I have noticed that many concrete structures from he 1950’s and 60’s are showing deterioration. It seesm that as the steel rebars rust, the expansion caused by the rusting causes the concrete to crak and spall off. Presumably, this willcontinue untill the rebars have all rusted away.
Does this mean that there is a finite life to these structures? How long (for example) can the Hoover Dam be expected to last?
I notice that now, rebars are covered in plastice to prevent rusting-will these structures have an infinite life? I’ve heard that Roamn concrete is still good-it gets harder with age. Is our portlant cememnt the same? Can we expect that our modern concrete structures willlast for thousands of years?
Well, some buildings will last until deliberately demolished, releasing the singing frog yet again to ruin the lives of greedy workmen.
For the record, his name is Michigan J. Frog. And he can really sing, I swear!
You’ll have to give me free beer before I buy that.
Concrete erodes just like a sedimentary rock. So it won’t last forever.
The short answer is yes, they all will eventually fall apart. Bridges and overpasses have a lot of extra stresses put on them – temperature changes on both the top and bottom surfaces, weight, vibration and so on – that don’t affect a dam in the same way, but sooner or later erosion wins.
The rebar is only gonna rust if it’s exposed to the elements (or if the moron contractor used seawater to mix the contractor which actually happened down on the coast here). Most structures are designed to protect the rebar but, over time and uesage, you may develop some cracking. (BTW, most cracking absolutely does not affect the strength of the structure, it just looks unsightly) The cracking may let water get to the rebar, producing the spalling (having the surface of the concrete crack off) the OP observed.
Also, if the structures used an inappropriately designed or mixed concrete mixture, you might get spalling. (that’s what’s happening to the Physics building here on the Univ of Texas campus. It was built in the late 60’s with crappy concrete and now there’s a lot of spalling)
Otherwise, concrete will just erode eventually like SmackFu and kunilou have noted. Bu we won’t use structures for that long. The structures will outlive their usefulness and be replaced long before they fail.
If you’re thinking about an apocalyptic, Mad Max, fall of civilization type situation, the structures would last a looooong time. You’d no longer have the everyday loading the structures were designed for so it’d be even longer til they’d fall. It’d vary by structure, of course. Bridges over creeks and rivers would probably go first. The numerous highway overpasses would probably last until the ground changed significantly and they lost support. Older concrete buildings (like the Empire State Building) would last much longer than the newer, mostly steel skyscrapers. I’m not a geologist but I think the Hoover Dam (and the new Yangtze River Dam) are less likely to fail than the rock valleys they’re embedded in.
Right. Another issue at work with the deterioration of much of our infrastructure and buildings – and here in SJ, where reinforced concrete is the material of choice for everything from a bridge to your garage, I get to see that a lot – is that even reinforced concrete needs maintenance. The rigid concrete itself will eventually develop thin cracking, and depending on what you used for aggregate it may even have an ab-initio porousness, either of which will let moisture get to the rebar and start its nasty work, unless you periodically and conscientiously apply sealant and patch cracks. I dunno if the rebar itself could probably benefit from being grounded to some electron-sacrificial metal to control corrossion.
Portland cement was created as an artificial imitation of the Pozzoli volcanic ash that was the basis for Roman hydraulic cement; is is also supposed to get harder with time (up to a point). If properly maintained, nothing prevents a well-designed, well-built modern concrete structure to last 2,000 years, either.
You’re all on target so far. The mechanism is that cracks occur in the concrete and allow the permeation of water, which rusts the reinforcing steel, which expands and spalls off the concrete. The sacrificial electromagnetic protection does exist, do a search for “cathodic protection” and you’ll find out more than you want to know.
Coating the reinforcing steel with epoxy does slow this process down. But keep in mind that on your average 9" thick bridge deck, the top mat of steel is only generally about 3" below the surface. Cracks will occur over time and eventually will find any defects in the bar coating and start the process. Another method commonly used to retard the process in bridges is to overlay the fresh deck with epoxy modified concrete, which is less permeable. Epoxy coating of reinforcement has been pretty much standard practice since the mid-1970s.
Hoover Dam should last for centuries. The environment is quite benign, the loads are not dynamic, the concrete is incredibly thick and the rebars are buried much deeper.
Reinforced concrete structures are also designed so that the concrete will fail before the steel will (assuming proper maintenance). The idea being that you can see the concrete cracking and deal with it before failure, but you can’t see the steel. At least you’d better not be able to see it.
An additional wear factor here in the northern part of the country is the use of salt on the roads during the winter. This salt combines with the melt water, gets into small cracks, and rusts out the rebar much faster.
A few years ago we had part of a parking garage collapse here in Minneapolis due to this. The cars picked up salt from the roads, it melted off as they sat in the parking ramp during the day, and eventually ate thru enough rebar that part of the floors collapsed. Apparently this could have been prevented had the owner done proper maintenance to the concrete floors when the first small cracks appeared.
So why is Wrigley Field shedding concrete?
Nitpick - the Empire State is a steel frame building, as are most New York skyscrapers from the Park Row building (1895) onward. Very, very few tall buildings in Manhattan ever used concrete as a structural element.
Yes and I believe the cladding is granite.
Poor concrete - poor maintenance. Both, or take you pick.
When I saw this thread I immediately thought of the German bunkers that I observed in Guernsey (they are all over the place) and in Trondheim, Norway. I doubt that any maintenance has been done on any of them and most of them are in good condition (at least from my untrained observation). To think they would last indefinitely would seem to violate some sort of law.