Iron rods rusting is the timebomb in all rebar structures?

[repoman]

Is this the main reason that most modern structures will have a very short lifetime compared to antique ones?

The rebar rusts and expands blowing up the structure given enough time.

 

[Philos]

Is this the main reason that most modern structures will have a very short lifetime compared to antique ones?

The rebar rusts and expands blowing up the structure given enough time.

repoman,

In the 1960s we called this 'concrete cancer'.

A.

 

[OLDMAN]

I was working on an overpass once, and we had to bend a transport rod to make a straight path for some rebar. The transport rod broke off completely when we tried to bend it. You should have seen the look on the foreman's face. But it makes one wonder just how rusty all the steel was in that bridge. It is still standing but only ten years old. It shouldn't collapse all at once like some do, but maybe a little by little.

 

[Bronzeage]

Is this the main reason that most modern structures will have a very short lifetime compared to antique ones?

The rebar rusts and expands blowing up the structure given enough time.

The longevity has more to do with the quality of the concrete and the mineral content to water in the original mix. A small amount of salt in the water will corrode the rebar, but it maybe decades before it shows. Another factor is whether the concrete is porous and is exposed to weather.

 

[Sarpedon]

Like most structures, the health is directly related to how well it sheds and repels water. Properly designed reinforced concrete can last as long as anything made by mortal hands. There are plenty of deteriorating newer structures, and plenty of very old ones that still stand with no problems. The main problem is that developers have no incentive to build high quality buildings. Buildings are built, and are quickly sold. The faster the developer can return a profit, the better for him. In the absence of regulation and discerning buyers, that translates to shoddy work. In bridges, it is a different problem. Since many bridges go over water, the environment is constantly moist. In cold climates. de-icing salt is often used. These two factors are not good for concrete. and poorly designed and protected or cheap concrete will suffer.

Ancient buildings, by contrast, seem better built, because only the well built ones are left. The crappy old buildings have been gone for centuries.

Also, it's Steel bars, not iron. Learn the difference.

 

[LordKiran]

Supposedly the longlasting nature of old roman buildings stems from the concrete they used, it just so happened that contact with water caused the growth of crystal matrices in the concrete that actually made it stronger!

Last I looked, scientists are still trying to figure out the precise composition of this concrete but that it has something to do with volcanic rock that was widely available in Italy at the time.

 

[Bronzeage]

Supposedly the longlasting nature of old roman buildings stems from the concrete they used, it just so happened that contact with water caused the growth of crystal matrices in the concrete that actually made it stronger!

Last I looked, scientists are still trying to figure out the precise composition of this concrete but that it has something to do with volcanic rock that was widely available in Italy at the time.

The volcanic ash which is plentiful in Italy made an excellent cement. The Romans learned how to process the ash by heating and other methods, and used it to build very high quality concrete, including concrete which hardened under water.

The formulas and engineering was lost for many centuries, but these days all have been re-engineered and is well understood.

 

[Loren Pechtel]

Two words: Basalt rebar.

It doesn't rust like iron rebar which not only means the structure will last longer but that you can do things that wouldn't be acceptable with standard iron rebar. With normal concrete code "requires" a thickness of at least 4 inches--even in low-stress situations where that's major overkill. With basalt rebar you're no longer required to bury it as deep in the concrete (because it doesn't need to be protected from rusting--the thickness "requirement" is 2x the required burying depth) and thus you can make a thinner wall.

 

[Sarpedon]

Fiberglass is similar but probably more sustainable.

 

[bilby]

Like most structures, the health is directly related to how well it sheds and repels water. Properly designed reinforced concrete can last as long as anything made by mortal hands. There are plenty of deteriorating newer structures, and plenty of very old ones that still stand with no problems. The main problem is that developers have no incentive to build high quality buildings. Buildings are built, and are quickly sold. The faster the developer can return a profit, the better for him. In the absence of regulation and discerning buyers, that translates to shoddy work. In bridges, it is a different problem. Since many bridges go over water, the environment is constantly moist. In cold climates. de-icing salt is often used. These two factors are not good for concrete. and poorly designed and protected or cheap concrete will suffer.

Ancient buildings, by contrast, seem better built, because only the well built ones are left. The crappy old buildings have been gone for centuries.

Also, it's Steel bars, not iron. Learn the difference.

Ancient buildings were also often over engineered - they were less sure about what would or would not stand up to the elements; so they erred on the side of caution.

Thick walls last millennia, even if fairly poorly made. Well made thin walls, with buttressing or flying buttressing, can be stronger, when new, but less long-lived. Modern structures are designed for a life span; and are highly unlikely to outlive that by an order of magnitude - if they did, it would represent wasted money.

Of course we also over engineer some structures; the concrete containment vessels of nuclear reactors will likely last tens of millennia, unless deliberately demolished.