is substantial.
I've not done anything at all with prestressed (other than a class on it in grad school) so I take your point that microcracking is important there, and possibly in regular RC.
I'd still like to see it demonstrated with research.
Given the frequency with which one has to replace moment-bearing prestressed components in bridges, and the comparative infrequency for compression-bearing RC ones, I cannot conceive that the answer will be anything except "microcracking greatly speeds chloride infiltration."
I do not have the time to go beg access to a JSTOR account and find out to what extent this topic has already been explored, unfortunately.
If steel is that much stronger, why use concrete at all? Just cost?
Something as simple as painting metal (or if you're feeling more advanced, plating with a corrosion resistant coating) can often reduce corrosion when the metal is bare. Has this been looked at for reinforced
Yup. Cost is the main reason to choose between a steel and concrete frame in building structures. There might be some possible architectural considerations--can't have architecturally exposed structural steel if you're using a concrete frame!--but often those will be downstream of "steel or concrete?" as an economic question.
In fairness, these considerations matter much less in building structures as compared to bridges and other exposed infrastructure applications.
And there, unfortunately, the accumulated lessons of a century tell us that in practice steel doesn’t do much better than concrete except in signature (ruinously expensive to replace) structures with dedicated funding streams. Otherwise, owners’ maintenance budgets are inconsistent, portions of exposed surfaces are inaccessible, coatings are applied imperfectly in the field, etc.
Plenty of steel girder bridges out there barely limping along within their 75-year design lives, and an increasing number of prestressed concrete girder ones living well beyond it.
Particularly enjoy posts like this that bring some relevant news to those of us that otherwise might miss it and tie in a whole bunch of other informative context as well. I know far more about concrete reinforcement than I did 30 mins ago.
Still curious why we would not use Roman elements in the recipes for modern concrete. Is it lost knowledge, not worth the cost, other performance issues?
I just think it’s amazing that without modern analytic tools, that they were able to figure out how to make, and use, such long lasting concrete. I think our powers of observation have atrophied.
I recently read a similar article on Roman concrete that said something completely different - that it was the high temperature it was mixed at that resulted in stronger chemical properties (nothing to do with rebar). The article didn't say how it was heated or how hot it was, just that its molecular composition was extraordinary.
Residential sidewalks, excluding driveways, shouldn't need reinforcement. Better self-healing of cracks would be valuable. My city has a huge backlog of sidewalk repairs.
Switching to pavers would be the trad solution, and have the added benefit of creating jobs. There are tools to speed the process of laying them down now too.
"creating jobs" basically simply means it would cost more (if it doesn't, it's not creating jobs on net). So that's not a benefit- it's a cost.
Not necessarily - it could mean that the costs are just distributed differently between materials and labor.
Here (given, suburbs, not city) the sidewalks normally fail because of tree roots lifting them. I don't think self-healing concrete would help.
or maybe they would heal gradually as the root grows gradually, like an expanding accordian rather than a bike-flipping shattered plate?
The sidewalk panel doesn't fail (as in the concrete breaking), it's still entirely intact. It's just lifted up on one side, so it no longer aligns with the adjacent panel — it fails by being a tripping hazard.
Thanks for writing about this. I happened to be in Rome when the paper came out and it was fun to visit the Pantheon with it in mind! Do you have any thoughts about fiber reinforcing and its effect on longevity?
Having spent a master’s program playing with GFRP and fiber-enhanced UHPC, I’ll venture out on a limb and say that the latter will remain uneconomical for most applications. It is used mainly for closure pours, bridge deck wearing surfaces and other high-strain/high-abrasion applications, within structures which are still mainly made of reinforced concrete with a 75- or 100-year design life.
GFRP reinforcement is becoming more common and affordable, though the sustainability of that fall in costs is debatable (mainly driven by Chinese state-owned firms selling both glass fiber and resin precursor chemicals below cost with subsidies to fill the gap).
GFRP applications are somewhat limited by both its brittle failure mode and relatively poor response to flexure compared to steel.
That said, I am seeing increased adoption and economic competitiveness for GFRP reinforcement and concrete-GFRP composite components including tub girders in infrastructure, even within a hundred year design life.
And, unlike many building structures, extending the design life of bridges and other transportation structures is of great value.
I wonder if it's a good thing that so many buildings will fall apart after 50 years because it lets you put something new in its place.
You are on to something. If you follow real estate news, you'll often see thirty year old skyscrapers being considered antiques and ripe for replacement. Before 9/11, there was a lot of talk about replacing the World Trade Center in such circles. It was considered a complex approaching obsolescence. Obviously, they were talking about tearing it down properly and safely, not crashing fuel laden airplanes into it.
Are we faced with such a binary choice? Could we not incorporate Calcium Oxide into reinforced concrete? I am no chemist, so I do not know what kind of reaction steel would have to this. Unless adding Calcium Oxide drastically alters the curing process (perhaps by slowing it down to an unacceptable degree, by modern standards), why could we not see it incorporated as an admixture in the future?
This is what I thought also, healing small cracks could prevent water from reaching the rebar and could be cheaper than using stainless steel for the rebar.
Concrete is a very porous material, and chlorides and CO2 will penetrate even without microcracking.
It's possible that this could reduce their intrusion enough to dramatically reduce the pH drop from these chemicals and therefore increase the life of the embedded reinforcement, make it worth spending the money. But it's also possible it won't make that much of a difference and therefore doesn't add value in proportion to the increased cost, or maybe only pencils out in some applications. More research will have to be done to see if it actually works. (And to make sure that there aren't hidden downsides--besides money--to this proposed addition.)
Thanks for writing this up. When I saw that tweet, my first thought was, "But this doesn't seem to address reinforcement corrosion directly?" This article saved me the time of having to dig around myself to make sure I wasn't crazy that this is still pretty far away from a practical improvement.
It'll be interesting to see if this has an effect on chloride/CO2 intrusion and consequent loss of reinforcement passivation. I don't recall that microcracking was a huge driver of that compared to porosity, but that's not something I've done much reading on in the last 20 years or so so I might be misremembering.
I find that difficult to imagine; a typical 150 foot precast girder might develop a camber of close to an inch after form release, so much so that it's common to wait a week and then "top" it with what's basically a glorified skim coat to mitigate microcracking.
But that figure will *grow* during storage and even after installation in a bridge as concrete creep allows prestressing to pull it further out of plane, routinely hitting 3-4 inches. All the microcracking thus induced is occurring in the top face, facing whatever moisture is leaching down through the deck or joints, and any drainage failures.
Now, of course the whole point of prestressing is that the vast majority of the section depth is under compression, so it's not like there's actual section cracking anywhere near the top face, but microcracking from camber application is substantial.
I've not done anything at all with prestressed (other than a class on it in grad school) so I take your point that microcracking is important there, and possibly in regular RC.
I'd still like to see it demonstrated with research.
Given the frequency with which one has to replace moment-bearing prestressed components in bridges, and the comparative infrequency for compression-bearing RC ones, I cannot conceive that the answer will be anything except "microcracking greatly speeds chloride infiltration."
I do not have the time to go beg access to a JSTOR account and find out to what extent this topic has already been explored, unfortunately.
If steel is that much stronger, why use concrete at all? Just cost?
Something as simple as painting metal (or if you're feeling more advanced, plating with a corrosion resistant coating) can often reduce corrosion when the metal is bare. Has this been looked at for reinforced
Yup. Cost is the main reason to choose between a steel and concrete frame in building structures. There might be some possible architectural considerations--can't have architecturally exposed structural steel if you're using a concrete frame!--but often those will be downstream of "steel or concrete?" as an economic question.
In fairness, these considerations matter much less in building structures as compared to bridges and other exposed infrastructure applications.
And there, unfortunately, the accumulated lessons of a century tell us that in practice steel doesn’t do much better than concrete except in signature (ruinously expensive to replace) structures with dedicated funding streams. Otherwise, owners’ maintenance budgets are inconsistent, portions of exposed surfaces are inaccessible, coatings are applied imperfectly in the field, etc.
Plenty of steel girder bridges out there barely limping along within their 75-year design lives, and an increasing number of prestressed concrete girder ones living well beyond it.
Really liked this post, thanks for putting in the time!
Thank you for the professional take on the issue.
Particularly enjoy posts like this that bring some relevant news to those of us that otherwise might miss it and tie in a whole bunch of other informative context as well. I know far more about concrete reinforcement than I did 30 mins ago.
Still curious why we would not use Roman elements in the recipes for modern concrete. Is it lost knowledge, not worth the cost, other performance issues?
I just think it’s amazing that without modern analytic tools, that they were able to figure out how to make, and use, such long lasting concrete. I think our powers of observation have atrophied.
I recently read a similar article on Roman concrete that said something completely different - that it was the high temperature it was mixed at that resulted in stronger chemical properties (nothing to do with rebar). The article didn't say how it was heated or how hot it was, just that its molecular composition was extraordinary.
https://www.foxnews.com/science/research-uncover-secret-made-ancient-roman-concrete-durable.amp
I believe that the heat came from the inclusion of quicklime, which heats up in an exothermic reaction.